Equipment suitable for the destruction of PFAS through an oxidation process and for transport to contaminated sites

Abstract

To destroy PFAS by oxidation in supercritical water (SCWO).SOLUTION: A method of destroying PFAS in a PFAS-containing aqueous mixture comprises the step for reacting PFAS with an aqueous oxidant under supercritical conditions to produce effluent and further comprises treating the effluent from the step for reaction with the aqueous oxidant with an alkali or alkaline earth element, thereby converting PFAS into an aqueous product mixture containing carbon dioxide and an alkali or alkaline earth fluoride.SELECTED DRAWING: Figure 6A

Description

[Technical Field]

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 62 / 948,765, filed December 16, 2019, and U.S. Provisional Patent Application No. 62 / 868,858, filed June 28, 2019. [Background technology]

[0002] Per- and polyfluoroalkyl substances (PFAS), including perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), and hundreds of other similar compounds, are widely used in the United States for many applications. There are significant concerns associated with these compounds due to widespread contamination with uncertainty regarding risks to human health and the environment. PFAS are molecules with chains of carbon atoms surrounded by fluorine atoms. The C—F bond is highly stable, allowing these compounds to persist in the natural environment. Some PFAS contain hydrogen, oxygen, sulfur, phosphorus, and / or nitrogen atoms. One example is PFOS: [ka]

[0003] Although some PFAS compounds (PFOA and PFOS) with known human health risks have been voluntarily phased out, residual contamination remains. Alternative PFAS compounds have been introduced with limited understanding of their health risks. Currently, only PFOA and PFOS are addressed in federal lifetime health advisories, and no maximum contaminant levels (MCLs) have been established to regulate acceptable levels of these and other PFAS compounds in drinking water. As of May 2020, 1,582 locations in 49 states have PFAS-contaminated drinking water sources. Current technologies used to treat PFAS-contaminated water are expensive, and managing spent media is costly and can result in long-term liability.

[0004] Many methods have been developed to remediate PFAS in the environment. For example, in US2019 / 0314876, Oberle et al. disclose a method and system for remediating soil containing PFAS in which the soil is heated, the PFAS are volatilized, captured, and condensed, steam is added, and the concentrated PFAS solution is then electro-oxidized.

[0005] In this invention, PFAS are destroyed by supercritical water oxidation (SCWO). In addition to this research, Deshusses of Duke University reported several earlier findings, leading to the issuance of a U.S. Department of Defense award in May 2020, entitled "Supercritical Water Oxidation (SCWO) for Complete PFAS Destruction." Summary of the Invention [Problem to be solved by the invention]

[0006] The application of SCWO to PFAS is relatively new and presents new challenges. SCWO of organic compounds has been known for some time and has been described in numerous papers and patents. For example, in U.S. Patent No. 4,861,497, Welch et al. disclose the use of liquid-phase oxidizing agents, such as hydrogen peroxide and ozone, in supercritical water to destroy organic compounds; tests using propylene glycol at 5,000 psia (pounds per square inch of air) and 750–860°F resulted in approximately 98% destruction. In U.S. Patent No. 5,232,604, Swallow et al. disclose the SCWO of organic compounds using an oxidizing agent, such as hydrogen peroxide, and a reaction rate enhancer, such as nitric oxide; in one example, sodium hydroxide and sodium nitrate were used to neutralize the hydrochloric acid formed in the oxidation of methylene chloride. In U.S. Patent Application Publication No. 2019 / 0185361, Aquarden Technologies noted that in the SCWO process, precipitation occurs in the zone where the fluid transitions from subcritical to supercritical, and designed a reactor with a residue outlet connection near this zone. In "Supercritical water oxidation of a model fecal sludge with the use of a co-fuel," Chemosphere 141 (2015) 189-196, Miller et al. reported the SCWO reaction of a fecal simulant in the presence of 48% excess oxygen. The use of a co-fuel can be used to generate a hydrothermal flame in an SCWO reactor, which is characterized by high temperatures, typically exceeding 1000°C. See "Supercritical Water Oxidation," in Advanced Oxidation Processes for Wastewater Treatment, (2018), 333-353.

[0007] Despite extensive previous efforts to develop systems for destroying PFAS, there remains a need for an efficient system for treating PFAS compositions and completely destroying PFAS. [Means for solving the problem]

[0008] First, water or soil samples can be treated to concentrate PFASs to a significantly reduced volume. In some cases, PFAS-containing media are stored in concentrated form and require no additional processing to concentrate. The concentrated PFAS mixture can be containerized and transported to a central facility for PFAS destruction. Alternatively, in some preferred embodiments, concentrated PFAS mixtures are treated on-site where they are generated. The concentrated PFAS solution is destroyed by oxidation. Suitable oxidation processes are described below. Most preferably, the oxidation process involves supercritical water oxidation (SCWO), which has been found to rapidly reduce PFAS concentrations by over 100,000-fold, e.g., PFOA from 1,700 parts per million (ppm) by weight to less than 5 parts per trillion (ppt). To enable efficient destruction with little or no external heat during steady-state operation, fuel can be added to provide some or all of the heat required to power the oxidation (or, in some cases, PFAS may be present with sufficient organic material to serve as fuel). The resulting effluent can be verified to contain little to no PFAS (typically less than 5 ppt) and then released into the environment as safe, clean water.

[0009] In one aspect, the present invention provides a method for destroying PFAS in a PFAS-containing aqueous mixture, comprising reacting the PFAS with an aqueous oxidizing agent, further comprising reacting with an alkali or alkaline earth element to convert the PFAS to a product mixture comprising carbon dioxide and an alkali or alkaline earth fluoride. The most preferred oxidizing agent is hydrogen peroxide.

[0010] In another aspect, the present invention provides a method (or system) for destroying PFAS in a PFAS-containing aqueous mixture, comprising pretreating the aqueous solution containing PFAS to separate the PFAS from salts, such as sodium chloride. The PFAS-containing aqueous solution is subjected to one or, preferably, multiple stages of reverse osmosis cascade to form a salt solution and a desalted solution. The desalted aqueous fraction obtained from reverse osmosis can be subjected to SCWO. The resulting salt solution is optionally treated with an acid and heated to a sufficient temperature (preferably at least 150°C, or at least 200°C, or 250°C, or up to 300°C, or up to 350°C) to volatilize the PFAS compounds remaining in the salt solution. This preferably results in a salt solution having 5 ppt or less of PFAS. The volatilized PFAS components are then condensed and captured, and optionally recombined with all or a portion of the desalted solution prior to SCWO. Alternatively, the condensed PFAS can be subjected to SCWO without recombining with the desalted solution. As with any aspect described herein, this pretreatment method can be used by itself or in combination with any of the other aspects or techniques described herein.

[0011] In a further aspect, the invention provides a method for destroying PFAS in a PFAS-containing aqueous mixture, comprising subjecting the PFAS-containing aqueous mixture to a separation procedure to separate the PFAS-containing aqueous mixture into a PFAS-enriched fraction and a PFAS-depleted fraction; heating at least a portion of the PFAS-depleted fraction to a temperature of at least 300°C to form a heated PFAS-depleted fraction; combining the heated PFAS-depleted fraction with the cooler PFAS-depleted fraction to form a heated PFAS-containing aqueous mixture prior to a supercritical destruction step; and reacting the heated PFAS-containing aqueous mixture with an oxidizing agent under supercritical conditions. Alternatively, the PFAS-containing aqueous mixture is separated into a salt-enriched fraction and a salt-depleted fraction (which is typical of reverse osmosis), the salt-depleted fraction is heated to a temperature of at least 300°C to form a heated salt-depleted fraction, and the heated salt-depleted fraction is combined with the cooler salt-depleted fraction to form a heated PFAS-containing aqueous mixture prior to the supercritical destruction step; and the heated PFAS-containing aqueous mixture is reacted with an oxidizing agent under supercritical conditions.

