Recycling of EDTA solution after soil washing of Pb, Zn, Cd and As contaminated soil

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<ul><li><p>in</p><p>100</p><p>knoycliherof</p><p>mgtheutiod EDr extentrand</p><p>1. Introduction</p><p>ly toxicin humd econing PTction)envirg agenashin</p><p>nteracrce.</p><p>for its complete degradation. Even partial EDTA recycling ratherthan chelant destruction could therefore improve the economicsof soil washing, both through chelant recovery and through savingsof energy for treatment of used soil washing solution.</p><p>lowed by electrochemical removal of PTTEs using a graphite anode.</p><p>2. Materials and methods</p><p>2.1. Soil washing</p><p>Soil was collected from a vegetable garden in the Mezica Valley,Slovenia. The valley has been exposed to more than three hundred</p><p> Corresponding author. Address: Centre for Soil and Environmental Science,Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana,Slovenia. Tel.: +386 01 423 1161; fax: +386 01 423 1088.</p><p>Chemosphere 86 (2012) 843846</p><p>Contents lists available at</p><p>Chemos</p><p>evier .com/locate /chemosphereE-mail address: domen.lestan@bf.uni-lj.si (D. Lestan).During soil washing, however, large quantities of used soilwashing solution are generated. EDTA complexes with PTTEs aretoxic, biologically inert and endure in the environment. Waste pro-cess waters from EDTA soil washingmust therefore be treated to re-move the EDTA and PTTEs before they are released. Advancedoxidation processes (Finzgar and Lestan, 2006) and electrochemicalmethods (Finzgar and Lestan, 2008) have been found to be effectivefor chemical destruction of the chelant. While EDTA is not a prohib-itively expensive chemical (approximately 1.5 Euro kg1), it ischemically stable and therefore requires a signicant energy input</p><p>hides. This method has been found to have limited applicationdue to the hazardous nature of the produced reagents and thesludge. None of these techniques has been commercialised. KnownEDTA recycling methods are generally sensitive to the presence ofmultiple toxic metals in the washing solution (Juang and Wang,2000). Most contaminated sites, however, contain multi-metalcontaminated soils.</p><p>In this feasibility study, a novel method of EDTA recycling fromused solution after washing Pb, Zn, Cd and As contaminated soilwas investigated. The method uses acid precipitation of EDTA fol-Soil contaminationwith potentialis ubiquitous and frequently resultssystem deterioration. Technically andiation techniques aimed at removurgently needed. Soil washing (extrathe focus of interest as a potentiallytechnique (Kim et al., 2003). ChelatinPTTEs from soil solid phases into thewuble complexes, without signicant ipreserving the soil as a natural resou0045-6535/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.chemosphere.2011.11.004trace elements (PTTEs)an poisoning and eco-omically feasible reme-TEs from the soil arewith EDTA has becomeonmentally sustainablets such as EDTA extractg solution as water-sol-tions with the soil, thus</p><p>Several approaches to recycling EDTA from used soil washingsolution have been proposed. Among these, substituting toxic met-als in EDTA complex with Fe3+ under acidic conditions, followed byprecipitation of the metals with phosphate and reclamation ofNa2EDTA in alkaline conditions (Kim and Ong, 1999) has becomethe focus of signicant interest. Electrochemical separation of toxicmetals and EDTA in a two-chamber electrolytic cell separated witha cation exchange membrane to prevent EDTA anodic oxidationhas also been introduced (Juang and Wang, 2000). Zeng et al.