RESEARCH ARTICLE

Cork stoppers as an effective sorbent for watertreatment: the removal of mercury at environmentally relevantconcentrations and conditions

Cláudia B. Lopes & JoanaR. Oliveira &Luciana S. Rocha &

Daniela S. Tavares & Carlos M. Silva & Susana P. Silva &

Niels Hartog & Armando C. Duarte & E. Pereira

Received: 14 May 2013 /Accepted: 27 August 2013 /Published online: 12 September 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract The technical feasibility of using stopper-derivedcork as an effective biosorbent towards bivalent mercury atenvironmentally relevant concentrations and conditions wasevaluated in this study. Only 25mg/L of cork powder was ableto achieve 94 % of mercury removal for an initial mercuryconcentration of 500 μg/L. It was found that under the condi-tions tested, the efficiency of mercury removal expressed asequilibrium removal percentage does not depend on theamount of cork or its particle size, but is very sensitive toinitial metal concentration, with higher removal efficiencies athigher initial concentrations. Ion exchange was identified asone of the mechanisms involved in the sorption of Hg ontocork in the absence of ionic competition. Under ionic compe-tition, stopper-derived cork showed to be extremely effectiveand selective for mercury in binary mixtures, while in com-plex matrices like seawater, moderate inhibition of the sorp-tion process was observed, attributed to a change in mercuryspeciation. The loadings achieved are similar to the majorityof literature values found for other biosorbents and for othermetals, suggesting that cork stoppers can be recycled as an

effective biosorbent for water treatment. However, the mostinteresting result is that equilibrium data show a very rarebehaviour, with the isotherm presenting an almost squareconvex shape to the concentration axis, with an infinite slopefor an Hg concentration in solution around 25 μg/L.

Keywords Mercury . Cork . Sorption . Recycling . Ioniccompetition . Kineticmodelling .Metal removal

Introduction

Water is essential to all forms of life and a fundamental aspect forenvironmental health and management. The rapid developmentand industrialization in many countries have been responsible forthe steadily increasing of industrial pollution. Although todaythere is an increased environmental conscience amongst popula-tions and policy makers, economic development is still the mainpriority, particularly in developing countries. The pollution aris-ing from the presence of metals in waste and surface watersrepresents a severe problem due to the scarcity of drinkablewater, and as consequence, the legislation regarding the dis-charge of potential toxic metal through wastewaters has becomemore strict (Figueira et al. 2011).

Mercury (Hg) is regarded as one of the most harmful metalsfound in the environment (Lopes et al. 2009; Zhao et al. 2010)and the European Parliament and the Council of EuropeanUnion have recently included mercury in the list of priorityhazardous substances (European Parliament 2008). Mercury isdescribed as a persistent and nonessential toxic element, beingreadily accumulated by organisms, posing a considerable po-tential threat to human health and natural ecosystems, even atvery low concentrations (Al Rmalli et al. 2008; Karunasagaret al. 2005; Lopes et al. 2010).

Responsible editor: Bingcai Pan

Electronic supplementary material The online version of this article(doi:10.1007/s11356-013-2104-0) contains supplementary material,which is available to authorized users.

C. B. Lopes (*) : J. R. Oliveira : L. S. Rocha :D. S. Tavares :C. M. Silva :A. C. Duarte : E. PereiraDepartment of Chemistry/CESAM and CICECO,University of Aveiro, 3810-193 Aveiro, Portugale-mail: claudia.b.lopes@ua.pt

S. P. SilvaCorticeira Amorim, S.G.P.S., S.A.S, Paio de Oleiros,Santa Maria da Feira, Portugal

N. HartogKWRWatercycle Research Institute, Nieuwegein, The Netherlands

Environ Sci Pollut Res (2014) 21:2108–2121DOI 10.1007/s11356-013-2104-0

The levels of mercury found in wastewaters come from avariety of global anthropogenic sources that comprise the indus-tries related with the production of chlor-alkali, paper and pulp,oil refinery, paint, fossil fuel burning, metallurgical processes,pharmaceutical and battery manufacturing (Karunasagar et al.2005; Lopes et al. 2009). As the regulations for wastewatertreatment became more stringent, alternative and cost-effectivetreatment technologies have been developed to replace some ofthe conventional and costlymethods, such as hydrometallurgicaltechnologies, ion exchange, electrodialysis and reverse osmosis(Chubar et al. 2003; Farooq et al. 2010). In the last two decades,special attention has been given to sorption methods due to theireconomic, ecological and technologic advantages (Chubar et al.2003; Farooq et al. 2010). Biosorption, which is the sorption ofcontaminants by sorbents of natural origin, has gained particularimportance, as these materials are cheap and effective and mostof the times represent unused resources that are widely availableand are environmentally friendly to use (Bayramoğlu and Arıca2008; Farooq et al. 2010).

Within the scope of this work, the potential to use stopper-derived cork powder as a biosorbent towards dissolved biva-lent mercury was evaluated. Cork is the outer bark of the oaktree known botanically as Quercus suber L. which is period-ically extracted from the tree. Although cork oak trees flourishonly in specific regions of the Western Mediterranean(Portugal, Spain, Southern France, part of Italy, NorthAfrica) and China, cork stoppers are spread worldwide dueto the wine industry. According to the ReCORK (2011) website, an estimated 13 billion wine cork stoppers are producedeach year, and as only a small percentage is recycled or reused,most end up as waste. In Portugal, a big effort has been made,particularly by the Green Cork program (www.greencork.org), which allows recycling 6 to 10 % of the total amount of corkstoppers in the Portuguese market (SICNoticias 2013). For5 years, the Green Cork program have already collected andrecycled more than 160 t of cork (SICNoticias 2013).Furthermore, during cork stoppers production, 20 % of theoriginal biomass ends up as a by-product/residue known as“cork powder” with no end use other than energy conversion(Gil 1997) and which therefore has been studied aiming atfinding new valorisation (Vilela et al. 2013).

Chemically, cork contains about 30 to 60 % (w/w) of suberin(an amorphous cross-linked polyester composed mainly of longchain aliphatic ω-hydroxyacids, α,ω-dicarboxylic acids andglycerol), 19 to 22 % (w/w) of lignin (an amorphous cross-linked aromatic polymer), 12 to 20 % (w/w) of polysaccharides(hemicelluloses and cellulose) and 13 to 16 % (w/w) of extrac-tive components (mainly triterpenic and phenolic compounds)(Chubar et al. 2003, 2004; Gandini et al. 2006; Pinto et al. 2009;Sousa et al. 2006). The mineral content is low (ca. 5 %), and themost abundant element is calcium (0.038–0.625 % (w /w)),followed by phosphorus, iron, magnesium and aluminium(Chubar et al. 2003; Machado et al. 2002).

