Expert Q&A Explores Activated Carbon's Role in PFAS Removal

Expert Q&A Explores Activated Carbon's Role in PFAS Removal

In the world of filtration technology, activated carbon stands out as a key player. Its applications in purifying water and air make it an essential component in various industries, from water treatment to food processing to environmental management. Neal Megonnell, vice-president of technical services at AqueoUS Vets, brings his wealth of knowledge to Treatment Plant Operator readers in this insightful Q&A. Here, he dives into the nuances of activated carbon, discussing its production, properties and performance in different scenarios, including its role in combating PFAS contamination.

What materials can be used to produce activated carbon?

Neal Megonnell: Activated carbon can be produced from any material that contains carbon. The standard activated carbon products you find throughout the world are typically made from various types of coal, coconut shells and wood-based materials. The primary products here in the U.S. would be coconut and coal-based materials. The coal materials include lignite coal, sub-bituminous coal or bituminous coal.

Does the starting material impact the final product?

Megonnell: Starting material impacts the final product in various ways: through its density, transport pore structure and adsorption pore structure. The adsorption pore structure is where adsorption takes place of the chemicals or compounds that you're trying to remove from air, water or applications such as edible oils. In the food industry, activated carbons — used quite frequently from starting materials such as coconut shells — will give a microporous carbon whereas a wood based carbon will get more than macroporous carbon. These pore structures lend themselves to the final application of the carbon being used.

What is the difference between direct activated and reagglomerated carbon?

Megonnell: There are a variety of manufacturing processes to make activated carbon. The typical process used here in the U.S. by the major producers, is what's known as reagglomeration, where the coal is ground into a powder, a binder is added and the material is then made into a briquette similar to what you'd see in your backyard charcoal grill. The material is sized and runs through the charring and activation process. 

Back in the 90s, the majority of the materials were coming in from Asia, which were lower-cost products compared to the domestic products. These were produced using a direct activation process where the coal was not ground to a powder initially, it was just sized, charred and activated from the coal itself. This led to a product that didn't work as well when compared to the reagglomerated domestic products. Those products are no longer introduced to the U.S. because of the high tariff that's been put on them due to an anti dumping lawsuit. There are other materials such as coconut shell, lignite coal and sub-bituminous coal that do not need to be reagglomerated to work because of the inherent pore structure within the starting material.

For PFAS applications, are the domestic bituminous coal-based carbons superior to other activated carbon products?

Megonnell: The domestic coal base materials are being touted as the gold standard in the industry for PFAS. However, there's sufficient data that the sub-bituminous material, and even some of the lignite-based materials, will perform equally or better when comparing on a volume basis than the domestic agglomerated bituminous coal-based materials.

Does the density of the activated carbon influence the performance of the activated carbon or have an impact on the cost to treat the water for PFAS?

Megonnell: The density of the activated carbon must be considered. Most of the data that's out there in the public will show bed volumes of water treated. The volume of carbon in the system equates to one bed volume. The data show in many cases equal bed volumes of water treated when comparing a higher and lower density carbon. A denser carbon means more weight in the vessel to treat the same amount of water. Carbon is sold on a weight basis, in dollars per pound. If the higher and lower density activated carbons work on an equal volume basis, the lower density materials are more cost effective on a dollars per 1,000 gallons treated basis than the higher density products.

What effect does iodine number have on the performance of an activated carbon for PFAS?

Megonnell: The iodine number is an ASTM standard test method within the activated carbon industry. It's a measure of total pore volume or total surface area of the activated carbon. It's a manufacturing quality control test method and that was never meant to predict performance in the real world for any application. Within the PFAS application, there again is sufficient data showing that some of the lignite coal-based materials, which are lower iodine number, perform equal or better to the higher iodine number bituminous coal base materials. So, there's no correlation between iodine number and PFAS performance.

What design criteria are critical when designing a treatment system for PFAS removal from water?

Megonnell: When designing a PFAS system, the critical parameters are the contact time and the hydraulic loading rate. The PFAS application has been shown to be a kinetically slow process, with slow adsorption. Contact times are longer than you would typically see compared with what I would refer to as standard activated carbon applications. These design criteria must be taken into account so the system performs well, and so that the media utilization is optimized.

Along with the critical design parameters, media selection is also critical to optimize performance. There is no silver bullet solution for PFAS. The selection of the optimal media whether it be GAC, IX or other media is based on water quality as well as the PFAS compounds present and the concentrations. Although the PFAS application is a focus, all activated carbon applications should be evaluated based on performance criteria and not test method specifications. Applications such as chlorinated organic solvents at low concentrations in groundwater would potentially be better treated with a coconut shell-based carbon, whereas color removal from surface water may be better treated with a more meso- and macroporous activated carbon such as a lignite or sub-bituminous based product.

What other compounds in the water affect the performance of the activated carbon for PFAS removal?

Megonnell: Activated carbon does not discriminate when it's trying to adsorb materials from water. Any organic compounds that are in the water will affect the PFAS performance. PFAS concentrations, unlike other activated carbon applications, are extremely low. Typical activated carbon applications over the years have been at ppb levels, while PFAS exists in the ppt level. Compounds do not compete equally for adsorption space. Higher concentration compounds and more strongly adsorbed compounds will take up adsorption space reducing the PFAS capacities. Other compounds to be aware of in water are inorganics such as iron and manganese. Iron and manganese can precipitate out in the water and coat the carbon and prevent it from adsorbing PFAS.

How does the use of activated carbon for PFAS removal differ from standard activated carbon drinking water applications such as TOC and THM removal?

Megonnell: The PFAS application is very different and it’s much lower concentrations than we typically would see for other compounds in drinking waters. For instance, TOC removal is generally at ppm levels. The chlorinated organic solvents that we've seen over the years through materials that were buried in the ground are typically ppb levels. The PFAS again is existing in low ppt levels with the EPA limit potentially as low as 4 ppt on some of these compounds. It's going to make the system design for PFAS a little different than what we experience over the years in terms of having multiple vessels in series to optimize media utilization.

And how is the spent activated carbon disposed of?

Megonnell: There are multiple ways to dispose of the spent activated carbon. It can be landfilled, incinerated, or it can be reactivated, potentially for further use. There's limited data on the efficiency of these processes. It's being worked on throughout the industry to show that the PFAS specifically in incineration and reactivation is being destroyed. In the landfill process the material is sent to a hazardous landfill to prevent it from reentering the environment.

Can activated carbon be thermally reactivated and reused in PFAS applications?

Megonnell: There is a small amount of data that suggests that the activated carbon can be thermally reactivated using standard reactivation kilns and destroying all the PFAS that is adsorbed on the carbon. That material, in theory, could be reused in an application, most likely not in drinking water. It would be reintroduced, for instance, in a wastewater application where the PFAS is not going to be a concern for the reactivated material.



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