Biosorption performance and mechanism insights of live and dead biomass of halophilic Bacillus altitudinis strain CdRPSD103 for removal of Cd(II) from aqueous solution

A halophilic multiple heavy metal-resistant bacterium, CdRPSD103, was isolated from the sea sediment and identified as Bacillus altitudinis strain CdRPSD103. This bacterial strain is able to tolerate and grow at high salt concentrations (up to 13% w/v NaCl). Biomass of Bacillus altitudinis CdRPSD103 is used in both live and dead conditions for Cd(II) biosorption. The effects of various operational parameters, such as pH, temperature, salinity, metal concentration, agitation speed, and biomass dosage, have been studied and optimised. Under optimal conditions, the removal of Cd(II) ranged from 99.45% to 55.24% using live biomass and from 98.81% to 53.13% using dead biomass, at Cd(II) concentrations ranging from 100 to 500 mg/L. The maximum biosorption (q_max) of Cd(II) was 280.2 and 269.6 mg per 1 g of live and dead biomass, respectively, at 500 mg/L of the initial Cd(II) concentration. The pseudo-second-order model was best fitted to this Cd(II) batch biosorption process and can be described as a two-step process (surface adsorption and intercellular accumulation). The adsorption isotherm was found in accordance with the Langmuir model, which represents the monolayer adsorption mechanism. Fourier transform infrared spectroscopy and field emission scanning electron microscopy with an energy dispersive X-ray analyses confirmed the possible interactions of bacterial cell surface ligands like hydroxyl, carbonyl carboxyl, and amine groups with Cd⁺² ions during the biosorption process by means of surface adsorption, ion exchange, and micro-precipitation. Transmission electron microscopic analysis confirmed the possible intracellular metal accumulation in live bacterial biomass. Therefore, CdRPSD103 is an effective bacterial strain for removing Cd(II) from saline metal-contaminated wastewater. The findings of this study can also be helpful for future widespread use of bacterial biomass, in both batch and continuous processes, to remove different hazardous metal ions from industrially polluted water systems.