Removal of Cd from contaminated farmland soil by washing with residues of traditional Chinese herbal medicine extracts

Soil washing is one of the effective methods for permanent removal of heavy metals from farmland soil, and selection of washing agents determines heavy metal removal efficiency. However, there is still a lack of cost-efficient and eco-friendly washing agents. In this study, three residues of traditional Chinese herbal medicine (RTCHM) extracts: residues of Prunus mume (Sieb.) Sieb. et Zucc. (RPM), residues of Schisandra chinensis (Turcz.) Baill. (RSC), and residues of Crataegus pinnatifida Bunge (RCP), were tested for their potential of Cd removal. The variations in amounts and compositions of dissolved organic carbon (DOC) and citric acid were responsible for the difference in Cd removal efficiencies of RTCHM extracts. Fourier-transform infrared spectrophotometer (FTIR) analysis showed that hydroxyl, carboxyl, and amine were the main functional groups of RTCHM extracts to chelate with heavy metals. The optimum conditions for RTCHM extracts were 100 g L-1 concentration, solid-liquid ratio 1:10, pH 2.50, and contact time of 1 h, and the highest Cd removal efficiencies of RPM, RSC, and RCP extracts reached 35%, 11%, and 15%, respectively. The ecological risk of Cd decreased significantly due to the decrease of exchangeable and reducible Cd fractions. RTCHM extracts washing alleviated soil alkalinity and had little effect on soil cation exchange capacity. Meanwhile, the concentrations of soil organic matter and nitrogen were enhanced significantly by RPM extracts and the activities of soil catalase and urease were also improved. Overall, among the tested extracts, RPM extracts was a much more feasible and environment-friendly washing agent for the remediation of Cd-contaminated farmland soil.