Surfactant-Enhanced Removal of Cu (II) and Zn (II) from a Contaminated Sandy Soil
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Batch tests were conducted to know the effectiveness of using surfactants only and surfactants with a complexing agent to remove Cu (II) and Zn (II) from an artificially contaminated sandy soil. SDS (sodium dodecyl sulfate), AOT (alpha-olefin sulfonate) and Tx-100 (Triton X-100) were the surfactants selected as the washing liquids. Complexing agent EDTA (ethylenediaminetetraacetic acid) was also selected for washing the soil. To avoid external factors from interfering with the cleaning process, artificial soil formed by a mixture of clean sand and bentonite was used to form contaminated soil samples. The amount of organic matter present was insignificant. Compared to extraction by distilled water, tests indicated that a six-fold increase in copper extraction occurred due to the presence of surfactants and/or the complexing agent EDTA. Compared to extraction by distilled water, zinc extraction by surfactants and or the complexing agent EDTA was nearly 1.2 to 1.3 times more. Effects of competition as well as interference associated with the adsorption and desorption of these metals are also very briefly reported.
KeywordsSurfactants SDS AOT Tx-100 Complexing agents Soil washing Site remediation Zinc Copper Heavy metals Soil pollution
Heavy metals have densities generally larger than 5 g/cm3. Cadmium, copper, lead, and zinc are commonly encountered hazardous heavy metals and are in the EPA’s list of priority pollutants (Cameron 1992). Heavy metals in soils generally occur in five fractions: exchangeable, carbonate, Fe–Mn oxide, organic and residual fractions. Sewage sludge, fertilizers, pesticides and wastes from metal industries are sources of Zn in the soil. Copper forms complexes with organic matter in the soil and is strongly held on inorganic and organic exchange sites present on soil particles. Hence, it is relatively more difficult to remove copper from soil (Alloway 1990; Cameron 1992). To enhance metal removal, leaching agents, pH adjusters, and surfactants and chelating agents may be added to the wash water (Deuren et al. 2002; Abdul and Gibson 1991). In the ex situ process, the contaminated earth is removed and washed with washing solutions. This type of soil washing is both cumbersome and more expensive (Davis and Singh 1995).
1.1 Adsorption and Desorption of Metals
Cation exchange, specific adsorption, organic complexation and co-precipitation are the mechanisms involved in the adsorption of ions by soil particles (Alloway 1990). Temminghoff et al. (1994) observed that copper desorption from a sandy soil and the complexation of copper by dissolved organic fractions are strongly dependent on pH. Wu et al. (1999) found that copper was preferentially sorbed on organic matter associated with the coarse clay fraction.
The effectiveness of CaCl2, HCl and EDTA (ethylenediaminetetraacetic acid) in removing lead from soils artificially contaminated with a lead compound [Pb (NO3)2] has been determined on the basis of systematic batch (Cline and Reed 1995) and column (Reed et al. 1996) tests. Concentrated HCl may cause the destruction of the soil structure due to mineral dissolution and lead to decreased soil productivity (Reed et al. 1996).
Recently, Christophi and Axe (2000) studied the important phenomena of interference and competition among copper, lead and cadmium while getting adsorbed to a goethite surface. They demonstrated that adsorption increased with metal electronegativitity (Cu > Pb > Cd) and gave a critical review of previous studies (Gadde and Laitinen 1974; Benjamine and Leckie 1981; Zasoski and Burau 1988) related to competition among metals to get adsorbed to soils.
1.2 Soil Pollutant Removal by Surfactants and Complexing Agents
Surfactants remove organics from soils using mechanisms of mobilization and solubilization. Surfactants increase mobility of hydrophobic organic compounds at lower concentrations. At high concentrations, they enhance the solubilization of many hydrophobic organic compounds by increasing the solubility of contaminants through micellar solubilization (Edward et al. 1994; Harwell 1992; Wilson and Clarke 1994; Huang et al. 1997; Duffield et al. 2003). Based on batch tests, Zhang and Lo (2007) have found the optimum conditions such as pH and surfactant concentration for the removal of marine diesel fuel from soils, using surfactant sodium dodecyl sulfate (SDS) and EDTA, in the presence and absence of lead in the soil.
