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Return to List of Fact Sheets1.0 IntroductionChlorinated phenolic compounds are prevalent in the pesticide industry and may be found in both industrial and domestic wastewaters. These compounds are thought to be highly toxic and lower chlorinated members have shown carcinogenic traits. The federal regulations presently applicable are fairly strict and encompass OSHA, RCRA, and the CAAA of 1990, to name a few.Due to their inherent toxicity and relative persistence, the pesticide industry substituted the use of chlorine with phosphorous, in many instances. This raw material substitution has produced a significant decrease in persistence, and a large drop in the environmental release of chlorinated phenols. The fate and transport of these compounds is largely dependent on whether they are in a neutral or ionic form. This will in part depend on the pH of the aqueous phase in which it enters, be it surface waters, ground waters, or partitioning to raindrops. With a pKa near ambient pH values, it is very likely that the chlorophenols will exist as an ionic species. Thus, increases in water solubility and mobility in the aqueous phase, a lower degree of sorption to solid phases (unless oppositely charged solid surfaces are available), and a decrease in its ability to volatize will be exhibited. In its neutral form, chlorinated phenols exhibit relatively low water solubility (although the phenol group does tend to form hydrogen bonding), moderate to high sorption, and in most cases, high volatility. 2.0 Physiochemical PropertiesThree compounds of interest in the organochlorine family are 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol. Synonyms frequently encountered include 2,4-DCP or DCP, 2,4,6-TCP or TCP, and PCP, respectively. These phenolic compounds are synthetically produced chemicals. Physically, the structures are composed of a benzene ring with varying numbers of chlorine substituents and one hydroxyl group on the ring. The physiochemical properties and structures are listed in Table 1 and Table 2, respectively.As the number of chlorine atoms substituted onto the ring increases, the pKa of the chlorophenol decreases. Since the pH of natural waters can easily vary from 6.0 to 8.0 and the pKa’s of chlorophenols range from 4.7 to 7.68, they are likely to be found in their conjugate base form (ionic species) in the environment. For food chain uptake, 2,4-dichlorophenol has a bioaccumulation factor (BCF) of 1.0 for trout, 1.53 for goldfish, and 2.41 for algae 9 (Mackay1997). However, PCP has a higher tendency to bioaccumulate, with BCF’s up to 10000 for some fish species, 1,000 for goldfish, 324 for mussels, and 78 for oysters (ATSDR 1994). Half-lives in the air range from 21.2 to 212 hours for 2,4-DCP depending on the rate of reaction with hydroxyl radicals, and 170 hours for 2,4,6-TCP (Mackay 1997). In surface waters, DCP’s half-life is estimated as 0.8 hours (summer) and 3.0 hours (winter), for distilled water and 5 and 23 days in seawater for summer and winter, respectively. The ground water half-life also may vary from 133 to 1032 days, depending on the subsurface conditions (Mackay 1997). Pentachlorophenol has a half-life in water ranging from 20 to 200 days. The half-life for 2,4-DCP in sediment is estimated to be approximately 116 days in July and 47 days in November. The half-life for 2,4,6-TCP in soil is estimated at 1700 hours (Mackay 1997). The dominant processes for transformation of PCP are photolysis and biodegradation. The solubility of DCP in water is approximately 4,500 ppm or mg/l. This is a significant drop off from 2-chlorophenol (20,000 ppm) but subsequently represents the trend in this chemical family as the number of chlorine substitutions increase (ATSDR 1997). The solubility drops to 434 mg/l in TCP and 14 mg/l in PCP. This also suggests a reason for the trend of the octanol water coefficient and sorption constants to increase with increasing chlorine substituents. TCP and PCP are more soluble in alcohol and ether than DCP. 3.0 Industrial Use of 2,4-DichlorophenolDichlorophenols are mainly found as intermediates formed during the production of 2,4-dichlorophenoxyacetic acid, which is a herbicide in the pesticide industry. This compound is also used in a feedstock mixture to produce the wood preservative, pentachlorophenol (Kent 1983). Other past commercial uses for 2,4-DCP include moth proofing and as an antiseptic. Currently, 2,4-dichlorophenol is still in use, and some of the industries producing this compound include Dow Chemical, Sandoz Agricultural Inc., and Vulcan Chemicals.Releases of 2,4-dichlorophenol are made available to the general public on the Toxic Release Inventory database. In 1995, which is the most recent TRI report available, 358,119 pounds of production-related 2,4-DCP waste was managed (TRI 1995). 