The Treatment of PFOA and PFOS (PFAS) with GC 8 X 30PF Granular Activated Carbon
Per-fluorinated chemicals, commonly known as PFOA (perfluorooctanoic acid) or PFO (perfluorooctanoate sulfonate) are among the top subject matters for water resource managers and other industry professionals. They are part of a larger range of persistent industrial chemicals – non-polymer and polyfluoroalkyl substances (PFAS), nicknamed “forever chemicals”. PFOA and PFOS are the two PFAS’s that have been produced in the largest amounts within the United States. (ATSDR 2015; EFSA 2008)
PFOA and PFOS are among other PFC’s that have been used in the production of various fluoropolymers. Due to their ability to repel oil and water, these compounds are used in the surface protection of products such as carpets and clothing treatments; coatings for paper, cardboard packing and leather products; industrial surfactants, wetting agents, additives and coatings; processing aids in the manufacture of fluoropolymers such as in non-stick coatings on cookware; membranes for clothing that are both waterproof and breathable; electrical wire casting; fire and chemical resistant tubing and plumbing thread seal tape as well as firefighting foam at airports and military bases.
Although dozens of PFC’s of varying chain lengths and compositions have been detected in water, the US Environmental Protection Agency (USEPA) has shown specific concerns regarding the longer chained PFC’s (with eight or more carbons), such as PFOA and PFOS. Given their solubility and stability in water, PFOS and PFAS have been detected in surface and groundwater in hundreds of locations in the US and around the world.
Due to their ubiquitous use in the past, PFO and PFOS are detected at higher concentrations and more frequently than PFC’s. They are also more toxic and more bio-accumulative than shorter chain PFC’s, such as the hexyl and butyl analogues. Exposure to some of these chemicals is linked to harmful health effects in the liver, kidneys, blood, and immune system.
Not All Activated Carbons are Created the Same
There are no bad activated carbons, just poorly applied activated carbons. In any applications it is essential to match the activated carbon to the specific treatment issue at hand.
Activated carbons are successfully being used for the treatment of drinking waters that have been identified as having long chain PFOA/PFOS. Specifically, re-agglomerated high-quality bituminous coal based granular activated carbons (GAC) have additional manmade cracks and crevices that increase the pathways that allow for a more homogenous activation and could potentially augment the transport pore structures. As compared to activated coconut GAC carbons, bituminous coal based granular activated carbons demonstrate a considerably greater removal of the target compounds. This is believed to be due re-agglomerated coal (GAC) having optimized transport pores which in turn enables access to the high energy pores that are effective in adsorbing the contaminants. Coconut GAC by contrast have relatively narrow transport pore structures which restrict access to their home abundant adsorption high energy pores.
Re-agglomerated bituminous coal based GAC manufactured by traditional methodologies are however still restricted in their ability to treat PFOA and PFOS as they exhaust their adsorption capacity far more quickly than they would in historic drinking water applications without PFAS being targeted for removal. Most studies fail to point this out and simply refer to how much PFA these carbons will remove in isolation and that they will require reactivations annually. This leads to costly, high frequency change outs of the existing activated carbon filter beds or alternatively leads to the set-up of additional large, deeper (x3) carbon beds specifically used to treat PFOA and PFOS.
The cost for the construction of a new GAC system specifically to treat PFOA and PFOS at the Sweeny Water Treatment Plant in North Carolina was recently estimated at $46 million dollars. That does not account for the cost of the additional new activated carbon and an approx. 400 day change out cycle.
“The added burden of cleaning PFAS from the water means that the plant will have to replace the activated carbon more frequently. Some of the existing beds had lasted for as many as ten years: now, they will be replaced every twelve months” (Janson – March 25, 2019, Volume 97, issue12 – C&EN).
During reactivation there is a loss of material and the reactivated carbon must be topped up with at least 10% virgin material. Meaning in the Sweeny case that the PFA beds are completely changed out with new virgin material in the same time frame as one reactivation cycle was required in their original beds.
What makes General Carbon’s re-agglomerated bituminous coal-based GC 8 X 30PF granular activated carbons applicable here is their ability to help overcome those obstacles by removing the PFOA and PFOS contaminants and delivering a more nominal in-service bed life.
