Fluorinated Surfactants and Fluorinated Polymer Materials (III): Personal Opinion on the Problems of PFOS

Xing Hang, Dou Zengpei, Xiao Zibing, Xiao Jinxin

Beijing FLUOBON Surfactant Institute, China

Abstract

Key words

fluorinated surfactant; fluoropolymer; PFOS; POPs; PFOA

Perfluorooctanesulfonyl fluoride, perfluorooctane sulfonic acid and their salts, and other corresponding derivatives (collectively known as PFOS) are classified as Persistent Organic Pollutants (POPs) due to their potential for long-range transport, persistence in the environment, bio-accumulation in food chain, as well as their possible toxicity to animals and humans.The production and application of PFOS are regulated and restricted globally.Environmentalists and toxicologists have carried out large-scale research and discussion on PFOS, and currently their voices prevail in the research field of PFOS, causing a global panic on the production and application of PFOS, so that the public also has some misunderstanding of PFOS.On the other hand, PFOS producers and industrial application departments have been drowned out, so they can only passively accept the regulation, and even take some dishonorable measures to evade the regulation.It is a pity that in this global PFOS discussion, surfactant researchers become bystanders, not only do not make a sound, but also get far away from the research of PFOS.This paper presents our personal opinion on PFOS from the perspective of surfactant researchers.

China and the problems of PFOS

China is a party to the “Stockholm convention”.On May 4th, 2009, in the fourth conference of the parties for the “Stockholm Convention on Persistent Organic Pollutants (POPs)”, China sent the Ministry of Environmental Protection and other relevant ministries and commissions of delegation.Our representative proposed that the current information of production, application, alternatives and management was not enough to support the PFOS to be listed in the convention, but in the end, China compromised in the case that the fund and other conditions were guaranteed (including try to make some important applications such as fire fighting, etc.to be listed as acceptable purposes), to obtain favorable conditions with relatively long buffer period for the research and evaluation of substitutes/alternative technologies in key areas such as fire fighting.In 2007, China implemented a national implementation programme for the first series of controlled substances (i.e.12 substances including DDT).On August 30th, 2013, the Standing Committee of the National People's Congress deliberated and approved the “Stockholm Convention on Persistent Organic Pollutants (POPs) ”.

On March 25th, 2014, Ministry of Environmental Protection united 11 other ministries and commissions and launched an announcement that the Amendment to annex A, B and C about adding nine POPs in the list of “Stockholm Convention on Persistent Organic Pollutants (POPs) ” and the Amendment to annex A about adding endosulfan in annex A will both go into effect.It announced that from March 26th, 2014, the production, transportation, use, import and export of PFOS are prohibited except for specific exemptions and acceptable purposes.For specific exemptions, substitute products should be developed as soon as possible and corresponding use of PFOS should be eliminated before the expiration of the exemption.For acceptable purposes, management and risk prevention should be strengthened, and efforts should be made to gradually phase out the production and use of PFOS.At the same time, the Chinese government sets up special funds from ministries and commissions such as the Ministry of Environmental Protection and the Ministry of Industry and Information technology, and, combined with preferential (or interest-free) loans from financial organizations such as the World Bank, encourages domestic PFOS manufacturers and scientific research institutions to carry out technological innovation, industrial upgrading and research and development of PFOS substitutes.

In addition to the active actions of government departments, Chinese universities and scientific research institutes also actively carry out the publicity, research, and development for PFOS elimination action.For example, in the last decade, the “Forum of POPs and academic seminar of POPs” sponsored by Tsinghua University Research Center of POPs hold every year; the School of Environment, Tsinghua University and the environmental protection FECO, Ministry of Environmental Protection also regularly or irregularly hold the charrette of PFOS list and alternative technology in the project of “China planning of PFOS listing investigation and strategic action”.These measures have played a positive role in promoting China's implementation of the Stockholm Convention.

In 2019, the major domestic PFOS producers in China have to eliminate the production of PFOS with the help of government subsidies for environmental protection.The immediate consequence is that the market price of perfluorooctanesulfonyl fluoride (POSF) is increased by 37% to 100%.This raw material sold will be mainly used in the high-tech field many of which are secret for commercial or military reasons, including but not limited to specific exemptions and acceptable purposes.This is partly because the performance of environmentally friendly alternatives cannot match that of PFOS derivatives, not to speak of some areas where the performance can be very decisive.

