REVIEW

Phosphate adsorption on metal oxides and metal hydroxides:A comparative reviewMengxue Li, Jianyong Liu, Yunfeng Xu, and Guangren Qian

Abstract: Phosphorus removal from wastewater is important for eutrophication control of water bodies. Metal oxides and metalhydroxides have always been developed and investigated for phosphorus removal, because of their abundance, low cost,environmental friendliness, and chemically stability. This paper presents a comparative review of the literature on the prepa-ration methods, adsorption behaviors, adsorption mechanisms, and the regeneration of metal (hydr)oxides (e.g., Fe, Zn, Al, etc.)with regard to phosphate removal. The contrasting results showed that metal hydroxides could offer an effective and economicalternative to metal oxides, because of their cost–benefit synthesis methods, higher adsorption capacities, and shorter adsorp-tion equilibrium times. However, the specific surface area of metal oxides is larger than that of metal hydroxides because of thecalcination process. Metal oxides with a higher pH at the zero point of charge have wider optimal adsorption pH ranges thanmetal hydroxides because of their surface precipitation in alkaline solutions. The regeneration of metal oxides using acids,bases, and salts and that of metal hydroxides using acids and bases has been critically examined. Further research on uniformmetal (hydr)oxides with small particle size, high stabilities, low cost, and that are easily regenerated with promising desorbentsare proposed. In addition, quantitative mechanism study and application in continuous-mode column trials are also suggested.

Key words: phosphate adsorption, metal hydroxides, metal oxides.

Résumé : L’élimination du phosphore des eaux usées est une manière importante de contrôler l’eutrophisation des plans d’eau. Lesoxydes métalliques et les hydroxydes métalliques ont toujours été développés et examinés aux fins de l’élimination du phosphore, enraison de leur abondance, leur coût minime, leur respect de l’environnement et leur stabilité chimique. Cet article présente une revuecomparative des littératures sur les méthodes de préparation, les comportements d’adsorption, les mécanismes d’adsorption et larégénération des hydr(oxydes) métalliques (p. ex., Fe, Zn et Al) en ce qui concerne l’élimination du phosphate. Les différences derésultats ont montré que les hydroxydes métalliques peuvent offrir une alternative efficace et économique aux oxydes métalliques, enraison de leurs méthodes de synthèse a coût avantageux, leurs fortes capacités d’adsorption et leur temps d’atteinte d’équilibred’adsorption plus court. Cependant, la surface spécifique des oxydes métalliques est plus grande que celle des hydroxydes métalliquesen raison du procédé de calcination. Les oxydes métalliques ayant un pH au point de charge nulle plus élevé ont une gamme de pH pluslarge pour l’adsorption optimale qu’en ont les hydroxydes en raison de leur précipitation de surface dans les solutions alcalines. Larégénération des oxydes métalliques au moyen d’acides, de bases et de sels et celle des hydroxydes au moyen d’acides et de bases ontfait l’objet d’examens critiques. On propose de plus amples recherches sur les (hydr)oxydes métalliques uniformes ayant une petitedimension de la particule, une haute stabilité, un coût avantageux et étant facilement régénérés a l’aide de désorbants prometteurs.De plus, on suggère aussi une étude quantitative des mécanismes et l’application d’essais de colonne en mode continue (« continuous-mode column trials »). [Traduit par la Rédaction]

Mots-clés : adsorption du phosphate, hydroxydes métalliques, oxydes métalliques.

1. IntroductionPhosphate (PO4) is often the limiting factor for primary produc-

tion by phytoplankton in lakes, and excess PO4 can result in eu-trophication. Phosphorus (P) in wastewater must be removed orreduced and subsequently (or simultaneously) recovered to avoideutrophication (Bhargava and Sheldrakar 1992). Metal oxides andmetal hydroxides (including industrial by-products and naturalmineral compounds containing these materials) are attractivecandidates as sorbents for P removal and recovery (Rittmann et al.2011), because metal (hydr)oxides are not only nontoxic and inex-pensive, but also environmentally friendly and chemically stable(Chu et al. 2009; Liao et al. 2005).

In previous studies, Yang Xiaofang et al. (2007) studied the ad-sorption of phosphate on pseudo-�-AlOOH and �-Al2O3 and showed

that pseudo-boehmite had a larger specific surface area, that�-Al2O3 was more acidic than pseudo-boehmite, and that pseudo-boehmite exhibited greater adsorption capacity than �-Al2O3 atpH 4 and 6. Moreover, in our previous study, the adsorption ca-pacity of lanthanum hydroxide-doped activated carbon fiber(16.1 mg/g) was found to be significantly higher than that of lan-thanum oxide-doped activated carbon fiber (9.5 mg/g) under thesame adsorption conditions (Zhang et al. 2012; Zhang et al. 2011).These results showed that the performances of metal oxides andmetal hydroxides for phosphate adsorption differ greatly. Thesedifferences were also reported by many other researchers in termsof the phosphate adsorption capacities, which range from 11.4 to111.1 mg P/g for various iron-based sorbents (Acelas et al. 2015;Mezenner and Bensmaili 2009; Yuan et al. 2014; Zhang et al. 2015).

Received 4 November 2015. Accepted 12 May 2016.

M. Li, J. Liu, Y. Xu, and G. Qian. School of Environmental and Chemical Engineering, Shanghai University, 333 Nanchen Road, Shanghai 200444,P.R. China.Corresponding authors: Jianyong Liu (email: liujianyong@shu.edu.cn), Guangren Qian (email: grqian@shu.edu.cn).Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink.

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Environ. Rev. 24: 319–332 (2016) dx.doi.org/10.1139/er-2015-0080 Published at www.nrcresearchpress.com/er on 16 May 2016.

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The removal efficiency depends widely on the chemical andphysical characteristics (i.e., particle sizes, surface functional groups,specific surface area, metal content, and stability) of the sorbentsand the environmental conditions, such as pH, ion strength, com-petitive ions, dosage, initial phosphate concentrations, and tem-perature present in solution. Thus, this efficiency may depend onthe different adsorption mechanisms of metal oxides and metalhydroxides. Although there are several reviews on phosphate ad-sorption from water and wastewater (Ramasahayam et al. 2014;Rittmann et al. 2011; Xu et al. 2012), the possible differences (i.e.,preparation methods, adsorption performances, and adsorptionmechanisms) between metal hydroxides and metal oxides remainunclear. To the best of our knowledge, no comparative review hasbeen published about the adsorption of phosphate from waterand wastewater using metal oxides and metal hydroxides.

This review focuses on the comparison of metal oxides andmetal hydroxides for P removal, mostly in batch experimentalsystems using synthetic water. A summary of relevant publisheddata (in terms of synthesis methods, adsorption isotherm andkinetics, competitive anions, and pH) with some of the latest im-portant findings is presented and the results have been compared.The adsorption mechanisms of metal (hydr)oxides and sorbentregeneration are also compared. Furthermore, future prospects ofmetal (hydr)oxides and their potential applications are discussed.

2. Synthesis methods of metal (hydr)oxidesSorbents for phosphate exist in many forms in nature. Metal

oxides and metal hydroxides are the most common forms. Theirease of synthesis and modification and the ability to control ormanipulate matter using surface chemistry could provide unpar-alleled versatility. It has been reported that preparation methodsplay a key role in determining the size distribution, morphology,specific surface area, stability, and surface chemistry of materials.The list and comparison of synthesis methods are shown inTable 1.

2.1. Synthesis methods of metal hydroxides

2.1.1. Sol-gel methodSol-gel is a solution chemistry-based technique to synthesize

pure, stoichiometric, and monodispersed nanoparticles (Corenoet al. 2003). This technique is based on the hydrolysis of liquidprecursors and the formation of colloidal sols. Metal precursors,metals, or metalloid elements surrounded by various reactive ligandsare the starting materials, and these undergo slow hydrolysis andpolycondensation reactions to form a colloidal system called a sol.The sol evolves and leads to the formation of a network contain-ing a liquid phase, called a gel (Zhang et al. 2004). This method canbe performed at low temperatures and is suitable for the large-scale production of nanoparticles with a relatively narrow sizedistribution (Qin 2007). Although this method may be a time-consuming process, it has been found to be effective for dispers-ing small metal hydroxide particles (Zhou et al. 2012).

2.1.2. Precipitation methodThe precipitation method is an easy and convenient way to

synthesize metal hydroxides from drying aqueous salt solutionswith the addition of a base. The control of the preparation processis difficult, and the prepared particles easily aggregate. However,through this method, a variety of metal hydroxides with bothcrystalline and amorphous nature could been synthesized (Chitrakaret al. 2006a; Guo et al. 2011; Xie et al. 2014; Zhu et al. 2009). BothX-ray diffraction analysis and scanning electron microscopy havedemonstrated that the materials have porous structures and irreg-ular and rough surfaces. Small-scale materials with a homoge-neous chemical composition can be obtained through a variety ofchemical reactions in solution (Liao et al. 2006). T

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2.1.3. Post-grafting and metal cation incorporation methodGenerally, it is possible to reinforce the chemical stability of the

sorbents by crosslinking treatments or grafting reactions. There-fore, a number of researchers reported the synthesis methods formetal hydroxides using a post-grafting and metal cation incorpo-ration process. Several types of functionalized materials wereused by grafting chelating ligands on the surface of mesoporousmaterials. The chelating functional groups can be bound tightly tometals, which can remove anions from water (Puanngam andUnob 2008; Yoshitake et al. 2003). Different organic functionalgroups (i.e., amine, thiol, carboxylic, aromatic, etc.) have beencovalently bonded to the structures of mesoporous materials via apost-synthesis grafting or co-condensation method. Materials likesilica gel, chitosan, and biomass, with different functional groups,have been used as a polymer support for the preparation of sor-bent material having functional groups (Manju and Anirudhan2000). To provide metal adsorption sites, metal salt solutions werestirred with functionalized materials, followed by washing usingdeionized water and 2-propanol. The substrate can be pretreatedby an aqueous solution of an anionic polymer, such as carbox-ymethyl cellulose. This method could bring about benefits forincreasing phosphate adsorption capacity (Chouyyok et al. 2010;Huang et al. 2013). Unfortunately, most improvements in the phos-phate adsorption capacity were accompanied by increasing costsand waste stream production.

2.1.4. Ion exchange methodThe ion exchange method is usually used when porous sorbents

have the function of cation exchange (Haghseresht et al. 2009). Ionexchange occurs between soluble ions, such as K+, Na+, Ca2+, andNH4

+, and metal ions, such as La3+ in La(NO3)3 • 6H2O (Shin et al.2005) or with the positively charged metal sol particles directly.The particle size could be controlled by selecting the pore size ofthe carriers to obtain high activity. However, apertures may notmatch well with metal ions or the positively charged metal solparticles. Moreover, some functional components incorporated inthe porous medium may release from the sorbent into aqueoussolution.

2.2. Synthesis methods of metal oxides

2.2.1. Metal hydroxides-drying or calcinationMetal oxides can be obtained after calcination or drying (>100 °C) of

metal hydroxides (Lv et al. 2013). This method is performed byheat-treating the metal hydroxides in the atmosphere. Activationin the process is needed to enhance the porosity and clean out thepores. Thus, a better porous structure can be obtained using thismethod.

2.2.2. Chemical methodThe chemical method has also been used to synthesize metal

oxides (Yang et al. 2011). In this method, metal salts are mixed withmesoporous or low-cost materials. In some cases, soluble starch isalso mixed into the water to create additional pores after thegreen granules are fired at high temperature (Chen et al. 2012).The sorbents are prepared successfully after drying the loadedcarriers under normal atmospheric temperatures or 100 °C andthen calcining at 300–600 °C (H. Li et al. 2009). Although researchhas shown that particles tend to aggregate, it has also shown thatthe chemical method can enhance the adsorption potentialthrough a simple and low cost process (Liu et al. 2008; Nguyenet al. 2014; Shin et al. 2004) and does not require high sinteringtemperatures.

2.2.3. Hydrothermal methodThe hydrothermal method has also been reported in the litera-

ture. In this technique, reactions are performed in an aqueousmedium in reactors or autoclaves at high vapor pressures andhigh temperatures. This process needs special devices and is a

time-consuming process that results in a low yield. However, ithas also been utilized to grow dislocation-free single-crystal par-ticles, and grains formed through this process may have superiorcrystallinity to those grown using other methods. Therefore, onecan obtain highly crystalline, pure, and well-dispersed nanopar-ticles using the hydrothermal technique (Wu et al. 2008).

2.2.4. Natural oxidesMost natural oxides are composed of clay (e.g., Sepiolite, Kanuma

clay), solid waste (e.g., sludge), and byproducts of the mining in-dustry (e.g., red mud), the steel industry (e.g., slag materials), orthe power plant industry (e.g., fly ash and bottom ash). In generalthey are composed mainly of oxides of Si, Al, Fe, Ca, Mg, and Ti indifferent proportions.

