GenX As A Substitute for PFOA: Treatability and Treatment Cost Perspectives

Background
The widespread application of per- and polyfluoroalkyl substances (PFAS) as water and stain repellents, and the main ingredient for firefighting foams [1] led to higher detection of PFAS in aquatic environment. Among PFAS, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are the most studied PFASs [2]. PFAS are chemically stable, and bioaccumulative and toxic in humans and fauna [3]. After recognizing the adverse effects of PFOS and PFOA (C8), they have been replaced with shorter-chain PFAS (C2–C7) [4]. Hexafluoropropylene oxide dimer acid, also known as GenX (C6), was developed particularly as a PFOA substitute by DuPont in 2009 [5]. The massive applications of GenX in the industry, as a less toxic substitute, has increased its importance more than other short-chain PFAS. Increased applications of GenX in oil-repellent coatings have led to higher detections of GenX in natural waters near production facilities [6,7].
Objectives 1. Examine treatability of GenX and PFOA by adsorbents and compare their performances for uptake of GenX and PFOA. 2. Evaluate costs of GenX and PFOA removal by activated carbon (AC) and anion exchange resins (AE). 3. Determine the suitability of GenX as a replacement for PFOA. Status The entire study has been completed. A full manuscript with all the details will be ready for submission upon acceptance of the abstract. Methodology Critical literature analysis of GenX and PFOA adsorption The kinetics and isotherms of GenX and PFOA removal by adsorption as well as governing factors including GenX and PFOA characteristics, pH, adsorbents characteristics, competing ions, adsorbent regeneration and adsorbent dosage were studied and analyzed. Simulation of GenX and PFOA with COSMO-RS A molecular simulation by COSMO-RS [8] was performed to predict the nature of interactions among GenX, PFOA, AC, and AE. The simulation helps determine the nature of interactions between GenX and PFOA with AC and functional groups, as well as ions present in AE. Treatment cost evaluation of GenX and PFOA by AC and AE The USEPA drinking water treatment technology unit cost model was developed using a bottom-up approach, known as work breakdown structure (WBS) for cost estimation [9]. The cost analysis for the wastewater treatment plant model is based on 5,500 households served and 3 people per household.
Results and Discussion
Critical literature analysis of GenX and PFOA adsorption GenX and PFOA adsorption by AC agrees well with both Freundlich and Langmuir isotherms (R2>0.9) and follows the pseudo-second-order model. The main driving force in GenX and PFOA adsorption by AC is hydrophobic interaction and by AE is ion exchange. GenX forms a fast, weak electrostatic bond with AC and can be replaced by PFOA for longer contact times (Figure 1). The optimal pH range for GenX and PFOA adsorption is 3 to 5 (Figure 2). The effect of dissolved organic matter (DOM) on GenX and PFOA adsorption by AC is inconclusive [6,10]. However, the presence of DOM decreases the adsorption of GenX and PFOA by AE. Simulation of GenX and PFOA with COSMO-RS The COSMO-RS simulation results agree with the experimental results showing the equilibrium adsorbent amount of GenX on AC would be less than PFOA for AC, while for AE, the GenX uptake is comparable to that of PFOA. Figure 3(a) shows that the interactions of GenX and PFOA with AC (ovalene) are mainly through van der Waals forces. The anionic forms of GenX and PFOA were used to evaluate their interactions with AE. The charge density values of GenX, PFOA, and AE (divinylbenzene, dimethylethanolammonium, and chloride ions) are shown in Figure 3(b) and indicate that a higher hydrogen bond donating AE such as dimethylethanolammonium is more efficient for removal of both PFOA and GenX. Treatment cost evaluation of GenX and PFOA by AC and AE Total capital costs, which include direct capital costs, indirect costs, and add-on costs are the same for the treatment of PFOA and GenX by GAC. However, the annual O&M costs for treating GenX are about 4.15 times higher than PFOA (Figure 4). The annual cost of treating almost 4 m3 of GenX per year is US$1.53, while it is around three times less (US$0.58) for PFOA (based on 30.3 years of useful life (reciprocal weighted average) and a 7% discount rate). The annual cost per household for treating GenX and PFOA contaminated water per year using AC is US$224 and US$85, respectively. The direct and total capital costs associated with AE (Type I strong base polystyrenic gel) are the same for both GenX and PFOA. The annual O&M costs for treating GenX and PFOA using the AE resin are comparable (US$156,594 for GenX and US$157,482 for PFOA). The slight difference in the annual O&M costs is probably due to the difference in the removal capacities of the AE resin for GenX and PFOA.
Conclusion GenX is less treatable by adsorption than PFOA. AC provides low removal efficiency for GenX (approximately 30%). AEs outperform AC in removing GenX. AEs with higher hydrogen bond donating characteristics are likely to have better removal capability for both PFOA and GenX. The effects of competing molecules such as DOM on GenX adsorption and associated mechanisms remain ambiguous. For AC, the estimated treatment (O&M) cost of GenX is 4.15 times higher than that of PFOA, while the GenX treatment cost is approximately the same as PFOA for AEs. As AC is the most common treatment method for GenX and PFOA, GenX as a replacement for PFOA is not favorable from treatability and treatment cost perspectives. Significance of Study GenX was adopted to replace PFOA because of its higher degradability in the environment than PFOA. Adsorption is the most common technology used to treat PFOA. However, this study demonstrated that treating GenX by AC and AE is more challenging than PFOA, and adsorption of GenX by AC costs more than that of PFOA.