Optimization of cultural conditions of Arthrobacter sp. Sphe3 for growth-associated chromate(VI) reduction in free and immobilized cell systems

The current study aimed to characterize Arthrobacter sp. Sphe3 ability to reduce Cr(VI) in suspended cell cultures as well as in immobilized form using Ca-alginate beads. Adaptation studies in the presence of 5 mg L−1 Cr(VI) showed a significant increase in specific growth rate from 0.25 to 0.3 h−1 and bioremoval percentage from 64% to 94% (p < 0.05), whereas Arthrobacter sp. Sphe3 could tolerate up to 50 mg L−1 Cr(VI). Optimization of culture conditions resulted in complete reduction of 45 mg L−1 Cr(VI) at 30 °C, pH 8 and 10 g L−1 of glucose. High glucose concentrations helped at reducing (80 ± 2.4)% of initial 100 mg L−1 Cr(VI), whereas the bacterial strain could tolerate 850 mg L−1 Cr(VI). Cr(III) formation was first evidenced by the appearance of a green insoluble precipitate in the medium. Cell biomass was successfully immobilized in Ca-alginate beads that were evaluated for their stability. Cell release was sharply decreased when 4% Na-alginate was used under non-shaking conditions. Biotransformation efficiency was enhanced when 25-50 mg cells mL−1 Na-alginate from the exponential growth phase were collected and co-encapsulated with either 1% glucose and 0.5% (NH4)2SO4, or 1% LB medium. Immobilized biocatalyst could be reused up to 6 continuous cycles in the presence of 10 mg L−1 Cr(VI), but its performance was lowered at higher metal concentrations comparing with free cells that significantly maintained their reducing ability up to 300 mg L−1 Cr(VI).