WO2021067786A1

WO2021067786A1 - System for degrading chemical contaminants in water

Info

Publication number: WO2021067786A1
Application number: PCT/US2020/054048
Authority: WO
Inventors: Michael S. Wong; Kimberly N. HECK; Bo Wang; Lijie DUAN; Sujin GUO; Paul Westerhoff
Original assignee: William Marsh Rice University; Arizona State University
Priority date: 2019-10-02
Filing date: 2020-10-02
Publication date: 2021-04-08

Abstract

A system for degrading contaminants may include a reactor containing a boron nitride photocatalyst. The reactor system may be configured to receive an aqueous liquid comprising at least one contaminant, and may also include a light source configured to direct light to the boron nitride photocatalyst, and an oxidant source in fluid communication with the reactor.

Description

SYSTEM FOR DEGRADING CHEMICAL CONTAMINANTS IN

WATER

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] The invention was made with government support under: Grant Number

NEWT- NSF 1449500 awarded by the National Science Foundation (NSF).

FIELD OF THE DISCLOSURE

[0002] Embodiments herein are directed toward systems and methods for the photocatalytic degradation of contaminants by boron nitride (BN) photocatalysts. Embodiments herein are particularly but not exclusively useful in the treatment of wastewater.

BACKGROUND

[0003] Contaminated water is becoming a growing problem in many areas of the world.

It is a grand challenge linked to public health, energy production, and sustainable development. The need to treat contaminated water spans across socioeconomic and cultural divides. Over 40 million Americans are not connected to a municipal water system and rely on the quality of the water available from alternative sources, such as wells. Moreover, climate change and industrial growth exacerbate fresh water scarcity. Thus, it is important to broaden access to clean drinking water from by developing new and efficient methods of cleaning water from a variety of sources (e.g., groundwater from wells, saltwater, brackish water, or recycled industrial water).

[0004] Conventional methods and systems for treating wastewater may employ coagulation, sedimentation, granular media filtration, reverse osmosis, ion exchange, adsorption (e.g, activated carbon), ultraviolet light radiation, ultrasonic energy, and other chemical-intensive processes to remove contaminants from water. Conventional separation techniques may generate waste streams that often require additional processing to treat the removed contaminants, such as regenerative treatments to destroy the contaminants· Advances in the water industry also include the development and use of photocatalysts, such as titanium dioxide, to detoxify contaminants from water.

[0005] Concern over water contamination has highlighted the lack of effective treatment with conventional approaches, particularly with detoxifying per- and polyfluoroalkyl substances (PFAS), such as perfluorooctanoic acid (PFOA).. Numerous specific industries and fire-fighting applications have used organic chemicals such as PFAS, and have contaminated both wastewaters and drinking water sources. There are few efficient conventional methods for removing perfluorinated compounds from water. Availability and cost of conventional wastewater treatment systems often impact the access to safe, potable water. Although photocatalysis offers advantages over conventional treatment approaches by using ambient conditions for reactions, air as the oxidant, and light as the energy source, identifying effective photoactive materials is challenging.

SUMMARY OF THE DISCLOSURE

[0006] In one aspect, embodiments disclosed herein relate to a system for degrading contaminants· The system may include a reactor containing a boron nitride photocatalyst and may be configured to receive an aqueous liquid comprising at least one contaminant· A light source may be provided and configured to direct light to the boron nitride photocatalyst, and an oxidant source in fluid communication with the reactor.

[0007] In another aspect, embodiments disclosed herein relate to a method for degrading contaminants· The method may include introducing an aqueous liquid comprising a contaminant to a reactor, which may contain a boron nitride photocatalyst. The method may also include introducing an oxidant to the reactor, activating the boron nitride photocatalyst by directing light waves from a light source at the boron nitride photocatalyst to form an activated boron nitride photocatalyst, and contacting the oxidant and the contaminant with the activated boron nitride photocatalyst to react the oxidant and the contaminant, thereby degrading the contaminant· [0008] In another aspect, embodiments disclosed herein relate to a membrane for use in a water purification system, the membrane including boron nitride affixed to a nanomaterial.

[0009] Other aspects and advantages will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0010] Fig. 1 illustrates a slurry reactor system according to embodiments herein.

[0011] Fig. 2 illustrates a fiber optic reactor system according to embodiments herein.

[0012] Fig. 3 illustrates the photocatalytic destruction of PFOA with either BN or

T1O2 photocatalysts.

[0013] Fig. 4 illustrates photocatalytic curves of the degradation of PFOA using different mass loadings of BN photocatalyst in a reactor.

[0014] Fig. 5 illustrates spectra collected on a Rigaku D/Max Ultima II diffractometer

(40 kV, 4940 mA) using Cu-Ka radiation.

[0015] Fig. 6 illustrates transmission electron microscopy images of BN with no defects (as received).

[0016] Fig. 7 illustrates transmission electron microscopy images of P25-Ti0₂.

[0017] Fig. 8 illustrates transmission electron microscopy images of ball milled BN.

[0018] Fig. 9 illustrates PFOA concentration over time using for BN.

[0019] Fig. 10 illustrates the rate of reaction for BN loading.

[0020] Fig. 11 illustrates PFOA concentration over time using for Ti0₂.

[0021] Fig. 12 illustrates the rate of reaction for T1O2 loading.

[0022] Fig. 13 illustrates terephthalic acid (·OH probe) generation of radicals by both

BN and T1O2.

[0023] Fig. 14 illustrates HPLC-DAD-detected PFOA concen tration-time and fluoride concentration profiles using BN, with and without 254 nm irradiation. [0024] Fig. 15 illustrates HPLC-DAD-detected PFOA concentration-time and fluoride concentration profiles using T1O2., with and without 254 nm irradiation

[0025] Fig. 16 illustrates HPLC-MS detected concentration-time profiles of PFOA and byproducts for BN, with corresponding total fluorine balance.

