Functionalized surfaces for the destruction of pathogens and organics
1. A method for capturing a contaminant, the method comprisingexposing a fluid stream comprising a concentration of a contaminant to a functionalized component comprising a high charge density binding component to reduce the concentration of the contaminant in the fluid stream;
- wherein the functionalized component comprisesa substrate having a surface;
the high charge density binding component attached to the surface and capable of binding a microorganism or organic molecule or organic material thereto; and
a photocatalyst bound to the high charge density binding component;
wherein the photocatalyst comprises a delaminated sodium nonatitanate;
and wherein the high charge density binding component comprises aluminum or gallium polycationic clusters.
The disclosure is directed to a surface having a binding component applied thereto for the adsorption or capture of pathogens and organic molecules or materials. The surface may be a component of a porous or nonporous substrate. The binding component may also bind a photocatalyst to the surface for photocatalytic destruction of the captured pathogens and organic molecules or materials.
|Hydrogen peroxide modified sodium titanates with improved sorption capabilities|
Patent #US 7,494,640 B1
Current AssigneeSavannah River Nuclear Solutions LLS
Sponsoring EntitySandia Corporation
|Functional surface coating|
Patent #US 20030022216A1
Current AssigneeAccelerate Diagnostics Inc.
Sponsoring EntityAccelr8 Technology Corporation
|Coatings with controlled porosity and chemical properties|
Patent #US 5,589,396 A
Current AssigneeSandia Corporation
Sponsoring EntitySandia Corporation
- 1. A method for capturing a contaminant, the method comprising
exposing a fluid stream comprising a concentration of a contaminant to a functionalized component comprising a high charge density binding component to reduce the concentration of the contaminant in the fluid stream; wherein the functionalized component comprises a substrate having a surface; the high charge density binding component attached to the surface and capable of binding a microorganism or organic molecule or organic material thereto; and a photocatalyst bound to the high charge density binding component; wherein the photocatalyst comprises a delaminated sodium nonatitanate; and wherein the high charge density binding component comprises aluminum or gallium polycationic clusters.
- View Dependent Claims (2, 3)
This application is a divisional of prior U.S. application Ser. No. 13/720,643, filed Dec. 19, 2012, and claims the benefit of U.S. Provisional Patent Application 61/577,349, entitled “FUNCTIONALIZED SURFACES FOR THE DESTRUCTION OF PATHOGENS AND ORGANICS,” filed Dec. 19, 2011, both of which are incorporated herein by reference in their entireties.
The present disclosure is related to U.S. patent application Ser. No. 13/253,964 filed Oct. 6, 2011, titled “Biomimetic Membranes and Methods of Making Biomimetic Membranes,” which is hereby incorporated by reference in its entirety.
The United States Government has rights in this invention pursuant to Contract No. DE-AC04-94AL85000 between the United States Department of Energy and Sandia Corporation, for the operation of the Sandia National Laboratories and Contract No. DE-NA0003525 awarded by the United States Department of Energy/National Nuclear Security Administration. The U.S. Government has certain rights in this invention.
The present disclosure is generally directed to functionalized surfaces, and is more particularly directed to functionalized surfaces for the capture and/or photocatalytic destruction of pathogens and organics.
Fresh water supplies are becoming alarmingly scarce in the face of a growing population that demands a high standard of living. Water availability is impacted by both increased discharge of waste into the environment and faster depletion of clean, subsurface sources. In response, it is projected that wastewater reuse (gallons/yr) will necessarily double by 2035. Impaired water such as wastewater requires due diligence for removal of disease-causing pathogens; again, due to increased and concentrated anthropogenic contaminants.
Microbiological pathogens, viruses, protozoa bacteria and organic contaminants in particular, are of great concern in municipal and at-the-source water supplies, particularly as these sources become scarcer and more polluted with increasing population coupled with increasing industrialization of developing countries. Filtration (over coagulation for instance) remains a method of choice for water treatment for both municipal plants and at-the-source or home treatment, given the ease of use, rapid treatment rate, and re-usability filter media (thus reduced cost). However, size-exclusion filtration is challenging for small pathogens such as viruses. Therefore, filtration by chemical affinity is an attractive option. Self-cleaning membranes or surfaces are also particularly attractive for water treatment, in that biofouling is a considerable challenge. Furthermore, the ability to clean filtration media extends its use and lifetime.
The primary issues relative to microbiological contaminants are 1) viruses are virtually impossible to filter by size exclusion due to their small diameter, and 2) biofouling of membranes is a ubiquitous problem, regardless of the type of water that is treated. Finally, the public is increasingly wary of using tap water, let alone treated waste-water for domestic use. Incidents such as the 1993 Milwaukee cryptosporidium outbreak have eroded public confidence in municipal water quality. However, confidence would build with development of highly effective, state-of-the-art treatment technologies.
What is needed is a method and system for treating fluid streams that is less expensive and more effective than current methods and systems.
One advantage of the present disclosure is to provide an improved fluid treatment method and system.
Another advantage of the present disclosure is to provide a fluid treatment method and system that effectively removes pathogens and/or organic material from municipal water.
Another advantage of the present disclosure is to incorporate in a membrane a coating for preventing or removing biofouling of the membrane and extending its useful lifetime.
Another advantage of the present disclosure is to provide alternatives to titanium dioxide as photocatalytic materials for coating surfaces.
Another advantage of the present disclosure is to provide a layering method of affixing photocatalytic agents to surfaces, using materials that are resistant to photocatalytic oxidation.
