The sequential use of washing and an electrochemical reduction process for the remediation of lead-contaminated soils
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The sequential use of washing and an electrochemicalreduction process for the remediation of lead-contaminated soilsAydeniz Demir a & Nurcan Kleli aa Department of Environmental Engineering , Mersin University , Mersin , TurkeyAccepted author version posted online: 01 Aug 2012.Published online: 06 Sep 2012.
To cite this article: Aydeniz Demir & Nurcan Kleli (2013) The sequential use of washing and an electrochemicalreduction process for the remediation of lead-contaminated soils, Environmental Technology, 34:6, 799-805, DOI:10.1080/09593330.2012.717107
To link to this article: http://dx.doi.org/10.1080/09593330.2012.717107
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Environmental Technology, 2013Vol. 34, No. 6, 799805, http://dx.doi.org/10.1080/09593330.2012.717107
The sequential use of washing and an electrochemical reduction process for the remediation oflead-contaminated soils
Aydeniz Demir and Nurcan Kleli
Department of Environmental Engineering, Mersin University, Mersin, Turkey
(Received 31 January 2012; final version received 26 July 2012 )
A two-step method for the remediation of three different types of lead (Pb)-contaminated soil was evaluated. The first stepincluded soil washing with ethylenediaminetetraacetic acid (EDTA) to remove Pb from soils. The washing experimentswere performed with 0.05 M Na2EDTA at 1:10 soil to liquid ratio. Following the washing, Pb removal efficiency from soilsranged within 5070%. After the soil washing process, Pb2+ ions in the washing solution were reduced electrochemically ina fixed-bed reactor. Lead removal efficiency with the electrochemical reduction at 2.0 V potential ranged within 5776%.The overall results indicate that this two-step method is an environmentally-friendly and effective technology to remediatePb-contaminated soils, as well as Pb-contaminated wastewater treatment due to the transformation of toxic Pb2+ ions into anon-hazardous metallic form (Pb0).
Keywords: lead; EDTA; washing; electrochemical reduction; contaminated soil
1. IntroductionThe lead contamination of soils has become a major con-cern all over the world. Lead can cause very serioushealth problems including damage to the kidneys, the liverand the reproductive system, basic cellular processes andbrain functions . Some physical, chemical and biologi-cal methods have been used for the remediation of heavymetal-contaminated soils. Among thesemethods, the wash-ing technique is the most common one used ex situ orin situ because it demonstrates a very high efficiency forheavy metal removal from both soils and sediments .Acids, bases or chelating agents are being used in general inthe soil-washing technique . Ethylenediaminetetraaceticacid (EDTA) is the most widely used chelating agent forsoil washing, since it forms strong complexes with mostpolluting heavy metals and facilitates their solubilizationfrom the soil into the washing solution . It is relativelyinexpensive compared to other chelants .
EDTA increases Pb2+ leaching due to the formation ofhighly soluble Pb2+-EDTA complexes as follows :
Pbads + Na2EDTA PbEDTA + NaadsPb2+ + EDTA4 PbEDTA2
The main problem associated with soil washing is thegeneration ofmetal ions in the solution phase (PbEDTA2),which requires a further treatment method to remove suchions from solution.
Corresponding author. E-mail: email@example.com
An alternative and effective process to lower the levelsof soil pollutants is the electrochemical method, which isalso called the electrokineticmethod. The electrochemicalmethod is a process in which a low-voltage direct-currentelectric field is applied across a section of contaminatedsoil to move contaminants. The principle of electroki-netic remediation is similar to a battery. After electrodes(a cathode and anode) are induced and charged, particles(e.g. ions) are mobilized by the electrical field. Ions andwater move toward the electrodes. Since it can work at dif-ferent pH values depending on the characteristics of theelectrodes , it has been used abundantly in the treat-ment processes ofmunicipal and industrial wastewater .Within the electrochemical method, special attention isgiven to the electrochemical reduction method. The elec-trochemical reduction method is the reduction of metal ionsfrom an electrolyte in an electric field and is synonymouswith electrowinning, electrolytic recovery, electroextrac-tion or electrodeposition . This method is a processwhich has the potential to keep certain heavy metals out ofthe environment . Electrochemical reduction is an alter-nativemethod for safe disposal. This technology is effectivein improving the treatment quality of industrial wastes,wastewaters and drinking waters on integration into a treat-ment plant or replacement of conventional processes thatare found to be less effective for eliminating specific organicand inorganic pollutants. This technique allows a decreasein the amount of metallic sludge produced by generatingcompact and less voluminous sludge, thereby resulting in
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a cost-saving process . Many researchers use electro-chemical methods for removing heavy metals from soiland wastewater. These studies mainly focus on in situ soiltreatment . In addition, complexing ligands, suchas EDTA, are also known to increase the electrochemicalremediation of soil .