These natural oxides are usually first dried, then crushed andsieved (<5 mm in diameter) because phosphate adsorption occursmostly in fine particles in batch experiments (Asaoka and Yamamoto2010; Yue et al. 2010). Moreover, the sorbents could also be acti-vated by chemical (acid or alkaline treatment), thermal, or chem-ical or thermal pretreatment (Lu et al. 2009; Ye et al. 2006) toimprove the sorption capacity. Pretreatment could be performedto change sorbent surface charges and remove undesired organiccompounds or competing ions from the sorbents and hence im-prove adsorption capacity and efficiency (Li et al. 2006; Xue et al.2009). However, in some cases, pretreatment gives adverse effects(Cheung and Venkitachalam 2000). Prior characterization studieson the biosorbent may help in selecting a suitable pretreatmentoption.

Adsorption in fixed-bed columns has shown relatively higherphosphate uptake as compared with other adsorption procedures(Xu et al. 2013). Column flow-through adsorption tests are con-ducted with synthetic phosphate solutions mostly using naturaloxides (Liesch 2010; Huang et al. 2009). This means that the sievednatural particles are small and thus the infiltration action throughthe packed column is well built (Zeng et al. 2004). Various metalcontents in natural oxides contribute more to the phosphate ad-sorption capacity. Thus it will be a good choice to use naturaloxides in column experiments.

2.3. Optimal synthesis methodsTo date, most reports have focused on the synthesis of small-

sized metal (hydr)oxides. Small-scale metal (hydr)oxides from mi-crometre to nanometre size are mostly reported. Studies haveshown that sorbents with small sizes could enhance activity andmechanical properties and significantly increase specific surfacearea. In addition, aggregation causes a loss of contact area for theadsorbate, which results in low adsorption capacities. Thus, sol-gel and hydrothermal methods for metal sorbents are ideal choicesfor fabricating well-dispersed sorbents in the nanometre range.The time-consuming process, special devices needed, and low yieldassociated with this technique could be overcome by updatingdevices and expanding dosages.

2.4. Comparison of the properties of the sorbents preparedthrough different synthesis methods

It is obvious that metal hydroxides and metal oxides have com-mon properties when synthesized using the previously mentionedsynthesis methods. Metal (hydr)oxides with various morpholo-gies, including crystallinity, porosity, roughness, and amorphousstructure, are more active in removing phosphate. However, incontrast to the methods used to prepare metal oxides, the prepa-ration process of metal hydroxides does not require calcination.With respect to energy consumption for calcination, the prepara-tion methods of metal hydroxide appear to be cost-beneficial.

As shown in Table 1, the specific surface areas of metal oxidesare greater than those of metal hydroxides. The application ofcalcination could not only accelerate the thermal degradation ofthe metal oxides, lead to the development of pores, and increase

Li et al. 321

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the surface area, but also lead to a subsequent loss of mass. Moretime is required to eliminate all of the moisture and most of thevolatile components in the precursor, resulting in the develop-ment of pores. As previous studies have shown, the extent ofadsorption may depend on the specific surface area; therefore, itis reasonable to conclude that the adsorption is, to some extent, asurface phenomenon driven by van der Waals forces. However,the adsorption of P could also be occurring through chemicalbonds at active sites formed on the sorbent surface at the time ofactivation.

Table 2 shows that the metal content of metal hydroxides ishigher than that of metal oxides. Metal content on the sorbentsurface could form surface active sites (e.g., metal–OH) for phos-phate adsorption (Özacar 2006; Deng and Yu 2012). The presenceof more OH groups on the sorbents creates more possibility ofphosphate–OH ion exchange, indicating a higher phosphate ad-sorption capacity. Thus higher metal content could provide moreactive sites for phosphate adsorption, which may result from theeffectiveness of the synthesis methods of metal hydroxides. Soboth metal content and surface area should be considered in thepreparation of efficient sorbents to achieve high adsorption ca-pacity.

Among La, Fe, Zn, and Al elements, La and Fe were found to bedetached vigorously during their performance (Huang et al. 2009;B. Li et al. 2009). In contrast, the results also show a minimalamount of released metal ions from the sorbents; however, it isdifficult to judge whether the stability of the metal oxides is betterthan that of the metal hydroxides because only a few studies haveshown the stability of the sorbents (Table 2). Commonly, the sta-bility of sorbents also has an effect on the adsorption capacity andis influenced by electrostatic and van der Waals interactions (Chenet al. 2007). Work is still needed to understand the mechanismsunderlying the enhancement of the stability of metal sorbents byreducing their surface energy, but this limits their large-scale ap-plication. The stability of metal sorbents could be greatly aug-mented by surface modifications with suitable functional groups, suchas aromatic, carboxylic acid, and amine groups. It should be notedthat the application of metal sorbents is strongly related to theintrinsic properties of the metal sorbents, which highly dependon the synthesis methods and modification mediums.

3. Adsorption behaviors of metal (hydr)oxidesA large number of studies have been carried out on metal

(hydr)oxides for potential use in batch systems using syntheticwaters. The batch mode of adsorption is static and conducted in aclosed system and therefore the data obtained are generally notapplicable to most real systems. However, the method is simpleand quick information can be obtained on the effects of manysolution variables on adsorption and can examine the mechanismof adsorption and compare the adsorption capacity of varioussorbents.

3.1. Adsorption isothermThe adsorption isotherm is significant for the explanation of

how the sorbent will interact with the adsorbate and helps eluci-date the adsorption capacity. Isotherms play an important role inunderstanding the adsorption mechanisms of metal sorbents (e.g., Fe,

Zn, Al, etc.). Table 3 shows the various isotherm results of P adsorp-tion by metal sorbents. The common adsorption isotherms employedin the literature follow the order Langmuir > Freundlich > multi-Langmuir ≈ Langmuir–Freundlich.

In practice, the two-parameter equations (Freundlich and Lang-muir) are more widely used than the three-parameter equations(multi-Langmuir and Langmuir–Freundlich) because of the incon-venience of evaluating three isotherm parameters. However, athree-parameter equation, which is reported in a few papers, canoften provide a better fit and is more representative of the iso-therm data than a two-parameter equation (Zeng et al. 2004).Much work is still needed to examine multi-parameter equationsfor phosphate adsorption. The Langmuir adsorption isotherm modelassumes that adsorption takes place at specific homogeneoussites within the sorbent and has been used successfully for manymonolayer adsorption processes. The Freundlich adsorption iso-therm model considers a heterogeneous adsorption surface thathas unequal available sites with different energies of adsorption.Freundlich does not predict saturation of the sorbent surface,whereas the Langmuir isotherm has been used to study the sur-face monolayer saturation, which is beneficial for comparing themaximum capacities of an adsorbate.

However, the theory behind the isotherm equation is based onthe adsorption of gases to uniform surfaces, and in the strict senseit can only be used to describe adsorption processes. Ca, Al, andMg being present in metal oxides also promotes precipitationreactions simultaneously with the adsorption processes at pH > 7–8.Veith and Sposito (1977) showed that the Langmuir equation candescribe precipitation and secondary precipitation reactionswhen studying well-defined and isolated reactions. But whenmore complex reactions occur, the Langmuir equation seldomapplies (Barrow 1978). Based on the guidelines of Del Bubba et al.(2003) and Arias et al. (2001) for comparison of sorbents, P adsorp-tion isotherms should be carried out using short-term isothermexperiments, using distilled water (to minimize precipitation re-actions) and pH values as far apart as the alkaline range.

Because standard procedures for batch adsorption experimentsdo not exist, a direct comparison of adsorption capacity is difficultbecause of the different applied experimental conditions (Cucarellaand Renman 2009). To illustrate the potential of the use of metalsorbents for the removal of P, a comparative evaluation of theadsorption capacities of metal sorbents under similar experimen-tal conditions is provided (Table 4). Among metal (hydr)oxides,hybrid sorbents (bi- or multi-metal sorbents) exhibit improvedadsorption capacities. Some sorbents, such as Fe–Mn binary oxide(Zhang et al. 2009), lanthanum hydroxide (Xie et al. 2014), iron(III)-loaded carboxylated polyacrylamide-grafted sawdust (Unnithanet al. 2002), and hydrated metal oxides dispersed within anionicexchange media (Acelas et al. 2015), have a relatively high capacityfor phosphate. So it is better for additional studies to develophybrid sorbents that are cost-effective and efficient for practicalapplications. Meanwhile, under similar experimental conditions,the phosphate adsorption capacity of metal hydroxides is slightlygreater than that of metal oxides, which is also in accordance withthe results reported by Zhang et al. (2015).

Enhanced phosphate removal by metal hydroxides may be at-tributed to the presence of more hydroxyl groups on the surface

Table 2. The metal contents and stabilities of metal (hydr)oxides.

AdsorbentMetalcontent Stability Adsorbents and references

Metal oxides 0.1%–46% 0%–0.09% of releasedmetal ions

Zr(IV) loaded okara (Nguyen et al. 2014), TiO2 loaded zeolite (Alshameri et al. 2014), coalash (Asaoka et al. 2010), bentonites (Zamparas et al. 2012), red mud (Yue et al. 2010)

Metal hydroxides 4%–68% 0%–7.4% of releasedmetal ions

Cu or Fe–nanoporous sorbents (Chouyyok et al. 2010), lanthanum hydroxide (Xie et al.2014), cerium–fibrous protein (Deng and Yu 2012), waste alum sludge (Babatunde andZhao 2010), Zr(IV)–chitosan (Sowmya and Meenakshi 2014)

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of metal hydroxides, which provide more adsorption sites com-pared with those found on the surfaces of metal oxides. The cal-cination process may influence weak phosphate adsorption onthe surface of metal oxides during sorbent synthesis. Some organ-ics and hydroxyl groups, which are the effective sites for phos-phate adsorption, could be decomposed after high temperaturetreatment (Wang et al. 2005). In addition, heat treatment may alsocause the sintering of particles, resulting in a loss of contact areafor the adsorbate and thereby a low adsorption capacity.

3.2. Adsorption kineticsThe study of adsorption kinetics illustrates how the solute up-

take rate controls the residence time of the adsorbate at the solu-tion interface. A rapid adsorption rate is as important as a highadsorption capacity in achieving a satisfactory adsorption perfor-mance.

To investigate the potential rate-controlling step and to obtainmore useful information regarding the P adsorption process ofthe sorbents, kinetic data were fitted into the pseudo-first-order(Yin et al. 2011), pseudo-second-order (Mezenner and Bensmaili2009), Elovich, and intra-particle diffusion models (Zhu et al. 2009)for metal sorbents (e.g., Fe, Zn, Al, etc.) by different researchers(Table 5). Summarizing from the literature, it is obvious thatpseudo-first-order kinetics or pseudo-second-order kinetics arethe most widely used in metal (hydr)oxides adsorption models.However, the kinetic adsorption data are usually better repre-sented by a pseudo-second-order model for metal (hydr)oxidesadsorption systems.

Fitting the kinetic data can be performed using the intra-particle diffusion model, which indicates that the phosphate ad-sorption process of the sorbents undergoes three successive steps:

(i) mass transfer (boundary-layer diffusion), (ii) adsorption of ionsonto sites, and (iii) intraparticle diffusion (Jiang et al. 2013; Moharamiand Jalali 2014). In many cases, there is a possibility that intra-particle diffusion will be the rate-limiting step, which is normallydetermined using the intra-particle diffusion model. However,the kinetic data could not be satisfactorily described only by theintra-particle diffusion model because of the minor contributionof the diffusion process to the whole kinetic process. Pseudo-first-order kinetics implies that one adsorbate (PO4

3−) may be adsorbedonto one surface site of sorbents. The applicability of the pseudo-second-order adsorption kinetic rate model indicates that chemi-sorption may be the rate-limiting step that controls these adsorptionprocesses. However, a few studies have used the Elovich model todescribe the adsorption of P from aqueous solutions (Zeng et al.2004; Zhu et al. 2009). These findings may conclude that the highapplicability of the Elovich model is in agreement with the factthat the Elovich equation is able to properly describe the kineticsof phosphate adsorption on highly energetic heterogeneous sys-tems (Chien and Clayton 1980; Sparks 1989).

The calculated kinetics favourable for phosphate adsorption bymetal hydroxides are presented in Table 4. It has been shown thatmetal hydroxides adsorbed PO4

3− within 0.02–20 h, which is amore rapid process than that of metal oxides (0.25–32 h). Thecontact time could be long enough to reach equilibrium, which isat least 24 h. The high rate of P uptake for metal hydroxides isprobably due to the availability of a large amount of binding siteson the surface of the metal hydroxides.

3.3. Effect of competitive anionsEnvironmental phosphate is always accompanied by other in-

organic anions in contaminated water. When a mixture of inor-

Table 3. Adsorption isotherm studies of phosphate removal from water by metal (hydr)oxides.

