TiO2-coated natural zeolite: Rapid humic acid adsorption and effectivephotocatalytic regeneration

Sanly Liu, May Lim, Rose Amal n

ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering, The University of New South Wales, Sydney NSW 2052, Australia

H I G H L I G H T S

� Coating TiO2 onto zeolite markedly improve its efficiency in adsorbing HA.� Most of the bulk organic (up to 80%) can be removed within 5 min of adsorption.� Immobilisation of TiO2 onto zeolite permits easier separation of the adsorbent.� Zeolite/TiO2 particles were readily regenerated by UVA irradiation.� After regeneration, the adsorbent can be reused repeatedly.

a r t i c l e i n f o

Article history:Received 6 September 2013Received in revised form24 October 2013Accepted 26 October 2013Available online 1 November 2013

Keywords:AdsorptionNatural zeoliteHumic substancesOxidationPhotocatalyticTiO2

a b s t r a c t

Natural zeolite coated with titanium dioxide (TiO2) was used as an adsorbent to rapidly remove humicacid (HA) from an aqueous solution. Coating TiO2 onto zeolite markedly improved its efficiency inadsorbing HA: most of the bulk organic matter (80%) could be removed within 5 min of adsorption by thezeolite/TiO2 particles at neutral pH, whereas less than 20% was removed by bare zeolite alone. Inaddition, immobilisation of TiO2 onto zeolite permitted easier separation of the adsorbent from thetreated water. Photocatalytic properties of TiO2 were exploited for the regeneration of the adsorbent. Theadsorbent was shown to be readily regenerated by photocatalytic oxidation and was still effective inremoving HA after 5 adsorption/regeneration cycles. These results indicate that TiO2-coated zeolite canbe a very attractive adsorbent for the rapid removal of HA from aqueous solutions.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Humic acid (HA) is ubiquitous in surface and ground water andconsists of a complex mixture of naturally occurring macromole-cular organic matter. The presence of HA in drinking water maylead to taste, colour, and odour problems. In addition, HA canpromote microbiological regrowth in the distribution system (Volkand LeChevallier, 2002). Although HA itself is not harmful tohuman health, its reactions with halogen-based disinfectingagents produces a range of disinfection by-products (DBPs) whichhave been associated with an increased risk of cancer (e.g. bladder,colon and rectal), reproductive and developmental problems(Richardson, 1998). With disinfection being an inevitable proce-dure in water treatment, effective removal of humic substances

often remains the most viable option to eradicate the potentialhealth risk of DBPs formation.

Adsorption is a removal method that does not produce anyharmful by-products in the treated water. It has also been found tobe superior to other drinking water treatment processes in termsof initial cost, flexibility and simplicity of design and ease ofoperation (Fernandes et al., 2010). Many adsorbents have beeninvestigated for use in the adsorption of HA including activatedcarbon (Daifullah et al., 2004; Ando et al., 2010), zeolites (Capassoet al., 2005), resin (Wang et al., 2010), iron oxides (Gu et al., 1995),and clay minerals (Feng et al., 2005). However, many of theseadsorbents need to be either disposed of or regenerated via a hightemperature/acid addition process before reuse, creating a sub-stantial amount of waste containing HA and spent adsorbent inhigh concentrations. This leads to high operation costs due to thedisposal or regeneration of spent adsorbent requirements.

Titanium dioxide semiconductor (TiO2) has attracted a greatinterest in the past decades owing to its photocatalytic properties.The removal of organics using TiO2 can be achieved via two

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Chemical Engineering Science

0009-2509/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ces.2013.10.041

n Corresponding author. Tel.: þ61 2 9385 4361; fax: þ61 2 9385 5966.E-mail address: r.amal@unsw.edu.au (R. Amal).

Chemical Engineering Science 105 (2014) 46–52

mechanisms: adsorption onto TiO2 surface and photocatalyticoxidation. It should be noted that adsorption usually occurs muchfaster than photocatalytic oxidation. When comparing the rates ofHA removal conducted at neutral pH, it has been reported thatequilibrium is achievable within 30 min in the case of adsorption(Wiszniowski et al., 2002), whilst photocatalytic degradationusually requires 60 min if not longer (Liu et al., 2008).

