Journal of Contaminant Hydrology 125 (2011) 39–46

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Journal of Contaminant Hydrology

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Column study of Cr(VI) removal by cationic hydrogel for in-situ remediationof contaminated groundwater and soil

Samuel C.N. Tang, Ke Yin 1, Irene M.C. Lo⁎Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China

a r t i c l e i n f o

⁎ Corresponding author. Tel.: +852 2358 7157; faxE-mail address: cemclo@ust.hk (I.M.C. Lo).

1 Current address: Residues and Resource ReclamaNanyang Avenue, Nanyang Technological University, 6

0169-7722/$ – see front matter © 2011 Elsevier B.V.doi:10.1016/j.jconhyd.2011.04.005

a b s t r a c t

Article history:Received 18 December 2010Received in revised form 13 April 2011Accepted 20 April 2011Available online 1 May 2011

Column experiments were conducted for examining the effectiveness of the cationic hydrogelon Cr(VI) removal from groundwater and soil. For in-situ groundwater remediation, the effectsof background anions, humic acid (HA) and pHwere studied. Cr(VI) has a higher preference forbeing adsorbed onto the cationic hydrogel than sulphate, bicarbonate ions and HA. However,the adsorbed HA reduced the Cr(VI) removal capacity of the cationic hydrogel, especially afterregeneration of the adsorbents, probably due to the blockage of adsorption sites. The Cr(VI)removal was slightly influenced by the groundwater pH that could be attributed to Cr(VI)speciation. The 6-cycle regeneration and reusability study shows that the effectiveness of thecationic hydrogel remained almost unchanged. On average, 93% of the adsorbed Cr(VI) wasrecovered in each cycle and concentrated Cr(VI) solution was obtained after regeneration. Forin-situ soil remediation, the flushing water pH had an insignificant effect on the release of Cr(VI) from the soils. Multiple-pulse flushing increased the removal of Cr(VI) from the soils. Incontrast, more flushing water and longer operation may be required to achieve the sameremoval level by continuous flushing.

© 2011 Elsevier B.V. All rights reserved.

Keywords:ChromiumCationic hydrogelIon exchangeIn-situ remediation

1. Introduction

Chromium (Cr) is a valuable metal with its price increasingin recent years (USGS, 2009). It is a common industrial metalutilized in various products and processes; however, it causesadverse impact on human health and the environment due toits high toxicity and mobility, as well as processing type Ahuman carcinogenic properties. Chromium can be released tothe environment due to leakage or spillage, thus contaminatingwater bodies in the surrounding environment (Eary and Davis,2007). Furthermore, soils have also been polluted as a result ofimproper storage or disposal, and chromium in soils can be

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leached into groundwater due to its high solubility andmobility(Dong et al., 2009).

Various technologies have been developed to remediatechromium-contaminated sites. Pump and treat is a conven-tional groundwater remediation technology, but the attrac-tiveness of this technology is declining, resulting from theenergy intensive pumping requirement (Higgins and Olson,2009). The permeable reactive barrier (PRB) is an increasingpopular remediation technology, widely applied in the recentdecade. Zero-valent iron (ZVI), a commonly used packingmaterial in PRBs, has been applied in many chromium-contaminated sites (Flury et al., 2009; Wilkin et al., 2005). Ingeneral, the Cr(VI) in groundwater is reduced to Cr(III) byZVI when the groundwater flows through a PRB, and isthen removed by the co-precipitation with ferric hydroxides(Lai and Lo, 2008; Puls et al., 1999). However, Lee and Wilkin(2010) reported that the precipitates blocked the reactivesurface of the ZVI in some PRBs. The precipitates also led topreferential channeling of groundwater flow, thus shortening

40 S.C.N. Tang et al. / Journal of Contaminant Hydrology 125 (2011) 39–46

the contact reaction time between Cr(VI) and ZVI (Bloweset al., 2000; Lo et al., 2006). Instead of recovery and reuse ofthe valuable chromium from the groundwater, chromium, inthe form of precipitates, is retained in the PRB and cannot berecovered and reused.