AdsorbentApplicableisotherm model Adsorbents and references

Metal oxides Langmuir Manganese–aluminum oxide (Wu et al. 2014) Fe–Zr binary oxide (Long et al. 2011), activatedred mud and fly ash (Li et al. 2006), alunite (Özacar 2003), ZrO2 (Liu et al. 2008)

Freundlich La(III)–modified zeolite (Ning et al. 2008), red mud (Huang et al. 2008), sepiolite (Yin et al.2011), palygorskites (Ye et al. 2006)

Freundlich–Langmuir ZnCl2 activated coir pith carbon (Namasivayam and Sangeetha 2004), modified bentonite(Zamparas et al. 2012), steel slag (Xiong et al. 2008), cobalt (Ogata et al. 2015)

Multi-Langmuir Ferric sludge (Song et al. 2011)Redlich–Peterson Iron oxide tailings (Zeng et al. 2004)

Metal hydroxides Langmuir Iron–activated carbon fiber (Zhou et al. 2012), La–lignocellulosic (Shin et al. 2005),Fe(III)–silica (Huang et al. 2013), waste alum sludge (Babatunde and Zhao 2010),Al- and iron-montmorillonite (Zhu et al. 2009)

Freundlich Zr(IV)–chitosan (Sowmya and Meenakshi 2014), zeolite (Li et al. 2009a), aluminum oxidehydroxide (Tanada et al. 2003)

Freundlich–Langmuir Fe(III)/Cr(III) hydroxide (Namasivayam and Prathap 2005), hydrated metaloxides–anionic exchange media (Acelas et al. 2014)

Table 4. The adsorption capacities and contact time of metal (hydr)oxides.

Adsorbent Adsorption capacity (mg P/g)Contacttime (h) Adsorbents and references

Metal oxides Single metal adsorbents (0.9–48) 0.25–40 Lanthanum–activated carbon fiber (Liu et al. 2011), iron–natural andengineered sorbents (Boujelben et al. 2008), aluminum–mesoporoussilicates (Shin et al. 2004), ZrO2 (Liu et al. 2008)

Hybrid metal adsorbents (3.5–345) 0.25–40 Manganese–aluminum oxide (Wu et al. 2012), Ferric sludge (Song et al. 2011),activated red mud and fly ash (Li et al. 2006), bottom slag and sewagesludge (Ge et al. 2013)

Metal hydroxides Single metal adsorbents (7.9–111) 0.02–20 Fe–skin split waste (Huang et al. 2009), cerium–fibrous protein (Deng and Yu2012), Zr(IV)–chitosan (Sowmya and Meenakshi 2014)

Hybrid metal adsorbents (9.8–415.2) 0.02–20 Cu or Fe–nanoporous sorbents (Chouyyok et al. 2010), Fe(III)/Cr(III) hydroxide(Namasivayam and Prathap 2005), hydroxyl aluminum- and hydroxyliron-montmorillonite complexes (Zhu et al. 2009)

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ganic anions is present in an aqueous solution, phosphateadsorption may then be affected by their competition for thesame sites on the metal sorbents. Researchers have concludedthat several inorganic anions in a solution would compete againsteach other for the available sites. Those with the greatest ionicpotential would be removed first, and if the sites were still under-saturated, then those with lower ionic potentials would be re-moved in sequence. The molecules with more electronegativityare attracted to the surface more strongly. It can be concludedfrom the literature that anions of higher valence have a moresignificant interfering effect than the monovalent anions in phos-phate adsorption on metal (hydr)oxides (SO4

2−, CO32− > F−, Cl−,

NO3−). Among the divalent anions, CO3

2− and SO42− appear to be

the most competitive anions for retarding the adsorption of phos-phate by metal (hydr)oxides. The decrease in the adsorption ca-pacity may be explained from the basis of the ion-exchangemechanism, in which divalent anions possess the highest affinityfor the sorbent material and compete most effectively with phos-phate adsorption.

Although the presence of an anion in a solution generally cre-ates competition for the adsorption sites, the total adsorptioncapacity of some metal oxides has been found to increase. Intypical systems, it is well known that certain additives, such assalts (NaNO3, NaCl, and KNO3), can accelerate the phosphate ad-sorption processes (Lü et al. 2013; Zhang et al. 2009). It could beinferred that anionic Cl− may participate in the adsorption processof phosphate ions (Chubar et al. 2005). The formation of an interme-diate complex of anionic and metal oxides may reduce the energy ofthe chelating reaction between H2PO4

− and metal oxides.Natural organic matter, which is mainly negatively charged organ

acid, such as humic acid and fulvic acid, binds very strongly to min-eral surfaces. Several experimental studies have been carried out onthe big effects of organic matter on phosphate adsorption to miner-als (Antelo et al. 2007; Borggaard et al. 2005; De Vicente et al. 2008).Adsorption of organic matter compounds may increase the negativecharge on the sorbents surface, or decrease the point of zero charge,thus making it more difficult for P sorption to occur (Erich et al. 2002;Tejedor-Tejedor et al. 1992). In contrast, organic matter (e.g., sodiumacetic solution) can also decrease final solution pH, which is benefi-cial to phosphate adsorption (Long et al. 2011). Meanwhile, it wasreported that competitive removal of phosphate in the presence of

organic matter was obtained. It is expected that organic matter pos-sesses affinity for the active sites and competes effectively with phos-phate for the sites (Geelhoed et al. 1998). However, fundamentalresearch on the interactions between inorganic phosphorus and neg-atively charged organic matters at metal (hydr)oxide surfaces is, untilnow, not advanced enough to allow for an understanding of thiseffect. Thus, the effect of organic matter on the adsorption of phos-phate on metal (hydr)oxides needs to be further investigated.

3.4. Effect of solution pH on phosphate adsorptionThe pH of the phosphate solution plays an important role in the

whole adsorption process and particularly on the adsorption ca-pacity. It influences not only the surface charge of the sorbent andthe stability of functional groups on the active sites of the sorbent,but also the solution phosphate chemistry.

Thermodynamic calculations have revealed that phosphate inaqueous solution exists as H2PO4

−, HPO42−, and PO4

3− at differentratios, depending on the solution pH, with pK1 = 2.15, pK2 = 7.20,and pK3 = 12.33, respectively. The adsorption ability of the surfaceand the type of active centres on the surface are indicated by asignificant factor called the point of zero charge (pHpzc). In therange of pH 2–12, when pH > pHpzc, the phosphate adsorptionmay be affected by the electrostatic repulsion and increasing com-petitive effect of OH− ions for the active sites on the sorbent.When pH < pHpzc, the sorbent surface is positively charged, fa-voring adsorption of the anions.

Changes in the pH values at approximately 2–12 in the adsorp-tion process as a function of the degree of phosphate adsorptionto metal sorbents have been shown in many studies. In general,phosphate adsorption capacity on metal (hydr)oxides tends to de-crease with increasing pH at pH 2–12. Table 6 shows that metaloxides and metal hydroxides have pHpzc values 3.2–11.0 and 2.3–9.5, respectively. The capacity of phosphorus adsorption is greaterat pH < pHpzc than that at pH > pHpzc. Thus metal oxides can getgreater adsorption capacity at pH < 3.2–11.0. Meanwhile, metalhydroxides can get greater adsorption capacity at pH < 2.3–9. Thephenomenon is discovered for the pH influence on P adsorption:adsorption was maximized nearly at pH 2–10 for metal oxides andpH 2–7 for metal hydroxides, with low adsorption out of optimumpH range. As already noted, a high pH will hinder P adsorption butfavor precipitation of Ca phosphates. The wider optimal pH range

Table 5. Adsorption kinetic studies of phosphate removal from water by metal (hydr)oxides.

Adsorbent Applicable kinetic model Adsorbents and references

Metal oxides Pseudo-first-order Sepiolite (Yin et al. 2011), alunite (Özacar 2003)Pseudo-second-order Modified bentonite (Zamparas et al. 2012), ferric sludge (Song et al. 2011), Fe–Zr binary oxide

(Long et al. 2011), NiFe2O4 (Jia et al. 2013)Elovich model Manganese–aluminum oxide (Wu et al. 2014), palygorskites (Ye et al. 2006)Parallel first-order Red mud (Huang et al. 2008)

Metal hydroxides Pseudo-first-order Zr–resin (Liu and Zhang 2015), LaAl–montmorillonite (Tian et al. 2009), aluminum oxidehydroxide (Tanada et al. 2003)

Pseudo-second-order Iron–activated carbon fiber (Zhou et al. 2012), iron lanthanum–activated carbon fiber (Liuet al. 2013), lanthanum–lignocellulosic (Shin et al. 2005), iron–eggshell waste (Mezennerand Bensmaili 2009), Zr–collagen fiber (Liao et al. 2006), Fe(III)/Cr(III) hydroxide(Namasivayam and Prathap 2005)

Pseudo-first-order,Pseudo-second-order

Waste alum sludge (Babatunde and Zhao 2010)

Elovich models Al- and Fe-montmorillonite (Zhu et al. 2009)

Table 6. The effect pH on the phosphate adsorption onto metal (hydr)oxides.

Adsorbents pHzpc Optimum pH Adsorbents and references

Metal oxides 3.2–11.0 2–10 Lanthanum(III)–ceramic (Chen et al. 2012), Fe–Zr binary oxide (Long et al. 2011), alunite (Ozacar 2006),porous clay (Yang et al. 2013), modified bentonite (Zamparas et al. 2012)

Metal hydroxides 2.3–9.5 2–7 Fe–Mg–La (Yu and Chen 2015), goethite and akaganeite (Chitrakar et al. 2006a), cerium–fibrous protein(Deng and Yu 2012), hydrous niobium oxide (Rodrigues and da Silva 2010)

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for metal oxides may contribute to the surface precipitationmechanism that occurs in alkaline solution. Moreover, some ofmetal oxides have higher pHpzc values than those of metal hy-droxides, which also can widen the optimal pH range. Wider op-timum pH ranges for metal oxides offer more opportunities tofacilitate the adsorption–regeneration application of metal oxides.

However, at very low pH values (pH �� pHpzc or pH ≤ 2) andvery high pH values (pH �� pHpzc or pH ≥ 12), the stability of themetal (hydr)oxidestructuresis impaired,whichdecreasesthephos-phate adsorption. Tokunaga demonstrated that La is soluble in adilute aqueous acidic solution, which is identical to the behaviorof La(OH)CO3 when it is exposed to low pH conditions (Tokunagaet al. 1997). Additionally, Chen reported that La(OH)3 precipitatesin alkaline solution (Chen et al. 2012).

3.5. Other parameters used for batch experimentsThere are many other operating parameters affecting phos-

phate adsorption in batch experiments, such as dosage, initialphosphate concentration, pH, and temperature. Thus, the effectsof these parameters are to be taken into account. Optimization ofsuch conditions will greatly help in the development of industrial-scale phosphate removal treatment process.

Generally, the increase in initial phosphate concentration willcause an increase in the capacity of the adsorbent. The phosphatesolutions could be obtained by dissolving potassium dihydrogenphosphate (KH2PO4) in deionized water and be adjusted withNaOH and HCl solutions to give initial pH 7, which is consistentwith actual wastewater. Adsorption generally increases with in-creasing temperature and room temperature (20–25 °C) is usuallyused. There is a tendency for higher P adsorption with lowerdosage. Thus P adsorption should be carried out at 1 g/L adsorbentdosage, 20–25 °C, using P concentrations up to 100 mg P/L at pH 7.

4. Adsorption mechanisms of metal (hydr)oxides

4.1. Possible adsorption mechanismsA mechanistic study of phosphate adsorption is of paramount

importance for the understanding the adsorbate–sorbent interac-tion; it can lead to optimization of the adsorption process and thesubsequent desorption and regeneration process. Mechanisticstudies of phosphate adsorption by metal (hydr)oxides have beencarried out with either the assistance of state-of-the-art analyticalequipment or the postulation from comprehensive experimental

observations on adsorption characteristics. Recently, it has beennow recognized that chemisorption (ion-exchange and electro-static attraction) is the most prevalent mechanism and that thepH is a main factor affecting adsorption. Surface precipitation andLewis acid–base interactions are also proposed by a few recentstudies.

4.1.1. Electrostatic forceThe electrostatic forces between metal sorbents and phosphate

ions are dependent on the pH. The pH-dependent adsorption in-dicates that the adsorption is dominated by surface complexation(outer sphere complex) (Yu et al. 2008). The electrostatic processesare illustrated in Fig. 1 at pH 2–12. When pH < pHpzc, the in-creased H+ in solution will react with the surface hydroxyl groupsto form some protonated hydroxyl groups. The electrostatic at-traction between a protonated hydroxyl group and phosphate willbe favourable for adsorption (Su et al. 2015). At the same time,electrostatic attraction can readily occur in conjunction with aspecific chemical adsorption due to an exchange reaction occur-ring at a lower pH range. When pH > pHpzc, the surface of thesorbents is enriched with increasingly negative charges, whichleads to the formation of deprotonated hydroxyl groups. As aresult, the electrostatic repulsion between them and the phos-phate increases with increasing pH (Lalley et al. 2016). Moreover,competition between hydroxide ions and phosphate ions for theadsorption sites also occurs, which is not beneficial for the subse-quent ion-exchange processes.

