Microwave based regeneration of spent activated carbon containing pfas

Abstract

Methods and systems for use in the regeneration of spent activated carbon are disclosed. Methods and systems are particularly beneficial for the regeneration of carbon that contains one or more poly- and perfluoroalkyl substances (PFAS) compounds but are also applicable to all other organic constituents in waters and wastewaters (e.g., natural organic matter, algal organic matter, effluent organic matter, taste and odor compounds, synthetic organic compounds). Methods include microwave heating at a fast ramp up rate to a high treatment temperature. Secondary treatments including treatment of gas in the microwave cavity with a recirculation system and / or an off-gases treatment system downstream of the microwave system can also be carried out.

Description

MICROWAVE BASED REGENERATION OF SPENT ACTIVATED CARBON CONTAINING PFASBackground

[0001] Poly- and perfluoroalkyl substances (PFAS) such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) are a class of synthetic compounds mass-produced since the 1940s. They exhibit excellent chemical and thermal stability and have been used in a wide range of industrial and consumer products such as firefighting foams, food packaging, water and stain repellents, cleaning products, paints, sealants, personal care products, non-stick cookware, etc. As a result of strong stability and persistence in a variety of media, PFAS are among those termed “forever chemicals.”

[0002] Unfortunately, exposure to certain PFAS may pose significant threats to human and animal health, including negative reproductive effects, developmental effects or delays, increased risk of some cancers, and reduced ability of the body’s immune system to fight infections. PFOA and PFOS have been the most widely used PFAS, and their production has been banned. Due to wide-spread use and disposal, PFAS have been detected in groundwater, surface water, wastewater, and tap water around the world. In fact, PFAS have been detected in concentration levels up to concentrations of mg / L (ppm); i.e., one million times the USA regulatory maximum concentration limits at some contaminated sites.

[0003] The United States Environmental Protection Agency (USEPA) announced its PFAS regulations on April 10, 2024: (i) enforceable Maximum Contaminant Levels (MCLs) at 4.0 ng / L for PFOA and PFOS, individually, (ii) MCLs of 10 ng / L for PFNA, PFHxS, and HFPO-DA (GenX Chemicals), and (iii) a hazard index (HI) of 1 for mixtures of four PFAS — PFHxS, PFNA, HFPO-DA, and PFBS.The European Union, Canada, China, as well as many other countries have proposed drinking water regulations aimed at establishing maximum contaminant levels for PFAS. The European Union has recommended limits of 100 ng / L for the combined total of 20 PFAS, including PFOA and PFOS, and 500 ng / L for the sum of all PFAS. In Canada, the maximum acceptable concentrations for PFOA and PFOS are 200 ng / L and 600 ng / L, respectively. Chinese drinking water standards stipulate maximum acceptable limits of 80 ng / L for PFOA and 40 ng / L for PFOS. Unfortunately, these proposed standards are far from being met. For instance, approximately 2,000 public watersystems in the United States are known to have mean concentrations of PFOA exceeding the regulation and 3,400 exceeding the regulation for PFOS.

[0004] PFAS compounds are non-volatile at room temperature, ionic at neutral pH, and amphiphilic, and traditional water treatment processes (coagulation, flocculation, sedimentation, sand filtration and disinfection) are ineffective at removing them. Advanced treatment technologies have been investigated to remove PFAS from water, including photocatalysis, electro-Fenton oxidation, plasma treatment, and sonochemical treatment. Although some of these processes have shown success in laboratory-scale experiments, considerable economic and design hurdles prevent scale-up for broader applications, e.g., the difficulty of integrating into existing treatment systems, high energy consumption, and harsh reaction conditions.

[0005] Granular Activated Carbon (GAC) adsorption is a well-established technology for removing synthetic organic contaminants and natural organic matter tfrom water and wastewater and has been designated as one of the best available technologies to remove PFAS from water and wastewater. However, the adsorption capacity of GAC decreases with the amount of water volume treated in full-scale treatment plants, and GAC shows a rapid breakthrough of PFAS compounds. Furthermore, GAC spent media containing PFAS necessitates proper handling and disposal, and limited options exist for disposal of solid wastes such as spent GAC containing PFAS.

[0006] Thermal treatment (e.g., up to 1000°C in direct, gas-fired rotary kilns or vertical furnaces) is the current common industry practice for managing PFAS-laden GAC, enabling the destruction of the contaminants while largely restoring the adsorption characteristics of the GAC. Unfortunately, conventional thermal treatment systems require large equipment facilities and are characterized by long processing times, high energy consumption, loss of GAC, reduced adsorption capacity of GAC, and long transport distances from the utilities to the regional regeneration facilities. Furthermore, the thermal destruction mechanisms of PFAS and the formation and identification of products of incomplete destruction by this process remain unknown.

[0007] What are needed in the art are more efficient methods for regeneration of spent activated carbon, and in embodiments for spent GAC laden with PFAS.Summary

[0008] According to embodiments, disclosed are methods for regenerating spent activated carbon that carries one or more PFAS compounds and exhibits an associated fluorine content. A method can include subjecting a bed comprising the spent activated carbon to microwave radiation, upon which the bed can increase in temperature at a rate of about 500°C per minute or greater to reach an average bed temperature of about 800°C or higher. The method also includes maintaining the bed at the average bed temperature of about 800°C or higher for a period of time of about 1 minute or longer to obtain a regenerated activated carbon.