Various materials have, therefore, been studied for theremediation of metal contaminated groundwater (e.g., Cu, As,U) through different physical and chemical processes in theform of sorptive or reactive barrier, such as zeolite (Camachoet al., 2010; Czurda and Haus, 2002), activated carbon (Douet al., 2010; Natale et al., 2008), chitosan (Gupta et al., 2009;Wan et al., 2004) and resins (Chiarle et al., 2000; Phillips et al.,2008). Among these materials, polymeric adsorbents, such aschitosan and resins, have shown a better performance, interms of their removal capacities and adsorbent reusability,than other adsorbents such as activated carbon and zeolite(Barakat and Sahiner, 2008; Ju et al., 2009). Cationic hydrogel,a type of polymeric adsorbent, has a large adsorption capacityand high adsorption rate (Barakat and Sahiner, 2008; Ozayet al., 2009), as well as being independent of pH (Tang et al.,2010).

Cationic hydrogelmaybea suitable PRBpackingmaterial forchromium-contaminated groundwater remediation, but itsperformance under various groundwater geochemical condi-tions remains unheeded. The pH of the environment is one ofthe critical parameters affecting the adsorption capacity, sincethe pH may alter the surface charges of adsorbents (Bodduet al., 2003; Jun et al., 2007). Moreover, groundwater usuallycontains a wide range of dissolved minerals, where the anionsin the background may compete with anionic contaminants(e.g., Cr(VI)) (Boddu et al., 2003). Natural organic matter(NOM) is also ubiquitous in subsurface environments. Sinceadsorption is one of the common methods of removing NOM(Anirudhan and Ramachandran, 2007; Boyer et al., 2008), thepresence of NOM is suspected of competing with Cr(VI) foradsorption sites. The environmental and economical sustain-ability of applying cationic hydrogel also depends on itsregeneration and reusability performance.

For soil remediation, soil flushing is one of the potentialtechnologies, where contaminants are dissolved into theflushing solution (Mulligan et al., 2001; Navarro andMartínez,2010). A tremendous amount of dilute contaminated water isproduced, and treatments are required before discharge.Instead of using extraction wells, collection systems packedwith the cationic hydrogel, can help to retain Cr(VI) from theflushing solution by adsorption and cut down on the amountof wastewater generated. On the other hand, since soil has acomplex structure, interactions between the contaminants insoils and theflushingwatermay greatly affect the efficiency ofsoil flushing. The solution pH, therefore, plays an importantrole in soil flushing, which affects the soil mineral dissolutionand the surface complex formation between the Cr(VI) andminerals (Stumm, 1997; Stipp et al., 2002). In addition, tailingis another problem in soil flushing with continuous pumping,where the residual concentrations do not meet treatmentstandardswith continuous pumping over long periods of time(USEPA, 2000). There is also a possibility that the dissolvedcontaminant concentration may rebound when pumping isdiscontinued.

In this study, cationic hydrogel was synthesized and appliedfor in-situ Cr(VI) removal from contaminated groundwater and

soils. For the in-situ remediation of Cr(VI) contaminatedgroundwater, the influences of background anions, groundwa-ter pH, the presence of NOM, and the regeneration andreusability of the cationic hydrogel were examined, by meansof column studies. For the in-situ remediation of Cr(VI)contaminated soils, the effect of the pH of the flushingwater (pH 5.5 and 8.0) and the operation mode (multiple-pulse) on Cr(VI) removal from contaminated soils, and theadsorption of Cr(VI) by cationic hydrogelwere also investigated.

