PFAS removal by activated carbon adsorption

Activated carbon adsorption (connect to: https://www.lenntech.com/library/adsorption/adsorption.htm) is one of the most suitable technology for emerging contaminants removal as to i.e. drinking water and wastewater utilities, because of its good removal efficiency, easiness of full-scale upgrading and minimal maintenance required.

According to Lenntech’s experiences, there are several factors needs to be considered when selecting activated carbon adsorption for PFAS removal: 1) carbon material/porosity, 2) feed water quality, 3) target PFAS characteristics and 4) contact time between PFAS and activated carbon during adsorption process.

Activated carbon porosity plays an important role in adsorption performance. In general, carbon with a combination of micropores and mesopores is favored than, e.g. microporous carbon. Because mesopore enables a fast diffusion of pollutants onto their final adsorption sites (micropores), it is then advantageous especially in a granular activated carbon (GAC) filter when contact time between PFAS and the carbon is fixed.

When applying carbon for PFAS adsorption, it is always advisable to know the feed water source and the water composition (by water analysis). For instance, in surface water, PFAS adsorption can be largely compromised due to the adsorption competition from co-present natural organic matter (measured as Total organic carbon, TOC), bearing in mind that the TOC concentration is in a mg/L magnitude as compared to PFAS in µg/L or ng/L. The higher the TOC level, the less removal of PFAS via adsorption. Normally, PFAS removal is higher in groundwater than in surface water because groundwater contains less TOC than surface water.

PFAS characteristic determines their adsorption efficiency as well. Investigations on GAC filter in a drinking water treatment plant showed a fast breakthrough of short-chain PFAS compounds, e.g. < C8 carboxylic acids and < C5 sulfonic acids [1], meaning that small molecular PFAS are more troublesome for GAC adsorption. Thus, instead of the typical empty bed contact time (EBCT), i.e. 10-20 min, longer EBCT is necessary for GAC filter design if small PFAS are the targets.

Maintenance of GAC filter is important. After adsorption saturation, GAC needs to be regenerated to restore adsorption capacity (usually via thermal processing of spent GAC). With a frequent GAC regeneration and replenish to makeup mass loss during regeneration, PFAS removal of > 89% to >98% can be maintained; while without regeneration, i.e. extending filter run time, removal rate is between 0 to 26% [2].

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References:

[1] Removal of perfluoroalkyl acids from the drinking water production chain, KWR

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