Synthesis of Biogenic High-Magnesium Calcite and its Experimental Immobilization Effect on Cd2+

Abstract

Biogenic carbonates usually differ significantly both in physical and chemical characteristics from those of the same chemical origin, and heavy metal immobilization is prominent among these differences. In this research, a biosynthetic method of high-magnesium calcite (HMC) generated by Bacillus velezensis (B. velezensis) in the presence of humic acid (HA) was developed to ascertain the immobilization characteristics of biogenic high-magnesium calcite (BHM) for Cd2+. The study showed that the presence of HA was crucial to the formation of crystalline BHM. The Cd2+ immobilization assay results indicated that significant amounts of Cd2+ could be adsorbed and the obtained data could fit the Langmuir equation (R 2 = 0.98, Q max = 65.36 mg/g), and the adsorption process conformed to the pseudo-second order kinetic model (R 2 = 0.99). In particular, BHM can maintain the adsorption capacity at above 100 mg/g even under acidic conditions (pH ≥ 3). Further analysis showed that the adsorption was mainly monolayer physical adsorption, and a small amount of chemisorption also occurred. Thermodynamic parameters (ΔG > 0, ΔH > 0, and ΔS < 0) indicated that the adsorption process was a non-spontaneous endothermic reaction. XRD and SEM-EDS analyses demonstrated that the culture precipitate obtained was mainly HMC, with numerous 10-nm pores based on Barrett-Joyner-Halenda theory of surface area. The results of TG-DTG/DTA analysis showed that the BHM contained about 31.31 ± 3.77% organic matter. Amorphous CdCO3 and the complex or chelate bearing Cd2+ formed in adsorption process could ensure low desorption, implying a novel prospect for remediation of heavy metal pollution using BHM.