Coupling Mechanisms for the Simultaneous Treatment of Phosphate- and Hexavalent-Chromium-Contaminated Wastewater
Yi Yang, Xintong You, Wei Shu, Qu Wei, Minyi Liu, Ying Li, Zhicong Chen, J. Paul Chen
Abstract
Phosphate and hexavalent chromium commonly coexist in wastewater, presenting greater risks to humans and living organisms. In this study, a bimetallic La(OH) 3 and CaO 2 -incorporated carbon nanotube (La–Ca–CNT) was developed as an innovative material for cost-effective treatment. Through the Box–Behnken design approach, low pH enhanced the removal of both phosphate and Cr(VI), while the chromium concentration had a negligible effect. Phosphate concentration improved phosphate adsorption, while it had an insignificant impact on chemical reduction of Cr(VI). A series of experiments and theoretical calculations revealed the coupling mechanism in which phosphate removal was negatively affected by Cr(VI) reduction, while Cr(VI) removal was not influenced by phosphate adsorption. Phosphate adsorption was an endothermic and combined physicochemical process with an activation energy ( E a ) of 8.6 kJ/mol and a Gibbs free energy (Δ G ) ranging from −32.2 to −42.8 kJ/mol; the treatment was achieved by a sequential process of proton transfer and ligand exchange. Chemical reduction of Cr(VI) followed an adsorption-controlled catalytic mechanism with a low E a value of 3.7 kJ/mol. In the coupling system, the removal of both phosphate and Cr(VI) included electrostatic attraction and surface complexation. This study elucidates the coupling mechanisms in the simultaneous treatment of phosphate- and Cr(VI)-containing wastewater, providing theoretical support for wastewater treatment.