Perfluorooctane sulfonic acid (PFOS, C₈F₁₇SO₃H), a perfluorinated compound, is a synthetic material. PFOS possesses superior heat resistance, chemical resistance, water repellency, oil repellency, rub resistance, and surface activity; hence, it is widely utilized in fire blowing agents, in coating agents for carpets, clothing, paper, and leather, in commercial stain preventing agents, in metal cleaners, and in aircraft hydraulic fluids. While PFOS is highly valued as a chemical agent for industrial and consumer uses, it is not easily degraded and is known to remain in the environment and biological materials for long periods of time [1,2].

Cerebellum tests using rats have shown that PFOS can induce neurotoxicity, and it can also cause reproductive toxicity in pregnant rats as indicated by a decrease in the survival rate of newborns. A recent study reported that the average blood PFOS concentration for Koreans was 6.06 ng/mL [3].

PFOS was registered as a persistent organic pollutant (POP) at the 4th Stockholm Convention on POPs held at Geneva, Switzerland, in May 2009. In South Korea (hereafter Korea), PFOS was designated as a POP by amending the enforcement decree of the POPs Control Act in April 11, 2011, and consequently, its manufacturing, export, and utilization have been severely restricted.

The 3M company voluntarily stopped PFOS production in 2002 and began developing alternatives such as perfluorobutane sulfonate with a reduced number of carbon chains; but they have not yet succeeded in developing a substance that can completely replace PFOS to date (UNEP/POPS/POPRC.6/Add.3/Rev.1, 2011.10.14). In Korea, Shin’s research team at Changwon National University (Department of Chemistry) are developing PFOS alternatives with a decreased or increased number of carbon atoms and a decreased number of fluorine atoms [4]. The objective of this study was to evaluate the biodegradation potential of five species four PFOS alternatives that were developed at Changwon National University. It is hoped that this research will aid in the identification of potentially useful PFOS alternatives.

Biodegradation rates of aniline (i.e., the control material in bottle 11) were 60.5% and 72.1% on days 7 and 28, respectively, thus confirming that the activated sludge (microorganisms) used in this study are behaving normally. The BOD values for bottle 6 with C₈F₁₇KO₃S, PFOS in a form of potassium salt reached 116.1 mg/L on day 7, 129.3 mg/L on day 14, and 139.6 mg/L on day 28 (Figure 1A). On the contrary, the BOD values for bottle 12 without test materials reached 120.8 mg/L on day 7, 137.8 mg/L on day 14, and 150.9 mg/L on day 28. The BOD values for bottle 7 with C₁₀H₈F₁₃KO₃S reached 126.0 mg/L on day 7, 151.0 mg/L on day 14, and 164.6 mg/L on day 28 (Figure 1B). The BOD values for bottle 8 with C₈H₈F₉KO₃S reached 128.3 mg/L on day 7, 150.0 mg/L on day 14, and 170.6 mg/L on day 28 (Figure 1C). The BOD values for bottle 9 with C₆H₈F₅KO₃S reached 122.5 mg/L on day 7, 141.0 mg/L on day 14, and 154.5 mg/L on day 28 (Figure 1D). The BOD values for bottle 10 with C₅H₈F₃KO₃S reached 119.3 mg/L on day 7, 133.8 mg/L on day 14, and 144.2 mg/L on day 28 (Figure 1E).

After calculating the biodegradation rates, we found that bottle 6 with C₈F₁₇KO₃S, which is the PFOS in a form of potassium salt, and bottle 10 with C₅H₈F₃KO₃S had negative values, thus indicating that biodegradation did not occur at all for 28 days. Biodegradation rates of bottle 7 with C₁₀H₈F₁₃KO₃S were confirmed at 6.8% on day 7, 17.1% on day 14, and 17.7% on day 28. Biodegradation rates of bottle 8 with C₈H₈F₉KO₃S were 9.3% on day 7, 15.1% on day 14, and 24.3% on day 28. Biodegradation rates of bottle 9 with C₆H₈F₅KO₃S were 2.0% on day 7, 3.7% on day 14, and 4.1% on day 28. Considering that the biodegradation rate for highly degradable materials is greater than 60% in general, we cannot conclude that all four PFOS alternatives used in the present study were well degraded. However, the biodegrading rate of C₈H₈F₉KO₃S was confirmed as the most superior (Table 1).

For three PFOS alternatives (bottle 7 with C₁₀H₈F₁₃KO₃S, bottle 8 with C₈H₈F₉KO₃S, and bottle 9 with C₆H₈F₅KO₃S) that were confirmed to undergo biodegration, the degrading microorganisms were separated and subjected to sequencing analyses. Samples from the C₁₀H₈F₁₃KO₃S treatment showed greater than 99% of high homology with Serratia marcescens, Acinetobacter spp., and Pseudomonas geniculata. Samples from the C₈H₈F₉KO₃S treatment had more than 99% of high homology with Bacillus spp., Serratia marcescens, and Chryseobacterium spp. Samples from the C₆H₈F₅KO₃S treatment had more than 99% of high homology with Bacillus cereus and Serratia marcescens (Table 2).

C₈F₁₇KO₃S, which is the PFOS in a form of potassium salt, as well as potential PFOS alternative C₅H₈F₃KO₃S with a decreased number of carbon and fluorine atoms did not undergo biodegradation at all. While C₈H₈F₉KO₃S, which has the same number of carbons as the parent compound but a reduced number of fluorines, showed the highest biodegradation rate followed by C₁₀H₈F₁₃KO₃S and C₆H₈F₅KO₃S with 6 carbon atoms and 5 fluorine atoms also exhibited susceptibility to biodegradation, but at a markedly low rate. Together, these results suggest that it may be it advantageous to develop PFOS alternatives with 8 carbons, the same as PFOS, but a reduced number of fluorines, as such chemicals are more susceptible to biodegradation than the parent compound.

Three microorganisms were confirmed to degrade C₈H₈F₉KO₃S and C₁₀H₈F₁₃KO₃S, which showed a relatively high biodegradation rate. Two microorganisms were identified during the biodegradation of C₆H₈F₅KO₃S, which exhibited a relatively low biodegradation rate. Such results follow the general principle that there are more species of microorganisms that can act as degradators for chemicals with a higher biodegradation rate.

Commercialization of PFOS alternatives should proceed with an overall consideration of ecotoxicity, human toxicity, functionalities, and economic feasibility. It is therefore necessary to perform additional studies regarding this matter.