Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Vinyl Chloride
Vinyl chloride (CH2CHCl; Chemical Abstracts Service registry number 75-01-4) is a colourless, flammable, explosive gas with a boiling point of -13.4°C and high volatility, as indicated by its Henry's Law constant of 2.8 kPa·m3/mol at 25°C and vapour pressure of 362.6 kPa at 20°C (U.S. EPA, 2000a). Vinyl chloride, also known as vinyl chloride monomer, chloroethene, monochloroethylene or ethylene monochloride, has a relative molecular weight of 62.5 (OEHHA, 2000; ATSDR, 2006). It is slightly soluble in water (1.1 g/L at 28°C) but highly soluble in fats and organic solvents. It polymerizes in light and in the presence of a catalyst. On combustion, it degrades to hydrogen chloride, carbon dioxide and traces of phosgene. Vinyl chloride has a pleasant, sweet odour at high concentrations (ATSDR, 2006; IARC, 2008). In air, 1 part per million (ppm) of vinyl chloride corresponds to 2.6 mg/m3 at 25°C and one atm of pressure.
Vinyl chloride can primarily enter the environment through anthropogenic sources, and may also be formed naturally in soil (Keppler et al., 2002). It is now produced and used with stringent methods for containment and recovery. Vinyl chloride can enter drinking water through leaching of the entrapped monomer from polyvinyl chloride (PVC) pipe. Releases to the aquatic environment are primarily a result of industrial discharges from chemical and latex manufacturing plants (IARC, 1979). Vinyl chloride can also be formed in groundwater and the environment by biodegradation of synthetic solvents, such as trichlorethene, trichloroethane and tetrachloroethene (Parsons et al., 1984; OEHHA, 2000; ATSDR, 2006).
Between 1986 and 1999, Canadian industrial manufacturing capacity for vinyl chloride ranged between 373 and 550 kilotonnes per year, whereas production rates ranged between 270 and 439 kilotonnes per year (CIS, 1999). The closing of vinyl chloride manufacturing plants has decreased Canadian production of vinyl chloride since 2000: the Dow Chemical Company closed its vinyl chloride facility in Sarnia, Ontario, in the early 1990s and its facility in Fort Saskatchewan, Alberta, in 2006; BF Goodrich closed its Shawinigan, Québec, plant in 1993, and Royal Polymer closed its plant in Sarnia, Ontario, in 2009. Only one PVC manufacturing facility currently exists in Canada located in Niagara Falls, Ontario (Environment Canada, 2008).
In 1982, 86% of the Canadian production of vinyl chloride was used to manufacture polyvinyl chloride, 4% was used to manufacture 1,1,1-trichloroethane and 10% was exported (Environment Canada, 1985). However, between 1986 and 1999, production of PVC increased while the use of vinyl chloride for 1,1,1-trichloroethane became negligible and the quantity exported decreased. The importation of PVC into Canada rose from 73 to 440 kilotonnes per year during that same period. After 1993, nearly 100% of vinyl chloride produced in Canada was used in the manufacture of PVC (CIS, 1999). The majority of PVC demand in Canada was for the manufacture of pipe and fittings. Rigid PVC used for pipes and fittings contains little or no plasticizer when produced for potable water applications. As such, PVC is sometimes called rigid or unplasticized PVC (Richardson and Edwards, 2009).
PVC is used in electrical wire, insulation and cables, industrial and household equipment, medical supplies, food packaging materials, building and construction products and piping. In addition, PVC is used as a raw material in the paper, glass, rubber and automotive industries. Vinyl chloride and polyvinyl chloride co-polymers are distributed and processed in a variety of forms, such as dry resins, plastisol, organosol and latex (U.S. EPA, 2000b). Concentrations of vinyl chloride monomer in PVC were reduced drastically between 1973 and 1975, ranging from 1 to 10 ppm (ECETOC, 1988). A wide variety of foods and drinks are packaged in containers or film made of polyvinyl chloride or vinyl chloride co-polymers, and residual vinyl chloride may migrate into food or beverages (ECETOC, 1988). Children's plastic toys, such as dolls, bath toys and squeeze toys are often manufactured with PVC. Vinyl chloride has also historically been detected in the interior of new cars (824 to 3120 μg/m3) (IARC 1979). The vinyl chloride content in the smoke of USA-manufactured cigarettes and small cigars ranged from 5 to 27 ng per cigarette (mean concentration of 11.35 ng/cigarette) (Hoffmann et al., 1976).
A PVC manufacturing facility located in Niagara Falls (Thorold), Ontario, released 2489 million litres of wastewater per day to the river between 2005 and 2007 and reported a vinyl chloride concentration of 1.46 µg/L in the wastewater released to the river (Oxy Vinyl, 2008). According to National Pollutant Release Inventory data, the total amount of vinyl chloride released into surface water in 2007 was 0.001 tonnes (NPRI, 2007). Vinyl chloride was historically released to water in Sarnia, Ontario; however, the amount has decreased since 1998, and no vinyl chloride release to water has been reported in Sarnia since 2000, with the exception of 0.297 tonnes in 2003.
