Simultaneous removal of catechol and Cr(VI) from tannery wastewaters through Fe(III)-mediating electron transfer

Cr(VI) and catechol are common pollutants in wastewaters of tannery industry. Cr(VI) reduction by catechol results in accumulation of Cr(V)-complex and toxic o-benzoquinone. Cr(VI) reduction can be enhanced by accelerating the electron transfer from catechol to Cr(VI) using Fe(III). This electron transfer allows the continuous production of reactive oxygen species (ROS) in advanced oxidation processes, which further degrades catechol. However, the effects of catechol on Fe(III)-based Cr(VI) reduction and ROS production are not clear. In this study, the electron transfer pathway between catechol, Fe(III) and Cr(VI) was investigated along with the catechol degradation by ROS. Results show that the process of Cr(VI) reduction follows well a pseudo-second-order kinetic model, and the activation energy decreases by 4189 J/mol after electrons are transferred by Fe(III). Aqueous chromium (50 mg/L) was completely precipitated as Cr(OH)3, FexCr1-x(OH)3 or Fe(OH)3-Cr(OH)3 at Fe(III) concentration of 10 mmol/L. Residual catechol and Fe(II) in wastewater activated percarbonate, thereby producing OH for over 480 min at pH 3.0 and generating OH and CO3− for over 5 min at pH 10.0. Catechol was degraded by these ROS to butenedioic, oxalic, and formic acids, which were then mineralized to H2O and CO2. Catechol (2 mmo/L) was fully removed from wastewater with TOC removal of 37%, and the degradation was not affected by SO42− and Cl−. This study provides new insights into the removal of pollutants from wastewaters via electron transfer mediated by Fe(III), and may aid in controlling the toxic effects of Cr(VI) and catechol-like organics in natural and engineered aquatic systems.