Metal contamination in the soil occurs as solubilized contaminant in the soil moisture, as adsorbed contaminant on the soil surface and as chemically fixed contaminant in soil compounds (Evanko and Davia 1997). Although surfactants have shown potential for remediation of heavy metal contaminated soils, research in the area of surfactant enhanced metal removal from soils is not very extensive. Doong et al. (1998) reported on surfactant enhanced desorption rates of cadmium, lead and zinc. They found that complexing agents such as EDTA and diphenylthiocarbazone (DPC) could alter the metal removal efficiency. Gadelle et al. (2001) noted that anionic surfactants Witconate AOK (sodium C14–16 olefin sulfonate) and T77 (sodium oleyl n-methyl taurate) effectively remove uranium [U (VI)] from acidic soils by cation exchange. Using surfactants SDS, CTAB (cetyl trimethyl ammonium bromide) and Tx-100 (Triton X-100), Mohammad and Jabeen (2003) studied the separation of heavy metal cations on soil amended silica gel layers. SDS has also been shown to be most effective in the micellar-enhanced ultra filtration (MEUF) method for heavy metal removal from water (Ahmadi et al. 1995).
Surfactants have low toxicity and relatively favorable biodegradability (Deshpande et al. 1999). Hence, one prefers them to complexing agents for site remediation. In a field setting, the presence of multiple competing ions is more frequent than the existence of a single contaminant. Only a few detailed studies (Christophi and Axe 2000) have dealt with the topics of competition and interference related to heavy metal adsorption/desorption in soils.
Effective metal removal from a soil is linked to the soil type and its horizon, age, cation capacity (CEC), pH, organic content and the interference effects of other inorganic soil contaminants (Reed et al. 1996). Soil matrix, hydraulic conductivity and the speciation of the metal can also alter the efficiency of soil remediation. The effectiveness of a soil washing solution is hence site specific. Modeling and mechanisms governing the performance of surfactants in the remediation of soils contaminated with Cu (II) and Zn (II) is complicated due to the complexity of soil system and the variety of factors involved. Consequently, the role of surfactants in site remediation is not completely clear.
Complexing agents are very effective in extracting heavy metals from the soil (Ellis et al. 1986; Eliot and Brown 1989; Peters 1999; and Reed et al. 1996). Hong et al. (1995) showed that chelate S-carbonxymethylcysteine (SCMC) removed and released copper reversibly. Based on systematic column-washing experiments, Davis and Singh (1995) concluded that efficient soil washing occurred when chelating agents are used at low flow rates. Davis and Hotha (1998) have also shown that the extent of lead removal is a function of the form of lead contaminating the soil and point out that EDTA alters the soil pH and is relatively expensive. Zhang and Lo (2006) have determined the optimum conditions related to metal concentration in soil and pH conditions, for effective removal of lead and/or zinc from soils, using EDTA.
1.3 Objectives of the Present Study
The present study mainly evaluates the potential of surfactants to enhance the remediation of sandy soils artificially contaminated by Cu (II) and Zn (II). To this end, batch experiments were conducted to investigate the sorption/desorption behavior of these two heavy metals in an artificially contaminated sandy soil. This ensured that adsorption or desorption mechanisms are not unduly influenced by external factors as in natural soils. These factors may include other metals, metal compounds and organics. For field applications, tests should be extended to include specific natural soils to include these effects. Distilled water, a complexing agent (EDTA), two anionic surfactants SDS and AOT (alpha-olefin sulfonate) and a nonionic surfactant Tx-100 (Triton X-100) were used as metal extracting agents. Effects of competition as well as interference associated with the sorption and desorption of metals were also noted. The selected surfactants have been used in the past as washing fluids and had no large harmful impact on soils.
2 Materials and Methods
The materials used mainly consisted of soil samples, target contaminants and surfactants. Cu (II) and Zn (II) in the form of Cu (NO3)2 1/2H2O and Zn (NO3)2 7H2O were the contaminants. HNO3 (70%) was used for digestion. Analytical-reagent grade chemicals were obtained from Fisher scientific (Canada). Distilled water was used for washing, diluting and for use as a control. Analytical-reagent grade SDS (sodium dodecyl sulfate), AOT (dioctyl sulfosuccinate), and Tx-100 (Triton X-100) were the surfactants used to represent anionic and nonionic surfactants. EDTA was the complexing agent chosen. Analytical-reagent grade surfactants and EDTA were obtained from Sigma Chemical Co. USA (1993).
