A review of the interaction of surfactants with organic contaminants in soil

The normal use of surfactants in household products leads to their presence in domestic sewage and, for strongly adsorbing surfactants, in sewage sludge. Surfactants present in sewage sludge-amended soil have the opportunity to interact with other xenobiotics present in the soil such as chlorinated organics and polycyclic aromatic hydrocarbons. Concern has arisen over possible effects arising from these interactions. The ability of surfactants to solubilise relatively insoluble xenobiotics is well known and has been exploited extensively in many industries. The use of surfactants to decontaminate ground water aquifers and in soil clean-up operations is well established, and both anionic and nonionic surfactants have been used to remediate land polluted with oils and hydrocarbons as well as many other organic contaminants. In addition to soil cleansing properties, some surfactants, even at very low concentrations, have been shown to enhance the biodegradation of certain xenobiotics in soil. However, at higher surfactant concentrations, it has been reported that degradation can be delayed due to the partitioning of xenobiotics into surfactant micelles. Surfactant: pollutant interactions in soil are very complex and depend heavily on a range of parameters including surfactant concentration in soil-water compared with critical micelle concentration (CMC), the adsorption characteristics of the surfactant and pollutant, solubility of the pollutant and the soil type. The most important parameter in terms of the ability of a surfactant to mobilise hydrophobic xenobiotics in contaminated soil is the surfactant CMC. In general, concentrations of surfactant in soil-water below the CMC have little or no effect on solubilisation of hydrophobic materials. Only when micelles are present does significant desorption of such pollutants from soil surfaces occur. Conversely, under some conditions, usually at concentrations well below the CMC, the presence of surfactant can enhance the adsorption of hydrophobic xenobiotics to soil. This has been attributed to partitioning of the xenobiotic into surfactant hemimicelles formed on the soil surface. In environments such as soils and sediments, adsorption of surfactants to surfaces results in much higher total surfactant concentrations being necessary to achieve micellisation in pore water than would be necessary in clean water systems. Therefore, much higher concentrations of surfactant are required than might be expected to cause significant changes in xenobiotic behaviour. Such high concentrations are not typical of those found in sludge-amended soil.