One-step removal of hexavalent chromium in wide pH range using thiourea dioxide: the role of reactive species

Abstract

One-step removal of hexavalent chromium in a wide pH range is of great significance. In this paper, a single thiourea dioxide (TD) and two-components thiourea dioxide/ethanolamine (MEA) were used as a green reducing agent for the efficient removal of Cr(VI), respectively. The reduction of Cr(VI) and the precipitation of Cr(III) were carried out simultaneously under this reaction system. The experimental results proved that TD was activated by amine exchange reaction with MEA. In other words, MEA promoted the generation of an active isomeride of TD by changing the equilibrium position of the reversible reaction. After adding MEA, the removal rate of Cr(VI) and total Cr could reach industrial water discharge standards in a wide pH range of 8–12. The change of pH, reduction potential and the decomposition rate of TD were investigated in the reaction processes. Meanwhile, reductive and oxidative reactive species were produced simultaneously during this reaction process. Further, oxidative reactive species (O₂˙⁻ and ¹O₂) were beneficial for the decomplexation of Cr(III) complexes and the formation of Cr(III) precipitation. The experimental results also demonstrated that TD/MEA was effective in practical industrial wastewater. Hence this reaction system has a significant industrial application prospect.