Patent Application
20230228722
Supercritical water oxidation (SCWO) is a process where oxidative reaction mechanisms are used to mineralize a target reagent within the supercritical phase of water, which is defined as temperatures and pressures exceeding 374° C. and 22.1 MPa, respectively. The oxidative reaction mechanisms are primarily driven by OH and HO2 radicals, generated through the addition of O2, air, H2O2, or another radical source. The SCWO process has been shown to rapidly oxidize and mineralize various reagents, such as biomass, sewage sludge, chemical warfare agent hydrolysate, pesticides, various industrial effluents, and other recalcitrant molecules. Notably, SCWO can be used to completely mineralize environmental contaminants, such as per- and polyfluoroalkyl substances (PFAS), per- and polyfluorocarbons (PFCs), pesticides, munitions, 1,4-dioxane, pharmaceuticals, microplastics, and others.
Analytical methods for detecting PFAS or PFCs in the environment are of particular interest, as contamination of groundwater and drinking water with PFAS is an emerging and urgent water security crisis. PFAS have been extensively used since the 1940s due to their chemical stability and distinct thermophysical properties. Widespread use of PFAS in manufacturing processes, consumer goods, and firefighting foams has directly contributed to contamination of soils and municipal drinking water supplies at concentrations significantly exceeding recommended exposure levels. Use of aqueous film-forming foams (AFFF) for firefighting training has particularly contributed to the water contamination problem. PFAS show high toxicity, even in the low part-per-trillion range, and the stability of the C—F bond means that PFAS are perpetually stable in the environment. Notable sites of PFAS contamination include military bases, airports, and locations near chemical manufacturing plants. Due to the stability of the PFAS molecule, and lack of destructive treatment options, PFAS are often found in high concentrations within such matrices as municipal sewage sludge, compost, livestock, and common food items.
Effective remediation of PFAS commonly involves “pump-and-treat” of contaminated water supplies or soil washing. Contaminated water passes through an industrial filtration system (e.g. reverse osmosis), producing clean water and a PFAS-rich brine. End-of-life destruction of the hyper-concentrated PFAS brine requires energy-intensive processing, as does pumping and filtration.
Current remediation methods for PFAS-impacted sites primarily include (a) filtration of PFAS from drinking water or (b) in situ fixation of PFAS in contaminated soils. The most commonly used filtration technologies include reverse osmosis (RO), or PFAS adsorption with granulated activated carbon (GAC) or ion exchange resins (IXR). All filtration and fixation technologies serve to capture PFAS molecules, but do not destroy PFAS molecules. End-of-life disposal with complete defluorination (cleaving the C—F bonds) is needed to eliminate the risk of subsequent environmental re-contamination, or future liability.
Due to widespread PFAS contamination of soils and aquifers, coupled with emerging regulations, there is a growing need for analysis methods to quantify target PFAS molecules and overall PFAS in a variety of matrices, including but not limited to: groundwater, drinking water, soil, tissue, blood, sewage sludge, compost, and landfill leachate. The typical analysis method for quantifying PFAS molecule concentrations (such as PFOS or PFOA) is liquid-chromatography, mass spectrometry (LC-MS/MS). Analytical standards and methods exist for PFAS analysis via LC-MS/MS, however, only ˜50 of the >5000 known PFAS have acceptable analytical standards and methods. Methods for attempting to quantify the total quantity of PFAS in a sample include adsorbable organofluorine (AOF) and extractable organofluorine (EOF) where a sorbent (such as ion resin) or solvent are used to isolate fluorocarbons for subsequent LC-MS/MS analysis. The total oxidizable precursor (TOP) assay is a method for indirectly estimating total PFAS content, through partially oxidizing PFAS molecules to key intermediate products, which can subsequently be analyzed by LC-MS/MS. Several recent studies have indicated that LC-MS/MS is inadequate to measure all organofluorine contaminants at sites with AFFF contamination.
The current method for quantifying total organofluorine (TOF) is combustion ion chromatography (CIC) where wet air oxidation (typically at temperatures between 900 and 1000° C.) is followed by ion chromatography. The oxidation step serves to convert organofluorine to gaseous HF, which is subsequently adsorbed and/or collected and introduced to a liquid matrix for analysis via ion chromatograph.
There is, however, significant skepticism that combustion ion chromatography can accurately measure total organic fluorine from PFAS-laden samples, with the current efforts pointing to TOF CIC not working well to measure fluorine from PFAS. Therefore, there remains a need for a method and device that can accurately and reliably measure TOF, especially with PFAS.
