Grantee Research Project Results
Final Report: Perstraction for the Removal of PFAs from Water
EPA Grant Number: SU840161Title: Perstraction for the Removal of PFAs from Water
Investigators: Almquist, Catherine B , Garza, Linda , Marcellino, Chris , Chen, Sean , Armstrong, Ryan , Flood, Megan , Goddard, Tai
Institution: Miami University - Oxford
EPA Project Officer: Spatz, Kyle
Phase: I
Project Period: December 1, 2020 through November 30, 2021 (Extended to November 30, 2022)
Project Amount: $24,979
RFA: P3 Awards: A National Student Design Competition Focusing on People, Prosperity and the Planet (2020) RFA Text | Recipients Lists
Research Category: P3 Awards , P3 Challenge Area - Safe and Sustainable Water Resources
Objective:
We demonstrated a novel perstraction process for the removal of perfluorooctanoic acid (PFOA) from water. The process utilizes a membrane barrier between an organic solvent and PFOA-contaminated water. The membrane barrier is required in this process, since significant foaming or emulsion formation occurs at the interface between organic solvents and PFAS-contaminated water. For this project, we synthesized membranes using polydimethylsiloxane (PDMS), which is a hydrophobic material. The inclusion of nanoparticles within the PDMS can vary the properties of the membrane, and so we investigated the effects of various nanoparticles imbedded in PDMS membranes on the absorption of selected solvents and of perfluorooctanoic acid (PFOA).
Summary/Accomplishments (Outputs/Outcomes):
This study represents a first time that perstraction was assessed as a process to remove perfluorooctanoic acid (PFOA) from water. In the perstraction process, PFOA permeates through a membrane from water to a solvent. The membrane used in this study was polydimethylsiloxane (PDMS). PFOA preferentially partitioned to alcohols (butanol, hexanol, and octanol) over water. In addition, ZnO and CuO particles in PDMS significantly enhanced the rate at which PFOA was absorbed in PDMS from deionized water due to ionic interactions. The perstraction of PFOA from deionized water into hexanol was demonstrated. However, perstraction was not successful at removing PFOA from tap water. While the application of perstraction to removing PFOA from water is limited, the idea was demonstrated and information contained within this manuscript is new.
The approach used in this study was as follows: (1) design and fabricate a perstraction test system; (2) measure the partition coefficients for PFOA in selected solvents; (3) determine the solubility and diffusivity of selected solvents in PDMS; (4) determine the uptake or absorption of PFOA in PDMS; and (5) demonstrate the perstraction process for removing PFOA from water. Each step in the experimental approach is described in more detail below.
1. Design and fabricate a perstraction test system. Four identical perstraction test systems were designed and fabricated out of aluminum blocks (7.6 cm × 7.6 cm × 15.2 cm). For each test system, the aluminum block was cut in half, and identical 100 cm3 cavities were machined in each half. The diameter of the cavity between the two halves is 4.45 cm. A membrane was cut to approximately 3.5 g and had dimensions of 5 cm in diameter and 1 mm thick. The membrane was placed between the two halves of the test system, and it completely covered the open area between the two halves. The assembly is secured together by 4 long screws. A schematic and a photo of a perstraction test system are shown in Figures 1a and 1b, respectively.
Figure 1. Perstraction test system. (a) A schematic of a lab-scale perstraction test system. (b) A photo of a perstraction test system used in this study. LHS = left-hand side where the PFOA aqueous solution was placed. RHS = right-hand side where the solvent was placed. The volume on each side of the membrane was 100 cm3.
Ports were drilled into the top of each half of the test system to facilitate loading and sampling the solutions on each side of the membrane. These openings were covered between sampling times during the experimental trials. The solutions in the vessels were stagnant, except when samples were obtained for subsequent analyses.
2. Measure the partition coefficients for PFOA in selected solvents. The partition of PFOA between water and selected solvents was assessed, and the results are summarized in Table 1. In all cases, the non-polar solvents had very low solubility for PFOA. In contrast, the alcohols all have partition coefficients > 1, indicating that PFOA preferentially separates from water to the alcohol phase. Based upon these results, 1-butanol, 1-hexanol, and 1-octanol were used as perstraction solvents for the rest of this study.
