Biosynthesis and characterization of cadmium carbonate crystals by anaerobic granular sludge capable of precipitate cadmium

Microbially induced carbonate precipitation (MICP) has been exploited as an efficient strategy to immobilize toxic metals in the form of carbonate salts. The present study investigated the cadmium carbonate (CdCO3) precipitation induced by an anaerobic granular sludge. The results revealed that anaerobic sludge showed high Cd utilization efficiencies (97.4% ± 1.1) after 18 h of incubation at 30 °C, and no differences were observed neither cadmium source nor substrate. According to SEM and X-ray diffraction results, the anaerobic sludge was able to precipitate Cd2+ as CdCO3 rhombohedral crystals in shape from 100 to 700 nm in size as a function of the source of cadmium and substrate used. FTIR results showed that 3 extra bands (1370, 848 and 688 cm−1) corresponding to CO32− anion appeared once CdCO3 was synthesized. TEM revealed the bioaccumulation of CdCO3 on the bacterial cell wall and the synthesis of crystals smaller than 100 nm. The synthesis of CdCO3 crystals was associated with K. pneumoniae and E. coli, which had a synergetic effect during CdCO3 precipitation. Furthermore, the pH increase around 8.9 ± 2.5 in all the cases suggested that the precipitation of CdCO3 was through ureolysis. [Display omitted] •An anaerobic granular sludge was able to precipitation Cd2+ as CdCO3 crystals.•No differences were observed neither cadmium source nor substrate.•Cd2+ precipitated as CdCO3 rhombohedral crystals from 100 to 700 nm.•Synthesis was associated to K. pneumoniae and E. coli.•The co-precipitation of cadmium with other metals was not observed.