The present invention is an apparatus and method for determining the total concentration or content of halogens in a liquid or solid sample, by using supercritical water oxidation (SCWO) or hydrothermal alkaline treatment (HALT). In one embodiment, the reaction conditions for SCWO are supercritical phase water conditions. In one embodiment, the reaction conditions for hydrothermal alkaline treatment for HALT are sub-critical phase water conditions. The herein described methods and apparatus can also be used to quantify the total content of any halogens, metals, or other elements which are not converted to gaseous products during the SCWO process.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
Referring to
In block 104, the method includes mineralization of the sample at high temperatures and pressures, such as supercritical water conditions. In one embodiment, supercritical water conditions may include a pressure greater than or equal to 22.1 MPa and a temperature of greater than or equal to 374° C. From block 104, the method enters block 106.
In block 106, the method includes cooling and expansion of the reaction products to ambient or near-ambient temperatures and pressures. In one embodiment, the produced ion are collected in an aqueous liquid product. The method includes collection of the liquid and solid reaction products in any suitable collection vessel. From block 106, the method enters block 108.
In block 108, the method includes the introduction of the collected reaction products to a suitable analytical instrument for analysis and quantification of the halogen of interest. In one embodiment, the products may be introduced to the analytical instrument through directly coupling the analytical instrument to the collection vessel or to the reactor vessel, for example, to inject the aqueous product directly to an ion chromatograph. In one embodiment, the method may include pH buffering of the sample to a pH amenable for introducing to the analytical instrument.
The methods of
In one embodiment of the method of
In one embodiment of the first method, the method includes contacting the sample in the reactor at a temperature in a range of 500° C. to 750° C. In one embodiment of the first method, the method includes contacting the sample in the reactor at a temperature in a range of 500° C. to 650° C.
In one embodiment of the first method, the method includes contacting the sample in the reactor at a pressure sufficient to maintain a supercritical phase of water.
In one embodiment of the first method, the temperature and/or pressure are sufficient to fully liberate the halogen atoms from the sample.
In one embodiment of the first method, the sample is a solid sample (e.g. sewage, soil, compost, tissue).
In one embodiment of the first method, the sample is a liquid sample (e.g. landfill leachate, groundwater, drinking water, blood).
In one embodiment of the first method, the analyzer is an ion chromatograph or an ion selective electrode.
In one embodiment of the first method, the concentration of F− ions is determined. In one embodiment of the first method, the analyzer is coupled to the collection zone to receive the cooled product.
In one embodiment of the method of
In one embodiment of the second method, the method includes contacting the sample in the reactor at a temperature less 374° C. and at a pressure sufficiently high to maintain a liquid phase of water.
In one embodiment of the second method, the alkaline amendment is selected from NaOH, KOH, LiOH, NH4OH, or a combination.
In one embodiment of the second method, the sample is a solid sample (e.g. sewage, soil, compost, tissue).
In one embodiment of the second method, the sample is a liquid sample (e.g. landfill leachate, groundwater, drinking water, blood).
In one embodiment of the second method, the analyzer is an ion chromatograph or an ion selective electrode.
In one embodiment of the second method, the analyzer is coupled to the reactor with a conduit and the method comprises transferring the product from the reactor to the analyzer after the cooling step.
In one embodiment of the second method, the method further comprises allowing the product to cool in the reactor, taking the cooled product out of the reactor, and analyzing the product.
In one embodiment of the second method, the sample includes per- and/or polyfluoroalkyl substances (PFAS).
In one embodiment of the second method, the method includes contacting the sample in the reactor at a temperature in a range of 250° C. to 374° C.
Referring to
In one embodiment, the reactor vessel 202 is constructed from a nickel-base alloy for corrosion resistance (e.g., Inconel 625, Hastelloy C-276), although other materials are also possible, including but not limited to: Inconel 600 and stainless steel 316.
In one embodiment as shown in
In one embodiment, an inductive heater, electric furnace, heated sand bath, or electric resistive heater 214 is used to generate supercritical phase water temperatures (>374° C.) and temperatures necessary for analysis (>300° C.) within the reactor vessel 202, although other heating methods may be used. In one embodiment, the heater 214 is used for generating sub-critical phase water temperatures (<374° C.)
In one embodiment, a pressure gauge or pressure transducer 216 monitors the internal pressure in the reactor vessel 202, and a thermocouple 218 measures the internal temperature in the reactor vessel, although both pressure and temperature sensors are optional. The signals from the pressure and temperature sensors may be used to control the heater 214 using a controller.
In one embodiment, the apparatus may include a pressure relief valve or rupture disc, and additional thermocouples to monitor temperature in locations such as the collection vessel, although both are optional.
