Define interdisciplinary team organization: including, communication, meetings, and milestones.

PROJECT DESCRIPTION

INTRODUCTION

Inorganic nanoparticles can be quite useful for the selective removal of a wide array of target compounds from contaminated waters. Their very high surface area to volume ratios and unusual surface chemistries can lead to favorable sorption characteristics and reaction kinetics. Unfortunately, it is difficult to take advantage of these properties in the real world due to the small size of the nanoparticles. Thus, the use of nano particles in fixed-bed columns, in-situ reactive barriers and other flow-through applications are not possible because the particles can simply suspend and flow away with the water. If they are confined in some fashion (e.g. by a filter or frit), their small size and tight packing result in extreme pressure drops. Clearly, the use of nanoparticles for water purification requires the development of methods whereby the nanoparticles may be immobilized in a fashion whereby they retain their intrinsic sorption/description, properties while providing improved mechanical strength, durability, and beneficial hydraulic properties for flowing water systems.  One objective of the proposed research is the exploration of such methods of immobilization using both inorganic and polymeric supports.

 

Arsenic in drinking water is chosen as the target contaminant for this research although the lessons learned in this research will be applicable to a large host of other pollution and water treatment problems. Arsenic’s toxicity to man and other living organisms has led to serious environmental problems and difficulties in procuring suitable drinking water in many parts of the world. In well-oxidized waters arsenic is present predominately as arsenate (H2AsO4-1 and HAsO4-2) while under reducing conditions it is usually present as arsenite (H3AsO3 and H2AsO3-1) [1]. Since the reduction and oxidation reactions of arsenic are particularly slow, both of these oxidation states can coexist irrespective of the redox conditions [2]. Thus, a successful adsorbent must be able to remove both arsenic(III) and arsenic(V) from water. The latter species are generally easiest to remove but the arsenic(III) species are 25-60 times more toxic than arsenate and are more mobile in the environment [3]. Adsorption on mineral surfaces is an important factor that controls the mobility and bioavailability of arsenic. Arsenate adsorption on clays and aluminum and iron oxides is greatest at low pH and decreases with increasing pH while arsenite has a maximum in adsorption to these materials at approximately pH 8.5 [4]. The maximum contaminant limit for arsenic in drinking water was recently lowered from 50 ppb to 10 ppb by the U.S. EPA due to its carcinogenicity even it a low concentrations. Unfortunately many aquifers are contaminated with sufficiently high levels of arsenic that they will require treatment. Water utilities must be compliant with the EPA directive by January 2006 but this leaves many rural owners of ground water wells at risk.  Furthermore, the difficulty of efficiently achieving the lower limit of arsenic means that the development of active nanosystems for arsenic remediation will be a boon to all producers and users of drinking water in arsenic-impacted areas.

 

A large variety of materials have been tested for removal of arsenic from water including adsorbents such as phyllosilicates, silica, and hydrous oxides of iron and alumina [3]. The most successful and heavily investigated materials have been iron oxides, especially ferrihydrite [5-12]. Nanoparticles of magnetite, hydrated iron oxide, alumina, zinc oxide, nickel oxide and zero-valent iron have all been reported to be effective for the removal of arsenic from water [references]  Recently, Apblett et al. discovered that hematite and zincite nanoparticles showed a significant capacity for adsorption of arsenate despite the relatively low reactivity of bulk hematite and zincite towards arsenate[13]. This is another example of the well-established ability of nanoparticulate materials to display unusual reactivity towards chemical weapons and environmental contaminants (e.g arsenate, heavy metals, and halocarbons) as compared to bulk materials. This investigation will utilize thermally-unstable metal complexes to synthesize nanoparticles since this approach is conducive to scaling to industrial production. The target materials will be a family of oxide spinels (ferrites) , their thiospinel analogues, and the individual  oxides and sulfides that are components of the spinels.

 

With the myriad of materials that can adsorb arsenic, a question arises as to what factors are important for the adsorption process. If one examines the surface chemistry of zinc oxide nanoparticles, it is found that the surface is primarily acidic – a surprising result considering the basicity of zinc oxide. This can be one factor for the outstanding ability of zinc oxide nanoparticles to adsorb arsenic (see below). Therefore, an investigation of the adsorption of arsenic from water would be incomplete without a careful examination of the surface chemistry of the particles. Therefore, an important component of this research will be the determination of the surface elemental composition and the surface basicity and acidity (in both Lewis and Bronsted senses) both before and after exposure to water and adsorption of arsenate and arsenite.