2.2 Soil Samples
The soil used in this study contained 98% of Ottawa sand and 2% of bentonite by weight. Sand was obtained from Geneq Inc., Canada, and bentonite was purchased from Sial Inc., Canada. The low content of bentonite ensured that in the secondary tests involving competition for adsorption sites for two metals, true competition for adsorption sites existed due to scarcity of such sites. Also, lower clay content is expected to reduce clogging of water passages in columns tests that are planned for future studies.
Characteristics of sand and bentonite
CEC (meq/100 g)
Organic matter % (at 550°C)
pH in water
Particle size Dm (mm)
0.044 (mesh 325)
0.42 (mesh 40)
2.3 Soil Sample 1
Soil sample 1
Cu (II) concentration (mg/kg soil)
Zn (II) concentration (mg/kg soil)
To get metal concentration, samples were digested by 70% of HNO3 and shaken at 60 oscillations/min for 24 h. Sample analysis by the atomic absorption (AA) Spectrophotometer (Perkin Elmer A Analyst 100) yielded metal concentrations to the nearest 0.1 mg/l.
2.4 Soil Samples 2 and 3
To study the interaction between Cu (II) and Zn (II), parallel experiments were conducted to compare the results with and without metal interactions. A metal salt solution containing 4,000 mg/l of only Cu (II) was added to the samples of sand and bentonite to form a new sample termed as soil sample 2. Metal salts containing 4,000 mg/l of only Zn (II) were added to the sand and bentonite samples to form another new sample termed as soil sample 3. The procedures to prepare soil samples 2 and 3 were identical to the procedures followed to prepare soil sample 1.
2.5 Batch Experiments
All soil samples were dried at 105°C for a minimum of 24 h before usage. Typically, in all batch experiments, 5.000 g (4.900 g of sand + 0.1000 g of bentonite) of the contaminated soil samples were weighed and placed in the reactor formed of 50 ml capacity plastic Nylon centrifuge sample tubes. For each test, 40 ml of washing solutions at varying concentrations were added to these reactors (sample tubes). All the gravimetric measurements were done with a sartorius balance capable of measuring to the nearest 0.001 g. The samples were equilibrated in a wrist action shaker at 60 oscillations/month for 24 h and later centrifuged for 20 min. The supernatant was taken for subsequent AA analysis to find the metal concentration. All tests were done in triplicate. The average value of the three tests and the extreme values (maximum and minimum) are shown in the sketches.
3.1 Sample 1: Effect of Surfactant Concentration
3.1.1 Sample 1: Cu [II] AND Zn [II] Removal by Surfactants
Figures 1, 3 and 5 show the removal of Cu [II] by surfactants. Cu [II] (Fig. 1) removal reaches a high value, when the concentration of Tx-100 is close to 0.5 mM (=2.17 CMC). No gain in Cu [II] removal occurs with an increase in the concentration of Tx-100 beyond 2.17 CMC. For SDS, Cu [II] removal rate reaches a peak near 1.25 CMC and remains constant at higher concentrations of SDS. For AOT, the peak removal rate is reached when the concentration reaches 1.1 CMC. Beyond 1.1 CMC, there is a decrease in Cu [II] removal. With an increase in concentration beyond CMC, the micelles that trapped the metal ions appeared as a precipitate and got attached to the soil particles and failed to reach the supernatant solution. Especially in the case of Cu (II), a greenish blue patch of matter was clearly discernible on top of the sediment in the sampling tubes. Consequently, at concentrations higher than the CMC, a further increase in Cu [II] concentration in the supernatant (Figs. 3 and 5) should not be expected. In tests with nonionic surfactant Tx-100, no metal precipitation was observed at surfactant concentrations above its CMC.
Results for surfactant Tx-100 show that desorbed Zn [II] concentrations increase with increasing surfactant concentration until Cs = 0.5 mM = 2.17 CMC (Fig. 2). For SDS (Fig. 4) and AOT (Fig. 6), Zn (II) removal reaches a peak at concentrations equal to 1.25 CMC and 1.11 CMC, respectively. For a higher surfactant concentration of SDS and AOT, Zn [II] removal does not get altered very much, compared to the peak Zn [II] removal rate.
