Synergistic effect of floatable hydroxyapatite-modified biochar adsorption and low-level CaCl2 leaching on Cd removal from paddy soil

The utilization of recycled biochar combined with chemical leaching is an appropriate method to remove cadmium (Cd) from paddy soil. Some Cd-rich soil clay particulates (particulate Cd) are reported to be removed via biochar adsorption and the potential impact of biochar on soil properties need further study. The removal efficiencies and mechanisms of Cd from soil by using floatable hydroxyapatite modified biochar (HBC) combined with CaCl2 were studied. Synergetic removal efficiencies of total Cd (46.5%) and bioavailable Cd (37.9%) from the paddy soil were achieved with 2% HBC and 1 mM CaCl2. The increased soluble Cd in soil pore water by CaCl2 leaching could be efficiently adsorbed on HBC, and removed by HBC collection, reducing the risk of the residual soluble Cd in soil pore water to rice plants caused by the inefficient drainage in the field. The suspendability of clay particulates in overlying water was little affected by the low-level CaCl2 based on Derjaguin-Landau-Verwey-Overbeek (DLVO) calculation. Moreover, low-level CaCl2 facilitated the accumulation of particulate Cd on the floating HBC via decreasing the interaction energy (by 25%) between clay particulates and HBC. HBC-mediated Cd migration contributed ~70% of total Cd removal, while soluble and particulate Cd removed through the drainage accounted for ~30%. Soil clay proportion maintained at 25.3% due to the replenishment of HBC residues. In addition, soil nutrient and physicochemical conditions were improved with HBC residues. This work provides a novel soil remediation method by using floatable biochar combined with low-level CaCl2 for Cd-contaminated paddy soil remediation.