Bioinduced Synthesis Of High Magnesium Calcite And Its Immobilization Of Cd(II)

Biogenic minerals refer to organic-inorganic mineral complexes induced and synthesized by the life activities of living organisms.The study of biogenic minerals will help to further understand many important biomineralization and environmental biogeochemical issues such as the biological migration and transformation of elements,the biological regulation of minerals and the environmental effects of mineralization products.In order to explore the synthesis mechanism induced by microorganism of high-magnesium calcite and its environmental effects,this paper used Bacillus velezensis as the experimental strain,comprehensively using biomimetic mineralization technology,microbial culture technology,chemical analysis technology,mineral characterization and cupping verification and other research methods,combined with X-ray diffraction(XRD),field emission scanning electron microscopy-energy dispersive spectrometer(FESEM-EDS),thermogravimetric analysis-differential thermal gravity(TG-DTA)and atomic absorption spectrophotometry(AAS)and other research methods.The synthesis mechanism of high-magnesium calcite induced the combination of microorganisms and humic acid,the physical and chemical characteristics of its mineralized products were explored,and the immobilization effect of Cd²⁺by biogenic high-magnesium calcite was further analyzed.The main findings are as follows:In the mineralization system of biomimetic chemistry or mineralization system of microorganism for high-magnesium calcite,organic functional groups such as hydroxyl and carboxyl groups serve as nucleation sites and their dehydration to Mg²⁺are one of the important factors in the synthesis of high-magnesium calcite.Humic acid or organic chemical reagents containing hydroxyl carboxyl groups can shorten the mineralization and synthesis time of crystalline high-magnesium calcite,and hydroquinone had the function of directional synthesis of high-magnesium calcite.Further studies on mineralization products show that the thermal decomposition of biogenic high-magnesium calcite undergoes four processes:water evaporation,organic matter oxidation and combustion,thermal decomposition to calcite,and final calcite decomposition.The presence of organic matter can greatly increase the microscopic stress and dislocation density of minerals,but the grain size and crystallinity of mineral crystal are greatly reduced.This kind of biogenic mineral induced by microorganisms is actually an organic-inorganic mineral complex with honeycomb morphology composed of organic matter(about 31.31%)and high-magnesium calcite.Analyzing the adsorption isotherm of Cd²⁺on biogenic high-magnesium calcite(BHM)induced by microorganisms showed that the adsorption behavior of Cd²⁺by BHM conformed to the Langmuir model(R²=0.98,Qmax=65.36 mg/g),and the maximum removal rate of BHM was 94.62%which was significantly higher than the removal rate of Cd²⁺by organic matter and high-magnesium calcite(44.11%and 66.62%respectively);the results of the adsorption kinetics study showed that the adsorption process of Cd²⁺by BHM was in great agreement with the pseudo-second-order kinetic model(R²=0.99);the thermodynamic parameters(?G>0,?H=21.00 k J mol^-1>0,?S=-40.39 J mol^-1 K^-1<0)indicated that the adsorption process of Cd²⁺by BHM was a non-spontaneous endothermic reaction which increased in order.In addition,BHM can maintain the unit adsorption capacity above 100 mg g^-1 under the condition of p H?3.BHM's acid resistance and great immobilization effect of Cd²⁺were closely related to its organic-inorganic composite structure,organic functional groups and rich mesopore distribution.Further studies had found that the possible formation of amorphous Cd CO3 and complexes or chelates contained Cd²⁺was the main factors for the high adsorption capacity and low desorption rate of Cd²⁺by BHM.The results of cup planting experiments with high-magnesium calcite and humic acid showed that the overall growth of amaranth(Amaranthus tricolor L)in the treatment group with humic acid or biogenic high-magnesium calcite or serpentine was better than that of the control group.Further research found that the heavy metal Cd²⁺content in the amaranth and the soil available Cd²⁺content in the treatment group were generally lower than those in the control group,which was related to the immobilization ability of Cd²⁺by high-magnesium calcite,humic acid and serpentine and the increase of soil p H.In summary,the above research results showed that negatively charged organic groups such as hydroxyl carboxyl groups can shorten the mineralization time of crystal high-magnesium calcite,and hydroquinone can directionally induce the synthesis of crystals high-magnesium calcite;biogenic high-magnesium calcite had great immobilization and passivation ability and low desorption characteristics for heavy metal Cd²⁺;the results of cup planting experiments showed that biogenic high-magnesium calcite had practical application potential for the remediation of heavy metal Cd²⁺pollution in soil.The results of this study provided a theoretical basis for the directional induced synthesis of high-magnesium calcite and its environmental effects.