Ureolytic bacteria-loaded modified biochar microcapsules for enhanced cadmium immobilization under acid conditions

Cadmium contamination in acidic environments remains a critical challenge due to enhanced metal mobility and the limited efficiency of conventional remediation technologies. Microbially induced carbonate precipitation (MICP) is a promising sustainable approach for heavy metal immobilization; however, its application under acidic conditions is severely restricted by reduced microbial activity and insufficient carbonate generation. Modified biochar was used as a functional carrier to develop adsorption-based microcapsules (AM) and entrapment-based microcapsules (EM) loaded with ureolytic bacteria for Cd²⁺ immobilization in acidic environments. Results showed that EM effectively protected bacteria under acid stress and maintained high urease activity, and Cd²⁺ removal achieved over 99.7 % in aqueous solutions, much greater than other groups at pH 3. The unconfined compressive strength of solidified soil increased with curing age and pH. At pH 7 and 14 curing days, AM exhibited the highest strength, whereas EM showed lower strength due to the limited mechanical stiffness of sodium alginate hydrogel beads. Both microcapsules reduced Cd²⁺ leaching, EM achieving the greatest reduction, 37.3 % lower than the control group. Cadmium immobilization was attributed to exchangeable Cd transformed into carbonate-bound and residual fractions through surface adsorption, complexation, and carbonate coprecipitation. The results of this study offer valuable insights for the remediation of heavy metal contaminated sites, especially in acidic environments.