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Wat. Sci. Tech. Vol. 37, No.8, pp. 65-71, 1998.© 1998 fAWQ. Published by Elsevier Science Ltd

Printed in Greal Britain.0273-1223/98 $19'00 + 0'00

SURFACTANT ENHANCEDREMEDIATION OF CADMIUMCONTAMINATED SOILS

Ruey-an Doong, Ya-Wen Wu and Wen-gang Lei

Department ofNuclear Science, National Tsing Hua University, Hsinchu,Taiwan 30043, Republic ofChina

ABSTRACT

An investigation involving the addition of surfactant to remediate cadmium-contaminated soils wasperfonned to detennine the optimal surfactant enhanced remediation system. Anionic (sodium dodecylsulfate, SOS), nonionic (Triton X-IOO, TXIOO) and cationic (cetyltrimethylammonium bromide, CTAB)surfactants were used to elucidate the extraction efficiency of surfactant. EOTA and diphenylthiocarbazone(OPe) were also added to enhance the extraction efficiencies of surfactants. Moreover, the pH effect wasexamined to detennine the optimal surfactant systems. The addition of anionic and nonionic surfactants canenhance the desorption rates of cadmium, lead and zinc, whereas the addition of cationic surfactant decreasedthe desorption efficiency of heavy metals. The desorption efficiency was found to increase linearly with theincreasing surfactant concentration below critical micelle concentration (CMC) and remained relativelyconstant above the CMC. Moreover, the addition of EOTA can significantly enhance the desorptionefficiency of heavy metals. Cationic surfactant was shown to be a more effective surfactant than nonionicand anionic surfactants in extracting heavy metals under acidic environment. The desorption efficiency ofheavy metal in the surfactantlEOTA mixture system was in the order of Cd > Pb> Zn. However, the additionof OPC lowered the heavy metal removals by 2 to 4 limes. Also, increasing pH value can decrease theextraction capabilities of nonionic and anionic surfactants. The results of this study demonstrate thatsurfactant in combination with complexing agents can be effectively used as chemical amendments to flushcadmium-contaminated soil by proper selection of type and concentration of surfactant and complexing agentat different pH values. © 1998 IAWQ. Published by Elsevier Science Ltd

KEYWORDS

Complexing agent; critical micelle concentration; desorption; heavy metals; remediation; surfactant.

INTRODUCTION

The contamination by heavy metals of soil environments has recently received much urgent attention. Manysuperfund sites are contaminated with toxic trace heavy metals in Taiwan. Among those sites, the cadmium­contaminated soil near Taoyuan County in northern Taiwan has been contaminated for two decades.According to the EPA guidelines, the 0.1 N HCI extractable concentrations of cadmium and lead, averaging19.4 mg/kg and 31.0 mg/kg, respectively, are sufficiently high to trigger an emergency cleanup.

The use of surface-active agents to enhance remediation of contaminants has recently received increasinginterest (Abdul et at., 1992; Shiau et al., 1994; Sabatini et ai., 1995; Doong et ai., 1996). The addition ofsurfactant can enhance the solubilization of many hydrophobic organic compounds, such as polynuclear

65

66 R.-A. DOONG et al.

aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and chlorinated hydrocarbons byincreasing the solubility of the contaminants via micellar solubilization (Edwards et at., 1991). Recently,Nivas et at. (1996) reported that the addition of surfactants can enhance the remediation of subsurfacechromium contamination. Moreover, several researchers reported that surfactant in combination with acomplexing agent has an even greater capability in extracting heavy metals from contaminated soils (Tondreet at., 1993; Ismael and Tondre, 1994). However, the role of surfactants in remediation of metal­contaminated soil is site specific, depending on the pH, matrix of soil, speciation of metal, and organiccontent. Also, the role of surfactant and complexing agent on the remediation of cadmium-contaminated soilstill remains unclear.