Table 1. Summary of partition coefficients for PFOA in selected solvents.
Aqueous Phase | Aqueous Phase | |
Solvent | Deionized water | Tap water 1 |
Hexane | N/A | 0.118 |
Cyclohexane | N/A | 0.003 |
Benzene | N/A | 0.025 |
1-butanol | 16.4 ± 14.5 | 71.6 ± 29.2 |
1-hexanol | 27.0 ± 16.7 | 31.8 ± 14.9 |
1-octanol | 12.4 ± 5.6 | 5.4 ± 6.5 |
PFOA solutions in tap water had measured pH values between 7.5 and 8.5, and so PFOA would be predominantly present in its ionized form. In contrast PFOA solutions in DI water had measured pH values < 3, and so a significant fraction of PFOA would be in its protonated or neutral form.
The partition coefficients for PFOA in alcohols are all similar when the aqueous phase is DI water. In contrast, the partition coefficients tend to decrease with increasing carbon chain length or decreasing polarity in the alcohol when tap water is used. It is likely that hydrophobic interactions between the solvent and the protonated form of PFOA were dominant in DI water, since PFOA would be present in its protonated form. However, the ionized form of PFOA would tend to prefer polar compounds and stay in the aqueous phase. In addition, tap water contains significant concentrations of minerals and ions, and so there would be significant competition for the ionic interactions of PFOA anions with other anions in the water with the polar head of the solvent.
3. Determine the solubility and diffusivity of selected solvents in PDMS. The solubilities and diffusivities of 1-butanol, 1-hexanol, and 1-octanol were determined. The effects of ZnO and CuO loadings (0 wt%, 5 wt%, 10 wt%, and 15 wt%) in PDMS on solvent absorption were also investigated. The results are summarized in Table 2. The solubilities of the alcohols in PDMS appear to decrease as the carbon chain in the solvent increases (1-butanol > 1-hexanol > 1-octanol). The diffusivities of 1-butanol and 1-hexanol in PDMS are similar, whereas the diffusivity of 1octanol was estimated to be much higher. The diffusivity of 1-octanol may have been enhanced by its more favorable thermodynamic interaction with PDMS. PDMS is a nonpolar polymer. The longer nonpolar hydrocarbon chain in 1-octanol compared with 1-butanol and 1-hexanol enhanced its diffusion within the polymer. The presence of imbedded particles in PDMS did not appear to significantly affect the observed solubilities or diffusivities of the alcohols in PDMS. This suggests that the interactions between the alcohols and PDMS dominate the rate of mass transfer of solvents in PDMS.
4. Determine the uptake or absorption of PFOA in PDMS. The absorption, or uptake, of PFOA in PDMS with and without imbedded particles was investigated using samples of membranes (~0.10 g and 1 mm thickness) in solutions of PFOA in deionized water. The concentrations of PFOA in the solutions were measured as a function of time over several days. Figure 2 shows the results of selected experimental trials with membranes containing several different imbedded particles. ZnO and CuO imbedded in PDMS significantly enhanced the uptake or absorption of PFOA from deionized water solutions more than all other imbedded particles investigated in this study. This result is likely due to the ionic interactions between ZnO and CuO in the PDMS and the carboxyl group on PFOA. However, PFOA in tap water does not significantly absorb or adhere to the membrane samples. This was an unfortunate result. Reasons for this result are likely due to the degree of ionization of PFOA in DI water compared to tap water as well as competitive ion interactions in tap water. The PFOA anions in water may interact with each other to form micelles, or they may interact with mineral cations in tap water.
Table 2. Summary of solvent absorption in PDMS membranes.
