Surfactant-enhanced remediation of contaminated soil: a review

Extracting aqueous solutions with or without additives are employed to solu bilize contaminants in soil. Since water solubility is the controlling remo ving mechanism, additives are used to enhance efficiencies. These additives can reduce the time to treat a site compared to the use of water alone. Ad ditives must be of low toxicity and biodegradable. The research in this are a has focussed mainly on halogenated volatile organic compounds (VOCs) acid is still quite limited for metal removal. Additives include surfactants, o rganic and inorganic acids, sodium hydroxide, which can dissolve organic so il matter, water-soluble solvents such as methanol, displacement of cations with nontoxic ones, complexing agents such as EDTA, acids in combination w ith complexing agents or oxidizing/reducing agents. Cationic, anionic and n onionic surfactants are particularly used for soil washing or Rushing. They contain both hydrophobic and hydrophilic portions, making them ideal for s olubilization of hydrophobic compounds. Numerous studies have indicated tha t surfactants enhance recoveries of non-aqueous phase liquids (NAPLs). Ther e have also been indications that pretreatment of soil with surfactant wash ing to solubilize hydrophobic compounds such as PAHs enhances biodegradatio n of these contaminants. A few in situ field studies have been performed wi th surfactants. Large-scale treatment has been done mostly for organic remo val. Soil pH, soil type, cation exchange capacity (CEC), particle size, per meabilities and contaminants all affect removal efficiencies. High clay and organic matter contents are particularly detrimental. Understanding the ch emistry of the binding of the contaminant and the hydrogeology of the site are very important. Once the water is pumped from the soil, it must be extr acted and then treated to remove the hydrocarbons and metals. Several techn ologies exist such as sodium hydroxide or sodium sulfide precipitation, ion exchange, activated carbon adsorption, ultrafiltration. reverse osmosis, e lectrodialysis and biological processes. Recycling of the surfactants is de sired to decrease treatment costs. This paper will provide an overview of the laboratory research, field demon stration and full-scale application of surfactants for the remediation of c ontaminated soil. The majority of pilot scale in situ flushing tests, parti cularly in the United States, have involved the use of surfactants and co-s olvents. There are only a few full-scale projects however. Recent laborator y scale efforts by the authors concerning the use of biosurfactants, biolog ically produced surfactants, to enhance the removal of copper, cadmium and zinc from contaminated soils and sediments are discussed. Three types of bi osurfactants were evaluated for their effectiveness. They included a lipope ptide called surfactin from Bacillus subtilis, a rhamnolipid from Pseudomon as aeruginosa and a sophorolipid from Torulopsis bombicola. The results ind icated the feasibility of removing the metals with the anionic biosurfactan ts even though the exchangeable fractions were not significant. (C) 2001 El sevier Science B.V. All rights reserved.