This study attempts to determine the optimal surfactant system for enhancing the remediation efficiency ofcadmium-contaminated soil. Anionic (sodium dodecyl sulfate, SDS), nonionic (Triton X-IOO, TXIOO) andcationic (cetyltrimethylammonium bromide, CTAB) surfactants were used to elucidate the extractionefficiency of surfactant. Also, complexing agents, such as EDTA and diphenylthiocarbazone (DPC) wereadded to enhance the extraction efficiencies of surfactants. Moreover, the pH effect was examined fordetermining the optimal surfactant systems.

MATERIALS AND METHODS

The surfactant, SDS, Triton X-IOO and CTAB were obtained from Merck Co., Darmstadt, Germany.Diphenylthiocarbazone(C6HsNHCSNHC6Hs) was obtained from Barker Chemical Co. Chloroform, used todissolve DPC during the preparation of the DPC reagent, was purchased from Tedia Co. (USA). Doublydistilled water (Millipore Co., Bedford, Mass., USA) was used throughout the experiment unless otherwisementioned. All other reagents were analytical grade and were used without further treatment.

The characteristics of the surfactants are described in Table I. The hydrophilic-lipophilic balance (HLB) andaggregation number values were obtained from the literature (Kile and Chiou, 1989; Shiau et at., 1994;Doong et at., 1996). The critical micelle concentrations (CMC) of SDS and Triton X-IOO were determinedby conductivity and turbidity measurements, respectively. Surfactant solutions of varying concentrationwere made with surfactant stock and deionized water. Multiple testing of each surfactant solution wasperformed to ensure consistent readings.

Table I. Characteristics of surfactants used in this study

surfactant type chemical formula M.W. HLB CMC aggregation(g) (mg/L) number

TritonX·lOO nonionic CSH17C6H4(OC2H4)loOH 625 13.5 268 100-155SDS anionic C12H2S0S03~a+ 288 40 2300 71CTAB cationic C16H33N(CHl)3+Bi 364 361

The soil used in this study was cadmium-contaminated soil obtained from Taoyuan County in northernTaiwan. It consists of 22% sand, 45% silt and 33% clay. The organic matter content of 0.0945 wasdetermined by combustion at 550°C. The pH value and the cation exchange capacity (CEC) were 5.8 and 8.8cmol/kg-soil, respectively. The average O.IN HCI extractable concentrations of cadmium (Cd), lead (Pb) andzinc (Zn) of the soil utilized in this study were 14.7 mg/kg, 29.2 mg/kg and 45.9 mg/kg, respectively.

Batch experiments were performed by containing 5g of soil with 50 mL surfactant solution in 125 mLErlenmeyer flasks. The series of reactors were equilibrated in an orbital shaker for 24 hr at 150 rpm and at25±O.2°C. Equilibrated samples were then centrifuged at 6000 rpm, filtered with 0.45 11m membrane anddigested with HN03 and HCI04 to remove soil particles and interferences. The supernatants were thenanalyzed for cadmium, lead and zinc by ICP-OES. The pH values of solutions were adjusted with phosphatebuffer solution to yield pH 2.3, 7.1 or 12.3.

Surfactant enhanced remediation of cadmium contaminated soils

RESULTS AND DISCUSSION

67

Effect of surfactants. Figure I illustrates the extraction concentration of zinc, cadmium and lead in thesurfactant-amended systems under neutral condition. The addition of anionic and nonionic surfactants canenhance the desorption of heavy metals from contaminated soil. SDS was shown to be a better surfactantthan Triton X-loo in extracting heavy metals. The enhancement ratio, based on the concentration ratio ofheavy metals extracted from surfactant-amended system to that of blank control, were 1.65 to 6.43 and 1.02to 4.11 for SDS- and TXloo-amended systems, respectively (Table 2). However, the amendment of cationicsurfactant lowered the desorption of heavy metals. As illustrated in Table 2, only 10% to 60% of thedesorption efficiency relative to that of blank control was demonstrated, implying that cationic surfactant isnot suitable for enhancing the desorption of heavy metals under neutral condition. This may be due to thestrong complexation of surfactant with soil matrix. Moreover, different desorption efficiencies among theheavy metals were observed. The desorption efficiency increased in the order of lead> zinc > cadmium,depicting that the desorption of lead was more easily achieved in the presence of anionic and/or nonionicsurfactants (Table 2).