However, practical applications of TiO2 as an adsorbent or aphotocatalyst in aqueous solutions are limited because of therecovery problems of fine TiO2 particles. Recently, attempts havebeen made to immobilise the TiO2 particles on different supports,such as silica (Lepore et al., 1996), activated carbon (Shi et al.,2010), clay (Paul et al., 2012; Chong et al., 2009; Kibanova et al.,2009), pumice stone (Rao et al., 2003), magnetite core (Beydounet al., 2000) and zeolites (Wang et al., 2010), to improve theirseparation from bulk water. Zeolites, in particular, have been shownto be promising due to their unique structures, uniform pores andchannels, high surface area and an excellent adsorption capacity forpollutants (O’Neill et al., 2001). Zeolites are three dimensionalaluminosilicate minerals with a porous structure that have valuablephysicochemical properties, such as cation exchange, molecularsieving, catalysis and adsorption (Wang and Peng, 2010). They carrypermanent negative charges on their structural framework and,therefore, usually possess high cation exchange capacity but havelittle affinity for anions or organic molecules in aqueous solution.Natural zeolites are cheap, abundant and easily available. Previousstudies have shown that modification of zeolites with cationicsurfactants improves their affinity for organic compounds inaqueous solution (Li et al., 2011; Zhan et al., 2010; Wang et al.,2006). However, the regeneration of the surfactant-modified zeo-lites produced substantial waste product. The frame of zeolite isknown to enhance the photocatalytic activity of the entrapped TiO2

by prolonging the separation of the photogenerated electrons andholes (Sasikala et al., 2010; Dubey et al., 2008). A TiO2-coated zeolitehas been studied for humic acid degradation and mineralisation(Lazau et al., 2011), however, the required timeframe for thereaction was 2 h.

In this study, a zeolite/TiO2 composite was synthesised using asol–gel process and used for adsorption of HA instead of in situphotocatalytic oxidation of HA. The adsorption process was selectedas the treatment process instead of the photocatalytic oxidation inorder to rapidly remove HA from the water. Since adsorption is aremoval-based method, it will not release any intermediates thatcould be more reactive to chlorine-based disinfectants than theparent HA compound into the treated water, as in the case of TiO2

photocatalytic oxidation if the treatment time is not sufficient(Liu et al., 2008). The physical properties, such as morphology,crystal structure, and porosity, of the prepared adsorbent werestudied to characterise the composite material. A zeolite/TiO2

composite adsorbent that is easily recoverable and regeneratedvia TiO2 photocatalytic oxidation, thus avoiding the production ofwaste or requirement for high cost regeneration process, will lead toa cost-effective and environmentally friendly water treatmentprocess.

2. Materials and methods

2.1. Photocatalyst preparation

Natural zeolite powders with a product name of ZELflocc wereobtained from Zeolite Australia Pty Ltd. 10 g of the zeolite powderswere dispersed in a solution containing 957.5 mL of ethanol and7.5 mL of water. The suspension was then stirred at 500 rpm.Titanium dioxide nanoparticles on the zeolite were prepared byhydrolysis of titanium tetraisopropoxide (TTIP). A diluted TTIP

solution was prepared by adding 1.75 mL of TTIP to 33.25 mL ofethanol and subsequently added into the zeolite suspensiondropwise at a rate of 30 mL/h. After aging for 3 h, the productswere repeatedly washed with Milli-Q water to remove any freeTiO2 particles that were not attached to the zeolite support. Boththe zeolite/TiO2 sample and free TiO2 particles that were collectedwhen washing zeolite/TiO2 sample were then dried at 80 1Covernight and calcined at 450 1C for 3 h in a muffle furnace.