Other features and advantages of the present disclosure will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the disclosure.
Wherever possible, the same reference numbers will be used throughout the drawings to represent the same parts.
According to an embodiment of the disclosure, a functionalized component is disclosed that include a substrate having a surface, and a high charge density binding component attached to the surface and capable of binding a microorganism or organic molecule or organic material thereto.
According to another embodiment of the disclosure, a method of capturing a contaminant is disclosed that includes exposing a fluid stream comprising a concentration of a contaminant to a high charge density binding component to reduce the concentration of the contaminant in the fluid stream. The high charge density binding component is attached to a surface of a substrate.
Other features and advantages of the present disclosure will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the disclosure.
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.
According to an embodiment, this disclosure is directed to functionalize ideal surfaces with 1) a chemical species that binds microorganisms and/or 2) a photocatalyst that is of ideal geometry (flat) and chemistry (anionic) for compatibility with the binding chemical species. High cationic charge is important for adsorbing the microorganisms by chemical affinity (rather than settling or some other physical mechanism). Hydrophobicity is also potentially important, but it can degrade under UV-irradiation for photocatalysis. In an embodiment, the functionalized surfaces may be applied to functionalizing membranes for water filtration, to improve filtration beyond size exclusion, into the chemical affinity realm. In another embodiment, the titanate layers alone may also be useful for antifouling; which is one of the biggest challenges of water treatment by filtration.
According to another embodiment, the present disclosure is also directed to the filtration of microorganisms and/or organic molecules and compounds by utilizing nano-engineered filtration membranes or filtration media that include a functionalized surface that captures pathogen and/or organic material. In an embodiment, the microorganisms may be pathogens. The nano-engineered filtration membrane or filtration media may also include a catalyst that catalytically destroys the captured pathogen and/or organic material.
Microorganism and/or organic material, which may be referred to as contaminants, capture by chemical affinity rather than size exclusion allows the use of larger-pore membranes, thus mitigating or eliminating problems related to very small pores, which includes high pressure (and energy) requirements to force the water through the membrane, pore clogging, and slow filtration rate. Furthermore, if filtration through a membrane or filter bed removes microorganism and/or organics with greater confidence, chemical treatment such as chlorination may not be required in some cases. Chlorination, though cheap and widely used, has the drawback of introducing soluble chlorocarbons into the water. Microorganism and/or organics destruction that can be manually initiated as by the disclosed invention allows for controlled cleaning to minimize biofouling, increase the lifetime of filtration media, decrease operation costs, increase efficiency of water treatment processes, and gain public confidence. These benefits help ensure a viable future for wastewater treatment technologies, and thus mitigate the worldwide growing water crisis.
The microorganisms may include viruses, biofilm-generating bacteria, protozoa, cysts, ova, fungi and pathogenic bacteria.
The organic material may include, but is not limited to pesticides, solvents, fuels, pharmaceuticals, polysaccharides, and other persistent organic compounds.
According to another embodiment of the present disclosure, a method is disclosed to functionalize a surface for filtration and/or purification of microorganisms and/or organic material. The functionalized surface may be formed on a membrane or filtration media.
Filtration media includes organic materials, such as, but not limited to membranes made of cellulosic and regenerated cellulose materials and/or polymers, such as, but not limited to polyethersulfone, and membranes, such as but not limited to polytetrafluoroethylene and other such products. Filtration media also includes inorganic materials such as, but not limited to glasses fibers, porous glasses, sand, and mineral compositions.
The filtration media may be in the form of a fibrous mass, plate, sintered plate, porous solid and gel material, mesh, lattice, or other physical constructs.
The filtration may be used for liquid and/or gas applications. The liquid application may be, but is not limited to municipal water purification, military applications in the field, water for industrial uses, household supplies, purification onboard ships, space vehicles, and other places where reliability and longevity of purification systems is critical.
The gas application may be, but is not limited to filtration of air for hospitals, submarines, airplanes, space vehicles, clean rooms, and other facilities needing where filtration systems must not be impaired by a buildup of contaminants.
The functionalized surface includes a first component, which may be referred to as the binding component. The binding component has an affinity for microorganism and/or organic material capture as well as serving as the binder for the second optional component, when present.
The binding component may be a compound whose molecules have a high charge density and are themselves resistant to oxidative degradation.
In an embodiment, the binding component may be an inorganic polycation. In an embodiment, the polycations may be polyoxometalate polycations. Polycations offer photodegradation resistance, which is problematic for comparably effective organic functional groups, and high charge density, important for capture of anionic contaminants.
In another embodiment, the binding component may be a silane coupling agent such as diethylene-triamino-propylsilane and numerous others. Silane coupling agents offer opportunities for photodegradation resistance as well as presenting a wide variety of functional groups to effect capture of contaminants.
In an embodiment, the binding component may be a silane with cationic functionalities, meaning that it presents a net positive charge to the surroundings after binding to a substrate such as a filter medium.
In another embodiment, the binding component may be an aminosilane or disilane having metal-based cations and/or polycations anchored thereto.
The binding component adsorbs the target material to the surface by presenting an opposite electrostatic charge. The binding component may also attach a photocatalyst to the surface by hydrophobic interactions, disulfide bonds, or other forms of affinity.
The optional, second component is a photocatalyst. The photocatalyst catalyzes destruction of microorganisms and/or organic compounds and materials. The destruction of microorganisms and/or organic compounds and materials is via controlled, UV-initiated production of reactive oxygen species.