Solvent | Membrane Material | Solubility (kg/m3) | Diffusivity 1 (1011 m2/s) |
1-butanol | PDMS 5 wt% ZnO in PDMS 10 wt% ZnO in PDMS 15 wt% ZnO in PDMS 5 wt% CuO in PDMS 10 wt% CuO in PDMS 15 wt% CuO in PDMS | 108 ± 9.7 | 2.6 ± 1.6 |
100 ± 4.6 | 1.6 ± 0.3 | ||
111 ± 6.9 | 1.3 ± 0.003 | ||
84.1± 6.5 | 1.5 ± 0.03 | ||
116 ± 11.4 | 1.8 ± 0.02 | ||
104 ± 20.6 | 1.6 ± 0.10 | ||
108 ± 9.6 | 1.4 ± 0.37 | ||
1-hexanol | PDMS 5 wt% ZnO in PDMS 10 wt% ZnO in PDMS 15 wt% ZnO in PDMS 5 wt% CuO in PDMS 10 wt% CuO in PDMS 15 wt% CuO in PDMS | 92.1 ± 12.2 | 1.5 ± 0.12 |
79.4 ± 4.4 | 1.5 ± 0.26 | ||
83.1 ± 6.7 | 1.2 ± 0.01 | ||
76.1 ± 0.8 | 1.4 ± 0.29 | ||
116 ± 1.0 | 1.4 ± 0.07 | ||
104 ± 5.1 | 1.6 ± 0.16 | ||
101 ± 5.1 | 1.4 ± 0.07 | ||
1-octanol | PDMS 5 wt% ZnO in PDMS 10 wt% ZnO in PDMS 15 wt% ZnO in PDMS 5 wt% CuO in PDMS 10 wt% CuO in PDMS 15 wt% CuO in PDMS | 52.1 ± 0.58 | 3.3 ± 0.45 |
41.3 ± 4.5 | 3.9 ± 0.62 | ||
40.4 ± 3.9 | 4.1 ± 0.22 | ||
41.7 ± 6.7 | 3.4 ± 0.22 | ||
44.4 ± 15 | 4.8 ± 3.8 | ||
48.6 ± 7.4 | 2.4 ± 0.54 | ||
44.7 ± 3.1 | 2.5 ± 0.27 |
1 The diffusivity values in the table were multiplied by 1011. Therefore, the diffusivities are the values in the table, Di × 10−11 m2/s.
Figure 2. PFOA uptake in PDMS with 10 wt% imbedded particles. Initial PFOA solution was 1000 mg/L in deionized water. The initial mass of each membrane sample was 0.1 g.
5. Demonstrate the perstraction process for removing PFOA from water. The perstraction process was demonstrated for removing PFOA from DI water into 1-hexanol through a PDMS membrane. The PFOA concentrations in the deionized water (blue markers and line) and in the corresponding 1-hexanol (orange markers and line) were recorded with time. These data are shown graphically in Figure 3. The sum of PFOA in the water and in the solvent is shown by the black dashed line. More than 70% of the PFOA was accounted for in the two solutions. The balance of the PFOA is presumably held up within the solvent-swelled PDMS.
Figure 3. Perstraction of PFOA from DI water into 1-hexanol through a PDMS membrane.
Conclusions:
Perstraction was demonstrated as a process by which PFOA can be removed from water. 1-Butanol, 1-hexanol, and 1-octanol are possible solvents for this process. However, the process was demonstrated only when the PFOA solution was in DI water and the membrane was PDMS without imbedded particles. The process appears to be dominated by hydrophobic interactions between the protonated or neutral form of PFOA, PDMS, and solvent.
The rate and extent of PFOA removal from DI water by PDMS was significantly enhanced when ZnO and CuO were imbedded in PDMS. However, PFOA was not extracted through the membrane when the PDMS membrane contained imbedded ZnO or CuO. This suggests that PFOA strongly adhered to the ZnO and CuO particles within the PDMS membranes.
PFOA was not effectively removed from tap water by perstraction, presumably due to the degree of ionization of PFOA. The ionized form of PFOA forms micelles in tap water, which would decrease access to the hydrophobic end of PFOA and reduce its interaction with PDMS.
While the application of perstraction to the removal of PFOA from water has limitations, the research generated new and interesting data regarding the interaction of PFOA with PDMS and PDMS imbedded with ZnO and CuO particles. The degree of ionization of PFOA must be considered when developing processes to remove PFOA from water.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 6 publications | 1 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Almquist CB, Garza L, Flood M, Carroll A, Armstrong R, Chen S, Marcellino C. Perstraction:a membrane-assisted liquid–liquid extraction of PFOA from water. Processes 2023;11(1):217 |
SU840161 (Final) |
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Supplemental Keywords:
PFOA, perstraction, polydimethyl siloxane, membranesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.