 

In many of the above examples that used non-aggregated or unsupported nanoparticles to remove arsenic from water, the nanoparticles had to be separated from the treated water by centrifugation or ultrafiltration. This is incompatible with rapid, continuous flow-through treatment of water. Several researchers have successfully employed polymers, carbon-nanotubes, ion exchange resins, and mesoporous silica to support nanoparticles for the purpose of more readily and effectively removing arsenic from water [References]. Anionic ion exchangers were particularly successful due to the polymer’s positive charge enhancing the adsorption of arsenic anions. In this investigation, we will continue to apply the use of materials already utilized for water purification as supports for nanoparticles. Potential supports include ion exchange resins, activated carbon, and zeolites.

 

OBJECTIVES

 

The ultimate target of this project is the development of a viable, publicly-acceptable material that could be used in an at-the-tap device for removal of arsenic from water, in household in-line water purification systems. In order to achieve this goal, the interdisciplinary team will apply nanotechnology and perform the following tasks:

 

  1. Identify a new nanomaterial that is both highly selective and has a high capacity for arsenate and arsenite.
  2. Probe the surface chemistry of nanoparticles to determine the properties that correlate with enhanced uptake of arsenic
  3. Develop methods to support the nanoparticles in order to provide hydrodynamic properties required for standard water treatment equipment.
  4. Perform a sociological investigation to provide public input into the project from an arsenic-affected community and to measure the acceptability to the public of the developed technology

 

Project objectives can be classified into one of four categories: those for the overall project, those related to the synthesis effort, those concerning nanoparticle characterization, those related to immobilization on supports, and those involved in the sociologic investigation. Overall objectives involve interdisciplinary activities among members of the research team.  Details of these objectives are discussed in the Plan of Work.  An anticipated Timeline is presented at the end of the Project Description.

  1. Overall (Team) Project Objectives
  • Define interdisciplinary team organization: including, communication, meetings, and milestones.
  • Define and maintain the focus on supported nanoparticle production processes from a unique, multidisciplinary perspective.
  • Integrate and manage individual experimental, characterization, and testing objectives for rapid process development and effective scale-up.
  • Interact with industry and the public to maintain a practical (commercial) focus.
  • Train qualified personnel to contribute to the growth of the burgeoning nanotechnology and water treatment industries in Oklahoma and the Nation.
  • Meet all required timelines, manage expenditures, and assure other federal and institutional requirements are performed.
  1. Synthesis of nanoparticles with high capacity and selectivity for arsenic
  • Focus the initial nanoparticle synthesis on zinc sulfide to provide immediate supported nanomaterials for investigation in water purification by the Engineering group.
  • Prepare and characterize nanoparticles of the target nanomaterials from precipitated precursors. Measure arsenate and arsenite capacity and selectivity.
  • Prepare and characterize nanoparticles of the target nanomaterials from water-soluble precursors. Measure arsenate and arsenite capacity and selectivity.
  • Prepare aqueous suspension of nanoparticles for surface coating of supports
  1. Characterization of Nanoparticles
  • Analyze size, surface, area, surface composition, acidity, and basicity in order to correlate these properties with arsenic adsorption capacity and selectivity
  • Determine the effects of exposure to distilled water and typical tap water on the properties of the nanoparticles
  • Identify the changes in surface chemistry when arsenate and arsenite are adsorbed on the nanoparticles
  1. Testing of Supported Nanoparticles for Water Treatment
  • Determine the hydraulic properties of the supported nanoparticles to ensure they will be applicable to water treatment
  • Measure the leachability of the nanoparticles from the supports
  • Characterize the performance of the nanomaterials for arsenic adsorption using column experiments
  • Determine the kinetics (mass transfer coefficients) for arsenate and arsenite uptake
  • Investigate the regenerability of the nanometric arsenic sorbants
  • Demonstrate the treatment of arsenic-containing drinking water from Yukon, OK in flow-through columns

  1. Sociological Investigation
    • Gain public input into the design of the nanoparticles and criteria of the experiments
    • Gauge the public acceptability of the new technology

 

PLAN OF WORK

 

  1. Synthesis of Nanoparticles

 