Traditionally each soil treatment method such as soilwashing or electrochemical reduction provides only the dis-placement of the pollutants. In this case, only the shape ofpollutants will be changed and pollutants will not beremoved completely. The application of only one of themethods of soil treatment is not adequate, so combined orsequential soil treatment systems, which are more effec-tive, are being developed to remove pollutants. In recentyears, combined treatmentmethods including electrokineticremediation have been studied for the in situ remediationof heavy metal-contaminated soils [4,18,19].
The main objective of this study is to assessment theapplicability of a sequential or combined use of soilwashingand the electrochemical reduction process to remediate Pb-contaminated soils.
2. Materials and methods2.1. Soil samples and propertiesThree surface soil samples collected at a depth of 030 cmwere identified as S1, S2 and S3. Two of them (S1 andS2) were collected from a vegetable garden (unpollutedsoils) and the other one (S3) was collected from a site withrecent Pb mining and smelting operations (polluted soil).Samples transported to the laboratory in plastic bags wereair-dried and crushed to pass through a 2mm sieve. Aftermixing thoroughly, they were stored in plastic bags priorto the laboratory analyses. Soil samples were analysed forpH and electrical conductivity in a 1:2 (m:v) ratio of soilto water solution suspension after 1 h of contact time ,carbonate (CaCO3) contentwasmeasured by a volumetricalcalcimetermethod as describedbyAllison andMoodie ,particle size analysis by a hydrometric method , Atter-berg limits according to ASTM D4318 procedure ,organic matter (OM) by theWalkleyBlack procedure ,cation exchange capacity (CEC) by the ammonium acetateprocedure , total soil surface area by ethylene glycolmonoethyl ether adsorption (EGME) , elemental analy-sis by X-ray fluorescence (XRF) spectrophotometry (modelXRF Rigaku Rix 2000), and for total Pb concentrationsaccording to DIN ISO 11466  using an atomic absorp-tion spectrometer (AAS) (model SENSAA). Controls of theanalytical procedure were performed using blanks and thereferences (CRM 7003 and CRM 483), which are treated inthe same way as the experimental samples.
2.2. Artificial soil contaminationAs S1 and S2 were unpolluted, they were contaminatedartificially with Pb. About 1 kg soil was thoroughly mixed
with 1 L of deionized water containing 1000mgPb2+(0.005M), which was in the form of dissolved salts ofPb(NO3)2. The slurry was then left to age at room temper-ature for almost three months with frequent and thoroughmixing.
2.3. Washing experimentsA washing procedure was applied to obtain the wash-ing solution and determine the effect of Na2EDTA on Pbremoval from contaminated soils. In these experiments,10 g soil was added to 100mL 0.05M Na2EDTA washingsolution. Then, polypropylene bottles were agitated usingan orbital shaker at a speed of 175 rpm at room temper-ature for 2 h. Peters and Shem  and Evangelista andZownir  observed that extraction of Pb with EDTAwasrapid, with an equilibrium time of 1 h. Therefore, an extrac-tion time of 2 h was chosen for the rest of this study. Aftermixing, the suspensions were centrifuged for about 5minat 5000 rpm. Following centrifugation, 2mL of supernatant(washing solution) were diluted with deionized water, acid-ified to a pH of 2.0 with 1:1 HNO3 and Pb analyseswere performed using AAS; the residue of supernatant wasused in the electrochemical reduction test. All tests wereperformed in triplicate and the results were presented asaverage of the triplicate washing.
