Simultaneous Sulfur Black 1 dye removal and Na₂S recovery via a sulfur-circular process: Advancing closed-loop...

Sulfur Black 1 (SB1) is a widely used dye in the textile industry. Its dyeing process generates significant amounts of wastewater containing high-molecular-weight, refractory, and toxic organic pollutants, along with sulfate. This study proposed a novel sulfur-circular process integrating persulfate-based advanced oxidation process (AOP), microbial sulfate reduction, and gas stripping-adsorption to simultaneously remove SB1 and recover sodium sulfide (Na₂S) from dyeing wastewater. In a 542-day trial, persulfate activation by zero-valent iron particles generated sulfate radicals (SO₄•^-) and hydroxyl radicals (•OH), achieving 98% oxidative degradation of SB1 in simulated dyeing wastewater. The AOP-treated wastewater, which contained biodegradable organic by-products and sulfate, was subsequently treated in an upflow anaerobic packed-bed sulfidogenic reactor (UAPSR). In the UAPSR, 42.7‒64.2% of sulfate was reduced, producing 60‒150 mg S/L dissolved sulfide. Meanwhile, the organic AOP by-products were efficiently mineralized, with a total organic carbon removal efficiency of about 80%. Fermentative bacteria (e.g., Lactococcus and Acidipropionibacterium) converted high-molecular-weight organic oxidation by-products into low-molecular-weight organics, which were further mineralized by sulfate-reducing bacteria (e.g., Desulfobacter and Desulfovibrio). Functional genes related to sulfur and carbohydrate metabolisms were also identified. Moreover, over 99% of dissolved sulfide in the UAPSR effluents was stripped and then absorbed with a NaOH solution, enabling the recovery of Na₂S as a low-cost solubilizer recycled for the SB1 dyeing process. This study offers essential fundamentals for developing cost-effective and closed-loop technologies to treat wastewater containing refractory dyes and other persistent organic pollutants.