Per- and Polyfluoroalkyl Substances (PFAS), often termed ‘forever chemicals’ due to their persistent nature, have emerged as a significant concern in water treatment engineering. These synthetic chemicals, found in various industrial and consumer products, are notorious for their resistance to degradation, leading to widespread environmental contamination and potential health risks.
The challenge in mitigating PFAS lies in their carbon-fluorine bonds, rendering conventional water treatment methods less effective. Fortunately, activated carbon treatment has proven to be an effective treatment technology for this contaminant.
Activated carbon, in its two primary forms – Powdered Activated Carbon (PAC) and Granular Activated Carbon (GAC) – offers a high degree of porosity and an extensive surface area, making it an ideal candidate for adsorbing PFAS from contaminated water sources. The process capitalizes on the affinity of activated carbon for organic molecules, particularly those as complex and stable as PFAS. This affinity is a result of the carbon’s surface chemistry and pore structure, which are enhanced through specific activation processes.
This article provides an in-depth analysis of activated carbon’s role in PFAS removal, encompassing its operational mechanisms, efficacy, and key practical aspects relevant to water treatment processes.
Key Takeaways
| Key Points | Description |
| High Efficiency in PFAS Removal | Activated carbon is effective in adsorbing a wide range of PFAS, especially long-chain compounds. |
| Versatility and Ease of Implementation | Both PAC and GAC can be integrated into various water treatment systems, adaptable to different scales and requirements. |
| Challenges with Short-Chain PFAS | Activated carbon is less effective against short-chain PFAS, necessitating alternative or supplementary treatment methods. |
| Operational Considerations | Regular replacement or reactivation of activated carbon is required, along with careful management of saturated materials. |
| Cost Implications | While offering significant benefits, the use of activated carbon involves operational and maintenance costs that must be factored into its long-term use. |
| Environmental and Regulatory Concerns | The disposal of spent activated carbon, particularly with concentrated PFAS, poses environmental and regulatory challenges. |
What is Activated Carbon?
Activated carbon is a specialized form of carbon processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions. Primarily used for purifying liquids and gases in industrial applications, it plays a crucial role in water treatment, particularly in the removal of complex contaminants like PFAS.
The two primary forms of activated carbon used in water treatment are:
1. Powdered Activated Carbon (PAC)
- PAC is finely ground carbon particles, typically with diameters less than 0.18 mm.
- Its fine particle size allows for rapid reaction and adsorption rates, making it suitable for applications requiring quick contaminant removal.
- PAC is generally used in batch processes or mixed with the water and then removed by filtration.
2. Granular Activated Carbon (GAC)
- GAC consists of larger carbon particles, usually ranging from 0.5 to 4 mm in diameter.
- The larger size of GAC makes it ideal for continuous flow applications, as it allows water to flow through the carbon bed and maximize contact time.
- GAC is typically used in fixed-bed column filters where water passes through a bed of granular activated carbon.
Check out this video about activated carbon.
The Activation Process
The production of activated carbon involves two key stages:
Step 1 – Carbonization
The raw material, often organic matter such as coal, wood, or coconut shells, is heated in a low-oxygen environment to remove water and volatile compounds. This process leaves behind a carbon-rich mass.
Step 2 – Activation
The carbonized material is then exposed to further processing, which involves heating it at high temperatures in the presence of activating agents like steam or carbon dioxide.
This stage develops a highly porous internal structure, vastly increasing the surface area. For instance, one gram of activated carbon can have a surface area of over 500 square meters, equivalent to about two tennis courts.
Properties for PFAS Adsorption
Activated carbon’s unique properties make it particularly suitable for adsorbing PFAS:
1. High Surface Area:
- The extensive surface area provides numerous binding sites for PFAS molecules.
2. Pore Structure:
- The intricate pore structure, including micro, meso, and macropores, allows for the adsorption of various PFAS compounds, irrespective of their molecular sizes.
3. Surface Chemistry:
- The surface of activated carbon can be modified to enhance its affinity for specific contaminants, including PFAS.
4. Physical Stability:
- Activated carbon maintains its integrity and adsorption capacity over a wide range of pH and temperature conditions, crucial for treating diverse water sources contaminated with PFAS.