[0009] Also disclosed is a system for regenerating spent activated carbon. For instance, a system can include a housing comprising a cavity configured for retaining spent activated carbon. The system can also include a microwave generator configured to direct microwave energy to the cavity. In addition, the system can include a control system configured to control the delivery of the microwave energy and thereby the temperature within the cavity. In some embodiments, a system can include thermal insulation surrounding at least a portion of the cavity and / or a thermal heater configured to deliver infrared radiation (i.e., heat) to the cavity. In some embodiments, a system can include a gas circulation system surrounding at least a portion of the cavity for the treatment of gas in the cavity that may contain some of the PFAS compounds and their decomposition products. Without wishing to be bound to any particular theory, the gas circulation system may allow for the further decomposition of PFAS and its degradation by-product. The circulation may circulate gas escaping from the GAC pores and / or an off-gas treatment component in fluid communication with the cavity. In some embodiments a system can further include a secondary treatment system in gas communication with the cavity configured for further treatment of off-gas generated within the cavity during a microwave activated carbon regeneration protocol.Brief Description of the Figures

[0010] A full and enabling disclosure of the present subject matter, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures in which:

[0011] FIG. 1 schematically illustrates one embodiment of a microwave activated carbon regeneration system as described herein.

[0012] FIG. 2 presents a typical time-dependent temperature profile of a PFAS- loaded GAC sample during microwave heating.

[0013] FIG. 3 graphically presents the effect of microwave heating rate on the defluorination and regeneration of different PFAS-laden GAC.

[0014] FIG. 4 schematically illustrates methods utilized in forming PFAS-laden GAC with different moisture content for examination as described herein.

[0015] FIG. 5 graphically presents the effects of moisture content on the defluorination extent of PFAS-laden GAC according to a microwave based approach.

[0016] FIG. 6 graphically compares the removal of various PFAS compounds by as-obtained field-spent GACs from different water treatment and reuse plants in the United States compared to the virgin GAC sample.

[0017] FIG. 7 graphically compares the removal of various PFAS compounds by microwave regenerated GACs compared to the virgin GAC sample of FIG. 6.

[0018] FIG. 8A is a graph showing the mass loss of F400 activated carbon when subject to multiple cycles of microwave heating. FIG. 8B is a graph showing the increase in BET surface area, total pore volume, micropore volume and mesopore volume for the F400 activated carbon.

[0019] FIG. 9A is a graph showing the mass loss of HD 3000 activated carbon when subject to multiple cycles of microwave heating. FIG. 9B is a graph showing the increase in BET surface area, total pore volume, micropore volume and mesopore volume for the HD 3000 activated carbon.Detailed Description

[0020] Reference will now be made in detail to various embodiments of the disclosed subject matter, one or more examples of which are set forth below. Each embodiment is provided by way of explanation of the subject matter, not limitation thereof. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present disclosure without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment, may be used in another embodiment to yield a still further embodiment.

[0021] In general, the present disclosure is directed to methods for regeneration of spent activated carbon that carry one or more PFAS compounds. Regeneration of spent activated carbon can restore adsorption capacity of the activated carbon and extend available service time while also reducing the cost and carbon dioxide (CO2)footprint of water treatment processes that utilize the activated carbon. For instance, disclosed methods can provide for defluorination of a starting material by about 80% or greater, or even higher amounts in some embodiments, such as about 85% or greater, or about 90% or greater.

[0022] Beneficially, activated carbon (AC) as may be treated by disclosed methods and systems is not particularly limited, and the adsorption capacity of PFAS-laden activated carbon of different physical and chemical characteristics can be treated. For instance, activated carbons of various surface areas and pore size distributions as well as activated carbons of any variety of surface chemistry, pH, and point of zero charge (PZC) are encompassed herein. In embodiments, the activated carbon can encompass GAC, but other forms of activated carbon are encompassed herein as well including, without limitation, pelletized activated carbon, powdered activated carbon, impregnated activated carbon, catalytic activated carbon, etc. In fact, disclosed methods can improve activated carbon characteristics in some embodiments by increasing surface area and pore size distribution of the materials. For instance, the BET surface area of the regenerated activated carbon may be greater than that of the spent activated carbon by between 5 and 50 percent, such as between 10 and 30 percent, such as between 15 and 25 percent. Additionally, the micropores of the regenerated activated carbon may have a microporosity of 85 to 110 percent of a virgin activated carbon, such as between 90 and 100 percent of a virgin activated carbon.

[0023] Disclosed methods and systems can remove other contaminants from a spent activated carbon material, in addition to PFAS. For example, and without limitation, disclosed methods can regenerate activated carbon that has been exhausted with other organic materials such as natural organic matter, effluent organic matter, and / or synthetic organic compounds.

