The special mechanisms of quasi-complexation and enhanced effect of “·OH action zone” in activated carbon combined ozonation

Abstract

Activated carbon (AC) is an excellent adsorbent and is widely employed as a catalyst in catalytic ozonation. However, the massive change in surface oxygen groups on AC before and after ozonation in this research indicated that AC was not a real catalyst. Moreover, the lower oxalic acid removal effect and the observed hydroxyl radicals (·OH) accumulative amount of pre-oxidative AC combined ozonation confirmed that AC functioned as an activator. We investigated the removal trend of oxalic acid in AC-activated ozonation under different pH, and it was consistent with its adsorption via adjusting the oxalic acid adsorbed on AC. Despite the accumulation concentration of ·OH reaching the highest value at pH = 4.7, the activated ozonation did not perform best. Considering the extremely short life of ·OH in an aqueous solution, an “action zone” existed, which was the effective region for ·OH to react with organics. This result proved that the AC adsorption would concentrate organics in the “action zone”, thus improving the degradation of organic contaminants. The solution containing oxalic acid had a higher accumulative concentration of ·OH than without oxalic acid, which meant a previously neglected pathway existed to produce ·OH. In addition, C2O4·- was reported in the solution containing AC, oxalic acid, and oxygen. Thus, the quasi-complexation reaction might occur between AC, oxalic acid, and oxygen. Overall, this study disclosed distinct mechanisms due to AC adsorption in the AC combined ozonation system.