[0026] Fig. 17 illustrates HPLC-MS detected concentration-time profiles of PFOA and byproducts for T1O2, with corresponding total fluorine balance.

[0027] Fig. 18 illustrates PFOA photodegradation over BN using EDTA, SOD, and

TBA as hole, superoxide/hydroperoxyl and hydroxyl radical scavengers, respectively.

[0028] Fig. 19 illustrates PFOA photodegradation over T1O2 using EDTA, SOD, and

[0029] Fig. 20 illustrates the proposed photooxidative mechanism of PFOA degradation over BN.

[0030] Fig. 21 illustrates XPS of B Is binding energies at different reaction times.

[0031] Fig. 22 illustrates XPS of N Is binding energies at different reaction times.

[0032] Fig. 23 illustrates absorbance spectra of BN through DR-UV.

[0033] Fig. 24 illustrates the Raman spectra of as-received and ball milled BN.

[0034] Fig. 25 illustrates the PFOA concentration-time profiles using as-received and ball milled BN.

[0035] Fig. 26 illustrates absorbance spectra of T1O2 through DR-UV.

[0036] Fig. 27 illustrates the PFOA photodegradation rates over as-received BN and

T1O2 in DI water and SDW.

[0037] Fig. 28 illustrates the PFOA concentration-time profiles in SDW with reinjections of PFOA for the BN case.

[0038] Fig. 29 illustrates the fluoride concentration profiles of the SDW with reinjections of PFOA for the BN case.

[0039] Fig. 30 illustrates the GenX and fluoride concentration- time profiles using BN with 254 nm irradiation. [0040] Fig. 31 illustrates the PFOA sorption isotherm obtained using the BN as the sorbent with the initial PFOA concentration in the range of 25-300 mg/L and the initial pH of 3 at 30 °C.

DETAILED DESCRIPTION

[0041] Embodiments disclosed herein are directed to a photocatalytic system that may degrade chemicals, such as pollutants typically found in driniking water or municipal and industrial wastewaters, thereby detoxifying the chemicals.

[0042] Embodiments disclosed herein may provide a broad set of stakeholders (e.g., industrial organizations, governmental organizations, and citizens) a secure source of clean water. Embodiments disclosed herein may broaden access to clean drinking water by cleaning water from a variety of potential sources (e.g., groundwater from wells, saltwater, brackish water, or recycled industrial water). Some embodiments of the present disclosure may be modular systems that offer compact embodiments of catalytic treatment systems. These modular systems may provide drinking water from the scale of a household, to a neighborhood to a remote town. In addition, these modular embodiments may also provide access to clean water in the event of a natural disaster.

[0043] In addition to drinking water, embodiments herein may improve the impact of oil and gas exploration and production operations on water supplies by helping to increase the quality of water cleanup for reuse and recycle. The environmental impact of water use in these industrial settings will be improved, saving energy and water resources.

[0044] Embodiments herein may relate to the design and manufacture of multifunctional nanomaterials to adsorb a wide variety of pollutants, including: oxo- anions, total dissolved solids, nitrates, salts, organics, foulants, sealants, viruses and microbes, among others. These nanomaterials may be immobilized in membranes that are packaged into system modules. The use of modules offers flexibility of targeted pollutant(s) and end-use application capacity or scale of delivered water rate. Novel photonic, catalytic, and/or photocatalytic engineered nanomaterials (ENMs) described herein may introduce new approaches to transform water treatment from a large, chemical- and energy-intensive process toward compact physical and catalytic systems. These innovations may benefit multiple stakeholders, from rural communities and locations hit by natural disasters to hydraulic fracturing oil and gas sites, where reuse of produced waters minimizes regional environmental impacts.

[0045] Contaminants enter the water system from a number of sources, such as industrial run-off and sewage from cities. Perfluoroc arboxylic acids (PFOA), part of the Per/polyfluorocarboxylic substances (PFAS) family, is an example of a particularly harmful contaminant found in the water supply. PFAS have been detected in water due to their application as surfactants, additives, firefighting foams, and lubricants. Toxicological studies indicated that PFASs bioaccumulate in humans and wildlife, potentially leading to developmental and reproductive problems, liver damage, and cancer. Concern over water contamination by harmful contaminants, such as PFOA, highlights the need for effective treatment approaches provided by embodiments of the present disclosure.

[0046] Generally, contaminants are degraded in aqueous liquids to detoxify pollutants that may otherwise cause harm on the environment. Wastewater is a representative example of an aqueous liquid that requires degradation of contaminants to protect the environment and drinking water sources. Removing perfluorinated compounds, such as PFOA, is particularly challenging in the treatment and detoxification of wastewater. Conventional PFOA degradation methods include advanced oxidation processes (AOPs). Ultraviolet-based AOPs promote direct and indirect photochemical reactions that show potential for efficient degradation of PFOA. Vacuum ultraviolet (VUV) and UV-C irradiation with high spectral irradiance decompose PFOA via photolysis. However, defluorination of PFOA is slow, making direct photolysis of PFOA unfeasible as an effective water treatment method. VUV and UV-C irradiation in solutions containing sulfite or persulfate also degrades PFAS, but continuous chemical addition is required.

[0047] Photocatalytic degradation of contaminants in aqueous liquids presents advantages over other methods. Advantages of photocatalysis include using ambient conditions for reaction, air as the oxidant, and light as the energy source. Photocatalysis also directly destroys targeted contaminants, thus rendering the contaminant non-toxic without additional processing. For example, physical separation methods often produce waste streams and may require regenerative treatments to destroy the separated contaminants·

[0048] Conventional photocatalyst systems may be operated in recirculating slurries that do not require continuous addition of reagent. PFOA degradation occurs through different reaction mechanisms, depending on the photocatalyst composition and reaction conditions. The conventional, commercially available photocatalyst is titanium oxide (TiC ),. Although TiC is used as a photocatalytic material, the rate of PFOA degradation is relatively slow and thus requires high-energy to degrade PFOA, which equates to a high operating cost for water treatment.