Despite so many efforts, China is still at the center of much of the blame for the PFOS problem.One of the main complaints is that 3M, an American company, had already phased out the production of PFOS from 2000 to 2002, while China took advantage of this opportunity to launch largescale production projects of PFOS.[1-4]In China, there are indeed many factories of electrochemical fluorination to produce PFOS, thus it is reasonable to be accused, but actually China was treated unjustly into a “scapegoat”.Firstly, the production of PFOS in China is greatly exaggerated.For example, Kannan et al.[1]pointed out that “PFOS has been produced in China since 2003 (several hundred tonnes annually)”.Although it is almost impossible to obtain accurate data of PFOS production, as far as the authors know, the data given by Kannan et al.are greatly exaggerated.Krafft et al.have pointed out in their review that[3]“In China, about 10,000 tons of PFOSbased compounds have been used in 2009, mostly for textile treatment.” Such a statement will obviously cause great misunderstanding to the public, because the 10,000 t here refers to the terminal formula product, and the PFOS concentration is very low, so the real amount of PFOS should be much lower.In addition, the 10,000 t fabric finishing agent is basically imported, not produced by China itself.In fact, the production of POSF in China is only a small part compared with the total historical production of POSF[5](the total cumulative production of POSF from 1970 to 2002 and the waste that cannot be used is about 122,500 t) and even compared with the production of 3M company.However, they're no longer in production but we're still in production (we should admit that it was true before 2019).In other words, we are still holding the bag for the historical problems with PFOS left over by 3M and others.Secondly, China basically produced the raw material of PFOS, i.e.POSF, which means that China is at the lower end of the value chain, with low profits.However, this raw material is exported to other countries and made into various terminal products, in which the content of PFOS is very low and below the limit stipulated by the PFOS regulatory department.In this industrial chain, only China is criticized.Figuratively speaking (maybe too extreme), if other countries made medicine with our POSF, they would be praised as bring evangel to human health, while we would be blamed as the “destroyer” of human health.As far as the list of PFOS-related terminal products is known, only a few of them can be truly manufactured and produced in China, but China produces POSF (a raw material of PFOS) is a surely well-known fact.

In order to avoid taking the blame for the fault of others, we must speed up technological innovation, make efforts in development of intermediate products and especially terminal formula products, and get rid of the situation of only producing raw materials as soon as possible.

The problems and difficulty in PFOS elimination action

At present, the research on PFOS-related problems has mainly focused on environmentology and toxicology, while the difficulty in PFOS problems is actually physicochemical problems.The gap between different majors brings great difficulties to the related research of PFOS.

The lack of data

One big problem with the research of PFOS is the lack of data.1) The production and emission data of PFOS are lacking.It is difficult to obtain complete data of production and emission of PFOS because the production information from enterprises is not open to public, and the formulas are usually confidential.Although the production and emission of PFOS are estimated in many literatures, it is difficult to evaluate the reliability of these estimated data, and the data in different literatures also vary greatly.2) Only a few data on the environmentrelated basic properties of PFOS are reliable.An understanding of the environmental fate of PFOS requires quantitative information on their partitioning among various environmental media.For example, parameters such as octanol-air (KOA) and octanolwater (KOW) partition coefficients are used to describe the equilibrium partitioning of the gas phase with a pure organic solvent and the aqueous phase respectively.Unfortunately, the understanding of the physicochemical properties and partitioning behaviour of PFOS are still limited.The scarce data publicly accessible are often uncertain, due to poor product definition and purity, trade secrets, complexity and diversity of environmental matrices or else.Little reliable data is available on colloidal and interfacial chemistry, self-association, surface film formation ability and characteristics, not to speak of effects on physiology and pharmacokinetics.The important film formation, stability and elasticity properties of defined PFOS-related compounds, relevant to emulsification, aerosol formation, adsorption on solids, coating, partitioning, etc.are barely documented.

