Remediation of Per-and Polyfluoroalkyl Contaminated Groundwater Using Cationic Hydrophobic Polymers as Ultra-High Affinity Sorbents
Field, James A.
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PublisherThe University of Arizona.
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EmbargoRelease after 05/18/2023
AbstractPer-and polyfluoroalkyl substances (PFAS) are a group of man-made chemicals which most known representatives are perfluoroctanoic acid (PFOA) and the perfluorooctane sulfonate (PFOS). These chemicals have been widely used in both industrial processes and consumer products and they are ubiquitous environmental contaminants. Many PFAS are toxic, bioaccumulative and very persistent in the environment. Due to the growing concern about the potential adverse effects of PFAS on human health, the U.S. Environmental Protection Agency has established a health advisory limit for drinking water for both PFOA and PFOS combined of 70 ng L-1. This very low advisory level together with the high resistance of PFAS to chemical and biological degradation make the removal of PFAS from aquatic bodies a complicated task, being the only currently feasible option the use of activated carbon or ionic exchange adsorption processes. These approaches are very costly due to the need for frequent bed replacement; thus, new treatment methods are required to efficiently remove PFAS from contaminated water. PFAS sulfonates and carboxylates contain a hydrophobic fluorocarbon chain and they are present as anionic species under environmental conditions. Based on these properties, this study proposed the synthesis and use of cationic hydrophobic polymers based on aniline and pyrrole as ultra-high affinity PFAS sorbents. These polymers were chosen because their positive charge and hydrophobic carbon backbone is expected to promote strong electrostatic and hydrophobic interactions with anionic PFAS. Polypyrrole (PPy) and six polymers based on aniline with different functional group substitutions were synthesized, namely, polyaniline (PANI), poly-o-toluidine (POT), poly-o-anisidine (POA), poly-o-ethylaniline (PEA), poly-sec-butylaniline (PSB), and polynaphthylamine (PNA). The impact of the different substitutions on the polymer properties as well as their effectiveness as PFAS adsorbents was evaluated. The use of precursors with different substituents had a major impact on the chemical composition of the polymers as shown by elemental analysis, as well as FTIR and XPS analysis. Likewise, the chemical structure of the precursors had a large impact on different physico-chemical properties of the polymers such as their specific surface area, zeta potential, isoelectric point, and acid-base behavior. From the seven synthesized polymers, PANI, POT, and POA presented the best PFAS adsorption characteristics. Individual isotherm experiments showed that POT, with a Freundlich coefficient of 78 (mg g-1)(mg L-1)-n, was the sorbent with the highest affinity for PFOA, followed by PANI and POA, with Freundlich coefficients of 46 and 30 (mg g-1)(mg L-1)-n, respectively. The Freundlich coefficient of PAC, 187 (mg g-1)(mg L-1)-n, was higher compared to that of POT. Nonetheless, at environmental relevant concentrations (ng L-1 to µg L-1), POT showed similar PFOA affinity and adsorption capacity as crushed granular activated carbon (PAC). Furthermore, evaluation of PFOA adsorption under increasing pH (pH range from 3 to 11) showed that the range of maximum sorption capacity varied depending on the polymers. This behavior suggests that the hydrophobic cationic polymers can be customized to enhance PFAS adsorption under different aqueous chemistry conditions. Contaminated groundwater often contains a mixture of PFAS, therefore, the adsorption of the fluorinated compounds listed in the EPA Third Unregulated Contaminant Monitoring Rule(i.e., perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), perfluorobutane sulfonate (PFBS), perfluorohexane sulfonate (PFHxS), and PFOS) as well as the fluorotelomer FtS 6:2 using the most promising sorbents was evaluated. Single solute isotherms as well as isotherms determined using a multicomponent PFAS mixture showed that both, the ionic head group and chain length, had a strong impact on the adsorption of the different PFAS. Perfluoroalkyl sulfonates were more effectively adsorbed than their carboxylic analogues with the same chain length. Furthermore, the adsorption of PFAS with the same functional head group increased with increasing fluorocarbon chain length, suggesting the importance of hydrophobic interactions between the fluorinated compounds and the adsorbent. This behavior was observed