(2005) separated Pb, Cd and Cu from EDTA with Na2S and Ca(OH)2under alkaline conditions, resulting in almost complete recovery ofmetals through precipitation in the form of insoluble metal sulp- 2011 Elsevier Ltd. All rights reserved.Technical Note</p><p>Recycling of EDTA solution after soil wash</p><p>Maja Pociecha, Domen Lestan Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101,</p><p>a r t i c l e i n f o</p><p>Article history:Received 28 July 2011Received in revised form 2 November 2011Accepted 3 November 2011Available online 26 November 2011</p><p>Keywords:Contaminated soilToxic metalsSoil washingEDTA recycling</p><p>a b s t r a c t</p><p>Soil washing with EDTA issoil. A practical way of recproblem. We demonstratetion to recover up to 50%(5330 mg kg1), Zn (3400100% of EDTA residual inAs concentration in the solWe employed the recoverewith the same potential fodepends on the EDTA conc53% and 61% of Pb, Zn, Cd</p><p>journal homepage: www.elsll rights reserved.g of Pb, Zn, Cd and As contaminated soil</p><p>0 Ljubljana, Slovenia</p><p>wn to be an effective means of removing toxic metals from contaminatedng of used soil washing solution remains, however, an unsolved technicale, in a laboratory scale experiment, the feasibility of using acid precipita-EDTA from used soil washing solution obtained after extraction of Pbkg1), Cd (35 mg kg1) and As (279 mg kg1) contaminated soil. Up towashing solution and 100%, 97%, 98% and 100% of initial Pb, Zn, Cd andn, respectively, were removed in an electrolytic cell using a graphite anode.TA and treated washing solution to prepare recycled soil washing solutionracting toxic metals from soil as the original. The efciency of soil washingation. Using twice recycled 30 mmol EDTA kg1 soil, we removed 44%, 20%,As, respectively, from contaminated soil.SciVerse ScienceDirect</p><p>phere</p></li><li><p>Cd and 279 91 mg kg As. The soil washing solution was ob-</p><p>37% HCl. The experiment was performed in triplicate. After shaking</p><p>plied by the operation time (initially 19 min of operation time</p><p>H4EDTA) in the washing solution obtained after electrochemical</p><p>2.6. Metal determination</p><p>Air-dried soil samples (1 g) were ground in an agate mill, sievedthrough a 160 lm mesh and digested in a microwave oven (CEM,MDS-2000) in 12 mL of aqua regia (20 min at 175 C) The diges-tions were conducted in triplicate. After cooling, digested sampleswere ltered through Whatman no. 4 lter paper (pore size2025 lm) and diluted with deionized water up to 100 mL. Thepseudo-total concentration of Pb, Zn, Cd and As were determinedby ame (acetylene/air) AAS with a deuterium background correc-tion (Varian, AA240FS). The metal concentration in the solutionswas determined by AAS directly. A standard reference materialused in inter-laboratory comparisons (Wepal 2004.3/4, Wagenin-gen University, Wageningen, Netherlands) was used in the diges-tion and analysis as part of the QA/QC protocol. The limit of</p><p>The rst step of the EDTA recycling process was acidication of</p><p>mosphere 86 (2012) 843846treatment (initial pH 10, nal pH 9.3, the solution contained5.3 g L1 Na). A washing solution with an equimolar concentrationof fresh Na2-EDTA in deionized water was also prepared. The pH ofthe washing solutions with recycled and fresh EDTA was adjustedto values between 4 and 8. Solutions (20 mL) were then used forextraction of contaminated soil (10 g) on a rotating shaker (1 h).After centrifugation at 2880g for 10 min, the concentrations ofPb, Zn, Cd and As in the solutions were determined. Each extractionwas done in triplicate.</p><p>2.5. EDTA determinationwere equal to 4 min of contact time). Ten mL samples were peri-odically collected and stored at 3 C for PTTE and EDTA analysis.The cathode was at the end etched with 30 mL of 65% HNO3 todissolve electro-deposited PTTEs. All the treatments were donein triplicate.</p><p>2.4. Efciency of recycled EDTA</p><p>Recycled washing solution with 30 mmol EDTA kg1 soil wasprepared by dissolving the recovered EDTA (acid-precipitated asthe acidied solution for 2 h, EDTA precipitated and was removedfrom the solution by centrifugation at 3760g for 10 min. EDTA wasdried at 60 C to a constant weight and stored as recycled EDTA forfurther experiments. Residual solution was further electrochemi-cally treated.