How Does Activated Carbon Work?
The effectiveness of activated carbon in adsorbing PFAS from water relies on its complex internal structure and the processes of diffusion and adsorption. Understanding these mechanisms is crucial for optimizing the use of activated carbon in water treatment.
The adsorption process typically involves the following stages:
1. Bulk Diffusion:
- Initially, PFAS molecules in the water must travel from the bulk solution towards the activated carbon. This movement, known as bulk diffusion, is influenced by the concentration gradient of PFAS in the water.
- During this phase, PFAS molecules move randomly but generally trend towards areas of lower concentration, which are typically near the surface of the activated carbon particles.
2. Film Diffusion:
- Once near the activated carbon, PFAS molecules encounter a boundary layer or a thin film of water that adheres to the surface of the carbon. This layer acts as a barrier to the adsorption process.
- Film diffusion is the process by which PFAS molecules migrate through this boundary layer to the actual surface of the activated carbon. The rate of film diffusion is influenced by the thickness of the boundary layer and the molecular size of the PFAS.
3. Pore Diffusion:
- After penetrating the boundary layer, PFAS molecules enter the porous structure of the activated carbon. Pore diffusion then occurs, which is the movement of PFAS molecules through the internal pore structure of the carbon.
- This stage is critical as the vast network of pores in activated carbon provides a large surface area for adsorption. The size, shape, and distribution of these pores affect the rate and extent of pore diffusion.
4. Surface Diffusion:
- Once inside the pores, PFAS molecules move along the internal surfaces of the pores. This movement, known as surface diffusion, leads the molecules to potential adsorption sites.
- Surface diffusion is influenced by the interaction between the PFAS molecules and the carbon’s surface, including any functional groups or chemical properties that might enhance adsorption.
5. Adsorption:
- The final stage is the actual adsorption of PFAS molecules onto the activated carbon. This can occur through various mechanisms, including physical adsorption (due to van der Waals forces) or chemical adsorption (involving stronger chemical bonds).
- The efficiency of adsorption depends on the compatibility between the PFAS molecules and the activated carbon’s surface properties, as well as the availability of adsorption sites.
The EPA provides a comprehensive overview of PFAS drinking water treatment technology options in their detailed fact sheet.
Use of Activated Carbon for Water Treatment
Activated carbon, particularly Powdered Activated Carbon and Granular Activated Carbon , plays a pivotal role in water treatment plants for the removal of PFAS. Their application, however, varies based on their physical characteristics and the specific requirements of the treatment process.
The following table outlines the maximum removal percentages of various PFAS compounds when treated with Activated Carbon.
| PFAS Compound | Maximum Removal |
| Perfluorobutanoic acid (PFBA) | 10% |
| Perfluorobutanesulfonic Acid (PFBS) | 90% |
| Perfluoropentanoic acid (PFPeA) | 40% |
| Perfluorohexanesulfonic acid (PFHxS) | 99% |
| Perfluorohexanoic acid (PFHxA) | 90% |
| Perfluoroheptanoic acid (PFHpA) | 90% |
| Perfluorooctanoic acid (PFOA) | 95% |
| Perfluorooctanesulfonic acid (PFOS) | 99% |
| Perfluorononanoic acid (PFNA) | 98% |
| Perfluorodecanoic acid (PFDA) | 90% |
Powdered Activated Carbon (PAC):
PAC is typically added directly to the water in treatment basins or contact tanks.
It is well-suited for short-term or emergency applications due to its rapid dispersion and high surface area.
After the adsorption process, PAC is removed from the water along with the adsorbed contaminants through sedimentation or filtration.
Explore the research on the effects of PFAS on human health detailed in this comprehensive study by the National Center for Biotechnology Information.
Granular Activated Carbon (GAC)
GAC is used in fixed-bed filters where water passes through a bed of activated carbon.
It is ideal for continuous, long-term treatment processes due to its larger particle size and reusability.
GAC filters are periodically backwashed to remove trapped particles and can be reactivated through thermal processes when their adsorption capacity diminishes.