[0012] Any of the aspects of the present invention may be further defined by one or any combination of the following: the step of reacting PFAS with an aqueous oxidizing agent occurs under supercritical conditions; the method is carried out in a mobile trailer; the method includes passing PFAS-contaminated water through a tank (wherein the pressure in the tank is relatively low such that the water is supersaturated with air and air bubbles are generated in the water to create a foaming mixture) and collecting the foaming mixture; prior to reacting PFAS with an aqueous oxidizing agent, the PFAS-containing aqueous mixture contains at least 100 ppm PFOA, and the method includes reducing the concentration of PFOA to at least 10 ppm. 6 or 10 7 or 10 8the PFAS is reacted with an oxidant in an oxidation reactor, and after exiting the reactor, the effluent is treated with a solution comprising NaOH, LiOH, or KOH to produce a neutralized solution, which is recycled to neutralize additional effluent; the neutralized effluent is at least partially evaporated into the air; the PFAS-containing aqueous mixture comprises at least 100 ppm PFAS by weight (in some embodiments, at least 500 ppm or at least 1000 ppm PFAS), and the method reduces the PFAS to 1 ppm or less, or 0.1 ppm or less, or by taking the PFOA concentration, the PFAS-containing aqueous mixture contains at least 100 ppm by weight of PFOA (in some embodiments, at least 500 ppm or at least 1000 ppm of PFOA), and the method converts PFOA to an effluent containing 1 ppm or less, or 0.1 ppm or less, or 0.01 ppm or less, or 1.0 ppb or less, or 0.1 ppb or less, or 0.01 ppb or less; in some embodiments, the PFOA range is 1 ppm to 5 ppt (parts per trillion).Alternatively, the PFAS-containing aqueous mixture comprises a PFOS concentration of at least 100 ppm by weight (in some embodiments, at least 500 ppm or at least 1000 ppm PFOS), and the method converts PFOS to an effluent containing 1 ppm or less, or 0.1 ppm or less, or 0.01 ppm or less, or 1.0 ppb or less, or 0.1 ppb or less, or 0.01 ppb or less; in some embodiments, the PFOA range is 1 ppm to 5 ppt (parts per trillion); the PFAS-containing aqueous mixture comprises 0.5 to 5, or 1 to 3, or about 2% by weight of organic fuel. the PFAS-containing solution is mixed with a solution containing 30 to 50 wt% H2O2, preferably at a weight ratio of 30:1 to 70:1 wt% PFAS solution:H2O2; the PFAS-containing solution is passed through an SCWO reactor with a residence time of 20 seconds or less, preferably 10 seconds, or 5 seconds or less, or 0.5 to 5 seconds; the PFAS-containing solution is added at a controlled rate of 50 to 150 mL / min (STP, standard conditions); no external heating is required after initiation; the PFAS-containing aqueous mixture contains at least 100 ppm PFOA, and the method reduces the concentration of PFOA to at least 10 ppm. 6 or 10 7 or 10 8 and in some embodiments up to about 10 9 the method is carried out in a mobile trailer; the method is carried out in a mobile trailer at a site contaminated with PFAS; the step of reacting PFAS with an aqueous oxidant occurs under supercritical conditions and occurs in a permeable wall reactor; the PFAS-containing aqueous mixture is a solids-containing mixture (preferably containing at least 5% or at least 10% by weight solids or at least 15% by weight solids); the method is carried out in a permeable wall reactor with a curved floor or ceiling.

[0013] The present invention also includes an apparatus for destroying PFAS, comprising a SCWO reactor of any of the reactor types described herein.

[0014] The present invention also includes a system for destroying PFASs that includes a SCWO reactor containing a PFAS-containing aqueous mixture, which can include any of the conditions and / or fluids described herein.

[0015] In another aspect, the invention provides a system for destroying PFAS, comprising: a first inlet conduit for passing a PFAS-containing aqueous stream to a mixing tee; a second inlet conduit for passing a heated stream of clean water to the mixed stream; a conduit connecting the mixing tee to an inlet of an SCWO reactor; an outlet of the SCWO reactor connected to a salt separator; the salt separator comprising an effluent outlet configured to pass clean water to a heat exchanger configured to heat the clean water exiting the heat exchanger and entering the mixing tee; or the effluent outlet configured to pass the effluent through the mixing tee.

[0016] In a further aspect, the present invention provides a method for destroying PFAS, which involves reacting PFAS under supercritical conditions under continuous operation while minimizing the effects of corrosion by supplying an externally applied electron flow throughout the system. From a historical perspective, when pipes are buried underground, such as water pipes, the pipes are connected to a magnesium block that acts as a sacrificial compound. The magnesium oxidizes, but iron pipes do not. In our case, the electronics board is designed to supply electrons (instead of a sacrificial electrode) that can be controlled to increase the amount of current (electron flow), preferably without increasing the DC voltage. The ability to control the electron flow is desirable because electron flow is affected by the surface density of the material at the interface between the fluid and the inner surface of the reaction vessel and the temperature of the reaction.

[0017] In another aspect, the present invention provides a method for destroying PFAS, comprising reacting the PFAS under supercritical conditions in the presence of a eutectic mixture of alkali and / or alkaline earth salts. Under supercritical conditions, the salts precipitate from the water. The precipitate can clog the reactor; however, by selecting a mixture of alkali and / or alkaline earth salts with a eutectic melting point lower than the conditions in the reactor, the salts remain molten, avoiding or reducing precipitation.

[0018] In a further aspect, the invention provides a method for destroying PFAS, comprising reacting the PFAS with an aqueous solution of hydrogen peroxide at a temperature of at least 100° C. and reacting with an alkali or alkaline earth compound (preferably calcium oxide) or a mixture thereof; resulting in the destruction of at least 90% of the PFAS.

[0019] In a further aspect, the present invention provides a method for water treatment and decontamination, comprising passing PFAS-contaminated water (where the water is saturated with air) through a tank; applying a vacuum in the tank to generate air bubbles in the water to create a foamed mixture; passing at least a portion of the foamed mixture through a cyclone air-water separator (where the mixture is subjected to centrifugal force); pumping air from the centrifuged mixture to form a degassed and centrifuged aqueous mixture; and collecting the degassed and centrifuged aqueous mixture. The degassed and centrifuged aqueous mixture typically contains at least 10 times the concentration of PFASs than the water passed through the tank.

[0020] In a further aspect, the invention provides a method of water treatment and decontamination, comprising passing PFAS-contaminated water through a tank, wherein the pressure in the tank is relatively low such that the water is supersaturated with air and air bubbles are generated in the water to create a foaming mixture; and collecting the foaming mixture.

[0021] In a further aspect, the present invention provides a method for minimizing corrosion using an external electron current source applied to the reactor.

[0022] In a further aspect, the invention provides a method for destroying PFAS, comprising passing an aqueous solution of PFAS through a plurality of electrodes; and subsequently reacting the formed HF with an alkali or alkaline earth element.

