| Organic pollutants and heavy metals are often found simultaneously in contaminated soils. These contaminated soils are threat to ecological security and need to be remediated urgently. However, remediation technologies for these co-contaminated soils are rather scarce. Soil washing is considered to be a highly effective and economical technique to treat these co-contaminated soils. Due to the consequent environmental risk brought by chemosynthetic agents, environment-friendly agents becomes more and more attractive in environmental remediation, such as biosurfactants and organic acid. Unfortunately, the high cost of biosurfactants become a big barrier to their practical application. In this sense, solutions that couldpromote the removal efficiency of co-contaminants by rhamnolipid biosurfactants, is worthy to exploring.In this study, representative organochlorine pesticides (OCPs), lindane, and typical heavy metals, Pb and Cd were chosen as the target pollutants to investigated:(1) the simultaneous removal of lindane, and Pb and Cd from simulated soils by rhmnolipids. (2) capacity of rhamnolipids-citric acid mixed systems to solubilize lindane, and to complex Pb and Cd in aqueous solutions. (3) The simultaneous removal of lindane, and Pb and Cd from simulated soils by rhmnolipids combined with citric acid. The main conclusions can be summed up as follows:(1) The solubilization of lindane increased as rhamnolipds concentration and ionic strength increased, while decreased as pH varied from 5.0 to 10.0. The complexation of Pb by rhamnolipids was strong than that of Cd and their complexation concentration increased as the rhamnolipids concentration increased. As pH rose from 5.5 to 10.0, the complexation of rhamnolipids with Pb was inhibited, while on the contrary, the complexation with Cd was promoted. The adsorption isotherms of rhamnolipids on the tested soil can be fitted using a linear equation. For the remediation efficiency, significant removal of lindane and metals was obtained when rhamnolipid concentration was above 5000 mg·L-1. At a rhamnolipid concentration of 40000 mg·L-1, the desorption rate of lindane, Pb and Cd was 76.9%,18.0% and 100%, respectively. The optimal pH value for three contaminants removal was 7.0.(2) The addition of citric acid reliably enhanced the solubilization of lindane by rhamnolipids, and the enhancing effect was positively correlated with citric acid concentration. The addition of citric acid also promoted the complexation of metals, especially for Pb. Moreover, Pb bound stronger than Cd with rhamnolipids and citric acid.(3) The addition of citric acid inhibited the adsorption of rhamnolipids at low citric acid concentration, while promote the adsorption of rhamnolipids at high citric acid concentration. What’s more, the addition of rhamnolipids inhibited the adsorption of citric acid, and rhamnolipids at high rhamnolipids works more effeciecntly than the low concentration alone. In addition, the addition of either citric acid and rhamnolipids promoted the dissolution of soil organic matter. The maximal desorption rate of lindane, Pb and Cd from tested soils was 89.3%,22.7% and 71.8%, respectively when 2% rhamnolipids and 0.1 mol·L-1 citric acid was employed. Combined effect could be attained by coupling use of rhamnolipids and citric acid, especially at the rhamnolipids concentration below 1%. High fractions of citric acid combined with rhamnolipids inhibited the removal of Cd. It also suggested that the removal efficiency of Pb and Cd by rhamnolipids combined with citric acid was strongly related to their speciation insoils, and metals in exchangeable stage could be removed more effectively. |
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