2.2. Analytical characterisation of the photocatalyst particles

The crystal structure of the sample was characterised bypowder X-ray diffraction (XRD) on a Philips X'Pert Pro MPD systemusing Cu Kα radiation with a scanning angle (2θ) of 20–80 1C andvoltage and current of 45 kV and 40 mA, respectively. The instru-ment was operated in the step scan mode at a scanning speed of0.13131/s and a step size of 0.026261. The composition of thesample was obtained from X-ray fluorescence (XRF) analysis on aPANalytical PW2400 Sequential WDXRF spectrometer. The mor-phology and microstructure of the photocatalyst were examinedby a Scanning Electron Microscope (Hitachi S900) operated in highvacuum conditions with spot size of 4.0 and accelerating voltage of4 kV. The Brunauer–Emmett–Teller (BET) surface area, adsorptionisotherms and pore size distribution of the catalysts were deter-mined using a Micromeritics Tristar 3000 nitrogen adsorptionapparatus at 77 K. Prior to the analysis, the powder was degassedunder vacuum in a Micromeritics VacPrep unit at 150 1C for atleast 2 h to remove adsorbed water or volatile organics. The diffusereflectance spectra of dry powders were measured using a Cary300 UV–vis spectrophotometer (Varian) equipped with an inte-grating sphere. BaSO4 was used as a reference sample.

2.3. Humic acid adsorption test

HA stock solution was prepared by mixing 5 g of Sigma Aldrichhumic acid (sodium salt) in 1 L of 0.1 M sodium hydroxide over aperiod of 1 day. The stock solution was filtered through a filterpaper with 11 mm pore size (No. 1, Whatman, UK) to remove allsuspended solids and stored at 4 1C. The dissolved organic carbon(DOC) concentration of the stock solution of HA is 1.6 g/L. Aqueoushumic acid solutions were prepared by diluting humic acid stocksolution in Milli-Q water containing 60 mg/L MgSO4, 60 mg/LCaSO4, 4 mg/L KCl, and 96 mg/L NaHCO3 to simulate the bufferingcapacity and ionic strength of natural water. pH of the aqueoushumic acid solutions was then adjusted to 7 with HCl or NaOH.Zeta potential of the bare zeolite and zeolite/TiO2 in aqueoussolutions containing the above salts (without HA) was measuredusing ZetaPALS (Brookhaven). Measurements of the adsorptionkinetics of humic acid on the adsorbent were carried out as batchexperiments, where 50 mL tubes, each containing 0.6 g of theadsorbent and 30 mL of aqueous humic acid solution, were shakenin a rotary suspension mixer (Ratek, RSM7) at ambient temperature.The initial dissolved organic carbon concentration of the humic acidsolution was 10 mg/L. One tube was taken out and the slurry wasfiltered through a 0.45 mm cellulose acetate membrane (Sartorius) atthe end of each time point. The effect of adsorbent dosage wasstudied in 50 mL tubes, each containing a different amount ofadsorbent and 30 mL of the aqueous humic acid solution (10 mg/LDOC), for a period of 24 h. To study the adsorption isotherms, 30 mLsolutions with varying initial HA concentrations were treated with0.6 g of the adsorbent for 24 h. The change in the organicsconcentration was measured by using Cary 300 UV–vis spectro-photometer at 254 nm. The percentage of humic acid adsorbed on

S. Liu et al. / Chemical Engineering Science 105 (2014) 46–52 47

the catalyst surface was calculated using the following equation:

% adsorption ¼ C0�Ct

C0� 100

where C0 is the initial concentration of humic acid and Ct is theconcentration of humic acid at time t.

2.4. Regeneration and reuse of zeolite/TiO2

To investigate the ability of regeneration and reuse of thezeolite/TiO2 particles, 30 mL of HA solution containing 10 mg/LDOC was mixed with 0.6 g of the adsorbent in a rotary suspensionmixer (Ratek, RSM7). After 2 h, the adsorbent was separated fromthe solution by centrifugation and the DOC concentration of thesupernatant was determined with UV absorbance at 254 nm.Following removal of the treated water for analysis, the adsorbentwas redispersed in 30 mL of Milli-Q water and regenerated underUVA light (three 20 W NEC blacklight blue fluorescent lamps withmaximum emission at 365 nm) overnight. After regeneration, theparticles were reused for adsorption with 30 mL of HA solutioncontaining 10 mg/L DOC. The regeneration and reuse steps wererepeated over 5 cycles.

3. Results and discussion

3.1. Characterisation of the zeolite/TiO2

The composition of the composite zeolite/TiO2 was analysedusing X-ray fluorescence. TiO2 made up approximately 1.55% of the

composite zeolite/TiO2 samples. The composite zeolite/TiO2 wasrich in silica.