[0023] In some embodiments, the entire process, or the SCWO portion of the process, can be characterized by having a PFAS concentration of at least 100 ppm by weight PFAS (in some embodiments, at least 500 ppm or at least 1000 ppm PFAS) of 1 ppm or less, or 0.1 ppm or less, or 0.01 ppm or less. Alternatively, the process can be characterized by having a PFOA concentration of at least 100 ppm by weight PFOA (in some embodiments, at least 500 ppm or at least 1000 ppm PFOA) of 1 ppm or less, or 0.1 ppm or less, or 0.01 ppm or less, or 1.0 ppb or less, or 0.1 ppb or less, or 0.01 ppb or less PFOA. In some embodiments, the PFOA range is 1 ppm to 5 ppt (parts per trillion). Alternatively, it can be characterized by reducing PFOS concentrations of at least 100 ppm by weight PFOS (in some embodiments, at least 500 ppm or at least 1000 ppm PFOS) to 1 ppm or less, or 0.1 ppm or less, or 0.01 ppm or less, or 1.0 ppb or less, or 0.1 ppb or less, or 0.01 ppb or less PFOS. In some embodiments, the PFOS range is 1 ppm to 5 ppt (parts per trillion). This process can also be characterized by the same level of destruction starting at PFAS concentrations below 100 ppm. In some embodiments, the PFAS-contaminated water contains at least 1000 ppt of at least one (or at least three, or at least four, or at least five, or at least six) compounds selected from the group consisting of: PFHxA (perfluorohexanoic acid), PFHpA (perfluoroheptanoic acid), PFOA, PFBS (perfluorobutane sulfonic acid), PFHxS (perfluorohexane sulfonic acid), PFHpS (perfluoroheptane sulfonic acid), PFOS, and combinations thereof, which are treated by the process and reduced by at least two orders of magnitude (or at least three orders of magnitude, or at least four orders of magnitude, or at least five orders of magnitude).In some embodiments, the PFAS-contaminated water contains at least 100 ppt of at least one (or at least three, or at least four, or at least five, or at least six) compounds selected from the group consisting of PFBA (perfluorobutanoic acid), PFPeA (perfluoropentanoic acid), PFHxA, PFHpA, PFOA, 6:2FTS (6:2 fluorotelomer sulfonic acid), and 8:2FTS (8:2 fluorotelomer sulfonic acid), and combinations thereof, which are treated by the process to reduce by at least two orders of magnitude (or at least three orders of magnitude, or at least four orders of magnitude, or at least five orders of magnitude), and / or to 5 ppt (or 1 ppt) or less.

[0024] Various aspects of the present invention are described using the term "comprising." However, in narrower embodiments, the present invention may instead be described using the term "consisting essentially of" or more narrowly, "consisting of." [Brief explanation of the drawings]

[0025] [Figure 1] Figure 1 is a schematic diagram of a water pretreatment system for treating PFAS-contaminated water before it passes through the SCWO reactor. [Figure 2] FIG. 2 shows a schematic representation of separation in an electric field. [Figure 3] FIG. 3 is a diagram of the laboratory-scale apparatus used to remove salts from solution. [Figure 4A] FIG. 4A shows a tube-in-tube heat exchanger. [Figure 4B] FIG. 4B is a cross-sectional view of the heat exchanger. [Figure 5] FIG. 5 shows a system for treating corrosive effluent containing aqueous HF at high temperatures. [Figure 6A] Figure 6A shows a SCWO system for the oxidation of PFAS. [Figure 6B] Figure 6B shows a compact SCWO system for the oxidation of PFAS. [Figure 7]FIG. 7 shows the identification of the test reactor used in the examples. [Figure 8] Figure 8 shows the results of SCWO destruction of the PFAS samples. [Figure 9] Figure 9 shows the results of SCWO fracture of PFAS field samples. [Figure 10] FIG. 10 shows a schematic of a SCWO reactor with curved walls in a transpiring permeable wall reactor. [Figure 11] FIG. 11 shows a spray drying system for removing solids. DETAILED DESCRIPTION OF THE INVENTION

[0026] In the present invention, PFAS-contaminated water has its conventional meaning. The source of PFAS-contaminated water can be soil, surface water, or groundwater in PFAS-contaminated areas. These areas can be industrial areas, particularly those where waterproof or non-stick coatings are applied. Another common source of PFAS-contaminated water is around airports or firefighting training areas exposed to aqueous film-forming foam (AFFF). Another source can be storage containers, which are usually accumulated for future destruction or disposal. Non-fluorinated organic compounds are typically present in PFAS-contaminated water, and chlorinated or brominated compounds may be present, especially in AFFF residues.

[0027] Pretreatment Debris and other solids can be removed from PFAS-contaminated water before the PFASs are destroyed. Typically, this can be accomplished by one or more filtration steps. In some embodiments, multiple filtration steps can be performed, removing progressively smaller particles. Filters can be valved so that only one filter or series of filters can be used. For example, one filter or set of filters can be cleaned or replaced while another filter or set of filters continues to operate. Filters can be any type of filter known to filter water, such as bag filters, cartridge filters, metal screens, or sand (preferably silica sand). Alternatively, or in addition, centrifugation can be used to remove solids.

[0028] PFAS-contaminated water can be subjected to softening treatments. These softening treatments can include one or any combination of the following: ion exchange resins, lime softening (aqueous calcium hydroxide solution to precipitate solids); chelating agents (e.g., treatment with EDTA); and reverse osmosis. Reverse osmosis is the preferred pretreatment method. Any pretreatment must consider the capture of PFAS in the pretreatment medium. Alternatively, or in addition, compounds such as organics can be removed by passing the water through hydrophobic clay to remove emulsified oil and granular activated carbon (both of which can absorb PFAS, so this use of adsorbents in pretreatment may require additional processing steps).

[0029] One embodiment of a pretreatment system is shown in Figure 1. PFAS-contaminated water from a source 110 (such as a feed tank) is pumped through a solids separator 112 and then through optional filters 114, 116, and 118. The solids-reduced water 120 can be treated in a softener 122 to remove / replace undesirable counterions (typically CA and Mg). Additional clean water 124 can be added if needed. Alternatively, or in addition, softened water 126 or water 120 can be passed through a reverse osmosis system 128. The softener 122 can be flashed, and the flash water sent to a purifier system 130, which can include granular activated carbon 132, and then discharged or sent to an evaporation holding tank.

[0030] Reverse osmosis (RO) systems can remove or concentrate PFAS from a water stream. PFAS-free (or PFAS-reduced) water passes through a membrane, while the PFAS and salts are sent to a salt solution stream. Implementing a cascade of RO membranes increases the efficiency of PFAS removal and throughput. In some embodiments, RO is utilized to increase the concentration of PFAS by at least 5-fold or at least 10-fold, and in some embodiments, by 5-30-fold or 5-20-fold.

[0031] Electric field separation is another concept that can be applied to water before it enters the reactor, or to the clean effluent, or both. This concept is shown schematically in Figure 2. A constant electric field is applied to the water. For water flowing toward the reactor inlet, the water carrying the PFAS compounds of interest can be concentrated and the problematic species can be removed. The effluent of some reactor processes can contain heavy metal atoms that can be removed by passing the water through an electric field. The water stream can be separated into two streams, three streams, or more (preferably two or three). The electrostatic plates can be easily replaced and cleaned.