In an embodiment, the photocatalyst may be a delaminated titanate. In an embodiment, the delaminated titanate may be a delaminated cesium or sodium titanate. In another embodiment, the delaminated titanate may be doped and/or treated with agents that may alter the catalytic properties of the titanate.
Agents for doping and/or treating the photocatalyst are numerous and provide possibilities of enhancing their catalytic efficiency, stability, and other desirable properties. Such dopants may include, but are not limited to nitrogen, lithium, niobium, or other elements or molecules.
Delaminated photocatalysts are readily attached to surfaces. In an embodiment, delaminated sodium titanates may be formed by a hydrothermal synthesis process as for example disclosed in U.S. patent application Ser. No. 13/253,964 filed on Oct. 6, 2011, titled “Biomimetic Membranes and Methods of Making Biomimetic Membranes,” the content of which is incorporated by reference in its entirety. The combination of an inorganic poly cation and delaminated sodium titanate catalyst on a surface showed synergistic functionality of capturing bacteriophage, followed by photocatalytic destruction.
According to another embodiment of the disclosure, the first component may a binding/catalyst component. According to this embodiment, the binding/catalyst component may be a highly charged or otherwise modified delaminated titanate that has dual functionality, serving as both the capturing component and the photocatalyst component.
In one embodiment, the disclosure may be directed to a binding component applied to a filtration medium. In another aspect of the disclosure, the disclosure may be directed to the binding component in conjunction with the photocatalyst applied to a filtration medium. For example, aluminum polycationic clusters might be bound to a surface to provide only capture functionality, while adding delaminated titanate nanosheets as a second component would add the ability to destroy captured contaminants via photocatalyzed oxidation.
In another embodiment, the disclosure may be directed to a binding component applied to a surface that is not a filtration medium but one intended for capture of contaminants from gas, water or other fluid around it. In yet another embodiment, the disclosure may be directed to the binding component in conjunction with the photocatalyst applied to a surface.
In an embodiment, the surfaces may be transparent and may provide transmission of electromagnetic radiation, including visible and/or UV light to the binding and/or photocatalytic component. In an embodiment, a delaminated titanate may be applied to a light-delivering surface, such as optically conductive fiber, light emitting diode, quantum dots, directly onto phosphors, or the walls of a transparent tube or pipe to create a backlit photocatalytic surface. Photocatalytic activity is thereby sited in locations otherwise difficult to illuminate by ultraviolet or other activating light. In another embodiment, delaminated titanates may be applied to a transparent surface in a fluid stream to reduce contamination by photocatalytic oxidation, or remove biofouling periodically or continuously.
Further in example, the organic polyion may be an ionic polyion that serves to both attract the polycationic cluster which attracts anionic contaminants. It also provides hydrophobicity to the surface which also renders it attractive to said contaminants.
Further in example, the organic polyion may be a polymer functionalized with an anion such as a sulfonate, carbonate, and nitrate. For example, the organic polyion may be polystyrene sulfonate.
The functional groups 116, which includes the binder and binder/photocatalytic component, may be may be deposited upon the surface 114 by exposing it to a pure solution of a functional component, which may or may not also require adjustment of solution conditions such as pH, for a predetermined time period. Functional components may be deposited in successive layers of same or alternating components, which may or may not involve adjustment of solution conditions, resulting in a complete coating with the desired properties.
In another embodiment, the functional group(s) may be selected for stability on that surface in aqueous conditions (i.e., does not readily detach).
In another embodiment, the functional group(s) may be selected for stability in a range of common aqueous conditions encountered in potable water including pH (˜6-8), ionic strength of the solution or may be selected for optimized functionality under particular aqueous conditions in accordance with chosen applications.
In another embodiment, the functional group(s) may be selected for stability in a fluid containing dissolved ions such as silica, phosphorous, Ca2+, Mg2+, Fe3+, etc.
In another embodiment, the functional group(s) may be selected for stability against photodegradation that will be performed by the attached photocatalyst designed to destroy the adsorbed microbe.
In an embodiment, cationic functional groups are used in general since most microbes, including viruses, are anionic under typical conditions, resulting in an electrostatic chemical affinity. Both organic and inorganic functional groups may be used. In an embodiment, the organic functional groups may be attached to the surfaces through silane coupling agents, and the inorganic functional groups may b e adsorbed onto the surface simply via electrostatic self-assembly.
In an example of the present disclosure, a filtration system was assembled including small columns filled with glass beads that had the bead'"'"'s surface functionalized by the chemical functional groups. In this configuration, the very large pores between the beads perform limited size exclusion filtration. The functionalized surface performs affinity or capture filtration. In this example, a 27 nm diameter bacteriophage, MS-2, which is a non-enveloped, chemically-resistant, waterborne RNA virus was used as a model contaminant due to its small size, ease of use in BSL-1 laboratory, and physical similarity to poliovirus, a notable waterborne pathogen, and ordinarily impractical to remove by filtration.
Filtration of MS2 bacteriophage using chemically modified surfaces was carried out in Pierce Spin Columns, which are autoclavable and have a bed volume of 2 mL (total capacity of 5 mL, catalog # P189896). These are dimensioned to sit inside standard centrifuge tubes, allowing loading and elution without pumps or tubing, using either gravity or centrifugation. The filtration medium and support for chemically-functionalized coatings was comprised of 0.5 mm diameter glass beads (Biospec). Before any coating procedure, the beads were cleaned with piranha, a 1:3 mixture of 30% hydrogen peroxide and concentrated sulfuric acid, for 4 hours, rinsed with deionized water, absolute methanol, deionized water, and dried at 100° C. under vacuum.