Little data (with considerable discrepancies) is accessible on the physicochemical properties of PFOSrelated compounds, which is a situation that requires consideration and rectification.Such data are needed for understanding the environmental andin vivobehaviour of PFOS.They should help determine which, for which uses, and to what extent, PFOS-related compounds are environmentally sustainable.[2]First of all, their application background is too professional and cause commercial and military secrecy.At present, most of the publicly reported work on fluorinated surfactants and fluoropolymers belongs to patents, and the researches on physicochemical properties are quite few.In contrast, the literature on the environmental science and toxicology for fluorinated surfactants and fluoropolymers has grown at an explosive rate in recent years.Unfortunately, the reports are largely negative.Another reason for the scarcity of data for physicochemical properties is the lack of reagent grade samples.The actual technical products often consist of blends (of largely unknown or unpublicized composition) of several surfactants and/or polymers.There is a definite need for better defined pure test material and standards for research, determination of such parameters as partition coefficients, critical solution temperatures, lipid solubilities that are essential for environmental monitoring and environmental processes understanding.However, currently, most of the fluorinated surfactants and fluoropolymers are industrial products, and the reagents are very rare.Some environmentally relevant fluorinated surfactants were synthesized by Lehmler et al.[6], and the synthesis of environmentally relevant fluorinated surfactants has been reviewed.Surfactant researchers should also devote themselves to this area, by synthesizing various fluorinated surfactants and testing their performance, so as to establish a more comprehensive and professional database of fluorinated surfactants.

Among the likely reasons for the lack of available data are insufficient communication and exchanges between the physical chemist and environmental chemist communities; the difficulty for the former to obtain sufficiently pure and well-defined test material; the fact that such systematic studies may not be considered sufficiently gratifying in the physical chemistry community in its race for novelty; and scarcity of specific funding.The paucity of public data could raise the question whether some products have been fully investigated and tested prior to commercialization.[2]

There are some literature works on the basic properties of poly- or perfluoroalkylated substances (PFAS) related to the environment.Ding et al.[2,7]have compiled experimental data and have predicted in silico for aqueous solubilities, vapour pressures, pKavalues for acids (acid dissociation constant), partition coefficients between air and water, octanol and air, organic carbon and water, bioconcentration, bioaccumulation and biomagnification factors for environmentally relevant PFAS.Bhhatarai et al.[2,8]develop useful quantitative structure/property relationship (QSPR) model based on, and validated by experimental data for aqueous solubility, vapour pressure and critical micellar concentrations of PFAS (such as perfluoroalkyl carboxylic acid (PFCA) and fluorinated telomer alcohol (FTOH)).However, these data are limited, often inaccurate and discrepancies can reach several orders of magnitude.These discrepancies may originate from purity or solubility problems, colloidal aggregation or adsorption behaviour, matrix effect, unsuitable analytical methods or models, lack of standardization of procedures, inadequate validation, etc.[2]A typical example is the pKafor perfluorooctanoic acid (PFOA), a simple parameter that has disagreement for a long time.The pKavalues of PFOA are reported to range widely from 0 to 3.8.[1]The environmental fate models that have been developed to describe the atmospheric transport potential of PFOA are highly sensitive to the values of pKaused.It is confusing that why PFCA have pKavalues approximately equal to their hydrogenated counterparts,[1]despite the inductively withdrawing character of the perfluoroalkyl chain that would be expected to depress the pKasignificantly.

The disadvantages of experimental methods

Take the degradation of FTOH as an example.One of the biggest challenges, when investigating FTOH degradation, is their relatively high volatility and poor water solubility, which results in a relatively high air-water partitioning coefficient.Furthermore, when dissolved in water, the tendency of the FTOH to sorb onto particles is very high.The striking partitioning behavior of FTOH complicates the understanding of the fate of these compounds in several ways: Firstly, mass balance is difficult to achieve in laboratory experiments, and this can only be remedied by working with closed systems, which in turn, do not represent natural environmental conditions.Secondly, owing to their high volatility, diverse atmospheric chemical reactions may occur, which may finally also lead to PFCA.[9]

There is another problem.Different compounds (PFOA, PFOS and other PFAS) can elicit markedly different, sometimes opposite effects and use different modes of action, and that a given PFAS can cause substantially different effects in different species, genders or strains.Extrapolation/interpolation of data, for example organ distribution, from one PFAS to another, even within a homologous series, is not granted.It is likely that at least part of these differences arise from the outstanding colloid and interfacial properties of PFAS.This also brings difficulty to the establishment of appropriate research methods.