3.1 Industrial Use of 2,4,6-TrichlorophenolThe major uses of 2,4,6-TCP are as an antiseptic and pesticide. Its used also included preserving wood, leather and glue, and preventing the build-up of mildew on fabric. In addition, 2,4,6-TCP is used as an intermediate to produce other chemicals. This compound is as a feedstock in the production of 2,3,4,6-tetrachlorophenol and pentachlorophenol (ATSDR 1990).North America and Scandinavia are the main regions of the world where 2,4,6-TCP has been used as wood preservatives (Kalliokoski and Kauppinen 1990). 2,4,6-TCP is presently not in use in the United States. However, it is produced as a by-product at a single site in Kingsport, MI (Tennessee Eastman Division) (TRI 1991). Recent data about the production of this compound are very limited. 3.2 Industrial Use of PentachlorophenolThe main uses of PCP in the US before being restricted were as pesticides (algaecide, herbicide, molluscicide, fungicide, and insecticide), disinfectant, and a wood treatment for utility poles. However, the EPA has now named it as a restricted-use pesticide meaning use is now limited to fence rows, drainage ditches and driveways. Some of the common names for this chemical are Dowicide-7, Woodtreat, and Penta Ready produced by such manufacturers as DOW Chemical Co. and Vulcan Chemicals. From 1983-1986, production of PCP in the US decreased from 45 million to 32 million pounds (ATSDR 1994).A stepwise chlorination of phenols is generally used to produce PCP; however, it can also be made by the hydrolysis of hexachlorobenzene. Contaminants such as other polychlorinated phenols, dibenzo-p-dioxins, and dibenzofurans can make up to 14% of commercial grade PCP, but limits have been set on the amount of these allowed. 4.0 Ecological Routes of Exposure for ChlorophenolsChlorophenols have been found in the water, air and soil of many ecosystems. The majority (85%) of past environment releases, for the three compounds of interest in this report (Penta, 2,4-DCP, and 2,4,6-TCP), were to surface water. In 1977, it is estimated that 741,000 pounds of 2,4-DCP were released as water emissions from U.S. production facilities, mainly as industrial discharges(ATSDR 1990). Interestingly, PCP, 2,4-DCP and 2,4,6-TCP were all found in waste effluents of iron and steel manufacturers, electrical components, photographic equipment, pharmaceuticals, and paper and pulp manufacturers (EPA 1987). 2,4,6-TCP showed the highest levels downstream of pulp and paper effluent discharge points, as compared to PCP and 2,4-DCP (Oikari et al 1985). Untreated and treated wastewater from POTW’s contained average levels lower than 1 ppm for both 2,4-DCP and 2,4,6-TCP.Approximately 12,000 pounds of Pentachlorophenol’s were released into U.S. municipal wastewater treatment facilities in the late 1970s (ATSDR 1994). Other releases of PCP to water have come from leather tanning and textile factories. Also, common pesticides such as lindane and hexachlorobenzene can be metabolized to PCP by plants, animals and microorganisms. All three chlorophenols have entered natural systems as agricultural run-off, which is considered to be a non-point source. It is estimated that 12,000 pounds of 2,4-DCP have been discharged directly to land in 1998 from production facilities. In addition, out of 471 waste disposal sites, 2,4-DCP was found at 14 sites and 2,4,6-TCP was found at two sites. Out of 1350 hazardous waste sites on the NPL, PCP has been identified in 246 of them. 2,4-DCP has also been detected in leachate wastes collected at municipal landfills (ATSDR 1997). According to 1995 TRI reporting, 105,687 pounds of chlorophenols were disposed of by deep well injection as well. A total of 1510 pounds of PCP were released to the soil in the US from manufacturing and processing facilities. Release to the soil also historically occurred due to pesticide use and deposition from the atmosphere in precipitation. Lastly, PCP, 2,4,6-TCP and 2,4-DCP may be produced from the chlorination of water and the breakdown of other chemicals found in wastewater, drinking water, and soil. The main source of air emissions for chlorophenols is by volatilization during production-related activities and the manufacturing of other end-use products (i.e. 2,4-D and 2,4-T). As much as 500,000 pounds of PCP were released to the atmosphere annually before it was declared a restricted use pesticide. In 1991, 12,508 pounds of PCP and 1432 pounds of 2,4-DCP and 86 pounds of 2,4,6-TCP were released as air emissions in 1991(TRI 1991). Furthermore, air emissions of 2,4,6-TCP are also found as flue gas condensates, emissions from waste burners, and fly ash from municipal incinerators. 2,4-DCP has been detected in the combustion of hazardous waste, coal, wood, and municipal solid wastes as well. 5.0 Environmental Risk Associated with ChlorophenolsThe environmental risk associated with chlorophenols should be analyzed in a way that addresses impacts on both humans and ecosystems. There are four basic steps followed in either case, to quantitatively estimate the degree of risk posed by a particular chlorinated phenol.The four steps in risk assessment are summarized in the following manner (Fjeld 1996):