[0032] An optional concentration step can be achieved by treating the PFAS-contaminated aqueous mixture with microbubbles to cause the PFAS to rise to the top of the mixture, where they can separate. A detailed description of a preferred concentration method is provided below.

[0033] Salts from aqueous solutions containing PFASs can be removed from solution by spraying them through a small-diameter nozzle into a very large-volume expansion vessel, where the water vaporizes, leaving behind salt-containing stalactites on the nozzle's exterior. The PFAS-containing aqueous solution can be heated and optionally pressurized in a heat exchanger before being forced through a small-diameter needle or other nozzle with a small exit orifice or orifice, causing rapid expansion to a large volume. In a preferred embodiment, the conduit leading to the nozzle is covered and heated by an external stream (which may be a hot recycle stream from the process). The pressure in the expansion vessel is lower, preferably at least five times, more preferably at least ten times lower, than the pressure in the conduit leading to the nozzle. The diameter of the nozzle orifice is preferably 2.5 mm or less, or 1.5 mm or less, and in some embodiments, 0.5 mm or less. The vapor flows downstream and can condense before entering the SCWO reactor. In this way, clogging and / or corrosion of the SCWO reactor can be significantly reduced. Solids that form on the nozzle 302 can be scraped off or fall off the nozzle. Optionally, solids can be removed from the vapor stream. In some embodiments, the invention includes: removal of salts as part of the PFAS destruction process; a salt removal device, preferably part of an SCWO system or device; and / or PFAS-containing salts deposited on a nozzle. The experimental device used to remove salts from an aqueous solution is shown in FIG. 3, which shows a salt stalactite 304. This device and its use are further optional aspects of the invention.

[0034] Concentration of PFAS from Water Using Vacuum Air Flotation (VAF) and Vacuum Enhanced Cyclone Separation (VECS) Given the surfactant properties of PFAS, they tend to partition at the air-liquid interface and concentrate at the air-water interface of surface water bodies. Wave action generates bubbles, forming foam. Laboratory studies have demonstrated that PFAS removal can be achieved by blowing air through PFAS-contaminated water (Meng et al., Chemosphere (2018), “Efficient removal of perfluorooctane sulfonate from aqueous film-forming foam solution by aeration-foam collection.”). In US2014 / 0190896, Dickson explored the use of vigorously mixed ozone to treat industrial waste; the lighter foam produced in this process is sent to a separation chamber by a foam concentrator. Prior art processes have issues related to the large amounts of air bubbling through the water, generating large amounts of foam that are difficult to manage, as well as concerns about the aerosols released.

[0035] In some embodiments of the present invention, the combination of VAF and VECS optimizes removal and overcomes the above-mentioned problems. The principle of VAF is to generate microbubbles (visible as cloudy water) within a treatment cell by injecting water saturated with air (nitrogen and oxygen) into the bottom of a cell maintained under high vacuum conditions. When the air-saturated water enters a low-pressure environment, the water becomes supersaturated with air, causing the air to drop out of solution and create microbubbles. Unlike bubbling air into a tank, these bubbles are so fine that they essentially fill the entire flotation chamber and rise very slowly, allowing for much greater air-PFAS contact than can be achieved through air sparging. Furthermore, a much smaller amount of air is introduced into the cell compared to that injected in sparging applications. This allows the PFAS to "scrub" and gently float to the top of the chamber. The treatment cell preferably has several flotation cells in series, separated by baffles. Forward flow is by gravity from one flotation chamber to the next. The use of series chambers improves treatment efficiency. For example, if each cell achieves 75% removal of PFAS, connecting three chambers in series will achieve 98% removal, and four cells will achieve greater than 99.5% removal.

[0036] Air-saturated water can be produced in a simple container where air is bubbled through the water. The water is preferably drawn from an uncontaminated source, possibly treated effluent. Because the flotation chamber is under vacuum, the air-saturated water is drawn into the flotation chamber, and therefore there is no need to pump this water out.

[0037] Foam from each chamber drains into separate foam collection sections of the tank. The tanks are all part of a single enclosure and are therefore under vacuum. This prevents aerosol release, and any leaks in the tank result in air leaking rather than aerosols. Foam from later chambers tends to be more dilute, allowing for the discharge to be sprayed to break up the foam, which is then recycled to the inlet of the treatment cell. Foam from the first chamber is highly concentrated and can be sucked out through a pipe connected to the same vacuum pump used to create the vacuum for the treatment cell. Reusing the airflow generates a high air flow rate, which creates a high flow velocity that can facilitate foam extraction. The foam-air mixture is then sent to a cyclone air-water separator, still under vacuum. Because bubbles become larger and less stable under vacuum, subjecting them to vacuum promotes their destruction. Within the cyclone air-water separator, high centrifugal forces are generated as the mixture enters the separator against the cylindrical wall, creating a centrifugal effect. The combination of centrifugal force and high vacuum aids in the destruction of the foam and separates the air from the concentrated PFAS solution. Air exits the top of the separator and is drawn into a vacuum pump (e.g., a liquid ring pump). Most of the vacuum pump's discharge is recycled to the treatment cell to recover the foam from the cell. A small amount of air is released. Theoretically, the amount of air released would be very small, similar to the amount of air dissolved in the clean water tank and injected into the bottom of the flotation chamber. This small amount of air is released to the atmosphere, but if deemed necessary, it can be passed through a demister unit to prevent entrained liquid from being carried away. The aqueous solution in the air-water separator and from the demister is pumped out and contained for disposal (e.g., on-site or off-site destruction) or further concentration (e.g., batch flocculation with settling to create sludge). Tests using simple aeration have shown that a concentration factor of 8400 or more can be achieved (Meng et al., 2018). For example, with an influent flow rate of 100 gpm, the amount of waste generated would be 17 gallons per day.Using this process, it can be further concentrated (which could be by one or two orders of magnitude, depending on the influent concentration), further reducing the volume. For example, if the influent contained 1 μg / L (1000 ppt) of PFAS and the solution could be concentrated to achieve 0.1% PFAS (1000 ppm), a concentration factor of 1 million to 1 has been achieved. In the example above, only 52 gallons per year would be produced instead of 17 gallons per day. Because a smaller amount of water can be applied to on-site or off-site destruction methods and on-site settling, the volume of sludge would be much smaller.

[0038] Regarding the cost-effectiveness of this system, consider that no chemicals or media are required and the entire process is primarily driven by a vacuum pump. The vacuum pump recirculates the air needed to draw in the air-saturated water and extract the bubbles from the treatment cell. A small blower is required to blow air into the clean water tank and pump the water out of the treatment cell. The baffled tank can be custom designed and has several baffled sections. The cyclone separator and demister can be commercial units. Once operational, no materials other than electrical machinery are required and little waste is generated. Using an on-site destruction unit with a small amount of concentrated solution can result in zero waste.

[0039] In another aspect of the present invention, the use of dissolved air flotation (DAF) facilitates the removal of PFAS. The principle of DAF is to generate microbubbles (visible as cloudy water) within a treatment cell by first creating pressurized water (usually at a pressure of approximately 75 pounds per square inch gauge [psig]) known as whitewater (saturated with air (nitrogen and oxygen)). When the whitewater enters an atmospheric pressure environment, the water becomes supersaturated with air, causing the air to drop out of solution and create microbubbles. Unlike bubbling air into a tank, these bubbles are very fine, essentially occupy the entire flotation chamber, and rise very slowly, allowing for much greater air-PFAS contact than can be achieved through aeration / air sparging. Furthermore, a very small amount of air is introduced into the cell compared to that injected in sparging applications. This allows the PFAS to "scrub" and gently float to the top of the chamber. The treatment cell preferably has several flotation cells in series, separated by baffles. Forward flow is by gravity, from one flotation chamber to the next. Using chambers in series increases treatment efficiency. For example, if each cell achieves 75% removal of PFAS, connecting three chambers in series will achieve 98% removal, and four cells will achieve >99.5% removal.