2. Materials and methods

2.1. Artificially contaminated soils

To prepare contaminated soils, Completely DecomposedGranite (CDG) soil, which is formed from weathering ofgranite and is the most commonly found soil type in HongKong (Ng et al., 2001), was taken 25 to 50 cm below theground surface at Clearwater Bay in Hong Kong. The CDG soilwas air-dried and passed through a 2-mm sieve before use.The chemical and mineralogical characteristics of the soil hasbeen reported in a previous study (Yan et al., 2010). One kg ofCDG soil was artificially contaminated bymixing with 270 mLof 2000 mg/L Cr(VI) solution, which was prepared bydissolving K2Cr2O7 (analytical-grade, Aldrich Chemical Com-pany) in ultrapure water. The contaminated soils were thenmixed thoroughly with a stainless steel spatula in a highdensity polyethylene container and allowed to equilibrate for1 h, and then dried in an oven at 105 °C for 24 h. The total Crcontent of the artificially contaminated soils, measured byacid digestion with HNO3–HCl–HF in a microwave digester(CEM MDS-2000), which was found to be 350±30 mg/kg,waswithin the range of reported values of field-contaminatedsoils/sediments.

2.2. Column experiments for in-situ remediation ofcontaminated groundwater

Column experiments were performed in 3.6-cm internaldiameter and 10-cm long columns. The cationic hydrogel wassynthesizedvia radical polymerizationof (3-acrylamidopropyl)trimethylammonium chloride (APTMCl) (75 wt.% solution inwater) asmonomers andN,N′-methylenebisacrylamide (MBA)as cross-linkers as described in a previous study by the authors(Tang et al., 2010). All hydrogel columns were packed in smallincremental stepswith a thoroughlymixed fusionof 150 g sand(BS 4550) and 1 g of the cationic hydrogel powder, giving auniform bulk density of 1.49 g/cm3, and a correspondingporosity of 0.40. For this column, one pore volumewas equivalent to 40 cm3. To simulate typical groundwater(AmericanWater Works Association, 2003), synthetic ground-water, containing 10 mM Na+, 0.8 mM Ca2+, 6.6 mM Cl−,3 mM HCO3

− and 1 mM SO42−, was prepared by dissolving

CaCl2·2H2O, Na2SO4, NaHCO3 and NaCl (analytical-grade,Aldrich Chemical Company) into ultrapure water, and 0.2 mMCr(VI) (about 10 mg/L) was added as a typical groundwatercontamination level. Various concentrationsof SO4

2− (0, 0.2, 0.5,1.0 and 2.0 mM) in the synthetic contaminated groundwaterwere also prepared. In addition, commercially availableAldrich humic acid (HA) was dissolved into ultrapure water(N18.1 MΩ·cm), followed by filtering through 0.45-μm acetate

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a

b

Chromate

Sulphate

Bicarbonate

No SO4

0.2 mM SO4

0.5mM SO4

1mM SO4

2mM SO4

Fig. 1. (a) Breakthrough curves of anions (Cr(VI), SO42− and HCO3

−) fromhydrogel column and (b) chromate breakthrough curves with different SO4

2−

concentrations in groundwater.

41S.C.N. Tang et al. / Journal of Contaminant Hydrology 125 (2011) 39–46

cellulose membranes (ADVANTEC) for preparing the humicacid stock solution. Dissolved organic carbon (DOC) wasemployed to express the concentration of HA. Five mg/L (asDOC) of HA was added to the influent in a set of columns forstudying the effect of HA on Cr(VI) removal by the cationichydrogel. Solutions at two initial pH values of 5.5 and 8.0 werealsopreparedbyadjusting thepHwith0.1 NNaOHor0.1 NHCl.The columns were oriented vertically and the syntheticcontaminated groundwater was introduced from the bottomat a flowrate of 0.3±0.05 mL/min to mimic the groundwaterflow reported in a site in Denmark (Lai et al., 2006). Theresidence time of water in the columns was 134 min. Theconfiguration of the column apparatus is shown in Supple-mentary Information Fig. S1b. The column effluent volume andthe time of collection were recorded. The concentrations ofchromate and sulphate were measured using a flame atomicabsorption spectrometer (AAS, Varian 220FS) and by ionchromatography (HPLC, Hewlett PackedHP1050), respectively.The concentrations of bicarbonate and HA in the effluentsweredetermined by a total organic carbon analyzer (TOC, Shimadzu5000A). Whenever necessary, sample dilution was undertakenbefore measurements were taken.