The complicacy and uncertainty in risk assessment

Human toxicity data are scarce, fragmentary, and often inconclusive, and the results reported in literature are usually inconsistent.For example, there have been varying opinions as to whether PFAS-containing consumer articles are a significant contributor to the total exposure.[1]So the risk assessment appears particularly difficult with PFAS.A toxic equivalency factors strategy does not seem applicable because: there is increasingly strong evidence that PFAS toxicity is mediated by a plurality of receptors; the responses elicited by different PFAS are widely different and inconsistent, including within a homologous series (i.e.they do not behave as a class); for a given compound the responses (and modes of action) depend widely on species and even strain and dose; additivity of effects had not been established and is improbable in view of the above points; and the available toxicological database is limited to very few compounds, mainly PFOA and PFOS.Dosimetric anchoring approaches developed for comparing specific toxicity studies appear foiled for lack of appropriate information, in particular about short-fluorocarbon-chain and other alternative PFAS.The uncertainty factors used to account for inter and intraspecies and other differences reach 2-3 orders of magnitude.The long-term effects on environment and human health of PFAS, long or short, remain uncertain.Economic and societal pressures are important.In view of the considerable amount of uncertainty, the precautionary principle should likely prevail.[3]

In summary, it appears that PFAS do not behave as a class, that their biological behavior is highly dependent on species, gender, etc.and unpredictable, and that, while most data focus on PFOA and PFOS, the biological effects of the other PFAS, in particular the more recently used alternatives to long-fluorocarbon-chain products, remain largely unknown.Relevance of animal data to human risk assessment is still being debated.The PPARα (peroxisome proliferator activated receptor α) case study Panel's opinion was that the rodent mode of action is not relevant or unlikely to be relevant to humans.Concerning PFAS other than PFOA and PFOS, risk assessment suffers dearly from lack of information about identity, sources, tonnages emitted, degradation products and metabolites.[3]

Knowledge and information gaps impede effective PFAS risk management[3]

Uncertainties about properties, pharmacokinetics and toxicity, environmental fate, and risk remain considerable for PFAS.Knowledge gaps about PFAS are many and seem growing as marketed products grow in number and complexity.The eventual fate of PFAA (fluorinated alkyl acids, including carboxylic acid and sulfonic acid), long or short, remains uncertain.They are likely to cycle through biota along with other POPs.Effects on endocrine and immune systems, developmental issues are still poorly understood, even for the most investigated PFAS.Many critical papers close with increasingly long TO DO lists.

Other than for a handful of PFAA, independent public information about the identity, chemistry, production, physicochemical properties, technical performances, environmental behavior, exposure, degradability, pharmacokinetics and toxicity is quite limited.This concerns, in particular, the multiple new low tonnage short-fluorocarbon-chain variants and long-fluorocarbon-chain polyether-type processing alternatives and new formulations brought on the market.Restricted sharing of information, especially from industry, is a serious obstacle to risk/benefit assessment and rational PFAS management.Lack of such information can legitimately raise the question whether a new product has been properly tested prior to marketing, and carries a liability.

Our tasks

From our standpoint as a surfactant worker, currently, although the experimental proofs of the impact of PFOS on the environment, especially on human health, are not sufficient, the production and use of PFOS still need to be regulated and restricted for the sake of health and safety.At present, the regulation and elimination of PFOS has become an established fact, unless there is new experimental evidence to prove the safety of PFOS.Environmental safety is a big thing; human health is also a big thing.Therefore, we must support and actively participate in the actions of PFOS elimination.