The chemical composition of cork, particularly its high con-tent of suberin, makes it a good candidate as biosorbent in watertreatment processes, and some studies have reported the remark-able properties of the cork in the bio-removal of metals (Chubaret al. 2003, 2004; Lopez-Mesas et al. 2011;Machado et al. 2002;Villaescusa et al. 2000). Natural andmodified corks have alreadybeen studied as biosorbent towards some metals such as cadmi-um, chromium, copper, nickel, lead, zinc and uranium (Chubaret al. 2003; Lopez-Mesas et al. 2011; Machado et al. 2002;Psareva et al. 2005; Villaescusa et al. 2000). However, recyclingcork stoppers as biosorbents rather than requiring fresh corkmaterial would allow increase the overall sustainability of usingcork as a biosorbent. To date however, it remains unclear to whatextent the metal sorbing capacities of cork are affected after useas stoppers.

In the present work, the potential of using recycled granulatedcork stoppers was evaluated for the first time, for the removal ofmercury from water, under realistic environmental conditions.The main objectives of the study were (1) to assess the efficiencyof the cork stoppers to clean water with low concentration levelsof Hg (microgrammes per litre range), (2) to evaluate the effect ofsome physico-chemical parameters such as the contact time,particle size, initial Hg2+concentration, biosorbent mass andpresence of other metals, (3) to assess possible differences be-tween cork stoppers and natural cork and (4) to evaluate thefeasibility of its application to natural waters, in particular sea-water used for aquaculture.

Material and methods

Chemicals

All chemicals used in this work were of analytical reagentgrade, and all solutions were prepared with ultra-pure water(18.2 MΩ cm, Milli-Q system). A certified standard stocksolution of mercury nitrate (998±2 mg/L), a cadmium nitrate(1001±2 mg/L) and a nitric acid solution (65 %) were pur-chased from Merck. All working solutions, including stan-dards for the calibration curves, were obtained by diluting thecorresponding stock solutions. A sodium hydroxide solution(0.1 mol/L) was used for pH adjustments, and the nitric acidsolution was used to acidify the sample solutions after collec-tion from the reaction vessel.

Biosorbent preparation and characterization

Two types of cork were studied: cork from used stoppers ofwine bottles (CS) and unused natural cork (NC). Both types ofcork were cut in small pieces and afterwards triturated using acoffee mill. Different fractions of the cork samples wereobtained using a mechanical sieve shaker, with 4 sieves, butonly two particle sizes were studied: a granulated fraction (G),

Environ Sci Pollut Res (2014) 21:2108–2121 2109

which comprises particle sizes between 0.5 and 1 mm and apowder fraction (P) that includes particle sizes ≤0.2 mm. Thecork samples were stored at room temperature in plasticcontainers until further use.

Chemical, structural and morphological features of thepowder fraction of stopper-derived cork were evaluated. Themain functional groups present in the cork were identified byFourier transform infrared (FTIR) spectrum, which wasrecorded on a Mattson 7000 FTIR spectrometer, using KBrpellets. The spectral range varied from 4,000 to 280 cm−1. Themorphological characteristics of stopper-derived cork wereevaluated by scanning electron microscopy (SEM) using S-4100 HITACHI equipment. This system was coupled withelectron dispersive spectroscopy (EDS) allowing the identifi-cation of the most abundant inorganic elements of the corkstoppers. Prior to SEM analysis, the cork samples were cov-ered with a thin layer of carbon and an electron accelerationvoltage of 20 kV was applied. The specific surface area of theparticles was determined with nitrogen adsorption BET mea-surements performed with a Gemini V2.0 Micromeritics in-strument. The pore size was calculated from the desorptionbranch using the Barret–Joyner–Hallenda method, and thepore volume was evaluated from the adsorbed amount.

Batch sorption experiments

Kinetic studies

The capacity of the cork biomass to remove Hg from spikedwater (ultra-pure, Cd2+ solution and seawater) was evaluatedunder various batch conditions, by contacting a certainamount of cork with an Hg solution for a required period oftime, under constant magnetic stirring at room temperature(21±1 °C) and neutral pH (ca. 6–7).

The kinetic sorption experiments were performed using a2-L volumetric flask as reaction vessel. The initial Hg2+

concentrations studied were 50 μg/L representing the guide-line value for Hg discharges from industrial sectors and500 μg/L simulating a situation of an accidental discharge.A sorbent-free trial was run as a control, in order to check Hg

losses that occurred by other means (e.g. adsorption on thevessel walls, Hg volatilization and/or retention by theMillipore membrane during the filtration step), rather thanthe interaction with the cork particles. To minimize Hg ad-sorption to the containers and avoid possible contaminations,all glassware used in the experiments was acid-washed priorto use. It was found that the Hg sorption that occurred in thecontrols increase with time and ranged [3–17 %].

Hg solutions were prepared daily by diluting the stocksolution to the desired concentration in water. Before thebeginning of each experiment an aliquot of the Hg solution(ca. 10 mL) was collected to confirm the initial Hg concen-tration (CHg,0). The kinetic sorption experiments began whenan accurately known amount of the cork biomass (m) wasadded to the Hg solution. Aliquots (ca. 10 mL) were collectedfrom the system Hg/cork, at increasing times and filteredthrough a pre-acid washed 0.45-μm Millipore membrane.The filtrate was adjusted to pH<2 with nitric acid and thenanalysed for Hg quantification. Each experiment was contin-ued until the Hg concentration in solution remained constant(CHg,e), corresponding to the solution–solid equilibrium. Allconcentrations values for the experiments were corrected bymeans of a recovery factor that accounts for the Hg lossesfrom the controls, and the specific experimental conditionsused in each experiment are summarized in Table 1.

The amount of Hg sorbed per unit of cork biomass, at timet , (qHg, milligrammes per gramme) was deduced from themass balance between the initial Hg concentration in solutionand the concentration at each time t :

qHg ¼CHg;o−CHg

� �V

mð1Þ

where V (litres) is the volume of the solution,m (milligrammes)is the dry weight of cork biomass, CHg,0 (microgrammes perlitre) is the initial concentration of Hg2+and CHg (microgrammesper litre) is its concentration at each time.

