Sunlight-driven sulfite/Cr(VI) photodegradation of potassium butyl xanthate in mining wastewater: pH-dependent radical evolution and the overlooked role of Cr(V)

Potassium butyl xanthate (PBX) is a widely used flotation collector that can persist in tailings waters and may generate toxic by-products. In sunlit tailings ponds, sulfite (S(IV)), Cr(VI), and PBX may coexist under certain conditions, yet the impact of coupled S(IV)-Cr(VI) photochemistry on PBX attenuation remains unresolved. We quantified PBX degradation in a sunlight/S(IV)/Cr(VI) system across pH 3-10, covering tailings-water variability and pH-driven speciation. The process enabled simultaneous PBX removal and Cr(VI) detoxification, with optimum performance at mildly acidic-neutral pH. At pH 6, k obs increased from 3.9×10 -3 min -1 (sunlight) to 2.35×10 -2 min -1 (sunlight/S(IV)) and 3.38×10 -2 min -1 (sunlight/S(IV)/Cr(VI)), confirming synergy. Quenching/probe tests and time-resolved EPR implicated SO 4 •− , •OH, and Cr(V), indicating pH-dependent coupling between the sulfite radical chain and the Cr(VI)/Cr(V)/Cr(III) cycle. DFT and LC-MS showed acidic hydrolysis/self-decomposition of PBX to CS 2 at pH 3, whereas near-neutral conditions favored radical/Cr(V)-mediated oxidation with efficient Cr(VI) reduction. Outdoor natural-sunlight tests using laboratory-generated flotation wastewater prepared from raw ore collected from the mine (with spiked S(IV) and Cr(VI)) achieved 90% PBX removal within 60 min at pH = 6, T ≈ 35-36 °C and solar irradiance ≈ 750-850 W m -2 ,indicating preliminary feasibility of the coupled process in a scenario-based flotation-wastewater matrix.