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Recently, a new type of bleaching sequence, Elemental Chlorine Free (ECF) light with one D stage, has been developed. It combines the efficiency and high selectivity of chlorine dioxide (ClO 2 ) bleaching with more environmental friendly oxygen based bleaching chemicals. This work examines the effect of pH on the formation of adsorbable organically bound halogens (AOX) in an intermediate D stage – a single ClO 2  stage at the middle of an ECF light bleaching sequence. Carbon dioxide (CO 2 ) is used to generate a bicarbonate buffer  in situ , stabilizing the pH during the bleaching. Near-neutral pH is hypothesized to decrease the formation of strongly chlorinating species, so that the AOX formation is reduced. The results indicate that a near-neutral pH D stage can reduce the AOX content in the effluents with up to 30%. The ISO brightness was unchanged to a lower ClO 2  consumption. The pulp viscosity was slightly higher after near-neutral pH D stage, but to its disadvantage a lesser delignification and removal of HexA was obtained. The degradation of HexA correlated well with the AOX, affirming earlier theories that HexA has a major impact on the AOX formation. The higher amounts of residual HexA and lignin resulted in more thermal yellowing of the pulps bleached with a near-neutral pH D stage.

Reduction of adsorbable organically bound halogens (AOX) formation at near-neutral pH chlorine dioxide bleaching of softwood kraft pulp