recycling of edta solution after soil washing of pb, zn, cd and as contaminated soil
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Chemosphere 86 (2012) 843–846
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Chemosphere
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Technical Note
Recycling of EDTA solution after soil washing of Pb, Zn, Cd and As contaminated soil
Maja Pociecha, Domen Lestan ⇑Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia
a r t i c l e i n f o a b s t r a c t
Article history:Received 28 July 2011Received in revised form 2 November 2011Accepted 3 November 2011Available online 26 November 2011
Keywords:Contaminated soilToxic metalsSoil washingEDTA recycling
0045-6535/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.chemosphere.2011.11.004
⇑ Corresponding author. Address: Centre for SoilBiotechnical Faculty, University of Ljubljana, JamnikSlovenia. Tel.: +386 01 423 1161; fax: +386 01 423 1
E-mail address: [email protected] (D. Lesta
Soil washing with EDTA is known to be an effective means of removing toxic metals from contaminatedsoil. A practical way of recycling of used soil washing solution remains, however, an unsolved technicalproblem. We demonstrate here, in a laboratory scale experiment, the feasibility of using acid precipita-tion to recover up to 50% of EDTA from used soil washing solution obtained after extraction of Pb(5330 mg kg�1), Zn (3400 mg kg�1), Cd (35 mg kg�1) and As (279 mg kg�1) contaminated soil. Up to100% of EDTA residual in the washing solution and 100%, 97%, 98% and 100% of initial Pb, Zn, Cd andAs concentration in the solution, respectively, were removed in an electrolytic cell using a graphite anode.We employed the recovered EDTA and treated washing solution to prepare recycled soil washing solutionwith the same potential for extracting toxic metals from soil as the original. The efficiency of soil washingdepends on the EDTA concentration. Using twice recycled 30 mmol EDTA kg�1 soil, we removed 44%, 20%,53% and 61% of Pb, Zn, Cd and As, respectively, from contaminated soil.
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1. Introduction Several approaches to recycling EDTA from used soil washing
Soil contamination with potentially toxic trace elements (PTTEs)is ubiquitous and frequently results in human poisoning and eco-system deterioration. Technically and economically feasible reme-diation techniques aimed at removing PTTEs from the soil areurgently needed. Soil washing (extraction) with EDTA has becomethe focus of interest as a potentially environmentally sustainabletechnique (Kim et al., 2003). Chelating agents such as EDTA extractPTTEs from soil solid phases into the washing solution as water-sol-uble complexes, without significant interactions with the soil, thuspreserving the soil as a natural resource.
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 washing must 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 kg�1), it ischemically stable and therefore requires a significant energy inputfor 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.
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and Environmental Science,arjeva 101, 1000 Ljubljana,
088.n).
solution 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 significant 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)2
under alkaline conditions, resulting in almost complete recovery ofmetals through precipitation in the form of insoluble metal sulp-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.
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-lowed by electrochemical removal of PTTEs using a graphite anode.
2. Materials and methods
2.1. Soil washing
Soil was collected from a vegetable garden in the Mezica Valley,Slovenia. The valley has been exposed to more than three hundred
844 M. Pociecha, D. Lestan / Chemosphere 86 (2012) 843–846
years of active lead mining and smelting. The soil sample containedof 5330 ± 690 mg kg�1 Pb, 3400 ± 190 mg kg�1 Zn, 35 ± 6 mg kg�1
Cd and 279 ± 91 mg kg�1 As. The soil washing solution was ob-tained after extraction of 75 kg of soil with 75 L of Na2-EDTA solu-tion (60 mM EDTA kg�1 soil) in a concrete mixer for 2 h. Afterextraction, the soil suspension was first filtered through a 2 mmsieve and the soil solid phase was then separated from the wastesoil washing solution in a chamber filter press (filter cloth thickness0.6 g cm�2, air permeability 22 dm3 dm�2 min�1, Ecotip, Slovenia).The concentration of EDTA, Pb, Zn, Cd and As in the waste soil wash-ing solution was 11600, 1110, 267, 7.1 and 61 mg L�1, respectively,the solution pH was 7.3.
2.2. EDTA recovery
EDTA was separated from the used soil washing solution byadjusting the pH of the solution to values between 1 and 2 using37% HCl. The experiment was performed in triplicate. After shakingthe acidified 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.
2.3. Electrochemical treatment
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 (flow rate 14 mL s�1). DC power supply (Elektronik Invent,Ljubljana, Slovenia) provided constant electrical current densityof 44 mA cm�2. 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-plied by the operation time (initially 19 min of operation timewere 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.
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2.4. Efficiency of recycled EDTA
Recycled washing solution with 30 mmol EDTA kg�1 soil wasprepared by dissolving the recovered EDTA (acid-precipitated asH4EDTA) in the washing solution obtained after electrochemicaltreatment (initial pH 10, final pH 9.3, the solution contained5.3 g L�1 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.
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Fig. 1. Reclamation of EDTA from the soil washing solution at different pH valuesafter acidification 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 < 0.05).
2.5. EDTA determination
The concentration of EDTA in the samples was determinedspectrophotometrically at 535 nm according to the procedure de-scribed by Hamano et al. (1993).