2.2.1. Regeneration and reusabilityThe ability of the cationic hydrogel to be regenerated and

reused was investigated by performing six successiveadsorption-desorption cycles. The contaminated syntheticgroundwater was first introduced into the columns to removeCr(VI) by adsorption for about 100 pore volumes (PV), andthen the influent was switched to 0.5 M NaCl for 5 PV for Cr(VI) desorption. After desorption, the influent was switchedto ultrapure water for 1 day, about 12 PV, for flushing thecolumns. Another cycle of the adsorption process was carriedout by switching the influent to the contaminated syntheticgroundwater again, and the cycle was repeated 5 times. Inaddition, to examine the effect of HA on Cr(VI) adsorption–desorption and reuse of the hydrogel, the regenerationprocess was also carried out for Cr(VI) removal with thepresence of HA in the influent.

2.3. Column experiments for in-situ remediation ofcontaminated soils

Column experiments for contaminated soils were per-formed in 3.6-cm internal diameter and 10-cm long columns.The columns were packed in small incremental steps with140 g of artificially contaminated soils to obtain a uniformbulk density of 1.39 g/cm3, corresponding to a porosity of 0.41.One pore volumewas equivalent to 41 cm3. The contaminatedsoil columns were connected to the hydrogel columns. Theflushing water was introduced from the bottom of the soilcolumns, and the effluent became the influent of the hydrogelcolumns. Tominimize the disturbance on soil flushing and thecollection of Cr(VI) by the hydrogel, a repeated set ofcontaminated soil column testswas run. The effluent collectedfrom the contaminated soil columns was used as a referencefor the influent of the hydrogel columns (connected to thecontaminated soil columns). The configuration of the columnapparatus is shown in Supplementary Information Fig. S1.Two pH values of the flushing water were studied, byadjustment to 5.5 and 8.0, using 0.1 N NaOH or 0.1 N HCl. In

comparison with the continuous mode flushing, a multiple-pulse flushing mode was adopted to investigate the effect ofthe operationmode on Cr(VI) leaching from the contaminatedsoils, and the subsequent Cr(VI) removal by the cationichydrogel. The set of columns (the contaminated soil columnsand the contaminated soil columns with the hydrogelcolumns) were flushed for 4 h per day and then the flushingwas stopped for 20 h, after which it was repeated for 5 days.The volume of column effluents and the time of collectionwere recorded. The concentration of chromate was measuredusing a flame atomic absorption spectrometer (AAS, Varian220FS).Whenever necessary, sample dilutionwas undertakenbefore taking measurements.

3. Results and discussion

3.1. Effect of anions on groundwater remediation

The effluent from the cationic hydrogel column wascollected, and the breakthrough curves of the bicarbonate,sulphate and chromate ions are shown in Fig. 1a. From aprevious study by the authors (Lo et al., 2011), theadsorption mechanism of the cationic hydrogel has beenproven to be through ion exchange. Therefore, these anionswere adsorbed to different extents by the cationic hydrogel.From the trend of the three breakthrough curves, it can beseen that the bicarbonate had the weakest competitivenessand chromate had the strongest competitiveness among

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42 S.C.N. Tang et al. / Journal of Contaminant Hydrology 125 (2011) 39–46

these anions. Although higher concentrations of bicarbonate(3 mM) and sulphate (1 mM) than chromate (0.2 mM) werepresent in the influent, the cationic hydrogel showed ahigher selectivity towards chromate, followed by sulphateand bicarbonate.

