Electrochemical EDTA recycling after soil washing of Pb, Zn and Cd contaminated soil

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  • Journal of Hazardous Materials 192 (2011) 714 721

    Contents lists available at ScienceDirect

    Journal of Hazardous Materials

    j our na l ho me p age: www.elsev ier .com

    Electrochemical EDTA recycling after soil washinPb, Zn

    Maja PocAgronomy Dep a, Slov

    a r t i c l

    Article history:Received 24 FeReceived in reAccepted 24 MAvailable onlin

    Keywords:ContaminatedToxic metalsRemediationEDTA recyclin

    st of Esolutit of tl Al anfromher f

    electationimpro, whi


    1. Introduction

    Metal posoning maywhile their ness due toexample, thwhich 240,0remediationsoil-washinbe enhancewhich formthe soil propsoils [2]. OnExtensive ethat ethylenmany metamedically tbody.

    The deverently hamp

    Corresponcal Faculty, UnTel.: +386 01 4

    E-mail add

    spent soil washing solution, preferably including chelant recycling.Although, several approaches have been proposed, none have been

    0304-3894/$ doi:10.1016/j.llution of soil is widespread across the globe. Acute poi- occur in humans due to a high intake of toxic metals,presence in the ecological cycles may cause chronic ill-

    metal bio-accumulation. In South Eastern Europe, forere are more than 1.8 million contaminated sites, of00 are in need of remedial treatment [1]. However, soil

    technologies have not been adequately developed. Ing technologies, the solubility and removal of metals cand by lowering the pH with acids or by adding chelants,

    soluble metal complexes. Acid enhancement changeserties, and is not effective in highly buffered, carbonate

    the other hand, chelants do not interact with the soil.valuations of numerous chelating agents have shownediamine tetraacetate (EDTA) is the most effective forls [3]. It is of interest that EDTA has also been usedo promote removal of especially Pb from the human

    lopment of EDTA-based soil washing technology is cur-ered by the lack of the cost-effective treatments of the

    ding author at: Centre for Soil and Environmental Science, Biotechni-iversity of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia.23 1161; fax: +386 01 423 1088.ress: domen.lestan@bf.uni-lj.si (D. Lestan).

    commercialised. Ager and Marshall [4] used zero-valent bimetallicmixtures (Mg0Pd0, Mg0Ag0) to precipitate Pb from the solutionwhile liberating EDTA in alkaline pH. Metals liberated from theEDTA complex were cemented to the surfaces of the excess mag-nesium or removed from the solution as insoluble hydroxides. Themethod is efcient but could be economically prohibitive. Honget al. [5] separated Pb from EDTA with Na2S and Ca(OH)2 underalkaline (pH 10) conditions, resulting in almost complete recoveryof metals through precipitation in the form of insoluble metal sul-phides. While Ca(OH)2 provided Ca2+ ions to compete for the EDTAligand (by replacing the chelated contaminant metal), Na2S wasused as an anionic precipitant to provide HS and S2 to competewith EDTA for the contaminating metals. This method has beenfound to have limited application due to the hazardous nature ofthe produced reagents and the sludge, the cost and operational dif-culties. Kim and Ong [6] recycled chelant from PbEDTA solutionby substituting Pb with Fe3+ under acidic conditions, followed byprecipitation of the released Pb with phosphate (Na2HPO4) at nearneutral pH. Fe3+ ions were then precipitated as hydroxides at highpH using NaOH, thus liberating EDTA. The process, alkaline pre-cipitation, is the simplest way to separate metals from chelant anddoes not use expensive or hazardous reagents. However, the cost ofthe process is affected by high reagent consumption: for Pb precip-itation a phosphate/Pb molar ratio of about 30 was necessary [6].Di Palma et al. [7] reduced reagent consumption by evaporating

    see front matter 2011 Elsevier B.V. All rights reserved.jhazmat.2011.05.077and Cd contaminated soil

    iecha, Damijana Kastelec, Domen Lestan

    artment, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljan

    e i n f o

    bruary 2011vised form 4 May 2011ay 2011e 31 May 2011



    a b s t r a c t

    Recycling of chelant decreases the coeffective when the spent soil washing we applied electrochemical treatmenand Cd contaminated soil. A sacriciacell at pH 10 efciently removed Pb Al consumption were signicantly higreplacement of NaCl with KNO3 as anthe Al anode) prevented EDTA degradwashing solution prior to electrolysis Zn and Cd, respectively, were removedThe recycled EDTA retained 86, 84 andrespectively./ locate / jhazmat

    g of


    DTA-based soil washing. Current methods, however, are noton contains more than one contaminating metal. In this study,he washing solution obtained after EDTA extraction of Pb, Znode and stainless steel cathode in a conventional electrolytic

    the solution. The method efciency, specic electricity andor solutions with a higher initial metal concentration. Partialrolyte (aggressive Cl are required to prevent passivisation of

    during the electrolysis. The addition of FeCl3 to the acidiedved Zn removal. Using the novel method 98, 73 and 66% of Pb,le 88% of EDTA was preserved in the treated washing solution.