The sorption kinetics of Hg on cork was studied using threekinetic models, namely the Lagergren pseudo-first-order mod-el (PFO) (Lagergren 1898), Ho’s pseudo-second-order model

Table 1 Experimental conditions used on the sorption studies

Effect Cork Particle size (mm) CHg,0 (μg/L) mcork (mg/L) Matrix

Particle size CS <0.2 and 0.5–1.0 50 and 500 17.5 Spiked ultra-pure water

Amount of cork CS <0.2 50 17.5, 25, 50 and 250 Spiked ultra-pure water500 25 and 250

Initial Hg2+ concentration CS <0.2 50 and 500 25 and 250 Spiked ultra-pure water

Type of cork NC and CS <0.2 500 50 and 500 Spiked ultra-pure water

Ionic competition CS <0.2 500 25 Cd2+ solution (2,000 μg/L)

50 250 Spiked natural seawater

Additional points for isotherm CS <0.2 50 0.5, 2.5, 10, 375 and 500 Spiked ultra-pure water

2110 Environ Sci Pollut Res (2014) 21:2108–2121

(PSO) (Ho and McKay 1999) and the Elovich model (Low1960), expressed respectively by the following equations:

q ¼ qe 1−e−k1t� � ð2Þ

q ¼ q2e k2t

1−qek2tð3Þ

q ¼ 1

βln 1þ αβtð Þ ð4Þ

where q and qe (both in milligrammes per gramme) are theamount of metal sorbed per gram of cork, respectively, at timet and at equilibrium; k1 (per hour) and k2 (grammes permilligramme-hour) are, respectively, the kinetic constants ofpseudo-first- and pseudo-second-order and α (milligrammesper gramme-hour) is the initial sorption rate and β (grammesper milligramme) is the desorption constant.

Equilibrium studies

The isotherm was obtained using the equilibrium data fromthe kinetic studies and five additional points obtained byvarying the mass of cork from 0.5 to 500 mg/L for an initialHg2+ concentration of 50 μg/L (Table 1).

The amount of Hg sorbed per unit of cork biomass, atequilibrium (qHg,e, milligrammes per gramme) was deducedfrom the mass balance between the initial Hg concentration insolution and the concentration at equilibrium, according with:

qHg;e ¼CHg;o−CHg;e

� �V

mð5Þ

The results were also compared by removal percentage (R ),which at equilibrium is defined by:

RHg;e ¼CHg;o−CHg;e

� �CHg;o

� 100 ð6Þ

Metals quantification and QA/QC procedures

All Hg analyses were performed by cold vapour atomic fluo-rescence spectroscopy, on a flow-injection cold vapour atomicfluorescence spectrometer (Hydride/vapour generator PSAnalytical Model 10.003, coupled to a PS Analytical Model10.023 Merlin atomic fluorescence spectrometer) and usingSnCl2 (10 % m /v ) as reducing agent. The Hg concentrationwas quantified through a calibration curve of five standardsprepared in a nitric acid solution (2 % v /v ), by dilution fromthe certified standard solution of Hg(NO3)2, and which con-centration ranged from 0.0 to 0.5 μg/L. In this range, the limit

of detection of the method is 0.02 μg/L and the precision andaccuracy are <5 %.

The Cd2+ solution was prepared by diluting the stock solutionin ultra-pure water to obtain a concentration tenfold the guidelinevalue for Cd discharges from industrial sectors (2,000 μg/L).Cd2+ quantification was performed by inductively coupled plas-mamass spectrometry (ICP-MS), on a Thermo ICP-MSXSeriesequipped with a Burgener nebuliser. The calibration curve forCd2+ quantificationwas obtained from standards (4–1,000μg /L)prepared by dilution of the certified standard solution ofCd(NO3)2 in a nitric acid solution (2 % v /v). The limit ofdetection of the method was 1.4 μg/L and the precision andaccuracy are <10 %.

Natural seawater characterisation

Natural seawater sample was collected at the shore of thePortuguese coast, in the Atlantic Ocean, near the abstraction ofwater for an aquaculture. The water sample was collected from adepth of 30 cm using polyethylene bottles that were rinsed withsurface water, before filling with the sample. The water wascharacterized in terms of pH, conductivity, suspended particulatematter (SPM), Hg concentration, major and minor elements. ThepH (8.1) and conductivity (54.7 mS/cm) of the seawater at 22 °Cwere record on a WTW meter. For SPM determination, 0.5–1.0 L of water was filtered through a pre-weighted 0.45-μm poresize cellulose acetate membrane filter (Millipore). The filterswere dried until constant weight and then re-weighed to estimatethe dry mass of SPM. For Hg determination, water samples werefiltered through a 0.45-μm pore size cellulose acetate membranefilter (Millipore), then acidified to pH below 2, with concentratednitric acid, and thenwater samples were analysed by cold vapouratomic fluorescence spectroscopy as described previously. Forthe determination of major and minor elements, filtrated(0.45-μm pore size cellulose acetate membrane filter) watersamples were analysed by inductively coupled plasma spectros-copy, on a Jobin–Yvon JY70 Plus Spectrometer. The content ofsuspended particle matter was 11.3 mg/L and the predominantcations in the water were sodium (>1,000 mg/L), potassium(650 mg/L), calcium (485 mg/L), magnesium (300mg/L), stron-tium (5 mg/L) and boron (3 mg/L). Amounts of lithium (351μg/L), zinc (31 μg/L), aluminium (23 μg/L), iron (12 μg/L), copper(5 μg/L) and barium (4 μg/L) were also detected. The concen-trations of several elements (e.g. As, Be, Cd, Co, Cr, Mn, Ni, P,Pb, Si, Ti and V) were below the quantification limit of themethod. The total Hg concentration in the seawater of 3.1 ng/Lis typical for non-polluted waters.

Error analysis

The parameters of the kinetic models considered in this workwere obtained by nonlinear regression analysis with theGraphPad Prism 5 program (trial version). This program uses

Environ Sci Pollut Res (2014) 21:2108–2121 2111

the least-squares as fitting method and the method ofMarquardt and Levenberg, blending the methods of lineardescent and Gauss–Newton for adjusting the variables. Thegoodness of the fit to the experimental data was evaluated bythe coefficient of determination (R2) and the standard devia-tion of residues (Sx/y). The relative error (E r) between exper-imental qHg,e and models’ estimation is also presented. Thestatistical parameters were calculated accordingly with thefollowing equations:

R2 ¼ 1 −

Xyi−byi

� �2

Xyi−yi

� �2 ð7Þ

Sx=y ¼X

yi−byi� �2

n−2

8><>:

9>=>;

1=2

ð8Þ

Er ¼yi−byi��� ���yi

� 100 ð9Þ

where n is the sample size, y i are the experimental values, yiis the mean of the experimental data and byi are the modelledor predicted values.