[0040] Whitewater can be produced in a pressure vessel where air is bubbled through water, or by introducing air into the outlet of a pump (e.g., a centrifugal pump) using a venturi injector or compressor. The water source is usually recycled exhaust. Whitewater water is then pumped into each flotation chamber.

[0041] The foam from the first chamber flows into the foam collection section of the tank, where it has time to break down into a small, concentrated PFAS solution. This first chamber is called the concentration stage because its purpose is not only to remove PFAS from the forward flow, but also to make the removed foam as concentrated as possible.

[0042] Each subsequent chamber (called a polishing stage) continues skimming from above, collecting the skimmed water and returning it to the concentration stage for further concentration, but these stages also have the ability to function similarly to a concentration stage through operational controls.

[0043] Using whitewater to generate microbubbles instead of aeration / sparging releases very little air from the water surface, resulting in little agitation at the surface. This prevents aerosol release and results in more manageable bubbles. It also reduces energy consumption by eliminating the need for a blower / compressor to generate large amounts of pressurized air.

[0044] For influents more highly contaminated with PFAS, more foam is generated from the initial chamber. If necessary, an optional cyclone separator can be applied, using a vacuum pump to draw the foam from the foam collection container. High air flow rates are generated by reusing the airflow, creating high velocities that facilitate foam extraction. The foam and air mixture is then sent to a cyclone air-water separator, still under vacuum. Because bubbles become larger and less stable under vacuum, placing the foam under vacuum promotes their destruction. Within the cyclone air-water separator, high centrifugal forces are generated as the mixture enters the separator against the cylindrical wall, creating a centrifugal effect. The combination of centrifugal force and high vacuum aids in foam destruction and separates the air from the concentrated PFAS solution. The air exits the top of the separator and is drawn into a vacuum pump (e.g., a liquid ring pump). Most of the vacuum pump discharge is recycled to the foam collection container for foam recovery. A small amount of air is released. Theoretically, the amount of air discharged would be as small as the amount of air entrained as part of the foam. This small amount of air would be vented to the atmosphere, but if deemed necessary, it could be passed through a demister unit to prevent entrained liquids from being carried away. If optional cyclone separation is required, consideration should be given to using a modification of this DAF technology called "PFAS Removal from Water Using Vacuum Air Flotation (VAF) and Vacuum Enhanced Cyclonic Separation (VECS)."

[0045] The concentrated PFAS solution is pumped out and contained for disposal (e.g., on-site or off-site destruction) or further concentration (e.g., evaporation). This liquid volume should be very small, but testing is needed to determine how small. Testing using simple aeration has shown that a concentration factor of 8400 or more can be achieved (Meng et al., 2018). For example, if the influent flow rate is 100 gpm, the volume of waste generated will be 17 gallons per day. This process can be used to further concentrate this (which could be by one or two orders of magnitude, depending on the influent concentration) to further reduce the volume. For example, if the influent contained 1 μg / L (1000 ppt) of PFAS and the solution could be concentrated to achieve 0.1% PFAS (1000 ppm), a concentration factor of 1 million to 1 would be achieved. In the example above, only 52 gallons per year would be produced, rather than 17 gallons per day. Small amounts of water can be applied to on-site or off-site destruction methods and on-site settling, resulting in much less sludge.

[0046] Considering the cost-effectiveness of this system, no chemicals or media are required, and the entire process is primarily driven by a water pump generating a pressure of approximately 75-100 psig. The baffled tank can be custom designed with several baffled sections. Once operational, no materials other than electrical equipment are required, and very little waste is generated. Using an on-site destruction unit with a small amount of concentrated solution can result in zero waste.

[0047] preheat The PFAS-containing water is preferably heated before (usually just before) entering the reactor. Heat from the reactor is used to heat the water entering the reactor. The use of a heat exchanger makes the process more energy efficient and compact, and extends the useful life of the reactor. Tube-in-tube heat exchangers are particularly desirable. The heat exchanger shown in Figure 4 was implemented and tested.

[0048] Supercritical Water Oxidation (SCWO) Start-up requires additional heat, generated by electricity or gas, to heat a heat sink such as a sand bath. If the supplemental fuel in the feed stream is oxidized, little or no external heat needs to be supplied after startup. The fuel can be any organic compound that is readily oxidized under the conditions in the oxidation reactor. Preferred fuels include alcohols such as methanol or isopropyl alcohol, acetone, or any other organic fuel, preferably water-soluble.

[0049] The alkali or alkaline earth element can be added to the oxidation reactor along with the concentrated PFAS solution. Alternatively, the alkali or alkaline earth element can be added to the effluent exiting the oxidation reactor. For example, little or no alkali or alkaline earth element can be added to the PFAS-contaminated aqueous mixture entering the reactor (except for Na, which is exchanged for less desirable counterions in an optional water softening pretreatment), and the effluent can be treated with an NaOH solution to neutralize the solution and convert the HF in the effluent to sodium fluoride and water. The alkali or alkaline earth element (preferably in the form of NaOH) can be added at any time following SCWO, preferably after the solution is no longer supercritical. The resulting basic solution can be recycled to re-neutralize the effluent, preferably in a continuous process where sodium hydroxide is added as needed. The neutralized effluent can be passed through a heat exchanger to heat the PFAS-contaminated aqueous mixture before the contaminated mixture enters the oxidation reactor; and the relatively cool effluent can then be recycled, released into the environment, or subjected to additional treatment on-site or at a water treatment facility.

[0050] Per- and polyfluoroalkyl substances (PFASs) are destroyed and converted to carbonates, fluoride salts, and sulfates. The destruction or conversion of PFASs is achieved through the synergistic effects of temperature, pressure, water, oxidizer, and, optionally, the addition of alkali or alkaline earth elements (or mixtures thereof). Alkali or alkaline earth elements can be added to the reactor feed or, preferably, the effluent to remove HF. Compounds such as CsOH, NaOH, KOH, or LiOH can be added to produce CsF, NaF, KF, or LiF, respectively. Ca(OH)2. A quench can be added at the end of the oxidation process to remove HF and form NaF or CaF2 (for example). This process is essentially the combustion of organic molecules in water, minus 1) the flame and 2) associated environmental pollutants that are harmful to the environment. The most notable products of the oxidation reaction are environmentally friendly carbon dioxide and water. Fluorides (such as CaF2) can be removed from the water and separated for further processing as needed. The process water can be recycled into the system.

