Chlorine and Chlorine Dioxide Bleaching Chemistry
To understand or appreciate the complex chemistry of bleaching with chlorine dioxide, it is necessaruy to set out some fundamental information about the various active chemical species involved. The oxidation level of the chlorine atom in chlorine dioxide (ClO2) is +4, while in elemental chlorine (Cl2) the oxidation level of the chlorine atom is +1. Based on reduction to chloride ion (Cl-), which has a oxidation level of -1, there is a change of 5 oxidation levels for ClO2 and 2 for Cl2. Chlorine dioxide thus has 2.5 times the oxidation potential of elemental chlorine and is therefore more effective (on a weight basis) for removing lignin from pulp by oxidation.
Elemental chlorine (Cl2), in addition to acting as an oxidizing agent, acts as a chlorinating agent. In the first stage of bleaching, about half the elemental chlorine applied to pulp combines with the lignin and the remainder oxidizes the lignin and is converted to chloride ion (Figure 1 A) (Kempf and Dence, 1970; Hardell and de Sousa, 1977). Following alkaline extraction, about 90% of the original elemental chlorine applied to pulp has been converted to chloride ion, the rest remaining as solubilized chlorinated organic material. Chlorine dioxide reacts differently with lignin. Chlorine dioxide is reduced by lignin to chlorite ion and hypochlorous acid. It is the hypochlorous acid which reacts with organic material to form chlorinated organic compounds (Figure 1 B). The relative concentration of Cl2, HOCl and OCl- is sensitive to pH (Figure 2) and careful control of the pH can reduce the formation of organochlorines.
A second difference between elemental chlorine and chlorine dioxide bleaching concerns the nature of the chlorinated products produced by the reaction of chlorine dioxide and elemental chlorine with lignin. Hypochlorous acid and elemental chlorine react differently with the chemical structures present in lignin (McKague and Reeve, 1994). For example, hypochlorous acid (a byproduct of chloride dioxide oxidation of lignin) reacts with double bonds to produce chlorohydrins (Figure 1 C). Elemental chlorine, on the other hand, reacts with double bonds to produce dichlorinated products (Figure 1 D). Chlorine is eliminated from chlorohydrins during subsequent alkaline extraction more readily than from dichlorinated products. Therefore, as a result of the different kinds of products formed by reaction of hypochlorous acid and elemental chlorine, less chlorinated organic material ultimately results from chlorine dioxide.
A third factor which contributes to the different nature of the chlorinated organic products produced in chlorine dioxide bleaching is the difference in the lignin present at the time of reaction. Hypochlorous acid is generated as a secondary product only by reduction of chlorine dioxide as the lignin is oxidized. Therefore, the lignin that is available to react with hypochlorous acid is more oxidized and contains fewer aromatic structures than that which reacts with elemental chlorine as the primary oxidant (Figure 1 A and B; McKague and Reeve, 1994). As a result, more of the hypochlorous acid is consumed by reaction with non-aromatic structures and thus less chlorinated aromatic material is formed.
The combination of the strong oxidizing capability of ClO2 (thus requiring less material), the reduction in the extent of chlorination, and the changes in lignin result in an approximately five to ten-fold reduction in the formation of organochlorine compounds when ClO2 is substituted for Cl2. In addition, the nature of these organochlorine compounds is very different (i.e. less chlorine substitution, less aromaticity). These factors influence the potential environmental properties of ClO2-derived organochlorines in terms of persistence, bioaccumulation potential, and toxicity.