Visible light-driven molecular oxygen activation by lead-zinc smelting slag and tartaric acid for efficient organic pollutant degradation and Cr (VI) reduction

The remediation of heavy metal and organic-contaminated wastewater remains a significant concern in environmental science, necessitating the development of cost-effective and efficient treatment systems. This study focuses on the setup of a highly effective molecular oxygen (O-2) activation system using lead-zinc smelting slag (LZSS) and tartaric acid (TA) under visible light. The LZSS-TA complex exhibited rapid photochemical reactions and demonstrated a remarkable capacity for O-2 activation, resulting in the in situ generation of hydrogen peroxide (H2O2) and 99 % degradation of para-chloroaniline (p-CA). The formation of H2O2 occurred through a two-step single electron transfer pathway, whereby C-centered radicals ((CHOHCHOHCOO-)-C-center dot) generated within the system initiated the reduction of O-2 to form O-center dot(2)-, as a crucial intermediate. The produced O-center dot(2)- is then reduced by Fe(II) to generate H2O2 via proton-coupled electron transfer (PCET) processes. Light-induced ligand-to-metal charge transfer (LMCT) process is the key to the production of C-centered radicals and Fe(II), which determines the production efficiency of H2O2 and (OH)-O-center dot. Additionally, this system enabled simultaneous oxidation of p-CA by 98 % and reduction of Cr(VI) by 100 % within 60 min. More importantly, O-2 activation by TA-assisted LZSS under natural sunlight/solar irradiation could efficiently degrade p-CA in both ultrapure water (94 %) and reservoir water (92.5 %) within 210 min. This study significantly contributes to the understanding of light-driven O-2 activation and lays the foundation for the design of cost-effective and efficient pollution remediation systems.