Historically, the amount of vinyl chloride released into air peaked at 44 tonnes in 1997 and has since been steadily decreasing. In 2007, 2.7 tonnes of vinyl chloride were emitted to the atmosphere by major Canadian industrial facilities; the largest vinyl chloride emissions in 2007 occurred in three Ontario cities: Thorold, Sarnia and Guelph. Releases were also regularly reported for Fort Saskatchewan, Alberta, before the 2006 closure of the vinyl chloride manufacturing plant there (NPRI, 2007).
Lesage et al. (1993) measured the vinyl chloride levels in leachate from a municipal solid waste landfill located in Guelph, Ontario: average vinyl chloride concentrations of 14 µg/L and 23 µg/L were found in samples (number not specified) collected in 1988 and 1989, respectively. In 2007, 0.538 tonnes of vinyl chloride were released to land, with releases occurring in Thorold, Ontario; Sarnia, Ontario; and Winnipeg, Manitoba (NPRI, 2007).
In a study conducted to investigate the potential for the production of vinyl chloride through natural soil processes, soil air and ambient air were analyzed for volatile chlorinated halocarbons. The concentrations of vinyl chloride in the soil air were significantly higher than those in ambient air, suggesting a natural formation in soil. Given that the vinyl chloride, trichloroethene and tetrachloroethene concentrations in ambient air were very low, the authors concluded that the significantly higher concentrations for vinyl chloride in soil air could not be attributed to ambient air contamination. The authors were able to produce vinyl chloride in the laboratory during follow up experiments using different pristine soils and model compounds. The authors proposed that vinyl chloride could be formed during the oxidative degradation of organic matter in soil, such as through a reaction between humic substances, chloride ions and an oxidant (ferric ions or hydroxyl radicals). The authors (Keppler et al., 2002) indicated that more detailed investigations are required to better understand the formation and significance of naturally produced vinyl chloride in soil.
Vinyl chloride is not usually detected in groundwater. Possible sources to groundwater include landfill leachate as well as spills of vinyl chloride or chlorinated precursor compounds. Vinyl chloride released to the soil does not adsorb onto soil particles; any that does not evaporate migrates readily to groundwater, where it can remain for months to years (Parsons et al., 1984). The half-life of vinyl chloride monomer at a concentration of 1 mg/L in open water at a 1 m depth is estimated to be 26 minutes, and 90% is lost by evaporation within 96 minutes (Verschueren, 1984).
Under aerobic conditions, vinyl chloride in samples taken from a shallow aquifer (groundwater) was readily degraded, with more than 99% being degraded after 108 days and approximately 65% being mineralized to carbon dioxide (Davis and Carpenter, 1990). Further, Bradley and Chapelle (2011) reported that even under hypoxic (initial dissolved oxygen concentrations about 0.1 mg/L) and nominally anoxic (dissolved oxygen minimum detection limit = 0.01 mg/L) conditions, first-order rates for vinyl chloride biodegradation (mineralization) were approximately 1 to 2 orders of magnitude higher in hypoxic groundwater sediment treatments and at least three times higher in hypoxic surface water sediment treatments than in the respective anoxic conditions.
Under anaerobic conditions, biodegradation of chlorinated compounds can result in vinyl chloride production. Vinyl chloride may be further degraded to less chlorinated and non-chlorinated ethenes, and possibly to carbon dioxide and ethane. However, these subsequent reactions often proceed at a slow rate under anaerobic conditions, and hence vinyl chloride may persist in groundwater (ATSDR, 2006). For example, Jacobs et al.(2007) reported that at some U.S. Superfund sites, vinyl chloride has been shown to persist for over 20 years. A pseudo-first order rate constant of 1.01 × 10-3 (months)-1 and a half-life of 57.2 years were estimated for the disappearance of vinyl chloride in water microcosms under anaerobic conditions without bioenhancement. However, anaerobic biodegradation has been shown to occur more rapidly under methanogenic or Fe(III)-reducing conditions (Bradley and Chapelle, 1996, 1997). Bradley and Chapelle (1996) reported that the addition of Fe(III) as Fe-EDTA to anaerobic aquifer microcosms resulted in rapid mineralization (15-34%) of [1,2-14C]vinyl chloride to 14CO2 within a period of 84 hours. Rates of vinyl chloride mineralization in Fe-EDTA-amended microcosms were comparable to those observed in aerobic microcosms (22-39% in 84 hours). The authors concluded that the addition of chelated Fe(III) may be an effective treatment for enhanced bioremediation of vinyl chloride contaminated ground water systems. In another study by Bradley and Chapelle (1997), the recovery of [1,2-14C]VC radioactivity as 14CO2 ranged from 5% to 44% in methanogenic microcosms and from 8% to 100% in Fe(III)-reducing microcosms for a 37 day period.
The low boiling point, high vapour pressure and low water solubility of vinyl chloride indicate that any vinyl chloride released to surface water will migrate rapidly to air. In waters containing photosensitizers, such as humic materials, sensitized photodegradation may also be important removal pathway (ATSDR, 2006).
When released to the atmosphere, vinyl chloride is expected to be removed by reaction with photochemically generated hydroxyl radicals (half-life of 1-2 days) liberating reaction products such as hydrochloric acid, formaldehyde, formyl chloride, acetylene, chloroacetaldehyde, chloroacetylchloranil, and chloroethylene epoxide (ATSDR, 2006).
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