The principal use of the monochlorinated phenols is as intermediates in the synthesis of the higher chlorinated congeners and certain dyes and pesticides. The chief use of 2,4-DCP is as an intermediate in the production of 2,4-D and other herbicides. 2,4-DCP is also used as an ingredient in antiseptics (Grimwood and Mascarenhas 1997).

The main route of entry of 2-, 3- and 4-CP and 2,4-DCP to the aquatic environment is likely to be as a result of discharges from plants manufacturing the compounds or from plants using the compounds as intermediates in the production of higher chlorinated phenols and other products, such as phenoxy herbicides.

Indirect sources include discharges from paper mills, where they are formed as by-products of the bleaching process, as a result of the disinfection of sewage, industrial wastes and drinking water with chlorine, and from the microbial breakdown of agricultural herbicides such as 2,4-D and subsequent run-off/leaching of the products.

Grimwood and Mascarenhas (1997) found that reliable data on the production levels of chlorophenols other than pentachlorophenol were not available in open literature. In 1975, the combined global production of all chlorophenols approached 200 million kilograms. More than half consisted of chlorophenols other than PCP, with 2,4-DCP, 2,4,5-TCP and 2,3,4,6-TCP predominating (WHO 1989). Krijgsheld and van der Gen (1986, cited in WHO 1989) reported European production levels of 4.5 and 9.1 million kilograms for total monochlorophenols and 2,4-DCP respectively (year not stated), while in 1972, total chlorophenol production in the UK was reported to be 1.14 million

Data for 2-CP from the South-West Region of the Environment Agency over the monitoring period 1992 to 1995 indicated that the great majority of concentrations at sites associated with routine monitoring in fresh and saltwaters were less than 0.2 microg l^-1.

Residues of all chlorophenol isomers have been detected in aquatic systems (WHO 1989). Generally, residues are present at measurable concentrations in discharges from such sources as manufacturing plants, wood-treatment facilities, municipal waste discharges and in receiving waters adjacent to these sources. Concentrations in other surface waters are more sporadic and usually low, although some isomers have been detected in some of the world's cleanest waters (WHO 1989).

These values are supported by the limited data reported in various other studies. For example, 2-CP, 3-CP and 4-CP have been detected at µg l^-1 levels in effluents from European sewage treatment plants and cooling water from power stations as a result of disinfection by chlorination (cited in WHO 1989), while in coastal areas and in rivers flowing through industrialised regions of the Netherlands, Piet and deGrunt (1975, cited in WHO 1989) reported that concentrations of monochlorophenols ranged from not-detected up to 20 microg l^-1 and dichlorophenols from not-detected up to 1.5 microg l^-1.

Chlorophenol concentrations in sediments are generally higher than in overlying water. This may be as a result of adsorption onto suspended solids in the water column and subsequent sedimentation. However, very few data are available for mono- and dichlorophenols.

At a site 2 km distant from a sulphate pulp mill, sediments in the Baltic sea were reported to contain a 2,4-DCP concentration of 0.9 µg kg^-1 (Xie 1983, cited in WHO 1989), while in a later survey, the same authors reported a sediment concentration of 16 microg kg^-1 2 km from the discharge and 0 microg kg^-1 5-10 km from the discharge.

Despite the high solubility of these compounds, some adsorption to the organic carbon content of aquatic sediments may occur as indicated by the moderate octanol-water (Kow) partition coefficients. However, the high solubilities and lower organic-carbon coefficients (Koc) for some soils, suggest that the lower chlorinated phenols may be susceptible to leaching to surface and ground waters. The low Henry=s Law Constants for these compounds suggest that volatilisation from surface waters is not likely to be an important removal route (Grimwood and Mascarenhas 1997).