In one embodiment, the apparatus 200 may be used as a “batch” reactor for both the SCWO and HALT processes. In one embodiment, the apparatus 200 may be used as a “continuous” reactor for both the SCWO and HALT processes.
In one embodiment of the disclosed methods, a solid sample (e.g. sewage, soil, compost, tissue) of known mass is loaded into a fixed volume of deionized (DI) water and H2O2 in a supercritical water oxidation reactor and completely mineralized, after which the liquid product is introduced to an ion chromatograph for analysis. As used herein, “mineralized” means fully decomposed and/or oxidized to the simplest molecular state.
In another embodiment of the disclosed methods, a liquid sample (e.g. landfill leachate, groundwater, drinking water, blood) of known mass is loaded into a fixed volume of deionized (DI) water and H2O2 in a supercritical water oxidation reactor and completely mineralized, after which the liquid product is introduced to an ion chromatograph for analysis.
In another embodiment of the disclosed methods, a liquid or solid sample of known mass is loaded into a fixed volume of deionized (DI) water with NaOH amendment (optimally >5 M-NaOH) and reacted sufficiently to cleave halogens from parent compounds, after which the product is introduced to an IC for analysis.
In varying embodiments of the disclosed methods, the liquid product can be analyzed for halogens, including, but not limited to: fluoride, chloride, bromide, iodide, and astatide via analysis methods including, but not limited to: ion chromatography, ion selective electrode.
In varying embodiments of the disclosed methods, pressures sufficient to keep the sample in the liquid (subcritical) or supercritical state (>22.1 MPa) are generated autogenically through expansion of the water at high temperatures, although a head of gas which is pressurized to an initial set pressure, or a high pressure pump or compressor may also be used to generate supercritical pressures.
In embodiments of SCWO processes, the oxidant source can include aqueous H2O2, although other oxidants may also be used, including but not limited to: compressed O2, compressed air. Known quantities of the sample and H2O2 are loaded manually through a hand valve, for example, but may also be introduced to the reactor vessel by a pump or multiple pumps.
In one embodiment, referring to
In one embodiment of the first apparatus, the sample is contacted with an oxidant.
In one embodiment of the first apparatus, the reactor comprises one or more of second inlets 222.
In one embodiment of the first apparatus, the one or more of second inlets are connected to an oxidant supply, a water supply, or both.
In one embodiment of the first apparatus, the collection zone further comprises a cooling zone 208 configured to reduce the temperature of the product.
In one embodiment of the first apparatus, the cooling zone 208 comprises a heat exchanger in substantial thermal contact with the collection zone (e.g., a cooling coil).
In one embodiment of the first apparatus, the collection zone further comprises a pressure regulator 230 configured to reduce the pressure in the collection zone.
In one embodiment of the first apparatus, the pressure is reduced through a capillary tube.
In one embodiment of the first apparatus, the pressure regulator comprises an expansion valve.
In one embodiment of the first apparatus, the collection zone further comprises a filter 232 (e.g., a size exclusion filter configured to remove solids from the product).
In one embodiment of the first apparatus, the collection zone further comprises a second inlet 234, for introducing a solution serving as a pH buffer or a total ionic strength adjustment buffer (TISAB).
In one embodiment of the first apparatus, the analyzer comprises a detector for halogen ions including one or more of F−, Cl−, Br−, I−, or At−.
In one embodiment of the first apparatus, the analyzer comprises an ion chromatograph (IC) or an ion selective electrode (ISE).
In one embodiment of the first apparatus, the concentration of F− ions is used to determine the total quantity of elemental fluorine in the sample.
In one embodiment of the first apparatus, the reactor is maintained at a temperature in a range of 500° C. to 750° C.
In one embodiment of the first apparatus, the reactor is maintained at a pressure sufficient to maintain a supercritical phase of water
In one embodiment of the first apparatus, the sample is a solid sample (e.g. sewage, soil, compost, tissue).
In one embodiment of the first apparatus, the sample is a liquid sample (e.g. landfill leachate, groundwater, drinking water, blood).
In one embodiment, referring to
In one embodiment of the second apparatus, the alkaline amendment is selected from NaOH, KOH, LiOH, NH4OH, or a combination.
In one embodiment of the second apparatus, the reactor further comprises a controller to maintain an internal reactor temperature in a range of 250° C. to 374° C.
In one embodiment of the second apparatus, a fluoride ion selective electrode or ion chromatography analyzer is coupled to the reactor to receive a cooled product after reaction of the reactants.
In one embodiment, the apparatus 900 can be referred to as a “continuous” reactor, and used for SCWO and HALT processes.
