Abstract

The most ubiquitous soil contamination problems in the world today are related to hydrocarbons and heavy metals. It is a common phenomenon to find a mixture of heavy metals and hydrocarbons in most contaminated sites in the US and also in Canada. The presence of these contaminants can destroy the balance in the natural habitat. Therefore there is a need for remediation to mitigate these effects on humans and the environment at large. Heavy metals, such as copper (Cu), zinc (Zn), and nickel (Ni), have received considerable attention with regard to their accumulation in soils, uptake by plants and contamination of groundwater by leaching. Since they cannot be degraded, they pose a serious problem to the environment. Hydrocarbons, e.g. diesel oil, also pose a similar risk when present in large quantities in soil. This research focuses on using biosurfactants; rhamnolipids, saponin and mannosyl-erythritol lipids to remediate a natural soil contaminated with a mixture of heavy metals and hydrocarbons (diesel fuel). The soil contained 894 mg/kg of zinc, 216 mg/kg of copper, 167 mg/kg of nickel and 228 mg/kg of the total petroleum hydrocarbons (TPH) content. Experiments carried out showed that after a series of five washings of the soil using biosurfactants; the highest removal of zinc (88% and 79%) was achieved using saponin (30 g/L), pH 3 and pH 5 respectively. The maximum copper removal (46%) was obtained with 2% rhamnolipids at pH 6.5. Highest nickel removal (76%) was obtained with saponin (30 g/L) pH 5. The TPH level in the soil after multiple washings dropped drastically from an initial concentration of 228 mg/kg to concentrations in the range of 14-67 mg/kg with biosurfactants and the control. Sequential extraction performed on the untreated soil showed that copper exists more in the organic fraction (50%), zinc in the oxide fraction (36%) and nickel exists more in the exchangeable and carbonate fractions (50%). After a series of five washings with biosurfactants it was evident that the oxide fraction of zinc, organic fraction of copper, exchangeable and carbonate fractions of nickel were substantially reduced, compared to the control and the untreated soil. The results of the study clearly indicated the feasibility of reducing zinc, copper, nickel and the total petroleum hydrocarbon content of a mixed contaminated soil with the anionic biosurfactants tested