Since chlorophenols are weak acids in aqueous solution, one of the major factors affecting environmental transport, degradation and toxicity is the degree to which the compounds are dissociated in natural waters. Under acidic conditions, chlorophenols exist primarily in the toxic molecular (undissociated) form, while under basic conditions, the dissociated form predominates. The pKa values (pH at which an acid compound is 50% dissociated) of 2-, 3- and 4-CP and 2,4-DCP indicate that at the pH range characterising most physiological and environmental conditions, these compounds will exist predominately in the more active undissociated form. Furthermore, as pH decreases, the proportion of molecules in the undissociated state will increase further, leading to yet higher activity as shown by parameters, such as adsorption to suspended solids and sediments and toxicity.

Chlorophenols are susceptible to photolysis and biodegradation. Photolysis is only expected to be an important process near the surface of water bodies (particularly in summer months). In deeper waters and sediments, aerobic and anaerobic biodegradation will be the main route of removal for chlorophenols. Photolysis of polychlorinated phenols appears to be higher than for monochlorinated congeners. Respective summer/winter half-lives for complete photomineralisation (i.e. breakdown to CO₂) of 6/14 and 53/334 days have been reported for 2,4-DCP and 4-CP in estuarine samples (Hwang and Hodson 1986). The higher rates in summer were attributed to higher irradiance in this season.

2-, 3-, 4-CP and 2,4-DCP all undergo microbial degradation under aerobic conditions via oxidative dechlorination and hydroxylation. Biodegradation is more rapid in sediments (and soils) as a result of more complex/active microbial communities and more favourable environmental conditions in these media (e.g. organic matter content, nutrient status, pH, etc.). The data also suggest that aerobic biodegradation is less rapid for meta- (3-) and para- (4-) substituted compounds and for the highly chlorinated congeners. This pattern is more apparent on an observation of the whole chlorophenol series from the mono- compounds through the di-, tri- and tetrachlorophenols up to pentachlorophenol. In addition, there is often a lag period associated with chlorophenol biodegradation in water and sediment. Such lag periods are usually attributed to the period required by resident micro-organisms to become physiologically acclimated to toxic compounds or to the period in which small populations of resident DCP degraders increases to large enough numbers, such that DCP biodegradation becomes detectable. In the water column and sediments of aquatic ecosystems, aerobic biodegradation half-lives of 2-36 days have been reported for the monochlorinated congeners at ambient temperatures. Corresponding values of 70-100% biodegradation of 2,4-DCP in 10-30 days have been reported. Some of these values may also incorporate removal by photolysis. Indeed, respective half-lives in estuarine water samples that account for both photolysis and microbial degradation of 4-CP and 2,4-DCP, have been reported to be 10/95 and 4/17 days in summer/winter, respectively.

Under anaerobic conditions in aquatic sediments, chlorophenols are biodegraded by reductive dechlorination (progressive replacement of the chlorines by hydrogen), usually by a consortium of several different microbial species. Under these conditions, meta- (3-) and para- (4-) substituted congeners appear to be more resistant to biodegradation. Moreover, in the complete mineralisation (i.e. reductive dechlorination to methane) of the higher chlorinated congeners, the breakdown of 4-CP is the rate limiting step. This is shown by values for complete biodegradation of 28-30, 15-61, 61 and 90 days for 2-CP, 3-CP, 4-CP and 2,4-DCP respectively. As with aerobic biodegradation, a lag period is usually associated with anaerobic biodegradation in aquatic sediments.

An exhaustive literature review on the toxicity of chlorophenols and dichlorophenols to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Grimwood and Mascarenhas 1997). The most sensitive groups of organisms have been identified.

Grimwood and Mascarenhas (1997) reviewed the data on the aquatic toxicity and concluded that saltwater data were considerably more limited than for freshwater organisms, but also that no one group of organisms was more sensitive than any other group. The majority of reported L(E)C50 data ranging from 0.6-19.5, 2.55-29.7 and 5-7 mg l^-1 for algae, crustaceans and fish, respectively, indicating moderate to high acute toxicity. These data mainly represent 4-CP and 2,4-DCP, with 2- and 3-CP data for fish only. The only long-term exposure data available for saltwater organisms relate to a mesocosm study conducted under field conditions in which a 4-CP and 2,4-DCP concentration of 1.0 mg l^-1 was found to cause severe inhibition of growth and biomass of natural phytoplankton communities (mixed species) (Kuiper and Hanstevit 1984).