4.1.2. Ion exchangeThe presence of OH groups in metal (hydr)oxides (e.g., Fe, Zn, Al,

etc.) creates the possibility of phosphate–OH ion exchange, indi-cating a ligand exchange reaction. This may occur through innersphere complexation, where when PO4

3− anions create covalentchemical bonds with metallic cations on the surface of the metal(hydr)oxides leading to the liberation of other anions that wereformerly attached to the metallic ions. But if boehmite (�-AlOOH)is well crystallized, it does not develop a high overall surfacecharge and has limited reactivity because the face remains un-charged and also does not adsorb ions like phosphate via ionexchange reactions.

Based on the effect of pH and desorption results, ion exchangemechanism could be animportant pathway for the removal of

Fig. 1. Phosphate sorption mechanisms of metal (hydr)oxides. [Colour online.]

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phosphorus. It is promising for P recovery as it is generally areversible process and has high efficiency with high selectivity forthe anions. (Kuzawa et al. 2006; Blaney et al. 2007).

The phosphate adsorption process, which coincides stronglywith the ion exchange mechanism, is presented in Fig. 1 at pH ≤pHpzc. Zhou et al. (2012) reported this mechanism for removingphosphorus using a hydrated ferric oxide-doped activated carbonfiber. At pH values greater than 2, the ion exchange took placebetween H2PO4

−, HPO42− ions and the surface OH− groups to form

complexation as follows:

(1) Fe � OH � H2PO4�¡ Fe � H2PO4 � OH�

(2) Fe � OH � HPO42�

¡ (Fe�)2HPO4 � 2OH�

At pH ≤ pHpzc or initial adsorption range (H2PO4 or HPO4 solu-tion), phosphate sorption onto sorbents is primarily the result ofan ion exchange between phosphate and hydroxide groups on thesurface of the sorbent (Liu et al. 2011). But ion exchange increasedsolution pH because of the exchanged hydroxide groups. Thus, itwill be better to adjust solution pH to the initial range. Mean-while, chemical modification can increase the number of activebinding sites in the material, improve the ion exchange proper-ties and form new functional groups that favor phosphate uptake.In the preparation procedure, metal content will increase and alsosurface hydroxide groups after combination of the existing prep-aration methods. So metal hydroxides may be chosen for the ad-sorption procedure because of their high metal content.

4.1.3. Lewis acid–base interactionAt lower pH values, because of the presence of excess hydrogen

ions in solution, metal active sites are protonated and become aweak acid (Lewis acid), acting as electron acceptors. Phosphateanions become a weak base (Lewis base), acting as electron do-nors. At alkaline pH, the metal active sites are deprotonated andnegatively charged along with phosphate species. Therefore, themetal active sites become a weak base (Lewis base), and phosphateanions become a weak acid (Lewis acid). This implies electrondonor–acceptor and Lewis acid–base interactions exist betweenthe metal active sites and phosphate anions (Fig. 1). Moreover, inthe Lewis acid–base interaction mechanism, it is likely that themetal active site reacts with oxygen anions in the phosphate toform M–O coordination bonds. When a phosphate molecule ap-proaches the metallic center, the cluster surface releases one H2Ogroup, and through acid–basic Lewis interaction, it is possible toform a monodentate complex. This monodentate complex may

continue to attack the unreacted adjacent –H2O/–OH species andform a bidentate complex (Acelas et al. 2015).

4.1.4. Surface precipitationWhen the concentration of components of the precipitate sur-

passes the solubility product of the precipitate, precipitation ofphosphorus with metal ions may take place (Sparks 2001). Thismechanism is described as fast and hardly reversible adsorption(Loganathan et al. 2014). In such cases, a finite volume exists adja-cent to the mineral surface that is oversaturated with respect toprecipitate formation (Ford 2006). Calcium is an important com-ponent of a number of materials used as sorbents (e.g., blast fur-nace, steel slags, and fly ash). With materials containing largeamounts of soluble Ca and a high pH, removal can occur directlythrough formation of Ca phosphate precipitates (Klimeski et al.2012). X-ray diffraction and scanning electron microscopic dataalso provide evidence for the formation of surface precipitates ofphosphate compounds of Ca, Zn, Al, and Mg on sorbents contain-ing components of these metals (Bowden et al. 2009; Khadhraouiet al. 2002; Oguz 2005).

4.2. Comparison of adsorption mechanisms between metaloxides and metal hydroxides

The reasons for metal (hydr)oxides to adsorb phosphate mayaccount for the electrostatic (Tian et al. 2009; Ou et al. 2007), ionexchange (Ning et al. 2008), and Lewis acid–base interactions (Blaneyet al. 2007) between the functional groups on the adsorbents andthe target anions in solution (Fig. 1). Summarizing from the liter-ature, a surface precipitation mechanism has been clearly observed formetal oxides. The phosphate adsorption process following the surfaceprecipitationmechanismisdemonstratedinalkalinesolutions.Thesur-face precipitation mechanism also promotes phosphate adsorption onmetal hydroxides, although few studies have investigated themechanism on metal hydroxides.

A quantitative analysis of the mechanisms involved in phos-phorus adsorption revealed that it is desirable to precisely controlthe adsorption performances of sorbents. To date, no study hasperformed a quantitative analysis of these mechanisms. The com-parison of the three mechanisms is roughly presented in Fig. 2according to the literature.

It could be observed that adsorption mechanisms are highlypH-dependent. In general, when pH ≤ pHpzc, the discerniblechange of solution pH implied that ion exchange occurs betweenthe surface hydroxide group and the phosphate. Zhang et al. (2011)revealed that the pH values at the initial stage of the adsorptionprocess increased. This indicated that phosphate sorption ontosorbent is primarily the result of an ion exchange between phos-

Fig. 2. The comparison of ion exchange, electrostatic attraction, and Lewis acid–base interaction mechanisms at different pH. [Colour online.]

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phate and hydroxide groups on the surface of the sorbent. Liuet al. (2011) also got the same result.

However, as initial pH increased (pH > pHpzc), the values ofsolution pH decreased. This is because of deprotonation of thecoordinated water molecules on metal active site (Wu et al. 2007),which revealed that the ion exchange mechanism was insignifi-cant under this pH range. This phenomenon implied that, in ad-dition to the ion exchange mechanism, phosphate may in part beabsorbed by coulombic attraction (Chitrakar et al. 2006b). Whenthe value of pH < pHpzc, the surface charge of the sorbent ispositive, which interacts with the anionic phosphate by electro-static forces. Moreover, at pH > pHpzc, the adsorption of phos-phate by means of electrical force may not occur. Hence, it waslikely that Lewis acid–base interaction mechanism governed. In2007, Blaney et al. (2007) proposed the same mechanism for theadsorption phosphate onto hydrated ferric oxide.

Overall, the main mechanism involved in the adsorption process var-ies with the change in solution pH. As the pH increase, electrostaticforces change from electrostatic attraction to electrostatic repulsion;in addition, the ion exchange mechanism becomes increasinglyweaker, whereas the Lewis acid–base interaction gradually domi-nates (Blaney et al. 2007; Liu et al. 2011, 2013).

Ion exchange seems to be more promising for phosphate recov-ery because it is generally a reversible process and has high effi-ciency with high selectivity for the anions. These results mayexplain why metal hydroxides have a better adsorption capacitythan metal oxides, which could be due to the abundance of hy-droxyl groups on the surface of metal hydroxides.

5. Sorbent regenerationThe regeneration of sorbents is the most difficult and expensive

part of adsorption technology. It accounts for >70% of the totaloperating and maintenance costs for an adsorption system (Gohet al. 2008). Meanwhile, a successful regeneration process, whichis either for reuse or for proper disposal, should restore the sor-bent to close to its initial properties for effective reuse (Delaneyet al. 2011; Wang et al. 2016).

5.1. Regeneration methodsAcids, alkalis, and salts have been used as metal oxides or P

desorbents (Table 7). Simple low-cost salts have proven to be suc-cessful in desorbing phosphate only from sorbents with weakadsorption strength, in which the adsorption mechanism results

from outer sphere complexation (Ning et al. 2008; Sowmya andMeenakshi 2014). Simple salts are found to be ineffective in des-orbing phosphate from sorbents that strongly adsorb phosphateby inner sphere complexation (Moharami et al. 2014; Zeng et al.2004). Acids and bases have been used successfully for removingphosphates from metal oxides (Table 7) because phosphate ad-sorption decreases at lower and higher pH values. In the stronglyacidic pH range, the decreased adsorption capacity of phosphatecould be attributed to the formation of a weak hydro phosphorussalt. In the alkaline pH range, the sharp decrease in the adsorptioncapacity may be due to the competition of hydroxyl ions with thephosphates for adsorption on sorbents. Furthermore, the sor-bents and phosphate species in the alkaline pH range are highlynegatively charged, providing unfavourable conditions for ad-sorption. The results show that metal oxides have similar desorp-tion efficiencies and regeneration times, which may be due to thewide optimal pH range (Table 6).

As shown in Table 7, it is obvious that only alkaline and acidsolutions have been used to desorb phosphate from metal hydrox-ides. The results show that the efficiency of desorption in alkalineenvironments is better than that in acids. The reason for this maylie in the low phosphate adsorption capacity at pH values above 7and the high phosphate adsorption capacity in the acidic pHrange (Table 6).

5.2. Regeneration comparison of metal oxides andmetal hydroxides

As demonstrated in Table 7, metal oxides and metal hydroxideshave similar desorption efficiencies and regenerate times in acidand alkali solutions. This may be due to the low phosphate adsorp-tion capacities in acid and alkali solutions. However, salts havemostly been used as metal oxides or P desorbents.

Acids and bases have been successfully used for removing bothouterspherecomplexation-andinnerspherecomplexation-sorbedphos-phate. However, high concentrations of acids and bases are notsuitable for certain metal oxides (oxides of Si, Fe, and Al) becausethey can dissolve or corrode parts of the sorbents or cause struc-tural changes, leading to problems associated with the regenera-tion and reuse of the sorbents (Cheng et al. 2009; Chitrakar et al.2006a; Delaney et al. 2011). The chemical precipitation mecha-nism, which occurs with metal oxides, may hinder desorption ofphosphate by alkalis; however, phosphate has been successfullydesorbed from metal oxides with salts. However, Table 7 also

Table 7. The metal (hydr)oxides regeneration and the adsorption mechanisms.

AdsorbentDesorption orregeneration reagent Mechanism Results Adsorbents and references

Metal oxides Acids E, I, C 70.29%–85% desorption efficiency Porous clay (Yang et al. 2013), lanthanum(III) loadedgranular ceramic (Chen et al. 2012)

Alkalis E, I 60%–93% desorption efficiency;regenerate 3–8 times

Zr(IV) loaded okara (Nguyen et al. 2014), Fe3O4

nanoparticles (Tu et al. 2015), NiFe2O4 (Jia et al.2013), Calcium–sepiolite (Yin et al. 2011)

Salts E, I, C 4.98%–97% desorption efficiency;regenerate 7–10 times

TiO2 loaded zeolite (Alshameri et al. 2014), bottomslag and sewage sludge (Ge et al. 2013),palygorskites (Ye et al. 2006), Cu–chelating resin(Jiang et al. 2013)

Mixture of salts andalkalis

E, I Regenerate 7 times La(III)-modified zeolite (Ning et al. 2008), activatedalumina (Urano et al. 1992)

Metal hydroxides Acid E, I 6%–49% desorption efficiency atpH = 5

Hydrous zirconium oxide (Rodrigues et al. 2012),hydrous niobium oxide (Rodrigues and da Silva2010)

Alkalis E, I 60%–90% desorption efficiency;regenerate 3–7 times

Fe–Mg–La composite (Yu and Chen 2015), LaAlmontmorillonite (Tian et al. 2009), zeolite (Liet al. 2009a), goethite and akaganeite (Chitrakaret al. 2006a), hydrous niobium oxide (Rodriguesand da Silva 2009)

Note: E, electrostatic; I, ion exchange; L, Lewis acid–base interaction; C, chemical precipitation.

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shows that phosphate can be desorbed using a combination ofsalts and bases (Ning et al. 2008). Bases are used to reduce theadsorption capacity of metal oxides, and salts desorb the phos-phates via an ion exchange process. Occasionally, special salts areused to regain the structure of the metal oxides that is lost duringthe desorption process (Urano et al. 1992).