Results and discussion

Biosorbent characterization

Figure 1 summarizes the main results obtained from the charac-terization of the powder fraction of used cork stoppers. Themicrograph of cork stoppers powder (Fig. 1a) reveals that thismaterial presents a homogeneous cellular structure formed bythin-walled cells, regularly arranged without intercellular spacein agreement with previously reported results on natural cork(Silvestre et al. 2011). The cells present an alveolar structure(very similar to a honeycomb) with rectangular prisms, mostlypentagonal and hexagonal. The morphologic features of the corkstoppers powder were also analysed after its use in the sorptionstudies with Hg (Fig. 1b). The alveolar structure of cork keeps itsprismatic shape of the cells and its regular arrangement. The EDSanalysis allowed identifying zirconium (Zr), potassium (K), sil-icon (Si), titanium (Ti), chlorine (Cl) and calcium (Ca) as themost abundant inorganic elements on the surface of the cork. Theidentification of some characteristic functional groups in

a

c

bFig. 1 Characterization of corkstoppers powder (CS-P) by SEMimages before (a) and after (b)sorption studies and by FourierTransform Infrared spectroscopy(c)

2112 Environ Sci Pollut Res (2014) 21:2108–2121

powdered cork stoppers was performed by FTIR (Fig. 1c). In thespectrum of cork stoppers powder, specific bands were assignedto the main components of cork: suberin, lignin and polysaccha-rides, with the suberin absorption bands being themost prevalent.The spectrum is characterized by a dominant C–H bands at2,923, 2,850, 1,464 and 1,373 cm−1 from aliphatic and olefinicC–H bonds in suberin, an intense C=O band at 1,741 cm−1 and aC–O–C band with two peaks located at 1,161 and 1,257 cm−1

from esters in suberin units. Moreover, the broad band at3,450 cm−1 indicates the presence of O–H groups. The peak at1,511 cm−1 corresponds to the C=C stretch in aromatic ring fromlignin and/or lignin-like. The polysaccharides units are charac-terized by the peaks at 1,031 and 1,089 cm−1 that correspond toC–H and C–Odeformation. The peak at 1,610 cm–1 correspondsto the C=C stretch in aromatic ring from suberin or to theextractive components in cork. These FTIR profiles are in agree-ment with previously published results for natural cork (Cordeiroet al. 1998; Rocha et al. 2001). The BET surface area of corkparticles was found to be 12.5 m2/g, and the pores have a size of4.4 nm and volume of 0.015 mL/g, which makes them suitablefor the sorption of several metallic contaminants.

Sorption kinetics of Hg in absence of ionic competition

Influence of cork particle size

Sorption experiments using two fractions of stopper-derived cork(less than 0.2 mm and between 0.5 and 1 mm) were carried outfor two initial metal concentrations (50 and 500μg/L) to evaluatethe effect of particle size of cork on sorption kinetics and effi-ciency towards Hg. The evolution of Hg concentrations in solu-tion for both particle sizes is shown in Fig. 2.

Despite the hydrophobic character of cork, only a fewmilligrammes gave rise to a rapid decrease on Hg concentra-tions in solution, for both particle sizes and initial Hg concen-trations. The time profile shows that Hg concentration insolution decreases suddenly in the first hours of contact and

then slows down approaching equilibrium, which is attainedafter 8 h of contact (Fig. 2a) when CHg,0 is 50 μg/L and after100 h for a concentration ten times higher (Fig. 2b). Clearly,for both initial concentrations, the removal was considerablyfaster for the powder fraction than for the granulated fraction,as can be seen from the initial slope of the time profile curveand confirmed by the kinetic constants (k1 and k2) and initialsorption rates (ν1, ν2 and α ) given, respectively, by the PFO,PSO and Elovich models (Table 2). This observation is anindication that there are significant internal and eventuallyexternal limitations to mass transfer. In fact, the smaller parti-cles offer inferior diffusion lengths, which in this case, are atmost 0.2/0.5=0.40 of the larger ones. Moreover, taking intoaccount the same unfavourable size limits (0.2 and 0.5 mm)and that the mass of cork is the same in both experiments, theexternal surface area for the small particles is at least 2.5 timeshigher than that of the bigger ones. In conjunction, these twofactors are incrementing significantly the mass transfer, i.e. themercury ions removal.

For the same initial Hg concentration, as the equilibrium isattained, the differences between the two particle sizes start toweaken, and at equilibrium, there are no relevant differences(relative standard deviation lower than 5 %) in terms ofremoval efficiency (50.5 vs 52.6 % for CHg,0=50 μg/L and91.4 vs 86.4% forCHg,0=500 μg/L, respectively for a particlesize <0.2 mm and [0.5–1.0]mm) and amount of Hg sorbed pergram of cork biomass (1.44 vs 1.50 mg/g for CHg,0=50 μg/Land 26.2 vs 24.4 mg/g for CHg,0=500 μg/L, respectively for aparticle size <0.2 mm and [0.5–1.0]mm). This observationclearly indicates that diffusion is preponderant on controllingthe transfer of Hg in the solid–liquid sorption process.

The modelling of the kinetic process by the three kineticmodels adopted in this study allows concluding that the sorp-tion kinetics of Hg for both particle size is well described bythe PSO and Elovich models, as confirmed by the high valuesof the determination coefficient (R2) and low values of thestandard deviation of y -residuals (Sy/x) (Table 2) and, except

a bFig. 2 Time profile of Hgconcentration in solution for particlesizes <0.2 mm, [0.5–1 mm] and forthe control samples. The plot (a)shows the removal curves forCHg,0=50μg/L andmCS=17.5mg/L and plot (b) shows the removalcurves for CHg,0=500 μg/L andmCS=17.5 mg/L

