Emerging Chemical Contaminants: The Case of Perfluorochemicals

Authors Avatar

Emerging Chemical Contaminants: The Case of Perfluorochemicals

Introduction

In recent times, an emerging class of global contaminant, the perfluorochemicals (PFCs), has been the subject of immense worldwide investigation. PFCs are a class of organohalogens in which the compound is fully saturated with fluorines. The unique physical and chemical properties of PFCs are of value in industry and commerce. Since the 1950’s sulfonated and carboxylated PFCs have been used in a wide variety industrial, commercial and consumer applications. Furthermore, these compounds are metabolically and photochemically inert, resisting both biotic and biotic degradation (Giesy & Kannan 2002). As a consequence, perfluorooctanesulfonate (PFOS) and related perfluorinated carboxylic acids (PFCAs) were recently found to be bioaccumulative, global contaminants of environmental and biological matrices (Giesy & Kannan 2001), including trace level concentrations in the human population (Olsen et al. 2003a; Olsen et al. 2003b). Furthermore, scientific findings suggest that many PFCs exhibit a range of toxicological effects, potentially posing public health concerns. Observed toxicological effects of sulfonated and carboxylated PFC exposure on laboratory animals suggest potential developmental, reproductive, and systemic toxicity (OECD 2002; Seacat et al. 2002; Seacat et al. 2003). Furthermore, perfluoroocatnic acid (PFOA), a polymerisation aid for fluoropolymer production (E.g. PTFE or TeflonTM), a trace residual found in fluorotelomer products and biodegradation product of fluorotelomers, is believed to have carcinogenic properties (US EPA 2002; Hogue 2005).

In light of this evidence, the 3M Corporation (the principle global producer of PFCs) in May 2000 voluntarily discontinued the production of materials based on perfluorooctanesulfonyl fluoride (POSF), a polymer of PFOS (3M 2000). Following this announcement, in a proposed Significant New Use Rule, the American EPA seeks to bring fully fluorinated alkyl sulfonate-containing compounds having 4–10 carbons under regulatory control (US EPA 2000). Furthermore, DuPont (a global producer of PFC products) and other global producers are committed the elimination of environmental emissions PFOAs as part of a global stewardship program (US EPA 2006).  

While it is recognised that PFCs are persistent in the environment, present in human blood, and that animal studies indicate toxic effects of concern, the sources and pathways leading to the presence of PFCs in the environment are as yet not well characterised, although it is likely that multiple pathways exist. This paper will attempt to bring to light current notions on the toxicological effects and mechanisms leading to environmental exposure of these ubiquitous and pervasive chemical contaminants.

Physical/Chemical Properties of PFCs

The chemical/physical properties that a fluorinated system imparts on pefluorinated organic compounds, makes them useful for many industrial and commercial applications. Furthermore, the perfluorinated system precludes environmental degradation of these compounds.

PFCs confer great thermal and chemical stability being able to maintain their properties under extreme conditions over long time periods. The stability of PFCs is a result of the carbon-fluorine bond, which is thermodynamically one of the strongest known (polarity and strength ~ 110 kcal/mol), rendering PFCs resistant to hydrolysis, photolysis, microbial degradation, and metabolism by vertebrates. The only known condition whereby PFCs are degraded is through high temperature incineration (Giesy & Kanan 2002; Renner 2004).

The partitioning behaviour of anionic sulfonated and carboxylated PFCs is unusual and has important implications regarding their fate in the environment and assimilation in organisms. Hydrophobic organic contaminants such as polychlorinated biphenyls (PCBs) are expected to partition into organic rich sediments (Higgins et al. 2005). The simple hydrophobic partitioning paradigm is not applicable to PFCs. PFOS and PFOA are surfactants forming three immiscible layers octanol/water and hence the soil adsorption coefficient and bioconcentratiion factors cannot be determined reliably. Data do suggest that PFOS has a greater tendency to remain in the dissolved phase than adsorb to particulate matter (3M 2000). Numerous studies have measured the presence of PFOS in aquatic systems (Moody et al. 2002; Moody et al. 2003). It is known that the solubility of PFCs in water decreases with increasing salt content (OECD 2002). If, PFOS were to adsorb to particulate the material would ultimately end up in sediment. Research carried out by Higgins et al. (2005) suggest the wide spread occurrence of PFOS and precursors in the sediments of an industrialised area. However, further study is required to understand better the binding potential of PFOS and related chemicals in various abiotic media.

The partitioning behaviour PFCs in biological systems is unlike that of hydrophobic organic contaminants. Unlike hydrophobic organic contaminants, PFCs are oleophobic and hydrophobic and therefore do not accumulate in the lipids of organisms (Simik 2005). In higher mammals, PFCs tend to bioconcentrate in blood proteins, subsequently accumulating in the liver and gall bladder (Renner 2004).

Join now!

As PFCs are moderately soluble and have little or no vapour pressure, their volatility is regarded to be negligible. These two latter properties determine the equilibrium distribution between water and air (Henry’s Law Constant) which suggests that PFCs are unlikely to partition into the gas-phase (3M 2000).

Furthermore, fluorotelomers (F(CF2)nCH2CH2OH) and sulfonamides have been cited as possible candidates for carboxylated and sulfonated PFC biospheric contamination. FTOHs are polyfluorinated compounds typically characterized by even numbered perfluorinated carbons and two non-fluorinated carbons adjacent to a hydroxyl group. Measured vapor pressures of FTOHs range from 140 to 990 Pa (Stock et al. 2004). ...

This is a preview of the whole essay