After first washing, soil residues in bottles were washedsequentially again two times in order to remove all of thePb from the contaminated soil. Following centrifugation,2mL of supernatant (washing solution) was again acidifiedwith 1:1 HNO3 and Pb analyses were performed.
The percentage of Pb removed was calculated usingan equation similar to the one earlier reported by Wuanaet al.  as:
Percentage Pb removed (%) = C1V1/Csms 100where C1 and Cs are the concentrations of Pb in the super-natant (in mgL1) and soil (mg kg1), respectively; V1 isthe volume of supernatant (in L) and ms is the dry mass ofthe soil (in kg).
2.4. Electrochemical reduction experimentsTo examine the efficiency of the electrochemical reductionof Pb2+ ions in the washing solution including EDTA-Pbcomplexes, electrochemical reduction tests were carried outin fixed-bed reactor as shown schematically in Figure 1.Electroanalytical measurements were carried out with apotentiostat (CH600A Model). Through initial electro-chemical tests in 100mg L1 Pb prepared at 0.05M EDTA,a reduction potential of 1.7V vs. Ag/AgCl was deter-mined as optimum. But, for a better understanding, allexperiments were also carried out below (1.5V) andabove (2.0V) this measured potential. In addition, a fur-ther experiment was performed at 2.5V for only S3.
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Figure 1. A fixed-bed electrochemical reactor.
The electrochemical reduction test was performed in thefirst washing solution in which the concentration of Pbis higher than in second and third washing solutions. Afixed-bed electrochemical reactor with a bed-thickness ofabout 25 mm was used for the electrochemical reduc-tion (Figure 1). Electrode sets consisted of Pb granulesas the cathode and platinum as the inert counter-anode.The electrical potential applied was held constant for eachrun for 2 h. After experiments, electrochemical reactor(including the electrodes) were cleaned with 5% (v/v) HClsolution and then rubbed with a sponge and rinsed withdeionized water. Prior to each experiment, Pb granules(99.99%) were washed with double distilled water and acti-vated electrochemically for 1 h at2.0V standard calomelelectrode (SCE) in 0.2M K2CO3 solution. After activa-tion, the granules showed a characteristic shiny metalliccolor.
During electrolysis, 1mL of samples were taken (15min) from the electrolyte. These samples were diluted withdeionizedwater, acidifiedwith 1:1HNO3 to pH2 and storedin low temperatures for use in further analyses with AAS.The pH and conductivity of the solutions in electrochemicalreactor were also measured.
Removal efficiency of the Pb depending on time wascalculated by the following equation:
R(%) = (Co/Ce) 100
where Co is the initial Pb2+ concentration (mgL1)and Ce is the equilibrium Pb2+ concentration at anytime (mgL1).
All data collected were subjected to a t-test (two-tailed),weighted mean (x) and standard deviation (SD) statisticalanalysis.
3. Results and discussion3.1. Some initial physical and chemical properties of
soils used in the studySome initial physical and chemical properties of soils usedin the study are given in Table 1. As shown there, the tex-ture classes of soils were silty clay loam soil (SiCL) forS1 and S2, the silty loam soil (SiL) for S3. The soil pHvalues of S1 and S2 were neutral (6.9 to 7.6), whereasS3 was a slightly acidic soil (6.1 to 6.8). The amount oforganic matter was very low (12%) in S1, medium (23%) in S2 and high (>4%) in S3. The lime content of S1and S2 was very high (1550%) and low for S3 (48%).The total CEC in S1, S2 and S3 were 27.96, 34.53 and5.58 meq 100 g1, respectively. The results of CEC value,the amount of aluminum and iron oxide, specific surfacearea and soil texture analysis showed that S2 had a muchhigher adsorption capacity compared to S1 and S3. The ini-tial total value o...