Antimicrobial Activities of Photocatalysts for Water Disinfection

Abstract

Water is an essential element for the survival of humanity and the continuity of the planet's development. Impacts of human activity on water resources have been a growing concern because of environmental contamination and public health problems. For that reason, strategies and technologies to promote water sustainability and ensure water availability for current and future generations, with suitable quality for its respective uses, are being researched and developed, with priority being given to modern, efficient, inexpensive, and fast techniques that can achieve adequate water disinfection. Water disinfection is an efficient process for removal, deactivation, or killing of microorganisms that are responsible for water-borne diseases, and it is applied in public water supply and reuse systems. Among disinfection techniques, a promising and efficient method used against a wide range of different species of organisms is photocatalytic water disinfection using a photocatalyst activated by ultraviolet (UV) or visible radiation for generation of reactive oxygen species (ROS) that damage microorganisms. We performed a review of the literature on the mechanisms of photocatalytic water disinfection and photocatalysts with microbial activity. It was found that disinfection may occur due to (1) attack by ROS on bacterial cells, (2) the effects of metal ion release on cellular proteins, (3) direct interactions between nanoparticles and bacterial cells, and (4) mechanical damage to cell membranes. Generation of ROS in the presence of light occurs with activation of the photocatalyst with appropriate radiation; however, the mechanism of disinfection when it is performed in the absence of light has not yet been completely defined. In order for industrial application of photocatalytic water disinfection to become feasible, it is necessary to develop photocatalysts that exhibit high antimicrobial activity and can be activated preferentially with solar radiation. Among the different types of photocatalysts are pure photocatalysts and nonmetals, halogens, metals, and rare earth-modified photocatalysts, as well as photocatalytic films, biofilms, and nanocomposites. Modified photocatalysts, especially fluorine- and cobalt-modified zinc oxide and silver-and nitrogen-modified titanium dioxide, are more efficient than pure oxides under visible radiation, which makes their use promising in solar-induced photocatalytic water disinfection because modified photocatalysts can be reused and are not harmful to human health and the environment, being in accordance with the principle of green chemistry. Photocatalytic films, biofilms, and nanocomposites include silver-and copper-modified potassium hexaniobate film; silver orthophosphate, titanium dioxide, and magnetite film; and titanium dioxide and tungsten(VI) oxide anchored on a reduced graphene oxide nanocomposite, which demonstrate high antimicrobial activity with a mortality rate higher than 97%. As few studies have been performed with native microorganisms and under real conditions, a case study was performed experimentally by the authors, involving application of a supported photocatalyst for disinfection of wastewater from whey processing during exposure to UV radiation. The photocatalyst, constituted by hematite and titanium dioxide supported on a glass sphere, showed high bacteriostatic activity against mesophilic microorganisms and irreversible damage to psychrophilic microorganisms' cell walls and their components. Therefore, this chapter contributes to the knowledge about use of photocatalysts as water disinfection agents and for control of waterborne diseases, especially those that are activated by solar radiation, thus making photocatalysis an efficient, viable, and environmentally safe alternative.