Investigation of Cr(VI) reduction and Cr(III) immobilization mechanism by planktonic cells and biofilms of Bacillus subtilis ATCC-6633

In this study, we investigated the Cr(VI) uptake mechanism of planktonic cells and biofilms of Bacillus subtilis (B. subtilis) ATCC-6633. Data showed that the effect of planktonic cells on the Cr(VI) uptake was quite different from that of biofilms. Planktonic cells had strong ability of Cr(VI) reduction, while biofilms possessed a great potential of Cr(III) immobilization. For planktonic cells, 100 mg/L Cr(VI) could be completely reduced. Both exopolymeric substances and cytoplasmic extracts contributed to high capacity of Cr(VI) reduction. After the reduction, noticeable Cr(III) precipitates were accumulated on bacterial surfaces, but 37.5% Cr(III) still remained in the supernatant. For biofilms, the biofilm debris became the main active ingredient of the Cr(VI) reduction. However, only 20 mg/L Cr(VI) could be reduced probably because of unavailability of reducing active sites during the biofilm formation. Further studies showed that biofilms had a better Cr(III) immobilization capacity than planktonic cells with 100% Cr(III) immobilized. Moreover, for the first time, we proposed a strategy combining the advantages of both planktonic cells and biofilms, and a successful Cr(VI) removal from typical Cr(VI)-containing plating wastewater was achieved through a 10-L pilot-scale experiment.