Synergistic Removal of Typical Heavy Metal and Organic Contaminants via FeS2/α-FeOOH/C from Electronic Industry Wastewater: Insights for Selective Degradation and Promotion

The simultaneous and sustainable removal of a typical heavy metal (Cr(VI)) and benzoic acid (HBA) via an FeS2/α-FeOOH/C/visible light system without consumption of extra reductive or oxidizing agents was performed to study the influence of HBA degradation on Cr(VI) reduction. The results showed that the influence order of different HBA options in accelerating Cr(VI) reduction was as follows: o-HBA > p-HBA > p-CBA > blank > m-HBA. With the addition of o-HBA, the Cr(VI) removal rate constant was increased by 1.5 times (0.047 to 0.119 min−1). Almost 100% of Cr(VI) and 70% of o-HBA were removed within 60 min under the initial Cr(VI) and o-HBA concentrations of 10 and 5 mg/L. Quenching experiments indicated that photogenerated e− and •O2− played an important role in Cr(VI) reduction, while photogenerated h+ and its derived •OH contributed to HBA degradation. Due to •O2− and •OH, separately ascribed to photogenerated e− and h+, the timely consumption of •O2− and •OH accelerated the separation and generation of photogenerated carriers, further improving light utilization efficiency and resulting in synergetic improved removal performance for both Cr(VI) and o-HBA. Theoretical calculations indicated the electron-donating ability of hydroxy groups in o-HBA was better than that in other HBA, making the activation energy for addition reaction with •OH lower. Hence, •OH would be consumed more quickly, leading to a higher promotion effect from o-HBA on Cr(VI) reduction. The real wastewater treatment experiment indicated the high applicability of synthesized FeS2/α-FeOOH/C for synergetic and sustainable removal of Cr(VI) and coexisting organic pollutants in electronic industry wastewater.