Fig. 1 shows the XRD spectra of zeolite and composite zeolite/TiO2 sample. The XRD spectrum of the zeolite was matched to thespectra of clinoptilolite, mordenite and quartz. The XRD spectrumof the zeolite/TiO2 displays a similar diffraction pattern to that ofthe bare zeolite; however, the intensity for clinoptilolite peaks islower as compared to the bare zeolite particles. This suggests thatthe TiO2 precursor interacts with the support decreasing thecrystallinity of the clinoptilolite. In addition, no diffraction peakscorresponding to typical TiO2 phase were found. This suggests thatthe TiO2 loading on the zeolite is too low to be detected by XRDanalysis.

The crystalline states of the free TiO2 particles (i.e. the particlesthat were not coated onto the zeolite and were collected whenwashing the zeolite/TiO2 sample) were also characterised by XRDanalysis. From Fig. 1(c), the diffraction peaks at 2θ¼25.31, 37.81,47.91, 54.91, 62.51, 68.91, 70.21, and 75.11 were matched to theanatase TiO2. This shows that the heat treatment to 450 ºC hassuccessfully transformed the amorphous TiO2 into crystalline TiO2.The small peak at 2θ¼30.81 could be matched to the brookitephase of TiO2. No other impurity peaks were observed. Weinferred from the XRD spectra of the free TiO2 particles that theTiO2 coated on zeolite consists primarily of the anatase phase witha minor admixture of brookite. The average crystallite size of TiO2

was calculated to be approximately 8 nm using the Scherrerequation, i.e.

D¼ kλβ cos θ

on the anatase (101) diffraction peak at 2θ¼25.31.Fig. 2 shows the SEM images of the zeolite surface before and

after coating with TiO2. The zeolite substrate has a broad particlesize distribution, ranging from 10 mm to 60 mm. It is evident fromFig. 2(b) (top image) that TiO2 coating resulted in a rougher surfacemorphology of the zeolite substrate. It could also be seen that theTiO2 coating is relatively uniform and there is no apparent sites ofuncoated zeolite.

The BET surface area of the bare zeolite particles was 11074.9 m2/g, and after TiO2 immobilisation, the surface area decreasedto 1670.05 m2/g. This suggests that the deposition of TiO2 ontothe zeolite surface causes blockage of zeolite pores. As the poresize of the zeolite (less than 10 Å) was smaller than the size of theTiO2 particles supported by the zeolite, TiO2 particles are coated onthe zeolite surface and not inside of the internal pores of thezeolite. In addition, the pore diameter for the zeolite is consideredto be too small for the titanium tetraisopropoxide (TTIP) mole-cules, and therefore during the preparation of zeolite/TiO2, thesorption of TTIP molecules on natural zeolite is limited to the siteson the external surface.

Diffuse reflectance UV–vis analysis was performed to quantifythe light absorption and determine the wavelength range forabsorption, which can be correlated with the band gap energy.The UV–vis diffuse reflectance spectra for the zeolite, zeolite/TiO2

and Aeroxide P25 TiO2 particles (Evonik) are presented in Fig. 3. Itcan be seen that the zeolite/TiO2 sample showed absorbance in thevisible range which suggests its potential to be activated by visiblelight. In addition, after the TiO2 immobilisation on the zeolite,higher absorbance at wavelengths less than 450 nm is observed.

3.2. Adsorption of humic acid on zeolite/TiO2

Adsorption of humic acid was measured at given contact timesusing 20 g/L of zeolite/TiO2. Fig. 4 shows that the rate of HAremoval is higher at the beginning, with most of the bulk organicsremoved after 5 min. In the initial stages of adsorption when the

5 15 25 35 45 55 65 75 85

Inte

nsity

(a.u

.)

2 theta (degrees)

anatase

brookite

clinoptilolite

mordenite

quartz

Fig. 1. XRD spectra of (a) zeolite sample, (b) zeolite/TiO2 sample and (c) free TiO2

particles that were not coated onto the zeolite during the synthesis process.