A very strange phenomenon is that: Currently, the reports about PFOS in literature are basically the studies from environmentalists and toxicologists, most of which have focused on the effects of PFOS on the environment and animal health (including human).In comparison, chemists are less involved.The ones that participate in this area are mainly analytical chemists, focusing on the analysis and testing of PFOS, while physical chemists, especially surfactant researchers, are less involved.Compared with the overwhelming reports in the journals of environmental science and toxicology, much fewer research papers on PFOS are found in important international journals of colloid and interface chemistry.At present, the opinions of environmentalists and toxicologist are dominant in the research field of PFAS, while the voices from professionals in colloid and interface chemistry are rare, which has impacted the entire field of PFAS, e.g., PFOS has caused a global panic even extending to the whole production and application of PFAS.The public have a lot of misunderstanding, and are even afraid of anything related to “fluorine”.

Currently, the fear of PFOS also extends to the field of colloid and interface chemistry.In recent years, we can find a phenomenon in professional journals that, because PFOS has been listed as POPs, it seems meaningless to study their physicochemical properties.This trend, however, limits the collection of PFOS data by physical chemists and objectively impedes the research of environmentalists and toxicologists.

Considering the reasons above, as surfactant researchers, we have the following views:

1) The research on physicochemical properties of PFOS should be strengthened.On the one hand, as a basic theoretical research, it should not be influenced by the market of the research object.On the other hand, in-depth research on the physicochemical properties of PFOS will promote the research on its environmentology and toxicology, and further promote the research on substitute products for PFOS.As mentioned above, there is a large gap in the current PFOS research, which provides many new research topics for surfactant researchers.But the fact is that, at present, many surfactant researchers blindly stay away from PFOS in their work.Now, PFAS has been developed into a fairly large, hot and complex field of science, merely with “hot” environmentology and toxicology (“cold” colloidal and interface chemistry on the contrary).And this “cold” means opportunity, that is, a multidisciplinary challenge is in our hands, and the task for surfactant researchers is how to make this field of colloid and interface chemistry “hot”.The challenge for Colloid and Interface chemists is to help fill in knowledge gaps about the physicochemical behavior of PFAS, investigate the likely role of their specific “super” hydrophobic, segregating, self-assembling and interfacial comportment in their apparently inconsistent pharmacokinetic and toxicity behaviors, and contribute to providing creative answers and answerable solutions to growing global demand for high-performance materials, while exercising responsible ecological and societal awareness.[3]

2) Surfactant researchers should communicate and collaborate with environmentalists and toxicologists.The question is a comprehensive and multidisciplinary question, which aims to advance the future of environmental PFAS management.It requires a very multidisciplinary and multidisciplinary effort, and should involve all scientific, industrial, regulatory and user groups worldwide.Future efforts aimed at advancing environmental PFAS management issues need to be eminently multidisciplinary and should concern all the actors of the global scientific, industrial, regulatory and user communities, worldwide.In the year of 2015, in the charrette of PFOS list and alternative technology in the project of “China planning of PFOS listing investigation and strategic action”, the author strongly suggested that they should cooperate with professional surfactant researchers more, and invite more surfactant researchers in their project.Herein, I strongly recommend that surfactant professionals actively contact environmentalists and toxicologists to seek cooperation opportunities.At present, the international funding for environmental science and toxicology research is very large, which is far beyond the funding in colloid and interface chemistry, and the same is true in China.Therefore, working with environmentalists and toxicologists can also provide more financial support for colloid and interface chemists.Moreover, physical chemists focusing on environmental issues may lead to a new field of science and new disciplines.

3) Surfactant researchers should join in the flow.Environmental scientists and toxicologists focus on the adverse impact of PFOS on the environment and human health.For surfactant researchers, they should focus on how to reduce and eliminate this adverse impact, such as the research on methods to reduce bioaccumulation and toxicity of PFAS (In theory, it can be done by changing the molecular structure), the indepth study of methods for PFAS degradation so that it can be degraded in the environment, etc.For example, Shaw et al.have reported thatGordoniasp.NB4-1Y can rapidly metabolize 70.4% of 6:2 FTAB and 99.9% of 6:2 FTSA under sulfur-limiting conditions.The breakdown products are complicated mixtures of PFAS, some of which are partly defluorinated by such a process of biotransformation.[10]There are many other ways that require the further involvement of surfactant researchers.

