Dynamic, Mechanistic, and Thermodynamic Modeling of Zn(II) Ion Biosorption onto Zinc Sequestering Bacterium VMSDCM

The surface of the bacterial cells before the biosorption of Zn(II) ion has been found rough, heterogeneous, and non-crystalline together with tremendous protrusions and negatively charged functional groups. The bacterium was characterized as rod shaped with Gram-negative type of cell wall structure. In reaction dynamics, pseudo-second-order kinetics with higher linear correlation coefficient (R-2) ranging between 0.97 and 0.99, lower sum of square errors (SSE) (0.035-0.081) and chi ((2)) (0.0013-0.009) provided a better explanation of sorption of Zn(II) ion on bacterium surface as compared to pseudo-first-order model. The removal of Zn(II) was governed by both film and intra-particle diffusion at onset and later stage of sorption of metal ion on the surface of bacterial cells. The R-2 (0.92-0.94) for intra-particle diffusion model was quite higher with lower values of SSE (9.56-16.33) and chi ((2)) (11.26-19.65) against the Bangham's model. The positive value of H (16.628x10(-6)kJ/mol) and S (5320.90kJ/mol/K) showed that the biosorption of Zn(II) ion across liquid phase on bacterial surface was endothermic with increased randomness at solid-liquid interface. The negative values of G demarcated the whole process as spontaneous in nature. In the present work, the distribution coefficient was found to be >0.5 at various temperature ranges. At the attainment of equilibrium, the residual concentration of Zn(II) ion in liquid phase was around 0.6mg/L, which was much below the limit described by United States Environmental Protection Agency (USEPA), i.e. 5mg/L.