Sol-gel-immobilized recombinant E. coli for biosorption of Cd²⁺

Recombinant Escherichia coli engineered with a metal-binding peptide was immobilized by entrapment in SiO₂ gel beads using the sol-gel method. Biosorption of Cd²⁺ ions by the immobilized cells was studied in both batch and continuous systems. Adsorption equilibrium could be established within 3 h and the kinetics was well described by the pseudo-second-order kinetic model. The equilibrium data were best described by the Langmuir isotherm with the maximum uptake capacity being 79.9 mg/g cell at 25 °C. More than 95% of the adsorbed Cd²⁺ could be removed with 0.1 M CaCl₂ during desorption. No loss in adsorption capacity was found up to five repeated adsorption/desorption cycles. From mass transfer analysis, only intraparticle diffusion effect was found to be important at low Cd²⁺ concentration (50 mg/dm³), while at high concentration (250 mg/dm³), both intraparticle and external mass transfer affected biosorption. Continuous removal and recovery of Cd²⁺ could be carried out by the immobilized cells in a packed-bed reactor.