Reagents
Reagents are prepared with aqueous perfluorooctanesulfonic acid (˜40 wt %, Sigma Aldrich), deionized (DI) water (ρ=18.2 MΩ-cm) and aqueous H2O2 (30 wt %, Fisher Scientific). NaF (>99%, Sigma Aldrich) is used to test fluoride recovery from the batch system. The reagent consists of perfluorooctanesulfonic acid (PFOS) and H2O2 premixed with DI water to target concentrations of ˜30 mg-PFOS/L and 68 g-H2O2/L. Two 5 mL samples of the unreacted reagent are analyzed via LC-QToF-MS to precisely quantify the initial PFOS concentration at 28.63 mg-PFOS/L. Small concentrations of perfluoroheptanesulfonic acid (PFHpS) and perfluorohexanesulfonic acid (PFHxS) are measured in each untreated sample, at respective initial concentrations of ˜0.31 mg-PFHpS/L and ˜0.014 mg-PFHxS/L, likely due to impurities in the PFOS used to prepare reagents.
Experimental Apparatus and Procedures
A 50 mL stainless steel 316 (SS316) batch reactor is used for all experiments, designed to operate at temperatures up to 500° C. and pressures up to 35 MPa. SS316 construction and external heating limits the maximum internal operating temperature to 500° C., although continuous SCWO reactors can be operated at temperatures up to 650° C. by leveraging internal heating and Inconel 625 construction. A schematic of the batch reactor apparatus 300 is shown in
To test fluoride recovery from the reactor, a NaF solution measured to contain 38.74 mg/L of F− was loaded into the reactor and brought to 500° C. The recovered liquid samples were measured to contain 42.02 mg/L of F−, on average. The slight increase in concentration is likely due to a minor loss of steam out of the reactor vent after quenching. Correcting for steam losses (a slight distillation effect) results in a “distillation-corrected” recovered fluoride concentration of 38.54 mg/L indicating good fluoride recovery within the system (no significant adhesion of fluoride to the reactor walls). Measured fluoride, PFOS, and PFHpS values are all corrected for this distillation effect.
Experimental Conditions
Temperatures are varied from 425 to 500° C., and residence times varied from 0 to 60 min, with experimental pressure maintained above the critical pressure of 22.1 MPa through autogenic expansion of the water. 10 mL of reagent are treated in each batch. All conditions are tested in duplicate. Preliminary blank tests show that the internal reactor temperature increases from 300° C. to 400° C. within 18.6 min, equivalent to a reactor heating rate of 5.4° C./min. During cooling the internal temperature drops from 400° C. to 300° C. within 1.6 min, equivalent to a reactor cooling rate of 62.5° C./min.
Product Analysis
Liquid samples from each experimental condition are analyzed via fluoride ion-selective electrode (ISE) for the resulting concentration of F+ ions. The fluoride ISE used is a Thermo Scientific Orion Star A214 pH/ISE/mV/Temperature Benchtop Meter Kit with a stainless steel automatic temperature compensation (ATC) probe, a mechanical stirrer probe, and a fluoride electrode. Calibration was performed using TISAB II standards at 0.1 ppm, 0.5 ppm, 1 ppm, 2 ppm, and 10 ppm. The slope of the calibration curve was calculated at −55.15 mV at the measured temperature of 16.0° C., within the expected instrument range of −54 mV to −60 mV.
Liquid products are also analyzed with targeted high-resolution LC-QToF-MS to screen, identify, and quantify the presence and concentration of 45 potential PFAS compounds in the liquid product. The 45 PFAS are automatically targeted via a custom algorithm, which includes C4 to C18 perfluoroalkanoic acids, C3 to C12 perfluoroalkane sulfonates, C6 to C12 fluorotelomer sulfonates, and C6 to C12 fluorotelomer alkanoic acids. LC-QToF-MS allows for quantifying remaining PFOS, as well as the production of intermediate liquid PFAS.
All samples analyzed via high resolution LC-QToF-MS are transferred to 15 mL pre-weighed polypropylene centrifuge tubes (Falcon; Corning, N.Y.) tubes. The following reagents are added to each aliquot: 63.8% water sample, 3.3% Fisher Scientific Optima LC-MS grade water (Hampton, N.H.) mixed with 0.1% of Optima LC-MS grade ammonium hydroxide, 13.6% Optima LC-MS grade methanol, 9.6% of Optima LC-MS grade isopropanol surrogate mixture, and 9.6% of Optima LC-MS grade methanol surrogate mixture. Surrogate concentrations are spiked at 74 pg/mL and 1.35 mL of sample was transferred into a 1.5 mL autosampler vial.