All silane coupling agents were purchased from Gelest, Inc, and these were used as received. Coating the beads with silane coupling agents was found to be most effective in a fresh mixture of absolute methanol, agent, and deionized water in volumetric proportions of 90:4:6, added in that order. Beads were exposed to the coating solution for 1-2 hours with continuous stirring, and then placed on filter paper in a Buchner funnel for vacuum-assisted draining and cleaning. The beads were rinsed twice with absolute methanol and at least 10 volumes of deionized water (using water first proved to be detrimental). The coated and cleaned beads were then annealed for an hour at 90-100° C., and were stored in a sealed glass container if not used right away. Secondary coatings, if applied (usually addition of a cationic metal (Al3+, Ba2+), were generally using aqueous solutions for one to two hours with stirring. Silane coupling agents (and their abbreviations) that were utilized include 1) aminopropylsilane (APS), 2) trimethyl-aminopropylsilane (TM-APS), 3) didecylmethl-aminopropylsilane (DDM-APS), 4) tributyl-aminopropylsilane (TB-APS), 5) diethylene-tri-aminopropylsilane (DE-TA-PS), 6) methyldisilane (MDS), and 7) benzyldisilane (BDS).
Aluminum polycations, [(GeO4)Al12(OH)24(H2O)12]8+ [(AlO4)Al12(OH)24(H2O)12]7+ and [(GaO4)Al12(OH)24(H2O)12]7+ (referred collectively as MAl127/8+; M=Ge, Al, Ga) were synthesized. The solution was adjusted to about one pH point above their natural pH for a final pH of between 5 and 6, and the aluminum concentration was 900-1000 ppm, as determined by ICP-MS. Beads were first coated with methyldisilane, then rinsed with deionized water and dried before use, but not annealed again. Analogous coating chemistries to glass slides, so that the coating could be characterized by UV-vis spectroscopy.
Dried, coated or control beads were loaded into the mini-columns at 2.5 grams each on the day of use. Air was displaced from the bed by adding deionized water, capping, inverting several times, and then allowing beads to settle under an excess of water. The beds were gravity-drained, then capped until needed. MS-2 containing solutions (105 pfu/ml concentration; pfu=plaque-forming unit) were added at 0.75 mL per column, a volume equal to the void volume of the settled bead bed, and allowed to reach equilibrium. One minute was allowed for binding, then elution was begun by first adding 0.25 mL of water, allowing it to sink in as eluate was captured in a centrifuge tube, followed by four more discrete additions of one in as eluate was captured in a centrifuge tube, followed by four more discrete additions of one milliliter apiece as the eluate pool was collected. This solution was then plaque-assayed (see below) for MS-2 population, post column filtration experiment.
Escherichia coli strain C3000, the required host organism for the MS2 model virus, was obtained from American Type Culture Collection (ATCC) and maintained in continuous cultivation at 36±1° C. either in tryptic soy broth or on tryptic soy agar (TSA, 1.4% agar).
The MS2 bacteriophage used in viral clearance experiments was ATCC strain 15597-B1O, reconstituted from the freeze-dried state according to ATCC instructions and expanded into an early log-phase E. coli culture in order to produce a high-titer seed stock for routine use. Numbers of individual MS2 were visualized as plaque forming units (pfu) by dilution with sterile Dulbecco'"'"'s phosphate buffered saline (DPBS) to a point where each live virus, when inoculated onto an agar plate of E. coli, would create a clarified zone (plaque) of infected and lysed bacteria.
For quantification, plaque assays were performed as described in the USEPA Manual of Methods for Virology. Briefly, test samples were diluted in sterile DBSS, with a target of 100-300 pfu mL−1, and combined with E. coli host cells in soft TSA (0.7% agar) at 44±1° C. The molten mixture was spread onto a solid feeder layer of TSA in a 100 mm petri dish and allowed to gel. Plaques were counted after 14 to 16 hours of incubation. For virus removal experiments, the target MS2 load was typically 105 pfu per mini-column.
As can be seen in
Furthermore, comparing the anionic controls, BDS (benzyldisilane) and MDS (methyldisilane) reveals the BDS actually has some binding affinity for MS-2, despite the fact that it carries a negative charge, while MDS captures negligible MS-2.
In addition, as DE-TA-PS also binds metal; the surface coating was modified with addition of Fe3+. While it was observed that this did not increase the affinity for MS-2, it may for other microbes that utilize iron in metabolic processes. The MDS and BDS provided a second silane functionality for coupling to metal cations or polycations such as divalent Ba2+, trivalent Al3+ and polycationic [MAl12O4 (OH)12(H2O)24]7+ (M=Al, Ga).
The anionic disilanes alone have minimal affinity for binding MS-2. However, the addition of the metal cation or polycationic clusters produced various coatings with affinity for MS-2. For example, MDS-GaAl12 is particularly effective in experiments measuring clearance of this virus from water. The BDS also provides a functional group that adsorbs in the UV-vis spectrum for characterization purposes.
In summary, both cationic charge and hydrophobicity exhibit good surface characteristics for capturing anionic microbial contaminants such as MS-2.
In another embodiment, aluminum polycations were used as the functional groups in experiments in solution as well as bound to surfaces, such as mica. Among aluminum polycations, GaAl12 increased removal of MS-2 compared to AlAl12 (Al13), which may be attributed to the poorer stability of Al13. Thus these studies proceeded with GaAl12 in particular.
