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The application of heterogenous supported catalysts for the treatment of greywater via in-situ generated H2O2

Richards, Thomas 2022. The application of heterogenous supported catalysts for the treatment of greywater via in-situ generated H2O2. PhD Thesis, Cardiff University.
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The work presented within this thesis looks primarily into developing a catalyst with the ability to synthesise H2O2 as well as reactive oxygen species that can act in combination as a biocide towards common bacteria and degrade an antibiotic. The current method for the synthesis of H2O2 on an industrial scale is the anthraquinone process, however this method necessitates large scale production to be economically viable due to the unselective hydrogenation of the carrier molecule resulting in the need for its periodic replacement alongside the overall complexity of the process. This large-scale production means that concentrated H2O2 (70 wt.%) then needs to be transported safely to the desired site of use. This leads to acid/halide stabilisers being added prior to transportation, which then makes the once environmentally friendly oxidant become a hazard as well as the solution needing to be diluted, given the desired concentration for common H2O2 use is around 3-5 wt.% H2O2. All these factors point towards the desire for a smaller-scale, more efficient method to produce H2O2 that could overcome all the drawbacks, including cost of stabilizers, dilution, transport, and storage, with the current industrial route. Water disinfection is currently reliant on chlorination, but ideally requires a route that avoids the formation of chemical residues. H2O2, a broad-spectrum biocide, can offer such an alternative, but is typically less effective than traditional approaches to water remediation. However, the results held within this thesis show that a catalytic approach to generating all H2O2 reactive oxygen species could form the basis of an alternative method for water disinfection. The first part of this work investigates the efficacy of AuPd catalysts prepared by an industrially relevant excess chloride wet co-impregnation procedure to synthesise H2O2 from molecular H2 and O2 in a batch regime. Subsequently, pelleted analogues of these materials were investigated for their activity towards H2O2 production and the remediation of Escherichia coli K12 JM109. The generation of reactive oxygen species, which include hydroxyl, hydroperoxyl and superoxide radicals (identified by electron paramagnetic spectroscopy), over the 1 wt.% AuPd/TiO2 catalyst during the synthesis of hydrogen peroxide was found to offer extremely high biocidal efficacy (8.1 log10). Comparison to traditional biocides, such as preformed H2O2 and NaOCl further demonstrated the efficacy of the catalytic approach, achieving rates of microorganism kill over 107 times more potent than conventional disinfectants. This approach could form the basis of an alternative method for water disinfection, particularly in communities not currently served by traditional means of water remediation or where access to potable water is scarce. iii Building on earlier studies into bactericidal and virucidal performance of a catalytic approach to water remediation this approach to oxidation was broadened to determine efficacy towards the remediation of organic contaminants found in water bodies via in-situ H2O2 generation. With a focus on the antibiotic metronidazole, a common antibiotic for the treatment of skin and mouth infections. Initial studies, using the optimal AuPd catalyst from previous investigations to microorganism kill, seemed to indicate the efficacy of the catalytic approach with in-situ H2O2 achieving far greater rates of conversion compared to that observed using commercial H2O2 However, extensive studies revealed that that while there may be a minor contribution from oxidative pathways the primary cause for the observed conversion of metronidazole was the catalysed hydrogenation of the metronidazole. Finally, extensive catalyst design was investigated with an aim to both improve the performance and stability of the AuPd catalyst studied for bactericidal efficacy and lower catalyst costs by find an alternative to Au. In this work a focus was placed on catalytic performance towards H2O2 production, in a batch regime. Initial studies into the well�established AuPd system demonstrated the key role of Pd: Au ratio on catalytic activity, under conditions that have previously been found to be optimal for H2O2 formation. Further investigations using the optimal Pd: Au ratio identified the role of the catalyst support in controlling particle size and Pd oxidation state and thus catalytic performance. Building on these studies, Pd was alloyed with a range of abundant secondary metals is subsequently explored. The performance of all catalysts towards H2O2 production was subsequently established under conditions approximating those used within earlier studies for water remediation. With an aim to ultimately transition into the flow regime previously utilised, the effect of pelleting the catalytic series was evaluated.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 27 April 2022
Last Modified: 28 Apr 2022 11:57

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