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

Abstract

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.