Formation of AOX and OCI in ECF-bleaching of birch pulp

How does the hexenuronic acid (HexA) in birch kraft pulp influence the formation of chlorinated organic substances in the effluent (AOX) and in the pulp (OCI)? Industrial birch chips cooked to different kappa numbers were oxygen delignified, treated in a hot acid stage, or not pretreated at all before D(EP)D bleaching. The AOX and OCI contents were determined and the filtrate from the first D stage was also stored for 10 days at pH 7 and 35°C, and the AOX levels were determined again. The amount of HexA removed in the first D stage correlated with the unstable part of the AOX formed in this stage, with this unstable part consisting of the AOX that is eliminated during 10 days of storage. This linear relationship is strong evidence that the unstable AOX originates from HexA. The levels of stable AOX and OCI on the other hand, correlated with the Klason lignin content of the pulp.