[0024] Microwave irradiation is a viable method for regenerating PFAS and organic exhausted activated carbons because of its heating capacity at the molecular level, which leads to homogenous and rapid temperature increases in the AC particle. The major driving force is the dielectric nature of carbon materials, which allows the conversion of low-power irradiation energy into a rapid and large temperature increase. Microwave heating can be considered as the reverse of conventional heating. Conventional heating utilizes conduction or convection to transfer energy in the form of infrared radiation to a material surface from an externalsource, which increases the temperature at the surface, followed by thermal conduction to heat the cooler interior regions of the material. Conversely, energy in the microwave spectrum can penetrate materials and simultaneously produce an increase in temperature throughout the volume of the material rather than conduction to the interior from an external surface, requiring the heating of the entire exterior cavity around the particles. Advantages of microwave heating include a uniform heating of a bulk material, an increase in energy transfer efficiency, and a reduction in the heating time process. Additional advantages include heating of interior pores of a porous material such as activated carbon, selective heating through directional control of the microwaves, greater control of the heating process, no direct contact between the heating source and the materials to be heated as well as reduced equipment size and reduced waste. For instance, microwave heating using a microwave generator as described herein may have overall energy consumption of less than that of a thermal process, such as less than half of that of a thermal process. As described above, the decreased energy requirements, but increased energy efficiency, may be attributed to the targeted efficiency of microwave heating, as compared to the thermodynamic inefficiency of thermal heating. Additionally, the microwave heating regeneration process as described herein may serve to regenerate activated carbon in less than half of the time required by a conventional thermal regeneration process, such as less than 30% of the time required by a conventional thermal regeneration process.

[0025] Additionally, the presently described method allows for a lower mass loss of activated carbon during regeneration. In a conventional thermal process, approximately 10 to 20 wt% of activated carbon must be replaced with virgin activated carbon, as the thermal regeneration process leads to carbon mass loss due to the presence of flue gases, e.g., steam and carbon dioxide. In the presently described microwave regeneration process, activated carbon mass loss may be less than 8 wt%, such as less than 6 wt%, such as less than 4 wt%, such as less than 2 wt%.

[0026] Activated carbon, due to its dielectric nature, is particularly suitable for microwave heating due to the delocalized pi (IT) electrons of sp2hybridized carbon. Specifically, the microwave-induced motion of electrons generates an increase in temperature throughout the material through Joule heating within the grain or arcgeneration at phase boundaries. Moreover activated carbon can exhibit high mechanical strength and good resistance to damage from the process..

[0027] FIG. 1 schematically illustrates one embodiment of a microwave-based system and method as disclosed herein. As illustrated, a system can include a housing 100 that defines a cavity therein configured to contain a bed of material 108 comprising an amount of spent activated carbon. The size of the cavity 110 is not particularly limited, and the system can be designed to process any desired amount of spent activated carbon, from a few grams to many kilograms, e.g., from about 1 kg to about 100 kg, in some embodiments.

[0028] The cavity 110 can be in communication with a microwave generator 112. For instance, the microwave generator 112 can be retained within the same housing 100 as defines the cavity 110 or can be a separate component, able to be retained so as to deliver microwave energy to the cavity 110. As utilized herein, the term “microwave” generally refers to electromagnetic energy having wavelengths on the order of 60 centimeters or less, e.g., from about 1 mm to about 30 cm, and frequencies ranging from about 300 MHz to about 300 GHz. Any suitable microwave generator 112 capable of generating an output frequency ranging from 300 MHz to 300 GHz, and having a power output of about 100 W or greater, such as about 500 W or greater, about 800 W or greater, or about 900 W or greater, such as from about 100 W to about 1500 W in some embodiments, would be suitable, e.g., a microwave generator 112 that includes a cyclotronic inverter system can be utilized.

[0029] The microwave generator 112 can be configured to deliver energy to the cavity such that a bed of material 108 that includes the activated carbon and that is retained in the cavity 110 will be heated quickly. For instance, activated carbon retained in the cavity 110 can be heated at a rate of about 500°C per minute or greater, such as about 530°C / min or greater, about 550°C / min or greater, about 580°C / min or greater, about 600°C / min or greater, about 630°C / min or greater, or about 650°C / min or greater in some embodiments.

[0030] The rapid heating of the bed 108 can continue until the activated carbon reaches a target temperature of about 800°C or greater, such as from about 800°C to about 1100°C, or from about 900°C to about 1000°C in some embodiments. The bed 108 can then be held at the target temperature for a period of time during which PFAS on the activated carbon can be destroyed, while minimizing the escape of intact PFAS from the pores of the activated carbon. For instance, the bed 108 can beheld at the target temperature for a period of about 1 minute or longer, such as from about 1 minute to about 5 minutes, or from about 2 minutes to about 3 minutes in some embodiments.

[0031] By way of example, and without limitation, FIG. 2 illustrates a temperature profile over the course of a typical processing protocol. As illustrated, upon delivery of microwave energy to a bed 108 that includes activated carbon retained in a cavity 110 of a device, a first phase of heating during which the bed 108 exhibits little increase in temperature can take place. During this phase, moisture can be removed from the activated carbon pores of the bed materials and as such, this phase is termed the “evaporation time” (tE). Following, the bed can be heated rapidly during the “ramping time” (ta) to reach the “target temperature time” (tTT), during which the activated carbon is exposed to the target temperature for a period of time. As indicated, the bed temperature can fluctuate somewhat over the course of the tTT, and as such the target temperature can be determined as the average temperature of the fluctuating bed temperature during the time period that the bed 108 is held at the target temperature.