[0051] One preferred system according to the present invention is designed to destroy PFAS at rates of at least 100 kg or at least 200 kg / hour of PFAS, and in some embodiments, in the range of 100-500 kg / hour of PFAS. Parallel units can be used to meet throughput requirements and schedules. Furthermore, tandem SCWOs can be used to increase the reactor cross-sectional area and improve throughput. This system can incorporate features such as: 1) multiple injection reaction points; 2) spiral reaction paths implemented using magnetic fields; and 3) "afterburner" flame subtraction. Conventional SCWOs have only a single head injection point for the PFAS-loaded / organic-loaded influent. The SCWO's performance can be turbo-boosted by using multiple injection points, thereby increasing the system's operational throughput by orders of magnitude. Multiple reactive species are generated within the reaction chamber, which, under the influence of a magnetic field, can be focused or diffused, depending on the need to amplify the interaction frequency or reaction cross-section. The afterburner configuration of the SCWO allows for further processing of the effluent before cooling if additional destruction efficiency is required.

[0052] The equipment can be designed for 1) fixed-location use or 2) transport to a site. Fixed-location configurations can be used in permanent treatment plants, such as permanently installed water utilities such as municipal water treatment systems. Portable units can be used in areas with low load requirements where temporary structures are appropriate. Portable units are sized to be transported in smaller enclosed spaces such as semi-trucks or trailers or shipping containers.

[0053] This design can be adapted to treat other organic contaminants by modifying the operating parameters without changing the equipment.

[0054] Passivation of the internal surfaces of SCWO reactors The interior surface of the reactor can be coated with a corrosion-resistant material (e.g., platinum aluminide, BC (boron carbide), SiC (silicon carbide), TaC (tantalum carbide), WC (tungsten carbide), a metal fluoride such as YF3 (yttrium fluoride), YN (yttrium nitride), LaF3 (lanthanum fluoride), LaN (lanthanum nitride), YbN, YbF3, or any lanthanide nitride or lanthanide fluoride, HfN (hafnium nitride), CeN (cerium nitride), CeF3 (cerium fluoride), TaN (tantalum nitride), Ta (tantalum), TaF (tantalum nitride), ZrN (zirconium nitride), ZrF (zirconium nitride), WC (tungsten carbide), WN (tungsten nitride), or a combination thereof).

[0055] Alternatively, or in addition, corrosion can be reduced by using sacrificial electrodes or impressed current cathodic protection.

[0056] Destruction in continuous or batch reactors SCWO systems operate by increasing the feed temperature and increasing the feed pressure. The pressure increase can be due to heating alone (which is preferred) or can be further increased via a compressor or high-pressure (reciprocating) pump. The temperature is increased by the application of heat via a conduit (for continuous reactors) or a reaction chamber (for batch reactors) and / or the addition of a fuel, such as an alcohol, which is oxidized to generate heat in the solution. Supercritical conditions are maintained for the oxidation; conditions within the reaction conduit or chamber are preferably in the range of 374°C to 1200°C and at least 220 bar, more preferably 221 to 300 bar. In some embodiments, the temperature within the SCWO reactor is maintained at 500°C or above, or 600°C or above, and in the range of 500 or 600 to 700 or 800°C.

[0057] Dealing with fluorine by-products from the destruction of PFAS A critical step in the process of the present invention is the handling of fluorine: HF (hydrofluoric acid) is corrosive and toxic and cannot be released into the environment.

[0058] The predicted complete oxidation reaction of PFOA is as follows:

number

[0059] An alternative or additional reaction is to add CsOH+NaOH or KOH+LiOH to the input to balance the ions and avoid HF.

[0060] CsF, NaF, KF, and LiF are more environmentally friendly salts than HF. example:

number

[0061] These salt combinations were carefully chosen because they have eutectic points lower than the reactor temperature, so the fluid moves as a molten material rather than a solid (the undesirable physical state of the fluid). For example: CsF-NaF, 23.5 mol% NaF in total salt mixture - eutectic point 609℃ KF-LiF, 50 mol% KF for the total salt mixture - eutectic point 485°C

[0062] These salts are insoluble in supercritical water but soluble in liquid water. The solid particles can foul and clog the reactor.

[0063] A corrosive effluent containing an aqueous HF solution at high temperature (e.g., about 700°C) can flow into a mixing pipe. Cooling water, typically containing hydroxy salts, can be fed into the mixing pipe to mix with the corrosive effluent. The cooled effluent contains dissolved fluoride salts, such as NaF. This system is shown in Figure 5.

[0064] A preferred SCWO reactor design is a continuous or semi-continuous system in which a (typically pretreated) PFAS-containing aqueous solution is fed to the SCWO reactor. In some embodiments, the aqueous solution is separated into a PFAS-enriched fraction and a PFAS-depleted fraction. The PFAS-depleted fraction is heated and combined with the PFAS-depleted fraction (either the same PFAS-depleted fraction, in which case the fractions are recombined), or another PFAS-depleted fraction, before entering the SCWO reactor. Reducing the time and amount of heating of the PFAS-contaminated solution (which may contain salts) can reduce corrosion, which may extend equipment life, reduce contaminants such as heavy metals in the effluent, and reduce clogging of the SCWO reactor.

[0065] As solids form in the SCWO reactor, it is desirable to tilt the reactor downward so that gravity pulls the solids downward and out of the reactor. In some embodiments, the flow path is straight and perpendicular (0°) to gravity; in some embodiments, the reactor is tilted relative to gravity, for example, at an angle ranging from 5 to 70° (relative to vertical), or 10 to 50°, or 10 to 30°, or 10 to 20°, and can be bent to reverse the flow direction to provide a compact device with consistently downward flow relative to gravity. Preferably, the reactor is a cylindrical pipe formed of a corrosion-resistant material. Desirably, the pipe has an internal diameter of at least 1 cm, preferably at least 2 cm, and in some embodiments, up to about 5 cm.

[0066] Flow through the components of the SCWO unit under supercritical conditions must be turbulent (Re at least 2000, preferably in the range of 2500-6000). The effluent from the SCWO reactor can enter the salt separator under supercritical conditions.

[0067] A preferred system 400 is shown in Figure 6A. PFAS-contaminated water (feedstock) 402 enters a mixing tee 404. The PFAS-contaminated water enters the tee at ambient conditions (room temperature) or a relatively low temperature to prevent corrosion of the inlet lines. In the mixing tee, the PFAS-contaminated water 402 is mixed with clean water 406 at a high temperature, which may be, for example, 650°C. The combined streams are brought to a combined temperature. Oxidant 426 can be in either stream 402 or 406, but is preferably in 406 to prevent premature reaction. Preferably, fuel (if present) is in the oxidizer-free stream. For start-up, the combined stream 408 passes through an SCWO reactor 410, which is preferably a vertical (gravity-dependent) tube surrounded by insulation and heating means. The temperature in the SCWO reactor is preferably in the range of 500-700°C. Effluent 412 enters salt separator 414, where salts 416 can be removed manually or through exhaust system 418, and exits the system as salt solution 420. Salt 416 can be a mixture of sodium chloride, sodium fluoride, sodium sulfate, sodium nitrate, or the corresponding salts of other alkali or alkaline earth elements. Supernatant 422 exits the separator and is sent through heat exchanger 424 (preferably oriented as shown so that the temperature of fluid 422 is highest where clean water 406 exits the heat exchanger). After passing through the heat exchanger, clean water stream 406 can optionally be passed through heater 430 to heat the water, preferably to above 600°C. After passing through the heat exchanger, effluent 432 exits the system and can optionally be neutralized any time after exiting the SCWO reactor. All or a portion of the effluent can be recycled back into the system and / or released to the environment or a water treatment facility.

[0068] An alternative reactor configuration is shown in Figure 6B; this is similar to the reactor in Figure 6A, except that the reactor is tilted relative to gravity.

