The Elementary Chlorine-Free (ECF) bleaching process for kraft pulp (KP) has prevailed in Japan. We have known that hexenuronic acid (HexA) in ECF bleached pulp plays an important role in brightness reversion (yellowing) of the pulp, and studied ways to remove HexA. Acidic sized paper made from ECF bleached pulp often causes the yellowing by heat and moisture. As a result, we noticed that peroxymonosulfuric acid (MPS, Caro’s acid) and HexA react effectively, and developed an ECF bleaching process which is equipped with a MPS stage. Consequently, the brightness reversion significantly decreased. MPS was found to be one of the bleaching chemicals which can remove HexA efficiently, and it has begun to be used in the first stage of a KP bleaching process in Japan. On the other hand, there is concern that removal of HexA may cause an increase in the environmental load from the effluent of the pulp bleaching process and a decrease in pulp yield. We have clarified the relationship between the amounts of HexA removed from pulp and the total organic carbon (TOC) in the filtrates of the washed pulp which were subjected to MPS treatment, hot acid (A∗) treatment, and chlorine dioxide (D) treatment, respectively. The environmental load in effluent was compared between the various methods for HexA removal. From the results, it was found that the effluent load of the MPS process was considerably lower compared with other processes. In addition, it was suggested that the reaction of HexA removal by MPS treatment should be different from that of cutting the bond between a xylan chain and a HexA residue.
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