Design of Risk-reducing Innovative-Implementable Small-System Knowledge (DeRISK) Center

Project: Research project

Description

The unique non-ligand forming chemistry and high solubility of nitrate have limited the range of available options for sustainable removal of nitrate from water, especially in context of small systems applicable to rural communities with underdeveloped infrastructure [REF Cotton 1972; Greenwood 1997]. Although very efficient in minimizing byproduct production, microbial based systems for treatment of nitrates are generally rendered impractical for use in small and rural communities because of the specifics stemming from operating a biological system. These systems often require highly trained personnel to control and manage the specifics of biological denitrification which include electron donor addition (e.g. methanol, hydrogen gas, etc), oxygen level control, and biomass management [REF: Park 2009; Rittman, 2010; van Ginkel 2009; McAdam, 2006; Xiao, 2010]. Other potentially applicable technologies, such as nitrate selective ion-exchange or membrane based technologies, do not comprehensively address the problem of nitrate treatment because they are designed to isolate and concentrate nitrate from water, consequently producing concentrated brines. High concentrations of sodium, potassium, chloride, nitrate, sulfate, and potentially other oxyanions create a major disadvantage for utilizing these technologies by necessitating costly management and disposal solutions [REF Ziv-El, 2009]. Zero valent iron has also been considered for treatment of nitrates; however, its effective use as a redox technology has been rendered inapplicable because of the challenges that stem from managing iron nitrogen byproducts [REF: Westerhoff 2003; Westerhoff and James 2003]. Photocatalysis is emerging as a novel redox technology for water treatment applications that could address these operational and byproduct challenges. Photocatalysts have an ability to create photogenerated conduction band electrons (e- cb) and valence band holes (hvb +) on their surfaces, which can engage in redox reactions with sorbed aqueous species. Titanium dioxide (TiO2) has been identified as the most efficient photocatalyst with high selectivity for nitrate reduction to innocuous byproducts such as nitrogen gas [REF: Doudrick 2013]. Addition of silver as a co-catalyst has amplified the efficiency and selectivity of TiO2 for photocatalytic nitrate reduction. Silver has ability to create strong Schottky barrier and trap e- cb, which are producing reducing radicals [REF Takai 2011]. For nitrate reduction, the TiO2 based photocatalysts require addition electron donor to decrease the electron-hole recombination and promote production of reducing radicals. Formic acid is the most efficient electron donor for reduction of nitrate to N-gases over TiO2 [REF: Zhang 2005; Doudrick 2012]. At this time, without formic acid nitrate is reduced to ammonia, which leads away from transitioning towards a zero-chemical addition to drinking water objective. Recent advances in water splitting photocatalysts offer new prospects for overcoming the challenges associated with chemical addition. Tantalate based photocatalysts have been successfully used to reduce nitrate using water as an electron donor, however, the nitrate removal rate and selectivity toward N2 were low [REF: Kato 2002]. Although studies have been dedicated to exploring novel photocatalysts for nitrate reduction, relatively little information is available on their applicability to drinking water systems and zero chemical addition. The innovative approach proposed in this study stems from the existing need to successfully translate existing bench-scale concepts into practical applications for photocatalytic reduction of nitrate in ion exchange brines and complex water matrices with zero chemical addition. Development of novel photocatalysts in combination with wavelength modulation supported by novel UV LED technology could pave the road for complete conversion of nitrate to nitrogen gases and development of integrative small photocatalytic systems for nitrate treatment. Identification of nitrate reduction intermediaries and pathways will enable tailoring reaction pathways and to develop novel treatment solutions characterized by high nitrate/nitrogen gases conversion yields. Considering that these research avenues have not been explored, we propose unexamined research areas that will permit direct treatment of ionexchange brines and water (surface and groundwater) without an external organic electron donor
StatusFinished
Effective start/end date8/1/147/31/18

Funding

  • EPA: Region 9: $250,000.00

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Nitrates
Photocatalysts
formic acid
Electrons
Brines
Gases
Byproducts
Water
Nitrogen
Catalyst selectivity
Silver
Drinking Water
Ion exchange
Iron
Potassium Chloride
Photocatalysis
Denitrification
Redox reactions
Level control
Biological systems