Self-mediated pH changes in culture medium affecting biosorption and biomineralization of Cd2+ by Bacillus cereus Cd01

Biomineralization is an interesting naturally occurring process of forming minerals by microorganisms, which offers an efficient way to sequester heavy metal ions within relatively stable solid phases. In this study, Bacillus cereus Cd01 was selected to investigate effects of self-mediated pH on biosorption and biomineralization of Cd 2+ in whole 72h cultivation period. Results revealed that strain Cd01-mediated pH decrease of the cultivation medium from 7.0 to 6.1 inhibited biosorption of Cd 2+ on Cd01 cells at the initial cultivation period, while an increased pH from 6.1 to 7.4 facilitated biosorption of Cd 2+ on Cd01 cells at the middle and late cultivation period. The reasons were mainly that self-mediated pH altered cell surface hydrophobicity and cell membrane fluidity of strain Cd01. Moreover, biosorption and bioaccumulation of Cd 2+ on Cd01 cells in the period of increased pH promoted biomineralization of Cd 2+ observed by the transmission and scanning electron microscopes. The analyses of energy dispersive spectroscopy, X-ray photoelectron spectroscopy and select area electron diffraction demonstrated that Cd 2+ loaded on Cd01 cells was biomineralized into polycrystalline and/ or amorphous cadmium sulfide and cadmium phosphate. These results suggest that strain Cd01 may play a potential role in biomineralization remediation of heavy metal contaminated soils.