Environ Sci Pollut Res (2014) 21:2108–2121 2113

Tab

le2

PFO,P

SOandElovich

sorptio

nkinetic

constantsfortheremovalof

Hgby

cork

stopperspowder

PFO

PSO

Elovich

Experim

entalconditio

nsBest-fitv

alues

Goodnessof

thefit

Best-fitv

alues

Goodnessof

thefit

Best-fitv

alues

Goodnessof

thefit

Sorbent

Matrix

mC0

qHg,e

k 1qe1

Er

v 1R2

Sx/y

k 2qe2

Er

v 2R2

Sx/y

αβ

R2

Sx/y

mg/L

μg/L

mg/g

1/h

mg/g

%mg/gh

g/mgh

mg/g

%mg/gh

Particlesize

effect

CS-P

SUPW

17.5

501.44

3.22

1.26

12.5

4.1

0.984

0.071

3.35

1.38

3.9

6.4

0.995

0.041

69.0

5.6

0.979

0.08

CS-G

1.62

0.33

1.57

2.8

0.51

0.984

0.080

0.21

1.81

11.5

0.69

0.991

0.061

1.13

2.44

0.975

0.10

CS-P

SUPW

17.5

500

26.2

0.86

21.0

19.8

18.2

0.867

2.92

0.052

23.1

11.9

27.9

0.950

1.79

141

0.30

0.989

0.85

CS-G

24.6

0.055

22.0

10.5

1.21

0.925

2.41

0.003

25.3

3.0

1.74

0.953

1.90

3.24

0.18

0.975

1.39

Massof

cork

andinitialHg(II)concentrationeffect

CS-P

SUPW

2550

1.11

9.29

0.94

15.1

8.76

0.954

0.086

14.3

0.99

11.2

13.9

0.980

0.056

2032

12.2

0.973

0.066

250

500.120

5.10

0.113

5.6

0.58

0.964

0.009

79.8

0.120

0.3

1.14

0.987

0.005

340

101

0.990

0.005

SUPW

25500

18.5

2.86

16.4

11.1

47.0

0.976

0.98

0.254

17.5

5.2

77.9

0.997

0.322

3217

0.56

0.975

1.008

250

500

1.88

20.1

1.62

14.0

32.4

0.935

0.163

18.1

1.70

9.8

52.0

0.970

0.110

8176

7.01

0.997

0.033

Ioniccompetitioneffect

CS-P

Cdsol.

25500

15.6

1.25

13.14

15.8

16.43

0.957

1.112

0.1

14.6

6.5

22.8

0.976

0.830

740.4

0.941

1.306

SSW

250

500.100

0.13

0.093

7.4

0.01

0.977

0.006

1.4

0.107

7.0

0.02

0.990

0.004

0.03

420.986

0.005

Forcomparison,

theexperimentalqHg,eisshow

ntogether

with

thatobtained

from

thefitting

correspondingto

first-andsecond-order

kinetics,as

wellas

therelativ

eerror(E

r)andthegoodness

ofthe

fittings.Initialsorptio

nrates:v

1=K1qeandv

2=k 2(q

e)2

CS-Pcork

stopperspowder,CS-G

cork

stoppersgranulated,SUPW

spiked

ultra-pure

water,C

dsolcadm

ium

solutio

n,SSW

spiked

seaw

ater

2114 Environ Sci Pollut Res (2014) 21:2108–2121

for one experimental condition, no significant differenceswere observed between the fittings provided by these twomodels (test F for a confidence level of 95 %). In general,the qe values estimated by the PSO model underestimate theexperimental values, showing a relative standard error thatranges from 3.0 to 12 %. The graphic representation of thefittings together with the experimental qHg,e values can beseen in the Electronic supplementary material.

Usually, in sorption studies, preference is given to the powderfraction since small particles have large surface area, offering alarge amount of available sorption sites, increasing the sorptionefficiency and favouring also the surface phenomena. Under theexperimental conditions taken, both fractions perform well,suggesting that the powder fraction could be used successfullyin batch studies, while the granulated fraction could be used infixed-bed columns, where the use of the powder fraction wouldcause an undesirable pressure drop. In environmental manage-ment practice, the powder could be used to respond quickly toaccidental spills, usually with higher concentrations, and thegranules could be used for the operational baseline treatment incolumn systems.

The concentration of some cations (Ca2+, K+, Mg2+ and Na+)in solution were monitored during the sorption kinetics, for thepowder fraction and for an initial Hg concentration of 500 μg/L.The results show that during the first hour occurred the release ofCa2+ and K+ ions from cork powder to solution, and after, theconcentration of both ions in solution remained constant. Theamount of ions released due to the sorption process was calcu-lated by subtracting the amount of ions released from the blankexperiment (in this case, those values were lower than the LODof the method) to the amount of ions measured in the sorptionexperiments. The release of Ca2+ andK+ ions from cork suggeststhe existence of an ionic exchange mechanism for Hg in the firsthour of contact. This fact is corroborated by the existence of asignificant correlation (r=1.000, P <0.05) for the first hour,between the total amount of Ca2+ and K+ released to solution(in microequivalent) and the amount of Hg2+ sorbed (also inmicroequivalent). Figure 3 shows the time profile of the Hg2+

sorbed and total Ca2+ and K+ desorption during the first 8 h. Thecoincident profile during the first hour, followed by a divergentpattern characterized by a continuous increase on the amount ofHg sorbed (microequivalent per gramme) and a plateau value onthe amount of the Ca2+ and K+ desorption (microequivalent pergramme), indicates the existence of more than one mechanisminvolve onHg sorption by cork biomass. Although at this stage itis not possible to anticipate all the exact mechanisms involved onHg sorption onto cork, it is clear that an ion-exchange processoccurs in the early stage of sorption.

Influence of cork amount and initial Hg2+ concentration

The amount of sorbent and the initial metal concentration areimportant parameters to obtain quantitative metal removal, since

they may influence the contact time necessary to reach equilib-rium, the amount of Hg sorbed and the possible occurrence ofside mechanisms (e.g. precipitation). Two initial Hg2+ concen-trations and different masses of stopper-derived cork powder(<0.2 mm) were used to evaluate the influence of these param-eters on sorption efficiency of cork towards Hg. For all massesand both initial concentrations, the same trend was observed: adecrease of Hg concentration in the liquid phase and an inferredincrease on the amount of Hg sorbed with contact time, evenwhen only a few milligrammes per litre of cork were used.Independently of themass of cork or the initial Hg concentration,the sorption process exhibited two distinct sorption stages, a faststage followed by a slow one. The slow period of sorption wasachieved after 8 h of contact when the initial concentration was50 μg/L (Fig. 4a) and after 24 h when the initial concentrationwas 500 μg/L (Fig. 4b).

