Biogenic hydroxyapatite synthesis by Bacillus subtilis: An efficient passivator for the reduction of cadmium contamination in agricultural soil

Abstract Cadmium (Cd) can cause various diseases and threaten human health through its accumulation in crops. In this study, Bacillus subtilis was used to prepare biogenic hydroxyapatite (B‐HAP), and chemical hydroxyapatite (C‐HAP) without microorganisms was produced as the control. The abilities of B‐HAP and C‐HAP to adsorb Cd in aqueous solutions and passivate the migration of Cd in actual soil were analyzed. In the adsorption experiments, the Cd concentration in the solutions decreased by 94.8% and 95.8% when 0.20 mmol of B‐HAP and C‐HAP were added. In the incubation experiment, the soil pH increased from 7.08 to 7.30 and from 7.08 to 7.43 when 0.021% of B‐HAP and C‐HAP were added. B‐HAP had little disturbance to the soil pH value. The results showed that Cd in the mobilizable state, at 0.021% B‐HAP and C‐HAP, respectively, decreased by 38.6% and 36.8% in the actual soil samples. The structure and morphology of B‐HAP were characterized using various techniques, which indicated that the adsorption mode of B‐HAP would change with time from ion exchange to specific adsorption. Therefore, B‐HAP can be used as an effective passivator for Cd removal from agricultural soil. The concentrations of Olsen‐P decreased by 35% when 0.021% B‐HAP was added after 104 days. This study provides insights into the development of novel passivators that benefit soil health and green development and has reference significance for the eco‐friendly regulation of low‐concentration‐Cd–contaminated agricultural soil.