Feasibility of Surfactant Use for In Situ Removal of Heavy Metals from Soil

摘要

One of the most widespread problems confronted in environmental cleanup is heavy metal contaminated soil. This is exemplified in a report from the U. S. Department of Energy (Riley et al., 1992) which noted that 40% of the waste sites on DOE facilities had soils affected with heavy metals. When found in the environment at high concentrations, the toxicity of metals, such as lead, cadmium, and copper, pose a significant health risk to both animals and humans. Leachate and runoff from soils contaminated with heavy metals potentially degrade both ground and surface waters. Also of concern is wind erosion, which tends to spread contamination over expanded areas. Present technologies for in situ environmental remediation of metals from soil are costly and often ineffective. Consequently, new methods need to be developed. According to the United States Environmental Protection Agency (1997). the most promising new in situ technologies include electrokinetics, phytoremediation, solidification/stabilization, and soil flushing. Soil flushing with surfactants may be an effective and economically attractive alternative. This study focused on the feasibility of using surfactants to remove heavy metals from soils. The effectiveness of anionic and cationic surfactants, in particular, were compared with the capability of other soil flushing treatments. The amount of research conducted on surfactant enhanced in situ environmental remediation of heavy metals has to date been very limited. Competitive adsorption at soil cation exchange sites and pH adjustment are the dominant mechanisms for cationic surfactants. Batch equilibrium tests done by several researchers show cationic surfactants, especially at low pH conditions, to be effective in desorbing divalent lead, cadmium, copper, and zinc metal cations (Pb~(-2), Cd~(-2), Cu~(-2), and Zn~(-2)) from soil particles (Beveridge and Pickering. 1983: Westall et al., 1994). Colloid mobilization, soluble complex formation, and pH adjustment are the three processes by which anionic surfactants remove cationic heavy metal contaminants from soil. Carleson and Moussavi (1988) found the anionic surfactant, sodium dodecyl sulfate, to be an effective chelating agent for Zn~(-2), while Tan et al. (1994) isolated a naturally occurring anionic biosurfactant capable of forming a soluble complex with Cd~(-2), Coarse grained soils and aquifer material commonly contain significant amounts of clay mineral and/or organic colloids which adhere to the surfaces of larger sand grains (Muecke, 1979). Due to their substantial charge density and specific surface area, these colloids provide a preferential location for electrostatic adsorption of heavy metals. Allred and Brown (1995) have shown several anionic surfactants to be extremely effective in mobilization of colloids present in a sandy soil. Initial screening of 13 surfactants and 7 other flushing agents was accomplished using falling-head permeability tests on a sandy soil treated with lead nitrate. Next, batch tests and saturated column experiments on a contaminated sandy soil obtained from a mining site in northern New Mexico were conducted using the best of the screened surfactants along with disodium ethylene diamine tetraacetic acid and nitric acid.