Chemisorptive biosorption of cd(II), Cr(VI), and Ni(II) by wastewater-derived Bacillus safensis PM22: mechanistic elucidation, surface characterization, and PGP potential.

Heavy metals such as cadmium (Cd), chromium (Cr), and nickel (Ni) pose severe risks to aquatic ecosystems and human health; however, the three-metal biosorption potential of Bacillus strains isolated from contaminated water remains poorly understood. In this batch biosorption study, the maximum adsorption potential of Bacillus safensis PM22 was achieved under multi-metal conditions at the optimum pH values of 6.0 (Cd), 6.0 (Cr), and 4.0 (Ni). The possible mechanisms and bacterial surface changes during the adsorption process were evaluated after optimizing the biosorption conditions at a fixed temperature of 35 ± 2 °C. The removal efficiencies and residual metal concentrations were determined using an atomic absorption spectrophotometer (AAS). The maximum removal efficiencies of 78.4% for Cd, 72.6% for Cr, and 65.3% for Ni were achieved at an initial concentration of 50 mg L-1 under optimal conditions. The adsorption capacity (qe) of PM22 increased (2-16 mg g-1 for Cd, 1-12 mg g-1 for Cr, and 0-12 mg g-1 for Ni) with increasing initial heavy metal concentration. Biosorption kinetics fitted the pseudo-second-order model (R 2 > 0.935; k₂: Cd = 0.018, Cr = 0.021, Ni = 0.011 g mg-1 min-1), suggesting chemisorption as the predominant mechanism. A Fourier-transform infrared spectroscopy (FTIR) analysis identified hydroxyl, ether, carbonyl, sulfonyl, nitryl, methylene, and methyl groups on the bacterial cell wall as active participants in metal complexation. Scanning electron microscopy (SEM) revealed changes in cell surface morphology, including increased surface roughness, cell deformation, and extracellular polymeric substances, in the presence of Cd, Cr, and Ni. To our knowledge, this is one of the first studies to simultaneously characterize the kinetic, equilibrium, and surface mechanisms of three-metal biosorption by a B. safensis strain associated with plant growth-promoting rhizobacteria (PGPR). Moreover, concurrent molecular confirmation of the CzcD metal-resistance and acdS ACC deaminase-encoding genes positions PM22 as a dual-function agent for heavy metal bioremediation and plant growth promotion under metal stress. Future studies should evaluate PM22 biosorption performance in real wastewater matrices and investigate biosorbent regeneration capacity across multiple adsorption-desorption cycles.