A challenge in developing nanoscale coatings on surfaces is finding direct methods of demonstrating and characterizing successful application of functional materials, since they generally cannot observe directly. Indirectly, an expected trend in MS-2 adsorption may be observed, but a direct characterization method is also desired. UV-vis spectroscopy may be if the functional group is photoactive. In this regard, two routes for characterizing chemically modified surfaces by UV-vis spectroscopy have been applied, both utilizing multi-layer coatings. In both examples, the GaAl127+ polycation is alternatively layered with an anionic photoactive specie, and multi-layered growth was monitored after each coating was applied. In one case, the photoactive specie may be an anionic cluster of approximately the same size and shape as the aluminum polycation; phosphotungstate [PW12O40]3−. The second is a polymer, for example polystyrene sulfonate.
According to an embodiment of the invention, the photocatalysts of the present disclosure possesses one or more of the following criteria: 1) geometry conducive to surface coating; 2) chemistry conducive to surface attachment; 3) potent photocatalytic activity; 4) compatibility with functionalities that adhere microbiological contaminants; and 5) economy and simplicity.
In an embodiment, the photocatalyst may be a layered titanate applied with or without the use of an intermediate binding layer, depending on the charge of the surface. In an embodiment, the titanate may be a delaminated (exfoliated) titanate, for example Cs-titanate and Na-titanate.
In an embodiment, a Cs-titanate parent material may be synthesized by solid-state reactions and is a non-hydrous phase.
In another embodiment, a layered sodium titanate material is formed hydrothermally. This form of layered sodium titanate may be referred to as sodium nonatitanate (SNT), which is the subject of U.S. patent application Ser. No. 13/253,964 filed Oct. 6, 2011, titled “Biomimetic Membranes and Methods of Making Biomimetic Membranes,” which is hereby incorporated by reference in its entirety.
The general formula for SNT is Na4Ti9O20.xH2O, (x between 0.5 and 2). Although the Cs:Ti ratio varies slightly for the Cs-titanate material, depending on the synthesis, an average formula is Cs0.36Ti0.91O2. Both structures have approximately double-layers of edge-sharing TiO6 octahedra; the Cs-titanate has approximately 90% occupancy in the double-layers. A major difference between the Cs-titanate and SNT is interlayer water that hydrates Na+ in SNT, whereas the Cs-titanate contains no water. Peroxide-modified, delaminated titanate sheets, where peroxide ligands replace some oxo and hydroxyl ligands, as indicated by the yellow color of these materials, were also prepared. Peroxide modification may increases activity of TiO2 photocatalysis, such as by repressing electron-hole recombination. Peroxide ligation of titanate materials can also increase surface area as shown in ion sorption studies, thereby also potentially providing enhancement for heterogeneous catalytic activity.
Methyl-orange and bromophenol blue photo-decomposition by four derivatives of delaminated titanates were compared to each other, as well as to Degussa P25 TiO2. The four titanate catalysts are: 1) NaTi, 2) peroxide-NaTi, 3) CsTi and 4) peroxide-CsTi, which denote the parent material and peroxide ligation (or not).
For synthesis of the Cs-titanate, ion-exchange and delamination of the titanate materials, a Turbula® System Schatz WAB (Basel, Switzerland) was used for high-speed mechanical shaking. Powders were placed in a 60 mL plastic tube with approximately 30 YSZ milling beads and 20-30 mL alcohol (i.e. methanol, isopropanol). The tube was capped tightly and sealed with electrical tape and mixed for 20-60 minutes. At various stages of their preparation, titanate materials were characterized by powder X-ray diffraction (XRD), thermogravimetry (TGA), infrared spectroscopy (IR).
Powder X-ray diffraction was carried out on a Bruker D8 Advance diffractometer in Bragg-Brentano geometry with Cu-Kα radiation and a diffracted-beam graphite monochromator. Infrared spectra, FTIR, (400-4000 cm−1) were recorded on a Thermo Nicolet 380 FT-ID equipped with a Smart Orbit (Diamond) ATR accessory. Thermal analysis was executed with a TA Instruments SDT 2960 for simultaneous thermogravimetric and differential thermal analysis (TGA-DTA) under air flow with a heating rate of 10° C./min. High resolution transmission electron microscopy (HR-TEM) was utilized to characterize the size of the Degussa P2 TiO2 that provided a stable colloid suspension by processing in the Turbula (see below). HRTEM was done on a JEOL 2010 F with Gatan energy filtered imaging, in the High Resolution Microscope User Facility at UNM in Earth and Planetary Science Department.
A layered Cs-titanate was formed using a 1:2.65 ratio of Cs:Ti. TiO2 (10 g, 0.125 mol) and Cs2CO3 (7.7 g, 0.047 mol Cs) were mixed as an alcohol slurry using the Turbula®. The alcohol was removed in vacuo with heating (40° C.) in a crucible, and the crucible with finely mixed, dry precursor powder was placed in an 800° C. oven overnight. Using the resulting powder, the mixing, drying and heating was repeated one time to obtain the pure desired phase (confirmed by XRD).
SNT was synthesized hydrothermally according to the following steps. Sodium hydroxide (NaOH; 10 g, 0.25 moles) was dissolve in 48 mL DI water in a 125-mL Teflon liner for a Parr reactor. Titanium (IV) isopropoxide (9.6 g, 0.033 moles) was added while stirring vigorously. A white slurry was formed. The reactor was closed and placed in a 170° C. oven for five days. Approximately 4 g of SNT was collected by pressure filtration as a white powder product.