4) We should look into the future.The most urgent problem of PFOS is not how much damage it does to the environment and human safety, but rather how little we know about it, and how much of the panic stems from the unknown.From the view of the future, if there is a deeper understanding of PFOS, perhaps the current PFOS problem will no longer be a “problem”.On the one hand, as mentioned above, we may be able to find ways to degrade PFOS, reduce its bioaccumulation and toxicity.On the other hand, in the future, the problem of PFOS may not be as serious as currently thought.In the history of scientific development, such things sometimes did happen, for instance, some surfactants that were previously banned as harmful to the environment and human body and were later released.Therefore, as surfactant researchers, we should not keep far away from PFOS research, but should be more involved in it.

In a word, PFOS should not be totally given up for the potential risk.Risk needs to be maintained at an acceptable level consistent with technological innovation, economic growth and the needs of protection of health and environment.Necessary and urgent research needs to be coordinated and improved.The expertise and information should be shared.Our challenge is to manage (or replace) PFAS to control or reduce pollution and health risks without losing the technological advantages and social benefits of their use, especially in developing countries where economic development comes first.

Other important PFAS

PFOS and PFOA are explicitly listed in the Stockholm Convention.There are many PFAS with the nature of POPs, and their production, application and regulation are inconsistent in many countries in the world.Some examples are shown as follows.

PFOA

PFOA is a kind of PFCA, which sometimes is named as C8.PFOA has been detected together with PFOS, which is typically the major perfluorinated contaminant, and other perfluorinated surfactants in many environmental samples ranging from surface waters to marine mammals indicating a global contamination with these chemicals.Interestingly, PFOA is not the only perfluoroalkanoic acid found in environmental samples.A series of PFCA ranging in chain length from 9 to 15 carbon atoms has been detected in several species collected at various locations in the circumpolar region and in harbour porpoises from Northern Europe.[6]

In 2012, the very long fluorocarbon-chain C11-14PFCA have been categorized as “vPvB (very persistent, very bioaccumulative)” potentially harmful pollutants and recorded in the Candidate List of Substances of Very High Concern (SVHC) for Authorization under the REACH (Registration, Evaluation, and Authorization of Chemicals) regulation.[3]PFOA and its ammonium salt (APFO) clearly fulfill the persistent, bioaccumulative and toxic (PBT) substance criteria.They were identified as such by the EU and joined the SVHC list in 2013 as toxic for reproduction.[3]In 2014, an International Agency for Research on Cancer working group also classified PFOA as possibly carcinogenic in humans.Fluoropolymers are no longer exempted from pre-manufacture (or importation) notification in the US.A proposed 2015 US Significant New Use Rule (SNUR) ensures that PFAS that have been phased out do not reenter the marketplace without review.[3]

Polyfluorinated alkyl phosphate ester surfactants (PAPs)

Polyfluorinated alkyl phosphate ester surfactants are found in three forms depending on the levels of phosphate ester substitutions: monoesters (monoPAPs), diesters (diPAPs) and triesters (triPAPs).Industrial PAPs mixtures consist primarily of diPAPs, with monoPAPs and triPAPs being produced as byproducts.[11]

PAPs are widely used in food-contact materials, from which they have the ability to migrate into food.[11]diPAPs were the major components of the fluorinated compounds in indoor dust samples.[11]Numerous phosphorus compounds, including fluorinated alkylated phosphate monoesters (n:2 monoPAPs) and diesters (n:2 diPAPs), fluorinated alkylphosphonic acids (PFPAs), fluorinated alkylphosphinic acids (PFPiAs) and fluorinated octane sulfonamidoethanol phosphate diester (SAmPAPs) are now found in surface waters, wastewater treatment plant (WWTP) sludge and effluents, sediments and human sera.Their degradation contributes to exposure to PFAA.[3]

Until now, the limited toxicological data currently accessible on PAPs (including some metabolic products of PAPs) give reason to believe that these compounds might have the ability to cause potentially adverse effects, as seen for other perfluorinated chemicals such as PFOS, PFOA and some FTOH.[11]The current lack of toxicological data on PAPs impairs the risk assessment of this group of compounds.It is evident that scientific and regulatory communities are only beginning to understand and effectively manage polyfluorinated compounds, including PAPs.[11]

