Combined Removal of Sr2+ by Montmorillonite-carbonate Mineralizing Bacteria

Abstract

Adsorption and biomineralisation are two important methods in the treatment of various radioactive heavy metal pollution. Among them, adsorption is considered by researchers as one of the most promising methods for the treatment of heavy metal wastewater due to its lower cost and better treatment effect, while biomineralisation is one of the hot spots of research at home and abroad today for its environmental friendliness and high operability. However, the combination of these two approaches in some way will be significant in expanding the treatment of radioactive heavy metal contaminated wastewater. In this work, a typical clay mineral, montmorillonite, a mineralising microorganism, carbonate mineralising bacteria and the radioactive heavy metal ion Sr2+ were selected as the objects of study, and a montmorillonitecarbonate mineralising bacteria composite system was constructed. The combined effect of clay minerals and microorganisms on the removal of Sr2+ was investigated by comparing the pH, carbonate concentration, Sr2+ concentration in the liquid phase and its fugitive state with time under the influence of montmorillonite and the constructed composite system of montmorillonitecarbonate mineralising bacteria, respectively. The results show that montmorillonite and carbonate mineralizing bacteria can remove Sr2+ through adsorption and microbial mineralization, respectively. In a single adsorption system, the removal rate of Sr2+ in liquid phase is proportional to the dosage of montmorillonite. In the constructed complex system, montmorillonite and carbonate mineralizing bacteria play different roles at different time and the overall effect of montmorillonite on Sr2+ adsorption decreases with increasing time, while the mineralizing effect of carbonate mineralizing bacteria increases with increasing time, that is to say, in the whole process of Sr2+ removal, the adsorption effect of montmorillonite is dominant in the early stage of the experiment, and the mineralizing effect of carbonate mineralizing bacteria is dominant in the later stage. The results of XRD and FT-IR analysis of the sediment at the end of the experiment show the formation of strontium carbonate precipitation, which verifies that carbonate mineralizing bacteria can achieve removal of Sr2+ through biomineralization. Under the liquid phase environmental conditions with the montmorillonite dosage of 40 g/L and the initial Sr2+ concentration of 1 10019 mg/L, the highest removal rate of Sr2+ by the montmorillonite single adsorption system is only 2848%, while the removal rate of Sr2+ by the constructed montmorillonitecarbonate mineralizing bacteria composite system is as high as 9792%. Therefore, compared with the traditional single montmorillonite adsorption system, the constructed montmorillonitecarbonate mineralizing bacteria composite system can effectively achieve the removal of Sr2+. Based on the adsorption of clay minerals and the biomineralization of microorganisms, the montmorillonitecarbonate mineralising bacteria composite system constructed in this study provides a reference for the future treatment of Sr2+ radioactive contaminated wastewater.

Keywords

• montmorillonite
• carbonate mineralizing bacteria
• sr2+
• adsorption
• microbial mineralization