[0032] To control the temperature of the bed 108 during a protocol, the system can also include a control system 114. A control system can be in communication with the cavity 110 via one or more sensors (e.g., temperature sensors for both AC and gas in the cavity, moisture sensors, etc.) as well as in communication with the microwave generator 112 as well as one or more additional components of a system, e.g., a heater 106, an off-gas treatment component 104, a purge gas flow, etc.). For instance, one or more temperature sensors, moisture sensors, etc. can be in communication with a programmable control system 114 (e.g., a PLC and the like) that can be programmed to automatically adjust the power delivered from the microwave generator 112 to the cavity 110 according to standard practices. By use of a control system 114, the temperature of the bed as well as other characteristics of the system can be controlled, e.g., temperature elsewhere in the cavity 110, one or more conditions within a secondary treatment component 104 (e.g., temperature, off-gas flow conditions from the cavity 110, pressure, purge gas flow, etc.).

[0033] In embodiments of the present disclosure, the system as described above may comprise a batch processing system, or a continuous system. Said continuous system may allow for a continuous feed of spent AC into the cavity 110 where it may be regenerated.

[0034] To further ensure destruction of PFAS as well as other organic matter contained in spent activated carbon of a bed 108, in embodiments at least one additional temperature within the cavity 110 can be controlled in addition to the temperature of the bed 108. For example, a temperature within the cavity 110 and adjacent to the bed 108 can be controlled so as to be maintained at an average temperature of about 800°C or greater at least during the tTTphase of a treatment protocol. An increased temperature within the cavity 110 and external to the bed 108 can be provided either passively or actively. For instance, in embodiments, a system can include thermal insulation surrounding the cavity 110, i.e., as a component of housing 100, which can maintain an elevated temperature within the cavity 110 during a protocol. In embodiments, a system can include a heater 106 that can be in communication with a controller 114 for actively maintaining an elevated temperature within the cavity 110 and external to the bed 108. For instance, thermal heating of the interior of the cavity 110 can be provided by use of a tube furnace, e.g., including a resistance coil wire or any other suitable heat source, as a heater 106 in thermal communication with the cavity 110. In those embodiments in which the system includes a heater 106 in conjunction with a microwave generator 112, a treatment protocol can generally include heating the interior of the cavity 110 to an elevated temperature prior to use of the microwave generator 112, as thermal heaters generally exhibit a much slower temperature ramp rate as compared to the rapid heating of the bed 108 of disclosed methods. Furthermore, gases within the cavity may be recycled through the bed 108, reducing the amount of heating required from heater 106.

[0035] In embodiments, the moisture content of spent activated carbon to be treated according to disclosed methods can be determined and optionally modified prior to the microwave heating of the materials. The moisture level of a spent activated carbon can affect a treatment process in multiple ways. For instance, water present in the pores of spent activated carbon following removal from a water treatment facility can inhibit the desired rapid increase of temperature during microwave processing. However, the presence of some water in activated carbon pores can also provide benefits during defluorination of PFAS compounds, as water can promote PFAS destruction via hydrolysis of perfluoroacyl fluorides, which are the most common intermediate thermal decomposition products of PFAS compounds. Specifically, in the presence of water, perfluoroacyl fluorides can react with water toform perfluorocarboxylic acids via the hydrogen fluoride elimination mechanism, which can be repeated by alternating HF elimination and hydrolysis reactions to form shorter and shorter chain perfluorocarboxylic acids and the eventual complete destruction of the compound. In the absence of water, the decomposition of perfluoroacyl fluorides occurs via direct cleavage of the intramolecular bonds, which can give a lower defluorination extent. For example, at a low moisture content (e.g., from about 1 wt% to about 5 wt% moisture), PFAS compounds on the spent activated carbon can volatilize during a treatment process and escape the activated carbon intact. This effect can be exacerbated when considering treatment of long- chain PFAS compounds (e.g., PFOA and PFOS) which can be adsorbed both in the micropores (pore size < 2 nm in width) and mesopores (pore size of 2 to 50 nm) of activated carbon and therefore can escape from the mesopores upon heating and volatilization.

[0036] Thus, to ensure improved destruction of PFAS compounds of spent activated carbon, it can be beneficial to include or maintain a moisture within the spent activated carbon to be treated according to the disclosed method. For instance, spent activated carbon to be treated by the microwave heating methods can exhibit a moisture content of about 20% or greater by weight of the activated carbon, such as from about 20 wt% to about 40 wt%, or from about 35 wt% to about 40 wt% in some embodiments. In some embodiments, an even higher moisture content can be utilized, such as about 40 wt% or higher, such as from about 65 wt% to about 70 wt%, for instance in those embodiments in which the spent activated carbon is laden with a large proportion of long-chain PFAS compounds.

[0037] The moisture content of spent activated carbon to be treated can generally be determined and modified as desired prior to location on the spent activated carbon within the cavity 110, as it has been previously shown that the mere presence of moisture surrounding the spent activated carbon has little effect on the materials, and in order to provide the best effect, the moisture of the materials should be within the pores of the activated carbon.