There are several requirements that must be taken into consideration in the development of nanoparticles that can adsorb arsenic. Perhaps the foremost among these is a high degree of selectivity and a high capacity for sorption of arsenic. Since the surface properties of nanoparticles are often radically different from bulk materials, it is difficult to predict exactly how a given nanoparticle will perform. For this reason, this project will focus on a small closely-related family of nanomaterials in order to both characterize the dependence of surface properties on crystallite size and which of these properties are most conducive for arsenic adsorption. A second factor that must be considered is the toxicity of the metal ions used: the metals must be non-toxic in case they are released from the support and are ingested. In the event that the sorbant materials are used on a large scale (e.g. at a municipal water treatment plant) or disposal in a landfill becomes problematic, the possibility of regeneration must be addressed. Arsenate and arsenite can be stripped by strong acid or base so any nanoparticulate material that is not amphoteric can be regenerated using one of these reagents. It is also important that the metals used be inexpensive. Within the above constraints, the proposed research will focus on spinels, M2+M3+X4 where M=Fe, Co, Zn and X=O, S, and their component oxides and sulfides, Fe2O3, Fe3O4, CoO, Co3O4, ZnO, ZnS, CoS, and FeS. Notably, several of the latter materials have been shown to adsorb arsenic from water (see below). All of these materials could be regenerated by base with the exception of amphoteric zinc oxide. However. the ZnO nanoparticles could still be applicable as a single-use adsorbent and they are also included in order to completely probe the influence of the metal ions on the surface chemistry and arsenic uptake of the nanoparticles. The methods that will be used to make nanoparticles are also conducive to isolation of ferrihydrite, FeO(OH), and metal-doped ferrihydrite. Thus, the properties and arsenic adsorption abilities of the parent FeO(OH) and zinc and cobalt-doped FeO(OH) will be determined. The doping level will be varied from 5 mole % up to the maximum level where phase separation or spinel formation occurs.

 

The synthesis of nanoparticles in this investigation will utilize the pyrolysis at low temperature (<150˚C) of thermally-unstable precursors. This is a viable method for fabrication of nanoparticles that can be scaled to the level of industrial production. It is also compatible with synthesizing nanoparticles within substrates without thermal degradation of the substrate. In order to produce nanoparticles readily on a large scale the Apblett research group has designed metal carboxylate complexes that decompose at low temperature to yield the metal oxide and small, volatile organic fragments. For example the salts of 2-oximinopropionate (also call pyruvic acid oxime and designated as PAO in this paper) serve as useful nanoparticulate ceramic and catalyst precursors [[Apblett, 1994 #31; Apblett, 1997 #23; Apblett, 1997 #24; Apblett, 1997 #22; Apblett, 1994 #30]].  These salts decompose in the range of 85 to 170˚C to yield carbon dioxide, acetonitrile, water and a metal oxide, hydroxide, or carbonate (Scheme 1). The metal-containing product is generally obtained as a high surface-area nanocrystalline material due to the extremely low deposition temperature and rapid volatilization of the organic decomposition products. Indeed, the great difficulty in using this approach to nanoparticle synthesis is keeping the nanoparticles from escaping the pyrolysis vessel.

 

 

 

Scheme 1. Pyrolysis of metal 2-oximinopropionate complexes.

 

The nanoparticulate metal oxides derived from these precursors have already proved to be useful for arsenic adsorption. For example, nanocrystalline ZnO, Co3O4, FeOOH, and FeeO3 (hematite) all showed a marked ability to adsorb arsenate from aqueous solution. Their adsorption capacities based on a Langmuir adsorption isotherm are shown in Table 1. The precursors for these salts, M(PAO)2·2H2O  (M=Fe, Co, Zn) were readily prepared using a precipitation reaction between aqueous metal salts and an aqueous solution of sodium pyruvic acid oxime. Notably, the three PAO salts are isostructural so that solid solutions of the metals are readily prepared. Thermal analysis showed an onset of decomposition of these complexes at approximately 100-120˚C. Note, however, that the materials in Table 1 were prepared at high temperatures required for complete removal of hydroxide and carbonate from the surface of the particles. Even under these conditions the oxides were nanocrystalline, had elevated surface areas, and performed well for adsorption of arsenic. It is expected that even more startling results will be realized for materials prepared at lower temperature. For example, ZnO can be obtained with a 9.2 nm size and a surface area of 300 m2/g by pyrolysis of Zn(PAO)2·2H2O  at 150˚C. This investigation will probe the properties of the nanoparticles deposited at the lowest possible temperatures as determined by thermal gravimetric analysis.

 

Table 1. Properties of nanoparticles derived from PAO complexes.