5.1 Toxicity of ChlorophenolsPentachlorophenol has been found in the urine of 100% of the children sampled in Arkansas at background concentrations greater than one ppb, with the mean being 14 ppb (Hill et al 1989). This study reflects the results of many others, suggesting that most people in the US be exposed to some levels of PCP due to its ubiquitous presence in the environment. 2,4-Dichlorophenol and 2,4,6-TCP were found in the urine of 20% and 10%, respectively, of the Arkansas children at concentrations only slightly greater than 1 ppb.The presence of these chemicals does not necessarily mean we are all at risk. The levels of toxicity vary for these compounds. In a study comparing 50% toxicity levels of chlorophenols (Janik and Wolf 1992), PCP was found to be the most toxic. Toxicity decreased as the number of chlorine substituents decreased (see Figure 1). This same pattern can be observed for; LD50’s of albino rats, bluegills, and fathead minnows and; for EC50’s of bull sperm inhibition, and IC50’s for Bacillus cultures. None of these levels fell below 175 ppb. While this might suggest that at levels found in the environment, there should be no toxic effect; all these tests were acute toxicity tests done under lab conditions with no confounding factors. Chronic low level exposure, like that found throughout the US, could very well pose a serious risk to an exposed population. Some current research shows that PCP and 2,4,6’TCP are carcinogenic in rodents, can induce chromosomal aberrations, and are associated with leukemia, malignant lymphoma, and soft tissue sarcoma’s in humans (Exon 1983, Kerkvliet. 1985, and DeMarini. 1990). This data, however, is mostly from research using technical grade chemicals and thus may reflect toxicity to contaminants in the pesticide instead of direct toxicity from the chemical itself. More research needs to be done on this subject. 5.2 Environmental RegulationsOn an international level, the International Agency for Cancer Research has listed all three compounds as group 2B carcinogens. In addition, pentachlorophenol has been given a guideline for drinking water by the World Health Organization of 10 mg/l (ATSDR 1990).There are applicable federal regulations for each compound in air and water and are also listed as hazardous compounds according to the Resource Conservation and Recovery Act of 1976. 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol must all meet pretreatment requirements and obtain discharge permits in order to send their effluent to the local POTW’s, as stipulated in the National Pollutant Discharge Elimination System. Ambient water quality criteria for 2,4-DCP and 2,4,6-TCP are 3.09 mg/l and 1.2 mg/l, respectively (ATSDR 1990). Both of these requirements are regulated according to the 1987 Clean Water Act. Any releases of pentachlorophenol and 2,4-dichlorophenol must be reported for all industries subject to TRI reporting as written in the Emergency Planning and Community Right-To-Know Act of 1986 ( SARA Title III). In terms of air pollution, PCP and 2,4,6-TCP are both listed in the Clean Air Act of 1990 in Title III, as Hazardous Air Pollutants (Wark et al 1998). Presently, only pentachlorophenol has OSHA listed requirements , and is 0.5mg/m3 (PEL-TWAa). Another major federal regulation deals with hazardous wastes listing. Pentachlorophenol and 2,4,6-TCP are both listed as F-027 wastes (acutely hazardous, non-specific). PCP (D-037) and 2,4,6-TCP (D-041) are both listed under toxicity characteristics with maximum allowable concentrations of 100 mg/l and 400 mg/l, respectively. As a specific waste from bottom sediment sludge produced in the treatment of wood preservers, PCP is also listed as K-043 (Clark 1997). 2,4-DCP is not listed and only 2,6-DCP makes the list as a pesticide waste from the synthesis of 2,4-D. The listed chlorophenols above must follow all pertinent requirements as promulgated in both RCRA of 1976 and SARA of 1986. 