Removal of hexenuronic acid to reduce AOX formation in hot chlorine dioxide bleaching of bagasse pulp

Abstract

In this study, the effects of the hemicellulose-linked hexenuronic acid (HexA) on the kappa number, brightness reversion and effluent properties under the hot chlorine dioxide bleaching (D-hot) stage were investigated. Moreover, the effects of the chlorine dioxide content on the residual HexA of bagasse pulp in D0 and D-hot stage was evaluated. HexA was determined by UV–vis, and the absorbable organic halogen (AOX), chemical oxygen demand (COD) and total organic carbon (TOC) in the bleached effluent were analyzed by using the AOX analyzer, COD analyzer and TOC analyzer, respectively. The results showed that the amount of chlorine dioxide could be reduced by D-hot pretreatment. The kappa number and brightness reversion of the bleached pulp obviously improved. Compared to the control, the AOX content decreased dramatically with an increase in temperature. However, the COD and TOC increased significantly. A kinetic model considering the removal of hexenuronic acid and its degradation was proposed. Degradation reactions of hexenuronic acid are first-order with respect to their content, and the pH influences the degradation of hexenuronic acid. The activation energy is 2.23 kJ/mol when the pH is 3.2, indicating that the degradation of hexenuronic acid is very rapid under these conditions.