Enhanced remediation of Cr(VI)-contaminated soil by incorporating a calcined-hydrotalcite-based permeable reactive barrier with electrokinetics

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<ul><li><p>Journal of Hazardous Materials 239 240 (2012) 128 134</p><p>Contents lists available at SciVerse ScienceDirect</p><p>Journal of Hazardous Materials</p><p>j our na l ho me p age: www.elsev ier .com</p><p>Enhanc socalcine e b</p><p>Jia Zhang a, Ga School of Env ab Australian Re r BioeQLD 4072, Aus</p><p>h i g h l </p><p> Combing e lated Cr(VI) rem Electrokin Permeable reactive barrier media (calcined hydrotalcite) stores chromate.</p><p>a r t i c l e i n f o</p><p>Article history:Received 24 AReceived in reAccepted 18 AAvailable onlin</p><p>Keywords:Permeable reaElectrokineticsHydrotalciteChromate remSoil contamina</p><p>a b s t r a c t</p><p>1. Introdu</p><p>Cr(VI) issuspected cthe entire psites of somleather tannadsorbed inpresence oftric potentigives rise to[1,2]. Thereresult, manincluding gepermeable </p><p> Correspon Correspon</p><p>E-mail add</p><p>0304-3894/$ http://dx.doi.opril 2012vised form 16 August 2012ugust 2012e 25 August 2012</p><p>ctive barrier</p><p>ovaltion and remediation</p><p>This paper describes the enhanced Cr(VI)-contaminated soil remediation via a combination of electroki-netics (EK) with a calcined-hydrotalcite-based permeable reactive barrier (PRB). First, this combinationproved to be feasible, and remarkably facilitated Cr(VI) remediation in a column test. Then, lightly-to-severely (0.161.65 mg/g) Cr(VI)-contaminated soil was remediated in a simulated test with the calcinedhydrotalcite as the PRB under an voltage of 1030 V (i.e. an electric eld intensity of 0.72.0 V/cm). Theobservations demonstrated that both PRB and EK are critical to efcient remediation and the high de-contamination efciency is supposedly attributed to the synergistic effect, for which EK concentratesanionic chromate to the anode region and PRB media (calcined hydrotalcite) absorbs and immobilizes it.Thus we have shown that the combined PRBEK system is highly adaptive and effective in remediationof a larger area contaminated with chromate and various anionic pollutants.</p><p> 2012 Elsevier B.V. All rights reserved.</p><p>ction</p><p> a highly toxic material, being both a mutagen and aarcinogen, and is quite soluble in water almost overH range [1]. It is a soil and groundwater pollutant ate industrial processes such as chrome plating, textile,ing as well as mine tailings, and moreover it may be</p><p> acid soil and reduced to the far less mobile Cr(III) in ferrous iron, sulphide, soil organic matter and elec-al. In recent year, rapid development in these industries</p><p> worldwide Cr(VI) soil contamination in various degreefore, its effective remediation is urgently desirable. As ay methods have been proposed for Cr(VI) remediation,ochemical xation, soil ushing/chromium extraction,</p><p>reactive barriers, electrokinetics and phytoremediation</p><p>ding author. Tel.: +61 7 33463809; fax: +61 7 33463973.ding author. Tel.: +86 21 66137758; fax: +86 21 66137758.resses: grqian@shu.edu.cn (G. Qian), gordonxu@uq.edu.au (Z.P. Xu).</p><p>[3]. Among them, permeable reactive barriers (PRB) and electroki-netics (EK) seem to be more readily applicable and effective.</p><p>Permeable reactive barriers (PRB) is a passive remediationtechnology, and has been proven effective in in situ Cr(VI)-contaminated groundwater treatment [1,4,5]. A PRB consists ofpermanent, semi-permanent or replaceable reactive media placedacross the groundwater ow path. When streaming through themedia, contaminants are converted into less harmful compoundsor being immobilized inside the PRB via some reactions [1,5]. Asa result, reactive media selection is always the focus for a studyof PRB. Reactive media is the main PRB body. Its efciency andduration of efciency are always the keys to a PRB design and oper-ation. The general reactive