Biosorption of Hexavalent Chromium by Bacillus megaterium and Rhodotorula sp. Inactivated Biomass

Abstract

Due to the adverse effects of hexavalent chromium (Cr6+) on human health and the quality of the environment, the scientific community has invested a lot of effort to solve this pollution problem. Thus, implementing sustainable alternatives for Cr6+ elimination by exploiting the capacity of microbial biomass to retain heavy metals by biosorption is considered an economic and eco-friendly solution, compared to the conventional physico-chemical processes. However, the ability of microorganisms to remove Cr6+ from liquid effluents can strongly be affected by biotic and abiotic factors. With these issues in mind, the main purpose of this paper was to investigate Cr6+ biosorption on Bacillus megaterium and Rhodotorula sp. biomass inactivated by thermal treatments, exploring the effects of some factors such as: pH, biosorbent dose, initial concentration of the metal in solution, temperature and contact time between the biosorbent and the metal ions on process effectiveness. The results showed that Cr6+ removal by biosorption on the selected microorganisms was strongly influenced by the pH of the solution which contains chromium, the reduction being the principal mechanism involved in hexavalent chromium biosorption. Equilibrium and kinetic studies were also performed, together with SEM-EDX and FTIR spectra, to explain the mechanisms of the biosorption process on the selected biomasses. Maximum uptake capacities of 34.80 mg/g biosorbent and 47.70 mg/g biosorbent were achieved by Bacillus megaterium and Rhodotorula sp., respectively, at pH 1, biosorbent dosage of 8 g/L, 25 °C, after a contact time of 48 h and an initial Cr6+ concentration in solution of 402.52 mg/L. The experimental results showed that Cr6+ biosorption by selected microorganisms followed the Elovich model, the values of the correlation coefficients being 0.9868 and 0.9887, respectively. The Freundlich isotherm model best describes the Cr6+ biosorption by Bacillus megaterium and Rhodotorula sp., indicating that a multilayer biosorption mainly controls the process and is conducted on heterogeneous surfaces with uniformly distributed energy.