Remediation of contaminated soil in copper mining areas using plant-based surfactants

Abstract

This research brings to knowledge the effectiveness of a novel plant-based saponin from Acacia concinna (shikakai) for the remediation of heavy metal contaminated soils. Shikakai has long been used for traditional hair treatment in India and other parts of the world but has not been used for soil remediation. The effectiveness of this saponin was compared with rhamnolipid (a class of microbial cleaning agent which has been studied widely and applied in soil remediation), EDTA (a chemical chelate known to be soluble and having many commercial applications including soil remediation), Sapindus mukorossi, commonly known as soapnut (a known plant-based biosurfactant that has been proved to enhance soil washing). The feasibility of soil washing for the removal of Pb and Cu, from soil collected from contaminated sites polluted by copper mining and industrial activities, as well as sandy loam soil spiked with Cd, Cu, Pb, and Zn were investigated in laboratory-scale batch and column experiments. Influencing parameters including the concentration of washing solutions and the pH of the washing solutions, soil-solution ratio and washing time were studied. These parameters were found to influence the removal efficiencies of the washing agents. The removal efficiency increased with an increase in surfactant concentration, washing time, and soil-solution ratio but decreased with an increase in solution pH. The results of washing contaminated soil from copper mining and industrial pollution, show that soapnut removed a cumulative of 66.98% and 72.98% of Cu from soil with low Cu contamination (C1) and high Cu contamination (C2) after triple wash cycles while shikakai removed 71.08% and 78.08% at the same conditions. While soapnut removed a cumulative of 78.98% and 77.98% of Cu, shikakai removed 82.91% and 83.90% of Pb from soil C1 and C2 after triples wash cycles. The batch washing experiments of the spiked soils show that the maximum removal efficiency of 92.82% was obtained when 6% of EDTA was used to wash Cu contaminated soil. At the same condition, soapnut, shikakai, and rhamnolipids achieved the removal efficiency of 77.07%, 76.92% and 56.31 respectively. The cumulative removal of heavy metals from column experiments ranged from 37.95 to 74.05% after washing with 10 pore volumes. The performance of column experiments demonstrates that the application of these cleaning agents in in-situ soil remediation can be an effective alternative to ex-situ remediation. The optimization of washing parameters, using response surface methodology (RSM) based on Box– Behnken Design shows that the optimal conditions of process parameters for Pb and Cu removal using shikakai were: (Conc. 3.3% and 3.7), (SSR 28.79 and 30.30), and (pH 3 and 3) respectively. The results proved the effectiveness of shikakai as a surfactant and as well as substitute for EDTA and rhamnolipid which have been known and applied for soil washing in several laboratory and field experimental projects.