[0049] Boron Nitride (BN) is generally considered an electrical insulator, due to its wide band gap. BN has also been investigated previously for its use in thermal (i.e., non-photo) catalysis. BN is also conventionally understood to act as a support material, wherein an active material (typically group 8b metals) is immobilized upon it. BN has also been proposed to treat organic compounds in water due to its high hydrophobicity and surface area. However, these methods only remove the organic compounds and do not detoxify them (i.e., further treatment remains necessary).

[0050] Embodiments disclosed herein are directed to treating contaminants in an aqueous liquid with boron nitride (BN) photocatalysts. Surprisingly, the BN photocatalyst may be activated by a light source (i.e., irradiated), including an ultraviolet light source, in the presence of an oxidant. The oxidant may be the ambient air, or may be provided by an oxidant source connected via flow line, wherein the oxidant (e.g., the added air or oxygen) flows into the system. The BN photocatalyst degrades the contaminants upon activation by the light, thereby decreasing the toxicity of the aqueous liquid.

[0051] Embodiments disclosed herein may include treating contaminants in an aqueous liquid with BN-Ti02 photocatalysts. The oxidant may be the ambient air, or may be provided by an oxidant source connected via flow line, wherein the oxidant (e.g., the added air or oxygen) flows into the system. Both the BN photocatalyst and the Ti02 photocatalyst may be present in the same system to degrade the contaminants upon activation by the light, thereby decreasing the toxicity of the aqueous liquid.

[0052] In embodiments of the present disclosure, boron nitride acts as a photocatalyst for degrading contaminants, such as PFOA, in an aqueous solution without the use of additional chemicals. It has been found that BN photocatalyst is active for the heterogeneous photodegradation of PFOA and may be as much as four times more effective than conventional photocatalysts, such as T1O2. In addition to degradation of stable and recalcitrant compounds like PFOA, embodiments of the present disclosure may also be used for the degradation of less-stable compounds, such as 1, 4-dioxane, pharmaceuticals, pesticides, and chlorinated solvents.

[0053] Photocatalytic reactor configurations may vary with light sources (ambient light, sunlight, ultraviolet light), photocatalyst handling (photocatalyst coated on or as a particulate suspended in a slurry, or coated on an immobilized surface), sources of wastewater (industrial runoff, sewage from cities), and sources of oxidants (ambient air, oxygen line).

[0054] In certain embodiments of the present disclosure, the BN photocatalyst may be present in one or more various forms, including in particulate form, coated on reactor surfaces, such as fixed-geometry surfaces, fluidized media, and contained in polymer nano-composites. The particular form of the BN photocatalyst used within a water treatment system is not particularly limited, other than the need to have sufficient surface area for contact with the reactants, oxygen and the contaminant(s), as well as sufficient exposure of the BN photocatalyst to light.

[0055] In some embodiments, the BN photocatalyst may be present in a coating that is disposed on one or more surfaces within a reactor. BN photocatalyst, for example, may be incorporated into a paste and deposited on a surface within or to be disposed within a reactor. The paste may be prepared in some embodiments by mixing BN photocatalyst nanopowders with a binder, such as polyethylene glycol, and condensed by evaporation until a target viscosity and BN photocatalyst content is reached. The paste may then be disposed on various surfaces by methods understood by those skilled in the art with the benefit of the present disclosure, such as fluidized bed coating, spray coating, and roller coating methods. Colloidal BN photocatalyst might also be directly coated on the support surface by pre-mixing the BN nanopowder and the support material in aqueous suspensions, followed by evaporation and calcination. BN suspensions may also be deposited onto chemically functionalized surfaces. BN could also potentially be grown directly onto the supporting materials.

[0056] In embodiments of the present disclosure, BN photocatalyst may be present on particles (i.e., particulate support materials), that may be used in a slurry reactor or fixed bed reactor, for example. BN photocatalyst may be applied to, deposited upon, or otherwise associated with suitable particulate support materials by various methods understood by those skilled in the art. Particulate support materials may include, among other compounds, silica, alumina, titania, and mixtures of these particulate support materials, therein. The size of the support materials may be in a range from lOnm to 10mm, for example, depending upon the reactor type (fixed bed or slurry). As a photocatalyst, the support materials are preferably low surface area support materials, and the BN photocatalyst may be disposed largely on an exterior or exposed portion of the particle such that the BN may be exposed to light during use. When used in a slurry bed reactor, particulate materials of sufficient strength should be used, such that contact with other catalyst particles does not result in damage to the catalyst that may render the catalyst unsuitable for the intended reaction. In other embodiments, BN particles themselves may be useful as a slurry or fixed bed catalyst.

[0057] In embodiments of the present disclosure, BN photocatalyst may be present on fixed-geometry surfaces. Herein, fixed-geometry surfaces may include immobilized surfaces, walls of the reactor, the outside surface of the light source, optical fiber structures, polymer rods, fixed bed membranes, and other equivalent means that may be coated with BN photocatalyst. These surfaces may be immobilized within a system and configured to allow a flow of a fluid across the outside surface area of the fixed- geometry surface. BN photocatalyst may be applied to the outside surface area of the fixed-geometry surface by coating methods understood by those skilled in the art. In certain embodiments, the fixed-geometry surface is the light source configured to conduct light waves through the inside of the structure, thereby exposing and activating the BN photocatalyst. [0058] Regardless of the manner in which the BN photocatalyst is disposed in a reactor, the BN photocatalyst may be disposed largely on the external surface of the support material or fixed geometry surfaces in a manner providing a large surface area and efficient exposure or the BN photocatalyst to light.