Proton exchange for Cs or Na was carried out with 1 molar HNO3 solution, again utilizing the Turbula®. Several grams of alkali titanate were combined with 75 mL nitric acid in a Teflon bottle for this process. In these exchange conditions, neither layered phase is observed to undergo any dissolution. The H+-exchanged powder was collected by centrifugation and washing; first with water then alcohol. Completeness of exchanged was checked by either energy dispersive spectroscopy (for Na or Cs), or thermogravimetry to 900° C., followed by X-ray diffraction of the heat-treated powder. If Na or Cs remained, sodium titanate or cesium titanate phases would be observed in the diffraction pattern. If the Na or Cs was or cesium titanate phases would be observed in the diffraction pattern. If the Na or Cs was completely removed, only TiO2 was observed. The Cs-titanate required three acid-exchange steps, whereas the acid-exchange of SNT was completed in a single step.
The layered titanates were delaminated by using a 40 wt % tetrabutylammonium hydroxide (TBA) aqueous solution diluted by approximately 50% water; and the H+-titanate powders are combined with the solution for Turbula® treatment for approximately 40 minutes. Again, the solid was isolated by centrifugation, washing and finally drying in a vacuum oven at 70 to 90° C. Both materials only required one step for complete exchange.
Peroxide treatment of the delaminated titanates were treated with peroxide by grinding the dried titanates to a fine powder with mortar and pestle, and suspended in 100% isopropanol at 0.5 wt. %. Hydrogen peroxide, 30% solution, was added dropwise with rapid stirring to a total of 2% of total volume, and allowed to react with continued stirring for 15 minutes. The yellowish peroxotitanates were separated from the mixture by repeated centrifugation and washing with isopropanol, and finally dried in vacuo at 70 to 90° C.
UV-vis spectroscopy was performed in either scanning (for characterization of titanate colloidal suspensions at concentrations of 20 μg/ml) or photometric mode (for quantifying dye concentrations) using a Shimadzu UV-3600 spectrophotometer and 1 cm quartz cuvettes.
Photocatalysis Experiments were performed using a germicidal lamp (GE model G15T8) as a photon source with a radiant energy peak at 254 nm and overall irradiance rated as 49 mW/cm2 at 2 cm distance. The target solutions were held at 18 or 26 cm from the source in multiple 5-mL volumes, contained in 6-well, polystyrene plates. In this way, up to twelve test articles could be exposed at once.
For most experiments, titanates were delivered in small volumes as colloidal suspensions created by shaking powdered preparations in deionized water with the Turbula® and YSZ beads, followed by centrifugation to remove all larger particles prone to settling. A colloidal suspension of Degussa P25™ TiO2 (Aldrich) was prepared in the same way as were those of the delaminated titanates. A sample of each suspension was thoroughly dried and its residue weighed to calibrate concentrations.
Photocatalytic activity at low pH was investigated using methyl orange (MeO), typically at 0.5 or 1.0×10−5 molar concentration in pH 4.7 to 5.0, 0.01 M potassium phosphate buffer. Photocatalytic degradation of bromophenol blue (BrB) at 10−5 M concentration was usually carried out at pH 8.0±0.1 in 5 mM sodium borate buffer, although other buffers and concentrations were used at times. Final molar concentrations of the dyes were calculated from readings of absorption at either 480 nm or 590 nm for MO and BrB, respectively, using standard curves.
Powder X-ray diffraction and infrared spectra of the delaminated titanates, NaTi and CsTi, from the SNT and Cs-titanate respectively, looked very similar to both each other.
NaTi and CsTi derived materials were very similar in their photocatalytic characteristics. However, the distinct advantage of the NaTi is the easier preparation. While CsTi involves two mixing and heating steps for the initial synthesis, the NaTi is just one simple step. More importantly, the exchange of H+ for the alkali, which is imperative for subsequent delamination with TBA-OH, occurs much more readily for NaTi. Complete proton exchange of CsTi was originally reported to require 3×24 hr exchange steps. With use of the Turbula®, we were able to completely the exchange in 2 hr exchange steps; yet three separate steps of providing fresh 1M acid solution was still necessary. On the other hand, H+ exchange of NaTi was complete in a single 2 hr contact time with 1M acid solution. This is a distinct advantage for both research and industrial applications. The obvious difference is the Cs in CsTi is not hydrated, whereas the Na in NaTi is hydrated in the parent material; and thus the hydrated ion is more readily mobilized in the aqueous environment.
Testing of the photocatalytic capacities of the delaminated titanates was initiated in the methyl orange, pH 4.7 to 5.0 system, using stable colloidal suspensions without the need of stirring over the 60 or 90-minute duration of UV exposures. The use of multi-well plates allowed the controlled comparison of several materials in a single session or the periodic withdrawal of samples from replicate wells without disturbance to the remainder. The ultraviolet light field previously was found to be suitably uniform using two mapping experiments that distributed 5-mL portions of a uniform titanate/MO mixture into 12 wells. Readings at 590 nm after 90 minutes exposure resulted in an average MO depletion of 2.82×10−5 mol/L/hr, with a standard deviation of 0.036, or relative standard deviation of only 1.3%.
At a concentration of 50 μg/mL in MO, photocatalytic oxidation rates of Cs-titanate and SNT-derived candidates, whether peroxide-treated or not, were close to one another (see
Performance differences between the titanates became apparent when they were used at suboptimal concentrations. Peroxide-treated versions of both SNT and Cs-titanate-derived catalysts routinely exhibited a modest advantage (2-19% oxidation rate improvement), as exemplified in
There was no consistent difference in the photocatalytic efficiency of the SNT versus the Cs-titanate-derived delaminated catalysts. However, as discussed above, the distinct advantage of the SNT is derived from the ease of preparation.