S. Liu et al. / Chemical Engineering Science 105 (2014) 46–5248

surface is still free from humic acid, the adsorption kinetics ismainly governed by the diffusion of humic acid molecules fromthe bulk solution to the surface. In the later stages of adsorption, alarge number of humic molecules are adsorbed on the zeolite/TiO2.Further adsorption can only take place if the arriving humic acidcan find available adsorption sites on the surface and can over-come the electrostatic repulsion between the adsorbed molecules

and those to be adsorbed, and, therefore, in the later stages theadsorption slows down.

The influence of adsorbent dosage on HA removal was studiedand the results are presented in Fig. 5. Zeolite/TiO2 particles aremuch more effective in removing HA than the bare zeolite. HAremoval by zeolite/TiO2 exceeds 80% compared to less than 20%obtained with the bare zeolite particles. The result from this study

Fig. 2. SEM images of (a) bare zeolite particles, and (b) as-prepared zeolite/TiO2 particles at two different magnifications (100k magnification for the top figure and 1kmagnification for the bottom figure).

0

0.5

1

1.5

2

2.5

F(R

)

0 350 400 450 500 550 600Wavelength (nm)

zeolitezeolite/TiO2P25

30

Fig. 3. UV–vis diffuse reflectance spectra of bare zeolite, zeolite/TiO2 and a commercial P25 TiO2 sample.

S. Liu et al. / Chemical Engineering Science 105 (2014) 46–52 49

is in agreement with the previous studies which also reported thatnatural zeolite is an inefficient adsorbent for the removal of humicsubstances from aqueous solutions (Wang et al., 2006; Zhan et al.,2011). The extent of HA adsorption on the bare zeolite particles ismuch less than on the zeolite/TiO2 particles although the barezeolite particles have a much larger BET surface area than thelatter. This is because the much smaller pores of the zeoliteparticles are not accessible for the HA molecules and, therefore,the available surface area obtained from the nitrogen adsorptionresults is not a true representation of the surface area available tothe much larger humic molecules. We can further infer that thesurface chemistry of the adsorbent plays a more important role ascompared to the amount of the available surface area for HAadsorption.

As the adsorbent dosage increased in Fig. 5, a greater surfacearea and a greater number of binding sites was available for theconstant amount of HA, which results in the increasing HApercentage removal with the increasing adsorbent dose. Anyfurther addition of the adsorbent beyond 30 g/L did not causeany significant change in the adsorption, i.e. the adsorption peakedat approximately 80%. Certain organic groups of HA are alwaysrecalcitrant to adsorption by zeolite/TiO2. The recalcitrant natureof certain HA fractions has also been reported with coagulationtreatment, activated carbon adsorption, and TiO2 photocatalyticoxidation (Liu et al., 2008; Fabris et al., 2008; Matilainen et al.,2006; Ng et al., 2012).

In order to determine which chemical components of the HAconstitute this refractory fraction, the ratios of absorbance at250 nm to that at 365 nm (E2/E3), and at 465 nm to that at665 nm (E4/E6) were determined from the UV–vis spectra of theHA before and after treatment. The quotients, E2/E3 and E4/E6,

have been found to have a good correlation with the molecularweight and aromaticity of humic substances, i.e. a low E2/E3 andE4/E6 ratio may be indicative of a relatively high degree ofcondensation of aromatic structures and a high molecular weight(Peuravuori and Pihlaja, 1997). The E2/E3 and E4/E6 values of theoriginal HA solution were 2.79 and 4.34, respectively. The E2/E3and E4/E6 values of HA after contact with zeolite/TiO2 at theoptimum loading were increased to 4.48 and 19.52, respectively,indicating that the leftover HA fraction had lower aromaticity andmolecular weight than the original HA, and these compounds arerefractory to adsorption by TiO2/zeolite. We further showed thatthe E2/E3 and E4/E6 values of HA after contact with bare zeoliteare relatively similar to the original HA solution (3.14 and 3.9,respectively), implying that there is no preferential removal of theorganic fractions represented by these quotients by the barezeolite.