[0059] One example of a reactor that may be used in certain embodiments is a slurry reactor. A photocatalytic slurry reactor may be configured for batch, semi-batch or continuous flow processing. The BN photocatalyst coated particulate, as described above, may be disposed in the slurry reactor with an aqueous liquid, such as wastewater. The slurry reactor may be configured with a light source, such as an ultraviolet lamp, which may be disposed internal or external to the reactor. The slurry reactor may also be configured for fluid communication with an oxidant source, such as a flow line to the reactor, an open surface of the reactor to the ambient air where agitation or other interaction with the ambient air may result in dissolution of oxygen within the aqueous liquid, or other equivalent means that may result in oxygen being dissolved in the aqueous liquid upstream before introduction of the aqueous liquid into the reactor.

[0060] Embodiments of the present disclosure may also include reactors configured to hold a stationary photocatalyst, such as the BN photocatalyst coated fixed-geometry surfaces as described above. Photocatalytic reactors with fixed, or stationary catalysts may be operated with batch, semi-batch, and continuous flow processing. The BN photocatalyst coated fixed-geometry surface may be fitted to be disposed in the reactor, wherein an aqueous liquid, such as wastewater, may be in fluid communication with the BN photocatalyst coated fixed-geometry surface. The reactor may be configured with a light source, such as an ultraviolet lamp, which may be internal or external to the reactor. The reactor may also be configured for fluid communication with an oxidant source, such as a flow line to the reactor, an open surface of the reactor to the ambient air where agitation or other interaction with the ambient air may result in dissolution of oxygen within the aqueous liquid, or other equivalent means that may result in oxygen being dissolved in the aqueous liquid upstream before introduction of the aqueous liquid into the reactor.

[0061] As shown in Fig. 1, embodiments disclosed herein may include a slurry reactor system 100. In slurry reactor system 100, an oxidant is present in the slurry reactor 12. The oxidant source may come from a line (not shown) to supply the oxidant, such as oxygen. The slurry reactor 12 may also be open to the surrounding environment wherein ambient air may act as the oxidant. The wastewater 14 enters through line 11 into a slurry reactor 12. The wastewater 14 from line 11 mixes with fluidized media, herein BN photocatalyst coated particles 13, to create a slurry. BN photocatalyst is coated on particles using methods understood by those skilled in the art. By pre-mixing BN nanopowders and the support materials with water, and then evaporating and calcining in the air atmosphere, BN might be coated on the support surface. The BN coating may also be polymer nano-composites wherein a paste is prepared by mixing BN nanopowders with a binder, such as polyethylene glycol, among others, and may be condensed by evaporation. The paste may then be applied to a particle surface to create BN photocatalyst coated particles 13.

[0062] As shown in Fig. 1, wastewater 14 flows inside the slurry reactor 12 and passes over the BN photocatalyst coated particles 13. As the wastewater 14 and BN photocatalyst coated particles 13 move in the slurry reactor 12, light source 15 exposes the contents of the reactor 12 to light waves, preferably ultraviolet light. In embodiments herein, ultraviolet light (mono- or poly-chromatic) activate the BN photocatalyst on the BN photocatalyst coated particles 13. The activated BN photocatalyst particles 13 then catalyze the reaction of the oxidant and contaminants, thereby degrading the water contaminants.

[0063] The time required for the photocatalytic treatment of the wastewater according to embodiments of the present disclosure may depend upon the concentration of contaminant, type of contaminant, type of BN photocatalyst application (i.e., BN photocatalyst coated on a fixed surface or present in a slurry), the BN photocatalyst loading, light exposure limitations, and other factors that may be readily understood by those skilled in the art with the benefit of the present disclosure. The treated wastewater 16, flows out of the slurry reactor 12 through line 17.

[0064] In embodiments of the present disclosure, a BN coating may be deposited and immobilized onto a fixed surface, such as glass, metal, or substrate. As shown in Fig. 2, fiber optic reactor system 200 depicts optical fiber rods 22 as a surface for the BN photocatalyst coating 23. BN photocatalyst coating 23 may be applied directly to the surface configured to direct ultraviolet light from a light source 25 to the BN photocatalyst coating 23 in the presence of the wastewater 26. The wastewater 26 enters the fiber optic reactor system 200 through line 21 and into the fiber optic reactor 27. As in Fig. 1, fiber optic reactor 27 is configured to draw an oxidant from a source, such as the ambient air via an open reactor vessel. The wastewater 26 passes over the BN photocatalyst coating 23 in the presence of ultraviolet light waves, and an oxidant source. The ultraviolet light waves originate from a light source 25, such as a lamp. The BN photocatalyst coating 23 reacts with the light waves and oxidant to degrade the contaminants in the wastewater via pathways described herein. The treated wastewater 28 flows out of the fiber optic reactor 27 though line 29.

[0065] In some embodiments of the present disclosure, the reactor systems, such as the slurry reactor system 100 and fiber optic reactor system 200, may be operated in batch, semi -hatch or in continuous flow modes. It is understood by those skilled in the art that the choice in modes may depend on a multitude of considerations, such as the location of the need for a reactor, the resources and tools available, and the immediacy of the need for clean water. For example, natural disasters require immediate action but may have limited resources and compromised logistics, thus batch or semi -hatch modes may be a more convenient choice.

[0066] Fig. 1 and Fig. 2 show embodiments of single-phase reactor systems. It will be understood by those in the art that embodiments of the present disclosure may be 2-, 3-, or multiple-phase reactor systems.

[0067] PFOA degradation is an intense area of study and innovation due, in part, to its recalcitrant characteristics. Experimental data of embodiments of the present disclosure demonstrate that BN photocatalyst is effective for the degradation of PFOA in aqueous liquids (e.g., wastewater) using ultraviolet light and oxygen, as illustrated by the following Examples.

[0068] Fig. 3 shows the photocatalytic disappearance of PFOA with BN photocatalyst and T1O2 photocatalyst. The reactions were performed in sealed quartz reactors. The light source originated from 254 nm UV-C lamps (output power of 4.8 W, the total spectral irradiance of 1.9 mW/cm²) equipped with a stir plate to simulate influent fluid flow in the reactor. A stock solution of PFOA (2 g/L) was prepared with DI water and diluted to 50 mg/L for all photocatalytic experiments. The sealed quartz reactor was bubbled with either air, oxygen, inert nitrogen, or hydrogen at 150 cc/min. Fig. 3 shows the base case disappearance of PFOA versus time for 50 mg of BN or Ti0₂. The experimental data shows that the BN is much more effective at removing PFOA than Ti0₂.