Methyl orange provides a convenient testing system, but is limited to a narrow range of acidic pH in which its color is stable. If the usefulness of photocatalysts is to include oxidative destruction of environmental contaminants, they must be efficacious at a higher pH, generally 7-9. Another organic dye, bromophenol blue (BrB) was selected because its absorbance spectrum is essentially stable between pH 6 and 9, a range conforming to most natural water sources.
In shifting the testing program from an acidic to a pH 8 environment, different buffers were evaluated. In an experiment that compared 5 mM sodium phosphate, sodium bicarbonate, and sodium borate buffers at pH 8.0±0.1, the depletion of BrB by delaminated titanate catalysis appeared to be enhanced in the phosphate by about 28 percent (with N=4, NaTi catalyst depleted BrB an average of 51.5±2.5% in borate and bicarbonate vs. 66±1% in phosphate in one hour) while this effect was not seen in TiO2. Several experiments were performed to confirm this phenomenon occurring for both the sodium- and cesium-derived catalysts, including an escalation of the phosphate content of 5 mM sodium bicarbonate buffer with the pH constant at 8.0 (see
Characterization of the different delaminated titanates via standard techniques such as X-ray diffraction, infrared spectroscopy and various microscopies (TEM, AFM, SEM) did not reveal obvious differences between the materials that we could link to variations in catalytic efficacy. UV-vis spectroscopy, measuring the adsorption bandgap energy, seemed the most sensitive technique in detecting differences between materials.
The photocatalytic activity of TiO2 and mesoporous TiO2 may be improved by doping TiO2 with approximately 1% phosphorous, presumably by increasing surface area and creating surface traps to delay e−/h+ recombination. This effect was not observed in prior experiments with Degussa P25 TiO2. However, the method of introduction of the phosphorous to the titanate materials was different. In the prior experiments, the phosphate was introduced before forming the TiO2 via calcination. According to the present disclosure, phosphorous was introduced only in the aqueous phase, so it would be incorporated into the delaminated titanates via surface adsorption or reaction with the surface. The surface of layered titanates is likely more reactive towards phosphate adsorption/reactivity than the TiO2.
Finally, the broad shoulder towards the visible wavelengths of the NaTi and peroxide-NaTi spectra indicate these materials may be more effective for photocatalysis compared to the CsTi materials, if a longer wavelength source was utilized in the application.
In summary, sodium nonatitanate-derived delaminated nanosheets are comparable in photocatalytic activity with Cs-titanate-derived catalysts. This is probably because the construction of the layers, edge-sharing TiO6 octahedra is similar for the two materials. The SNT-derived materials are very attractive for further development, since both their initial synthesis and processing is much simpler: From initial synthesis to final delamination, SNT requires approximately three processing steps compared to seven-nine steps for Cs-titanate. The primary differences are in the initial synthesis and the acidification prior to delamination.
Furthermore, sodium is much less costly than cesium. Since SNT is synthesized hydrothermally and Cs-titanate is synthesized via solid-state processing, both have advantages and disadvantages for chemical modifications such as by doping with other elements to enhance activity, selectivity, or shift the absorption band-gap. Both forms were shown to be superior to standard TiO2 when tested as colloidal suspensions under a germicidal UV light. This is probably a combined effect of higher surface area and a shifted bandgap that provides a better match with the UV-source.
Further treatment of these preparations with hydrogen peroxide appears to confer some degree of enhancement in most photodegradation experiments. Treatment with millimolar concentrations of sodium phosphate was seen to accelerate photocatalysis by the delaminated titanates rather significantly. These findings offer some new possibilities of simple chemical modifications to enhance photocatalytic activity.
The assembly and function of the entire ‘package’ of binding component and photocatalyst is now discussed. The titanate photocatalyst and GaAl12 cationic glue were assembled on mica surfaces, chosen for their pristine and atomically flat nature that is necessary for Atomic Force Microscopy (AFM) surface imaging and adhesion studies. In another embodiment, the substrate may be membranes such as polyethersulfone, sintered glass, or any material in current or future use in filtration membranes that is or may be rendered refractory to photocatalysis. The cationic function group [GaO4Al12(OH)24(H2O)12]7+ polycation was utilized to both adhere the titanate sheets to the mica surfaces, as well as provide an electrostatically attractive surface for the microorganism such as MS2 bacteriophage. In another embodiment, other functional groups may be used. Therefore, a cation-anion-cation multilayered sandwich is assembled on the mica, see
The photocatalysts remain active in the surface adsorbed state. MS2 adsorbed to 1) GaAl12 polycation only, and 2) GaAl12-titanate-GaAl12 were imaged. The photocatalytic destruction of the phage was demonstrated.
Other experiments were designed to determine whether delaminated titanates could be bound to a solid surface and retain photocatalytic ability. To this end, Cs- and Na-derived, delaminated titanates were bound to freshly-exposed muscovite mica (Ted Pella) surfaces that had been previously coated with [GaO4Al12(OH)24(H2O)12]7+ polycations, denoted GaAl12, synthesized as described prior. Four 1-inch square mica pieces coated only with GaAl12 (at 1 mg/mL in deionized water, with five minutes contact) and four with GaAl12 followed by a coating with the titanate were rinsed thoroughly with deionized water and dried at room temperature. Each piece was placed beneath 0.5 mL of 2.5×10−5 M MO and exposed to UV light for three hours, after which all of each MO load was recovered with limited rinsing, the final volumes equalized, and their absorbance was read to determine the amount of dye remaining.