One mL of sample was injected in a SCIEX X500R QTOF System (Framingham, Mass.). The analytical column used was Phenomenex Gemini C18, 3 mm×100 mm×5 μm (Torrance, Calif.). Prior to the column, a Phenomenex C18 SecurityGuard™ 4 mm×2 mm (Torrance, Calif.) and two Agilent Zorbax 4.6 mm×12.5 mm×6 μm DIOL guard columns (Santa Clara, Calif.) are used. A Phenomenex Luna 5 μm C18(2) 100 Å LC column 30×3 mm (Torrance, Calif.) is used as a delay column to aid in chromatographic separation. The column oven temperature is set to 40° C. The eluent mobile phases used are (a) Optima LCMS grade water with 20 mM Fisher Scientific HPLC grade ammonium acetate (Hampton, N.H.) and (b) 100% Optima LC-MS grade methanol. The autosampler rinse solution was 100% Optima LC-MS grade isopropanol. Eluent flow rate is held at 0.60 mL/min, and composition is ramped from 90% (a) to 50% (a) over the first 0.5 min, then to 1.0% (a) at 8 min and held until 13 min, then ramped to 90% (a) at 13.5 min and held constant until 20 min.
Electrospray ionization in negative mode (ESI−) with SWATH® Data-Independent Acquisition for both TOF-MS and MS/MS mode are used. Precursor ion data is collected for mass-to-charge ratios (m/z) of 100 to 1200 for 1283 cycles with a total scan time of 842 ms and accumulation time of 20 ms, with ion spray voltage set at −4500 V and temperature set to 550° C. The ion source, curtain, and collision gas for collisionally activated dissociation (CAD) are set to 60 psi, 35 psi, and 10 psi, respectively. For QTOF scanning, the collision energy is set to −5 V and the declustering potential to −20 V, both with no spread. Product ion (MS/MS) scanning is conducted for m/z 50 to 1200 Da. The accumulation time for each SWATH window is 50 ms and collision energy was −35 V with 30 V spread. The instrument is mass calibrated every 5 injections using SCIEX ESI Negative Calibration Solution.
Results & Discussion
Results show high fluorine conversion for experiments conducted at 500° C., with a maximum fluorine conversion of 78.2% achieved after 60 min of reaction (
The fluorine mass balance shows total fluorine recovery >75% for experiments conducted at 475 and 500° C. (
Effects of Reaction Temperature & Residence Time
Increasing the reaction temperature results in an increased extent of destruction for PFOS, PFHpS, and increased fluoride production. Increasing residence time also increases the extent of PFOS destruction and fluoride production. However, the maximum PFOS destruction efficiency of 70.0% achieved after 60 min. of reaction at 500° C. is not sufficient to lower the resulting PFOS concentration below the EPA health advisory level of 70 ng/L. Additionally, the longer residence time of 60 min. does not produce a significant increase in PFOS destruction for experiments conducted at 500° C. One possible explanation may be partitioning of oxidant and PFOS within the reactor. As H2O2 decomposes to O2 at higher temperatures, O2 may partition toward the bottom of the reactor. The density of O2 is 0.1158 g/mL while the density of water is 0.0897 g/mL (both at 500° C. and 25 MP a), thus density-driven stratification could drive O2 toward the bottom of the batch reactor and limit interaction of O2 with PFOS molecules. Additionally, PFOS likely partition to the reactor walls within the hydrothermal environment, due to the hydrophobicity of the PFOS molecule coupled with the polar solvent behavior of supercritical water, consistent with the known phenomenon of PFAS readily sorbing to various soils and minerals and partitioning to phase interfaces. Regardless of the physical explanation, the destruction efficiencies of PFOS and PFHpS, shown in
Reaction Mechanism
It appears that C—S bond cleavage is the initial reaction step for the destruction of perfluoroalkane sulfonates under SCWO conditions. SCWO is known to favor free-radical mechanisms driven by hydroxyl (OH) and hydroperoxyl (HO2) radicals, formed here through H2O2 decomposition. Carter and Farrell (Carter, K. E.; Farrell, J. Oxidative destruction of perfluorooctane sulfonate using boron-doped diamond film electrodes. Environ. Sci. Tech. 2008, 42, 6111-6115) previously investigated the energy barriers to hydroxyl radical attack on the PFOS molecule at the (i) —SO3 site, (ii) a —F site, and (iii) C—C bond site via density functional theory (DFT), in the context of electrochemical oxidation. The —SO3 site is the most energetically accessible site for reaction, with an activation barrier of 122 kJ/mol. C—C bond cleavage is the next most energetically available reaction, with an activation barrier of 169 kJ/mol, and C—F bond cleavage is the least available, with an activation barrier of 241 kJ/mol.