Sulphate, like chromate, is oxyanionic, and has a similarmolecular structure to chromate. Due to the similar structure,high competitiveness and common presence in naturalenvironments, sulphate has often been used to study itseffect on Cr(VI) adsorption efficiency (Neagu, 2009; vanBeinum et al., 2006). In Fig. 1b, the Cr(VI) breakthroughcurves shifted to the left when the concentration of sulphatein the effluent increased. The higher the sulphate concentra-tion, the less the capacity for Cr(VI) removal. Moreover,sulphate posed a significant effect on the reduction of Cr(VI)removal in the low concentration range (0–0.5 mM), wherethe Cr(VI) removal capacity of the cationic hydrogel droppedfrom 59.3 mg/g (under 0 mM SO4

2−) to 43.1 mg/g (under0.5 mM SO4

2−). However, the extent of the reduction of Cr(VI)removal decreased with increasing sulphate concentrationsfrom 1 mM to 2 mM, where the Cr(VI) removal capacitydropped from 27.3 mg/g (under 1 mM SO4

2−) to 24.5 mg/g(under 2.0 mM SO4

2−).

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Vef

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Fig. 2. Cr(VI) breakthrough curves with and without HA present in theinfluent (a) during adsorption process, (b) during desorption process, and(c) during adsorption process after regeneration.

3.2. Effect of humic acid on groundwater remediation

The breakthrough curves of Cr(VI) with and without HAwere similar (Fig. 2a), which indicates that HA had a slightinfluence on the Cr(VI) removal by the cationic hydrogel inthe first cycle of adsorption. With the presence of HA, theamount of Cr(VI) being adsorbedwas affected in the later partof the adsorption process (after pore volume of 80), becauseHA was also adsorbed by the cationic hydrogel (Supplemen-tary Fig. S2a), indicating that some sites were occupied by theHA. Although some studies reported that HA competed withCr(VI) for kaolin adsorption sites (Li et al., 2010), HAexhibited almost no influence on Cr(VI) adsorption onto thecationic hydrogel in the first adsorption cycle. This is probablydue to the difference in the adsorption kinetics of Cr(VI) andHA onto the cationic hydrogel. From a previous study of theauthors (Tang et al., 2010), the kinetics of Cr(VI) adsorptionwas very fast, reaching equilibrium within 5 min, but it took200 min for HA adsorption to reach equilibrium. Therefore,when the adsorption process continues, less sites areavailable for Cr(VI) adsorption because the adsorbed HAaccumulates on the hydrogel, and thus the amount of Cr(VI)removal is reduced in the latter part of the process.

For regeneration of the cationic hydrogel, the effluentCr(VI) concentration from both columns, with and withoutadsorbed HA, were nearly the same (Fig. 2b), which indicatesthat the desorption of Cr(VI) was not affected by the adsorbedHA on the hydrogel. This resulted in 98% of the adsorbedCr(VI) being recycled. During the desorption process, only 51%of the adsorbed HA was also desorbed out (SupplementaryInformation Fig. S2b). Fig. 2c reveals the effect of HA on thesubsequent Cr(VI) adsorption process. It was found that thecapacity for Cr(VI) removal decreased. It is probably due to theblockage of the adsorption sites by the adsorbedHA remainingon the cationic hydrogel, and thereby less sites are availablefor the Cr(VI) adsorption.

3.3. Effect of pH on groundwater remediation

Since the speciation of Cr(VI) is pH dependent, two typicalgroundwater pH values, 5.5 and 8, were used to study theinfluence of pH on Cr(VI) removal by the cationic hydrogel.The results show that the Cr(VI) removal was slightly affectedby pH (Fig. 3), in which the removal capacity was found to be27.8 and 29.6 mg Cr/g at pH 8 and 5.5, respectively. The slightdifference in the removal capacity is perhaps due to thespeciation of Cr(VI) and the surface charges of the cationichydrogel. The speciation of Cr(VI) was estimated using theVisual MINTEQ (Gustafsson, 2008), which showed that themajority of the Cr(VI) is in the form of HCrO4

− (86.7%) at pH5.5, while CrO4

2− is dominant (97.8%) at pH 8.0. Therefore, the

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Fig. 3. Influence of two initial groundwater pH values on Cr(VI) break-through curves.

