Hexavalent chromium bioremediation using Hibiscus Sabdariffa calyces extract: Process parameters, kinetics and thermodynamics

Abstract

Hexavalent chromium toxicity remains an indispensible toxicity to bioremediation. It has environmental and health impact in areas of hexavalent chromium discharges. This persistent problem associated with hexavalent chromium toxicity had led to employment of both physical and chemical means of control, which suffers major drawbacks. These drawbacks include; incomplete metal removal, high energy requirement, high reagent consumption and high cost of implementation. Bioremediation via bioreduction can be explored using plants rich in antioxidants. Hibiscus sabdariffa is a plant rich in antioxidants. In this study the bioreduction potential of Hibiscus sabdariffa calyces extract in bioremediation of toxic hexavalent chromium (Cr VI) to less toxic trivalent chromium (Cr III) was examined. In a batch mode, the effect of process parameters viz pH, temperature, initial Cr (VI) concentration, contact time and bioreductant concentration was assessed on the bioreduction process. Hexavalent chromium was quantified using diphenyl carbazide complexing agent that forms violet colour. The kinetic and thermodynamic of the process was also investigated. The extract (1 mg/ml) was able to bioreduce 72% of 10 mg/L hexavalent chromium solution in 15 min. The optimized condition of the process was achieved at pH 2.0 and 25 °C. Kinetics studies on the bioreduction revealed pseudo second-order as the main kinetic path the reaction takes with R2 values of 0.928, 0.993 and 0.997. The extract has the potential to reduce hexavalent chromium which is toxic to trivalent chromium (less toxic).