The media filtration landscape

Advances in chemistry and material science have led to the proliferation of materials designed for contaminant removal via a variety of mechanisms. These materials, referred to collectively as filtration media, vary in cost, ideal operating chemistries, design parameters, and disposal routes.

Three common adsorptive filtration medias that dominate the market today: granular activated carbon (GAC), ion exchange resin (IX), and surface modified clay (SMC). In addition to these, there are several novel materials developed for specific applications, namely PFAS removal. This article will focus on treatment mechanisms for these media and how commonly encountered co-contaminants impact treatment efficacy.

Treatment mechanisms 

Each community’s unique hydrogeology and industrial history determines water quality parameters that dictate the design of contaminant removal systems. Filtration media must be selected to align with these conditions. By understanding the underlying treatment mechanisms for each media, system designers can better select the materials that minimize the lowest total cost of ownership and maximize mechanical component performance.

Granular Activated Carbon

GAC removes contaminants through adsorption, where organics adhere to the surface of carbon particles. Its effectiveness comes from a highly porous structure and large surface area. GAC is tailored for specific applications by adjusting surface chemistry, pore volume size, and mean particle distribution to optimize contaminant transport—governed by bulk, pore, and film diffusion—and sequestration, which involves physisorption, chemisorption, and pore entrapment.^[1]

GAC is produced from various raw materials and manufacturing processes. Common carbon sources, such as coconut shell, bituminous, sub-bituminous, and lignite carbon, require different processing, and are used for the removal of different contaminants. Coconut carbons are more microporous, and are more effective at removing smaller contaminants, such as benzene or 1,2,3-trichloropropane. Coal-based carbons are more mesoporous, and are more effective at removing larger contaminants, such as PFOA or trihalomethanes. After use, GAC may be re-activated and reused, typically in non-potable applications.

Ion Exchange Resin

IX resin removes contaminants via electrostatic interactions, where functional groups are designed to selectively exchange with targeted contaminants. Functional groups are either cationic (i.e., sulfonic or carboxylic acid), or anionic (i.e., quaternary ammonium or tertiary amine), depending on the charge of the targeted contaminants. Most PFAS, including PFOA and PFOS, which exist as perfluorooctanoate and perfluorooctane sulfonate anions, respectively, and are removed with anion exchange resins. In ion exchange resins, the main transport mechanism is film diffusion, and sequestration is achieved via electrostatic binding.^[2]

IX resins are synthesized from various polymer backbones and functionalization processes. Common resin matrices include polystyrene-divinylbenzene (PS-DVB) and acrylic-based polymers. The cross-linking of PS-DVB resins offer higher mechanical strength, making them more suitable for removing inorganic ions like nitrate, sulfate, or PFAS in high-pressure systems. Polystyrene is also naturally hydrophobic, which improves affinity for PFAS. In contrast, acrylic-based resins are more hydrophilic, which increases transport kinetics and makes the resins better suited for removing short chain organic acids or natural organic matter (NOM). Some ion exchange resins are regenerable; however, most PFAS resins tend to be single-use due to cost advantages.

Surface Modified Clay

SMC media removes contaminants through adsorption or ion exchange mechanisms, depending on clay modifications. During the manufacture of SMC materials, the ions that form clay’s natural crystalline structure are substituted—or “modified”—with functional groups for specific treatment tasks. SMCs were designed to overcome challenges associated with granular activated carbon, namely, transport issues caused by the presence of large organic molecules (i.e., humic acid), and emulsified oil and grease.^[3]

Organo-modified clays, or organoclays, are modified with large organic ions (i.e., quaternary ammonium), which make the surface of the clay hydrophobic. The primary removal mechanism of contaminants in organoclay materials is hydrophobic adsorption, making them more selective for contaminants like PFAS when compared to GAC.^[4] Inorgano-modified clays, or inorganoclays, are modified with inorganic ions (i.e., metal oxides or hydroxides), which make the surface of the clay hydrophilic. The primary removal mechanism in organoclay materials is ion exchange, whereby target contaminants like lead or cadmium, are exchanged with the modified ion in the crystal lattice. SMCs are disposed of after use.