[0038] Defluorination of PFAS (as well as destruction of other organic compounds present in the spent activated carbon) can be accomplished in a microwave regeneration system within the activated carbon bed itself as well as in the cavity areas external to the bed for materials that may volatilize during the microwave treatment and escape the activated carbon intact. In some embodiments, a systemcan further include an off-gas treatment component 104 in fluid communication with the cavity 110, which can be utilized to facilitate destruction of PFAS, partially degraded PFAS by-products, or any other contaminants that may have escaped destruction within the cavity 110 of the system.

[0039] An off-gas treatment component 104 can in embodiments utilize a thermal treatment method. For example, and as illustrated in the system of FIG. 1 , an off-gas treatment component 104 can be in thermal communication with a heater 106. A heater 106 can be the same heater as used to control a temperature within the cavity 110 or a different heater, as desired. Moreover, a thermal treatment component 104 can be controlled with the same control system 114 as may be used for controlling other components of a system or can utilize a separate control system, as desired. An off-gas treatment protocol can in embodiments include retaining the off-gas from a cavity 110 at a temperature of about 800°C or greater, such as about 800°C to about 1200°C, or from about 900°C to about 1000°C in some embodiments for a period of time of about 5 minutes or less, such as from about 1 minute to about 3 minutes, or about 1 minute to about 2 minutes in some embodiments. Thus, residual PFAS compounds and / or their degradation products and / or other organic materials on the spent activated carbon that escape from the bed 108 and the cavity 110 can be removed at the off-gas treatment component 104.

[0040] The present invention may be better understood with reference to the examples set forth below.Example 1

[0041] The effect of heating rate was evaluated on the microwave regeneration of several PFAS-laden GAC samples having a moisture content of approximately 65- 70%.

[0042] Filtrasorb® 400 (F400) GAC obtained from the Calgon Carbon Corporation was used in all experiments. Four PFAS were examined including two perfluoro alkyl carboxylic acids - perfluoro butanoic acid (PFBA) (> 99%) and PFOA (> 98%), and two perfluoro sulfonic acids - PFBS (> 98%) and PFOS (98%). These four compounds can also be classified as long-chain PFAS, including PFOA and PFOS as eight carbon chain compounds; and short-chain PFAS, including PFBA and PFBS as four-carbon chain compounds. PFOA and PFBS were obtained from TCI America, PFOS was obtained from Matrix Scientific, and PFBA from ACROS Organic. PFOA and PFOS were obtained in the solid form, and PFBA and PFBS asliquid samples. All PFAS compounds were used as received without further treatment. The specifications and properties of PFBA, PFBS, PFOA and PFOS are shown in Table 1, below.Table 1

[0043] PFAS compounds were dissolved in distilled deionized water (DDW) (5 mg / L PFAS in 200 mL water). The aqueous samples thus formed were then adsorbed on GAC (5.0 g F400 GAC, 2-7 days) to obtain PFAS-laden GAG samples. Prior to microwave regeneration, the GAC samples were filtered with the filtered samples exhibiting a moisture content in the range of 65% to 70%. The H2O content was determined as the wt / wt% of H2O in the air-equilibrated sample (a function of relative humidity).

[0044] A research-grade microwave system with programmable control mode was used to provide a controlled microwave operating environment. In the programmable mode, a target temperature of 980°C (> 950°C) within the GAC bed was initially set to observe the highest expected defluorination extent for the PFAS compounds. To reach the target temperature value, one of six heating rates was set, i.e., either 100°C / min, 200°C / min, 300°C / min, 400°C / min, 533°C / min, or 653°C / min. After reaching the target temperature, the system controller maintained the GAC bed temperature near the target temperature by automatically adjusting the microwave power. A typical temperature profile of the GAC bed is presented in FIG. 2.

[0045] As indicated in the representative temperature profile of FIG. 2, a first phase termed the “evaporation time” (tE) occurs during which inter- and intra- granular moisture content is removed from GAC pores. The next phase is a rapidheating period to the target temperature, which is referred to as the “ramping time” (tR). The final phase is termed the “target temperature time” (trr), during which the GAC is exposed to the target temperature for a period of time. The tE can vary with the moisture content of the sample but was generally from about 15 seconds to about 45 seconds. The ramping time depends on the selected heating rate (°C / min). For the target temperature region, the temperature profile of the GAC bed showed variation as the power cycled, with approximately 200°C fluctuations.

[0046] The fluorine extraction was performed by soaking 5 g GAC samples in 200 mL of distilled deionized water (DDW) on a shaker for up to 72 hours. Then the samples were passed through a 0.45 mm filter, and the filtrates analyzed using a Dionex ICS-2100 ion chromatography equipped with a Dionex ADRS 600 suppressor. The standard calibration solutions (1 , 5, 10, 25, 50, 100, 250, 500, 100 and 2000 ppm) were prepared by diluting concentrated fluoride solution (2000 μg / L, > 99.9%, Sigma) with pre-determined amounts of DDW (US EPA method 300). .

[0047] FIG. 3 illustrates the impact of the microwave heating rate to the targeted bed temperature on the defluorination of the different PFAS-laden GAC. The comparative analysis of the obtained results showed that the heating rate had a greater effect on PFOA and PFBS as their extent of defluorination was lower compared to PFOS and PFBA at the same heating rate. The extent of defluorination exhibited distinct trends specifically for heating rates less than 400°C / min. PFBA showed > 90% F- mass balance recovery irrespective of the heating rate whereas PFOA and PFBS showed an increase with respect to the increase in heating rate. %F- mass balance recovery initially increased for PFOS at 200°C / min and 400°C / min and then slightly decreased at 533 and 653°C / min.

