# Ion Exchange MIEX(r) GOLD Resin as a Promising Sorbent for the Removal of PFAS Compounds
## 3 Results and Discussion
### PFAS Removal Efficiency of MIEX(r) GOLD Resin  
The removal efficiency of the resin was studied against six different PFAS compounds, which are PFBA, PFBS, PFHxA, PFHxS, PFOA, and PFOS. In all cases, the weight ratio of PFAS to resin was maintained at 1:4400 (i.e., 0.5 mL eq. to 0.11 g resin was used for 25 mL of each 1 ppm PFAS solution). Significantly, >99.999% removal of PFOA, PFOS, and PFHxS was estimated. On the other hand, 99.21 \(\pm\) 0.12%, 99.8 \(\pm\) 0.01%, and 99.65 \(\pm\) 0.00% removal of PFBA, PFBS, and PFHxA, respectively, were quantified (Figure 3b). Figure 2: **(a)** The zeta potential (mV) for pH range 3–11, and **(b)** FTIR spectrum of MIEX® GOLD resin. Notably, the removal efficiency was found higher for longer-chain PFAS in comparison with shorter-chain PFAS. For example, PFOA (eight-carbon chain) showed more affinity for resin compared to shorter chain PFBA (four-carbon chain) and PFHxA (six-carbon chain). This was because the increased length of the alkyl group causes an increase in the hydrophobic character of PFAS compounds, which is more favorable to a passage from the solution to be adsorbed on resin. Regardless of smaller molecular size, PFBA and PFBS are less attracted or adsorbed to resin compared to longer chain homologs, which suggests no significant impact of size exclusion phenomena over the ion exchange phenomena of PFASs [7]. The results also showed that PFAS compounds with the sulfonic acid head group (e.g., PFHxS and PFBS) have slightly higher affinity to the resin compared to compounds with the carboxylic acid group (e.g., PFHxA and PFBA). Similar affinity was observed with other sorbents such as aminated sorbents, which exhibit higher affinity, efficiency, and selectivity toward sulfonic acid-containing PFASs [27]. This is mainly because the sulfonic acid head group of PFASs provides greater electrostatic interaction compared to the carboxylic acid head group toward the cationic quaternary ammonium group of adsorbents [8; 27]. Figure 4 shows the schematic illustration of PFAS adsorption onto MIEX(r) GOLD resin. Negatively charged PFAS compounds in water can bind to the positively charged quaternary ammonium ion on the binding site of the resin surface by exchanging with the chloride ion. This ionic exchange caused the successful removal of PFAS compounds from contaminated water. This underlying mechanism of PFAS removal by MIEX(r) GOLD resin is similar to that of dissolved organic carbon removal by this resin [28]. The regeneration of resin is performed in MtOH (methanol) with saturated NaCl. The presence of a Na+ cation attracts the negatively charged PFAS from the positively charged quaternary ammonium  
Figure 3: (**a**) Chemical structure of different PFAS compounds, and (**b**) Removal efficiency of MIEX® GOLD resin for PFBA, PFBS, PFHxA, PFHxS, PFOA, and PFOS. ion of resin beads. At the same time, an excess amount of chloride in the solution binds to the positively charged quaternary ammonium ion and helps to replace or desorb the adsorbed PFAS. The maximum adsorption capacity of MIEX(r) GOLD resin for PFOA and PFHxS was also determined. The result showed >90% adsorption capacity of MIEX(r) GOLD resin for both PFOA and PFHxS. The adsorption test for PFHxS was performed in two scales: (i) 0.11 g resin was exposed to 0.121 g PFHxS, and (ii) ten times larger scale by adding 1.1 g resin to 1.23 g PFHxS. Both scales showed similar results with an average of 1.01 \(\pm\) 0.04 g PFHxS adsorption per gram of resin. This adsorption capacity for PFHxS (1.01 g/g) is significantly higher than other adsorbents, for example, magnetic activated carbon (0.13 g/g), few-layered porous graphite (0.612 g/g), and montmorillonite (0.31-0.37 \(\times\) 10\({}^{-3}\) g/g) [29; 30]. The adsorption capacity of MIEX(r) GOLD resin for PFOA was also tested and 1.05 \(\pm\) 0.01 g PFOA adsorbed per gram of resin was achieved.