[0069] Loading of the photocatalyst may impact the activity and rate of PFOA loss. For example, loading a quartz cell with a solid can lead to suspension turbidity which can negatively affect light flux throughout the reaction vessel. Fig. 4 shows exploratory experiments wherein the BN photocatalyst mass was varied between 0-100 mg to determine the optimum BN photocatalyst load mass. The maximum rate of PFOA loss was observed between 50-100 mg BN photocatalyst. Therefore, the mass of photocatalysts (in the case of the experiment, h-BN) used for each experiment was kept at 50 mg. Embodiments of the present disclosure may apply the load ratio captured in the experiment to maximize the photocatalysis pathways of BN while not affecting the light flux through the reaction vessel.

[0070] With the photocatalyst load determined to be 50mg for the exploratory experiment, the experiment proceeded to test the effectiveness of photocatalysts, including Ti0₂ and BN, at degrading PFOA. The photocatalysts were added into a 50 ml flask containing 20 ml DIW and 50 mg/1 PFOA. The quartz flask was then sealed with a rubber septum, and bubbled with either air, oxygen, inert nitrogen, or hydrogen at 150 cc/min for 15 min. The suspensions were stirred constantly during the photocatalytic processes to ensure the uniform dispersion of photocatalysts. The experiments were conducted for up to 6 hours to calculate the activity of photocatalysts. Aliquots of reaction fluid were filtered (0.2 pm) from the suspension to determine the pH, concentration of PFOA, and concentration of released fluoride ion.

[0071] Table 1 reviews published literature reaction conditions and the half-life of

PFOA (i.e., the amount of time to reduce PFOA concentration by half of its initial value) under these conditions, and compares them to the methods of the present disclosure. As shown in the last two sets of data in Table 1 for BN reaction systems, the technology of the present disclosure not only degrades PFOA extremely rapidly, but without the use of excess chemicals, additional detoxifying processes, or energy intensive (high wattage) processes. Additionally, the PFOA half-life can be further reduced by simply exposing to more intense radiation (green entry).

Table 1

[0072] Experimental data found BN to be more than 20 times more active than ΉO2 for the treatment of PFAS when used in embodiments of the present disclosure. Experimental data based on embodiments of the present disclosure found BN active for the heterogeneous photodegradation of PFOA at room temperature using ultraviolet light (254 nm wavelength), and four times more effective than TiCh. [0073] Embodiments herein have been shown effective for the removal of perfluorinated compounds via their reaction with presumed radicals generated during the photocatalytic reaction. Because of the activity of these radicals, they may be used to destroy other less-stable organic compounds, such as 1,4-dioxane, pharmaceuticals, pesticides, or chlorinated solvents in water. Experiments provide evidence that multiple reactive oxygen species are involved in PFAS defluorination in the presence of BN, including superoxide and single oxygen radicals.

[0074] PFOA degradation can undergo stepwise decarboxylation/defluorination in which PFOA composes to perfluorheptanoic acid (Cl), which decomposes to perfluorohexanoic acid (C6), and so on. The shorter chain homologues of PFOA may react directly with the surface B-H bonds formed during irradiation with the UV light. The surface bound fluoride may be released from the BN surface as the concentrations of PFOA and its fragments decrease. Furthermore, the present disclosure does not require the use of additional chemicals, unlike most conventional treatment technologies require; water with atmospheric air is effective for the degradation of PFOA by the photocatalytic BN and a UV-C light source.

[0075] BN advantageous photocatalytic properties with PFOA may be, in part, due to its hydrophobic surface. PFAS is a surfactant with a polar headgroup and non-polar carbon-fluorine chain. PFAS’s hydrophobic moiety interacts with the otherwise hydrophobic surface of BN. Other photocatalysts are usually metal oxides (e.g., titanium dioxide) and as such exhibit surface charge and are not considered hydrophobic in nature. Unlike some carbon photocatalysts that start out as being partially hydrophobic, oxidation of the surface by light produces oxygen functional groups that increase polarity, and thus would not maintain their hydrophobicity. The unique hydrophobic surface of BN, and its ability to attract PFAS, is a distinctive quality of its ability to bring PFAS compounds close enough to the surface to facilitate heterogeneous catalysis.

[0076] As seen in Fig. 31, the PFOA sorption isotherm obtained using the BN as the sorbent with the initial PFOA concentration in the range of 25-300 mg/L and the initial pH of 3 at 30 °C. The isotherm was then fitted according to the linearized forms of the Langmuir model using the following equation: C_e C_e 1 R_e Rm b X q_m where C_e is the equilibrium solution concentration (mmol/L), q_e is the amount of PFOA adsorbed at equilibrium (mmol/g), q_m is the maximum adsorption capacity (mmol/g), b is the adsorption equilibrium constant (L/mmol). The fitted results of the isotherm parameters for PFOA absorption at pH 3 are summarized in Table 2. The results show that PFOA can be absorbed on the surface of BN, which benefits the photodegradation.

Table 2 _ q^{m qm b} R² (mmol/g) (mmol/m²) (L/mmol) _

0.013 0.0005 5.86 0.99

[0077] BN was not expected to be an active photocatalyst, particularly in the UV-C region (i.e., light wavelength of about 254 nm), as its reported band gap is very large at 5.8 eV (corresponding to a higher energy wavelength of 213 nm). However, experiments employing embodiments of the present disclosure showed photocatalytic degradation of PFOA over BN photocatalyst, wherein the BN photocatalyst material comprised defects, i.e., edge defects or B or N vacancies. The BN spectrum showed a small peak absorbance at 208nm wavelength, and at 254nm wavelength. It has been found that BN photocatalyst surface defects are necessary for light absorption and photodegradation capability. Large surface area BN photocatalyst, such as hydrothermally exfoliated BN photocatalyst, with high surface defect content may result in higher PFOA photodegradation activity. Defects may be intentionally introduced in the BN photocatalyst through various methods known in the art, such as ball milling. Ball milled BN photocatalyst demonstrated improved performance for PFOA degradation. In one experiment, the photocatalytic degradation rate increased from 0.24 mg to 0.44 mg of PFOA/Lmin after ball milling, two to four times greater than that of T1O2 data.