For the same initial Hg2+ concentration, the amount of metalsorbed per gram of cork biomass at equilibrium, increasedconsiderably with the decrease of the amount of sorbent. For,respectively, 250 and 17.5 mg/L of cork, the equilibrium valuesincreased from 0.12 to 1.44 mg/g for an initial Hg2+ concentra-tion of 50 μg/L and from 1.88 to 26.2 mg/g for an initialconcentration tenfold higher. This observation reveals that inthe range of experimental conditions used, the sorption capacityof the cork biomass was not achieved since a plateau value ithas not been reached. Furthermore, the removal percentage isnot correlated (*P <0.05) with the amount of sorbent, unlikeresults observed for other materials (Ghodbane and Hamdaoui2008; Lopes et al. 2009; Ofomaja and Ho 2007; Riaz et al.2009). In those cases, a large increase on the removal percent-age with the increase of sorbent amount is reported, which isnaturally attributed to an increase in binding sites and surfacearea (Ghodbane and Hamdaoui 2008; Iftikhar et al. 2009; Riazet al. 2009). In the case of cork, for the same Hg2+ concentra-tion, the removal percentage increased only slightly with in-creased mass of sorbent (from 50.5 to 61.9 % forCHg,0=50 μg/L and from 91.4 to 94.2 % for CHg,0=500 μg/L), and whateverthe mass of cork used, the residual concentration of Hg2+ insolution hardly changed (from 24.7 to 19.0 μg/L for CHg,0=50 μg/L and from 32.04 to 28.9 μg/L for CHg,0=500 μg/L).Some authors (Ghodbane and Hamdaoui 2008; Iftikhar et al.2009; Kahraman et al. 2005) suggest that increasing the amountof sorbent could lead to the overlapping or aggregation ofsorption sites resulting in a decrease in total sorbent surfaceavailable to metal ions and an increase in diffusion path length.However, in this study, it was noticed that for the same amountof cork and an increase of tenfold on the initial Hg2+ concen-tration, no major differences were recorded on the residual Hgconcentration in solution. Also, both the amount of Hg sorbedand the removal percentage increased considerably with theincrease of the initial Hg concentration at least for CHg,0∈[50;500]μg/L (the removal efficiency increased from 51–62 to 91–94%, while the amount of Hg sorbed increased ca. 12–13 fold),

Environ Sci Pollut Res (2014) 21:2108–2121 2115

and so the hypothesis of overlapping or aggregation of thesorption sites is discarded.

Overall, the results emphasise that, for the initial concentra-tions tested, whatever the amount of cork used, the residual Hgconcentration will not reach the extremely low values (0.07 μg/L) established by the Water Framework Directive as environ-mental quality standards for surface waters. However, for aninitial concentration of 500μg/L, it is possible to achieve residualconcentrations lower than the limit for wastewater discharges(50 μg/L). In fact, it was observed that the mean of the residualHg concentrations obtained in the essays carried out to evaluatethe effect of mass of cork and initial Hg concentration was 24.7±4.9 μg/L. This value suggests that under the experimental con-ditions tested, the cork biomass is able to reduce Hg levels to thesame equilibrium concentration CHg,e, independently of Hg andcork concentrations.

By changing the mass of sorbent or the initial metal con-centration, the kinetic of the sorption process was alsochanged. It was noticed that the initial sorption rates (ν1, ν2

and α ) given by the kinetic models have the same trend of theexperimental qHg,e values, decreasing with increasing theamount of sorbent and increasing with increasing the initialmetal concentration. Under the experimental conditions used,both PSO and Elovich models were able to fit appropriatelythe kinetic data, and although there is no significant differ-ences between the fittings of the two models for the lowestinitial Hg concentration (test F for a confidence level of95 %), the PSO model presents systematically higher R2 andlower Sy/x values than the Elovich model, when the amount ofsorbent is 25 mg/L. Moreover, the qe values estimated by thePSO model matched accurately the experimental points (rel-ative errors between 0.3 and 11 %). The fittings of the allkinetic models adopted, together with the experimental qHg,e

values, can be seen in the Electronic supplementary material.

Influence of cork type

The efficiency of the cork stoppers to remove Hg from spikedultra-pure water was compared with unused natural cork, fortwo experimental conditions: 25 and 250 mg/L of cork and500 μg/L Hg2+. The time profile of Hg concentration followsthe same trend for both cork types: a quick decrease during thefirst eight hours of contact with the cork biomass, followed bya minor and slow decrease until the achievement of equilibri-um. The comparison between the two types of cork revealsthat they display a coincident profile during the entire range ofcontact time, independently of the amount of cork used (datanot shown since no significant difference was found).Independent of the amount of cork powder used, the two typesof cork display similar values in terms of amount of Hg sorbedper gram of cork and removal efficiency, as confirmed by the

mean values between the two types of cork (X ) and therelative standard deviation (rsd): qe =18.3 mg/g, rsd=2.7 %;

Re =91.8%, rsd=2.8% for 25mg/L of cork and qe =1.88mg/

g, rsd=0.2 %; Re =94.4 %, rsd=0.3 % for 250 mg/L of cork.

Fig. 3 Time profile of Hg sorbed onto cork and Ca and K desorption forCHg,0=500 μg/L and mCS=17.5 mg/L

a bFig. 4 Time profile of Hgconcentrations in solution usingdifferent amounts of cork for aCHg,0=50 μg/L and b CHg,0=500 μg/L.

2116 Environ Sci Pollut Res (2014) 21:2108–2121

The similarity between the results obtained with the applica-tion of the two kinds of cork is in keeping with the observedsimilarity of the chemical nature of both corks, i.e. the stop-pers are mainly composed by natural cork and the presence ofsome additives resulting from both the manufacture and finaltreatment of the stoppers, as well as the exposure of thestoppers to the bottled wine, have no observable effect onthe removal process.

Sorption kinetics of Hg under ionic competition

Binary system of Hg/Cd

According to the Water Framework Directive, mercury andcadmium are the only metals classified as priority pollutants,and since their releases to the environment are under strictregulation, it is important to study and find out efficientsorbents for their total removal from water. Although theremoval of these priority pollutants from water has alreadybeen studied for other sorbents (e.g. Otero et al. (2009)),research on their removal from multi-component systems issparse (Kadirvelu et al. 2008; Sulaymon et al. 2010), and onlyrecently, Cardoso et al. (2013) studied the competitive remov-al of both metals from water using a titanosilicate material asion-exchanger.

The time profile curves of Hg and Cd removal (Fig. 5),from a solution containing both metals in concentrations thatmimic an accidental disposal with a ten times exceedance oftheir guideline values for wastewater discharge, show distinctpatterns for each metal. Foremost, the removal was onlyefficient for Hg (91.6 %) while only 6.6 % for Cd with theequilibrium amount of Hg sorbed by the cork of 15.8 mg/gand of Cd sorbed only 5.2 mg/g. This result points out that thecork powder is much more selective for Hg than for Cd. Bycalculating the equilibrium, selectivity of the system (S ) ispossible to quantify this qualitative conclusion. The selectivityis given by the ratio between the distribution coefficients (K)of both cations at equilibrium, i.e. SHg/Cd=KHg/KCd, wherethe distribution coefficient is defined by the ratio between theamount of metal sorbed at equilibrium (q e, millimoles pergramme) and the metal concentration in solution also at equi-librium (C e, millimoles per litre). Accordingly, the equilibri-um selectivity value found for SHg/Cd is 156.