43S.C.N. Tang et al. / Journal of Contaminant Hydrology 125 (2011) 39–46

concentration of CrO42− (with two negatively charged

species) is higher at pH 8.0 than at pH 5.5, so that moreadsorption sites are required. Moreover, at pH 8.0, the surfacecharges of the cationic hydrogel became less positive (fromthe results of the supplementary experiment on the zetapotential measurement, Supplementary Information Fig. S3).Therefore, less positively charged cationic hydrogel for theadsorption of more negatively charged Cr(VI) (mainly CrO4

2−)leads to a slight drop in the removal capacity at pH 8.0.

3.4. Regeneration and reusability of cationic hydrogel

The concentration of Cr(VI) in the effluent during the sixconsecutive adsorption–desorption cycles is shown in Fig. 4.

Fig. 4. Concentration of Cr(VI) in the effluent during the

The adsorption capacity stayed at about 27 mg/g for all the sixcycles, while 93% on average of the adsorbed Cr(VI) wasrecycled from each cycle. This indicates a higher adsorptioncapacity and better reusability of the cationic hydrogel thansome other reported adsorbents, such as polyaniline (ad-sorption capacity about 4 mg/g) (Kumar and Chakraborty,2009) and modified activated carbon (adsorption capacityabout 10 mg/g) (Choi et al., 2009). After regeneration, thestrength of the Cr(VI) solution became more concentrated,rising from 10 mg/L in the synthetic contaminated ground-water to as high as 400 mg/L in the desorption effluent. Theamount of contaminated water was greatly reduced (around90% by volume) after the process. This can massively lowerthe treatment cost of contaminated water and enhance therecycling and reuse of Cr(VI), thus becoming moreeconomical.

3.5. Effect of water flushing pH on soil remediation

Contaminated soil columns were flushed at two differentpH values, and the concentrations of Cr(VI) (Fig. 5a) and thepH values of the effluents (Fig. 5b) are presented. Theeffluents from the contaminated soils were treated by thehydrogel columns. It was found that the pH of the flushingwater showed an insignificant effect on Cr(VI) leaching,where the total Cr(VI) leached out from the soils was19.15 mg at pH 8.0, and 18.46 mg at pH 5.5, respectively. Ithas been reported that Cr leaching is significant only whenpHb5 (Jing et al., 2006). Fig. 5b shows that the initial pHvalues were changed after passing through the soil columns,probably because of the change in the concentration of Cr(VI)

six successive adsorption and desorption cycles.

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Fig. 5. Soil flushingwith water of different pH values (a) Cr(VI) concentrationin the effluent of contaminated soil columns, (b) pH of the effluent ocontaminated soil columns, and (c) Cr(VI) concentration in the effluent ohydrogel columns connected to contaminated soil columns.

44 S.C.N. Tang et al. / Journal of Contaminant Hydrology 125 (2011) 39–46

ff

and the soil surface properties. As a result, the initialgroundwater pH of 5.5 was increased to 6.3 at equilibrium,and the initial groundwater pH of 8.0 dropped at thebeginning and then increased back to 8.0 at equilibrium.The concentrations of Cr(VI) in the effluent of the hydrogelcolumns are shown in Fig. 5c. Most of the Cr leached out fromthe soils was removed by the cationic hydrogel and thus theconcentration of Cr(VI) in the effluent remained at a lowlevel. However, the column with a lower initial pH showed abetter performance in the Cr(VI) removal.

Similar to that reported in the previous section, thespeciation of Cr(VI) and the surface charges of the cationichydrogel were varied at different pH values. HCrO4

− domi-nates (67.1%) at pH 6.3, while CrO4

2− dominates (96.9%) at pH8.0, and hencemore sites are required to remove CrO4

2− by the

hydrogel via ion exchange at pH 8.0. In addition, the surfacecharges of the cationic hydrogel became less positive at pH 8.0(Supplementary Information Fig. S3). Therefore, less Cr wasremoved by the hydrogel with the alkali water flushing as aresult of the change in Cr(VI) speciation and the surfacecharges of the hydrogel.