    of Pb, Zn and Cd extraction potential from contaminated soil,

    2011 Elsevier B.V. All rights reserved.

  • M. Pociecha et al. / Journal of Hazardous Materials 192 (2011) 714 721 715

    the washing solution volume by 75%, precipitation of EDTA underacidic conditions and recycling the residual EDTA using alkalineprecipitation, as described above. Juang and Lin [8] tempted elec-trolytic recovery of Cu and chelant from equimolar EDTA solutionin a two-chbrane Neosoxide coatewere used amal currentrecovery ratrations. ReEDTA solutimore proneand degradis requiredtons are genmembrane

    The EDTdemonstratdisadvantagthe mentionmulti-contaseparated, tand Cu, comcurrent efof the meta

    In orderexchange melectrochemventional sunder alkaldemonstrating solutiontheir EDTA method is g

    In this smethod forrecovery frtion of muinnovation,several metand the rechas not yetelectricity ccorrosion oefciency o

    2. Theoret

    The expfrom a spena potential ode, Al ionsthe cathode



    Al3+ and Ohydroxidesmeric Al hydAl13O4(OH)amorphousarea and cotivity (electto EDTA oxi

    Table 1Selected properties of two contaminated soils (A and B) used for extractions.

    Soil properties Soil A Soil B

    pH (CaCl2) 6.57 7.0ic matmol C

    %) ) ) e

    llowutionly on


    tals a

    2e itateditated

    3 + Mile Mtro-l(OH

    ions (d Al

    3 + Ogativd EDn. ED

    formAl antenty of

    in a sthe sy dec


    il pro

    s weich

    lead he to

    60 mconds pas4 m

    is wasoilswatec maty (C

    by t

    il extraction

    obtain soil washing solutions of various Pb, Zn and Cd con-tions, we placed 0.5 kg of air-dried soil (A and B) and 1000 mLous solution with different EDTA (disodium salt) concentra-

    from 7.5 to 50 mM EDTA) in 1.5 L asks. Soil was extracted onamber cell separated with a cation exchange mem-epta CM-1 to prevent EDTA anodic oxidation. Iridiumd on titanium (Ti/IrO2) and stainless steel electrodess the anode and cathode, respectively. Under the opti-

    density (139 A m2) and initial catholyte pH of 2.2, thete of Cu was >95% at sufciently high CuEDTA concen-covery of other divalent metals, including Pb, from theiron was possible but less efcient. The method is further-

    to operational problems, such as membrane foulingation, and a high feed of the spent washing solution

    to prevent EDTA precipitation in acidic media (pro-erated in the cathode chamber) and deposition on thesurface [9].A recycling procedures described above have beened on a laboratory scale. In addition to their specices, single metalEDTA solutions were handled in alled studies. However, most contaminated sites containminated soils. Juang and Wang [9] tested a membranewo-chamber electrochemical treatment for a binary, Pbplexed solution and found lower metal recovery and

    ciency than for single metal contamination, regardlessl concentration ratios.

    to overcome problems with the use of a cationembrane and EDTA precipitation, we proposed a novelical treatment of spent washing solution in a con-

    ingle chamber electrolytic cell (without membrane)ine conditions (pH 10) using a sacricial Al anode. Weed the method feasibility for treatment of spent wash-

    with a single toxic metal, Pb [10] and Cu [11] andcomplexes. The theoretical explanation of the proposediven in the section below.tudy, the feasibility of the proposed electrochemical

    simultaneous removal of Pb, Zn and Cd, and EDTAom a washing solution obtained after EDTA extrac-lti-metal contaminated soil was evaluated. This is an

    since simultaneous treatment of EDTA complexes withals is ineffective with currently proposed methods [9]ent electrochemical method using a sacricial Al anode

    been tested [10,11]. The factors governing the speciconsumption and specic Al consumption after electro-f the anode were examined. Strategies to improve thef the EDTA recycling were investigated.

    ical background

    erimental set-up for electrochemical EDTA recoveryt soil washing solution is explained in Section 3.3. Whendifference is applied between the Al anode and a cath-

    are generated from the anode and hydroxyl ions from:

    : Al Al3+ + 3e (1)de : 2H2O + 2e H2 + 2OH (2)H ions react further to form various monomeric Al, such as Al(OH)2+, Al(OH)2+ or Al2(OH)24+ and poly-roxides, such as Al6(OH)153+, Al7(OH)174+, Al8(OH)204+,

    247+ or Al1


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