Sequential removal of cyanide and phenol using zerovalent iron and Rhodococcus pyridinivorans PDB9T NS-1 encapsulated in a freeze-dried Ba-alginate nano-biohybrid system

Abstract

Phenolic compounds and cyanides are the major pollutants in coke-oven wastewater, posing serious human health hazards and ecosystem damage. Although the microbial biodegradation of phenolic pollutants is an eco-friendly and efficient technique, even very low cyanide concentrations inhibit the growth of phenol-degrading microorganisms. Furthermore, below pH 10, cyanide is converted into HCN gas (pKa = 9.2). Therefore, cyanide treatment requires a pH greater than 10; however, microbial growth is not possible at such a high pH. Thus, in the present study, a sequential nano-microbial system of nano zero valent iron (nZVI) and Rhodococcus pyridinivorans PDB9T NS-1, encapsulated in freeze-dried Ba-alginate beads, was designed for the removal of cyanide and a phenol from synthetic wastewater. The structural and morphological analyses of nZVI and bacterial-immobilized freeze-dried Ba-alginate beads were performed using XRD, FTIR, SEM, EDX, and XPS techniques. The results revealed that at an initial cyanide and phenol concentration of 100 mg L-1 and 300 mg L-1, respectively, nZVI achieved almost complete cyanide removal after 120 min, followed by 95% phenol biodegradation by R. pyridinivorans PDB9T NS-1 encapsulated in freeze-dried Ba-alginate beads. The cyanide removal process was found to follow a pseudo-second-order kinetic model, suggesting that cyanide removal occurred via chemisorption, with a sorption capacity of 103 mg g-1. The thermodynamic and intraparticle diffusion models revealed that the cyanide removal process was endothermic and spontaneous, with nZVI and exterior mass transfer primarily governing it. Additionally, the nanocomposite demonstrated strong reusability, remaining effective across four adsorption cycles.