Immobilization of Cu by Bacillus subtilis DBM and the Role of Extracellular Polymeric Substances

The mechanisms involved in immobilization of soil Cu and the role of extracellular polymeric substances (EPS) in Cu(II) adsorption by Bacillus subtilis DBM were investigated in this study. Adsorption and desorption experiments with intact DBM cells revealed that complexation with surface functional groups and intracellular accumulation were involved in the immobilization of soil Cu. The removal of EPS using cation exchange resin resulted in a 26.6% decrease in the Cu(II) adsorption capacity relative to untreated cells. Compared to intact cells, EPS-free cells showed a 9.9% decrease in the proportion of complexed Cu(II), while the intracellular fraction increased by 8.0%. Surface complexation modeling indicated that the total concentration of complexation sites on the intact DBM cell surface was 1.11 mmol/g dry biomass, which was decreased by 17% to 0.92 mmol/g after EPS removal. Infrared analysis revealed that the pKa values of the carboxyl and phosphate groups in the DBM cell wall differed from those in the EPS. Carboxyl, carbonyl, hydroxyl, amino, and phosphate groups were involved in binding Cu(II) by both intact and EPS-free cells, and Cu(II) was more likely to combine with organic rather than inorganic phosphates. The presence of the EPS increased the binding potential of surface functional groups and may help to prevent heavy metals from entering the cells.