Study On Kinetics And Morphology Of Calcium Carbonate Precipitation Catalyzed By Bacterial Carbonic Anhydrase

The precipitation process of calcium carbonate(CaCO₃)is very slow in nature. In the reactive system of CaCO₃-H2O-CO2, the reaction CO2 + H2O ? H + + HCO₃- is a rate-limiting step. A biological catalyst could be used to accelerate this reaction. Carbonic Anhydrase(CA)is a zinc-containing metalloenzyme that can remarkably catalyze the above reaction, and with a zinc ion as active centre. Our previous studies found that microbial CA had significant promoting effect on calcite precipitation. In this paper, the kinetics of CaCO₃ precipitation catalyzed by bacterial CA at different initial pH, enzyme concentrations and Ca²⁺ concentrations through the gaseous diffusion systems were further investigated, and the morphology of CaCO₃ crystals obtained in the precipitation process was analyzed using XRD, FTIR and FESEM. The main results are as follows.(1) The change of deposited Ca²⁺ during the process of CaCO₃ precipitation catalyzed by bacterial CA at different initial pH was well fitted by exponential model. In the experimental pH range (pH 6.0⁸.0), the precipitation rate of CaCO₃ increased with initial pH. The results of XRD and FESEM analysis indicated that the CaCO₃ crystals were mainly calcite in the presence of bacterial CA. The crystals were not regular and their size was bigger. With increasing contact time, the crystals gradually changed from prism shape to pyramid-like shape or assembled into irregular polyhedral shape.(2) The change of deposited Ca²⁺ during the process of CaCO₃ precipitation catalyzed by bacterial CA at different enzyme concentrations was well fitted by exponential model. The enzyme concentrations of 0.2².0 U/mL were beneficial to CaCO₃ precipitation, however, the CaCO₃ precipitation was inhibited when the enzyme concentration was at 8.0 U/mL, which indicated that overhigh enzyme concentration was not more favorable for CaCO₃ precipitation catalyzed by bacterial CA. The results of XRD, FTIR and FESEM analysis showed that there were significant differences in the morphologies of CaCO₃ crystals among different enzyme concentrations. Vaterite was present at lower concentration of CA, and the higher concentration of CA favored the formation of calcite.(3) The change of deposited Ca²⁺ during the process of CaCO₃ precipitation catalyzed by bacterial CA at different initial Ca²⁺ concentrations was well fitted by not only exponential model but also polynomial model. The precipitation rate of CaCO₃ increased with the initial concentration of Ca²⁺, but overhigh initial concentration (100 mmol/L) of Ca²⁺ had a certain influence on the effect of CaCO₃ precipitation catalyzed by bacterial CA. The results of XRD, FTIR and FESEM analysis showed that the initial Ca²⁺ concentration had greater effect on the morphology of CaCO₃ crystals formed in the presence of bacterial CA. The lower initial Ca²⁺ concentration favored the formation of vaterite and the higher initial Ca²⁺ concentration favored the formation of calcite.In summary, the initial pH, enzyme concentration and Ca²⁺ concentration, et al., could not only influence the rate of CaCO₃ precipitation catalyzed by bacterial CA, but also greatly influence the polymorph and morphology of CaCO₃ crystals. Therefore, different crystalline feature of CaCO₃ materials could be induced by bacterial CA under above different initial conditions. The role of bacterial CA in CaCO₃ precipitation was related to the electrostatic adsorption of CA enzyme protein on Ca²⁺ and the selective adsorption of CA enzyme protein on the crystal faces, except for the enzymatic catalysis. The results in this paper provide scientific base for enriching the theory of karst dynamics, and also provide a new idea for preparation of CaCO₃ biomineralization materials.