2.6. Metal determination
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 filtered through Whatman no. 4 filter paper (pore size20–25 lm) and diluted with deionized water up to 100 mL. Thepseudo-total concentration of Pb, Zn, Cd and As were determinedby flame (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 ofquantification for Pb, Zn, Cd and As were 0.1, 0.01, 0.02 and0.09 mg L�1, respectively. Reagent blank and analytical duplicateswere also used where appropriate, in order to ensure accuracyand precision in the analysis.
2.7. Statistics
The Duncan multiple range test was used to determine the sta-tistical significance (p < 0.05) between different treatments, usingthe computer program Statgraphics 4.0 for Windows.
3. Results and discussion
Initial extraction using 60 mmol kg�1 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 specific Znfractionation in the residual soil fraction of the Tessier’s sequentialextraction procedure (Finzgar and Lestan, 2008).
The first step of the EDTA recycling process was acidification ofthe 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, acidification of used soil washing solution leadsonly to partial EDTA precipitation. The percentage of precipitatedEDTA decreased statistically significantly 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).
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Fig. 2. Concentration of Pb, Zn, Cd and As and in the soil washing solutions during electrochemical treatment at pH 2, 4, 8 and 10. Error bars represent standard deviationfrom the mean value (n = 3).
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.
Electrodeposited (%) In solution (%) Precipitated (%)
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
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
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
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
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After 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 fluorine. The important parameter of an anodic oxidationprocess is therefore the selection of the anode material, with a suf-ficient 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 modified 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 signifi-cantly. The final pH values after electrochemical treatment were5.8–6.3, 8.3–8.4, 8.9–9.2 and 9.1–9.2 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, 6.8–8.7 V for a solutionwith pH 4, 7.0–8.4 V for a solution with pH 8 and 6.6–8.2 V for asolution with pH 10.
As shown in Fig. 2, the electrochemical removal of PTTEs wasmore effective when an alkaline initial pH of the treated solution
was applied. Fe precipitated abruptly (presumably as insoluble Fehydroxides) from solutions with an initially alkaline pH. These data
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Fig. 3. Concentration of Pb, Zn, Cd and As in the soil washing solution afterextractions of soil with fresh and recycled EDTA (30 mmol kg�1) solution atdifferent pH values. The closed square at pH 6.5 denotes washing solution preparedfrom twice recycled EDTA. Error bars represent standard deviation from the meanvalue (n = 3).
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may indicate that EDTA complexes with Pb, Zn, Cd and Fe are moreeasily degradable in alkaline than in acidic conditions. This is alsoevident from measurements of the EDTA concentration. After105 min (contact time) of the electrochemical treatment, the EDTAconcentration decreased from an initial 11600 mg L�1 to a final101 ± 49, 77 ± 37, 78 ± 5 and 40 ± 24 mg L�1 in solutions with initialpH 2, 4, 8 and 10, respectively. In relation to degradation products,Johnson et al. (1972) reported that a Pt anode oxidised EDTA intoCO2, formaldehyde and ethylendiamine. The final concentration ofPTTEs in the solution (treated under the most efficient regime, withan initial pH 10) was 6.1 ± 5.3, 10.8 ± 9.5 and 0.4 ± 0.03 mg L�1 ofPb, Zn and Cd. Concentration of As was below the limit ofquantification.
The share of Pb, Zn, and As removed from the soil washing solu-tion by electro-deposition on a stainless-steel cathode increasedwith increasing pH of the electrochemical treatment (Table 1). Cdwas removed mainly by precipitation, regardless of the pH of thesolution. The possible mechanism of PTTE precipitation was byabsorption on flocks of Fe oxides and hydoxides, which were visi-ble as reddish debris. Removal of metallic (and organic) pollutants
by coagulation and electro-coagulation with Fe ions is a wellknown method of waste-water treatment (Wong et al., 1997).
Finally, recycled soil washing solutions of different pH wereprepared by dissolving H4EDTA (reclaimed by acidic precipitation)in the washing solution obtained after electrochemical treatment(initial pH 10). H4EDTA was presumably dissolved as Na2EDTA.Fig. 3 shows the potential of the recycled soil washing solutionfor Pb, Zn, Cd and As removal from the contaminated soil at differ-ent pHs of extraction. The results indicated no statistically signifi-cant difference between the efficiency of the recycled EDTAsolution and that of freshly prepared EDTA solution (Duncan test,p < 0.05). The most effective was soil extraction at pH 5, for bothwashing solutions. The extraction efficiency did not deteriorateeven when the recycled washing solution was prepared from twicerecycled EDTA (Fig. 3). Using twice recycled EDTA 44 ± 2%, 20 ± 1%,53 ± 1%, 61 ± 11% of Pb, Zn, Cd and As, respectively, was removedfrom the soil.
4. Conclusions
The results of our initial study indicate that acidic EDTA precip-itation coupled with electrochemical removal of PTTEs and resid-ual EDTA could become a feasible option for treatment andrecycling of used soil washing solutions containing EDTA com-plexes with Pb, Zn, Cd and As, as part of a soil washing process.After electrochemical treatment, PTTEs were easily removed fromthe washing solution as insoluble precipitates and as electro-deposits on a cathode. Both process water and EDTA can be recy-cled in a closed process loop; the recycled EDTA solution retainedall the soil PTTE extraction capacity. The novel treatment methodfor used soil washing solution is robust and characterised by sim-ple equipment, brief retention time and easy operation.
Acknowledgement
This work was supported by the Slovenian Research Agency,Grant L1-2320.
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