Abstract: The remediation of farmland soil contaminated by heavy metal is a difficult environmental issue in China. In this study, cadmium (Cd)-contaminated farmland soil from Mashigou, Songxian County of Henan Province was collected as the test soil, and five treatments, including hydrated lime, calcite, zeolite, montmorillonite and biochar, were added and assessed using an indoor culture test method. The Tessier modified sequential seven-step extraction method was used to analyze the Cd speciation content in the treated soil. The passivation effect of different treatments on Cd pollution at 1%, 3% and 5% was then assessed in weakly acidic soil. Results show that the five treatments increased the soil pH in the order of:hydrated lime (2.00-2.50 unit increase) > calcite (0.86-1.38 unit increase) > montmorillonite (0.83-1.19 unit increase) > biochar (0.07-0.38 unit increase) > zeolite (0.11-0.20 unit increase). The five treatments all reduced the content of ion exchangeable Cd in soil, but their effects were substantially different. The passivating sequence was as follows:hydrated lime > calcite > biochar > montmorillonite > zeolite. After 50 days of culture with treatments at 1%, 3% and 5%, the reductions in exchangeable Cd were 76.65%-90.66% with hydrated lime, 46.12%-53.77% with calcite, 9.36%-33.03% with biochar, 11.72%-19.14% with montmorillonite, and only 0%-10.94% with zeolite. The treatments changed the proportion of Cd forms in soil to different degrees. The proportions of weakly bound Cd (carbonate bound, humic acid bound, iron and manganese oxide bound) increased significantly, and the proportions of strongly bound Cd (strong organic bound, silicate residual state) also increased to a certain extent. In general, hydrated lime and calcite greatly reduced exchangeable Cd, biochar had an intermediate effect, and montmorillonite and zeolite had the minimal effects on exchangeable Cd. Considering the effects of these treatments on the soil pH and Cd passivation efficiency, 3% calcite is the best passivating agent for weakly acidic Cd-contaminated soil under laboratory culture test conditions. As a natural non-metallic mineral, calcite showed efficient passivation, and it is cheap, easy to obtain and environmentally friendly, suggesting its potential for in-situ remediation of weakly acidic Cd contaminated soil.

Key words: cadmium, passivator, weakly acidic soil, ion exchange state, occurrence form, calcite