Degradation of typical flotation reagents using lead-zinc smelting slag as mediator for persulfate activation: Effect of gallic acid and Cr(VI) on the removal performance and fate of reactive oxygen species

To remediate the Cr(VI)-organic co-contaminants in a non-ferrous mining area, a gallic acid (GA) accelerated lead-zinc smelting slag (LZSS, a mine-sourced waste) mediated peroxodisulfate (PDS) Fenton-like system was constructed for degradation of two typical flotation reagents (benzotriazole and N-hydroxyphthalimide). LZSS acting as an in-situ Fe source in the Fenton-like process, could continuously release Fe species, while GA as a chelate with reducing properties was able to accelerate the rate-limiting step of Fe(III)/Fe(II) cycle to enhance the production of reactive oxygen species (ROS). In the LZSS/PDS/GA system, produced SO4•-, •OH and Fe(IV) jointly contributed to the contaminant removal through radical/nonradical pathways. However, when Cr(VI) coexisted with organic pollutants in the LZSS/PDS/GA system, the reduction of Cr(VI) consumed the electrons that otherwise would have been available for activation of PDS, resulting in fewer different ROS being produced. The increased concentration of GA, as an electron donor, promoted the production of SO4•-, but this promoting effect gradually diminished with increasing Cr(VI). Overall, the dominant ROS gradually transformed from Fe(IV) to SO4•-/•OH as the GA level increased or the Cr(VI) level decreased. Therefore, regulation of the relative roles of ROS by adjusting either the GA dosage or the Cr(VI) levels in the wastewater can improve availability of ROS for further specific removal of pollutants. This study offers an all-in-one solution for utilization of LZSS industrial waste and degradation of flotation reagents, and it also provides a new insight into the advanced environmental application of GA in remediation of Cr(VI)-organic co-contamination.