</p><p>2.3. Electrochemical treatment</p><p>Flow-through electrolytic cells (constructed from polyacrila-mide) equipped with graphite anode and stainless cathode (elec-trode surface 68 cm2, distance between electrodes 13 mm) wereused. Washing solution (500 mL) was circulated from a magneti-cally stirred jar through the electrolytic cells using a peristalticpump (ow rate 14 mL s1). DC power supply (Elektronik Invent,Ljubljana, Slovenia) provided constant electrical current densityof 44 mA cm2. The initial pH of the solution was adjusted to se-lected values by drop-wise addition of 5 M NaOH. The contacttime of the solution in the electrolytic cell was calculated as theratio of cell volume and volume of the washing solution, multi-tained after extraction of 75 kg of soil with 75 L of Na2-EDTA solu-tion (60 mM EDTA kg1 soil) in a concrete mixer for 2 h. Afterextraction, the soil suspension was rst ltered through a 2 mmsieve and the soil solid phase was then separated from the wastesoil washing solution in a chamber lter press (lter cloth thickness0.6 g cm2, air permeability 22 dm3 dm2 min1, Ecotip, Slovenia).The concentration of EDTA, Pb, Zn, Cd and As in thewaste soil wash-ing solution was 11600, 1110, 267, 7.1 and 61 mg L1, respectively,the solution pH was 7.3.</p><p>2.2. EDTA recovery</p><p>EDTA was separated from the used soil washing solution byadjusting the pH of the solution to values between 1 and 2 usingyears of active lead mining and smelting. The soil sample containedof 5330 690 mg kg1 Pb, 3400 190 mg kg1 Zn, 35 6 mg kg1</p><p>1</p><p>844 M. Pociecha, D. Lestan / CheThe concentration of EDTA in the samples was determinedspectrophotometrically at 535 nm according to the procedure de-scribed by Hamano et al. (1993).the used soil washing solution using HCl. EDTA is known to bepoorly soluble in acidic media and to precipitate in its protonated(H4EDTA) form (Wong et al., 1997). Acidic precipitation as a meansof chelating agent recovery was previously reported by Zoltan(1991) for EDTA containing steam generator cleaning solutions.As shown in Fig. 1, acidication of used soil washing solution leadsonly to partial EDTA precipitation. The percentage of precipitatedEDTA decreased statistically signicantly with the pH of the trea-ted washing solution. At pH 1, up to 50% of the initial EDTA concen-tration was recovered from the solution (Fig. 1).</p><p>a ab</p><p>ab b</p><p>0</p><p>10</p><p>20</p><p>30</p><p>40</p><p>50</p><p>60</p><p>1 1.3 1.7 2pH</p><p>Rem</p><p>oved</p><p> ED</p><p>TA (%</p><p>)</p><p>a</p><p>b</p><p>ab</p><p>ab</p><p>a ab</p><p>ab b</p><p>a ab</p><p>ab b</p><p>a ab</p><p>ab b</p><p>a</p><p>b</p><p>ab</p><p>ab</p><p>Fig. 1. Reclamation of EDTA from the soil washing solution at different pH valuesquantication for Pb, Zn, Cd and As were 0.1, 0.01, 0.02 and0.09 mg L1, respectively. Reagent blank and analytical duplicateswere also used where appropriate, in order to ensure accuracyand precision in the analysis.</p><p>2.7. Statistics</p><p>The Duncan multiple range test was used to determine the sta-tistical signicance (p &lt; 0.05) between different treatments, usingthe computer program Statgraphics 4.0 for Windows.</p><p>3. Results and discussion</p><p>Initial extraction using 60 mmol kg1 EDTA to obtain used soilwashing solution for further EDTA recycling experiments removed72%, 27%, 71%, and 80% of Pb, Zn, Cd and As from the contaminatedsoil. As expected, Zn was the least extractable. This had also beenobserved in our previous studies for soils from the same contami-nated site. The low extractability was explained by the specic Znfractionation in the residual soil fraction of the Tessiers sequentialextraction procedure (Finzgar and Lestan, 2008).after acidication with HCl. Error bars represent standard deviation from the meanvalue (n = 3). Letters (a, b) denote statistically different EDTA removal from solutionaccording to the Duncan test (p &lt; 0.05).