5.3 Treatment and Disposal of ChlorophenolsA biological method for removal of chlorophenols in drinking water and wastewater is enzymatic detoxification using the horse radish peroxidase enzyme (Freeman et al. 1988). The addition of this enzyme with hydrogen peroxide to solution causes enzymatic cross-linking of the substrate forming insoluble polymers, which then precipitate out of solution. In addition, a recent study of ozone pretreatment to enhance chlorophenol biodegradation has shown promising results and research indicates significantly greater levels of COD removal (Cozzens 1995).Supercritical water-oxidation, a thermal treatment method that involves heating a dilute solution to the "critical" values of water, (374oC and 22 Mpa) seems to be an attractive alternative. Above this critical point, the organic compound undergoes complete oxidation to CO2 and water (Freeman et al. 1988). One can also consider a conventional physiochemical treatment system. The process entails primary flocculation and sedimentation in sequence, which is followed by adsorption of the supernatant to activated carbon. Many commercial pesticide applicators have installed such a system (Bridges1988). As far as effective disposal methods, pentachlorophenol must be treated with either sodium bicarbonate or a sand-soda ash mixture prior to incineration. The recommended disposal method for 2,4-DCP and 2,4,6-TCP is incineration, as well. Complete combustion must be achieved in order to avoid the formation of some extremely toxic by-products, such as polychlorinated dibenzofurans (ATSDR 1990). Furthermore, 2,4,6-trichlorophenol and PCP are listed as hazardous for landfill disposal and have limits of .04 mg/l and 1.0 mg/l, respectively (Freeman et al. 1988). 5.4 Pollution Prevention ActivitiesThere are two major activities being utilized in industry to either reduce or re-use chlorinated phenols. A significant reduction has been achieved through the substitution of organochlorines with organophosphates. These compounds have lower pKa’s and are less persistent with a shorter half-life. Even though organophosphates are less effective as pesticides, in terms of residence time (due to a lower half-life), this raw material substitution definitely decreases the impact on various ecosystems.As a separation process, chlorophenol wastewaters can be extracted with a solvent mixture of no. 2 fuel oil and amyl alcohol still bottoms at a 97% removal efficiency, as classified under EPA-600/52-81-043 (Freeman et al. 1988). This recovery technique is designed to increase production efficiency by decreasing raw material purchases and disposal costs, while ultimately lowering overall waste generation. The numbers back up industry’s effort to reduce the generation of chlorine-substituted phenolic wastes, as shown in the public access TRI reports from 1987,1992, and 1995. Since 1987, the total release and transfer of chlorophenols to the environment has decreased by 90%, going from 1,463,932 pounds, to 344,763 pounds, to 14,496 pounds, respectively (TRI 87’, 90’, 95’). Thus, the overall risk associated with exposure to chlorophenols, has been significantly reduced. 6.0 Chemical Fate of Chlorophenols determined by EXAMS ModelAll input values used in the EXAMS model are available in Table 3. The results obtained in the output from the EXAMS model suggests that a discharge of chlorophenols into a pond-like system will mainly be found in the sediment. Pentachlorophenol exhibits the highest degree of accumulation in the sediment as compared to 2,4-DCP and 2,4,6-TCP. As shown in Figure 2, the percent distribution in the benthic sediment for PCP, TCP, and DCP, is 98.98%, 86.87%, and 67.48%, respectively (Burns 1982). Therefore, since the model gives output of the ultimate chemical fate between only the sediment and the water column, one can easily interpret the percent distribution in the aquious phase. These results are consistent with chlorophenols’ relatively low solubility and high Kow. The number of chlorines substituted on the phenolic aromatic compound and subsequently the overall size, obviously plays a significant role in accumulation. With less chlorine atoms substituted onto the benzene ring a smaller degree of accumulation in the sediment is found.Due to a higher percent distribution in the sediment, PCP has the greatest tendency to persist in this ecosystem, as compared to 2,4,6-TCP and 2,4-DC. PCP will subsequently have the lowest bioavailability and thus, is less likely to be metabolized by microorganisms. Coupling this fact with a high BCF suggests that PCP will bioaccumulate (and bioconcentrate from one aquatic species to another) the most and pose the greatest risk to human exposure through food chain transport. Even though TCP and DCP are less persistent, a higher concentration in the water column, as compared to PCP, suggests that they may pose a risk to human exposure through ingestion