Material Crystallite Size (nm) Preparation

Temperature (˚C)

Surface

Area (m2/g)

Arsenic

Capacity (mg/g)

Fe2O3 30 436 140 0.8
Co3O4 6.4 276 59 1.4
FeO(OH) 1.0 285 300 2.1
ZnO 25 389 37 2.1

 

 

It would also be beneficial to develop water-soluble PAO saltss that can be impregnated as a single-source precursor into the various substrates. Fortunately, the oxime (-C=N-OH) protons in the complexes are acidic so that they can be reacted with ammonium hydroxide to give a water-soluble salts.  Notably, the deprotonation of the PAO salts to form anionic complexes could have a major influence on the surface chemistry of the deposited nanoparticles. Therefore, this will be explored by isolation, characterization, and synthesis of nanoparticles using the ionic complexes.

 

M{O2C(=NOH)CH3}2 + NH4OH ® [NH4]+1[M{O2C(=NO)CH3}{O2C(=NOH)CH3}]-1 + H2O

 

The metal sulfides will be synthesized by pyrolysis of metal xanthates (ethyl dithiocarbonates) that decompose at 70-150˚C to yield metal sulfides and diethyldithiocarbonate (Scheme 2). This decomposition generates nanometric metal sulfides with high surface areas (100-300 m2/g).  The Apblett group has found that ZnS prepared in this manner had a very high adsorption capacity of 12.1 mg/g for arsenic (as arsenate). The iron, cobalt, and zinc xanthates are all insoluble in water and therefore could be precipitated on and within supports in the same manner as described above for PAO complexes: impregnation with aqueous metal salts followed by treatment with aqueous potassium xanthate. Pyrolysis in air below below 150˚C will yield the metal sulfides. The one exception is iron(II) xanthate which must be heated under an inert gas to yield FeS. In air, this precursor decomposes at 80˚C to yield nanocrystalline ferrihydrite and can serve as an alternative precursor for this material. The Apblett research group has found that water-soluble analogues of the xanthates can be prepared by reaction of choline or isethionic acid with aqueous carbon disulfide and potassium hydroxide. This replaces the ethyl groups of xanthate with charged alkyl groups, -CH2CH2-NMe3+1 and  -CH2CH2-SO3-1, respectively. The metal complexes formed with these dithiocarbonate derivatives are extremely water-soluble. The Apblett group has also found that co-precipitated metal ions form thiospinels such as ZnFe2S4 when heated to 150˚C.

Scheme 2. Pyrolysis of metal xanthate complexes

 

In the initial stages of this research the above precursors methods will be used to synthesize unsupported nanoparticles of Fe2O3, CoO, Co3O4, ZnO, ZnS, CoS, FeS, ZnFe2O4, CoFe2O4, ZnCo2O4, ZnFe2S4, CoFe2S4, ZnCo2S4, FeOOH, Zn0.5Fe0.95OOH, and Co0.5Fe0.95OOH. These will be characterized and tested for adsorption of arsenite and arsenate as described below. If any of the bimetallic species display improved sorption behavior over the parent oxides or sulfides, the ratios of the metals will be varied to determine the one that maximizes arsenic adsorption. The six most promising materials will subsequently be developed into supported nanoparticle systems for water treatment. However, early in the study the various precursors for zinc sulfide nanoparticles will be used to create supported nanoparticles since these materials have already shown significant promise in early testing.

 

 

  1. Immobilization of Nanoparticles

Gary: I need your references including titles. If you already have them in an Endnote library send that. If not, I can put them in Endnote format if you send me the stuff. Remember that we need titles.

 

In the arsenic adsorption experiments discussed above, a key step was the filtration using a fairly expensive 0.20 µm membrane filter. The fine nature of the nanoparticulate oxide and sulfides required this since the particles readily pass through normal sintered glass frits and also suspend too readily for decanting. Unfortunately, filtration is not a viable approach for rapid water treatment. Therefore, an important aspect of the project is the development and testing of strategies to integrate nanoparticles into existing water treatment equipment and document their performance.  Integration into existing equipment has the highest likelihood of getting these new materials into the marketplace quickly.  The objective is to develop supported nanoparticles that have two key components:

(1) A new nanomaterial that is highly selective and with high capacity for arsenic that will be identified by the procedures described above

(2) A support with the hydrodynamic properties expected for standard water treatment equipment.

 

This objective can be achieved by embedding the nanoparticles into existing commercial products such as ion exchange resins, granular activated carbon or zeolites.  We will also use macroporouspoly(styrene-co-divinylbenzene) beads that are similar to the ion exchange resin beads but bear no charged groups and are used for sorption of organic species from water.  For the purpose of this investigation the zeolite material (materials?) that will be investigated are _.  Gary: the zeolite will need large pores, I suppose chabazite is one choice but you may have others in mind. Also, does it make sense to use cation exchangers when the species we wish to adsorb are anionic? This could make sense if exchange of counterions into the cation exchanger  helps drive the adsorption of arsenic. By starting with a known treatment material the hydrodynamics in practice are well known.  Another advantage of using current commercial materials is that these materials can continue to perform the function for which they were designed, while also supporting the nanoparticles that will increase their efficiency.  All four types of supports are worthy of consideration since they each have unique physical and chemical characteristics.