These activation energy barriers are consistent with the activation energy values of other recalcitrant, heteroatomic chemical species which can be destroyed by SCWO processing. Bianchetta et al. (Bianchetta, S.; Li, L.; Gloyna, E. F.; Supercritical water oxidation of methylphosphonic acid. Ind. Eng. Chem. Res. 38, (1999) 2902-2910) studied the continuous SCWO kinetics of methylphosphonic acid (MPA), a surrogate species for chemical warfare agents, and measured an activation energy of 186±36 kJ/mol for MPA conversion. Experiments conducted at 400° C. resulted in <5% conversion, while experiments conducted at 550° C. resulted in >99.9% conversion. It follows that SCWO processing of PFOS, with similar activation energy barriers for breaking C—C and C—F bonds via oxidative radical attack, will require similar temperatures to achieve high conversion levels.
Should C—F bond cleavage occur as an initial reaction step, some production of fluorotelomer sulfonates or fluorotelomer alkanoic acids may be witnessed. However, none are detected during targeted LC-QToF-MS screening. No perfluoroalkane sulfonates, aside from PFOS, PFHpS, and PFHxS, (all in decreasing levels), or perfluoroalkanoic acids are detected as reaction intermediates. Alongside the observation of significant PFOS conversion with correspondingly high fluoride production, it appears that the initial C—S cleavage step is also the rate-limiting step for overall conversion. This is well-supported by the ˜100% fluoride mass balance at 475 and 500° C.
Following cleavage of the sulfonate group, the C—C bonds of the perfluorooctyl radical are susceptible to hydrolysis and/or radical attack. Wu et al. previously demonstrated hydrolysis of PFOA within 30 minutes in hot-compressed water at 200° C., suggesting that hydrolysis of the intermediate PFAS formed after C—S bond cleavage is driven by reaction with water molecules, which are abundantly present in SCWO.
Reagents
Heptadecafluorooctanesulfonic acid solution (˜40 wt %, synonym: PFOS), perfluorooctanoic acid (95 wt %), perfluoroheptanoic acid (99 wt %), and heptafluorobutyric acid (98 wt %, synonym: PFBA) from Sigma Aldrich are used for reagent preparation. Deionized (DI) water (p=18.2 M-Ω) is used for preparation of all liquid reagents. The initial reagent contains PFOS, PFOA, PFHpA, and PFBA, at a concentration of ˜1 g/L each. A compressed air cylinder (Breathing Grade) from Praxair is used as the oxidant source in all experiments. Reagent grade ethanol (99.8%, Fisher Scientific) is diluted with DI water and used as a pilot fuel for all experiments.
Experimental Apparatus and Procedures
Continuous SCWO reactors can achieve processing temperatures of 500 to 650° C. and residence times of 5 to 60 s, with characteristically fast reaction kinetics due to the high reaction temperatures, high density of the reaction environment, and enhanced mixing of reacting species.
The SCWO reactor used for this study is a continuous, downflow reactor which operates using a pilot fuel as an internal heat source. The SCWO reactor consists of four influent lines (i) preheated ethanol-water mixture, (ii) preheated compressed air-water mixture, (iii) feedstock, and (iv) quench water. HPLC pumps continuously introduce the influents into the reactor, with individually selectable mass flow rates. Only water is run through the system during preheating; once the desired temperatures for ignition are reached, the ethanol and compressed air are introduced by switching three-way solenoid valves on the pump inlet lines. Reactor influent lines are preheated by resistive cartridge heaters before injection into the reactor through a custom, co-flow nozzle.
The reaction section is a titanium-lined vessel, with an internal volume of ˜1120 mL. The titanium liner has an inner diameter (ID) of 24.25 mm and sits within a stainless steel 316 (SS316) outer tube with an ID of 25.4 mm and an outer diameter (OD) of 38.1 mm.
After passing through the reactor section, the effluent is quenched through a heat exchanger and subsequently throttled across a back-pressure regulator (BPR), which is used to control the internal pressure. Liquid and gaseous products are collected on the effluent line after product quenching.
Steady-state operating temperatures are varied from 510 to 640° C., and with residence times held near-constant around 25 s, and experimental pressure maintained above 25 MPa. Table 1 shows the full range of experimental conditions.