15 EJ_ Pb

~Cdl

'-"'

-, 10

8

I5

0crAB

Figure I. The desorbed concentration of lead, cadmium and zinc in the surfactant-amended systems.

Table 2. The enhancement ratio and desorption efficiency of heavy metals with the amendment of differenttypes of sUlfactants

enhancement ratio· desorption efficiency (%)••surfactant

Zn Pb Cd Zn Pb Cd

SDS 6.43 2.49 1.65 29.6 51.8 15.5TXIOO 4.11 1.02 1.53 18.9 20.8 14.4CTAB 0.53 0.10 0.60 2.46 1.91 5.60

• :enhancement ratio is the concentration ratio of heavy metals extracted from soil by surfactant tothat of distilled water.

•• :Desorption efficiency is defined as the percentage of the extractable concentration by surfactantsto that ofO.IN HCI extractable concentration.

68 R.-A. DOONG el ai.

Surfactant concentration is an important factor influencing the desorption efficiencies of heavy metals.Figure 2 illustrates the desorption of cadmium with the amendment of different concentrations ofsurfactants. The desorbed concentration was found to linearly increase with the increasing surfactantconcentration below critical micelle concentration (CMC), and remained relatively constant above the CMC.At equilibrium, the desorbed concentrations of cadmium were 1.87 mglkg and 3.27 mglkg in TXlOO- andSDS-amended systems, respectively. This corresponds to 12.7% and 22.3% desorption efficiency ofcadmium, respectively. Also, a 1.5- to 2- fold increase in maximum Cd removal relative to that of blankcontrol was obtained, suggesting that the addition of nonionic and anionic surfactants can enhance thedesorption of heavy metals. However, the addition of cationic surfactant linearly decrease the desorptionefficiency of cadmium with the increasing concentration below CMC. The ratio of cadmium removal bycationic surfactant to that of blank control was 0.53, showing that the desorption of heavy metals could behindered when cationic surfactant was added to the system.

4.0

3.5

i 3.0

'I25

20

1.5 1-:=001___._s:li

I --....-CfAB

0.5

0.00 2 4 6 8 10

aveFigure 2. The desorbed concentration of cadmium in the different concentrations of surfactants.

The possible mechanisms for the extraction of heavy metals by surfactants include ion exchange,precipitation-dissolution and counterion exchange (Rosen, 1979). Navis et al. (1996) depicts that counterionexchange could promote dissolution of precipitated heavy metals when the concentration of surfactantexceeds the CMC, thereby enhancing the removal of chromium from the soil. Another mechanism forenhanced heavy metal removal is ion exchange. Since micelles are not directly involved in ion exchange, theexchangeable ions will increase below the CMC and remain relatively constant above the CMC. However,the results obtained in this study indicate that the enhancement of heavy metal removal mainly occurs belowthe CMC and slowly increases above the CMC. This suggests that counterion binding may not be a majormechanism in Cd extraction. Ion exchange may be the dominant mechanism for enhancing cadmiumextraction from contaminated soil in this study.

Effect at" complexint: at:ents. Recent research has demonstrated that the addition of ligands together withsurfactants can enhance the micellar solubilization of copper ions in a micellar-enhanced ultrafiltrationsystem (Tondre et al., 1993). To further understand whether a micellar-solubilized complexing agent canenhance heavy metal removal relative to use of surfactants only, solubilization assays were conducted usingEDTA and DPC a~ the complexing agents. Figure 3 illustrates the desorbed concentrations of lead, zinc, andcadmium with the amendment of different concentrations of EDTA only (no addition of surfactant). The

Surfactant enhanced remediation of cadmium contaminated soils 69

addition of EDTA can significantly enhance the removal of heavy metals and the extraction efficiencyincreased with an increasing concentration. However, the extent of desorption differed among the heavymetals. Better desorption efficiency was found for cadmium and least for zinc. Also, different extractingagent concentrations at maximum heavy metal removal was observed. The optimal concentrations of EDTAin enhancing the desorption were ImM for Cd and Zn and 50mM for Pb, showing that only small amount ofEDTA is enough for significantly enhancing the desorption of cadmium and zinc.