[0078] Various wavelength ranges may work with embodiments herein. In some embodiments, the wavelength range may be limited to UV-C, UV-B, and UV-A. Visible light may not work as effectively as ultraviolet light, thus water treatment using sunlight may be slow.

[0079] Examples [0080] In the following examples, perfluorooctanoic acid, hexagonal boron nitride

(BN, powder, ~lpm, 98% purity, lot number STBH7651), titanium dioxide (TiC , P25 nanopowder, ~21 nm particle size, >99.5%), silicon dioxide (SiC , nanopowder, 10-20 nm particle size, 99.5% purity) (as obtained from Sigma-Aldrich), as well as deionized water (DI water) were used to collect data on the photodegradation of PFOA. All materials were of analytical grade and used as received.

[0081] The crystallite structures of BN and T1O2 were characterized using X-ray powder diffraction (XRD). As shown in Fig. 5, spectra were collected on a Rigaku D/Max Ultima II diffractometer (40 kV, 49 40 mA) using Cu-Ka radiation, and were typical of reported T1O2 and h-BN, wherein “a” and “r” refer to the anatase and rutile phases of P25-Ti0₂. Transmission electron microscopy (TEM) images were obtained using a JEOL 2010 TEM operating at an accelerating voltage of 200 kV. Fig. 6 shows BN before defects, Fig. 7 shows R25-Ή0₂, and Fig. 8 shows ball milled BN. X-ray photoelectron spectroscopy (XPS) data was obtained using a PHI Quantera System with monochromatic A1 KR radiation. XPS samples were prepared by placing 50 mg of material in an 80°C oven overnight to evaporate water before analysis. DR-UV of the powders was obtained by diluting with 60 wt% BaSCri, pressing into a wafer, and measured using a Shimadzu UV-2450 spectrometer. Nitrogen physisorption was performed following 5 hours (“h”) of evacuation at 150°C and BET surface areas are shown below in Table 3. Raman spectra was collected from 100 /cm to 3200 /cm on a Raman microscope with 532 nm excitation.

Table 3

[0082] The photocatalytic reactor used was fabricated. It was equipped with six 4W

UVC Ushio G4T5 low pressure mercury germicidal lamps and a stir plate. Table 4 shows the reported literature values of PFOA photocatalysts using UVC irradiation. The spectral irradiance values for the experimental system, as shown in Table 4, were measured using a UVP UVX radiometer equipped with a 254 nm sensor placed on the stir bar of the UV reactor. In a typical experiment, the optimum amount of photocatalyst (2.5 g/L BN, or 0.5 g/L T1O2) was added to a 100 mL quartz round bottom flask containing 20 mL of DI ¾0 spiked with 50 ppm (0.12 mM) PFOA and capped with a septum before placing in the reactor box and turning on light and stirring. Initial pH for most experiments was 6.5. Aliquots were removed and filtered with 0.20 pm syringe filters. Optimal loadings of BN and T1O2 were determined by dosing the reactor with varied masses of photocatalyst, then choosing the concentration at which the rate of PFOA disappearance was maximum, as shown in Figs. 9-12. PFOA concentration over time using various amounts of photocatalyst are shown in Fig. 9 (for BN) and Fig. 11 (for T1O2). The rate of reaction for various photocatalyst loading are shown in Fig. 10 (for BN) and Fig. 12 (for T1O2). Each loading in Fig. 10 and Fig. 12 was carried out twice, with nearly identical reaction rates. Due to changes in pH with time, degradation rates were determined using only the first points (i.e. <60 min) in the degradation profile. To assess possible leaching of BN or T1O2, select filtered aliquots were analyzed by ICP-OES. No leached B, N, or Ti was detected.

Table 4

[0083] Experiments using terephthalic acid (·OH probe) confirmed generation of radicals by both BN and T1O2, with BN producing relatively more ·OH than T1O2, as shown in Fig. 13. Superoxide/hydroperoxyl radical generation was also confirmed using nitrotetrazolium blue. [0084] The concentration of PFOA was determined by high-performance liquid chromatography with diode array detector (HPLC-DAD, 1260 Infinity II Agilent, USA) with a WPH Cl 8 column (4.6 mm x 250 mm, 5 pm). The mobile phase was 50 v% acetonitrile and 50 vol% of 5 mmol/L Na₂HP0₄ with a flow rate of 0.8 mL/min, a sample injection volume of 50 pL, and detected at 210 nm. The concentration of released fluoride was measured by ion chromatography (IC, (Dionex Aquion, 4 x 250 mm IonPac AS23, AERS 500 Carbonate Suppressor). pH measurements were recorded using an Orion Star A111 pH probe. Chemical actinometry using potassium ferrioxalate actinometer to determine photons adsorbed in the flasks in the photoreactor was done using standard methods. The rate of photons adsorbed was determined to be 2.6 pEin/L/s. Defining an apparent quantum yield (QY) as molecules of PFOA degraded per photon reaching the reaction fluid:

where D| PFOA | is PFOA degraded in time interval At, V is the reaction fluid volume, and g is the adsorbed photons in time interval At, as determined by chemical actinometry, we calculate that the T1O2, as-received BN, and ball milled BN have QYs of 0.18%, 0.4%, and 0.7%, respectively (i.e, as-received BN and ball milled BN are roughly 2 and 4 times more photon efficient than T1O2).