Product Analysis
Liquid samples are analyzed via fluoride ion-selective electrode (ISE) for the resulting concentration of F− ions, and via targeted high-resolution LC-MS/MS to quantify the presence and concentration of 28 potential PFAS compounds in the liquid product. A full list of PFAS analytes (MI List) is available in Table 5. Between fuel, water, and oxidant injection, the overall feedstock is diluted by a factor of 10-20; Destruction and defluorination efficiencies are corrected accordingly. Measured fluoride yields and defluorination efficiencies are shown in Table 1.
Based on the results shown in Table 1, SCWO temperatures >600° C. and residence times >20 s are ideal for near-complete conversion of PFAS to fluoride. Longer residence times, higher oxidant loading, and/or higher temperatures can be used to drive more complete conversion.
Reagents
Heptadecafluorooctanesulfonic acid solution (˜40 wt %, synonym: PFOS), perfluorooctanoic acid (95 wt %), perfluoroheptanoic acid (99 wt %), and heptafluorobutyric acid (98 wt %, synonym: PFBA) from Sigma Aldrich are used for reagent preparation. Deionized (DI) water (p=18.2 M-Ω) is used for preparation of all liquid reagents. The initial reagent contains PFOS (C8HF17SO3), PFOA (C8HF15O2), PFHpA (C7HF13O2), and PFBA (C4HF7O2), at concentrations of ˜1 g/L each. This translates to an overall initial F loading in the reagent of 2.634 g-F/L. Full PFAS destruction and defluorination should yield a measured F− concentration near to this value.
Experimental Apparatus and Procedures
A 50 mL stainless steel 316 (SS316) batch reactor is used for initial experiments, designed to operate at temperatures up to 500° C. and pressures up to 35 MPa. Details of the reactor apparatus are shown in
During batch experiments, the reactor is placed into the furnace and brought to the desired experimental temperature, defined as the experimental “zero time”. After reaction for the desired residence time the reactor is removed from the furnace and fan-cooled to room temperature. The gaseous products are vented, the system is opened, and liquid products are collected in HDPE sample containers. The system is rinsed with DI water between experiments.
In these experiments, 15 to 35 mL of reagent are loaded into the reactor. NaOH (>97 wt %, Fisher Scientific) is used for reagent preparation in some cases (see Table 2). HCl (36.5-38.0 wt %, Millipore Sigma) is used for pH buffering post-treatment in experiments where NaOH is used. The reaction temperature is 350° C., with a reaction pressure of −25 MPa generated autogenically through expansion of the liquid. The reaction time is 60 min for all experiments. Total ionic strength adjustment buffer (TISAB) from Fisher Scientific is mixed with the reaction product prior to analysis with fluoride selective electrode (FSE). TISAB assists with pH buffering, and also ensures that F− ions are not complexing during analysis.
In some experiments, a sapphire or quartz liner was used (see Table 2). The final fluoride concentration is calculate based on the equation
C1V1=C2V2
where the concentration of fluoride in the effluent (C1) is related to the measured fluoride concentration in the buffered solution (C2) based on the volume change (dilution) between the two. Defluorination efficiency is calculated by dividing the fluoride concentration by 2634 mg/L, which is the known starting concentration of fluorine in the mixture.
Results
Table 2 shows that reaction at 350° C. for 60 min without alkaline amendment does not results in significant defluorination. Addition of 5 M-NaOH results in measured defluorination of 84.2%.
A significant goal of these experiments was to assess the stability of various liner/reactor materials. A single-crystalline sapphire liner was added to test whether a stable ceramic liner material could be used to mitigate corrosion of the reactor surface. Several experiments conducted with sapphire did not produce notable fluoride yields, leading to the premise that the SS316 reactor surface may be catalytically driving the reaction. However, addition of a SS316 tube into the sapphire liner still did not produce measurable fluoride yields. Ultimately it was discovered that the sapphire was reacting with the alkali amendment and fluoride to produce an insoluble fluoride complex. For this reason, the use of a sapphire liner was abandoned for future experiments.
A quartz liner was also tested. The quartz was found to be unstable under the highly alkaline conditions and suffered notable damage during the experiment. However, the resulting fluoride yield was ˜100%, indicating that a non-reactive liner material could be used to mitigate corrosion in the reaction vessel. The use of quartz was also abandoned for future experiments due to its instability under the reaction conditions.
Based on the results presented here, the ideal conditions for PFAS conversion to fluoride in subcritical water involve reaction at (i) T˜350° C., (ii) P˜25 MPa, (iii) [OH−]˜5 M, (iv) reaction time ˜60 min. pH buffering and TISAB addition is needed for analysis via FSE, while pH buffering alone is sufficient for analysis via ion chromatography (IC).
