Chiang Mai Journal of Science

     The overlapping effects of a high geological background and anthropogenic contamination in the karst area of Guizhou province have led to serious soil cadmium (Cd) pollution. The use of microorganisms as soil remediation agents for Cd-contaminated soils has garnered considerable attention. This study systematically investigated the isolation and identification of Cd-resistant bacteria from karst soil, focusing on microbial growth and tolerance mechanisms in extreme environments. In this study, a novel Cd-resistant strain was isolated from highly Cd-contaminated soil in a lead-zinc (Pb-Zn) mining area. Using a combination of morphological analysis, physiological-biochemical studies, and 16S rRNA gene sequencing, the strain was identified as Bacillus sp. Ld-1. Minimal inhibitory concentration tests revealed that Bacillus sp. Ld-1 exhibits high Cd²⁺ resistance, tolerating up to 500 mg/L. The equilibrium data closely followed the Langmuir model, indicating a monolayer adsorption process for Cd²⁺. The Langmuir model applied to the experimental data revealed a maximum absorption capacity of 4.54 mg/g for the strain. Additionally, a pseudo-second-order kinetic model provided the most accurate description of the biosorption kinetics. When Bacillus sp. Ld-1 was present at a concentration of 10 g/L with Cd²⁺ levels of 1-25 mg/L, good clearance rates (>87%) were achieved. Scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) revealed that under treatment with 200 mg/L of Cd²⁺, the bacterial surface became rough and accumulated Cd²⁺, indicating extracellular adsorption as a key pathway for Cd²⁺ removal by this strain. In conclusion, the isolated Bacillus sp. Ld-1 has high resistance to heavy metals and shows effective Cd adsorption, making it a promising biological resource for remediating Cd-contaminated soil in karst regions.