The effect of ionic interaction on the sorption process mayalso be represented by the ratio of the amount of metal sorbedin the presence of the other metal, qe

binary; to the amount ofmetal sorbed when it is present alone in the solution, q e

single.At equilibrium, if qe

binary/qesingle<1, the sorption is inhibited

by the presence of other metal; if qembinary/qe

single=1, there isno observable interaction; and if qe

binary/qesingle>1, the sorp-

tion is favoured by the presence of other metal. For this multi-component system, the value of qe

binary/qesingle for Hg was

found to be 0.85, indicating that the sorption capacity of cork

towards Hg is only slightly inhibited by the presence of Cd.Additionally, the modelling results (Table 2) also show thatthe Hg initial removal rate is slower in the binary system thanin the single one. Furthermore, the modelling of the kineticresults under ionic competition shows that there is a goodagreement between the experimental data and the fittingsaccomplished by the PSO model (Electronic supplementarymaterial), which accurately estimates the experimental qHg,e

values (relative errors lower than 7 %). However, no signifi-cant differences were observed between the best two fittings(test F for a confidence level of 95 %).

Hg/seawater system

Most of the sorption studies available in literature are onlycarried out in ultra-pure water and in some scarce cases toindustrial effluents; nevertheless, the salt waters are also oneof the last receptors of pollutants (through submarine outfalls)and the source of water used in many aquacultures. Theefficiency of cork powder to reduce Hg levels in seawaterwas evaluated for an initial concentration of 50 μg/L andusing a cork mass of 250 mg/L. Figure 6 shows the normal-ized concentration of Hg in spiked ultra-pure water and inspiked seawater. The complexity of the matrix on the sorptionof Hg onto cork powder is observable, both on the kinetic andequilibrium behaviour. By increasing the complexity of thematrix from ultra-pure water to seawater, the following trendswere observed: (1) the initial removal rate decreased consid-erably, as seen from the different slopes of the removal curvesfor the first hours (Fig. 6) and confirmed by the kineticconstants (k1and k2) and the initial sorption rates (ν1, ν2 andα ) given by the kinetic models; (2) the equilibrium wasattained latter; and (3) both the removal percentage and theof amount of Hg removed per weight of cork at equilibriumdecreased slightly, respectively, from 62 to 48 % and from0.12 to 0.08 mg/g.

The effect of seawater matrix on the sorption process wasalso assessed by calculating the ratio of the amount of metalsorbed in the Hg/seawater system, qe

sw to the amount of metalsorbed in the Hg/ultra-pure water system, qe

upw. The valuefound was 0.66 and confirms a moderate inhibition of thesorption process, under the experimental conditions tested.The degree of inhibition observed in this study can be relatedwith the relatively low concentration of Hg in comparisonwith the vast concentration of other ions, but mainly to theformation of negative Hg chloro-complexes. In the spikedseawater, more than 95 % of the Hg is complexed with Cl−

ions as HgCl42− (74.2 %) and HgCl3

− (21.5 %). The specia-tion data were obtained using the Visual Minteq program. Arecent study evaluated the effect of NaCl electrolyte on thesorption of Hg onto unmodified rice husk and concluded thatthe presence of Cl− ions has a strong effect on metal sorptiondue to the formation of HgCl4

2− and HgCl3− complexes

Environ Sci Pollut Res (2014) 21:2108–2121 2117

(Rocha et al. 2013), which have lower affinity by the sorbentsurface than the Hg(OH)2 species. In the case of cork sorbent,it was identify that the early stage of sorption is characterizedby the existence of the ion-exchange mechanism, which canbe strongly inhibited due to the Hg speciation in seawater(absence of Hg2+, specie easily ion-exchangeable and domi-nance of negatively charged species HgCl4

2− and HgCl3−, not

exchangeable with Ca2+ and K+ ions).Even so, it must be highlighted that under extremely com-

petitive conditions like seawater, the stopper-derived corkpowder was able to almost halve the mercury levels.

Furthermore, the modelling of the sorption kinetic of Hg inseawater by the kinetic models adopted in this study showsthat all models describe very well the quantity of Hg(II)removed in the early stage of sorption, although there areslightly deviations between the experimental and the fitteddata, particularly in the inflexion zone. The highest determi-nation coefficient and the lowest standard deviation of

residuals were obtained for the PSO model, which was ableto accurately estimate the q e value obtained experimentallyfor the Hg/seawater system (relative error of 7 %). However,for a confidence level of 95 %, there is no significant differ-ence between the goodness of the fit of the best two models (Ftest), for the tested matrix.

Sorption isotherm

The isotherms describe the equilibrium distribution of thesorbate between the liquid and solid phase and are an essentialtool for designing any sorption system. Moreover, the com-parison of isotherm parameters for distinct sorbents allows thequantitative comparison of their sorption capacity andstrength. Figure 7 shows the sorption isotherm for the systemHg/cork stoppers powder for 0<CHg,e<30 μg/L. The iso-therm exhibits a very unfavourable trend, with an almostsquare convex shape to the concentration axis (C e). In termsof the theoretical classification of BDDT (Brunauer, Deming,Deming, Teller), its shape falls within type III, which is alsofound, e.g. in the adsorption of bromine on silica gel at 79 °Cand water on glass (Do 1998). The most remarkable feature ofthe Hg/cork stoppers powder system is that, at high concen-trations, the sorption is extremely enhanced as if the interac-tions between solute and sorbent were smaller than those inthe liquid (Do 1998). The trend evidenced in Fig. 7 is verypromising for real applications, since it establishes that pow-der cork derivate from stoppers can be used to treat veryconcentrated solutions, being always able to drop the mercurylevel down to ca. 25 μg/L.

Comparison of cork efficiency towards other metalsand with other biosorbents

The comparison of the sorption performance of differentbiosorbents or even of the same biosorbent towards differentmetals is not a trivial task. Kratochvil and Volesky (1998)

Fig. 5 Time profile of normalized concentrations (C /C0) of Hg in asingle-component system and of Hg and Cd in a multi-component systemfor CHg,0=500 μg/L;CCd,0=2000 μg/L and mCS=25 mg/L

Fig. 6 Time profile of normalized concentration (C /C0) of Hg in spikedultra-pure water and in spiked natural seawater for CHg,0=50 μg/L andmCS=250 mg/L

Fig. 7 Equilibrium data for the system Hg/cork stoppers powder

2118 Environ Sci Pollut Res (2014) 21:2108–2121

suggested that sorption of metal by the two biosorbents mustbe compared only at the same equilibrium concentration, sincesorbents can have distinct behaviour at low and high equilib-rium concentrations. Moreover, the same authors suggest thatthe comparison of single-sorbate sorption performance is bestbased on a single-sorbate isotherm measured under the sameoperating conditions, such as pH, temperature and ionicstrength. However, most of the times, this condition is verydifficult to accomplish and the comparison of the sorptioncapacity given by an equilibrium model as Langmuir is oftentaken as indicative.