in experiments with both POT and PAC. Adsorption process are generally very sensitive to the aqueous chemistry (e.g., pH, ionic strength, ionic composition) as well as the presence of co-contaminants, such as natural organic matter (NOM), that compete for active sites on the sorbent surface and decrease the adsorption of the targeted compounds. PFOA adsorption capacity of the polymers was found to decrease with increasing NOM concentration, but to a much lower extent compared to PAC. At the highest NOM concentration tested (2 mg L-1), the PFOA adsorption capacity of POT and PANI decreased by 40 and 60%, respectively, whereas PAC lost nearly all its adsorption capacity under the same conditions. This observation could be due to the involvement of both hydrophobic and electrostatic interactions on PFAS adsorption by the polymeric adsorbents, while PFAS adsorption by PAC only relies on hydrophobic interactions. On the other hand, increasing ionic strength had an important impact on the PFOA adsorption capacity of PANI and POA, which decreased by 93% when the ionic strength was as low as 5 mM NaCl. In contrast, POT only lost 20% of its initial PFOA adsorption capacity in the presence of very high NaCl levels (500 mM). The impact of tionic strength on the adsorption process was highly depended on the ionic species. In contrast with NaCl, divalent salts at ionic strength of 5 meq L-1 (CaCl2 and Na2SO4) caused a substantial increase in the affinity of the polymers for PFOA, up to six times in the case of POA at a salt concentration of 5 meq L-1. Under the same conditions, the divalent salts did not alter the PFOA adsorption capacity of PAC. The synthesized polymers had a relatively low specific surface area (1-26 m2g-1) compared to GAC, a widely used PFAS adsorbent (814 m2 g-1). The low specific surface area (SSA) of the polymers suggests that it may be possible to enhance significantly the PFAS adsorption capacity of these materials by increasing their SSA. Different approaches were explored to obtain polymers with high SSA such as the synthesis of nano-sized- and crosslinked polymers. The synthesis of nano-sized polymers led to a large increase in the fraction of small size pores (size < 1 nm), however this increase was not accompanied by an enhancement of the SSA and PFOA adsorption capacity of PANI. Crosslinking of PANI with paraformaldehyde led to a very high increase of the SSA of the polymer (up to 20-fold) as well as an increase in the fraction of small size pores (< 5 nm). However, a proportional increase in the PFOA adsorption capacity of the crosslinked polymer was not observed, and its Freundlich constant was just doubled. The results obtained also indicated that the increase in SSA was strongly dependent on the dose of crosslinker utilized. Due to the morphology of the synthesized polymers, which are fine powders, their use in packed-column adsorption processes would be complicated due to high pressure losses and polymer washout. Thus, synthesis of polymers grafted on a granular material was performed, using GAC and cellulose as supporting materials. Our results confirmed that the studied polymers could be grafted on GAC and that the synthesized composites presented high SSA and high affinity for PFAS compounds. The adsorption capacity of the composite materials was similar to that GAC and even higher when small amounts of PANI were grafted on the GAC surface. In contrast with the GAC/polymer composites, PANI grafted on microcrystalline showed a very low PFAS adsorption capacity. Predicting the rate at which adsorption takes place is important for adsorber design. Therefore, kinetic experiments were performed to characterize the rate of PFOA adsorption by the most promising materials. The kinetics of PFOA adsorption by the evaluated polymers were comparable to that of a leading PAC, except for crosslinked PANI that had a 70% higher second-order adsorption rate than PAC. Overall, these results indicate that the most effective hydrophobic cationic polymers (PANI, POT, and POA) offer great promise for the removal of PFAS from contaminated water. These sorbents display high PFAS adsorption affinity and capacity, rapid PFAS adsorption kinetics, and they can be grafted on a granular material to facilitate their use in continuous-flow absorbers. The involvement of electrostatic and hydrophobic adsorption mechanisms on PFAS adsorption by the new materials provides clear advantages for their application under variable aquatic chemistry conditions (e.g. presence of contaminants, NOM, ionic species) when compared to conventional adsorbents such as granular activated carbon.
Degree ProgramGraduate College