[0069] oxidizing agent Two tested sources of reactant oxygen used in supercritical water oxidation to destroy PFAS are oxygen gas (O2) and hydrogen peroxide (H2O2). In addition to or instead of these two species, other reactant oxygen sources or oxidants can be added to destroy PFAS in the oxidation reactor. Other oxidants are oxyanion species and peroxyacids. These include ferrate (FeO4 2- ), percarbonates (e.g., C2K2O6), permanganates (e.g., KMnO4), potassium peroxymonosulfate (commercially known as Oxone), peroxybenzoic acid, and ozone (O3).

[0070] The oxyanion species is ferrate (FeO4 2- ), percarbonate (CO4 2- ), permanganate (e.g. MnO4 - Oxyanionic species are a category of chemicals that include, but are not limited to, oxyanionic species, oxones, and oxones. A general tendency of these species is that the rate of oxygen release increases above ambient conditions. The present system can operate at high temperatures and pressures to achieve supercritical water conditions. Under these same conditions, oxyanionic species readily decompose to release oxygen. Therefore, oxyanionic species can be incorporated as a reactant oxygen source; however, adding elements such as Mn can result in the formation of additional solids that clog the reactor.

[0071] Peroxyacids and other peroxyacids are generally considered oxidizing agents. Historically, they have been used primarily in the oxidation of alkenes to epoxides, Baeyer-Villiger oxidation of ketones to esters and lactones, and oxidation of heteroatoms to oxides (amines to amine oxides, sulfides to sulfoxides and sulfones, selenides to selenoxides, and phosphines to phosphine oxides). In the present invention, peroxyacids can be oxidizing agents for the destruction of PFAS.

[0072] Ozone is unstable in neutral aqueous solutions and typically decomposes into O and other species. Thus, ozone is a potential source of reactant oxygen for the process of the present invention.

[0073] A preferred oxidizing agent is hydrogen peroxide, which can be added in excess (e.g., at least 50% or at least 100% excess or in the range of 50% to 300% excess) and reacts to form dioxygen and water.

[0074] fuel 0.5-5, 1-3, or about 2 wt% organic fuel (e.g., isopropyl alcohol) burns with enough energy to heat the reactor to 450-600°C. At startup, it must be heated (electrically) from 20°C to 450°C. The IPA is then ignited, combusted, and heats the fluid in the reactor to 600°C (the target reactor temperature). Once this is done, heat from the reactor is used to preheat the fluid (instead of an electric sand bath) so that the reaction proceeds isothermally at steady state. However, a sand bath or other heat sink can be used as a heat exchanger to heat the preheat tube from the reactor coil.

[0075] Fuel addition operates at any scale and flow rate you choose. For example, if you need to treat 100 gallons of PFAS-contaminated water with 2.54 gallons of isopropyl alcohol (density = 0.785 g / mL), you can treat it with 2540 gallons of IPA if you have 100k gallons available. Alternatively, you can calculate the amount of fuel based on the heat required. For example, the oxidation of IPA generates 1912 kJ / mol, and IPA has a molecular weight of 60.1 g / mol, so if you need approximately 20 mg / mL of PFAS water, you will need 0.64 kJ / mL of PFAS solution.

[0076] They also calculated that if the PFAS solution contained 7 wt% PFAS (which is normally not feasible as it far exceeds the solubility limit), it could be run isothermally at steady state. The breakdown would be exothermic, consistent with the heat required to heat the fluid.

[0077] Destruction by the synergistic action of high temperature, high pressure, and hydrogen peroxide The PFAS in water is mixed with hydrogen peroxide and calcium oxide. The solution is mixed and placed in a pressure vessel maintained at elevated temperature (e.g., at least 250°F or at least 300°F, in some embodiments ranging from 250 or 300°F to 500°F) and at a pressure of at least 2 atmospheres, preferably at least 3 atmospheres, and pressurized, for example, with nitrogen to 5 atmospheres, for 3 hours. The precipitate formed in the reaction vessel can be separated from the supernatant liquid. PFASs have very low reactivity. Surprisingly, it has been discovered that by reacting PFASs with a combination of aqueous hydrogen peroxide and an alkali or alkaline earth element (or the high temperatures and pressures described above), greater than 99% destruction can be achieved under relatively mild conditions. Calcium oxide is added to the reaction mixture to suppress the formation of HF, which results in the formation of calcium fluoride. Thus, the present invention involves reacting PFASs with an aqueous solution of hydrogen peroxide and calcium oxide at a temperature of at least 100°C, preferably in the range of 120-200°C, or 130-170°C, or 140-160°C. Preferably, at least 90%, or at least 95%, or at least 99% of the PFASs are destroyed by this method.

[0078] Additional Terms Any of the processes of the present invention can be characterized by one or any combination of the following: a temperature profile that heats the oxidation reactor to a temperature of at least 450°C to ignite the fuel being fed; in some preferred embodiments, the PFAS-containing solution is mixed with a solution containing 30-50 wt% HO, preferably at a weight ratio of 30:1 to 70:1 (weight percent ratio), or in particularly preferred embodiments, at a weight ratio of about 50:1 PFAS solution:HO; in some embodiments, the PFAS-containing solution is passed through the SCWO reactor for a residence time of 20 seconds or less, preferably 10 seconds, or 5 seconds or less, or 0.5 to 5 seconds; in reactors where PFAS is destroyed under supercritical conditions, the reactor volume is based on the volume containing the supercritical fluid conditions; a preferred reactor configuration is a continuous plug flow reactor. In some tests, the concentrated PFAS feed is delivered to the oxidation reactor at a rate of about 50 mL / min; in some embodiments, the rate is controlled between 50 and 150 mL / min (standard conditions); this rate can be adjusted to achieve the desired conditions. The feed can include fuel and oxidizer. Preferably, no external heating is required after start-up. In some embodiments, the PFAS-containing aqueous mixture (preferably after concentration pretreatment) contains at least 100 ppm PFOA, and the method reduces the PFOA concentration to at least 10 ppm. 6 or 10 7 or 10 8 and in some embodiments up to about 10 9 To decrease.

[0079] Any of these conditions may be utilized or acquired by the mobile unit.

[0080] Supercritical oxidation treatment of PFAS-contaminated solids An input unsorted solid sample (e.g., soil) with unknown particle size distribution is subjected to the fluidizing pressure of a variable flow SCWO. Controlling the flow allows for particle density inversion and separation of the soil into its granular components (sand, silt, clay, organic matter). This fluidizing "bed" causes rapid percolation of the SCW through the solid sample, sorting it by its size distribution based on the pressure. The chamber "ceiling" becomes a conventional percolation barrier for the SCW, preventing salt accumulation and allowing further percolation of the SCW by the solids.

[0081] Curving the "floor" and "ceiling" regions creates a natural pressure gradient along the angle, naturally reducing fluidization pressure and gravity without complex pressure regulation. As particles drift through the chamber, they prefer the angled location where they fluidize past the reversal point, naturally separating the soil from the salt and exhaust gases. Larger particles are exposed to longer processing times while the gases and salts are removed. The washed soil exits during the pressure cycle. The chamber exhaust can collect salts and exhaust gases in the salt solution chamber, similar to a conventional TWSCW reactor. This is shown in Figure 10, along with a permeable wall, showing the preheated PFAS feedstock entering the reactor, the gases and solution exiting at the top, and the solids exiting at the bottom.