Factors Affecting the Efficiency of Activated Carbon in PFAS Removal
The efficiency of activated carbon in removing PFAS from water is influenced by several factors, as summarized in the following table:
| Factor | Description | Impact on PFAS Removal |
| Carbon Type | PAC and GAC have different surface areas and pore structures. | PAC is more effective for rapid, short-term treatment, while GAC is suited for long-term, continuous removal. |
| Dose | The amount of activated carbon added to the water. | Higher doses generally increase PFAS removal efficiency but can be more costly. |
| Contact Time | The duration for which PFAS is in contact with the activated carbon. | Longer contact times usually enhance PFAS adsorption, especially important for GAC systems. |
| Water Quality | Characteristics of the water, including pH, temperature, and the presence of competing contaminants. | Certain water quality parameters can affect the adsorption process, either enhancing or hindering PFAS removal. |
| PFAS Characteristics | Types and concentrations of PFAS in the water. | Different PFAS compounds have varying affinities for activated carbon, influencing removal efficiency. |
The application of PAC and GAC in water treatment for PFAS removal is a nuanced process, heavily dependent on the specific conditions and requirements of each treatment facility.
Delve into the latest findings on environmental impacts of PFAS in the study published by Environmental Sciences Europe.
Advantages and Limitations of Using Activated Carbon for PFAS Removal
The use of activated carbon, both Powdered Activated Carbon (PAC) and Granular Activated Carbon (GAC), in treating PFAS-contaminated water presents a range of advantages and limitations. Understanding these is crucial for water treatment engineers to make informed decisions about the application of this technology.
Advantages
Activated carbon offers several benefits in the removal of PFAS from water sources:
- High Efficiency: Activated carbon is highly effective in adsorbing a wide range of PFAS compounds, particularly long-chain PFAS.
- Versatility: Both PAC and GAC can be used in various water treatment setups, from small-scale to large municipal systems.
- Ease of Implementation: Activated carbon systems can be integrated into existing water treatment processes without significant modifications.
- Additional Contaminant Removal: Besides PFAS, activated carbon can remove a variety of organic compounds, improving overall water quality.
- Proven Technology: Activated carbon is a well-established method with a long history of use in water treatment, providing a level of reliability and predictability.
Limitations and Challenges
Despite its advantages, the use of activated carbon for PFAS removal also comes with several limitations and challenges:
- Limited Effectiveness for Short-Chain PFAS: Activated carbon is less effective in removing short-chain PFAS compared to long-chain PFAS.
- Saturation and Reactivation: Both PAC and GAC have a finite adsorption capacity and require regular replacement or reactivation.
- Residual Management: The disposal of spent activated carbon, especially if it contains concentrated PFAS, poses environmental and regulatory challenges.
- Operational Costs: The cost of activated carbon, especially for large-scale applications, and the expenses related to its maintenance and disposal can be significant.
- Competitive Adsorption: The presence of other organic compounds in water can compete with PFAS for adsorption sites on activated carbon, reducing its effectiveness.
Table: Comparison of PAC and GAC in PFAS Removal
The following table compares the characteristics of PAC and GAC in the context of PFAS removal:
| Feature | Powdered Activated Carbon (PAC) | Granular Activated Carbon (GAC) |
| Particle Size | Fine particles | Larger granules |
| Application | Ideal for batch processes and short-term use | Suited for continuous, long-term treatment |
| Reactivation | Generally disposed of after use | Can be reactivated and reused |
| Implementation | Easily added to existing systems | Requires specific infrastructure like fixed-bed filters |
| PFAS Removal Efficiency | High for short-term applications | Consistent over longer periods |
For guidance on mitigating exposure in your home, the Minnesota Department of Health offers strategies on PFAS home treatment.
Conclusion
The utilization of activated carbon, both Powdered Activated Carbon (PAC) and Granular Activated Carbon (GAC), in the treatment of PFAS-contaminated water is a significant step forward in addressing a pressing environmental and public health issue. This technology, with its proven efficiency in adsorbing a wide range of PFAS compounds, offers a reliable solution in the ongoing battle against water pollutants.
However, it is important for design engineers to balance the advantages of activated carbon with its limitations, particularly in terms of its effectiveness against different types of PFAS, the operational costs, and the challenges associated with the disposal and reactivation of spent carbon.