3.6. Effect of operation mode on soil remediation

In the multiple-pulse soil flushing mode, the soil columnswere flushed for 4 h each day, after that the flushing wasstopped for 20 h. This procedure was repeated for 5 days(Fig. 6a). Fig. 6b shows the concentration of Cr(VI) in theeffluent from the soil columns for five successive flushingcycles, in which the concentration of Cr(VI) increased slightlyat the beginning of each cycle. The increases of theconcentration of Cr(VI) were due to the rebound effect afterpumping was turned off (USEPA, 2000). The periods in whichno flushing was provided, allowedmore time for Cr(VI) in thesoil to reach equilibrium between the soil and porewater, andthereby promoted the desorption of Cr(VI) into the porewater (Palmer andWittbrodt, 1991).When the soil was beingflushed in the next cycle, a higher concentration of Cr(VI) wasobtained in the effluent. For continuous flushing, although ahuge amount of flushing water was applied, the leaching wasnot as satisfactory as that of the multiple-pulse soil flushing.This was probably due to the limited residence time for waterin the continuous flushing case, thus reducing the time for theconcentration in the pore water to equilibrate to the Cr(VI)concentration in the soil. Multiple-pulse soil flushing allowsan effective operation mode which shortens the operationtime and reduces the amount of flushing water needed toachieve the same removal. A mass of 18.8 mg Cr(VI) wasleached out using multiple-pulse flushing in 5 cycles, whilea double flushing period, meaning twice the amount ofthe flushing water, was required for continuous flushing inorder to obtain the same amount of leached Cr(VI). Nearlyall the Cr(VI) in the effluent of the multiple-pulse flushingwas adsorbed by the cationic hydrogel, and therefore thecorresponding effluent was found to have Cr(VI) belowdetectable concentrations (Fig. 6c).

4. Conclusions

Cr(VI) can be effectively removed from contaminatedgroundwater and soil by cationic hydrogel. The cationichydrogel had the highest selectivity towards Cr(VI), followedby sulphate and bicarbonate. However, the removal capacityof Cr(VI) by the cationic hydrogel was reduced by increasingthe sulphate concentration. HA was also removed by thecationic hydrogel, but could not be thoroughly desorbed afterregeneration. The reduction in the Cr(VI) removal capacity isprobably due to blockage of adsorption sites by the adsorbedHA.Moreover, the speciation of Cr(VI) and the surface chargesof the cationic hydrogel are pH dependent, which results in aslight influence on the Cr(VI) removal. About 93% on averageof the adsorbed Cr(VI) can be recovered after regeneration,and a small volume of concentrated Cr(VI) solution can beobtained. For soil remediation, the pH of the flushing waterhad an insignificant effect on the amount of Cr leached, but thespeciation of Cr(VI) and the surface charges of the cationic

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Fig. 6. (a) Multiple-pulse flushing operation, (b) Cr(VI) concentration in the effluent during multiple-pulse flushing operation, and (c) Cr(VI) concentration in theeffluent of hydrogel columns connected to contaminated soil columns.

45S.C.N. Tang et al. / Journal of Contaminant Hydrology 125 (2011) 39–46

hydrogelwere affected, thereby influencing Cr(VI) removal bythe hydrogel. Furthermore, multiple-pulse soil flushing,coupled with cationic hydrogel, has been demonstrated as amore efficient way for Cr(VI) removal from contaminatedsoils. Although the cationic hydrogel shows promising resultsfor Cr(VI) removal from contaminated groundwater andsoil, further study or pilot test should be required beforeapplication on fields. The site characteristic should also beinvestigated and taken into consideration.

Acknowledgement

The authors wish to thank the Research Grants Council ofthe HKSAR Government for providing financial support underthe RPC07/08.EG03 and the General Research Fund 617309for this research study.

Appendix A. Supplementary data

Supplementary data to this article can be found online atdoi:10.1016/j.jconhyd.2011.04.005.

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