 

Kralik et al. (2000) discuss the advantages of a polymer support for metal nanoparticle catalysts.  The advantages presented for a polymeric support would also be important in this application as well.  The details of their experimental methods are of particular significance to this proposal.  Key points discussed by the authors include characterization of the catalyst and their specific properties, effects on mass transfer properties and selectivity, and deactivation due to interaction with the polymer support.  Narayanan and El-Sayed (2005) also indicate that the three dimensional shape and position of the nanoparticles within the support can impact the performance of the particles.

 

The hydrodynamics of flow through a packed bed is well established (e.g. Ergun Equation).  Mass transfer removal processes using either adsorption or ion exchange is well studied where the mass transfer coefficient (in the dimensionless Sherwood number) is typically a function of the flow properties (Reynolds number) and the molecular diffusion properties (Schmidt number).  We have done extensive correlations of MTC data for the removal of trace concentrations of ionic species from water (Chowdiah et al. (2003) and Lee et al. (1997)).  Given the basic properties (capacity and selectivity) of the adsorbent or exchange materials we can accurately predict their performance in a water treatment cycle.  Secondary properties do become an issue in selection of material; for example leachables and degradation.

 

Some recent literature discusses the importance of mass transfer and looks for ways to get nanomaterials into large particle configurations in order to address hydrodynamics.  Sebastien et al. (2002) discuss the mass transfer resistances of two forms of a nanomaterial developed into both a powder and a larger particle configuration.  They found that the larger particles were crystalline and had improved mass transfer properties to compressed powder.  Westerhoff and Hristovski (2005) considered adsorption of arsenic on TiO2, looking at alternative methods of using nanoparticles effectively.  One method utilizing agglomeration had significantly improved mass transfer and adsorption properties than commercially available porous metal hydroxide media.

 

Three types of immobilization will be considered:

  1. A coating layer of particles near the surface of the support. This could be achieved by adsorption of charged nanoparticles to the surface from aqueous suspensions. Therefore, the nanoparticles of the materials described above will be synthesized as aqueous suspensions by performing the decomposition of the water-soluble precursors in water in pressurized vessels at 120˚C, a temperature suitable for decomposition of both PAO and dithiocarbonate complexes. If necessary, sodium citrate or sodium mercaptoethylsulfonate will be added to prevent the nanoparticles from agglomerating. Particle sizes will be measured by transmission electron microscopy and dynamic light scattering prior to deposition of the nanoparticles onto the substrates. Many of these nanoparticles will be positively or negatively charged at typical drinking water pH  and thus, they could be adsorbed onto an ion-exchanger or zeolite with opposite charge although the binding may be weak and pH-sensitive.  However, the metal sulfide nanoparticles derived from choline dithiocarbonate and isethionic dithiocarbonate precursors would bear positively-charged and negatively charged residues (respectively) from the choline and isethionic acid residues on their surface, These would provide excellent tethering to an oppositely charged support. Similarly, the oxide nanoparticles could be tethered by treatment with a reagent such as sodium citrate.
  2. Impregnation of the particles into the support to a specific depth. This can most easily be accomplished by an in situ precipitation method when using a charged substrate such as ion-exchange resins. or zeolites. The procedure would involve impregnation of the substrate with a solution of the metal nitrates or chlorides to incipient wetness followed by treatment with an aqueous solution of NaPAO or potassium xanthate. This will cause the PAO or xanthate salts to precipitate close to and on the surface of charged substrates. The reason for this is the Donnan exclusion that hinders inward migration of ions with the same charge as the ion exchanger causing precipitation to occur as a band close to the surface. For example, the Apblett research group has prepared outer shells of nickel metal nanoparticles about 10 µm thick by treatment of a metal-loaded cation exchanger with sodium borohydride. [Reference]. Subsequent firing would yield the supported nanoparticles restricted to depths that may be controllable by the amount and concentration of reagents used or multiple deposition steps prior to thermal treatment.
  3. Even distribution of the particles throughout the support matrix. This can be achieved by the in situ precipitation method when using non-charged substrates such as granular activated carbon and neutral poly(styrene-co-divinylbenzene) beads. A second method is to impregnate the substrates with water-soluble precursors followed by heating to deposit the nanoparticles.  The ammonium salts of deprotonated PAO complexes and the water-soluble analogues of the xanthates with charged alkyl groups, -CH2CH2-NMe3+1 and  -CH2CH2-SO3 will be suitable for this approach. Note that with anion-exchange resins the anionic metal complexes would ion-exchange directly into resin ensuring distribution of the deposited nanoparticles throughout the resin bead. Similarly the cationic analogues of xanthates would be useful for placement of metal sulfide nanoparticles within zeolites. The placement of nanoparticles within cation exchangers will not be pursued since it has been shown that Donnan exclusion prevents the uptake of arsenic [reference].