[0048] Without wishing to be bound to any particular theory, variations in effect were attributed to the differences in size, functionality, and thermal stability of the selected PFAS compounds. The thermal stability of PFBS requires higher temperatures for destruction (about 450°C), and as a result a slow heating rate still resulted in the loss of some volatilized PFBS molecules without destruction. However, the effect of PFAS structure on fluorine recovery became less important with increasing heating rate. This was attributed to reaching the high target temperature very quickly, and as a result the PFAS compounds could be destroyed non-selectively. Overall, a higher heating rate was shown to increase the decomposition of PFAS compounds in the GAC pores during a microwaveregeneration process, e.g., 80-100% PFAS defluorination, nearly independent of PFAS structures, while minimizing volatilization and / or escape of PFAS molecules from the GAC pores.Example 2

[0049] PFAS-laden GAC samples were formed with various moisture content according to the general scheme illustrated in FIG. 4. PFAS compounds as described in Example 1 were dissolved in double distilled water 10 (5 mg / L PFAS in 200 mL water). The aqueous samples thus formed were then adsorbed to GAC 12 (5.0 g F400 GAC, 2-7 days) to obtain PFAS-laden GAC samples 14. After adsorption (5.0 g F400 GAC, 2-7 days) and before microwave regeneration, GAC samples were filtered 16 to provide sample material 18 exhibiting a moisture content in the range of 65% to 70%. A portion of this filtered sample material was heated 15 in an oven at 105°C for either 5-7 min. to provide sample material 17 exhibiting a moisture content in the range of 1% to 5%, or for 3 min. to provide sample material 19 exhibiting a moisture content in the range of 34% to 40%.

[0050] To check the consistency of the microwave system, samples were run as duplicates and all characterization parameters were checked for both duplicate samples.

[0051] A summary of the findings related to the effect of moisture content and PFAS type on the defluorination extent of PFAS-laden GACs following the microwave regeneration process is presented in FIG. 5.

[0052] As a general trend for all PFAS compounds examined, an increase in the initial moisture content of GAC resulted in an increase of 5-10% for fluorine mass balance recoveries. This increase was attributed to the hydro-pyrolysis of PFAS compounds. It has been shown that water can promote PFAS destruction via hydrolysis of perfluoroacyl fluorides, which are the most common intermediate thermal decomposition products of PFAS compounds. In the absence of water, the decomposition of perfluoroacyl fluorides occurs via direct cleavage of the intramolecular bonds giving lower defluorination extent. The higher defluorination rate with increasing initial moisture content for longer chain PFAS compounds also suggested that there was higher residual water in the carbon pores.

[0053] The short-chain PFAS compounds (PFBA and PFBS) showed > 90% defluorination extent under all moisture content conditions. Change in moisture content from 1-5% to 65-70% yielded 8% and 4% increases in the defluorinationextents of PFBA and PFBS, respectively. The higher defluorination extent under all moisture content conditions was attributed to the length and size of these compounds, which can penetrate and adsorb more strongly in the micropores of GAC. As a result, they may not be able to escape from the GAC pores fast enough and are more effectively defluorinated regardless of the moisture content.Example 3

[0054] Two GACs with different pore size distributions were utilized including a microporous GAC (“GAC A”; F400 available from Calgon Carbon Corporation) and a mesoporous GAC (“GAC B”; HD3000 available from Norit Americas, Inc.). The GACs were loaded with PFOA as described above to a loading level of 0.2 mg PFOA per g GAC. Both of GAC A and GAC B had high moisture contents, with GAC A having a content of 65 to 70%, and GAC B having a moisture content of 80 to 90%. The samples were regenerated by use of a microwave system, as described above, at a microwave power of 900 W at a bed temperature of about 1000°C.

[0055] GAC B showed a fluorine mass balance recovery for PFOA defluorination about 60% at the tested conditions while the fluorine mass balance recovery of GAC A was about 96% under the same conditions (i.e., MW power, GAC bed temperature and PFOA loading). The different defluorination behaviours of PFOA in GACs A and B was attributed to easier volatilization and escape of volatilized PFOA molecules from mesoporous GAC B as compared to the microporous GAC A. Upon exposure to microwave heating, when PFOA-laden GACs reach an adequate volatilization temperature (~192°C), escape of volatilized PFOA molecules would be easier from the mesopores of GAC B than from the micropores of GAC A, where adsorption energy is higher.

[0056] In a separate experiment, GAC A was loaded with a higher PFOA loading of 20 mg PFOA per g of GAC. Microwave regeneration of this GAC showed a much lower defluorination (17%), indicating escape of PFOA molecules from outer regions of carbon pores or less penetration of excess PFOA molecules in narrower pore regions of the carbon.

[0057] The microwave used in these experiments had a quartz cavity and did not have any insulation. To create a zone with a high temperature above the GAC within the microwave cavity, the microwave unit was connected directly, in-line, to a high- temperature (950°C) tube furnace.

