Washing of Cadmium(II) from a Contaminated Soil Column

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  • This article was downloaded by: [Memorial University of Newfoundland]On: 10 October 2014, At: 00:03Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

    Journal of Soil ContaminationPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/bssc19

    Washing of Cadmium(II) from a Contaminated SoilColumnAllen P. Davis a , Dilip Matange a & Mohammad Shokouhian aa Environmental Engineering Program, Department of Civil Engineering, University ofMaryland, College Park, MD 20742Published online: 22 Sep 2010.

    To cite this article: Allen P. Davis , Dilip Matange & Mohammad Shokouhian (1998) Washing of Cadmium(II) from aContaminated Soil Column, Journal of Soil Contamination, 7:3, 371-393

    To link to this article: http://dx.doi.org/10.1080/10588339891334276

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  • Copyright 1998, CRC Press LLC Files may be downloaded for personal use only.Reproduction of this material without the consent of the publisher is prohibited.

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    Journal of Soil Contamination, 7(3):371393 (1998)

    INTRODUCTION

    HERE are numerous documented instances of contamination of surface andsubsurface soils by heavy metals. Cadmium is a metal of major environmen-

    tal concern due to its high toxicity and relatively high mobility in the terrestrialenvironment (Boekhold et al., 1991). At the present time, there are limited tech-nologies available for treating metal-contaminated soils. Thus, development ofnew and effective techniques for soil treatment is essential.

    Washing of Cadmium(II)from a Contaminated Soil Column

    Allen P. Davis*, Dilip Matange, andMohammad Shokouhian

    Environmental Engineering Program;Department of Civil Engineering; University ofMaryland; College Park, MD 20742

    The washing of cadmium (from CdO(s)) froma soil column employing either an acidsolution or EDTA (a strong metal chelator)was examined. For Cd(II) levels of 50 to1000 mg/kg, the fraction removed wasessentially independent of the initial Cd(II)concentration. The most efficient washingof cadmium was achieved using an acidwash solution at pH 2.5. Lower Cd(II) re-movals were found at lower pH, apparentlydue to inhibition of CdO(s) dissolution byconstituents released from the soil underhighly acidic conditions. EDTA wash solu-tions were employed at EDTA:cadmiummolar ratios ranging from 1:1 to 10:1. Up to90% removal of total Cd(II) was achievedat the 10:1 ratio after the passage of thefirst 50 PV of wash solution. Although higherchelate levels enhanced Cd(II) removal,the utilization efficiency of EDTA for cad-mium decreased.

    KEY WORDS: EDTA, acid solution, CdO(s), Cd(II).

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    One successful technology for removing heavy metals from contaminated soilis soil washing. In the soil washing process, the soil is excavated and the soilparticles are vigorously mixed with a wash solution. Metal-rich soil particles areremoved, other metals affiliated with the soil are solubilized, and the wash solutionand clean soil are separated. Generally either a strong mineral acid or a strongmetal chelator is used to chemically enhance the metal removal. For example,several experiments were carried out by Esposito et al. (1989) on soil artificiallycontaminated with either 20 or 1000 mg Cd(II)/kg soil, as well as other heavymetals. Addition of the chelating agent ethylenediaminetetraacetic acid (EDTA)was found to increase the efficiency of metal removal. A 3:1 molar ratio of EDTAto total metal contaminant present in the soil produced optimum metal removal.Peters and Shem (1992) also found EDTA to be efficient in the washing of leadfrom an artificially contaminated soil. Tuin and Tels (1990b) found that 75 to 81%of the Cd in contaminated soils could be removed via washing with 0.1 N HCl. Ofsix metals washed from the soil, cadmium was the easiest removed.

    A number of soil parameters must be considered in order to evaluate theeffectiveness of a soil washing process. Soils with a greater percentage (>50%) ofsand and gravel respond better to soil washing due to the difficulties of removingcontaminants from chemically active clay material (Van Benschoten et al., 1994).Other factors that affect metal extraction efficiency from soils include the metalcontamination history of the soil, the liquid-soil extraction ratio, initial metalconcentrations, and extraction kinetics (Tuin and Tels, 1990b; Van Benschoten etal., 1994).

    The in situ process of soil washing, in which the washing solution is in somemanner forced through the in-place soil matrix, is known as soil flushing. Effi-ciency of metal removal via soil flushing is less controllable than with excavationand soil washing due to the variable in situ soil characteristics. The characteristicsof the washing fluid that is applied to the contaminated soil are chemicallymodified through interactions with the soil. Thus, the efficiency of a soil flushingsystem varies as the flushing continues. Davis and Singh (1995) examined theflushing of a soil column contaminated specifically with zinc(II) solids. IncreasedZn(II) removal efficiency was found at lower pH and with increasing concentra-tions of EDTA. Also, the form of the Zn(II) contamination played an important rolein the overall washing efficiency.

    In the present study, several parameters that affect the efficiency of soil flushingfor Cd(II) removal are investigated. The soil is artificially contaminated employingCdO(s) to fix the Cd(II) loading. A specific cadmium phase is used because it islikely that highly contaminated soils contain metals present as distinct geochemicalphases in addition to sorbed species, as noted in several cases for lead-contami-nated soils (Hessling et al., 1990; Van Benschoten et al., 1997). Tuin and Tels(1990a) noted the majority of cadmium in the exchangeable fraction based onsequential extraction on actual contaminated soils, although the metal was distrib-uted among all fractions. Nevertheless, subsequent sorption phenomena are impor-

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    tant to the success of the flushing process since metals that become dissolved fromthe metal solids still will interact with the soil surface. Specific parameters inves-tigated include effects of initial cadmium loading of the soil, pH of the washsolution, and addition of a strong chelating agent (EDTA) at various concentra-tions.

