Fundamental aspects of reactions during chlorine dioxide generation and use

The thesis focuses on the fundamental aspects of reactions during chlorine dioxide generation process and the decrease in AOX formation during chlorine dioxide delignification of chemical pulps.; The chemistry involving chlorous acid, which is a major intermediate in the chlorine dioxide generation processes, was studied. It was concluded that chloride plays a crucial role on the chlorine dioxide formation from both the disproportionation of chlorous acid and the reaction between chlorous acid and hypochlorous acid. Under the typical condition of a commercial chlorine dioxide generator, it was found that chlorine dioxide is produced only if chloride is present. The underlying mechanisms of the chloride effect on the reactions were discussed. The kinetic study on the HOCl-ClO₂ − reaction under slightly acidic condition shows that chlorine dioxide is formed through two distinct pathways, one is promoted by chloride, the other is independent of chloride. The overall kinetics of the chlorine dioxide generation from the reaction is: d&sqbl0;ClO2&sqbr0;/dt=&cubl0;k1 +k2&sqbl0;Cl^-&sqbr0;&cubr0;&sqbl0;ClO ^-2&sqbr0;&sqbl0; HOCl&sqbr0;&sqbl0;H⁺&sqbr0; ; Based on the development of chlorine dioxide, chlorous acid and chloride determined experimentally, the mechanism of the H₂O₂-based ClO₂ generation process was proposed. It was concluded that the HClO₃-HClO₂ reaction is responsible for the formation of chlorine dioxide, and chloride is a catalyst in such a reaction.; It was experimentally confirmed that hydrogen peroxide can rapidly react with hypochlorous acid/chlorine under typical chlorine dioxide generation and bleaching conditions. The addition of hydrogen peroxide as a co-reducing agent in the methanol-based chlorine dioxide generation process not only eliminates the chlorine byproduct but also increases the chlorine dioxide generation rate. Furthermore, the addition of hydrogen peroxide in an equivalent amount to the chlorine contained in chlorine dioxide solution decreases the organically bound chlorine significantly. Implications of these experimental findings on the operation of commercial chlorine dioxide generators and bleaching sequences were suggested.