media often used is zero-valence iron,and has been used to treat Cr(VI)-contaminated soil remediation viareductive precipitation [2,6,7]. However, the consequent precipita-tion due to iron corrosion usually decreases the PRB permeabilityand prohibits the ller activity, which affects the longevity of thebarrier materials [8,9]. Thus, one choice is to use sorbents as thePRB media to adsorb contaminants. Particularly, a family of anionicclay materials, hydrotalcite-like materials (HTs) have shown a high</p><p> see front matter 2012 Elsevier B.V. All rights reserved.rg/10.1016/j.jhazmat.2012.08.039ed remediation of Cr(VI)-contaminatedd-hydrotalcite-based permeable reactiva, Yunfeng Xua, Wentao Lia, Jizhi Zhoua, Jun Zhao</p><p>ironmental and Chemical Engineering, Shanghai University, Shanghai 200072, PR Chinsearch Council Centre of Excellence for Functional Nanomaterials, Australian Institute fotralia</p><p>i g h t s</p><p>lectrokinetic with permeable reactive barrier to remove Cr(VI) from simuoval efciency has been remarkably enhanced in the combined system.etic concentrates chromate under the electric eld./ locate / jhazmat</p><p>il by incorporating aarrier with electrokinetics</p><p>uangren Qiana,, Zhi Ping Xub,</p><p>ngineering and Nanotechnology, The University of Queensland, Brisbane,</p><p> soil.</p></li><li><p>J. Zhang et al. / Journal of Hazardous Materials 239 240 (2012) 128 134 129</p><p>adsorption capability for various anions [1013], such as chro-mate (Cr(VI)), exhibiting the potentiality as PRB reactive media toremove this soil contaminant. HTs are exemplied by the natu-rally existing hydrotalcite (Mg6Al2(OH)16CO34H2O) [14]. Its mostimportant pmore, the cHT-like strufrom aqueo[15]. As ourbe achievedcan be regesolution anPRB reactiv</p><p>On the omethod of sCr(VI) remetrodes are inarea. Betweis applied. Athe electrodis especiallyedly due toremoval eftaminated shas a strongsuppressedantacid mahydrotalciteto increase </p><p>ThereforremediationPRB with EKsystems to soils, as detthe feasibilthe Cr(VI) rknowledge,[2124] usisynergistic or emphasiPRBEK syscontaminat</p><p>2. Materia</p><p>2.1. Materi</p><p>Hydrotalized as the4 h. The calchemical fo</p><p>Loam aninated soilsmaximum wtively. In ge100 meshesor 1.65 mg(dry loam orinitial watethat all liqutures were in refrigeranated soils was used ininated soils</p><p>(A) Pth; W = width; H = height; unit = millimeter).</p><p>g. Filter paper was used to isolate CHT from the contaminatedd the anode.</p><p>BEK system</p><p>he PRBEK reactive column (Fig. 1A), the middle column 70 cm3) was lled with 130 g of contaminated loam, whichhigher osmosis, and about 10 g of CHT was placed in thee (anode). When the owing solution (0.01 M KCl) was</p><p>uced from the right inlet via a constant-ow pump at a ow 0, 0.1, 0.2 or 0.3 ml/min (0.00, 1.85 104, 3.70 104, and</p><p> 104 cm/s, since general loam owns a hydraulic conductivityt 1.50 103 cm/s), a 30 V direct current (DC) was suppliedrect-current (DC) source (Model TPR-3C) to create an elec-ld intensity of about 2 V/cm. Three additional tests weren under 0.1, 0.2 and 0.3 ml/min but without voltage sup-ter running for about 3, 6, 9, 12, 18 and 24 h, the experimentopped, and all contaminated soil was collected. The Cr(VI)t left in the collected soil was then analyzed and thus theal percentage was calculated.</p><p>mulation of PRBEK remediation</p><p>can be referred to Fig. 1B, the middle cubic was radiallyed (i.e. expanded column) to simulate the real situation</p><p> soil was contaminated in a large area.o experiments, e.g. with and without CHT ller, were con-</p><p> to examine the synergistic effect of PRBEK combination). The cubic inside was lled with CHT, and nine sampling</p><p> (AI) were selected on the surface displayed by the diagonalroperty is the high anion exchange capacity. Further-alcined form can be spontaneously reconstructed intocture in water, which adsorbs anions into the interlayerus phase, promising