[0058] Upon connection of the microwave to the tube furnace, a second GAC B sample was subjected to the regeneration protocol, upon which the total fluorine mass balance recovery increased to 95% as compared to the 60% recovery obtained previously. The additional 35% of fluorine detected in the tube furnace system indicated the destruction of escaped volatilized fraction of PFOA and / or the thermal decomposition of by-products under the high-temperature condition of thecombined microwave-tu e furnace system (950°C). Similarly, when the tube furnace was connected to the microwave system for a high loading GAC A sample, overall fluorine mass balance recovery increased from 17% to over 90% (i.e., the extent of defluorination).Example 4

[0059] A tube furnace was connected in-line with the microwave regeneration system such that the off-gas of the microwave system was passed through a high temperature (950°C) chamber with 30 min detention time, and under a N2 gas flow at a pressure of 2.5 psig. After a regeneration run, both the microwave system and the connected tube furnace system were washed with DDW and methanol to determine the overall fluorine and PFAS mass balance recoveries. PFAS-laden GAC A and GAC B, as described above, were regenerated with the modified microwave system.

[0060] The connection of the tube furnace to the microwave system increased the total fluorine mass balance recovery of GAC B from about 60% to 95%. In addition, LC-MS / MS results showed that PFAS compounds were below the detection limits in the regenerated GAC. The additional 35% of fluorine detected in the tube furnace system indicated the destruction of escaped volatilized fraction of PFOA and / or the thermal decomposition of by-products under the high-temperature condition of the tube furnace system (950°C).Example 5

[0061] Field-spent GACs that had been in service for several months were collected during replacement after exhaustion from four water treatment plants around the United States. Multiple samples were taken from each plant at various locations within the treatment facility, as indicated in Table 2, below.Table 2* Obtained from pilot-scale.1F400: Filtrasorb®400 activated carbon product produced by Calgon Carbon Corporation2F300: Filtrasorb®300 activated carbon product produced by Calgon Carbon Corporation.

[0062] The samples were characterized for surface area and pore size distribution. The GAC samples were exposed to microwave regeneration under the conditions of a heating rate of 653°C / min and maintaining the GAC at 800°C - 1000°C for 2-3 min. Regenerated GAC samples at moisture levels of 1%-5% and 65%-70% were also characterized for surface area and pore size distribution.

[0063] Batch adsorption experiments were performed for PFAS compounds including PFOA, PFOS, PFNA, HFPO-DA, PFHxS, PFBS, and PFBA, before and after regeneration, to examine the changes in the removal of the field GACs. The batch adsorption experiments were conducted in two different background matrices (DDW and ionic strength (IS) of 0.02 mol / L), and initial PFAS concentrations of 1 and 2 μg / L, respectively. Ionic strength solution was prepared with 15 mM NaHCOs and 5 mM NaCI to obtain 0.02 mol / L IS solution, and its pH adjusted to 6.0 using HCI.

[0064] PFAS removal efficiencies of MW regenerated samples varied between 80-100% (FIG. 7) as compared to the virgin F400, while field-spent GAC samples had removal efficiencies ranging from 0 to 60% (FIG. 6).

[0065] As shown in Table 3 below, following the microwave regeneration protocol almost all of the PFAS adsorption sites were recovered for the spent field GACswhich were loaded with several organic and inorganic compounds in addition to PFAS. .

[0066] Table 3, shows BET surface area and micropore volume of the top samples of the field GACs before and after microwave regeneration. For comparison, the virgin F400 had a 1019 m2 / g BET surface area and 0.328 cm3 / g micropore volume. As shown in Table 3 below, after the microwave regeneration process, GAC characteristics were restored to near original levels. The field GAC from Plant C, although using F400, had distinctly different characteristics believed to be due to its having been utilized and thermally reactivated for decades at the thermal reactivation facility on the site at this particular waste water treatment plant. As is clear from the data in Table 3 for the virgin F400 and Plant C field GAC, the repetitive thermal reactivation of the same GAC for decades resulted in a very substantial increase in the mesopore volume of the carbon (over 800% compared to that of the virgin F400 GAC) as a direct result of the widening (and loss) of important microporosity. This large increase in mesopore volume in the field GAC from Plant C is the reason why the microwave regeneration of it apparently increased its mesopore volume by over 1000% when compared to the mesopore volume of the virgin GAC. In fact, the very large increase in mesopore volume was a result of the repetitive reactivation of the F400 GAC in Plant C, which occurred at the expense of micropore volume - a range of pore size critical to the effective adsorption of PFAS compounds.Table 3

[0067] As shown in Table 3 above, the BET surface area of the regenerated activated carbon was 10 to 30% greater than that of the spent activated carbon prior to regeneration. Additionally, the total pore volume increased by 10 to 30 % over the spent activated carbon. The regenerated activated carbon, as shown in Table 3 above, had a BET surface area that was 87 to 99 % of that of virgin activated carbon, and a micropore volume that was between 31 and 98 % of the virgin activated carbon. Further, most samples of regenerated activated carbon had a micropore volume within the range of 89 to 101 % of that of the virgin activated carbon, a finding that is of critical benefit to the efficiency of adsorption of PFAS compounds on activated carbon, and subsequently to the efficiency of regeneration of the spent carbon through a microwave heating processt. Additionally, the total pore volume of the regenerated activated carbon was restored to greater than 85% of the total pore volume of the virgin activated carbon, such as greater than 90% of the total pore volume of the virgin activated carbon, such as greater than 95% of the total pore volume of the virgin activated carbon. The mesopore volume of theregenerated activated carbon may be greater than 70% of the mesopore volume of the virgin activated carbon, such as greater than 80% of the mesopore volume of the virgin activated carbon, such as greater than 95% of the mesopore volume of the virgin activated carbon, such as greater than 100% of the mesopore volume of the virgin activated carbon.Example 6