Reaction conditions can be altered and still yield acceptable results. Temperatures as low as 250° C. will still yield defluorination, but longer residence times and/or higher [OH−] loading is needed to achieve ˜100% defluorination. Temperatures higher than 350° C. may also be used, but care must be taken to manage the expansion of water at these temperatures, as pressure must be kept within a safe operational range.
Pressure can be any value high enough to keep the reactants liquid (or liquid-like) at the reaction temperature. [OH−] loading can also be reduced, but longer reaction times and high temperatures would be needed for ˜100% defluorination. Alternately, [OH−] loading could be increased to the solubility limit of the hydroxide in liquid water, which would speed reactions. Longer reaction times may also be used to ensure that near-complete defluorination is achieved.
The efficacy of the HALT process toward the destruction of PFAS in a landfill leachate matrix was studied. Landfill leachate is a complex liquid matrix, containing high levels of total suspended solids, (TSS), total dissolved solids (TDS), and chemical oxygen demand (COD), which can cause interference with many PFAS destruction processes. Leveraging both a batch HALT reactor and a continuous HALT reactor, the rates of PFAS destruction in the two system configurations are compared.
Materials & Methods
Leachate from an active landfill in Mexico is provided from an industrial partner, with characteristics shown in Table 3. A 100 μg/mL analytical standard of perfluorooctanesulfonic acid (PFOS) in methanol (Supelco) is used to dope leachate to a target initial PFOS concentration of 100 μg/L. The resulting initial PFAS concentrations in the leachate is shown in Table 4, as measured by commercial LC-MS/MS; several other PFASs are detected, likely due to impurities in the PFAS used for doping. All other PFASs screened for in EPA Method 537.1 (Table 5) are below detection limits. NaOH (>97 wt %, Fisher Scientific) is used for reagent preparation. HCl (36.5-38.0 wt %, Millipore Sigma) is used for pH buffering post-treatment.
aMeasured,
binformation supplied by project partner.
Batch Reactor
A 50 mL stainless steel 316 (SS316) batch reactor is used for initial experiments, designed to operate at temperatures up to 500° C. and pressures up to 35 MPa. Details of the reactor apparatus are shown in
During each experiment, the reactor is placed into the furnace and brought to the desired internal temperature, defined as the “zero time”. After reaching the desired residence time the reactor is removed from the furnace and cooled to room temperature. The gaseous products are vented, the reactor is opened, and liquid products are collected in HDPE sample containers. The system is rinsed several times with DI water between experiments.
During batch experiments, the reactor is placed into the furnace and brought to the desired experimental temperature, defined as the experimental “zero time”. After reaction for the desired residence time the reactor is removed from the furnace and fan-cooled to room temperature. The gaseous products are vented, the system is opened, and liquid products are collected in HDPE sample containers. The system is rinsed with DI water between experiments.
Continuous Reactor
A continuous, tubular reactor is used to evaluate the effect of system configuration on observed PFAS destruction rates. A process flow diagram (PFD) of the continuous system is shown in
After reaction in both reactor configurations, HCl is added to buffer the solution pH to a value between 5 and 7 (ideally between 5 and 5.5), as measured by pH strips. The dilution effect of HCl addition is factored into correcting the measured effluent characteristics.
Experimental Conditions
The overall loading of NaOH is maintained at 5 M. The batch reactor is operated at 350° C., with a residence time of 120 min. The internal pressure reaches >25 MPa autogenically, through liquid expansion. The continuous reactor is operated at a constant internal temperature of 350° C., and residence times of 15, 30, 45, and 60 min. Internal pressure is held at 25 MPa.
Product Analysis
Untreated and batch-reactor-treated landfill leachate is analyzed for 18 PFAS analytes via LC-MS/MS using EPA Method 537.1 (SGS; Orlando, Fla.). Samples treated in the continuous reactor are analyzed for 26 PFAS analytes via LC-MS/MS, using the Michigan list (SGS; Orlando, Fla.). Table 5 shows the analytes included in analysis. Untreated and treated landfill leachate is analyzed for TOC and TN with a Shimadzu TOC-VCSH instrument.
Results & Discussion
Measured levels of PFAS in reactor effluent shows that total PFAS levels are reduced by >95% during continuous HALT processing at all tested residence times.
These results clearly demonstrate several positive conclusions related to using the HALT process to treat landfill leachate. Namely, (i) HALT processing can destroy >99% of PFOS in batch and continuous reactor configurations, and (ii) PFAS destruction kinetics in a continuous HALT reactor are faster than in a batch HALT reactor.
While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 63/023,866, filed on May 12, 2020, incorporated herein in its entirety by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/029173 | 4/26/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63023866 | May 2020 | US |