a high removal of anionic pollutions</p><p> earlier work has proved that Cr(VI) remediation could by calcined hydrotalcite [16]. In addition, the adsorbentnerated conveniently by anion exchange with NaHCO3d then calcination [12], i.e. HTs could be reused as thee media after regeneration.ther hand, electrokinetics (EK) is an active remediationoils polluted with charged species, and tested feasible indiation in both bench and eld-scale [17,18]. In EK, elec-stalled vertically or horizontally in a contaminated soilen the electrodes, a low direct current voltage gradients a result, the pollutant ions move in soil pores towardes by electromigration and electroosmosis [19,20]. EK</p><p> useful in a low osmosis soil where PRB works limit- low groundwater hydraulic conductivity. However, itsciency is usually hindered by acidication of the con-oil. It is reported that if cooperating with a barrier which</p><p> acid neutralizing capacity, this acidication would be, thus enhancing EK efciency [20]. HT adsorbent is anterial, and it is our hypothesis that merging a calcined--based PRB with EK will give rise to a synergistic effectthe remediation efciency.e, this article aims to verify our hypothesis that Cr(VI)</p><p> can be synergistically enhanced through combining a. To this end, we set up two specic PRBEK remediationsimulate the Cr(VI) removal from Cr(VI)-contaminatedailed in Fig. 1. The former set-up was utilized to proveity of PRBEK combination, and the latter to simulateemediation in a much broader area. To the best of our</p><p> although the synergy of PRB and EK has been reportedng zero-valent iron as the PRB reactive media, PRBEKmechanism from a broader area has not been reportedzed in public literature. We found that the combinedtem much more efciently removes Cr(VI) from theed soil.</p><p>ls and methods</p><p>als</p><p>lcite (Mg/Al = 3, HINWOUN CHEMICAL Co. Ltd.) was uti- PRB reactive media after calcination under 723 K forcined hydrotalcite was named as CHT, with a nominalrmula of Mg3.0AlO4.5 (MW = 172).d kaolin were chosen to mix with Cr(VI) as the contam-</p><p> because they stand for low and high osmosis, and theirater content is about 40% and 56% (saturation), respec-</p><p>neral, loam or kaolin was rst dried and milled through sieve. Then, the contaminated soils were spiked at 0.16Cr(VI))/g (dried solid soil) by adding Cr(VI) solution to</p><p> kaolin at a solid/liquid weight ratio of 3:1. Namely ther weight content in columns was 25%, which ensuredid was captured with the solid soil. Finally, the mix-thoroughly mixed by mechanical rabbling and storedtor for further use. The pH of these simulated contami-was about 6.0. About 130 g of loam or 1000 g of kaolin</p><p> experiments with or without 10 g of CHT. Both contam- were easily lled in the designed equipment with slight</p><p>Fig. 1. (L = leng</p><p>tappinsoil an</p><p>2.2. PR</p><p>In t(abouthas a left sidintrodrate of5.55 of abouby a ditric ealso ruply. Afwas stamounremov</p><p>2.3. Si</p><p>As expandwhere</p><p>Twducted(Fig. 2pointsRBEK reactive column; (B) expanded PRBEK reactive column</p></li><li><p>130 J. Zhang et al. / Journal of Hazardous Materials 239 240 (2012) 128 134</p><p>Fig. 2. PRBEKpoints. (Note t</p><p>stripes betwwith 30 V aCr(VI)-pollulling strucIn general, tinuously, athe amount</p><p>To invesdiation, a voof a severeprevious veling points experimentCr(VI)-contfor analysiSupplemen</p><p>2.4. Cr(VI) c</p><p>The wetwhole columwas weighereacting withe Cr concinductivelyAES) (Mode</p><p>3. Results </p><p>3.1. Feasibi</p><p>Fig. 3 prsystem undcentage wasoil after trin soil. If noin a quickeexample, w80% Cr(VI). ow rate ofany owingremoved byCHT lled inreports elseelectroplatifor 5 days [</p><p>0</p><p>80</p><p>100</p><p>r(VI) rns. 0.of 0 V </p><p> undoth ove.easeK sysfromhat thighsam</p><p> signanios HCowlyalso vem</p><p>ion dore, cvene</p><p> knowanodPRB ontity, </p><p> via EK can be quickly absorbed by PRB media placed nearby,reduces the Cr(VI) concentration gradient and acceleratesVI) removal. system to treat Cr(VI) in an expanded area. AI stand for samplinghat AI were on the same vertical plane.)