Insight into the Cd2+ biosorption by viable Bacillus cereus RC-1 immobilized on different biochars: Roles of bacterial cell and biochar matrix

Biosorption of cadmium in solution by viable bacteria immobilized on biochar was tested under culture conditions, and the relative importance of biosorption by bacterial cell and biochar matrix was investigated. Viable Bacillus cereus RC-1 was immobilized on the biochars derived from rice straw (RSB-beads), chicken manure (CMB-beads), and sewage sludge (SSB-beads), respectively. The biosorption characteristics of immobilized beads was investigated under variable pH, incubation time, and initial Cd2+ concentration. The best results were obtained with RSB-beads that adsorbed Cd2+ of 158.77 mg/g, and the biosorption by these immobilized cells was superior when compared to cells in suspension. High biosorption capacity was the result of simultaneous adsorption resulting from biochar and bacteria, and accumulation resulting from bacteria, in which the bacterial cells contributed more than the abiotic biochar matrix to total removal, but only when initial Cd2+ concentrations were less than 180 mg/L. The biochars played a dual role of adsorbent and carrier, and was struggling to provide nutrients to the bacteria. The relative contribution of each mechanism was determined, which demonstrated that ion-exchange was predominant in RSB-beads (contributing 39.5%–45.7% depending on initial Cd2+ concentration) and in CMB-beads (41.4%–46.8%). In contrast, ion-exchange was a minor fraction for SSB-beads (15.6%–20.1%), where complexation was the most important component (56.9%–66.8%). We propose a model regarding the roles of different mechanisms, which may provide insights into the biosorption process by microorganism immobilized onto a biochar matrix for heavy metal removal, as an important step towards improving microbial remediation.