Immobilizing of lead and copper in aqueous solution using microbial- and enzyme-induced carbonate precipitation

Inappropriate irrigation could trigger migration of heavy metals into surrounding environments, causing their accumulation and a serious threat to human central nervous system. Traditional site remediation technologies are criticized because they are time-consuming and costly. In the past few years, the microbial-induced carbonate precipitation (MICP) is deemed as an alternative to traditional technologies due to its easy maneuverability. The enzyme-induced carbonate precipitation (EICP) has attracted attention as bacterial cultivation is waived before catalyzing urea hydrolysis. The present work applied MICP and EICP technologies to the lead (Pb) and copper (Cu) immobilization. The effect of the degree of urea hydrolysis, mass and species of carbonate precipitation, and chemical and thermodynamic properties of carbonates on the immobilization efficiency was investigated. Results showed that NH4+ concentration reduced when subjected to the effect of Pb2+ or Cu2+ toxicity, and for a given Pb2+ or Cu2+ concentration, it was much higher under MICP than under EICP. Further, the immobilization efficiency against Cu2+ is way below that against Pb2+. The reduction in the immobilization efficiency against Pb2+ and Cu2+ appeared to be due to two precipitates respectively (i.e., cotunnite and atacamite). Their chemical and thermodynamic properties were not as good as those of calcite, cerussite, phosgenite, and malachite. Their degradation, when subjected to harsh pH conditions, results in a reduction in the immobilization efficiency.