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

    85%

    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

    soil

    g

    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

    enia

    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

    Attheanode

    Atthecatho

    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

    A2

    tals a

    2e itateditated

    3 + Mile Mtro-l(OH

    ions (d Al

    3 + Ogativd EDn. ED

    formAl antenty of

    in a sthe sy dec

    teria

    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 Al13(OH)345+ [12]. Finally, they all transform into

    Al(OH)3 and combine to form ocks with a large surfacensiderable absorption capacity. Due to the high Al reac-ro-positivity), Al is oxidized at the anode preferentiallydation.

    OrganCEC (mSand (Silt (%Clay (%Textur

    To aing solpossib

    M-EDT

    Me

    M2+ +precipprecip

    Al(OH)

    Whby election. Aconditcharge

    Al(OH)

    This nechargesolutioAl ionsof the tion postabilithigherwhile slightl

    3. Ma

    3.1. So

    Soilnia, whactive from t3980 The seused a587 1analysin the CaCl2organicapacitexture

    3.2. So

    To centraof aquetions (ter (%) 14.2 7.2+ 100 g1) 20.7 20.3

    51.0 38.542.5 55.46.5 6.1Sandy loam Silty loam

    electrochemical EDTA recycling from the spent wash-s, metals (M) are liberated from the complex with EDTA,

    the outside of the electric double layer of the cathode:

    M2+ + EDTA4 (3)re reduced and deposited onto the cathode,

    M(s) (4)

    as insoluble hydroxides or absorbed and co- on Al hydroxide ocks:

    2+ Al(OH)O2M + 2H+ (5) are removed from the spent washing solution (mostlydeposition), the EDTA remained preserved in the solu-)3 is typically amphoteric and, under the alkalinepH 10) maintained during electrolysis, forms negativelyhydroxide:

    H Al(OH)4 (6)e charge of Al hydroxide ocks explains why negativelyTA is not removed and remains recycled in the washingTA is presumably recycled as AlEDTA complex, since

    in abundant concentrations during electro-corrosionode and since they are not electrodeposited (the reduc-ial Al3+/Al is very low, 1.66 V, [13]). Furthermore, theAlEDTA complex formation has been reported to beolution with pH 9 than in solutions with pH 7 and 4 [14]tability of, for example, the Pb and CuEDTA complexreases in solutions with pH > 9 [15].

    ls and methods

    perties

    re used from two locations in the Mezica Valley, Slove-has been exposed to more than three hundred years ofmining and smelting. The rst soil (soil A) was collectedp 30 cm layer of a vegetable garden and it containedg kg1 Pb, 1170 60 mg kg1 Zn and 26 1 mg kg1 Cd.

    soil (B) was collected from abandoned arable land, nowture, and was contaminated with 1431 39 mg kg1 Pb,g kg1 Zn and 12 0 mg kg1 Cd. Standard pedologicals made on homogenized soil samples (Table 1). The pH

    was measured in a 1/2.5 (w/v) ratio of soil and 0.01 Mr solution suspension. Soil samples were analyzed fortter by WalkleyBlack titrations [16], cation exchangeEC) by the ammonium acetate method [17] and soilhe pipette method [18].

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

    a rotating shaker (3040 GFL, Germany) for 24 h at 16 rpm. Approx-imately 850 mL of the washing solution was obtained from eachask after separation of the washing solution from the solid soilphase by centrifugation at 2880 g for 10 min. Fine particles wereremoved fr80 g m2).

    To concefactors impused the samfresh soil.

    3.3. Treatm

    The elec1 L glass aically stirrecathodes (dand the sur1:1. The elewater to kebelow 35 Cdeterminedwas measujana, Slovencalculated b

    tcon =top V

    Vo

    where tcon itrolytic cellvolume of ting solutionequalled 3.7

    During tlated to pH The voltageaddition of ples of treacentrifugedsupernatantrolysis wasPb from thestudy the pon EDTA rectrations of determinedthe washintrochemica

    3.4. Use of r

    The recyobtained afof the wawith 30 mMmeasured acentration wfresh EDTAeach, in ve

    3.5. EDTA d

    The conspectrophoHamano etwashing so

    El. cu rrent density96 mA cm-2

    0

    50

    100

    150

    200

    250

    300

    350

    400

    450

    El. cu rrent density96 mA cm-2

    Fig

    TA chss ofform

    of anic red yl-1volum

    mg L

    etal d

    drieh a 1000)ere ltereion

    Pb, Zith aetal cy. A risonetherQA/Q

    wernd analytical duplicates were also used where appropriate,r to ensure accuracy and precision in the analysis.

    lculation of the specic electricity consumption

    cic electricity consumption (q) for treatment of the washingn was calculated using the following equation (Eq. (8)):

    I tm

    (8)

    U is the voltage measured during the experiment (in V), I the electrical current (in A), t the operation time at breaking

    (in h) and m the amount of metals (Pb, Zn, Cd and Fe) (in g)ed from the washing solution in operation time t. The break-nt was determined by segmented regression using packageented in program R [20] using data on the metal (Pb, Zn,

    Fe) concentrations change vs. time. Regression indicatedgments of linear relation, as shown in Fig. 1. The breakingom the washing solutions by ltration (lter paper

    ntrate metals in the washing solutions (used to assessortant for specic electricity and Al consumption), wee washing solution for two or three extractions of the

    ent of the washing solutions in the electrolytic cell

    trolytic cell was composed of electrodes placed in ask in which 500 mL of washing solution was magnet-d. The Al anode was placed between two stainless steelistance = 10 mm). The overall anode surface was 63 cm2

    face area ratio between the cathodes and anodes wasctrode cell was cooled using a cooling mantle and tapep the temperature of the treated washing solution. Electrical current densities were adjusted to one of four

    values: 64, 96, 192 and 288 mA cm2. The cell voltagered with a DC power supply (Elektronik Invent, Ljubl-ia). The contact time of electrochemical treatment wasy Eq. (7):

    con

    p(7)

    s the contact time of the washing solution in the elec-, top is the operation time of the experiment, Vcon is thehe electrolytic cell and Vop is the volume of the wash-

    in the experiment. Initially, 30 min of operation time8 min of contact time.he process, the pH of the washing solution was regu-10 by drop-wise addition of 5...

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