[0068] Spent activated carbon samples were subjected to repetitive microwave heating for a total of 10 cycles. As shown in FIGS. 8A, 8B, 9A and 9B, wherein FIGS. 8A and 8B correspond to F400 samples and FIGS. 9A and 9B correspond to HD 3000 samples, after the first heating cycle, subsequent activated carbon loss was below 2 wt% for cycle numbers 2 thru 10. In the initial Cycle 1, mass losses of 7.7% (FIGS. 8A) and 16% (FIG 9A), respectively, were incurred for the F400 and HD 3000 samples, which is attributable to mass losses of organic material within the activated carbon, not mass loss of activated carbon, per se.

[0069] Additionally, FIGS. 8B and 9B show the corresponding increases in surface area and pore volumes for the microwave cycling of the F400 and HD 3000 samples, respectively. In FIG. 8B, the BET surface area is represented by the vertical bars, whereas the top line corresponds to the total volume of all the pores in the F400 activated carbon, the middle line represents the volume of the micropores in the activated carbon, and the bottom line represents the mesopores in the activated carbon. Similarly, in FIG. 9B, the BET surface area is represented by the vertical bars, whereas the top line corresponds to the total volume of all the pores in the HD 3000 activated carbon, the middle line represents the volume of the micropores in the activated carbon, and the bottom line represents the mesopores in the activated carbon.

[0070] While certain embodiments of the disclosed subject matter have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the subject matter.

Claims

WHAT IS CLAIMED IS:

1. A method for regenerating a spent activated carbon, the spent activated carbon carrying a poiy- or perfluoroalkyl substance, the method comprising: subjecting a bed comprising the spent activated carbon to microwave radiation such that the bed increases in temperature at a rate of about 500°C per minute or greater to an average bed temperature of about 800°C or higher; maintaining the bed at the average bed temperature for a period of time of about 1 minute or longer to form a regenerated activated carbon; wherein the regenerated activated carbon has a fluorine content that is about 20 wt% or less of that of the spent activated carbon.

2. The method of claim 1 , wherein the microwave radiation is delivered to the bed from a microwave generator operating at a power output of about 500 Watts or greater.

3. The method of claim 1 , wherein the bed is held at the average bed temperature for a period of time of from about 1 minute to about 5 minutes.

4. The method of claim 1 , further comprising maintaining an area adjacent to the bed at an elevated temperature while the bed is maintained at the average bed temperature.

5. The method of claim 4, wherein the elevated temperature is about 800°C or higher.

6. The method of claim 1 , wherein the spent activated carbon has a moisture content of about 20 wt% or greater.

7. The method of claim 6, wherein the spent activated carbon has a moisture content of from about 20 wt% to about 40 wt%.

8. The method of claim 6, wherein the spent activated carbon has a moisture content of from about 65 wt% to about 70 wt%.

9. The method of claim 1 , further comprising modifying a moisture content of the activated carbon prior to subject the bed to the microwave radiation.

10. The method of claim 1 , further comprising heat treating an off-gas from the bed.

11. The method of claim 10, wherein the heat treating comprises retaining the off- gas at a temperature of about 800°C or greater for a period of time.

12. The method of claim 11 , wherein the period of time is about 5 minutes or less.

13. The method of claim 2, wherein the overall energy consumption of microwave regeneration is less than that of a thermal process.

14. The method of claim 1 , wherein less than 8 wt% of activated carbon is lost.

15. The method of claim 1, wherein the regenerated activated carbon has a BET surface area 10 to 30% higher than that of the spent activated carbon.

16. The method of claim 1, wherein the regenerated activated carbon has a BET surface area 87-96% of that of a virgin activated carbon.

17. The method of claim 1 , wherein the regenerated activated carbon has a micropore volume of from 87 to 98% of that of a virgin activated carbon.

18. A system for regenerating a spent activated carbon, the spent activated carbon carrying a poly- or perfluoroalkyl substance, the system comprising: a housing defining a cavity configured for retaining a bed comprising the spent activated carbon; a microwave generator configured for delivering microwave radiation to the bed; a control system configured to control the delivery of the microwave radiation from the microwave generator to the bed such that the bed is heated at a rate ofabout 500°C per minute or greater to an average bed temperature of about 800°C or higher.

19. The system of claim 18, further comprising thermal insulation surrounding at least a portion of the cavity.

20. The system of claim 18, further comprising a heater configured to deliver infrared radiation to the cavity.

21. The system of claim 18, further comprising a gas recirculation system for circulating gas in the cavity during microwave regeneration to further decompose PFAS and its degradation by-products escaping from the spent activated carbon pores and / or an off-gas treatment component in fluid communication with the cavity.

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