The presence of heavy metals and metalloids in water represents nowadays one of the most important environmental problems. These species have infinite lifetimes, and chemical or biological treatments present severe restrictions or are economically prohibitive. Hexavalent chromium, mercury, uranium, arsenic, or lead are on the list of priority pollutants of most environmental agencies, with more and more exigent limits of discharge or concentration in drinking water. From the beginning of the development of heterogeneous photocatalysis, the transformation and deposition of metals or metalloids were visualized as processes with promissory potential application to remove these pollutants from water. Three types of mechanisms can be considered for these processes, all of them taking place through successive monoelectronic electron transfer steps: (a) direct reduction by photogenerated electrons; (b) indirect reduction by intermediates generated from electron donors (reducing radicals); (c) oxidative removal by holes or hydroxyl radicals. This presentation is an overview of the work performed in our laboratories with the cases of hexavalent chromium and arsenic being treated in profundity. In the case of hexavalent chromium, direct reductive photocatalysis and indirect reduction by intermediates coming from ethylenediaminetetraacetic acid (EDTA) or citric acid added as electron donors are the main processes governing the removal of Cr(VI) by TiO2 photocatalysis. In the case of arsenic, removal can proceed by oxidation of As(III) to As(V), a very much studied process. However, reductive photocatalysis has been less studied and can take place under specific conditions, leading to the removal of As species by the formation of As(0) on the surface of the photocatalyst. While for As(III) direct reduction by photogenerated electrons is possible, As(V) reduction only proceeds in the presence of an electron donor such as methanol. The mechanisms taking place in these cases will be postulated in this presentation and the possible application to real systems will be discussed.
Audience Take Away:
- Explain how the audience will be able to use what they learn? An understanding of the mechanistic pathways involved on chromium and arsenic photocatalytic removal will help the audience to use the concepts to optimize experimental conditions on lab- scale and/or operational parameters on field scale to improve the rate and yield of the removal processes.
- How will this help the audience in their job? Same answer as before. Additionally, the presentation will emphasize the problem of pollution of water by heavy metals and arsenic and the effects on human health of these elements, especially in drinking water.
- Is this research that other faculty could use to expand their research or teaching? The experimental setup for the experiments is very simple and can be implemented easily in other university laboratories.
- Does this provide a practical solution to a problem that could simplify or make a designer’s job more efficient? Photocatalysis is a very simple and low-cost technology, as TiO2 is a very stable, reusable, and cheap material and solar light can be used to start the pollutants removal process.
- Will it improve the accuracy of a design, or provide new information to assist in a design problem? Reactors can be easily designed, and scaling is even possible.
- List all other benefits. Photocatalytic processes use concepts from several disciplines such as physics, chemistry, chemical engineering, and environment. Thus, it is an interdisciplinary subject that can improve the knowledge of the state of the art of this theme by the audience.