Oxalate enhanced reduction of Cr(VI) by coke-making by-product—Coke powder

Coke powder (CP), derived from the carbonization of coal, was systematically investigated for its enhanced activity in Cr(VI) reduction by oxalate (Ox). The effects of three operating variables—Ox concentration, CP dosage, and pH—on the reduction of Cr(VI) were optimized using response surface methodology (RSM) based on the Box–Behnken design. Approximately 51.9 % of Cr(VI) could be reduced in the CP/Ox system in the dark, which increased to over 99.4 % with the introduction of visible light irradiation. pH plays a crucial role in the removal of Cr(VI), with lower pH levels being more conducive to Cr(VI) removal. Mechanism investigations implied that the ash in CP was adverse to Cr(VI) removal in the dark because it occupied active sites. While, the ash was beneficial to Cr(VI) removal under visible light irradiation since it could activate Ox to generate O2•−. The high efficiency of Cr(VI) reduction could be mainly attributed to the aromatic structure and graphite layers of CP, which promote the transfer of electrons from Ox to Cr(VI). As revealed by electrospray ionization-mass spectrometry (EIS-MS), the possible pathway of Cr(VI) reduction in the CP/Ox/Vis system was elucidated. These findings provide new insights into the resource utilization of solid waste and the treatment of heavy metal sewage. [Display omitted] •Enhanced Cr(VI) reduction by activated oxalate with coke powder is studied.•Main influence factors of Cr(VI) reduction are probed by RSM.•Cr(VI) reduction is closely associated with carbonaceous components in coke powder.•Ash enhances Cr(VI) reduction with visible light irradiation due to O2•− generation.•The Cr(VI) reduction pathway is revealed by EIS-MS.