(446a) Removal of Low Trace Ppb Level Perfluorooctanesulfonic Acid (PFOS) with ZIF-8 Coatings Involving Adsorbent Degradation

Per- and poly-fluoroalkyl compounds (PFAS) are group of anthropogenic and environmentally persistent organic chemicals, composed of hydrophobic-oleophobic aliphatic alkyl chain with the hydrogen atom either totally (per-) or partly (poly-fluoroalkyl) replaced by fluorine atoms¹. These compounds though widely used in industrial and commercial applications as surfactant, are considered top priority pollutants².

Recently, ZIF-8 has also been suggested as promising candidate for removal of per- and poly-fluoroalkyl compounds (PFAS)^3,4. However, besides the limited literature, two major gaps are identified. First, the PFOA/PFOS removal adsorption experiments on ZIFs were performed at higher initial concentration (100 – 500 mg/L)^3,4 than those environmentally relevant (μg/L level)². So, even though high sorption capacities have been reported, the equilibrium PFAS concentration of the aqueous solution remains significantly (> 100 times) above the limit imposed by the EU directive 2020/2184 (0.5 μg/L)¹. Secondly, no clear understanding of the adsorption mechanism has been proposed. While Chen et.al. concluded that crystal structure and surface functionality are important parameters affecting PFOA adsorption on ZIFs³, no major insight in the PFOS adsorption mechanism on ZIF-8 and ZIF-67 were provided in the study by Konno et.al.⁴.

In this work, we report low trace level removal of PFOS i.e., 20 – 500 μg/L (ppb), from aqueous media by using a ZIF-8-based structured adsorbent. A ZIF-8 coated copper sheet (ZIF-8@Cu) composite was synthesized following a direct in-situ synthesis route at room temperature⁵ and was employed for PFOS removal when present in low trace ppb level concentration. The ZIF-8 coated copper sheet showed the highest removal rate (98 %) in comparison to different commercial AC and all-silica zeolites. The removal rate remained consistent over a wide range of concentration and additionally, no adsorbent leaching from the composite was noticed, which eradicated the need of additional steps such as filtration, centrifugation, unless needed for other adsorbents studied here. The composite displayed fast uptake with saturation reaching within 4h, irrespective of the initial concentration. However, the morphological and structural characterization revealed surface degradation of ZIF-8 crystals, along with a decline in the crystal size. The adsorption of PFOS on the ZIF-8 crystals was attributed to chemisorption, as the surface degradation surges with increase in PFOS concentration or with cyclic exposure at low concentrations. Methanol seemingly removed surface debris (partially), thus providing access to ZIF-8 crystals beneath surface debris. Overall, the findings demonstrate that at low trace level PFOS concentrations ZIF-8 can be considered a possible candidate for PFOS removal, as though it suffers slow surface degradation, it also removes efficiently PFOS molecules from aqueous solutions.

References

1. European Commission. Official Journal of the European Union 2019, 1–61 (2020).

2. Pauletto, P. S. & Bandosz, T. J. J Hazard Mater 425, 127810 (2022).

3. Chen, M. J. et al. Microporous and Mesoporous Materials 236, 202–210 (2016).

4. Konno, H., Nakasaka, Y., Yasuda, K., Omata, M. & Masuda, T. Catal Today 352, 220–226 (2020)

5. Sharma, R., Van Assche, T. R. C., Baron, G. V. & Denayer, J. F. M. Microporous Mesoporous Mater. 343, 112163 (2022).