[0082] SCWO is currently only performed on a large scale for aqueous samples or colloids suspended in a carrier liquid. This approach naturally processes solids without pre-sorting or water suspension. This design also allows for rapid scaling and parallelization. Because the SCWO reactor is exothermic, the entire process is thermally and electrically self-sustaining after initial startup. This reactor design applies to spent activated carbon, chemically contaminated rock, municipal solid waste treatment, and soil samples.

[0083] Other solutions include traditional permeable wall reactors, tubular reactors, countercurrent tanks, or film-cooled reactors. Permeable wall reactors can be useful for SCWO destruction of PFAS, especially solids. The permeable walls prevent prolonged contact of salts and acids with the reactor walls while turbulent flow carries away the solids.

[0084] The application of fluidization eliminates many of the problems of SCWO scaling. First, soil samples can be processed at high volumes and rates (e.g., at least 10 kg / hr or at least 50 kg / hr, and in some embodiments, up to approximately 500 kg / hr or 300 kg / hr) without prior sorting or pretreatment. Second, fluidizing SCW significantly increases particle penetration, preferentially penetrating smaller particle sizes over larger ones. This improves destruction rates because contaminants such as PFAS prefer to bind to smaller soil particles. Finally, the use of a curved design eliminates hydraulic complexity from the variable fluidization design, making the system more robust. This also offers significant opportunities for further innovation, such as intentionally creating turbulence within the reactor volume through time-varying fluidization pressure.

[0085] Mobile Unit An example of a mobile unit is one that can be transported (and preferably operated within) a trailer. For example, the system can be transported (and optionally operated) on a trailer measuring no more than 29 feet (8.8 m) in length, no more than 8 feet 6 inches (2.6 m) in width, and no more than 13 feet 6 inches (4.1 m) in height. While these dimensions define the preferred size of the mobile system, workers in this area will understand that other dimensions are available for the mobile unit.

[0086] Post-processing Because the oxidation process destroys essentially all PFAS, the treated effluent can be safely released into the environment. In some embodiments, at least a portion of the effluent evaporates into the air. The PFASs are destroyed, and the remaining contaminants (metals, NaF, etc.) are safe because they tend to have very high vapor pressures and do not evaporate with the water. The effluent can be subjected to treatments such as reverse osmosis and / or other processes (e.g., Metsorb™) to remove metals or other contaminants before the effluent is released or disposed of.

[0087] Precipitates such as fluoride salts can be filtered or centrifuged from the effluent. The PFAS-free effluent can be passed through a heat exchanger, where the effluent is cooled by PFAS-contaminated water flowing into a reactor. The effluent can be flashed and / or evaporated. Other techniques, such as RO, chelation, and / or passage through adsorbents, can be used to remove impurities such as metals from the effluent.

[0088] As shown in Figure 11, solids can be removed by spray drying. The effluent can be sprayed into a stream of hot air. The water quickly evaporates from the droplets, leaving behind dry solid particles. The solids can be collected in a cyclone and / or filter. The clean effluent can be released into the air and / or the water can be condensed. [Example]

[0089] A number of tests were conducted using the SCWO apparatus shown schematically in Figure 7. The reactor was immersed in a bed of hot sand. As the solution passed through the piping, the temperature rose to 700°C (supercritical conditions were established at approximately 374°C). The oxidant solution (containing either O2 or H2O2) was mixed with the PFAS-containing stream in a T-joint before passing through the high-temperature reactor. The cleaned effluent then passed through the reactor and out.

[0090] Tests of the SCWO process using an aqueous solution containing 1700 ppm PFOA resulted in emissions containing less than 5 ppt PFOA.

[0091] A trial run (single pass through the SCWO reactor) was performed using a sample prepared to simulate AFFF concentrations (386,000,000 ppt of PFAS). After SCWO destruction, six of the eight PFAS analytes were reduced to less than 5 ppt, with an overall destruction efficiency of greater than 99.999%, as shown in Figure 8.

[0092] A field sample containing 8,600 ppt of PFAS was passed through the reactor, reducing all 12 PFAS compounds to less than 5 ppt, resulting in an overall destruction of 99.95%. The before and after concentrations are shown in Figure 9.

Claims

1. 1. A method for destroying PFAS in a PFAS-containing aqueous mixture, comprising reacting the PFAS with an aqueous oxidizing agent under supercritical conditions to produce an effluent, and further comprising treating the effluent from the step of reacting with the aqueous oxidizing agent with an alkali or alkaline earth element to convert the PFAS to an aqueous product mixture comprising carbon dioxide and an alkali or alkaline earth fluoride.

2. 10. The method of claim 1, wherein the step of reacting the PFAS with the aqueous oxidizer is conducted under supercritical conditions and in a SCWO reactor having a residence time of 20 seconds or less.

3. 3. The method of claim 2, wherein the PFAS-containing aqueous mixture contains at least 100 ppm PFOS and the effluent contains 1 ppm to 5 ppt PFOS.

4. 3. The method of claim 2, wherein the PFAS-containing aqueous mixture comprises at least 100 ppt of at least one compound selected from the group consisting of PFBA, PFHxA, PFHpA, PFOA, 6:2FTS, and 8:2FTS, and wherein the method reduces the concentration of the at least one compound to 5 ppt or less.

5. 3. The method of claim 2, wherein an organic fuel is added to the PFAS-containing aqueous mixture prior to the reacting step.

6. The method of claim 5 , wherein the organic fuel comprises alcohol or acetone.

7. 6. The method of claim 5, wherein after adding the organic fuel, the PFAS-containing aqueous mixture comprises 0.5 to 5 wt. % organic fuel.

8. 3. The method of claim 2, wherein the flow during the supercritical oxidation step is consistently downward with respect to gravity.

9. 3. The method of claim 2, further comprising pretreating the PFAS-containing aqueous mixture to separate salts.

10. The method of claim 1 , wherein the oxidizing agent comprises hydrogen peroxide.

11. The PFAS-containing aqueous mixture contains at least 100 ppm of PFOA, and the method destroys PFOA, reducing the PFOA concentration in the effluent to at least 10 6 2. The method of claim 1, wherein the amount of hydroxylase is reduced by 2 times.

12. The method of claim 1 performed in a mobile trailer.

13. A method for destroying PFAS in a PFAS-containing aqueous mixture, comprising reacting the PFAS with an aqueous oxidant under supercritical conditions to produce an effluent, and further comprising treating the effluent from the step of reacting with the aqueous oxidant with an alkali or alkaline earth element to convert the PFAS to a product mixture comprising carbon dioxide and an alkali or alkaline earth fluoride; 1. A method comprising, prior to the step of reacting PFAS with an oxidizing agent, supersaturating the PFAS-containing aqueous mixture with air to form a supersaturated PFAS-containing aqueous mixture, delivering the supersaturated PFAS-containing aqueous mixture to a tank, and reducing the pressure in the tank to generate air bubbles in the aqueous mixture and produce a foamed mixture.

14. 14. The method of claim 13, wherein treating the effluent with an alkali or alkaline earth element comprises treating the effluent from reacting with an aqueous oxidant with NaOH, LiOH, or KOH.

15. 15. The method of claim 14, wherein the PFAS-containing aqueous mixture contains at least 100 ppm PFOA and the effluent contains 1 ppm to 5 ppt PFOA.

16. 14. The method of claim 13, wherein the PFAS-containing aqueous mixture is obtained from the foamed mixture.

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