PFAS removal by ion exchange

Beside activated carbon adsorption (connect to ‘PFAS removal by activated carbon adsorption’ page), Ion exchange  is another option which is gaining the market nowadays. Like GAC, ion exchange resin beads are packed in a vessel and operated in a passing-through mode.

In typical surface water/groundwater with neutral pH condition, PFAS are negatively charged due to dissociation of the carboxylic or sulfonic functional groups in most PFAS molecules. The positively charged anionic exchange resin (AER) is very promising for the acidic PFAS removal with sulfonic ‘heads’, removal percentage is up to 90-99% (no bed volume data was available) [1].

Comparing to granular activated carbon (GAC), which prefers hydrophobic substances due to its hydrophobic nature, AER, on the other hand, has a great affinity to hydrophilic PFAS, e.g. perfluoroalkyl acids (PFAA). The mechanism is mainly via exchanging with the carboxylic or sulfonic acid heads of the PFAA compounds [2].

As mentioned in another page (connect to ‘PFAS removal by activated carbon adsorption’ page), GAC adsorption of PFAS is sensitive to competition from the co-present organic matter. The efficiency of AER for PFAS is, however, more effected by the co-present anions. So in case the feed water contains high total dissolved solids (TDS), AER might not be the ideal option for PFAS removal.

Ion exchange is a fast reaction process as compared to adsorption, requiring much shorter contact time (usually 2 - 3 minutes) between the resin beads and pollutants, this translates into a smaller system configuration and less footprint, saving capital costs. Ion exchange was also tested to have higher throughout and longer bed life, in general 6-18 months, as compared to 9 -12 months for GAC filters on PFAS removal [3].

Since ion exchange requires chemical regeneration for capacity recovery, a high-concentrated PFAS-containing waste stream will be produced, which is even difficult to treat. To avoid waste regenerant production, single pass (or single use) ion exchange is considered, the waste resin beads (basically hydrocarbon material-made) are then landfilled or incinerated. 

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References:

[1] PFAS: Drinking Water Treatment, EPA, Calgon Carbon, Pittsburgh PA, 1 March, 2018.

[2] Remediation technologies and methods for per- and polyfluoroalkyl substances (PFAS), March 2018.

[3] Webinar: Carbon and Resin Solutions For PFAS Removal in Your Community or Industry.