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Reaction Mechanisms and Kinetics of Reductive Transformation of Toxic Heavy Metals and Nitrate

  • Author(s): Chen, Gongde
  • Advisor(s): Liu, Haizhou
  • et al.
Creative Commons 'BY' version 4.0 license
Abstract

Contamination of hexavalent chromium Cr(VI), pentavalent vanadium(V) and nitrate is threatening the availability of drinking water resources. Their reductive transformation into environmentally benign products is a viable and sustainable strategy. The goal of this dissertation is to investigate the reaction kinetics and mechanism of reductive transformation of Cr(VI), vanadium(V), and nitrate.

First, highly reductive TiO2 nanocrystals were synthesized for photocatalytic Cr(VI) reduction in drinking water matrices. The synthesized catalyst with surface-bonded diethylene glycol exhibited a high electron-releasing capacity. Under UV irradiation, Cr(VI) was efficiently reduced with a fast precipitation of trivalent chromium(III) hydroxide Cr(OH)3(s). The synthesized catalyst was reused for multiple cycles and exhibited excellent performance in Cr(VI)-contaminated groundwater.

Second, a novel denitrification process was developed for nitrate removal in drinking water matrices. Reactive radicals generated from nitrate photolysis oxidized formate into formate radical (CO2·-). Kinetic modelling and principal component analyses showed that synergistic nitrate photolysis with CO2·--induced reduction of nitrogen intermediates enabled synchronous removal of dissolved nitrogen and organic carbon. pH and dissolved organic matter at levels similar to those in groundwater had a negligible impact on the dentification performance. The proposed process exhibited strong application potential in treating nitrate-contaminated groundwater.

Third, the redox chemistry of vanadium(V) species and their transformation products were investigated with state-of-the-art rotating ring-disk electrode techniques. VO2+ and NaxHyV10O28(6-x-y)- predominantly exhibited faster intrinsic reduction kinetics than HxV4O12+x(4+x)-, and HVO42- The reduction of vanadium(V) underwent a one-electron transfer process except for that of NaxHyV10O28(6-x-y)- with a two-electron transfer. Ring electrode current showed that the reduction product of NaxHyV10O28(6-x-y)- was stable, while the other three vanadium(V) species had half-lives from milliseconds to seconds. Phosphate complexation favored the reduction of vanadium(V) and inhibited the re-oxidation of its reduction product in water treatment.

Finally, radical-induced photochemical reduction of Cr(VI) and nitrate in the spent ion-exchange regenerant brine was investigated to increase the sustainability of ion exchange processes. CO2·- has been demonstrated to be more effective than alcohol-derived carbon-centered radicals to remove Cr(VI) in the spent brine. The reduction of Cr(VI) was favored at a high dosage of formate and low pHs and was not affected by ionic strength. Chloride transformed NO2· into less reactive NO· and Cl2·- and inhibited Cr(VI) reduction. Co-removal of Cr(VI) and nitrate was achieved with an extended reaction time.

In summary, novel redox-based treatment processes have been developed to remove Cr(VI) and nitrate from drinking water and wastewater matrices. Fundamental redox chemistry of vanadium(V) species and their reduction products have been unveiled electrochemically by rotating ring-disk electrode. The developed treatment processes can be integrated into decentralized water treatment and reuse facilities to remove Cr(VI) and nitrate from local water resources. The redox chemistry of vanadium could help design effective treatment processes for vanadium(V) removal. Future work will be conducted to design treatment modules to remove Cr(VI) and nitrate in actual water matrices and extend the application of rotating ring-disk electrode for other aquatic contaminants.

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