The experimental conditions used here are very differentfrom the ones of other studies, showing how difficult this taskis. Although results are not entirely comparable, Table 3 pre-sents the qHg,e range and the corresponding experimentalconditions reported in the different studies, and not the sorp-tion capacity.

The effectiveness of cork stoppers towards Hg, in terms ofamount of metal sorbed per unit of weight, is quantitativelysimilar to its efficiency towards other metals (Table 4). Incomparison with other available biosorbents, cork stoppersare clearly as good in their performance to uptake Hg ions(Table 3). However, it must be emphasised that the majority ofthe previous sorption studies have been performed with initialmetal concentrations orders of magnitude higher and thereforedo not represent the actual environmental conditions of lowerconcentrations that occur in effluent and wastewater dischargesor that can result accidentally, while the effectiveness of corkstopper-derived biosorbent to treat waters with low level of Hgcontamination is representative of real world conditions.

Conclusions

For the first time, stopper-derived cork was used as biosorbentto decrease mercury levels in different types of solutions atenvironmentally relevant concentrations and conditions.

It is now clear that stopper-derived cork can be effectivelyused as biosorbent towards one of the most toxic metals, Hg,and that its capacities to sorb this metal are not affected by itsprevious utilization as stoppers by the wine industry. Theefficiency of the process depends on the operating conditions,but a few milligrammes per litre of stopper-derived cork areable to achieve sorption efficiencies that can reach 95 %. Itwas found that the higher is the mercury concentration, thegreater is the efficiency. The amount of Hg sorbed at equilib-rium (in milligrammes per gramme) does not depend onsorbent particle size but increases with increasing initial metalconcentration and decreases with increasing the amount ofsorbent. The sorption kinetic is well described by the pseudo-second-order model, by the Elovich model or by both, and theinitial sorption rate is very sensitive to factors, such as particlesize, amount of sorbent, initial Hg concentration and presenceof other ions. Moreover, diffusion seems to be preponderanton controlling the transfer of Hg in the solid–liquid process.

In absence of ionic competition, more than one mechanismsare involved in the sorption process of Hg onto cork, with theion-exchanging prevailing in the beginning of the process. Underionic competition, the stopper-derived cork was equally effective(92 %) and very selective (SHg/Cd=156) for Hg in binary mix-tures (Hg and Cd). In seawater, the process was likely affected bythe presence of Cl− ions, which led to a change on Hg speciation

Table 3 Experimental conditions and the amount of metal sorbed per weight of sorbent, of cork towards other metals and of other biosorbents towardsmercury

System pH CM,0 (mmol/L) mcork (g/L) C0/m ratio qM,e (mmol/g) Reference

Hg2+/cork 7 (0.025–0.25)×10−2 0.025 0.01–0.1 (0.60–92.7)×10−3 This study

Cu2+/cork 5 0.16–9.44 2 0.08–4.72 (47.2–329)×10−3 Chubar et al. (2003)

6–7 0.079–1.57 1 0.079–1.57 (10.7–41.5)×10−3 Villaescusa et al. (2000)

Zn2/cork + 5 0.15–9.18 2 0.075–4.59 (52.0–384)×10−3 Chubar et al. (2003)

Ni2/cork + 5 0.17–6.82 2 0.085–3.41 (49.4–179)×10-3 (Chubar et al. 2003)

6–7 0.085–1.70 1 0.085–1.70 (8.00–69.8)×10−3 Villaescusa et al. (2000)

Cr3+/cork 4 0.19 2 0.038 65.4×10−3 Machado et al. (2002)

Cr6/cork + 2.0–4.7 0.19 6.6 0.028 327×10−3 Fiol et al. (2003)

Pb2+/cork 5 0.097 10 0.0097 (13.5–19.8)×10−3 Lopez-Mesas et al. (2011)

Cd2+/cork 5 0.18 10 0.09 (8.00–14.2)×10−3 Lopez-Mesas et al. (2011)

Hg/cork 7 (0.025–0.25)×10−2 0.025 0.01–0.1 (0.60–92.7)×10−3 This study

Hg/Bacillus subtilis 5 0.025–0.25 5 0.005–0.05 (24.4–359)×10-3 Wang et al. (2010)

Hg/Eucalyptus bark 5 0.50 1–16 0.03–0.5 (20.0–200)×10−3 Ghodbane and Hamdaoui (2008)

Hg/Unmodified rice husk 6 (0.025–0.25)×10-2 0.025 0.01–0.1 (0.79–8.97)×10−3 Rocha et al. (2013)

Hg/coconut coir 6 0.050–0.37 2 0.025–0.185 (24.7–164)×10−3 Anirudhan et al. (2008)

Hg/moss 5.5 0.050–1.99 4 0.0125–0.5 (10.0–364)×10−3 Sari and Tuzen (2009)

Environ Sci Pollut Res (2014) 21:2108–2121 2119

(formation of HgCl3− andHgCl4

2− complexes) and consequentlyto the inhibition the ion-exchange mechanism.

At last, the sorption process evidenced a rare but very inter-esting behaviour well depicted by the particular isotherm mea-sured. For equilibrium solution concentrations lower than 20 μg/L, the system is approximately irreversibly (flat branch) but thenbecomes essentially vertical, which means that stopper-derivedcork can be used to accomplished excellent removal of concen-trated solutions down to ca. 20–25 μg/L. To achieve a total Hg-free solution, other sorbents, like core shell magnetic particles(Lopes et al. 2013), may be applied to polish the water treatment.

This work is the beginning of understanding the corkpotentialities for metal water treatment and highlights a newvalorisation of cork stoppers.

Acknowledgments The authors thank Fundação para a Ciência e aTecnologia (FCT) (PTDC/MAR-BIO/3533/2012; PEst-C/MAR/LA0017/2011), FSE and POPH for funding. The authors C.B. Lopesand L.S. Rocha also thank their Post-DOC grants (SFRH/BD/45156/2008; SFRH/BD/47166/2008).

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