Although several procedures and conditions were employed experimentally, the most successful protocol for adsorbing MS2 viruses to mica was the following. Mica (Ted Pella, Hi-Grade) was cleaved with a new scalpel blade and immediately immersed in GaAl12 (1000 ppm Al, pH 4.5 in water) for 10 minutes, then rinsed with purified water and allowed to dry. Subsequent coatings were achieved by applying the desired solution or suspension dropwise to the upper, newly cleaved and treated surface for a total of 0.5 mL on a 1-inch square of mica. After ten minutes, the liquid was aspirated from the mica, which was then rinsed with purified water and allowed to dry.
MS2 virus solutions used for coating were assayed at 0.2 to 1.0×1010 viable pfu/mL in 0.01 M phosphate buffer, pH 6. However, due to the serial subculturing used to generate the virus stock and the natural attrition rate of MS2, it is likely that viable viruses were greatly outnumbered by noninfectious capsids. For adsorbing MS2 onto titanate sheets, the delaminated titanate was applied to mica first, than counter-coated using a second treatment with GaAl12. After ten minutes, unbound GaAl12 was rinsed off, and MS2 solution was applied dropwise as described minutes, unbound GaAl12 was rinsed off, and MS2 solution was applied dropwise as described.
Tapping mode atomic force microscopy (AFM) using a Veeco Dimension 5000 Tool was used to examine surface-adhered titanate layers and MS2 adsorbed onto cleaved mica surfaces. With this AFM the mica substrate root mean square roughness was measured at 50 pm in a 1 um scanned area, indicating a noise floor for the system. Height and distance calibration was verified using standard calibration samples.
The use of colloidal suspensions of the titanate nanosheets is convenient for screening experiments utilizing different materials, conditions and contaminants; but surface-bound titanates have greater potential for practical use in remediating organic and microbial contamination in water. Furthermore, the delaminated titanate layers have the ideal morphology for adsorption onto a flat surface. A binding component or coupling agent may be used to affix the nanosheets to high-quality mica surfaces (specifically for AFM characterization). The titanate layers are anionic under most pH conditions, and therefore we considered cations that would adhere readily to the anionic mica surface when deposited from an aqueous solution. While organic cations can be utilized (polymers for instance), they might themselves be vulnerable to catalytic photo-oxidation. Inorganic polycations may be used as an adherence layer. Use of the aluminum tridecamer [AlO4Al12(OH)24(H2O)12]7+ (Al13) has in fact been reported for exactly this purpose. (34) However, prior studies have shown that the related Ga-centered aluminum cluster, [GaO4Al12(OH)24(H2O)12]7+, GaAl12 is more stable and carries a higher charge in aqueous solution, (1) so we opted to utilize this ‘cationic glue’ instead.
Dense coverages of delaminated titanate from both CsTi and NaTi, as well as the peroxide derivatives, were assembled on GaAl12-coated mica surfaces. Typical CsTi and NaTi coatings imaged by AFM are shown in
Photocatalytic degradation of methyl-orange utilizing layers derived from SNT affixed on mica surfaces demonstrated these catalysts are also functional in this form. Average remaining concentration of MO after the first UV exposure of the solid-phase catalyst squares was 0.16×10−5 M as oppose to the control squares with 1.30×10−5 M MO remaining, a reduction of 88% for three hours exposure time. The photocatalytic activity of Degussa P25 TiO2 was demonstrated for comparison and these are compared in
The TiO2 nanoparticles reduced the MO concentration to around 56%, again demonstrating the superior performance of the titanate nanosheets. The same mica pieces were rinsed off with deionized water, dried overnight, and used the next day in a repeat experiment. In the second trial of the same surfaces without additional coating, the remaining MO concentration recovered from the titanate squares was 0.14×10−5 M as oppose to the control squares with 1.29×10−5 M MO remaining, a reduction of 89%, again over three hours. There is no loss in photocatalytic activity, demonstrating the robust nature of these materials, and the appropriate choice of an inorganic ‘glue’ to bind the anionic titanate layers to the anionic substrate. Although mica was chosen to optimize conditions for AFM imaging, most filtration substrates or media are in fact anionic, so the inorganic polycation is important for more practical applications as well.
The disclosure is further directed to using delaminated titanates for selectively precipitating rare-earth metals out of solution. This is a property of delaminated titanates that is neither photocatalytic nor necessarily surface-bound. Rare-earth metals (RE) are used extensively in hybrid vehicles, wind turbines, ‘supermagnets’ and display and lighting applications. Given the current shortage of rare-earths on the global market, issues around mining, separating and recycling rare-earths have become exceedingly important. Sorption and precipitation of rare-earths may be applied in recycling of rare-earths, cleaning mine drainage, and other mining separation processes.
It was also determined that the titanate layers can be recycled for reuse. The Eu can be removed with acid, the titanate delaminated again, and reused for Eu sorption. This cycle is summarized in the equation below:
It should be noted that in a solution with sodium as a competitive ion (200:1 Na:Eu ratio), the Eu is removed selectively: 92% of the Eu is removed, indicating that successful uptake of rare earth elements is practical from saline waters.
While the disclosure has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. For example, telescoping or linear devices may be hydraulically driven, and/or these devices may be driven with hydraulics, air, water, or electricity or any combination thereof.
In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the disclosure.