    MATERIALS AND METHODS

    The methodology used for this work is similar to the one that was used by Davisand Singh (1995) for the washing of zinc(II)-contaminated soil. The soil for thisstudy was taken at a depth of two feet from an open grass field at the Universityof Maryland, College Park, MD. Some of the physical and chemical properties ofthe soil were determined by the Department of Agronomy, University of Maryland,indicating a sandy loam soil fractionated as 74% sand, 16% silt, and 10% clay. Thesoil pH was 6.1 and the cation exchange capacity (CEC) was 3.41 meq/100 g. Thesoil was air-dried and sieved through a 1.18-mm opening sieve. Batches of 500 gof the soil were dry-spiked with powdered cadmium oxide (Fisher Sci.) to producecontamination ranging from 50 to 1000 mg Cd/kg soil. The contaminated soil wasshaken overnight in a plastic bottle to disperse the cadmium solids throughout thesoil.

    The cadmium content of the unspiked soil was determined using a strong acidextraction, following the methodology presented by Berrow and Stein (1983). Onegram of soil was mixed with 10 mL of 6 M HCl in a 100-mL Pyrex beaker andheated to complete dryness on a steam bath. This process was repeated four times.The residue was extracted with 0.06 M HCl, filtered, and the filtrate was analyzedfor cadmium by graphite furnace atomic absorption spectrophotometry. The totalcadmium in the original soil was calculated as 0.2 mg/kg, and thus is negligible ascompared to the 50 to 1000 mg/kg loading of the contaminated soil.

    In each washing experiment, 11.9 g of contaminated soil was packed in aPlexiglass column (1.9 cm internal diameter, 5 cm length), producing a 3-cm soilcolumn with a bulk density of 1.4 g/cm3. Aluminum foil-covered rubber stoppersand a small amount of glass wool were used to seal both ends of the column. Washsolutions of various concentrations of acid or EDTA were used to remove cad-mium from the soil, all at a fixed ionic strength of 5 102 M NaNO3 (E.M.Science). Water used in all experiments was deionized using a Hydro-Servicereverse osmosis/ion exchange apparatus (Model LPRO20). The effects of changein pH of the wash solution from 6 to 2, and change in concentration of EDTA(tetrasodium, Fisher; 1.6 105 to 1.1 103 M, equivalent to total solutionEDTA:total soil Cd(II) molar ratios from 1:1 to 10:1) at pH 6 were studied.

    The washing solutions were introduced from the bottom of the column tosaturate the soil at a flow rate of 3 mL/min (velocity = 1.06 cm/min). Approxi-mately 5.5 h were required to pump 1 L (250 pore volumes) of extraction solution

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    through the soil column. A fraction collector (ISCO Retriever II) using 15 and 30mL vials was employed to capture the column effluent samples, which wereacidified with HNO3 (Fisher). Dilutions up to 500 times were made and sampleswere analyzed for cadmium using a Varian Tectron Model AA5 atomic absorp-tion spectrophotometer (AAS), flame configuration. A detection limit of approxi-mately 0.1 mg/L Cd(II) was obtained with a linear range up to 3 mg/L. All labmaterials and glassware were acid-washed to ensure complete removal of anyadsorbed metals.

    Batch experiments were also carried out to evaluate the Cd(II) adsorptioncharacteristics of the soil with respect to pH variation. Solutions of three differentconcentrations of Cd(NO3)2 (6 to 30 mg/L as Cd) were prepared (I = 5 102 MNaNO3). Twenty 125 mL Nalgene bottles were each filled with 100 mL of theprepared solution. To 19 bottles, 1 g of soil was added. The initial pH of the bottleswas adjusted over the range of 1 to 8. The bottles were placed overnight in a shaker,after which the final pH of each sample was measured. The samples were filteredusing 0.2 m membrane (Gelman) filters (disregarding the initial filtrate to mini-mize possible effects from filter sorption), and analyzed for dissolved Cd(II) usingAAS. The amount adsorbed was calculated from the difference between the samplewithout soil and the final dissolved Cd(II) concentrations. Experiments were alsocompleted with the addition of 103 M Fe(NO3)3 (at low pH only) and 103 MCa(NO3)2 to the Cd(II) solution to evaluate effects of these metals on Cd(II)adsorption characteristics.

    Finally, several experiments were completed to investigate preliminarily thedissolution kinetics of the CdO(s) that was used as the contaminant source. Solu-tions of 5 102 M NaNO3 (300 mL) were prepared and adjusted to pH 2.0, 2.5,and 3.0 using HNO3. One gram of CdO(s) was added to these solutions; subse-quently ten samples were taken at one minute interval. The samples were rapidlyfiltered (0.2 m) and analyzed for dissolved Cd(II) using AAS. Dissolution experi-ments at these pH values were also completed individually in the presence ofCa(NO3)2, Fe(NO3)3, Al(NO3)3, and NaH2PO4, each at 103 M.

    RESULTS AND DISCUSSION

    Effect of Cadmium Loading on Washing Efficiency

    Figure 1A shows the cumulative Cd(II) removal efficiency of a wash solution atpH 6 for initial cadmium loadings of 50 to 1000 mg Cd(II)/kg. Only 11 to 22% ofthe total cadmium in the soil column is removed after the passage of 250 porevolumes (PV) of the wash fluid. Approximately one half of the removal occurswithin the passage of the first 50 PV. The removal efficiency is greatest for 100 mgCd(II)/kg and lowest for 300 mg Cd(II)/kg, although the difference is not large.The fraction Cd(II) removed by the wash was independent of the initial cadmium

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