The adsorption isotherms of HA on zeolite and zeolite/TiO2 areshown in Fig. 6. It could be seen that the adsorption capacity of HAby zeolite was low but it was enhanced greatly by loading withTiO2. The adsorption of humic acid on a solid surface is a complexprocess that depends on the chemical nature of the zeolite surfaceand the HA as well as the solution properties. The chemicalbehaviour of humic acid is controlled by the two functionalgroups: the carboxyl and phenolic-OH groups (Lu et al., 2000).The carboxyl group starts to dissociate its proton at pH 3 and thehumic molecules become negatively charged at pH43. This hasimplications on the adsorption behaviour. Phase Analysis LightScattering studies show that the zeta potential of bare zeolite atthe pH range studied was highly negative (see Fig. 7). The lowadsorption efficiency of humic acid onto the zeolite is likely due tothe fact that the zeolite possesses highly negative structuralcharges on its frameworks at pH 7 which means that the electro-static attraction with HA is minimal.

After TiO2 immobilisation, the surface charge of the particleswas positive at low pH and the surface charge of the zeolite/TiO2

particles decreases with the increase in pH. Note that the iso-electric point (IEP) of the zeolite/TiO2 particles was found to bearound 6 which is nearly identical to that of titanium dioxide(Jaffrezic-Renault et al., 1986) consistent with the successfuldeposition of the TiO2 particles onto the zeolite core. A decreaseof the negative surface charge favours the adsorption of HA ontothe surface of the TiO2 supported on zeolite. Wiszniowski et al.(2002) studied the adsorption of humic acids onto TiO2 using FTIRDRIFTS technique and found that humic acids are adsorbed on TiO2

mainly by the carboxylate groups. Substantial HA adsorptionoccurs at neutral pH (above the point of zero charge of TiO2)which indicates that besides electrostatic interaction there could

0

0.2

0.4

0.6

0.8

1

1.2

0 500 1000 1500 2000

Am

ount

ads

orbe

d (m

g/g)

Time (min)

Fig. 4. Uptake kinetics of HA on zeolite/TiO2 (Adsorbent loading¼20 g/L, pH 7).

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

Perc

enta

ge re

mov

al (%

)

Adsorbent dose (g/L)

zeolite/TiO2

zeolite

Fig. 5. Effect of zeolite and zeolite/TiO2 dosage on the adsorption of HA. Adsorptionconditions: pH 7, temperature 25 1C, initial dissolved organic carbon content¼10 mg/L.

0

1

2

3

4

5

6

0 20 40 60 80 100 120

Am

ount

ads

orbe

d (m

g/g)

Equilibrium concentration (mg/L)

zeolite/TiO2zeoliteLangmuir fitFreundlich fit

Fig. 6. Adsorption isotherms of HA at pH 7 by zeolite and zeolite/TiO2: points referto experimental data and lines refer to Langmuir (solid line) and Freundlichisotherm equation (dotted line) fitting of the experimental data. Adsorbent dosage:20 g/L; temperature 25 1C; adsorption was carried out for 24 h.

S. Liu et al. / Chemical Engineering Science 105 (2014) 46–5250

be other adsorption mechanisms occurring for the zeolite/TiO2,such as hydrogen bonding interactions with the hydroxyl surfacegroups of TiO2 or hydrophobic interactions (Song et al., 2012; Yanget al., 2009). In addition, TiO2 can interact with the deprotonatedsurface groups of HA via ligand exchange to form surface com-plexes, as reported by Yang et al. (2009) and Sun and Lee (2012).

The adsorption of HA onto zeolite/TiO2 increased rapidly with theincreased initial HA concentrations. This is due to the increase in thedriving force which is the humic acid concentration differencebetween the liquid and the solid phases. The Langmuir and Freun-dlich models were adopted to fit the isotherm data. The constants inthe Langmuir isotherm can be determined by plotting (Ce/qe) versusCe and making use of the Langmuir equation rewritten as:

Ce

qe¼ Ce

qmþ 1qmKL

where qm (mg/g) and KL are the Langmuir constants (L/mg)representing the maximum adsorption capacity for the solid phaseloading and the Langmuir constant related to the binding strength,respectively.

The Freundlich model is an exponential equation that can beexpressed as

qe ¼ KFCe1=n

where qe is the amount of HA adsorbed per unit mass of adsorbent(mg/g), Ce is the concentration of HA in solution at equilibrium(mg/L), KF represents the extent of adsorption and is the amount ofHA adsorbed per unit mass of adsorbent (mg/g), and n is theheterogeneity factor related to the intensity of adsorption whichvaries with the heterogeneity of this material. The values of KF and1/n were determined from the slope and intercept of the ln(qe)versus ln(Ce) plots.

