Environ. Sci. Technol. 1996, 30, 1890-1895
Biological Dechlorination of Model Organochlorine Compounds in Bleached Kraft Mill Effluents YING ZHENG AND D. GRANT ALLEN* Department of Chemical Engineering and Applied Chemistry, Pulp & Paper Centre, University of Toronto, 200 College Street, Toronto, Ontario, M5S 3E5 Canada
Models of organochlorine compounds present in bleached kraft pulp mill effluent, with well-defined structure, can be used to provide information on the relationship between their chemical structure and their behavior toward biological dechlorination. The biological dechlorination of three monomeric model compounds, 4-methyl-5-chloromuconolactone monomethyl ester (MCME), 2-chloro-3-methylmuconolactone (CMML), and 3-chloro-4-methylcatechol (CMCA), and a dimeric model, 2-(4′-methyl-2′muconylmethyl)-3-chloro-4-methylmuconic acid dilactone (CMDL), were studied. The first two monomers are products of simulated bleaching reactions (sequentially with ClO2 and NaOH) using a monomeric model compound of residual lignin, 4-methylguaiacol (MG), and are considered to be monomeric models of organochlorines in the ClO2 bleachery effluents. Similarly, the dimer model is a product of a dimeric lignin model reacting with ClO2 and NaOH and is considered to be a model of organochlorines of higher molecular weight. CMCA was chosen as a model compound for the phenolic portion in the effluent. It was found that MCME and CMDL do not undergo biological dechlorination. CMML did not dechlorinate in the absence of biomass for over 35 days, but it was dechlorinated by biomass from a pulp mill activated sludge system within 8 days. CMCA was found to be chemically unstable and rapidly dechlorinates when dissolved in water. The recalcitrance of MCME and CMDL to biological dechlorination is postulated to result from the fact that they both possess structural features that represent a dead-end intermediate of the ortho-cleavage pathway that is common to many aerobic bacteria.
Introduction There has been increased public concern about the effects that chlorinated compounds may have on plant and animal * To whom correspondence should be addressed; telephone: (416) 978-8517; fax: (416) 978-8605; e-mail address:
[email protected].
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 6, 1996
communities in the environment. The pulp and paper industry is one of the sources of organochlorine compounds in recepient waterways. Organochlorine compounds are formed during the bleaching of wood pulp with chlorine (Cl2) and chlorine derivatives such as chlorine dioxide (ClO2). They occur in bleached kraft mill effluents (BKMEs) and are frequently collectively characterized and quantified under the group parameter adsorbable organic halogen (AOX), which is a measure of the total amount of chlorine that is associated with organic compounds. Biological treatment systems such as activated sludge and aerated lagoons have been widely used to improve the quality of pulp and paper mill effluents. These treatment systems are generally designed to remove biochemical oxygen demand (BOD), suspended solids (SS), and toxicity. Their capabilities in removing AOX have also been well recognized. In these systems, AOX is removed through volatilization, biosorption, and biological dechlorination (14). However, the extent of removal varies from mill to mill, ranging from 20 to 70% (3, 5). Regulatory agencies world-wide have begun to impose more and more stringent limits on the discharge of AOX in BKMEs. In Ontario, Canada, for instance, the regulations for AOX discharge limit are 2.5 kg/tonne of air-dried pulp (2.5 kg/ADt) in 1992 and 1.5 kg/ADt in 1996, with proposals to reduce the discharge limit further in the future (6). To meet these regulations, the pulp and paper industry has been actively searching for ways to improve their effluent quality. While internal process modification, such as 100% substitution of chlorine dioxide for chlorine, has resulted in significant reduction in AOX, color, and acute toxicity of BKMEs (7, 8), enhanced performance of external effluent treatment facilities is expected to play an important role in maximally eliminating AOX. AOX has often been divided into two main groups based on molecular weight separation in ultrafiltration membranes: low molecular weight (LMW, 1000) material (1). The HMW material is a major fraction (g80% in spent liquors) of AOX in BKMEs when chlorine is used as a bleaching agent (9, 10) and is over 50% of the AOX when 100% chlorine dioxide is used as a bleaching agent, the so-called elemental chlorine-free (ECF) bleaching (11). Clearly, in order to have effective biological treatment and some understanding of the environmental fate of organochlorine compounds in BKMEs, it is important to understand the biodegradation patterns of such HMW material and the monomeric units of which it is comprised. Due to the extreme complexity of the HMW material in effluent (11-15), in this study we looked at model compounds with well-defined structures that are likely present in the HMW material. The objectives of this study are (i) to determine the extent to which dechlorination of model compounds occurs by mixed microbial communities similar to those in a typical biological treatment system and (ii) to correlate their behavior with their chemical structures and relevant microbial metabolic pathways so that the mechanism of biological dechlorination can be understood. Furthermore, the question of whether or not the phasing out of chlorine and chlorine derivatives is environmentally beneficial is still under debate (16-21). Are all chlorinated compounds
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1996 American Chemical Society
problematic? From this study we hope to gain some understanding of the environmental fate of organochlorine compounds as well.
Background on Model Compounds Chemistry of Model Compounds. The choice of model compounds was based on the characteristics of the HMW material in BKMEs and the availability of suitable models. Recent research by others in our group at the University of Toronto’s Pulp & Paper Centre (22) has shown that the sequential reactions of a monomeric model compound of residual lignin, 4-methylguaiacol (MG), with chlorine dioxide and sodium hydroxide lead to the following products (reactions 1 and 2); O
OH OCH3
O
dark
+ ClO2 CH3
+ byproducts (1)
COOCH3
70 °C