[0085] As seen in Fig. 14, Fig. 15, HPLC-DAD-detected PFOA concentration-time profiles are shown using BN (Fig. 14) and Ti02 (Fig. 15) with and without 254 nm irradiation. Also shown in Fig. 14 and Fig. 15 are the fluoride concentration profiles for BN (Fig. 14) and Ti02 (Fig. 15). PFOA concentrations unexpectedly decreased with irradiation time using BN. PFOA did not degrade with 254 nm light without BN or T1O2. PFOA had a half-life of 1.2h for BN in the reaction system, exhibiting a photocatalytic rate of 0.24 mg of PFOA/Lmin. PFOA degraded using T1O2, as expected, with a half-life of 2.4h at a rate of 0.11 mg of PFOA/Lmin. As shown in in Fig. 15, fluoride ions were detected after irradiation began. After 240 min., about 52% of the total initial fluorine was released as F for BN, as compared to about 40% for T1O2. Also, after 240 min., PFOA concentrations were not measurable in the BN case, suggesting the formation of shorter-chain PFAS byproducts. [0086] As discussed above, previous studies indicate PFOA can undergo stepwise decarboxylation and defluorination. Fig. 16 shows concentration profiles that typify reactions occurring in series (i.e., PFOA is degrading in a stepwise fashion over BN). Fig. 17 shows PFOA is also degrading and forming shorter chain intermediates over Ti0₂.

[0087] To probe the involvement of different ROSs, photocatalytic reactions in the presence of ethylenediaminetetraacetic acid (EDTA), superoxide dismutase (SOD), or ίέtί-butanol (TBA) as ROS scavenging agents were carried out. EDTA completely inhibited BN and Ti0₂ activity, indicating photogenerated holes are critical to PFOA degradation. Fig. 18 and Fig. 19 show PFOA photodegradation over BN (Fig. 18) and Ti0₂ (Fig. 19) using EDTA, SOD, and TBA. SOD initially inhibited BN and Ti0₂ activity before PFOA degradation became apparent (at around 30 minutes), indicating the importance of Ό₂/ΌOH species. TBA had a partial inhibitory effect, indicating that ΌH radicals are also involved in PFOA degradation. In all, this suggests a co dependent mechani sm, as shown in Fig. 20, in which holes and radical species degrade PFOA and related byproducts, rather than a single radical that is responsible for defluorination.

[0088] To explore reactions similar to that of SiC photocatalyst known to those skilled in the art, XPS was performed on BN before and at different reaction times, as shown in Fig. 21 and Fig. 22. Fig. 21 and Fig. 22 show XPS of B Is (Fig. 21) and N Is (Fig. 22) binding energies at different reaction times. The higher binding energy shifts of the B Is and N Is peaks (after 180 min.) to the formation of surface B-F and N-F bonds, respectively. PFOA and its shorter chain homologues can react directly with surface fi ll bonds formed during irradiation. The binding energy peaks shifted back after 360 min., and may be as a result of the release of the surface-bound fluoride into solution from the BN surface as the concentrations of PFOA and its fragments decreased. A reductive reaction pathway over in-situ hydrogenated BN sites may be coexisting with the photo-oxidative pathway (the oxidative pathway being shown in Fig. 20). The B-H surface species would be regenerated via photocatalyzed reaction with either proton or water species, as has been reported for BN and fluorinated BN in photocatalyzed water splitting. The XPS results imply concurrent hydrodefluorination during BN catalyzed PFOA photodegradation. [0089] The BN band gap should be too wide for 254 nm light absorption, yet PFOA underwent apparent photocatalytic degradation over BN. The absorbance spectrum of BN was measured, as shown in Fig. 23, as well as for T1O2, as shown in Fig. 26, through DR-UV. As expected and shown in Fig. 26, T1O2 has a wide absorbance throughout the UV region, extending from 200 to 400 nm. In comparison, the BN spectrum showed a small peak absorbance at 208 nm. As seen in Fig. 23, BN had very small, but non-zero, absorbance at 254 nm, which may be due to defects within BN, such as edge defects. The Raman spectra of the BN material was collected, which arises from in-plane vibrations of N and B in opposing directions. Its broadness and location are measures of defectiveness. As shown in Fig. 24, the BN (as received) was relatively defective. The full width at half-maximum was 12/cm, which was broader than the value of the bulk powder. The E2_g mode blue- shifted to 1367/cm, from 1371/cm for bulk BN.

[0090] After introducing defects via ball milling, the resulting material showed an increased absorbance in the UV-C range, as seen in Fig. 23, and a broader E2_g Raman peak, and a slight blue-shift was observed, as seen in Fig. 24. As shown in Fig. 25, the ball milled BN showed improved performance for PFOA degradation. The photocatalytic degradation rate increased from 0.24 to 0.44 mg of PFOA/Lmin after ball milling. These rates are 2 to 4 times greater than that of T1O2. The apparent quantum yields for as -received BN and ball milled BN were calculated to be 0.4% and 0.7%, respectively, higher than the value of 0.18% for T1O2. Thus, BN surface defects are necessary for light absorption and photodegradation capacity and predict that large- surface area BN, with high surface defect content, will show higher PFOA photodegradation activity.

[0091] Simulated drinking water was used to determine the effects of a realistic water matrix on BN photocatalytic properties. The water composition is shown below in Table 5.

Table 5

[0092] Fig. 27 shows the PFOA photodegradation rates over as-received BN and T1O2 in DI water and SDW. As shown in Fig. 27, BN outperformed T1O2, whose level of degradation after 120 min. dropped to 15%. Fig. 28 shows the PFOA concentration time profiles in SDW with reinjections of PFOA for the BN case. BN maintained activity following three cycles of PFOA degradation over 19 h, during which time the T1O2 was able to degrade only the initial spike over 16 h, as seen in Fig. 28. Fig. 29 shows the corresponding fluoride concentration profiles of the SDW with reinjections of PFOA for the BN case. As shown in Fig. 29, at the end of the experiments, the level of free fluoride in the BN system was 48 ppm, representing about 43% of the total fluorine added, while the T1O2 system had 25 ppm F^~, representing about 74% of the initial fluorine added.