Products of telomerization

At present, the Stockholm Convention mainly restricts and prohibits fluorinated surfactants produced by electrochemical fluorination such as PFOS, and it has not made clear provisions on fluorinated surfactants produced by telomerization.Currently, the fluorinated surfactants produced from F(CF2)n(CH2)mI which are raw materials synthesized by telomerization are mainly the products ofn=6.Although the Stockholm Convention has not yet stipulated for the fluorinated telomer derivatives, in the long run, restriction or even prohibition will come soon.Some companies have developed fluorinated surfactants with 6 fluorinated carbons.Although they are not belong to the list of Stockholm Convention, it can be expected that, after the elimination of PFOS, those fluorinated surfactants

with 6 fluorinated carbons could be the next.It has been reported that the human serum elimination half-life of PFOS has a geometric mean of 1,751 days (95% confidence interval 1,461-2,099) while PFHxS has a geometric mean of 2,662 days (95% confidence interval 2,112-3,555).[12]The fluorinated surfactants with 6 fluorinated carbons seem to have no obvious advantage over the one with 8 fluorinated carbons in bioaccumulation.

6:2 fluorotelomer sulfonamidoalkyl betaine (6:2 FTAB) is usually applied in aqueous film-forming foam (AFFF) and is frequently detected, along with one of its suspected transformation products, 6:2 fluorotelomer sulfonate (6:2 FTSA), in terrestrial and aquatic ecosystems impacted by AFFF usage.

Human or ecological risks associated with 6:2 FTSA and 6:2 FTAB have been rarely examined.The only study that assesses the health risks associated with 6:2 FTSA found that 6:2 FTSA exhibits moderate hepatotoxicity relative to that reported for legacy PFOS and PFOA.[10]The health risk could also be a problem for the fluorotelomer surfactants with 6 fluorinated carbons.

Long fluorocarbon-chain PFAS

Long-fluorocarbon-chain PFAS refers to PFCA with 7 and more fluorinated carbons (e.g.PFOA) and perfluoroalkane sulfonates (PFSA) with 6 and more fluorinated carbons (e.g.PFHxS, PFOS), and substances that have the potential to degrade to longfluorocarbon-chain PFAA.An incomprehensible phenomenon is that PFAS with longer fluorocarbon chain are occasionally used as alternatives to PFOS despite their nature of POPs.It is realized that bioaccumulation and likely also health risks increase with increasing fluorocarbon-chain length.[5]Therefore, we believe that the use of longer PFAS as PFOS substitutes is simply an attempt to circumvent the Stockholm Convention.It is unacceptable and should be resisted.

Very persistent long-fluorocarbon-chain PFCA is also frequently detected in human serum.C7-11and C13PFCA and 4:2 to 8:2 diPAPs were found in most inhabitants of two German cities, as well as C4-10PFAS and a wide range of their precursors.Increasing concentrations of perfluorononanoic acid (PFNA) likely indicates indirect exposure via biotransformation of FTOH-based substances.[3]

PFOS substitutes

At present, PFOS substitutes are mainly short fluorocarbon-chain PFAS and fluorinated polyethers.The controversy over these two types of substances continues.They will be introduced in detail in the next serial article.

Summary

Currently, environmental scientists, toxicologists, surfactant researchers, manufacturers and consumers have put forward different views on PFOS.From the view of surfactant researchers, we should not “blindly follow the flow” or give up the research of fluorinated surfactants such as PFOS.Conversely, we should try our best to study how to reduce the impacts of these POPs by use of the relationship between structure and properties of fluorinated surfactants.We believe that as more surfactant researchers join the PFOS research team, there will be more means to inhibit or reduce the adverse characteristics of PFOS, and there will be more and better fluorinated surfactants without the characteristics of POPs.

In the next serial article we will discuss the strategies for replacement of PFOS, introduce PFOS substitutes, and analyze the performance and environmental behavior of short-fluorocarbon-chain substitutes; We will further discuss the research and optimization of alternative products to reduce environmental and health risks in response to the growing global demand for high-performance materials, as well as the sustainability issues in the use of these PFAS.