Strong enhancement on acetaminophen removal in thermal-activated peroxymonosulfate with sodium metaborate: Performance, mechanism, and control of chlorinated by-products formation

This study introduced sodium metaborate (NaBO₂) into thermal/PMS system to enhance acetaminophen degradation. The added NaBO₂ possessed better performance than sodium tetraborate in accelerating the degradation of acetaminophen with thermal/PMS system over the wide pH range from 3.0 to 11.0. NaBO₂ could be partly hydrolyzed to the strong buffer capacity of the pair of H₃BO₃/B(OH)₄⁻ to well keep the reaction solution pH at 9.4–9.7 regardless of the initial pH. HOOB(OH)₃⁻ and HOOBO were produced via the reactions between PMS and B(OH)₄⁻/BO₂⁻. ¹O₂ and HO^• were the main ROS accountable for acetaminophen degradation in NaBO₂/thermal/PMS system. ¹O₂ was mainly formed from the self-decomposition of PMS and the reactions between PMS and HOOB(OH)₃⁻/HOOBO. HO^• was mainly formed from the thermolysis of the O-O bonds of PMS, HOOB(OH)₃⁻ and HOOBO. Three elimination pathways of acetaminophen were proposed according to the identified nine elimination intermediates, and the reaction solution toxicity was significantly decreased in NaBO₂/thermal/PMS system. Improving NaBO₂ dosage, PMS concentration and reaction temperature could accelerate acetaminophen removal. Cl⁻ and Br⁻ significantly promoted acetaminophen removal, while humic acid had the adverse effect. NaBO₂/thermal/PMS system had well anti-interference capacity for actual water constituents. Moreover, the added NaBO₂ could significantly inhibit the production of chlorinated by-products via the transformation of the free chlorine to the inert product of B(OH)₃OCl⁻ in Cl⁻-containing water treatment with NaBO₂/thermal/PMS system. This study provides an effective method for improving the thermal/PMS system oxidation capacity and suppressing the chlorinated by-products production in Cl⁻-containing water treatment.