[0093] Finally, the BN was tested for photodegradation of a short-chain compound

(i.e., GenX). Fig. 30 shows the GenX and fluoride concentration-time profiles using BN with 254 nm irradiation. As shown in Fig. 30, roughly 20% degraded in 2 h under the same reaction conditions, with an estimated half-life of at least 5 hours. This is the first reported heterogeneous photocatalytic decomposition of GenX. [0094] Systems herein may be effective for other fluorinated compounds and other contaminants. For example, systems according to embodiments herein may be effective for fluorinated and non-fluorinated organic compounds, such as Gen-X, 1,4-dioxane, pesticides, chlorinated solvents, pharmaceuticals (e.g., levofloxacin, sitagliptin, and fluoxetine, among others), and generally less stable organic compounds than PFOA.

[0095] The system directly and efficiently degrades persistent organic pollutants directly rendering them nontoxic. There is no need to pretreat the water with another technology. The technology does not require additional solvents. Furthermore, there is no need to use chemical additives.

[0096] Embodiments of the present disclosure may provide a secure source of clean, safe water to a broad set of stakeholders (e.g., industrial organizations, governmental organizations, and citizens). Embodiments of the present disclosure offer flexibility of targeted pollutant(s) and end-use application capacity or scale of delivered water rate. Embodiments of the present disclosure may transform water treatment from a large, chemical- and energy-intensive process toward compact physical and catalytic systems.

[0097] Embodiments of the present disclosure may be applicable in the treatment of wastewater, thereby providing potable water to serve stakeholders from households, to neighborhoods, and to remote towns. Scalability may be achieved in a wide variety of applications with proper unit design.

[0098] Embodiments of the present disclosure may also provide access to drinking water during times of natural disasters. Natural disasters may destroy existing wastewater treatment facilities, leaving the affected population vulnerable to contaminants in the drinking water. Some embodiments of the present disclosure may be implemented on the fly with commercially available supplies to set up a wastewater treatment system, therein providing the population with access to clean water.

[0099] In addition to drinking water, embodiments of the present disclosure may improve the water "footprint" of oil and gas exploration and production operations by helping to increase the quality of water cleanup for reuse and recycle. Embodiments of the present disclosure may improve the environmental impact of water use in industrial settings, thus saving energy and water resources. [00100] Component technologies used in embodiments herein may include fouling- resistant, high-permeability membranes that use engineered nanomaterials (ENMs) for surface self-cleaning and biofilm control; capacitive deionization with highly conductive and selective electrodes to remove sealants (divalent ions); rapid magnetic separation of paramagnetic nanosorbents for easy reuse; nanophotonics-enabled direct solar membrane distillation for low-energy desalination; disinfection and advanced oxidation/reduction using nanocatalysts; and template-assisted nanocrystallization for scaling control. Embodiments herein may utilize ENM interactions with water pollutants and substrate materials, providing integrated unit processes that immobilize, support, or recover ENMs. Systems disclosed herein may be resilient, economical, and highly efficient.

[00101] While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims.

Claims

CLAIMS What is claimed:

1. A system for degrading contaminants, including fluorinated compounds and other wastewater contaminants, comprising: a reactor containing a boron nitride photocatalyst and configured to receive an aqueous liquid comprising at least one contaminant; a light source configured to direct light to the boron nitride photocatalyst; and an oxidant source in fluid communication with the reactor.

2. The system of claim 1, wherein the reactor is a slurry reactor, and wherein the boron nitride photocatalyst is in particulate form or comprises boron nitride affixed to a surface of a particulate support.

3. The system of claim 1, wherein the boron nitride photocatalyst is contained in a coating disposed or dispersed on a surface of and/or within the reactor.

4. The system of claim 3, wherein the coating is disposed on an exterior surface of the light source.

5. The system of claim 1, wherein the boron nitride photocatalyst is contained within a membrane disposed within the reactor.

6. The system according to claim 1, wherein the reactor is a batch reactor.

7. The system according to claim, wherein the reactor is a continuous flow reactor.

8. The system according to any one of the preceding claims, wherein the light source is an ultraviolet light.

9. The system according to claims 1-7, wherein the light source is configured to emit light at wavelengths of 200nm to 400nm.

10. The system according to claims 1-7, wherein the light source is configured to emit light at wavelengths of 250nm to 400nm.

11. A method for degrading contaminants, comprising: introducing an aqueous liquid comprising a contaminant to a reactor, the reactor containing a boron nitride photocatalyst; introducing an oxidant to the reactor; activating the boron nitride photocatalyst by directing light waves from a light source at the boron nitride photocatalyst to form an activated boron nitride photocatalyst; contacting the oxidant and the contaminant with the activated boron nitride photocatalyst to react the oxidant and the contaminant, thereby degrading the contaminant·

12. The method of claim 10, further comprising disposing a coating comprising the boron nitride photocatalyst on a surface of the reactor or on a surface disposed within the reactor.

13. The method of claim 10 or claim 11, further comprising forming the boron nitride photocatalyst, the forming comprising at least one of ball milling boron nitride and hydrothermally exfoliating boron nitride.

14. The method of claim 12, further comprising mixing the boron nitride photocatalyst with a binder.

15. The method of claim 12, further comprising affixing the boron nitride photocatalyst on a nanomaterial to form a boron nitride photocatalyst containing nanomaterial and immobilizing the boron nitride photocatalyst containing nanomaterial in a membrane.

16. The method according to claims 10-13, wherein the light source is an ultraviolet light.

17. The method according to claims 10-14, wherein the light source emits light at wavelengths of 200nm to 400nm.

18. The method according to claims 10-14, wherein the light source emits light at wavelengths of 250nm to 400nm.

19. A membrane for use in a water purification system, the membrane comprising boron nitride affixed to a nanomaterial.