Development of Photocatalytic Materials and Systems for the Removal of Selenium from Industrially Impacted Water

Abstract

Selenium (Se) contaminated water derived from global industrial activities such as power generation, oil extraction and refining, coal and mineral mining, metal smelting, and agricultural irrigation can bioaccumulate in aquatic organisms and presents is toxic to many organisms, including humans. Se represents an extremely difficult contaminant to remove from wastewater due to its solubility, toxicity and state of matter over different oxidation states. At low concentrations, Se is an essential trace dietary element and consumed in foods and supplements. However, at higher concentrations Se becomes toxic, leading to selenosis in animals. Since the therapeutic window for Se is narrow, a slight increase in concentration can lead to toxic effects. Se exists naturally in inorganic forms, with selenite (SeO32-) and selenate (SeO42-) being the predominant species of interest due to their toxicity and solubility.

The objective of this thesis research focuses on (1) evaluating photocatalytic treatment of Se-rich industrial wastewaters and (2) the development of catalyst materials to improve photocatalytic activity, selectivity and recoverability. The industrial wastewaters considered in this research are flue gas desulphurization wastewater (FGDW), mine-impacted water (MIW) and synthetic mine-impacted brine (SMIB). Photocatalysis reduction on TiO2 was found to effectively and selectively remove selenate in the presence of many dissolved species commonly found in industrial wastewater, providing a powerful alternative to conventional Se removal techniques. Catalyst materials were synthesized to improve both their activity and selectivity towards Se reduction products.

This work demonstrates, for the first time, that photocatalysis using TiO2 can be effective at removing Se from raw flue gas desulphurization wastewater (FGDW), which is produced during the operation of coal-fired power plants. Selenate was reduced to less than 1 µg/L as Se in FGDW with concentrations of many competing co-existing ions exceeding 2,500x that of selenate. This work also uncovered the mechanisms of electron transfer through kinetic modelling, which have substantial impact on the understanding of photocatalytic reduction in a complex Se-TiO2 photocatalytic system. The simultaneous generation of solid elemental selenium (Se0) and hydrogen selenide (H2Se) through two consecutive first-order reductions is reported under a direct Z-scheme photocatalyst arrangement between photodeposited Se and TiO2.

In addition, the photocatalytic reduction on TiO2 was evaluated for selenate removal from mine-impacted water (MIW) and was shown to remove Se to less than 1 g/L. In this study, we uncover a unique advantage of photocatalytic reduction of selenate in MIW, largely the ability to selectively reduce selenate from more than 500 ug/L to less than 1 ug/L. The significant Se decrease was observed in the presence of the more thermodynamically favourable electron acceptor, nitrate and at high concentrations of sulfate. Selective photocatalysis is highly desired in complex water sources that contain a variety of dissolved species in addition to the target species for efficient use of the UV energy supplied. The electron transfer mechanism proposed involves electrons from the TiO2 conduction band being responsible for the reduction of selenate to Se0 while both carbon dioxide radicals and Se conduction band electrons are considered responsible for the further reduction of Se0 to H2Se.

The production of brine from MIW enables a reduction in water volume of 6-8 times, while increasing the concentration of target species in the water, such as selenate. As a result, the photocatalytic reduction of selenate in synthetic mine-impacted brine (SMIB) was also thoroughly investigated. Considering the two possibilities for Se reduction products (Se0(s) vs. H2Se(g)), the ability to control the generation of a particular product was explored during the photocatalytic reduction of selenate over TiO2 in SMIB. Photocatalytic reduction can effectively remove Se from an initial Se concentration of > 3,300 ug/L in SMIB to < 2 ug/L Se. An increase in solution temperature led to a marked increase in selenate removal kinetics and an increase in selectivity towards H2Se(g), while increasing the concentration of formic acid led to an increase in selenate removal kinetics and an increase in the selectivity towards Se0(s). A bivariate response surface analysis was used to present the selectivity of Se reduction product as high as 99% gaseous H2Se or > 85% solid Se0, under varying reaction conditions. Finally, a two-pronged electron transfer model is proposed to explain the selectivity towards Se0(s) vs. H2Se(g) under varying conditions: (i) Se0(s) is produced by direct reduction of selenate by TiO2 conduction band electrons and (ii) H2Se gas is produced by electrons transferred into Se0, followed by a reduction of Se0 to H2Se or through a direct reduction by carbon dioxide radical.

Finally, this approach provides flexibility towards the final state of Se after treatment, which allows for two different possible options of Se capture and recovery; direct solid Se capture from the catalyst and scrubbing processes to recover gaseous H2Se. A materials engineering approach was then implemented to achieve enhanced tunability towards desired Se reduction products. Heterogenous nanoscale photocatalysts were synthesized by depositing noble metal nanoparticles (Au, Ag, Pt and Pd) onto TiO2, which demonstrated work-function dependent bimodal selectivity of final products during the photocatalytic reduction of selenate to Se0 or H2Se. The Se-noble metal-TiO2 (Se-NM-TiO2) photocatalytic system is structured in a direct Z-scheme arrangement, when Au, Ag or Pt are used, allowing for high selectivity towards H2Se. In contrast, Pd acted as an electron sink which decreased the reducibility of the photogenerated electrons, ultimately causing a higher selectivity towards Se0. Au-TiO2 offers the largest H2Se selectivity of all catalysts tested, while Pd-TiO2 (highest work function) offers the highest selectivity to solid Se0 generation. This study elucidates electron transport mechanisms and Fermi level equilibration via quantized double-layer charging effects of the Se-NM-TiO2 system.

Overall, this thesis advances the understanding of photocatalytic reduction of selenate in FGD, MIW and SMIB. It expands the knowledge of Se speciation during and after photocatalytic treatment and elucidates electron transfer mechanisms responsible for the two-stage reduction of selenate in impacted water. Photocatalytic treatment of Se in these complex waters provides a selective, chemical-reductant-free catalytic reduction process capable of removing Se to < 1 ug/L. This thesis advances the understanding of photocatalytic advanced reduction processes, primarily towards the reduction of selenate and expands our current understanding of the complex Se-TiO2 heterogeneous semiconducting photocatalyst system. Finally, the ability to selectively reduce selenate in complex industrial waters allows for the development of new wastewater treatment system configurations to effectively treat complex water streams.