 

Each method has specific advantages that would allow engineers to control the material to specific process requirements.  The primary difference between the three will be increased support mass transfer resistance with increasing nanoparticle depth.  This variable gives engineers control of the adsorption process over a range of operating conditions.

 

An example application would be loading nanoparticles onto a commercial anionic exchange resin – typically 0.5 mm diameter.  Operation with this bead size is well understood and equipment for its use is readily available.  The hydrodynamics and pressure drop is well understood.  However, specific questions must to be addressed prior to including nanoparticles within the bead matrix.  These include:

  1. Once the nanoparticles are embedded onto or into the support is the attachment secure? If not, what is the leachability of nanoparticles from the support?
  2. Are there any nanoparticle-resin interactions that impair or enhance selectivity and capacity? This can be answered by comparing supported to unsupported nanoparticles
  3. Can an anionic material still be regenerated with base? If so, are the nanoparticles regenerated or affected as well?
  4. How does arsenic migrate through the nanoparticle/resin support matrix? For exchange materials, some arsenic will be exchanged onto ion exchange sites as well.  Arsenic held on ion exchange sites will leach off the support easier, but may then bind on the nanoparticles.
  5. What is the overall performance of the material in laboratory column studies?
  6. Which nanoparticle/support combination works best?

 

 

In general, the experimental methods used to evaluate new exchange/adsorption materials include:

  • Determining the location of nanoparticles embedded on or within the support using electron microscopy;
  • Packing chromatographic columns with the material and injecting known solutions to obtain retention time and peak height in order to calculate basic selectivity and capacity properties.
  • Preparing a laboratory test column (usually 2.54 cm ID x 20 to 100 cm bed depth) and obtaining mass transfer coefficient, equilibrium leakage and particle throw.

 

With these experimental data we have the computer program capabilities to define performance of large-scale equipment and at-the-tap devices.

 

 

  1. Characterization of Nanoparticles and Supported Nanoparticles

 

Samples of the various precursors both neat and impregnated into supports will be heated in a microprocessor-controlled muffle furnace to temperatures corresponding to the temperature at which the organic ligand decomposes, as identified in thermal gravimetric analysis experiments. The resulting nanoparticles and supported nanoparticles will be characterized as follows:

XRD Analysis             The XRD patterns of the as-prepared materials will be recorded and a search match program along with the ICDD database will be used to identify any known crystalline phases present in the sample.  Scherrer analysis will be employed to estimate the crystallite size [44] of the nanoparticles. The XRD analysis will be repeated after exposure of the nanomaterials to pure water, tap water, and aqueous arsenate and arsenite to check for formation of new crystalline phases. In particular, formation of arsenic-containing phases would tremendously enhance the adsorption capacity for arsenate or arsenite but would hinder the regeneration of the nanoparticles.

Infrared Spectroscopy            The vibrational spectra of the ceramic materials will be obtained using diffuse reflectance infrared spectroscopy (DRIFTS), a method that is highly effective for qualitative and quantitative analysis of powders [45]. The spectra are expected to provide detailed information concerning the nature and identity of decomposition products and sulfate, hydroxide or carbonate on the surface of the particles

Surface Area and Pore Size Distribution Analysis     Specific surface area measurements of the nanoperticle systems will be performed by adsorption of N2 at low temperature using the Brunauer, Emmett, and Teller (BET) technique [46].  The desorption isotherm will be used to determine the distribution of pore sizes according to the Barrett-Johner-Halenda method [47]. The changes in pore size and surface area with sintering temperature will be determined by analyzing the samples heated to various temperatures as described above.