100

~Zn

~Cd

--6--Pb

20

°o!:-----'-~=-- .........--40-=---'--~ro;---L...--~---'---7,looIDTAcon:ematDn(nM)

Figure 3. The desorption efficiency of zinc, lead, and cadmium in the EDTA-amended systems. The concentrationof EDTA ranged from I mM to 100 mM.

~Zn

--o--Pb____ Cd

.",.

.=

oO~---'---5r----'-----;1;inO--"---111c5--"---~20ope con;ertrati>n (nM)

Figure 4. The desorbed concentration of heavy metals in the different concentrations of OPe.

70 R.-A. DOONG et al.

The addition of DPe lowered the desorption efficiencies of heavy metals. As shown in Figure 4, the additionof DPC lowered the desorption efficiency of cadmium and zinc by 3 to 4 times. No obvious enhancementeffect on the removal of lead was observed. The minimum desorbed concentrations of cadmium and zincoccurred at 0.4 mM and 4 mM, respectively. This result implies that DPC would hinder the remediation ofcontaminated soils.

Figure 5 illustrates the desorption of heavy metals with the amendment of different surfactants andcomplexing agents under acidic condition (pH = 2.3). Different extraction capabilities of surfactants andcomplexing agents were demonstrated. Solubilization of complexing agents into cationic surfactant solution(CTAB) was shown to be a more effective treatment than nonionic and anionic surfactants in extracting leadunder acidic condition. However, no significant difference in cadmium desorption was observed,demonstrating that the enhancement of desorption of heavy metals was primarily attributed to EDTA.Noting that the addition of DPC lowered the heavy metal removals by 2-4 times, suggesting that EDTA is amore effective complexing agent than DPe for enhancing the extraction of heavy metals.

40

I35

~30_Pb

!1222I Zn'-"s:: 25

'j 20

e! 150()

~10

5

~

Figure S. TIle desorbed concentration of heavy metals with the surfactants and complexing agents under acidiccondition (pH =2.3).

soPh

,~'-"s::.~

30

5e! 200()

~0

~

Figure 6. The pH effect surfactants and complexing on the desorption agents.

Surfactant enhanced remediation of cadmium contaminated soils 71

Effect of pH. Figure 6 illustrates the pH effect on the desorption of lead with the amendment of differentsurfactants and complexing agents. The extraction capabilities of nonionic and anionic surfactants decreasedwith the increase of pH value in the EDTA-amended solutions. This implies that counterion binding and/orprecipitation may be the major mechanisms for the extraction of lead under alkaline environment. Littleenhancement of lead in DPC-amended system was observed relative to that of blank control, suggesting thatthe extraction of DPC-amended system was independent of pH. Moreover, the comparison between Figures5 and 6 demonstrates that pH is a more important factor than the nature of surfactant in remediating lead­contaminated soils.

CONCLUSIONS

The results of this study demonstrate that surfactant in combination with complexing agents can beeffectively used to treat the cadmium-contaminated soil by proper selection of type and concentration ofsurfactant and complexing agent at different pH values. Anionic surfactant was demonstrated to be mosteffective in remediating the cadmium-contaminated soil. The addition of EDTA can significantly enhancethe desorption of heavy metals, whereas the amendment of DPC lowered the extraction efficiency by 2 to 4times. The anionic surfactant with EDTA was shown to be the most effective treatment in remediatingcadmium-contaminated soils.

ACKNOWLEDGEMENTS

The authors would like to thank the National Science Council, R.O.C. for the financial support for this studyunder Contract No. NSC86-2621-P-007-002.

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solubilized complexing agentsof various HLB using ultrafiltration. Langmuir, 9, 950-955.