The fitting results for the zeolite/TiO2 sample and the isothermparameters are presented in Table 1. Both the Langmuir and Freun-dlich isotherms showed reasonably good fits to the experimentaldata; however, the Langmuir isotherm presents a better fit. Langmuirisotherm assumes monolayer adsorption onto a surface containing afinite number of adsorption sites, whereas the Freundlich isotherm

assumes heterogeneous surface energies. Since HA has a high negativezeta potential at neutral pH, the adsorption is probably dominated bya monolayer adsorption rather than multilayer adsorption due to thelarge electrostatic repulsion between the adsorbed molecules and themolecules to be adsorbed. The adsorption capacity of the zeolite/TiO2

particles from the Langmuir isotherm is 5.94 mg/g. This value ishigher than the reported 2 mg/g of HA adsorption on clinoptilolite-rich tuffs (Capasso et al., 2007) and comparable to the 6.9 mg/gadsorption capacity of humic acid on a commercially availableactivated carbon (Ferro-Garcia et al., 1998) or the 8 mg/g adsorptioncapacity of humic acid on TiO2 microspheres (Sun and Lee, 2012).

3.3. Regeneration of zeolite/TiO2 after HA adsorption viaphotocatalytic oxidation

The regeneration of adsorbent, i.e. restoration of adsorptioncapability, is a crucial factor in practical application of theadsorbent. As synthesised zeolite/TiO2 particles were tested forsuccessive cycles of HA adsorption and regeneration via photo-catalytic oxidation. The results are presented in Fig. 8. It can beseen that after five successive cycles of adsorption–regeneration–resorption there is no significant decrease of the adsorptioncapability of the zeolite/TiO2. This indicates that the TiO2 coatingon the zeolite support is stable and is not detached during theadsorption/regeneration process. In addition, the photocatalyticoxidation process is able to degrade the HA adsorbed on thephotocatalyst surfaces resulting in the recovery of the adsorptioncapacity. Therefore, it is feasible to regenerate and reuse thezeolite/TiO2 for the adsorption of HA.

4. Conclusions

We demonstrate for the first time that surface immobilisation ofTiO2 onto zeolite can enhance the adsorption of humic acid. Inaddition, the immobilisation of TiO2 onto zeolite particles permitseasier separation of the adsorbent (compared to free suspendedTiO2 nanoparticles) from the treated water. While natural zeolitehad minimal affinity for humic acid, zeolite coated with TiO2 is aneffective adsorbent for removing humic acid with high aromaticityand molecular weight at neutral pH. In addition, the TiO2 coatedzeolite particles were readily regenerated by UVA light irradiation tooxidise the adsorbed HA. The adsorption capacity after five succes-sive adsorption–regeneration cycles was maintained, demonstratingthe potential of zeolite/TiO2 as a promising HA adsorbent that is costeffective and will not generate substantial waste.

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

0 2 4 6 8 10 12

pH

Zeta

pot

entia

l (m

V)

Fig. 7. Zeta potential of zeolite and zeolite/TiO2 as a function of pH in the absenceof HA at 25 1C.

Table 1Parameters of the Langmuir and Freundlich adsorption isotherms for zeolite/TiO2.

Model Parameters Value

Langmuir qm (mg/g) 5.94KL (L/mg) 0.04R2 0.9933

Freundlich KF (mg/g) 0.351/n 0.61R2 0.9821

0%

20%

40%

60%

80%

100%

1 2 3 4 5

Number of cycles

Perc

enta

ge re

mov

al

Fig. 8. Comparative HA adsorption capacities of the zeolite/TiO2 in five adsorption–regeneration cycles.

S. Liu et al. / Chemical Engineering Science 105 (2014) 46–52 51

Acknowledgements

This research was supported under the Australian ResearchCouncil Linkage Projects funding scheme (LP100100481).The authors would like to acknowledge the financial support fromDCM Process Control, Water Corporation and facilities provided bythe UNSW Mark Wainwright Analytical Centre. We also thank ZarNi Oo for his assistance in preparation of the adsorbent and theadsorption study.

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