In general, because of the risk of high salinity, a high concen-tration of salts can lead to problems if the phosphate in the de-sorbed solution is to be recovered or used for fertilizing andirrigating crops (Johir et al. 2011). Therefore, desorbents of metalhydroxides seem to be more environmentally friendly, and metalhydroxides may be more stable when applied for regeneration,regardless of whether the desorbents dissolve or destroy the sor-bents. Moreover, as to phosphate recovery, metal (hydr)oxides areappropriate for subsequent direct use as nutrient-bearing soilamendments or P fertilizer (He et al. 1994; Notario et al. 1995); thisapplies in particular to metal hydroxides that have high P-capturecapacities and rapid P adsorption mechanisms.

However, like adsorption experiments, batch desorption exper-iments are not convenient for application on an industrial scale,in which large volumes of wastewater are continuously gener-ated. It is also imperative to analyze continuous desorption data,which can provide valuable information for improving the designand operation of phosphate desorption processes from adsorbents.

6. Future perspectivesTo date, there are still many open questions and challenges

requiring further detailed investigations within this field. The futuretrends in improving the performances of metal (hydr)oxides forphosphate adsorption may lie in the following aspects.

6.1. High-performance sorbents

6.1.1. Synthesis methodsIt is still difficult to synthesize uniform metal (hydr)oxides with

narrow size distributions and high stabilities on a large scale,which is important for phosphate adsorption.

For example, chemical and precipitation methods are the low-cost methods to synthesize metal (hydr)oxides on a large scale;however, these methods always result in serious aggregation.Post-grafting and metal cation incorporation processes can rein-force the chemical stability of the sorbents but usually increasethe costs and produce excessive waste. Sol-gel and hydrothermalmethods are often used to synthesize particles with uniform mor-phology, high purity, and narrow particle size distribution. However,both of these methods are usually intricate and time-consuming.Therefore, a combination of the existing methods may be a novelstrategy for the synthesis of metal (hydr)oxides with the con-trolled size and demanded shape and high stability required foruse in wastewater treatment. For example, the combinations ofthe post-grafting and metal cation incorporation process and thechemical precipitation-assisted hydrothermal method, the post-grafting and metal cation incorporation process-assisted sol–gelmethod, or the post-grafting and metal cation incorporation process-hydrothermal method may overcome the drawbacks of each sin-gle method.

6.1.2. Low-cost sorbentsThe cost factor should not be ignored. Lower production costs

with higher removal efficiencies of the sorbents would make thephosphate adsorption process more economical and efficient.

Certain types of waste materials containing metal (hydr)oxideshave been used to lower phosphate concentrations and could bepossible alternatives as low-cost sorbents. However, most of themare strongly pH-dependent and have problems with the regener-ation process. The conditions for the production of low-cost sor-bents need to be optimized using surface modifications for higheruptake of pollutants. Although chemically modified low-cost sor-

bents may enhance P adsorption, the cost of the chemicals andtechnologies used must be considered to produce “low-cost” sor-bents. Thus, the selection and identification of an appropriatelow-cost sorbent is one of the keys to achieving the maximumadsorption of specific types of pollutants. Moreover, the releasingof toxic metals from the resultant waste into treated water shouldbe continuously monitored.

To further improve the adsorption efficiencies and reduce thecosts, it is necessary to develop novel phosphate-specific sorbents.In particular, rare earth elements have attracted special attentionfor their high phosphate adsorption capabilities, nontoxic prop-erties, and environmental friendliness. However, the high priceand scarcity of rare earth elements hinder their use in practicalapplications. Therefore, combining transition elements with rareearth elements and low-cost sorbents may achieve both goals:finding the ideal phosphate removal capacity and having low op-erating costs.

6.2. Sorbents regenerationTo enhance the economic feasibility of the process, regenera-

tion studies need to be performed in detail with the pollutant-laden sorbents to recover the adsorbate as well as the sorbent.Based on the literature, few regeneration frequency studies havebeen reported, which may be because of the low desorption effi-ciencies of the sorbents or the destruction of the sorbents afterdesorption. Thus, future research must explore highly efficientmaterials that are expected to be easily regenerated over severalcycles of operations without significant loss of sorption capacity.The uses of ion exchange mechanisms for water treatment havedemonstrated good selectivity for phosphate, high phosphate cap-ture capacities and easy regeneration capabilities. Therefore, theanion exchange materials (e.g., anion exchange resins) may beinvolved in phosphorus adsorption and sorbent regeneration inthe future study.

6.3. DesorbentsReuse of the regenerating solutions would alleviate the problems of

storage and discharge of the solution. In some studies, phosphatehas been successfully desorbed by using NaCl or NaOH; however,high-strength NaCl solutions that are sent to wastewater treatmentplants inhibit the necessary biological processes (Cañedo-Argüelleset al. 2013; Panswad and Anan 1999) and land applications of wastestreams containing high concentrations of NaCl are detrimentalto the soil makeup and plant growth (Bernstein 1975). Becausehigh Cl− concentrations can cause difficulty in the reuse of recov-ered P, OH− salts may play an important role in regeneration tests.Furthermore, potassium is an essential nutrient for higher plants.Therefore, K salts and OH− salts may be promising desorbents forthe reuse of recovered P solutions. But appropriate concentrationsof desorbents should be considered to avoid dissolution or thecorrosion of parts of the sorbents in the future study.

6.4. Adsorption mechanismThe mechanism through which metal (hydr)oxides affect the

phosphate adsorption process remains unclear, and no literaturediscusses the quantitative mechanism involved in the process,which is beneficial to preparation of sorbents and application inindustry.

Kinetic studies, thermodynamics, and isotherm models are in-sufficient to explain the mechanisms that were involved. Moreresearch on the metal sorbents-phosphate interface and quantita-tive analysis of the contributions of functional groups to thismechanism should be performed. Although there are disparitiesbetween some X-ray adsorption fine structure and Fourier trans-form infrared spectroscopic studies, many spectroscopic, macro-scopic, and modelling studies have indicated that complexes formon the sorbent surfaces. There is much work to be done on thestudy of the relationships between the functional groups of sorbents

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and adsorption performances, which would help in predicting themechanism. The use of isotope tracing, temperature-programmeddesorption, nuclear magnetic resonance, and models is recommended.Moreover, regeneration experiments need to be considered for thispurpose.

Further investigations must be performed, and the results willhelp determine metal sorbents with improved functional groupsand tissue regeneration abilities.

6.5. Application in actual wastewaterMost studies on phosphate removal by metal sorbents have

been performed in batch experiments and only a few have beenreported in fixed-bed column systems, which are more relevant toreal operating systems in natural waters. Sieved natural oxidesappear in mostly column experiments, which are fine particles.However, it was also observed that the columns packed withsieved natural oxides are occasionally clogged by fine particles.This observation suggests that granulation of particles should beconsidered when using them for column adsorption.

Meanwhile, the phosphate-containing industrial wastewaterdischarged into lakes, rivers, and other natural waters is one ofthe major sources of excessive phosphate in water bodies. There-fore, it is also necessary to remove phosphate from industrialwastewater. Phosphate-uptake from seawater by goethite (Gaoand Mucci 2001, 2003), aluminium oxy-hydroxide (Tanada et al.2003), manganese oxide (Yao and Millero 1996), and Zr(IV)-loadedresin RGP (Zhu and Jyo 2005), have recently been presented in afew studies. There is a need to investigate the simultaneous re-moval of many co-existing pollutants (e.g., inorganic anions andorganic anions).

Such works in column systems and actual wastewater couldcontribute to understanding the phosphate removal process us-ing metal sorbents.

7. ConclusionThis comparative review summarized the preparation meth-

ods, adsorption behaviors, adsorption mechanisms, and the re-generation of metal (hydr)oxides (e.g., Fe, Zn, Al, etc.). Themajority of these substrates have been tested in laboratory exper-iments; others have also been investigated in column experi-ments. The essential differences in experimental conditions thathave prevailed in the experiments contribute to difficulties innormalization of the results obtained. But under similar experi-mental conditions, comparative evaluations were discussed.

Hydroxyl groups on the sorbent surface were reported to playimportant roles in the adsorption of phosphate. The effective syn-thesis methods of metal hydroxides give enough active sites forthe phosphate adsorption. It was reported that metal hydroxidesdisplayed their dominant superiority for the higher phosphateadsorption capacity than that of metal oxides. What is more, thesynthesis methods of metal hydroxides are generally cost-benefitwithout calcination. The sol-gel and hydrothermal methods arepossible ideal choices for the synthesis of nanoscale sorbents withgood dispersancy.

From the economic perspective, the equilibrium time of phos-phate adsorption onto metal hydroxides (0.02–20 h) was shorterthan the 0.25–32 h of adsorption onto metal oxides. Desorbents(alkaline and acid solutions) of metal hydroxides are more envi-ronmentally and P-containing metal hydroxides exhibit betterpotential for P recovery. However, metal oxides, with a higherpHzpc, 3.2–11.0, have wider optimal adsorption pH ranges, 2–10,than metal hydroxides, because of their surface precipitation inalkaline solutions.

The competing ions have similar effects on phosphate adsorp-tion onto metal (hydr)oxides. Nitrate and chloride do not interferewith phosphate adsorption, whereas sulfate and carbonate ap-pear to be the most competitive anions with phosphate for ad-sorption. The common mechanisms of phosphate removal for

both metal oxides and metal hydroxides are electrostatic forces,ion exchange, and Lewis acid–base interaction. For metal sorbents(e.g., Fe, Zn, Al, etc.), chemisorption (ion-exchange and electro-static attraction) is the most prevalent mechanism. Besides Ca, Al,and Mg, which remove phosphate by precipitation at high pH, theremoval was highest at pH > 7–8. Ion exchange seems to be morepromising for phosphate recovery because it is generally a revers-ible process and has high efficiency with high selectivity for theanions.

Although metal (hydr)oxides are widely produced and used,there is still much work to be done. Further research must focuson uniform metal (hydr)oxides with small particle size, high sta-bilities, and low cost with high phosphate sorption capacities.Future research also needs to explore the sorbents that are easilyregenerated over several cycles of operations without significantloss of sorption capacity. Appropriate concentrations of K saltsand OH− salts may be promising desorbents for the reuse of recov-ered P solutions. At the same time, it is important to clearly anal-yse the metal sorbent-phosphate interface and the quantitativecontributions of functional groups to the mechanism. Further-more, these trials need to be extended to continuous-mode col-umn trials, which are more applicable to real operating systemson actual wastewater.

AcknowledgementsThis work was financially supported by the National Natural

Science Foundation of China (grant No. 41472312) and the Pro-gram for Innovative Research Team in University (grant No. IRT13078).

ReferencesAcelas, N.Y., Martin, B.D., López, D., and Jefferson, B. 2015. Selective removal of

phosphate from wastewater using hydrated metal oxides dispersed withinanionic exchange media. Chemosphere, 119: 1353–1360. doi:10.1016/j.chemosphere.2014.02.024. PMID:24630462.

Alshameri, A., Yan, C., and Lei, X. 2014. Enhancement of phosphate removalfrom water by TiO2/Yemeni natural zeolite: Preparation, characterizationand thermodynamic. Microporous Mesoporous Mater. 196: 145–157. doi:10.1016/j.micromeso.2014.05.008.

Antelo, J., Arce, F., Avena, M., Fiol, S., López, R., and Macías, F. 2007. Adsorptionof a soil humic acid at the surface of goethite and its competitive interactionwith phosphate. Geoderma, 138(1–2): 12–19. doi:10.1016/j.geoderma.2006.10.011.

Arias, C.A., Del Bubba, M., and Brix, H. 2001. Phosphorus removal by sands foruse as media in subsurface flow constructed reed beds. Water Res. 35(5):1159–1168. doi:10.1016/S0043-1354(00)00368-7. PMID:11268836.

Asaoka, S., and Yamamoto, T. 2010. Characteristics of phosphate adsorptiononto granulated coal ash in seawater. Mar. Pollut. Bull. 60(8): 1188–1192.doi:10.1016/j.marpolbul.2010.03.032. PMID:20403625.

Babatunde, A.O., and Zhao, Y.Q. 2010. Equilibrium and kinetic analysis of phos-phorus adsorption from aqueous solution using waste alum sludge. J. Hazard.Mater. 184(1–3): 746–752. doi:10.1016/j.jhazmat.2010.08.102. PMID:20846787.

Barrow, N.J. 1978. The description of phosphate adsorption curves. J. Soil Sci.29(4): 447–462. doi:10.1111/j.1365-2389.1978.tb00794.x.

Bernstein, L. 1975. Effects of salinity and sodicity on plant growth. Ann. Rev.Phytopathol. 13(1): 295–312. doi:10.1146/annurev.py.13.090175.001455.

Bhargava, D.S., and Sheldarkar, S.B. 1992. Effects of adsorbent dose and size onphosphate-removal from wastewaters. Environ. Pollut. 76(1): 51–60. doi:10.1016/0269-7491(92)90116-r. PMID:15092008.