Novel medias

There are many emerging media technologies designed specifically for PFAS removal. These can broadly be grouped into metal-organic and inorganic frameworks (referred to here collectively as MOFs for simplicity), and functionalized organic polymers. MOFs consist of a three-dimensional crystalline lattice formed by organic or inorganic linkers and metal ion clusters, with best-in-class materials leveraging a hybrid approach, with inorganic scaffolding, designed to create a large stable surface area in the material, and organic functional groups, designed to remove PFAS through a combination of hydrophobic and interactions and electrostatic attraction.^[5] Organic functionalized polymers are polymers designed to have a high surface area with functional groups that attract PFAS through a combination of hydrophobic, electrostatic, and dipole interactions, though mechanisms are still being explored. Functionalized beta-cyclodextrin polymers are common, though others are being tested.

Treatment mechanisms and performance under different water quality conditions for each media are summarized in Table 1. It is important to note that all medias struggle in the presence of co-contaminants; however, the degree to which performance is affected vary. For example, the presence of TOC will interfere with GAC, IX, and SMC performance, but SMC is generally more resistant.

Design considerations

Three media related factors impact pressure vessel design; Empty Bed Contact Time (EBCT), the contact time required for contaminants to effectively transport to available adsorption or ion exchange sites; adsorption capacity, the mass of contaminants captured prior to saturation; and hydraulic loading rate (HLR), the flow rate per unit of surface area wherein an effective mass transfer zone (MTZ) is established.

Empty Bed Contact Time

Filtration media with rapid transport kinetics generally require shorter EBCTs. GAC requires a longer EBCT as contaminants transport through the large carbon surface area. Novel media designed specifically for PFAS have rapid transport kinetics and shorter EBCTs. EBCT dictates the volume of media required for a specific flow, as given by:

Medias with shorter EBCTs require less overall volume but tend to cost more per unit. To understand total unit economics, adsorption capacity must also be considered. EBCTs for the treatment medias discussed are shown in Table 2.

Adsorption capacity

Adsorption capacity dictates how long a media will last in the field. Adsorption isotherms are used to estimate capacity; however, these are commonly done under ideal conditions (i.e., PFAS in deionized water), and do not represent media lifetime in real-world water matrixes. For example, GAC and IX have similar adsorption capacities, but because GAC is less selective, adsorption sites are taken up by other contaminants and in-situ bed life tends to be shorter. Adsorption capacity for the medias discussed are shown in Table 2.

Hydraulic Loading Rate

Each media has a range of hydraulic loading rates where effective mass transfer zones are established in a media bed. If a system is designed with an HRL that is too high, ideal kinetics will not be established; too low, and preferential channels will form. HRL is calculated by the equation below:

A higher hydraulic loading rate means that more flow can be passed through a smaller diameter vessel; however, more energy is required to pump the water through the system.

Effective water treatment system design requires aligning media properties with site-specific water chemistry and operational constraints. Engineering decisions must balance adsorption capacity, EBCT, and HLR to optimize performance and cost. Proven media like GAC, IX, and SMC offer reliable options, while emerging materials such as MOFs and functionalized polymers provide high-efficiency PFAS removal; however, the conditions under which these materials may excel is still being proven. Understanding each media’s transport and sequestration mechanisms enables precise system sizing, minimizes footprint, and ensures long-term operational reliability in complex water matrices.

References

[1] (Hung, Yung-Tse, et al. "Granular activated carbon adsorption." Physicochemical treatment processes. Totowa, NJ: Humana Press, 2005. 573-633.)

[2] Arden, Thomas Victor. Water purification by ion exchange. Springer Science & Business Media, 2012.

[3] Lee, Seung Mok, and Diwakar Tiwari. "Organo and inorgano-organo-modified clays in the remediation of aqueous solutions: An overview." Applied Clay Science 59 (2012): 84-102.

[4] Yan, Bei, Jian Wang, and Jinxia Liu. "STXM-XANES and computational investigations of adsorption of per-and polyfluoroalkyl substances on modified clay." Water research 201 (2021): 117371.

[5] Liu, Fuqiang, et al. "A comprehensive review of novel adsorbents for per-and polyfluoroalkyl substances in water." ACS ES&T Water 4.4 (2024): 1191-1205.