Ball-Milling-Modified Biochar with Additives Enhances Soil Cd Passivation, Increases Plant Growth and Restrains Cd Uptake by Chinese Cabbage

Biochar is a popular amendment in Cd polluted soil. However, the performance of bulk biochar is still less than satisfactory, so effective modification is very important to improve its capacity to adsorb Cd. In the present study, biochar derived from reed straw was modified by ball milling with the addition of either potassium hydroxide (KOH) alone (QK) or combined with attapulgite (QKA). Both batch experiments and pot cultivation were conducted to elucidate the adsorption mechanisms of Cd by modified biochar and their effects on Cd passivation and plant uptake in Cd polluted soil. The results showed that QK and QKA could provide higher pH values, and more oxygen-containing functional groups and minerals compared with bulk biochar (YC), promoting the complexation, ion exchange and precipitation of biochar to cadmium (Cd). The modified biochar was more inclined to multi-layer, non-ideal surface and chemical adsorption, which was an endothermic process. Compared to non-biochar addition (CK), the application of QK or QKA significantly promoted the values of pH, EC, CEC, available potassium and organic matter in soil. The addition of QK, QKA and YC decreased the availability of Cd by 22.61%, 22.32% and 14.16%, accompanied by the increase of residual Cd by 47.96%, 47.60% and 37.27%, respectively, indicating the more effective passivation of the modified biochar (QK and QKA). Compared to CK, biochar applications could significantly improve Chinese cabbage growth, and decrease Cd content in the aerial/edible part of plants by 42.97, 18.16 and 7.29%, respectively, for QK, QKA and YC. With the application of QK, Cd concentrations in the aerial/edible part of Chinese cabbage were reduced to 0.15 mg kg−1 (lower than 0.2 mg/kg, the leafy vegetables national safety standard). Generally, the performance of QK on the remediation effects and vegetable production was better than that of QKA, indicating the potential of QK for the remediation of Cd-contaminated soil and the safe production of vegetables.