Blaney, L.M., Cinar, S., and SenGupta, A.K. 2007. Hybrid anion exchanger fortrace phosphate removal from water and wastewater. Water Res. 41(7): 1603–1613. doi:10.1016/j.watres.2007.01.008. PMID:17306856.

Borggaard, O.K., Raben-Lange, B., Gimsing, A.L., and Strobel, B.W. 2005. Influ-ence of humic substances on phosphate adsorption by aluminium and ironoxides. Geoderma, 127(3–4): 270–279. doi:10.1016/j.geoderma.2004.12.011.

Boujelben, N., Bouzid, J., Elouear, Z., Feki, M., Jamoussi, F., and Montiel, A. 2008.Phosphorus removal from aqueous solution using iron coated natural andengineered sorbents. J. Hazard. Mater. 151(1): 103–110. doi:10.1016/j.jhazmat.2007.05.057. PMID:17611022.

Bowden, L.I., Jarvis, A.P., Younger, P.L., and Johnson, K.L. 2009. Phosphorusremoval from waste waters using basic oxygen steel slag. Environ. Sci. Tech-nol. 43(7): 2476–2481. doi:10.1021/es801626d. PMID:19452904.

Cañedo-Argüelles, M., Kefford, B.J., Piscart, C., Prat, N., Schäfer, R.B., andSchulz, C.-J. 2013. Salinisation of rivers: An urgent ecological issue. Environ.Pollut. 173: 157–167. doi:10.1016/j.envpol.2012.10.011. PMID:23202646.

Chen, N., Feng, C., Zhang, Z., Liu, R., Gao, Y., Li, M., and Sugiura, N. 2012.Preparation and characterization of lanthanum(III) loaded granular ceramic

Li et al. 329

Published by NRC Research Press

Env

iron

. Rev

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

Nan

jing

Inst

itute

of

Geo

grap

hy a

nd L

imno

logy

, CA

S on

01/

17/1

7Fo

r pe

rson

al u

se o

nly.

for phosphorus adsorption from aqueous solution. J. Taiwan Inst. Chem. Eng.43(5): 783–789. doi:10.1016/j.jtice.2012.04.003.

Cheng, X., Huang, X., Wang, X., Zhao, B., Chen, A., and Sun, D. 2009. Phosphateadsorption from sewage sludge filtrate using zinc–aluminum layered doublehydroxides. J. Hazard. Mater. 169(1–3): 958–964. doi:10.1016/j.jhazmat.2009.04.052. PMID:19443104.

Cheung, K.C., and Venkitachalam, T.H. 2000. Improving phosphate removal ofsand infiltration system using alkaline fly ash. Chemosphere, 41(1–2): 243–249. doi:10.1016/S0045-6535(99)00417-8. PMID:10819207.

Chien, S.H., and Clayton, W.R. 1980. Application of Elovich equation to thekinetics of phosphate release and sorption in soils. Soil Sci. Soc. Am. J. 44(2):265–268. doi:10.2136/sssaj1980.03615995004400020013x.

Chitrakar, R., Tezuka, S., Sonoda, A., Sakane, K., Ooi, K., and Hirotsu, T. 2006a.Phosphate adsorption on synthetic goethite and akaganeite. J. Colloid Inter-face Sci. 298(2): 602–608. doi:10.1016/j.jcis.2005.12.054. PMID:16455102.

Chitrakar, R., Tezuka, S., Sonoda, A., Sakane, K., Ooi, K., and Hirotsu, T. 2006b.Selective adsorption of phosphate from seawater and wastewater by amor-phous zirconium hydroxide. J. Colloid Interface Sci. 297(2): 426–433. doi:10.1016/j.jcis.2005.11.011. PMID:16337645.

Chouyyok, W., Wiacek, R.J., Pattamakomsan, K., Sangvanich, T., Grudzien, R.M.,Fryxell, G.E., and Yantasee, W. 2010. Phosphate removal by anion binding onfunctionalized nanoporous sorbents. Environ. Sci. Technol. 44: 3073–3078.doi:10.1021/es100787m. PMID:20345133.

Chu, L., Yan, S., Xing, X.-H., Sun, X., and Jurcik, B. 2009. Progress and perspec-tives of sludge ozonation as a powerful pretreatment method for minimiza-tion of excess sludge production. Water Res. 43(7): 1811–1822. doi:10.1016/j.watres.2009.02.012. PMID:19282018.

Chubar, N.I., Kanibolotskyy, V.A., Strelko, V.V., Gallios, G.G., Samanidou, V.F.,Shaposhnikova, T.O., et al. 2005. Adsorption of phosphate ions on novelinorganic ion exchangers. Colloids Surf., A, 255(1–3): 55–63. doi:10.1016/j.colsurfa.2004.12.015.

Coreño, J., Martínez, A., Coreño, O., Bolarín, A., and Sánchez, F. 2003. Calciumand phosphate adsorption as initial steps of apatite nucleation on sol-gel-prepared titania surface. J. Biomed. Mater. Res., Part A, 64A(1): 131–137. doi:10.1002/jbm.a.10395.

Cucarella, V., and Renman, G. 2009. Phosphorus sorption capacity of filter ma-terials used for on-site wastewater treatment determined in batch experiments-acomparative study. J. Environ. Qual. 38(2): 381–392. doi:10.2134/jeq2008.0192.PMID:19202009.

De Vicente, I., Jensen, H.S., and Andersen, F.Ø. 2008. Factors affecting phosphateadsorption to aluminum in lake water: implications for lake restoration.Sci. Total Environ. 389(1): 29–36. doi:10.1016/j.scitotenv.2007.08.040. PMID:17900664.

Del Bubba, M., Arias, C.A., and Brix, H. 2003. Phosphorus adsorption maximumof sands for use as media in subsurface flow constructed reed beds as mea-sured by the Langmuir isotherm. Water Res. 37(14): 3390–3400. doi:10.1016/S0043-1354(03)00231-8. PMID:12834732.

Delaney, P., McManamon, C., Hanrahan, J.P., Copley, M.P., Holmes, J.D., andMorris, M.A. 2011. Development of chemically engineered porous metal ox-ides for phosphate removal. J. Hazard. Mater. 185(1): 382–391. doi:10.1016/j.jhazmat.2010.08.128. PMID:20934247.

Deng, H., and Yu, X. 2012. Adsorption of fluoride, arsenate and phosphate inaqueous solution by cerium impregnated fibrous protein. Chem. Eng. J. 184:205–212. doi:10.1016/j.cej.2012.01.031.

Erich, M.S., Fitzgerald, C.B., and Porter, G.A. 2002. The effect of organic amend-ments on phosphorus chemistry in a potato cropping system. Agric., Ecosyst.Environ. 88(1): 79–88. doi:10.1016/S0167-8809(01)00147-5.

Ford, R.G. 2006. Structural dynamics of metal partitioning to mineral surfaces.In Natural attenuation of trace element availability in soils. Edited byR. Hamon, M. McLaughlin, and E. Lombi. New York: CRC Press/Taylor &Francis Group. pp. 73–88.

De Sousa, A.F., Braga, T.P., Gomes, E.C.C., Valentini, A., and Longhinotti, E. 2012.Adsorption of phosphate using mesoporous spheres containing iron andaluminum oxide. Chem. Eng. J. 210: 143–149. doi:10.1016/j.cej.2012.08.080.

Gao, Y., and Mucci, A. 2001. Acid base reactions, phosphate and arsenate com-plexation, and their competitive adsorption at the surface of goethite in 0.7M NaCl solution. Geochim. Cosmochim. Acta, 65(14): 2361–2378. doi:10.1016/s0016-7037(01)00589-0.

Gao, Y., and Mucci, A. 2003. Individual and competitive adsorption of phosphateand arsenate on goethite in artificial seawater. Chem. Geol. 199(1–2): 91–109.doi:10.1016/s0009-2541(03)00119-0.

Ge, S., Zhang, H., Ye, H., Zhang, H., Zhao, K., Sun, Q., et al. 2013. Sorptionbehavior of phosphate on an MSWI bottom slag and sewage sludge co-sintered adsorbent. Water, Air, Soil Pollut. 224(11): 1683. doi:10.1007/s11270-013-1683-1.

Geelhoed, J.S., Hiemstra, T., and Van Riemsdijk, W.H. 1998. Competitive inter-action between phosphate and citrate on goethite. Environ. Sci. Technol.32(14): 2119–2123. doi:10.1021/es970908y.

Goh, K.-H., Lim, T.-T., and Dong, Z. 2008. Application of layered double hydrox-ides for removal of oxyanions: a review. Water Res. 42(6–7): 1343–1368. doi:10.1016/j.watres.2007.10.043. PMID:18061644.

Guo, H., Li, W., Wang, H., Zhang, J., Liu, Y., and Zhou, Y. 2011. A study of

phosphate adsorption by different temperature treated hydrous cerium ox-ides. Rare Met. (Beijing, China), 30(1): 58–62. doi:10.1007/s12598-011-0197-5.

Haghseresht, F., Wang, S., and Do, D.D. 2009. A novel lanthanum-modifiedbentonite, Phoslock, for phosphate removal from wastewaters. Appl. ClaySci. 46(4): 369–375. doi:10.1016/j.clay.2009.09.009.

He, Z.L., Yang, X., Yuan, K.N., and Zhu, Z.X. 1994. Desorption and plant-availability of phosphate sorbed by some important minerals. Plant Soil,162(1): 89–97. doi:10.1007/BF01416093.

Huang, W., Wang, S., Zhu, Z., Li, L., Yao, X., Rudolph, V., and Haghseresht, F.2008. Phosphate removal from wastewater using red mud. J. Hazard. Mater.158(1): 35–42. doi:10.1016/j.jhazmat.2008.01.061. PMID:18314264.

Huang, W.-Y., Li, D., Yang, J., Liu, Z.-Q., Zhu, Y., Tao, Q., et al. 2013. One-potsynthesis of Fe(III)-coordinated diamino-functionalized mesoporous silica:effect of functionalization degrees on structures and phosphate adsorption.Microporous Mesoporous Mater. 170: 200–210. doi:10.1016/j.micromeso.2012.10.027.

Huang, X., Liao, X., and Shi, B. 2009. Adsorption removal of phosphate in indus-trial wastewater by using metal-loaded skin split waste. J. Hazard. Mater.166(2–3): 1261–1265. doi:10.1016/j.jhazmat.2008.12.045. PMID:19136202.

Jia, Z., Wang, Q., Liu, J., Xu, L., and Zhu, R. 2013. Effective removal of phosphatefrom aqueous solution using mesoporous rodlike NiFe2O4 as magneticallyseparable adsorbent. Colloids Surf., A, 436: 495–503. doi:10.1016/j.colsurfa.2013.07.025.

Jiang, H., Chen, P., Luo, S., Tu, X., Cao, Q., and Shu, M. 2013. Synthesis of novelnanocomposite Fe3O4/ZrO2/chitosan and its application for removal of ni-trate and phosphate. Appl. Surf. Sci. 284: 942–949. doi:10.1016/j.apsusc.2013.04.013.

Johir, M.A.H., George, J., Vigneswaran, S., Kandasamy, J., and Grasmick, A. 2011.Removal and recovery of nutrients by ion exchange from high rate mem-brane bio-reactor (MBR) effluent. Desalination, 275(1–3): 197–202. doi:10.1016/j.desal.2011.02.054.

Chen, K.L., Mylon, S.E., and Elimelech, M. 2007. Enhanced aggregation ofalginate-coated iron oxide (hematite) nanoparticles in the presence of cal-cium, strontium, and barium cations. Langmuir, 23(11): 5920–5928. doi:10.1021/la063744k. PMID:17469860.

Khadhraoui, M., Watanabe, T., and Kuroda, M. 2002. The effect of the physicalstructure of a porous Ca-based sorbent on its phosphorus removal capacity.Water Res. 36(15): 3711–3718. doi:10.1016/s0043-1354(02)00096-9. PMID:12369518.

Klimeski, A., Chardon, W.J., Turtola, E., and Uusitalo, R. 2012. Potential andlimitations of phosphate retention media in water protection: A process-based review of laboratory and field-scale tests. Agri. Food Sci. 21(3): 206–223.

Kuzawa, K., Jung, Y.-J., Kiso, Y., Yamada, T., Nagai, M., and Lee, T.-G. 2006. Phos-phate removal and recovery with a synthetic hydrotalcite as an adsorbent.Chemosphere, 62(1): 45–52. doi:10.1016/j.chemosphere.2005.04.015. PMID:15951001.

Lalley, J., Han, C., Li, X., Dionysiou, D.D., and Nadagouda, M.N. 2016. Phosphateadsorption using modified iron oxide-based sorbents in lake water: Kinetics,equilibrium, and column tests. Chem. Eng. J. 284: 1386–1396. doi:10.1016/j.cej.2015.08.114.