of drinking water (if the water is not properly treated). The output referring to each compound’s persistence is located in Figure 3. The data indicates that the time needed for natural processes to remove 95% of PCP, 2,4,6-TCP and 2,4-DC are 27 years, 30 months, and 15 months, respectively (Burns 1982). In terms of the compound’s ability to partition itself into the atmosphere, the rate of volatilization is greatest for 2,4,6-TCP. This characteristic, as shown in Figure 3, is consistent with the fact that TCP has highest henry’s law constant of the three compounds. According to the EXAMS model output describing a mass distribution of the total chemical load, one can see that 17.57% of 2,4,6-TCP, 15.35% of 2,4-DCP, and 9.28 % of PCP, will be transported from the system by volatilization (Burns 1982). This indicates that volatilization is not a dominant removal process for the discharge of chlorophenols into a pond-like ecosystem. The model utilized does not consider the half-life of each compound for various transformation mechanisms. Thus, the output data is lacking information relevant to hydrolysis, oxidation, and biodegradation for the chlorophenol’s analyzed. Thus, the information describing effects of these parameters on the overall fate and transport is just not available. One must also realize that if the pKa’s were considered in this model, trends associated with each compound would change significantly. In a natural water system, these organochlorines may very well exist as ionic species. This is due to their pKa being lower than the pH of the surface water. Under this scenario, the chlorophenol’s water solubility will increase, which will subsequently lower their persistence by increasing bioavailability to degradation. 7.0 ConclusionOne can conclude that chlorinated phenols are not the most benign synthetic chemicals produced by man. Though some industries have moved to other pesticides, they still are being used in great enough numbers to impact natural ecosystems. The information discussed in this report should give any reader a basic understanding of how the chemicals are produced, where the most significant releases are coming from, what pertinent risks does a community have from these compounds, and what is the ultimate fate and transport of chlorophenols in a natural system.In terms of the Exposure Analysis Modeling System, one can conclude that pentachlorophenol is the most persistent compound of the three analyzed. Inevitably, this means that it will be the most difficult to remediate or treat. In our pond-like environment, pentachlorphenol may be sorbed deeply into the bottom sediment and thus, be unavailable for natural biodegradation. Furthermore, all three have high Kow’s and in a surface water, may have to be dredged up and treated elsewhere, a very costly procedure. At the same time, the environmental engineer on a project dealing with surface water contamination must look closely at the pH in the aqueous phase to determine the actual solubility of chlorophenols. If the compounds are in the conjugate base form they will be more water soluble, and treatment objectives may need to focus on contaminant removal from the aqueous phase. In addition, mobility of chlorophenols becomes an issue and one must assess the risk of exposure to downstream communities. As industry and society in general, begin to move
away from end-of-pipe treatment and begin to look "upstream" to minimize
waste at the source of generation, chlorophenol contamination may become
a forgotten issue. The incentive is there for industry to reduce waste
generation through TRI reporting requirements. In other words, if everyone
knows who is polluting, one can be sure that industry will do its best
to reduce waste output and promote good community relations, in order to
remain competitive with new "greener" companies. Unfortunately, we have
already released a great amount of chlorophenols into the environment.
Therefore, an extremely important task for all environmental engineers
in the twenty-first century is to find solutions to the problems created
by are forefathers.
aPEL-TWA = Permissible Exposure Limit – Time-Weighted Average Appendix
Figure 1. The effect of varying numbers of chlorine substituents of chlorophenols
on toxicity levels in various test systems expressed in unit relative to
2,4-DCP (Janik and Wolf 1992).
Figure 2. Chlorophenol Volatilization Rate at Steady State (EXAMS Model)
Figure 3. Persistence of Chlorophenols in the Environment. Represented
by the time it takes for natural processes to remove 95% of the chemical
(EXAMS Model).
Figure 4. Chlorophenol Accumulation at Steady State (EXAMS Model)
References
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