ANALYSIS OF IN-SITU PRODUCED HYPOCHLOROUS ACID AND ITS USE IN UNDERSTANDING CHLORATE FORMATION IN CHLORINE DIOIXDE DELIGNIFICATION

摘要

In chlorine dioxide delignification and bleaching, the byproduct chlorate is unfriendly to environmental and its formation wastes the oxidation power of ClO_2. Thus it is important to understand its mechanism of formation in order to improve chlorine dioxide bleaching efficiency and environmental friendliness. It has been known that the formation of chlorate is highly dependent on pH conditions, which decreases with increasing bleaching end pH. However, this phenomenon was not well explained. Our result turned out that chlorate formation increased slightly when the bleaching end pH is increased from 1.8 to 2.5 but decreased greatly when the pH continued in increasing. To elucidate the above phenomenon, the behaviors of HClO, Cl_2, ClO~-, HClO_2 and ClO_2~- according to pH were discussed after dividing the bleaching pH ranges into several parts. Hypochlorous acid was analyzed by adding DMSO, a very effective hypochlorous acid scavenger, to the bleaching stages and detecting the produced DMSO_2 by HPLC. It was found that chlorite is available throughout the whole pH ranges and the formation of hypochlorous acid decreases with increase end pH, this gives strong support that chlorate is mainly formed through the reaction of in-situ produced hypochlorous acid with chlorite. However, according to Emmenegger-Gorden's mechanism, chlorine favors the regeneration of chlorine dioxide while HClO favors chlorate formation, thus lowering the pH from 4 to the acidic end should decrease chlorate formation. The two opposite effects leaded to the maximum formation of chlorate at around end pH 2.5. The study suggested that a better understanding of chlorate formation should combine the mechanism of hypochlorous acid - chlorite reaction with that of Emmenegger - Gorden's.