To provide some context we have outlined briefly below a well known and historically proven example of the type of contaminant that causes these types of challenges and the how General Carbon developed specialist activated carbons to address that.
Methyl-tertiary butyl-ether (MTBE) was added to gasoline as a fuel additive to facilitate ignition and improve air quality. However, MTBER has now contaminated a significant amount of groundwater (which constitute 50% of USA drinking water) sources because it is water soluble and migrates relatively quickly from underground leaks compared to hydrocarbons from leaking storage tanks. It has a bad taste and odor in drinking water at low trace concentrations. It needs to be removed but general GAC manufactured by conventional methodologies can struggle due to their variability in performance in lot to lot numbers and in the manner and distribution of their pore structure.
In term of GAC treatment, typical PFC removal applications would be trace removal applications, i.e. the adsorption of soluble organic compounds of low molecular weight, present at low contaminant concentrations. This would suggest that GACs exhibiting higher energy pore structures would outperform GACS having larger adsorption pores and lower adsorptive energy. However, for that to be effective as outlined by McNamara et-al- January 2018- Journal AWWA, you would also need an activated carbon with optimized transport pores to access all those high energy pores.
“The most common groundwater treatment for PFOA and PFOS is extraction and filtration through granular activated carbon. However, because PFOA and PFOS being PFCs have moderate absorbability, the design specifics (of the activated carbon) are therefore especially important in obtaining acceptable treatment (EPA 2016b, 2106c)”.
To elaborate, it is generally accepted that adsorption space or pores need to be 1.3 to 1.8 times larger than the adsorbing molecules volume. Thus, small molecules are best adsorbed in the smallest high adsorption energy pores. The term pore and adsorption space are used interchangeably. Physical adsorption of small molecules is a nanotechnology phenomenon. Adsorption of molecules depends on what we call the power of three, which is the molecular distance between adsorbate and the graphitic platelet i.e. one-molecular layer away is a relative adsorption force of one, two molecular layers away the relative force is 1/8, three molecular layers away from the GAC graphitic platelet the relative adsorption force compared to the one molecular layer is one twenty-seventh (1/27th) and three molecular layers away from graphitic platelet in negligible physical adsorption.
A key objective of the activated carbon industry is the control and distribution of these pore structures or adsorption spaces. They are created during the activation process and they are divided firstly by size (Macro, Meso, and Micro- also referred to as superhighway, highways, and general dirt roads) and secondly in terms of their structure between two scales- (disorganized and disordered). The more disordered the more developed, detailed, controlled, and less disorganized the pore structure of the activated carbon is. In this regard “carbons fall into two groups anisotropic or graphitizable carbons and isotropic or non-graphitizable carbons, activated carbons fall into the latter group (Marsh and Rodriguez-Reinoso-2006)”.
A Transformational Approach
It is the ability of General Carbon’s partner’s patented RODECS technology to control both the pore size and the distribution structure with pinpoint accuracy, in real times, and repeatedly. This enables General Carbon to create specific re-agglomerated bituminous coal based granular specialist activated carbons with the characteristics necessary to consistently address the challenges associated with PFOA and PFOS removal.
The RODEC procedures an extremely homogenous product, repeatedly, removing the variability associated with conventional activated carbon methodologies. General Carbon also control the RODECS to maximize the development of the overall pore structure (transport pore system and high energy pores) in other activated carbons that are particular to a specific set of contaminants in water, in air, or in other applications.
The RODEC’S unique ability to do this vs. General Carbon’s competitors is due to an exceptionally well controlled sealed processing chamber, highly instrumented operation, and an intelligent process informative guiding system. We call it the AI in ACI. The result is an activated carbon with a specifically targeted and more optimal bed life for PFOA and PFOS applications which make them incredibly efficacious and more cost-effective vs re-agglomerated bituminous coal based GAC manufactured by traditional methodologies.
PFOA and PFOS is a key area of need where the lack of variability and optimally developed pore structures in General Carbon’s re-agglomerated bituminous coal-based specialist granular activated carbon, GC 8 X 30PF, can be directly applied to deliver consistent results and great customer value.