Effectiveness of surfactants in removing Cu [II] and Zn [II] from soil sample 1
Optimal Concentration (mM)
Ratio to CMC
Max. Cu (II) removed (mg/l)
Ratio to Cu (II) removed by water
Max. Zn (II) removed (mg/l)
Ratio to Zn (II) removed by water
At equilibrium, distilled water removed 0.2 mg/l of Cu (II) and 25.7 mg/l Zn (II) through solubilization. The highest desorbed concentrations of Cu (II) were 2.17 and 1.25 mg/l and 1.11 ppm in Tx-100 (Fig. 1), SDS (Fig. 3) and AOT (Fig. 5) amended washing systems, respectively. This corresponds to 7.98, 6.25 and 5.99 times greater mobilization of soil bound Cu [II] by surfactants than that mobilized by distilled water.
The desorbed concentrations of soil bound Zn (II) were 32.4, 36.8 and 34.67 mg/l, respectively, in Tx-100 (Fig. 2), SDS (Fig. 4) and AOT (Fig. 6) amended washing systems. This corresponds to 1.26, 1.43, and 1.35 times greater mobilization of soil bound Zn [II] by surfactants than that mobilized by distilled water. The optimal concentration was generally at a concentration close to or in excess of CMC. For the present tests, reported critical micelle concentrations (CMCs) are based on data related to CMC in water. Loss of surfactant occurs due to adsorption to soil. Hence, the surfactant attains CMC at higher concentrations when soil is present.
Most natural (soil) surfaces are negatively charged and are expected to adsorb relatively less anionic surfactants resulting in lower losses. Hence, they have higher efficiency in removing metals from soils (Harwell 1992). Bourbonais et al. (1995) too expect anionic surfactants to exhibit better metal removal efficiency than nonionic surfactants, because amphoteric and cationic surfactants tend to form strong complexes with soil minerals. However, present results, indicate that significant differences in metal extraction (Table 3) were not present.
3.2 Sample 1: Removal of Cu [II] and Zn [II] Using EDTA and Surfactants
3.3 Samples 1, 2 and 3: Metal Interaction and Competition
Batch tests indicated that surfactants such as SDS, AOT and Tx-100 are effective in enhancing removal of Cu [II] and Zn [II] from an artificially contaminated sandy soil that contained 98% clean sand and 2% Bentonite. Tests confirmed the relatively large affinity of Cu [II] to bind strongly with soil particles and resist desorption, when distilled water is the washing fluid. However, distilled water alone was able to remove a high percentage of Zn [II], confirming the relatively weak affinity of Zn [II] to soil particles. Higher rates of Zn [II] and Cu [II] removal generally occurred at concentrations close to 2.17 CMC for the non-ionic surfactant T-x 100 (Figs. 1, 2 and 3). For both SDS and AOT, higher metal removal rates for Zn [II] and Cu [II] occurred at concentrations only slightly higher than their CMCs. This suggests that the micelles indirectly cause the mobilization and removal of these metals and that relatively, there is more surfactant loss to the soil in the case of the non ionic surfactant Tx-100.
The highest desorbed concentrations of Cu (II) in Tx-100, SDS and AOT amended washing systems, respectively, correspond to 7.98, 6.25 and 5.99 times greater mobilization of soil bound Cu [II] by surfactants than that mobilized by distilled water alone. The desorbed concentrations of soil bound Zn (II), respectively, in Tx-100 (Fig. 2), SDS (Fig. 4) and AOT amended washing systems correspond to 1.26, 1.43, and 1.35 times greater mobilization by surfactants than that mobilized by distilled water alone.
The mixture of EDTA and SDS (10 mM) removed 95% of Cu (II). The removal of Cu (II) was also improved by the addition of Tx-100 to EDTA. The combination of SDS (10 mM) and EDTA was extremely effective in removing almost all of Zn (II) from the sandy soil.
In tests involving adsorption of Cu (II and Zn (II), the presence of Zn (II) marginally reduced the retention of Cu (II). However, in the presence of Cu (II), the retention of Zn (II) is significantly reduced. This confirms the higher affinity of Cu (II) for the soil adsorption sites.
Cu (II) desorption decreases when Zn (II) is also present in the soil. Quantitatively, when Zn (II) is also present in the soil, Cu (II) desorption in water decreases from 0.63% to 0.23% and in SDS from 3.1 to 1.7%. However, when Cu (II) is present, Zn (II) desorption increases to 17.7% from 5.3% in water and to 25.4% from 7.1% in SDS.
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