Chlorine dioxide reactions with lignin model compounds and kraft pulps

Chlorine dioxide (ClO₂), a free-radical oxidizer, was reacted with a non-phenolic lignin model compound (1-(3,4,4-methoxyphenyl) ethanol), a kraft pulp, and a methylated kraft pulp to determine the main sources of chlorate formation during wood pulp bleaching. Inorganic compounds were measured using ion exchange chromatography and capillary electrophoresis. Analysis of reaction samples indicate that chlorate forms largely by inorganic reactions which involve hypochlorous acid (HOCl). Under alkaline reaction conditions, hypochlorite (ClO−) oxidizes chlorine dioxide to chlorate (ClO₃−), lowering reaction efficiency. Under aqueous acid conditions, hypochlorous acid reacts with chlorite (ClO ₂−), forming a di-chlorine dioxide (Cl₂O ₂) intermediate, which further reacts to form chlorate via hydrolysis. Chlorine dioxide reaction efficiency is improved when chlorite is added to a reaction medium. This is because chlorite scavenges hypochlorous acid and hypochlorite ions, suppressing chlorine dioxide oxidation. Furthermore, chlorite ions react with the di-chlorine dioxide intermediate (Cl₂O ₂) to form chlorine dioxide, rather than the hydrolysis product (ClO ₃−).