Li, B., Lu, X., Ning, P., and Yang, Y. 2009. Nitrogen and phosphate removal byzeolite-rare earth adsorbents. Environmental Science and Information Appli-cation Technology, 2009. ESIAT 2009. International Conference on, 2:599–602. doi:10.1109/ESIAT. 2009.172.

Li, H., Ru, J., Yin, W., Liu, X., Wang, J., and Zhang, W. 2009. Removal of phosphatefrom polluted water by lanthanum doped vesuvianite. J. Hazard. Mater.168(1): 326–330. doi:10.1016/j.jhazmat.2009.02.025. PMID:19297092.

Li, Y., Liu, C., Luan, Z., Peng, X., Zhu, C., Chen, Z., et al. 2006. Phosphate removalfrom aqueous solutions using raw and activated red mud and fly ash. J. Hazard.Mater. 137(1): 374–383. doi:10.1016/j.jhazmat.2006.02.011. PMID:16621271.

Liao, X.-P., Ding, Y., Wang, B., and Shi, B. 2006. Adsorption behavior of phos-phate on metal-ions-loaded collagen fiber. Ind. Eng. Chem. Res. 45(11): 3896–3901. doi:10.1021/ie051076c.

Liesch, A.M. 2010. Wastewater phosphorus removal by two different types ofandesitic volcanic tephra. J. Nat. Res. Life Sci. Edu. 39(1): 40–44. doi:10.4195/jnrlse.2010.0001se.

Liu, H., Sun, X., Yin, C., and Hu, C. 2008. Removal of phosphate by mesoporousZrO2. J. Hazard. Mater. 151(2–3): 616–622. doi:10.1016/j.jhazmat.2007.06.033.PMID:17658689.

Liu, J., Wan, L., Zhang, L., and Zhou, Q. 2011. Effect of pH, ionic strength, andtemperature on the phosphate adsorption onto lanthanum-doped activatedcarbon fiber. J. Colloid Interface Sci. 364(2): 490–496. doi:10.1016/j.jcis.2011.08.067. PMID:21937053.

Liu, J., Zhou, Q., Chen, J., Zhang, L., and Chang, N. 2013. Phosphate adsorption onhydroxyl–iron–lanthanum doped activated carbon fiber. Chem. Eng. J. 215–216: 859–867. doi:10.1016/j.cej.2012.11.067.

Liu, X., and Zhang, L. 2015. Removal of phosphate anions using the modifiedchitosan beads: Adsorption kinetic, isotherm and mechanism studies. PowderTechnol. 277: 112–119. doi:10.1016/j.powtec.2015.02.055.

Loganathan, P., Vigneswaran, S., Kandasamy, J., and Bolan, N.S. 2014. Removaland recovery of phosphate from water using sorption. Crit. Rev. Environ. Sci.Technol. 44(8): 847–907. doi:10.1080/10643389.2012.741311.

Long, F., Gong, J.-L., Zeng, G.-M., Chen, L., Wang, X.-Y., Deng, J.-H., et al. 2011.

330 Environ. Rev. Vol. 24, 2016

Published by NRC Research Press

Env

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. Rev

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nd L

imno

logy

, CA

S on

01/

17/1

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nly.

Removal of phosphate from aqueous solution by magnetic Fe–Zr binary ox-ide. Chem. Eng. J. 171(2): 448–455. doi:10.1016/j.cej.2011.03.102.

Lü, J., Liu, H., Liu, R., Zhao, X., Sun, L., and Qu, J. 2013. Adsorptive removal ofphosphate by a nanostructured Fe–Al–Mn trimetal oxide adsorbent. PowderTechnol. 233: 146–154. doi:10.1016/j.powtec.2012.08.024.

Lu, S.G., Bai, S.Q., Zhu, L., and Shan, H.D. 2009. Removal mechanism of phos-phate from aqueous solution by fly ash. J. Hazard. Mater. 161(1): 95–101.doi:10.1016/j.jhazmat.2008.02.123. PMID:18434007.

Manju, G.N., and Anirudhan, T.S. 2000. Batch lead and cadmium ions bindingand exchange properties of polymer-coated hydrous iron(III) oxide. J. Sci. Ind.Res. India. 59(2): 144–152.

Mezenner, N.Y., and Bensmaili, A. 2009. Kinetics and thermodynamic study ofphosphate adsorption on iron hydroxide-eggshell waste. Chem. Eng. J. 147:87–96. doi:10.1016/j.cej.2008.06.024.

Mohamad, S., Bakar, N.K.A., Ishak, A.R., Surikumaran, H., Pandian, K., Raoov, M.,et al. 2014. Removal of phosphate by paper mill sludge: Adsorption isothermand kinetic study. Asian J. Chem. 26(12): 3545–3552. doi:10.14233/ajchem.2014.16227.

Moharami, S., and Jalali, M. 2014. Effect of TiO2, Al2O3, and Fe3O4 nanoparticleson phosphorus removal from aqueous solution. Environ. Prog. SustainableEnergy, 33(4): 1209–1219. doi:10.1002/ep.11917.

Namasivayam, C., and Prathap, K. 2005. Recycling Fe(III)/Cr(III) hydroxide, anindustrial solid waste for the removal of phosphate from water. J. Hazard.Mater. 123(1–3): 127–134. doi:10.1016/j.jhazmat.2005.03.037. PMID:15955623.

Namasivayam, C., and Sangeetha, D. 2004. Equilibrium and kinetic studies ofadsorption of phosphate onto ZnCl2 activated coir pith carbon. J. ColloidInterface Sci. 280(2): 359–365. doi:10.1016/j.jcis.2004.08.015. PMID:15533408.

Nguyen, T.A.H., Ngo, H.H., Guo, W.S., Nguyen, T.V., Zhang, J., Liang, S., et al.2014. A comparative study on different metal loaded soybean milk by-product‘okara’ for biosorption of phosphorus from aqueous solution. Bioresour. Tech-nol. 169: 291–298. doi:10.1016/j.biortech.2014.06.075. PMID:25062541.

Ning, P., Bart, H.-J., Li, B., Lu, X., and Zhang, Y. 2008. Phosphate removal fromwastewater by model-La(III) zeolite adsorbents. J. Environ. Sci. (Beijing,China), 20(6): 670–674. doi:10.1016/s1001-0742(08)62111-7.

Notario, d.P.J.S., Arteaga, P.I.J., Gonzalez, M.M.M., and Garcia, H.J.E. 1995. Phos-phorus and potassium release from phillipsite-based slow-release fertilizers.J. Controlled Release, 34(1): 25–29. doi:10.1016/0168-3659(94)00116-C.

Ogata, F., Imai, D., Toda, M., Otani, M., and Kawasaki, N. 2015. Adsorption ofphosphate ion in aqueous solutions by calcined cobalt hydroxide at differenttemperatures. J. Environ. Chem. Eng. 3(3): 1570–1577. doi:10.1016/j.jece.2015.05.028.

Oguz, E. 2005. Sorption of phosphate from solid/liquid interface by fly ash.Colloids Surf., A, 262(1–3): 113–117. doi:10.1016/j.colsurfa.2005.04.016.

Ou, E., Zhou, J., Mao, S., Wang, J., Xia, F., and Min, L. 2007. Highly efficientremoval of phosphate by lanthanum-doped mesoporous SiO2. Colloids andSurf., A, 308(1–3): 47–53. doi:10.1016/j.colsurfa.2007.05.027.

Özacar, M. 2003. Adsorption of phosphate from aqueous solution onto alunite.Chemosphere, 51(4): 321–327. doi:10.1016/s0045-6535(02)00847-0. PMID:12604084.

Özacar, M. 2006. Contact time optimization of two-stage batch adsorber designusing second-order kinetic model for the adsorption of phosphate ontoalunite. J. Hazard. Mater. 137(1): 218–225. doi:10.1016/j.jhazmat.2006.01.058.PMID:16530939.

Pan, B., Wu, J., Pan, B., Lv, L., Zhang, W., Xiao, L., et al. 2009. Development ofpolymer-based nanosized hydrated ferric oxides (HFOs) for enhanced phos-phate removal from waste effluents. Water Res. 43(17): 4421–4429. doi:10.1016/j.watres.2009.06.055. PMID:19615711.

Panswad, T., and Anan, C. 1999. Impact of high chloride wastewater on ananaerobic/anoxic/aerobic process with and without inoculation of chlorideacclimated seeds. Water Res. 33(5): 1165–1172. doi:10.1016/S0043-1354(98)00314-5.

Puanngam, M., and Unob, F. 2008. Preparation and use of chemically modifiedMCM-41 and silica gel as selective adsorbents for Hg(II) ions. J. Hazard. Mater.154(1–3): 578–587. doi:10.1016/j.jhazmat.2007.10.090. PMID:18063298.

Qin, J. 2007. Nanoparticles for multifunctional drug delivery systems-licentiatethesis. The Royal Institute of Technology, Stockholm.

Ramasahayam, S.K., Guzman, L., Gunawan, G., and Viswanathan, T. 2014. Acomprehensive review of phosphorus removal technologies and processes.J. Macromol. Sci., Part A: Pure Appl. Chem. 51(6): 538–545. doi:10.1080/10601325.2014.906271.

Rittmann, B.E., Mayer, B., Westerhoff, P., and Edwards, M. 2011. Capturing thelost phosphorus. Chemosphere, 84(6): 846–853. doi:10.1016/j.chemosphere.2011.02.001. PMID:21377188.

Rodrigues, L.A., and da Silva, M.L.C.P. 2009. An investigation of phosphate ad-sorption from aqueous solution onto hydrous niobium oxide prepared byco-precipitation method. Colloids Surf., A, 334(1–3): 191–196. doi:10.1016/j.colsurfa.2008.10.023.

Rodrigues, L.A., and da Silva, M.L.C.P. 2010. Thermodynamic and kinetic inves-tigations of phosphate adsorption onto hydrous niobium oxide prepared byhomogeneous solution method. Desalination, 263(1–3): 29–35. doi:10.1016/j.desal.2010.06.030.

Rodrigues, L.A., Maschio, L.J. , Coppio, L.de.S.C., Thim, G.P., andPinto da Silva, M.L.C. 2012. Adsorption of phosphate from aqueous solutionby hydrous zirconium oxide. Environ. Technol. 33(12): 1345–1351. doi:10.1080/09593330.2011.632651. PMID:22856308.

Shin, E.W., Han, J.S., Jang, M., Min, S.-H., Park, J.K., and Rowell, R.M. 2004.Phosphate adsorption on aluminum-impregnated mesoporous silicates: sur-face structure and behavior of adsorbents. Environ. Sci. Technol. 38(3): 912–917. doi:10.1021/es030488e. PMID:14968882.

Shin, E.W., Karthikeyan, K.G., and Tshabalala, M.A. 2005. Orthophosphate sorp-tion onto lanthanum-treated lignocellulosic sorbents. Environ. Sci. Technol.39(16): 6273–6279. doi:10.1021/es048018n. PMID:16173592.

Song, X., Pan, Y., Wu, Q., Cheng, Z., and Ma, W. 2011. Phosphate removal fromaqueous solutions by adsorption using ferric sludge. Desalination, 280(1–3):384–390. doi:10.1016/j.desal.2011.07.028.

Sowmya, A., and Meenakshi, S. 2014. Zr(IV) loaded cross-linked chitosan beadswith enhanced surface area for the removal of nitrate and phosphate. Int. J.Biol. Macromol. 69: 336–343. doi:10.1016/j.ijbiomac.2014.05.043. PMID:24938204.

Sparks, D.L. 1989. Kinetics of soil chemical processes. Academic Press.pp. 190–206.

Sparks, D.L. 2001. Elucidating the fundamental chemistry of soils: past andrecent achievements and future frontiers. Geoderma, 100(3–4): 303–319. doi:10.1016/s0016-7061(01)00026-x.

Su, Y., Yang, W., Sun, W., Li, Q., and Shang, J.K. 2015. Synthesis of mesoporouscerium–zirconium binary oxide nanoadsorbents by a solvothermal processand their effective adsorption of phosphate from water. Chem. Eng. J. 268:270–279. doi:10.1016/j.cej.2015.01.070.

Tanada, S., Kabayama, M., Kawasaki, N., Sakiyama, T., Nakamura, T., Araki, M.,and Tamura, T. 2003. Removal of phosphate by aluminum oxide hydroxide.J. Colloid Interface Sci. 257(1): 135–140. doi:10.1016/s0021-9797(02)00008-5.PMID:16256465.

Tejedor-Tejedor, M.I., Yost, E.C., and Anderson, M.A. 1992. Characterization ofbenzoic and phenolic complexes at the goethite/aqueous solution interfaceusing cylindrical internal reflection Fourier transform infrared spectroscopy.2. Bonding structures. Langmuir, 8(2): 525–533. doi:10.1021/la00038a036.