 

Electron Microscopy   Scanning electron microscopy will be performed on the various nanoparticulate products after they are first sputter-coated with gold or carbon to make the particles electrically-conductive. This will allow the characterization of the surface topography of larger particles and the degree of aggregation of the particles and will also provide an assessment of the size of primary particles with sizes above 3 nm. Transmission electron microscopy will also be used to study the particle size and morphology of the nanoparticles, and their distribution within the supports. The samples of neat nanoparticles will be prepared by ultrasonically dispersing a few milligrams of the ceramic powder in a few milliliters of alcohol and evaporating a drop of the resulting suspension on a TEM grid.  Supported nanoparticle systems will be embedded in resin and then sliced with a diamond knife to obtain a thin cross-section.  In both microscopies, elemental mapping by EDAX will be used to determine the composition of the nanoparticles.

 

Dynamic Light Scattering  Particle sizing of neat nanoparticles suspended in water will also be performed on a Dynamic Light Scattering apparatus. The particle suspensions will be made by ultrasonically suspending the particles in either pure water or water containing sodium citrate as a defloculating agent.

 

X-ray Photoelectron and UPS Spectroscopies  The surface of nanoparticles often does not reflect their bulk composition, Ions of one type or another can segregate at the surface, gases (e.g. CO2 and H2O) can be adsorbed from the air, oxidation of sulfide to sulfate, or displacement of sulfide by hydroxide or oxide all can and do occur. Therefore, the surface chemistry of the nanoparticles will be determined by X-ray Photoelectron and UPS spectroscopies of the as-deposited neat nanoparticles, nanoparticles exposed to pure water and Stillwater, OK drinking water. nanoparticles used to treat arsenic-containing solutions in distilled water, and nanoparticles used to treat arsenic-containing water from Yukon, OK. Nick: please add XPS and UPS boilerplate

 

Determination of Brønsted- and Lewis-Acid Sites    It was discovered that the zinc oxide nanoparticles discussed above had surfaces that were profoundly acidic (148 µmol/g acid sites versus 57 µmol/g base sites) a surprising result considering bulk ZnO is basic. It is thought that the ability of the nanoparticles to adsorb arsenate when bulk ZnO does not may be due to this acidity (indeed the adsorption of arsenic in the sequence Fe2O3, Co3O4, and ZnO roughly correlates with the surface acidity. The acid properties of catalyst materials and sorbants are often determined using probe molecules coupled with either infrared or NMR spectroscopic analysis [48]. The hydroxyl groups on the surface of metal oxides have varying acidity that is dependent on the local structure of the metal to which they are attached. If these groups can react with bases, the catalyst will display Brønsted acidity:

M-OH + B ® M-O + BH+

Metal ions on the surface provide Lewis acid sites. On the other hand sulfide and oxides ions provide both Lewis and Bronsted base sites. By reacting the nanoparticles with suitable bases, B, and then obtaining the infrared spectra, the intensity of the IR vibrations of BH+ can be used to determine the number of acid sites.  The use of bases with a range of pKa values will provide a method of estimating the acid strength of the hydroxyl groups.  Lewis acid sites present on the surface of a powder due to coordinatively-unsaturated metals can also be determined by infrared spectroscopy and probe molecules:

M + :B ® M-B

If B-H and M-B have differing IR spectra, the adsorption of a single base can provide characterization of both Brønsted and Lewis acidity. Many nitrogenous bases are useful for this purpose: pyridine, ammonia, and butylamine being used most often [48]. Each of these bases will be adsorbed onto the oxide materials produced in this investigation and the infrared spectra will be recorded Nick: you may want to chose just one probe or go with all 3 or chose others. As well, temperature-programmed desorption will be used to determine the amount of each base taken up by the oxide and the strength of their interactions with the nanoparticle. Nick: Please add appropriate details. Similarly, Lewis base sites can be probed using trimethylborate or CO2 adsorption/desorption.

The surface acidity and basicity of the supported and unsupported nanoparticles before and after exposure to water, aqueous arsenate, and aqueous arsenite will be determined. It is hoped that the latter two experiments when compared to the first two will shed light on which surface properties are important for arsenate and arsenite adsorption.

 

  1. Measurement of Arsenate and Arsenite Capacity and Selectivity

 

Adsorption isotherms will be measured using batch reactions at pH 4.6, 7.0, and pH 9.2 and initial arsenic (arsenite or arsenate) solution concentrations of 20, 50, 100, 250, 500, 1000, and 2000 ppm. Note that the pH range corresponds to that used by Raven et al. when investigating the adsorption of arsenic by ferrihydrite. 10 ml of each of the solutions will be placed 15-mL glass vials and 0.050 g of the nanoparticles will be added to the vials (the amount of supported nanoparticle systems used will be adjusted to give a comparable mass of nanoparticles). The vials will then be capped and placed on a rotary mill for 24 hours to ensure complete reaction, After 24 h, the supernatant will be passed through a 0.20 µm membrane filter, the pH will be measured and the sample will be analyzed for arsenic content by atomic absorption spectroscopy. The adsorption data will be analyzed by both the Langmuir and Freundlich, equations (reference) since both have proven useful for modeling the absorption of arsenic by various nanoparticulate systems, The equation that gives the best fit to the data will be used to calculate the adsorption capacity. In the case of supported nanoparticles, the arsenic adsorption capacity of the support (if any) will also be determined

