Chlorination Products

It is well established that hypochlorous acid (HOCl) and chlorine (Cl2) are responsible

for the chlorination products found in chlorine dioxide-bleached pulps (see

Section 7.4.7) [12,25–27]. Both are in a pH-dependent equilibrium [see Section

752 7Pulp Bleaching

7.4.2, Eq. (66)]. In general, the pH influences the stability [28] and the mechanism

of chlorine dioxide oxidation of aromatic compounds [29]. Normally, hypochlorous

acid and chlorine are found in very low concentrations, are consumed rapidly, and

cannot be detected in bleaching filtrates [30].

There are two possible explanations for the higher organic chlorine at pH 2 [31].

One explanation is that chlorous acid reduction by Cl– is occurring [Eq. (76)

[32,33]. The half-life of this reaction at pH 2 and 25 °C has been reported to be as

low as 17 min when chloride ions are in excess of chlorite [32,33], indicating that

– under the correct conditions – hypochlorous acid forms fast through:

HClO2_Cl__H_ _ 2HOCl _76_

The net result would produce more hypochlorous acid, which could further

react, increasing the amount of chlorinated products at pH 2 [Eq. (77).

Lignin _ HOCl _ Lignin _ Cl _ H2O _77_

Another explanation for the higher amount of organic chlorine at low pH is

that more elemental chlorine is present at pH 2. Hypochlorous acid is in equilibrium

with elemental chlorine through a hydrolysis reaction. The rate of chlorination

reactions [Eq. (78)] increases when more elemental chlorine is present

because of its greater oxidation potential compared to hypochlorous acid:

Lignin _ Cl2 _ Lignin _ Cl _ H2O _ H__Cl_ _78_

The formation of chlorinated organic compounds is influenced by, for example,

hexeneuronic acid (HexA) [29,34,35], and can be reduced by using dimethylsulfoxide

(DMSO) [36,37], lower chlorine dioxide charges [38], or by adding sulfamic

acid [4,38]. However, polychlorinated products such as dibenzo- p -dioxins and

dibenzofurans [39, 40] and chloroform [41,42] have also been found in the process

water of kraft pulp bleaching mills. The concentrations of dioxins were found to

be below 1p g TEQ L–1 (TEQ = toxic equivalency quantity). Approximately 30% of the

chloroform produced in ECF (ECF = elemental chlorine free) bleaching of hardwood

oxygen-delignified kraft pulp are discharged to bleaching effluents [42]. Chloroform

in the effluents was not decomposed by activated sludge, and more than

97% was emitted to the air by volatilization. Therefore, an ecological risk of using

chlorine dioxide in bleaching is immanent [43]. However, Nakamata et al. [42] suggested

to introduce ECF bleaching into all mills. This would lead to a considerable

fall from the chloroform discharge from ECF and chlorine bleaching mills down

to about 3% (based on data from 1999). Chlorinated compound concentrations in

final effluent (following secondary treatment) have been found to comprise a negligible

risk to aquatic organisms in comparison with the known toxicological

thresholds [43,44]. The ecological risk associated with (mono)-chlorinated compounds

[45] that derived essentially from glucuronoxylan is expected to be negligible

since they are easily degraded and not detected in the final mill effluent [46].

7.4 Chlorine Dioxide Bleaching 753

7.4.5

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