Tian, S., Jiang, P., Ning, P., and Su, Y. 2009. Enhanced adsorption removal ofphosphate from water by mixed lanthanum/aluminum pillared montmoril-lonite. Chem. Eng. J. 151(1–3): 141–148. doi:10.1016/j.cej.2009.02.006.

Tokunaga, S., Wasay, S.A., and Park, S.-W. 1997. Removal of arsenic(V) ion fromaqueous solutions by lanthanum compounds. Water Sci. Technol. 35(7): 71–78. doi:10.1016/s0273-1223(97)00116-9.

Tu, Y.-J., You, C.-F., Chang, C.-K., and Chen, M.-H. 2015. Application of magneticnano-particles for phosphorus removal/recovery in aqueous solution. J. Tai-wan Inst. Chem. Eng. 46: 148–154. doi:10.1016/j.jtice.2014.09.016.

Unnithan, M.R., Vinod, V.P., and Anirudhan, T.S. 2002. Ability of iron(III)-loadedcarboxylated polyacrylamide-grafted sawdust to remove phosphate ionsfrom aqueous solution and fertilizer industry wastewater: Adsorption kinet-ics and isotherm studies. J. Appl. Polym. Sci. 84(13): 2541–2553. doi:10.1002/app.10579.

Urano, K., Tachikawa, H., and Kitajima, M. 1992. Process development for re-moval and recovery of phosphorus from wastewater by a new adsorbent. 4.Recovery of phosphate and aluminum from desorbing solution. Ind. Eng.Chem. Res. 31(6): 1513–1515. doi:10.1021/ie00006a013.

Veith, J.A., and Sposito, G. 1977. On the use of the Langmuir equation in theinterpretation of “adsorption” phenomena. Soil Sci. Soc. Am. J. 41(4): 697–702. doi:10.2136/sssaj1977.03615995004100040015x.

Wang, S., Boyjoo, Y., Choueib, A., and Zhu, Z.H. 2005. Removal of dyes fromaqueous solution using fly ash and red mud. Water Res. 39(1): 129–138. doi:10.1016/j.watres.2004.09.011. PMID:15607172.

Wang, T., Xu, X., Ren, Z., Gao, B., and Wang, H. 2016. Adsorption of phosphateon surface of magnetic reed: characteristics, kinetic, isotherm, desorp-tion, competitive and mechanistic studies. RSC Adv. 6: 5089–5099. doi:10.1039/C5RA25280F.

Wang, W., Zhang, H., Zhang, L., Wan, H., Zheng, S., and Xu, Z. 2015. Adsorptiveremoval of phosphate by magnetic Fe3O4@C@ZrO2. Colloids Surf., A, 469:100–106. doi:10.1016/j.colsurfa.2015.01.002.

Wu, W., He, Q., and Jiang, C. 2008. Magnetic iron oxide nanoparticles: synthesisand surface functionalization strategies. Nanoscale Res. Lett. 3(11): 397–415.doi:10.1007/s11671-008-9174-9. PMID:21749733.

Liao, P.H., Wong, W.T., and Lo, K.V. 2005. Release of phosphorus from sewagesludge using microwave technology. J. Environ. Eng. Sci. 4(1): 77–81. doi:10.1139/s04-056.

Wu, K., Liu, T., Xue, W., and Wang, X. 2012. Arsenic(III) oxidation/adsorptionbehaviors on a new bimetal adsorbent of Mn-oxide-doped Al oxide. Chem.Eng. J. 192: 343–349. doi:10.1016/j.cej.2012.03.058.

Wu, K., Liu, T., Ma, C., Chang, B., Chen, R., and Wang, X. 2014. The role of Mnoxide doping in phosphate removal by Al-based bimetal oxides: adsorptionbehaviors and mechanisms. Environ. Sci. Pollut. Res. 21(1): 620–630. doi:10.1007/s11356-013-1937-x.

Wu, R.S.S., Lam, K.H., Lee, J.M.N., and Lau, T.C. 2007. Removal of phosphate fromwater by a highly selective La(III)-chelex resin. Chemosphere, 69(2): 289–294.doi:10.1016/j.chemosphere.2007.04.022. PMID:17531289.

Xie, J., Wang, Z., Lu, S., Wu, D., Zhang, Z., and Kong, H. 2014. Removal andrecovery of phosphate from water by lanthanum hydroxide materials. Chem.Eng. J. 254: 163–170. doi:10.1016/j.cej.2014.05.113.

Xiong, J., He, Z., Mahmood, Q., Liu, D., Yang, X., and Islam, E. 2008. Phosphateremoval from solution using steel slag through magnetic separation. J. Hazard.Mater. 152(1): 211–215. doi:10.1016/j.jhazmat.2007.06.103. PMID:17703877.

Li et al. 331

Published by NRC Research Press

Env

iron

. Rev

. Dow

nloa

ded

from

ww

w.n

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sear

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com

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itute

of

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grap

hy a

nd L

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, CA

S on

01/

17/1

7Fo

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al u

se o

nly.

Xu, P., Zeng, G.M., Huang, D.L., Feng, C.L., Hu, S., Zhao, M.H., et al. 2012. Use ofiron oxide nanomaterials in wastewater treatment: a review. Sci. Total Environ.424: 1–10. doi:10.1016/j.scitotenv.2012.02.023. PMID:22391097.

Xu, X., Gao, B., Tan, X., Zhang, X., Yue, Q., Wang, Y., and Li, Q. 2013. Nitrateadsorption by stratified wheat straw resin in lab-scale columns. Chem. Eng. J.226: 1–6. doi:10.1016/j.cej.2013.04.033.

Xue, Y., Hou, H., and Zhu, S. 2009. Characteristics and mechanisms of phosphateadsorption onto basic oxygen furnace slag. J. Hazard. Mater. 162(2–3): 973–980. doi:10.1016/j.jhazmat.2008.05.131. PMID:18614283.

Yan, L.-G., Xu, Y.-Y., Yu, H.-Q., Xin, X.-D., Wei, Q., and Du, B. 2010. Adsorption ofphosphate from aqueous solution by hydroxy-aluminum, hydroxy-iron andhydroxy-iron-aluminum pillared bentonites. J. Hazard. Mater. 179(1–3): 244–250. doi:10.1016/j.jhazmat.2010.02.086. PMID:20334967.

Yang, J., Zhou, L., Zhao, L., Zhang, H., Yin, J., Wei, G., et al. 2011. A designednanoporous material for phosphate removal with high efficiency. J. Mater.Chem. 21(8): 2489–2494. doi:10.1039/c0jm02718a.

Yang, S., Zhao, Y., Chen, R., Feng, C., Zhang, Z., Lei, Z., and Yang, Y. 2013. A noveltablet porous material developed as adsorbent for phosphate removal andrecycling. J. Colloid Interface Sci. 396: 197–204. doi:10.1016/j.jcis.2012.12.077.PMID:23484953.

Yang, X., Wang, D., Sun, Z., and Tang, H. 2007. Adsorption of phosphate at thealuminum (hydr)oxides–water interface: Role of the surface acid–base prop-erties. Colloids Surf., A, 297(1–3): 84–90. doi:10.1016/j.colsurfa.2006.10.028.

Yao, W., and Millero, F.J. 1996. Adsorption of phosphate on manganese dioxidein seawater. Environ. Sci. Technol. 30(2): 536–541. doi:10.1021/es950290x.

Ye, H., Chen, F., Sheng, Y., Sheng, G., and Fu, J. 2006. Adsorption of phosphatefrom aqueous solution onto modified palygorskites. Sep. Purif. Technol.50(3): 283–290. doi:10.1016/j.seppur.2005.12.004.

Yin, H., Yun, Y., Zhang, Y., and Fan, C. 2011. Phosphate removal from wastewa-ters by a naturally occurring, calcium-rich sepiolite. J. Hazard. Mater. 198:362–369. doi:10.1016/j.jhazmat.2011.10.072. PMID:22088501.

Yoshitake, H., Yokoi, T., and Tatsumi, T. 2003. Oxyanion adsorptions by mono-,di-, and triamino-functionalized MCM-48. Bull. Chem. Soc. Jpn. 76(4): 847–852. doi:10.1246/bcsj.76.847.

Yu, S.M., Chen, C.L., Chang, P.P., Wang, T.T., Lu, S.S., and Wang, X.K. 2008.Adsorption of Th(IV) onto Al-pillared rectorite: Effect of pH, ionic strength,temperature, soil humic acid and fulvic acid. Appl. Clay Sci. 38(3–4): 219–226.doi:10.1016/j.clay.2007.03.008.

Yu, Y., and Chen, J.P. 2015. Key factors for optimum performance in phosphateremoval from contaminated water by a Fe-Mg-La tri-metal composite sor-bent. J. Colloid Interface Sci. 445: 303–311. doi:10.1016/j.jcis.2014.12.056. PMID:25635604.

Yuan, X., Bai, C., Xia, W., Xie, B., and An, J. 2014. Phosphate adsorption charac-teristics of wasted low-grade iron ore with phosphorus used as natural ad-sorbent for aqueous solution. Desalin. Water Treat. 54(11): 3020–3030. doi:10.1080/19443994.2014.905974.

Yue, Q., Zhao, Y., Li, Q., Li, W., Gao, B., Han, S., et al. 2010. Research on thecharacteristics of red mud granular adsorbents (RMGA) for phosphate re-moval. J. Hazard. Mater. 176(1–3): 741–748. doi:10.1016/j.jhazmat.2009.11.098.PMID:20006430.

Zamparas, M., Gianni, A., Stathi, P., Deligiannakis, Y., and Zacharias, I. 2012.Removal of phosphate from natural waters using innovative modified ben-tonites. Appl. Clay Sci. 62–63: 101–106. doi:10.1016/j.clay.2012.04.020.

Zeng, L., Li, X., and Liu, J. 2004. Adsorptive removal of phosphate from aqueoussolutions using iron oxide tailings. Water Res. 38(5): 1318–1326. doi:10.1016/j.watres.2003.12.009. PMID:14975665.

Zhang, G., Liu, H., Liu, R., and Qu, J. 2009. Removal of phosphate from water bya Fe-Mn binary oxide adsorbent. J. Colloid Interface Sci. 335(2): 168–174.doi:10.1016/j.jcis.2009.03.019. PMID:19406416.

Zhang, J., Shen, Z., Shan, W., Chen, Z., Mei, Z., Lei, Y., and Wang, W. 2010.Adsorption behavior of phosphate on Lanthanum(III) doped mesoporous sil-icates material. J. Environ. Sci. (Beijing, China), 22(4): 507–511. doi:10.1016/s1001-0742(09)60141-8.

Zhang, J., Shen, Z., Mei, Z., Li, S., and Wang, W. 2011. Removal of phosphate byFe-coordinated amino-functionalized 3D mesoporous silicates hybrid mate-rials. J. Environ. Sci. (Beijing, China), 23(2): 199–205. doi:10.1016/s1001-0742(10)60393-2.

Zhang, K., Zhang, X., Chen, H., Chen, X., Zheng, L., Zhang, J., and Yang, B. 2004.Hollow titania spheres with movable silica spheres inside. Langmuir, 20(26):11312–11314. doi:10.1021/la047736k.

Zhang, L., Wan, L., Chang, N., Liu, J., Duan, C., Zhou, Q., et al. 2011. Removal ofphosphate from water by activated carbon fiber loaded with lanthanumoxide. J. Hazard. Mater. 190(1–3): 848–855. doi:10.1016/j.jhazmat.2011.04.021.PMID:21530079.

Zhang, L., Zhou, Q., Liu, J., Chang, N., Wan, L., and Chen, J. 2012. Phosphateadsorption on lanthanum hydroxide-doped activated carbon fiber. Chem.Eng. J. 185–186: 160–167. doi:10.1016/j.cej.2012.01.066.

Zhang, L., Gao, Y., Xu, Y., and Liu, J. 2016. Different performances and mecha-nisms of phosphate adsorption onto metal oxides and metal hydroxides: acomparative study. J. Chem. Technol. Biotechnol. 91(5): 1232–1239. doi:10.1002/jctb.4710.

Zhou, Q., Wang, X., Liu, J., and Zhang, L. 2012. Phosphorus removal from waste-water using nano-particulates of hydrated ferric oxide doped activated car-bon fiber prepared by Sol–Gel method. Chem. Eng. J. 200–202: 619–626.doi:10.1016/j.cej.2012.06.123.

Zhu, M.-X., Ding, K.-Y., Xu, S.-H., and Jiang, X. 2009. Adsorption of phosphate onhydroxyaluminum- and hydroxyiron-montmorillonite complexes. J. Hazard.Mater. 165(1–3): 645–651. doi:10.1016/j.jhazmat.2008.10.035. PMID:19038495.

Zhu, X., and Jyo, A. 2005. Column-mode phosphate removal by a novel highlyselective adsorbent. Water Res. 39(11): 2301–2308. doi:10.1016/j.watres.2005.04.033. PMID:15939450.

332 Environ. Rev. Vol. 24, 2016

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