Immobilizing of lead and copper using chitosan-assisted enzyme-induced carbonate precipitation

Abstract

Enzyme-induced carbonate precipitation (EICP) is considered as an environmentally friendly method for immobilizing heavy metals (HMs). The fundamental of the EICP method is to catalyze urea hydrolysis using the urease, discharging CO32− and NH4+. CO32− helps to form carbonates that immobilize HMs afterwards. However, HMs can depress urease activity and reduce the degree of urea hydrolysis. Herein, the potential of applying the chitosan-assisted EICP method to Pb and Cu immobilization was explored. The chitosan addition elevated the degree of urea hydrolysis when subjected to the effect of Cu2+ toxicity where the protective effect, flocculation and adsorption, and the formation of precipitation, play parts in improving the Cu immobilization efficiency. The use of chitosan addition, however, also causes the side effect (copper-ammonia complex formation). Two calcium source additions, CaCl2 and Ca(CH3COO)2, intervened in the test tube experiments not only to prevent pH from raising to values where Cu2+ complexes with NH3 but also to separate the urease enzyme and Cu2+ from each other with the repulsion of charges. The FTIR spectra indicate that the chitosan addition adsorbs Cu2+ through its surface hydroxyl and carboxyl groups, while the SEM images distinguish who the mineral are nucleating with. The findings shed light on the potential of applying the chitosan-assisted EICP method to remedy lead- and copper-rich water bodies.