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Application of photoassisted reduction and oxidation methods for the treatment of environmental pollutants

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Keywords
  • Engineering
  • Civil|Engineering
  • Sanitary And Municipal|Environmental Sciences
Disciplines
  • Chemistry

Abstract

A TiO$\sb2$/VIS system was developed to photobleach 15 organic dyes with varying chemical structures using visible light. Triphenylmethane dyes were observed to be easier to bleach than anthraquinone dyes. Compounds with ionized groups had a faster photobleaching rate, possibly because of a higher affinity for the TiO$\sb2$ surface. Photobleaching kinetics of the dyes could be approximated as a pseudo-first-order reaction. The TiO$\sb2$/VIS method was ineffective for the treatment of poorly water-soluble solvent dyes. The TiO$\sb2$/VIS method also suffered from retardation by the presence of acetonitrile, alcohols, HCl, $\rm H\sb3PO\sb4,$ humic substances and dissolved organic matter. Similar retardation was also observed in the $\rm TiO\sb2/O\sb2/UV$ system for the treatment of toxic organochlorines. When applied to a mixture of compounds, the $\rm TiO\sb2/O\sb2/UV$ system selectively favored the decomposition of methylene blue. For example, 4-chlorophenol was barely degraded until the methylene blue was consumed.^ Another innovative technique, the photosensitized reduction method, degraded 2,3-dichlorodioxin, DDT and toxaphene effectively via an electron transfer relay mechanism which involved "visible light", a reductant and a photosensitizer. When sunlight was used, photodechlorination of perchloroethylene was about 14 times faster than when a 150-watt spotlight was used. Sodium borohydride was found to be a superior sacrificial reductant to triethylamine. Humic substances, though retardants in TiO$\sb2$ catalyzed photo-oxidation, became effective reductants in the dye-photosensitized approach. Some nontoxic natural dyes were good photosensitizers, with about the same efficiency as methylene green. Riboflavin was about 10 times faster. When Chlorofresh (copper chlorophyllin) was used as the photosensitizer and triethanolamine was used as the reductant, the relative photoreduction rates decreased in correspondence with the order of electronegativity of the following compounds: hexachloroethane $>$ carbon tetrachloride $>$ bromoform $>$ perchloroethene $>$ 1,2-dibromoethane. Moreover, glycerin accelerated the reaction, perhaps by dissociation of chlorophyll (the photosensitizer) aggregates. Simple dechlorinated products were observed. A sequential dechlorination mechanistic pathway was proposed. Overall, this research has proven the feasibility of utilizing sunlight, humic substances and nontoxic dyes to render a toxic compound less toxic and to enhance the natural carbon regeneration rates. ^

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