A Review and Assessment of the Ecological Risks Associated with the Use of Chlorine Dioxide for the Bleaching of Pulp
Harold Bergman2, Robert Huggett3
Table of Contents
From an environmental risk assessment perspective, chlorination of organic substances usually produces an incremental effect on properties that is generally understood and, to an extent, predictable. Substituting chlorine dioxide for elemental chlorine in the first stage of bleaching results in approximate five to ten-fold decrease in formation and discharge of organochlorines. This also results in a decrease in the degree of chlorine substitution which reduces the persistence, toxicity and the potential for bioaccumulation of the organochlorine compounds formed. In particular, the formation of highly hydrophobic chemicals, such as dioxins, is substantially decreased or virtually eliminated by increasing chlorine dioxide substitution to 100%. Chlorine dioxide thus reacts differently from elemental chlorine by different chemical processes and produces different organochlorine products, in smaller amounts.
A large number of organochlorine chemicals of widely differing properties may be produced from the bleaching of pulp. These range from the highly hydrophobic, persistent and bioaccumulative, toxic substances, such as 2,3,7,8-TCDD, to the higher molecular weight, non-hydrophobic, water soluble material which makes up part of the surrogate parameter Adsorbable Organic Halogen (AOX). Much of the organochlorine material characterized as AOX is of unknown structure but, for the most part, these compounds are water-soluble and non-hydrophobic and will not bioaccumulate to a biologically significant or be transferred via the food chain. They should not be confused with the hydrophobic chemicals.
Production of chlorinated phenols in mills using 100% chlorine dioxide substitution in the first stage of bleaching showed high margins of safety (greater than 100-fold) for risks to aquatic organisms. If dilution factors are higher than ten (found in most North American receiving environments), risk quotients for the 95th percentile concentrations of these substances will have margins of safety for aquatic organisms of more than 1000-fold. Although no toxicity data are available for several of the chlorinated substances that may be found in effluents, it is unlikely that the effects of these compounds will differ substantially from their better known homologues and they also present little risk to aquatic organisms.
Assessing effects of 100% chlorine dioxide substitution on populations and communities in the receiving environment is confounded by the fact that addition of nutrients to environments where turnover or dilution rate is low can cause changes in the community which mask toxicity-mediated effects. The isolation of specific, process-related, effects at the population level from this web of causality is difficult but, in the few studies conducted, responses at these levels of biological organization are not discernable, and levels of bioaccumulated substances and biomarkers such as mixed function oxidase have decreased. The effects of effluents from mills with high chlorine dioxide substitution as studied in microcosms suggests that this process results in significant reductions of risks to populations and communities as compared to bleaching with elemental chlorine.
Based on the weight of evidence from laboratory studies, field studies and the application of predictive models, the substitution of chlorine dioxide for elemental chlorine in the first stage of bleaching will result in reduction in the quantities of organochlorines produced; in the degree of chlorine substitution in the organochlorines produced; in the degree of persistence of the organochlorines produced; in the potential for bioaccumulation and food chain transfer; in potential toxicity; and, in adverse ecological effects. Based on the data available, mills bleaching with chlorine dioxide (100% substitution) and employing secondary treatment and with dilution typical of North American mills, present an insignificant risk to the environment from organochlorine compounds. However, sufficient evidence exists from observations at unbleached mills to suggest that other non-chlorinated compounds which are released or formed during the production of pulp may cause responses such as induction of mixed function oxidases and changes in hormone levels. The identification of these compounds and the elucidation of their mechanisms of action should be a priority.