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.

Sequential washing and eluent regeneration with agricultural waste extracts and residues for facile remediation of meta-contaminated agricultural soils.

Washing with organic acids and dissolved organic carbon (DOC) is a promising technique for effective removal of potentially toxic metals from agricultural soils and the two key factors are the screening of inexpensive, high-efficiency, and environmentally friendly washing agents and the safe treatment of waste eluent. We used extracts from agro-forestry wastes (pineapple peel, lemon peel, grapefruit peel and gardening crabapple fruit) to develop a facile two-stage sequential washing method (extracts and/or citric acid (CA) and coupled with extracts) and regenerated waste eluent. The washing efficiencies of Cd and Cu were significantly increased by pineapple peel (PP) using two-stage sequential washing with the sequence of PP + CA-PP > CA-PP > PP-PP. The potential pollution risk from soil Cd was lowered by 33.0% from moderate to low risk, and soil nutrient contents increased. 80.9% of Cd and 81.3% of Cu in waste eluent were efficiently removed by the PP residues. The removal mechanisms of metals in soils and eluents by PP washing agents and residues can be attributed to acid activation, cation exchange and complexation between metal ions and carboxyl groups. Therefore, the PP extracts and residues are potentially suitable for the removal of Cd and Cu from polluted agricultural soils and washing waste eluents.