 

The selectivity of the nanoparticles for arsenate and arsenite versus benign anions found in ground water such as phosphate, sulfate, and carbonate will be determined using similar batch experiments as described above. However, the arsenite and arsenate solution will be spiked with one of the competing ions in a molar ratio of 1:1, 1:5, and 1:10 arsenic:competing ion. The magnitude of the drop in adsorption of arsenate or arsenite with increasing competing ion concentration will be inversely proportional to the selectivity for arsenic adsorption

 

The unsupported nanoparticles that exhibit the best combination of arsenic adsorption capacity and selectivity will be targeted for development into supported nanomaterials. Nevertheless, the selectivity and capacity of the nanoparticles on the support will also be measured since the support is likely to enhance the capacity and may change the selectivity.

 

 

MANAGEMENT, OUTREACH AND EDUCATIONAL PLAN

 

The Team

 

The chemical engineering ion exchange laboratory has been performing resin testing and column tests for more than 20 years.  Some of these tests have been to evaluate property data of ion exchange resins – for example, defining the selectivity coefficient of dimethylamine (DMA) used for pH control in nuclear water chemistry.  Experimentation has been performed to evaluate amine-resin interaction between mixed beds of cation and anion exchange resins and ethanolamine (a project performed in collaboration with Prof. Apblett).  Experimentation has been performed to generate laboratory data in order to model the performance of ion exchange resins for ultrapure water applications.  OSU has developed an accurate ion exchange model that has proven to be valid for nuclear and coal fired power plant chemistry and microelectronics grade water.  The model has also been used for select environmental applications and the fate of trace metals.  The laboratory has the ability to do both small column testing and Ion Chromatography packed-column testing.

 

The educational benefits of this project will be tremendous since the students involved in the project will be involved in the interplay between three separate disciplines of science and engineering and will participate as part of a truly collaborative team.  Beyond the personnel provided for in the budget, undergraduate students will also be recruited to work on various aspects of this project.  Avenues of involvement for such students include the Freshman Scholars Program plus a variety of research fellowships and senior projects for academic credit.  The experience of working in an interdisciplinary team on a project that is developing critical technology for the future will produce well-qualified personnel that can help meet the human resource needs of industry and academia.  Components of this research will also have a broader impact on the graduate and undergraduate student population since results will be discussed in a Materials Chemistry course and in our nanotechnology course offerings that we initiated as part of the state-wide “Nanonet” NSF EPSCOR project.  OSU is a leader in satellite course offerings of all kinds and this could help facilitate the outreach to students at other universities. In recognition of the interdisciplinary nature of nanotechnology research, we propose during the summer months to have the students and postdoctoral fellows rotate through the three constituent laboratories for several days so that they can gain experience in all aspects of the project. All personnel will also be involved in assisting the political scientists in conducting the public focus groups. We also propose to hold joint group meetings of all involved personnel at least three times per semester to provide for cross-fertilization of research ideas.

For broader public education, we will also establish and maintain a web site that details our efforts in this research and delineates the promise of nanotechnology. Our University publicity office will also aid us in disseminating promising results through local media. Professor Apblett is also involved in public outreach concerning nanotechnology through the local section of the American Chemical Society and is expanding this effort to include OSU’s Center for Science Literacy. One of the major customers for the developed technology is industry. In recognition of this, we have already committed ourselves to provide lectures concerning the technology at Nomadics, Inc. We anticipate expanding this effort to the numerous other Oklahoman companies with a strong interest in nanotechnology and water purification.

OSU is strongly committed to the recruitment and retention of graduate and undergraduate students from underrepresented minority groups. The University’s location makes it a particularly important venue for involvement of Native Americans in science and engineering. With the assistance of our Vice President of Multicultural Affairs, Earl Mitchell (need the proper name of the new guy), we will vigorously pursue the recruitment of minority students to this project. The project has already attracted a female chemistry doctoral candidate, Christine Dewan, a first year student who is performing research on the PAO complexes while being supported as a teaching assistant.

 

 

 

 

 

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