</p></li><li><p>01020304050607080</p><p>As (m</p><p>g L-</p><p>1 )</p><p>-200</p><p>0</p><p>200</p><p>400</p><p>600</p><p>800</p><p>1000</p><p>1200</p><p>1400</p><p>0 20 40 60 80 100 120Contact time (min)</p><p>Fe (m</p><p>g L-</p><p>1 )</p><p>mos0</p><p>200</p><p>400</p><p>600</p><p>800</p><p>1000</p><p>1200</p><p>Pb (m</p><p>g L-</p><p>1 )</p><p>pH 2pH 4pH 8pH 10</p><p>0</p><p>50</p><p>100</p><p>150</p><p>200</p><p>250</p><p>300</p><p>8</p><p>4</p><p>6</p><p>pH 2pH 4pH 8pH 10</p><p>pH 2pH 4pH 8pH 10</p><p>50</p><p>100</p><p>150</p><p>200</p><p>250</p><p>Zn (m</p><p>g L</p><p>)Zn</p><p> (mg </p><p>L-1 )</p><p>(mg </p><p>L)</p><p>(mg </p><p>L-1</p><p>M. Pociecha, D. Lestan / CheAfter partial recovery, the remaining EDTA with all the ex-tracted PTTEs remained in the used soil washing solution. In orderto remove residual contaminants, we applied electrochemical oxi-dation using a graphite anode. Electrochemical oxidation on theanode occurs due to the generation of active oxygen species, fore-most hydroxyl radicals, which are very powerful oxidants, secondonly to uorine. The important parameter of an anodic oxidationprocess is therefore the selection of the anode material, with a suf-cient oxygen overvoltage before H2 (cathode) and O2 (anode)form. This electrochemical window allows the production of hy-droxyl radicals (Oliveira et al., 2007). Various anode materials havebeen studied: graphite, Pt, various noble metal oxides (PbO2, IrO2,TiO2, SnO2) on a titanium substrate and boron-doped diamond an-ode (Troster et al., 2002). We used graphite since it is non-toxic,inexpensive and does not release metal ions back into the solution(to form undesirable EDTA complexes later in the process). Th elec-trochemical reactions modied the pH of the treated solutions. Ininitially acidic solutions, the electrochemical system generated en-ough OH for the pH to diverge toward neutral and alkaline values,while the pH of initially alkaline solutions changed less signi-cantly. The nal pH values after electrochemical treatment were5.86.3, 8.38.4, 8.99.2 and 9.19.2 for solutions with initial pHvalues set to 2, 4, 8 and 10, respectively. The voltage betweenthe electrodes also varied during the treatment: from 7.4 to17.3 V for a solution with initial pH value 2, 6.88.7 V for a solutionwith pH 4, 7.08.4 V for a solution with pH 8 and 6.68.2 V for asolution with pH 10.</p><p>As shown in Fig. 2, the electrochemical removal of PTTEs wasmore effective when an alkaline initial pH of the treated solution</p><p>0</p><p>2</p><p>0 20 40 60 80 100 120Contact time (min)</p><p>Cd </p><p>Cd </p><p>Fig. 2. Concentration of Pb, Zn, Cd and As and in the soil washing solutions during electfrom the mean value (n = 3).phere 86 (2012) 843846 845was applied. Fe precipitated abruptly (presumably as insoluble Fehydroxides) from solutions with an initially alkaline pH. These data</p><p>rochemical treatment at pH 2, 4, 8 and 10. Error bars represent standard deviation</p><p>Table 1Balance of removal of Pb, Zn, Cd and As from soil washing solutions with differentinitial pH after electrochemical treatment. The relative concentration of PTTEselectro-deposited on the cathode and in the solution were measured; the relativeconcentration of precipitated PTTEs was calculated as a difference. Means (n = 3) andstandard deviation of results are presented.</p><p>Electrodeposited (%) In solution (%) Precipitated (%)</p><p>PbpH 2 33 3 1 1 66 4pH 4 45 5 1 1 54 6pH 8 70 3 2 0 28 3pH 10 69 8 0 0 30 8</p><p>ZnpH 2 36 0 26 2 37 1pH 4 47 7 10 8 43 15pH 8 66 3 8 1 26 4pH 10 51 9 3 3 45 12</p><p>CdpH 2 21 1 13 1 66 2pH 4 24 2 5 4 71 6pH 8 26 0 5 1 69 1pH 10 25 4 2 1 72 5</p><p>AspH 2 14 0 0 0 86 0pH 4 25 2 0 0 75 2pH 8 49 3 0 0 51 3pH 10 53 9 0 0 47 9</p></li><li><p>1500</p><p>846 M. Pociecha, D. Lestan / Chemosphere 86 (2012) 8438460</p><p>300</p><p>600</p><p>900</p><p>1200Pb</p><p> (mg </p><p>L-1 )</p><p>Fresh EDTARecycled EDTA</p><p>2-times recycled EDTA</p><p>0</p><p>100</p><p>200</p><p>300</p><p>400</p><p>Zn (m</p><p>g L-</p><p>1 )</p><p>6</p><p>8</p><p>10</p><p>g L-</p><p>1 )</p><p>Fresh EDTARecycled EDTAmay indicate that EDTA complexes with Pb, Zn, Cd and Fe are moreeasily degradable in alkali...</p></li></ul>

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