Optimal remediation strategies for simultaneously immobilizing arsenic and cadmium using combination of vermicompost and zero-valent iron in two soil types

Abstract

Arsenic and cadmium co-contamination has attracted extensive attention in agriculture, application of organic–inorganic combined amendments has been proven as an effective mean of remediation. But research about the effective remediation strategy for different soils is limited. In this study, mixture of vermicompost (VC) and zero-valent iron (Fe 0 ) in different ratios (1:1,V1F1; 2:1, V2F1; 5:1, V5F1) were selected as amendments to reduce bioavailability of As and Cd in paddy soil and laterite through the water spinach ( Ipomoea aquatica Forsk) pot experiments. The untreated soil and soil treated with individual VC and Fe 0 were designed as controls. Application of the combined amendments simultaneously decreased the soil available As and Cd by 15.5%–30.6% in paddy soil and 17.4%–21.7% in laterite. As- and Cd-absorbance in shoot of water spinach decreased by 43.5%–72.0% in paddy soil and 29.3%–47.6% in laterite. The combined amendments also significantly improved the soil properties (pH, OM, CEC, enzyme activity) and promoted the plant growth (biomass, height, SPAD values). Structural equation modelings (SEMs) revealed that Fe 0 played a more important role in lowering As bioavailability by directly reducing migration and availability of soil As, while VC played a more important role in lowering Cd bioavailability by improving soil properties and yield. Calculating from SEMs, in the combined amendments, the total effects of Fe 0 on reducing shoot As was −0.84 and −0.28, the total effects of VC on reducing shoot Cd was −0.67 and −0.93, in paddy and laterite, respectively. Taken together these observations, V1F1 was effective in the remediation of paddy soil with high As concentration, V2F1 was effective in the remediation of laterite with high Cd concentration and acidic property. This work provide clear remediation strategies for different As and Cd co-contaminated soils, but large scale field experiments are still needed to verify the strategies in the future. • Combination of VC and Fe 0 were used as amendment agents. • The effectiveness of VC and Fe 0 on As and Cd phytotoxicity were quantified by SEMs. • VC and Fe 0 affected the migration of As and Cd through different pathways. • V1F1 was optimal for paddy soil with high As concentration. • V2F1 was optimal for laterite with high Cd concentration and acidic nature.