The lowest reported toxicity data for 4-CP include 5-day NOECs, of 0.39 (total cell volume) and 1.08 (total cell count) mg l^-1 for the diatom Skeletonema costatum (Cowgill et al 1989), and a 96 hour NOEC (mortality) of 3.2 mg l^-1 for juveniles of sheepshead minnow Cyprinodon variegatus, while the lowest reported data for 2,4-DCP include a 96-hour LC50 and a 72-hour EC50 (growth) of 2.55 and 0.6 mg l^-1 for the grass shrimp Palaemonetes pugio and diatom Phaeodactylum tricornutum respectively (Rao et al 1981, cited in WHO 1989 and Kusk and Nyholm 1992). Toxicity data for 2- and 3-CP are limited to just one or two fish values, with the lowest being 96-hour LC50s of 6.6 and 4.0 mg l^-1 for sole Solea solea and flounder Platichthys flesus respectively (Smith et al 1994).

In fish, polychlorophenols appear to be Type II 'polar' narcotics. In other words, compounds that cause narcosis associated with a specific mode of action. This has been identified as an uncoupling of electron transport (oxidative phosphorylation) in mitochondria. However, the mechanism of toxicity in invertebrates and of the monochlorophenols appears to be less specific (Grimwood and Mascarenhas 1997).

The toxicity of chlorophenols to aquatic organisms rises with increasing degree of chlorination and substitution away from the ortho- (2-) position. The higher toxicity of the more highly chlorinated congeners can be ascribed to an increase in lipophilicity which leads to a greater potential for uptake into the organism. Ortho-substituted congeners are generally of lower toxicity than the meta- and para- substituted compounds, as the close proximity of the ortho-substituted chlorine to the OH group on the molecule appears to 'shield' the OH, which apparently interacts with the active site in aquatic organisms, causing the observed toxic effects (Grimwood and Mascarenhas 1997).

Toxicity also depends on the extent to which the chlorophenol molecules are dissociated in the exposure medium, with increased toxicity observed with a decrease in pH. This is because the more toxic non-dissociated form predominates at lower pH, while at higher pH, the less toxic dissociated form is predominant. Moreover, the pKa values of 2-, 3- and 4-CP and 2,4-DCP indicate that at the pH range characterising most environmental conditions, these compounds will exist predominately in the more active non-dissociated form.

No data could be located for sediment dwelling organisms.

Grimwood and Mascarenhas (1997) concluded that the vast majority of aquatic organisms did not readily accumulate monochlorophenols or 2,4-DCP to high levels, with BCFs in fish ranging from 3.8-34.0 at neutral pH, and depuration half-lives in the order of hours to days. Since toxicity is directly linked to bioaccumulation, Grimwood and Mascarenhas (1997) also concluded that all the issues concerning the effects of degree of chlorination and dissociation on the toxicity of chlorophenols, also related to the uptake and bioaccumulation of these compounds. Therefore, an observation of bioaccumulation data for the whole chlorophenol series would show that the higher chlorinated congeners (e.g. tri-, tetra- and PCP) are accumulated to higher levels, with BCFs ranging from 10²-10³.

Potential effects include:

• toxicity of 2-chlorophenol to algae, invertebrates and fish at concentrations above the EQS of 50 microg l^-1 (annual average) and 250 microg l^-1 (maximum allowable concentration) in the water column;
• toxicity of 2-4-dichlorophenol to algae, invertebrates and fish at concentrations above the EQS of 20 microg l^-1 (annual average) and 140 microg l^-1 (maximum allowable concentration) in the water column.
• potential for bioaccumulation for the higher chlorinated (tri-, tetra- and pentachlorophenol) compounds in the series.

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