Abstract
Environmental pollution caused by heavy metals such as Cr(VI) is posing serious threats to humans and other living organisms. Bioreduction of toxic Cr(VI) to low toxic Cr(III) using extracellular polymeric substances (EPS) has gained attention in recent decades due to its cost-effectiveness and environmentally friendly nature. However, the effect of naturally occurring anions such as oxalate, citrate, and phosphate which have the potential to alter the sorption process needs to be investigated. Thus, the adsorption of Cr(VI) by EPS of Pseudomonas fluorescens was investigated in the presence of citrate, oxalate, and phosphate. Zeta potential measurements, spectroscopic and thermodynamic analyses were performed to study the biosorption process. After interacting with Cr(VI), the zeta potential of EPS became less negative suggesting the presence of cationic Cr species. The adsorption capacity of EPS for Cr(VI) was significantly (p < 0.05) enhanced in the presence of oxalate and citrate relative to free EPS and EPS-phosphate. This observation was attributed to the ability of oxalate and citrate to enhance Cr(VI) bioreduction to Cr(III). The maximum adsorption capacities were respectively 91.1, 48.4, 38, 30.2 mg g−1 for EPS-oxalate, EPS-citrate, free EPS, and EPS-phosphate. Cr adsorption isotherms fitted the Langmuir, Dubinin-Radushkevich (D-R), and Freundlich equations well. The adsorption process was both spontaneous and exothermic with ΔGads values of -19.0, -21.5, -20.8, and -26.7 kJ mol−1 and ΔHads of -165.8, -100.9, -274.6, and -104.4 kJ mol−1 for EPS-Cr(VI), EPS-Cr(VI)-phosphate, EPS-Cr(VI)-oxalate, and EPS-Cr(VI)-citrate, respectively. This study serves as a reference for optimizing the adsorption and bioreduction of Cr(VI).
Published Version
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