</p><p>een two graphite electrodes. Both cases were appliednd about 10 g of CHT to treat about 1000 g of lightlyted soil (0.16 mg/g). In order to avoid destruction of theture in the system, sampling was taken very carefully.100 mg of wet sample was taken from each point con-nd then Cr(VI) was extracted from the collected soil for</p><p> determination.tigate the inuence of electrokinetic voltage on reme-ltage of 10, 20 or 30 V was applied in the remediation</p><p>ly Cr(VI)-polluted soil (1.65 mg/g). Different from thertical plane sampling points, the corresponding samp-were reselected in the middle horizontal plane in this, as shown in Supplementary Data Fig. S1. Similarly,aminated wet soil was taken and Cr(VI) was extracteds. All column test parameters were summarized intary Data Table S1.</p><p>oncentration analysis</p><p> sample taken from the sampling points (100 mg) or then was dried at 60 C overnight, and the dried sample</p><p>d. Cr(VI) in the dried sample (50 mg) was extracted byth 10 ml of 5% HNO3 under ultrasonication for 1 h andentration in the extracted solution was determined by</p><p> coupled plasma - atomic emission spectroscopy (ICP-l Prodidy, Leeman).</p><p>and discussion</p><p>lity of PRBEK combination</p><p>0</p><p>20</p><p>40</p><p>60</p><p>Rem</p><p>ov</p><p>al </p><p>(%)</p><p>Fig. 3. Cconditiovoltage </p><p>sludgewere bwe remsystem</p><p>IncrPRBECr(VI) Note twith a at the and EKCrO42</p><p>(such athus slanion the mothe anTherefeffecti</p><p>It isto the tively, CrO42</p><p>proximregionwhich the Cr(esents Cr(VI) removal (%) in the combined PRB and EKer different conditions. In this test, Cr(VI) removal per-s calculated by comparing the Cr(VI) amount left in theeatment for some time with the initial Cr(VI) amount</p><p> DC voltage was applied, the higher ow rate resultedr removal of Cr(VI) from the column, as expected. Forhen the ow rate was 0.1 ml/min, it took 20 h to removeThe same Cr(VI) removal percentage took only 8 h at the</p><p> 0.3 ml/min. When only DC voltage was applied without, the Cr(VI) removal seemed a bit fast, with 80% Cr(VI)</p><p> 4 h. The removal efciency with the voltage applied and the left side of the column seems much higher than thewhere [25,26]. Peng and Tian removed 34% Cr(VI) fromng sludge under an electric eld intensity of 1.5 V/cm25], and Hanay et al. removed 34% Cr(VI) from sewage</p><p>To furthtem in remthe aqueouumn in theto Fig. 1B),particular p</p><p>3.2. Applica</p><p>After feaumn tests, conducted to test applcontaminatCr(VI) remoation time 30252015105</p><p>Time (h)</p><p> 0 ml/min / 30 V</p><p> 0.1 ml/min / 0 V</p><p> 0.1 ml/min / 30 V</p><p> 0.2 ml/min / 30 V</p><p> 0.3 ml/min / 30 V</p><p> 0.2 ml/min / 0 V</p><p> 0.3 ml/min / 0 V</p><p>emoval prole in the PRBEK reactive column under different running1 ml/min/0 V stands for running at a ow rate of 0.1 ml/min, under aand with 10 g of the CHT barrier adjacent to the anode.</p><p>er 2.03.3 V/cm for 8 days [26]. Their water contentsabout 70%, bigger than this work (33.3%). In our case,d 80% Cr(VI) under 2.0 V/cm within 4 h in the PRBEK</p><p>d rates of Cr(VI) removal were observed in the combinedtem. As shown in Fig. 3, it took only 3 h to remove &gt;99%</p><p> the soil under 30 V at the ow rate of 0.10.3 ml/min.he effect of the ow rate was constrained within 3 h,er ow rate resulting in a bit higher removal percentagee time point. It was obvious that combination of PRBicantly speeded up the Cr(VI) removal. It seems thatn adsorbed in the soil is exchanged with aqueous anionsO3, Cl and H2PO4/HPO42) in the owing water and</p><p> removed from the soil. Under the electric eld, CrO42</p><p>migrates to the anode, which does not only superposeent of CrO...</p></li></ul>

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