Direct and indirect oxidation removal of chloride ions from sulfuric acid wastewater using photoactivated PMS/PS: efficiency and mechanism

Recycling is the current development direction for the treatment of sulfuric acid wastewater, where the concentration of chloride (Cl(-i)) impurity below 50 mg L-1 is a prerequisite. In this study, a clean, cheap and effective advanced oxidation approach was proposed for the removal of Cl(-i) using photoactivated peroxymonosulfate (PMS) or persulfate (PS). The removal efficiency of Cl(-i) via PMS or PS under ultraviolet (UV) light or sunlight was successively determined, and the mechanism for the removal of Cl(-i) was explored. For PMS, the contribution ratios of direct oxidation via PMS, indirect oxidation via hydroxyl (center dot OH) radicals and indirect oxidation via sulfate (center dot SO4-) radicals were 13.3%, 14.9% and 71.8%, respectively, under UV light. For PS, the contribution ratios of center dot OH and center dot SO4- radicals under UV irradiation were 28.3% and 71.7%, respectively. Under suitable conditions, both PMS and PS showed excellent performance for the removal of Cl(-i) with a residual concentration of Cl(-i) below 50 mg L-1. Owing to its higher stability in air, lower dosage under sunlight, shorter reaction time and lower power consumption, PMS exhibited more advantages in the aspects of storage convenience, electricity and reagent cost, and application possibilities under sunlight compared to PS. PMS and PS showed excellent performance in removing Cl(-i) as Cl2 from sulfuric acid wastewater under UV irradiation. Indirect oxidation via center dot SO4- radicals played an essential role in removing Cl(-i).