Washing Removal of Heavy Metals from Contaminated Farmland Soils Under Low Liquid-Solid Ratio by Agricultural Waste Extract Enhanced with Mgda

Abstract

Washing with plant extracts is a promising and low-cost technique for the permanent removal of toxic metals from farmland soils. However, the current used plant extracts are inefficient in removing heavy metals unless under high liquid-solid ratios (> 10:1), which hampers potential field application in farmland. To enhance metal removal efficiencies, we developed a binary washing agent that comprise of Prunus mume (Sieb.) Sieb. et Zucc. residue extract (RPM) sourced from agricultural waste, and combine with Methylglycinediacetate acid (MGDA) as enhancer. Results showed that removal efficiencies of Cd and Cu were enhanced significantly by increasing the proportion of MGDA in the mixtures, and the optimum mixing ratio was 50 g L-1 RPM + 0.34 g L-1 MGDA (RPMG3). Under a liquid-solid ratio of 2:1, the Cd and Cu removal efficiencies for RPMG3 achieved 31.9% and 9.2% in alkaline soil and 35.2% and 22.3% in acid soil, respectively. The mixture of RPM and MGDA showed synergistic effects on metal removal, and reduced soil Cd and Cu contents below the risk screening values through acid activation, metal ion exchange and complexation of functional groups. The potential ecological risks of Cd in the soil were reduced significantly due to the decrease of exchangeable and reducible Cd fractions after RPMG3 washing. After RPM and RPMG3 washing, soil organic carbon and nutrients concentrations, soil catalase, urease and β-glucosidase activities were all increased. Meanwhile, RPMG3 washing had the best promotion effect on wheat seed germination and growth. Thus, RPMG3 is a potential environment-friendly washing agent, which can effectively remove heavy metals from farmland soils at a low liquid-solid ratio.

Keywords: Heavy metals, Farmland soil, Plant extract, Methylglycinediacetate acid, Soil washing