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 nanoparticles 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/desorption, 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:
Identify a new nanomaterial that is both highly selective and has a high capacity for arsenate and arsenite.
Probe the surface chemistry of nanoparticles to determine the properties that correlate with enhanced uptake of arsenic
Develop methods to support the nanoparticles in order to provide hydrodynamic properties required for standard water treatment equipment.
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.
A. 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.
B. 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
C. 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
D. 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
E. 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 syn
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