**2.2 Biological mechanisms**

448 Solar Radiation

hydroxyl radicals. Among various AOPs, photocatalytic processes are very attractive for the mineralization (conversion to carbon dioxide, water, and other mineral species) of aqueous pollutants and inactivation of pathogenic microorganisms (Gelover et al., 1999; Bandala and Estrada, 2007; Bandala et al., 2007 & 2008). The use of AOPs for water disinfection, using solar radiation as the energy source, usually referred to as *enhanced photocatalytic solar disinfection* (ENPHOSODIS), has allowed the efficient deactivation of highly resistant microorganisms. Specifically, heterogeneous and homogeneous photocatalysis are the AOPs with the most technological applications because of their ability to remove organic pollutants and their capability to inactivate nuisance microorganisms. Regarding heterogeneous photocatalysis, the use of titanium dioxide (TiO2) as a catalyst has been widely tested and proven effective for deactivating several microorganisms as well as carcinogen cells (Dunlop et al., 2008; Rincon and Pulgarin, 2003; Reginfo et al., 2008; Castillo-Ledezma et al., 2011). In comparison with the heterogeneous arrangement, homogeneous processes also rely on the generation of hydroxyl radicals. Nevertheless, it has been proposed that other highly oxidant species could be involved in pollutant degradation and microorganisms deactivation. Fenton and Fenton-like processes are among the most widely studied methodologies (Bandala et al., 2009; Guisar et al., 2007; Bandala et al., 2011). From the economic point of view, the possibility of using solar energy to promote both homogeneous and heterogeneous photocatalytic processes is an interesting alternative to the use of these technologies in developing countries (Bandala et al., 2011; Blanco et al., 2007). The aim of this chapter is to review the state-of-the-art in the use of solar driven Fenton-like processes for the deactivating waterborne pathogens. It also the goal of this work to discuss the advantages and potential limitations of these treatment processes while analyzing the challenges and opportunities for the application of such technologies at real scale in poor,

isolated regions in developing countries with no access to safe drinking water.

**2. Chemical and biological mechanisms involved in homogeneous** 

Fe2+ +H2O2 → Fe3+ +HO• + OH<sup>−</sup>

[FeOH]2+ +H2O2↔ [Fe(OH)(HO2)]+ +H+

• + HO<sup>−</sup>

(1) (2) (3) (4) (5) (6) (7) (8) (9)

Fe2+ +HO• → Fe3+ +OH<sup>−</sup> Fe3+ +H2O ↔ [FeOH]2+ +H+ Fe3+ + H2O2↔ [FeHO2]2+ +H+

[FeOH]2+→ Fe 2++ HO• [FeHO2]2+ → Fe 2+ + HO2• [Fe(OH)(HO2)]+ → Fe 2++ HO2

RH + HO• → R• + H2O

Table 1. Chemical reactions involved in the Fenton reaction (Fe (II) and H2O2).

al., 2008; Gallard and De Laat, 2000; Gallard et al., 1999).

The chemical mechanisms involved in the Fenton reaction are well known since early of the past century. The reactions of iron (II) salts with hydrogen peroxide have been widely studied for decades and the main reactions involved are summarized in Table 1 (Orozco et

**photocatalysis 2.1 Chemistry** 

In general, photocatalytic processes, both homogeneous and heterogeneous, in the presence of iron and hydrogen peroxide have been demonstrated effective against a wide variety of resistant microorganism such as viruses (Kim et al., 2010), helminth eggs (Bandala et al., 2011a,b; Guisar et al., 2007), bacteria and spores (Dunlop *et al*., 2008; Bandala *et al*., 2009; Bandala et al., 2011c; Sichel *et al*., 2009; Castillo-Ledezma et al., 2011).

The main mechanism involved in deactivating pathogenic microorganisms is suggested to be related with the cellular damage produced by so-called reactive oxygen species (ROS), mainly hydroxyl (HO•) and superoxide radical (O2 •) as shown in Tables 1 and 2. According to different studies, these ROS are able to modify and eventually destroy the structure of the cell membrane (Alrousan *et al*., 2009, Malato *et al*., 2009), mainly as the result of lipid peroxidation (Dunlop *et al*., 2008; Alrousan *et al*., 2009). The initial damage is produced in the outer lipopolisaccarid and peptidoglycan walls, followed by lipid peroxidation and protein and polysaccarides oxidation (Malato *et al*., 2009; Dalrymple *et al*., 2010) affecting the regulatory function of the cell membrane for the internal and external interchange. The damage produced will further produce failure in the cell's respiratory activity and decrease its permeability, allowing the attack of inner cell components leading to its death (Alrousan *et al*., 2009).

Some studies have also demonstrated that microorganism's deactivation is also improved by the presence of iron derivatives, which have been suggested to show an important inhibitory activity in important microbiological processes such as biofilm generation (Dunlop *et al*., 2008). Cells are used to regulate iron adsorption as a defense mechanism against hydroxyl radical; however once hydroxyl radicals are generated in the intracellular media, as a result of the Fenton-like process by direct attack of the ROS, they are free for reacting with biomolecules (Darlymple *et al*., 2009).

During cellular metabolism some ROS are produced, such as superoxide (O2 •), hydroxyl radical and hydrogen peroxide (H2O2), as a result of cell respiration. However, these oxidizing species are in equilibrium with the immune system defense mechanism through anti-oxidizing enzyme production related to superoxide dismutase (SODs), catalase (CAT) and glutathione peroxidase (GPX) families (Castillo-Ledezma et al., 2011). When microorganisms are exposed to a major oxidative stress, for example ROS produced during a photocatalytic process, enzyme production is no longer capable of eliminating excess radical allowing deep cell damage. In the same way, ROS may produce additional oxidative stress in the cells through Fenton and Heber-Weiss reaction (Dunlop *et al*., 2008) generating damage in all the cell components including proteins, lipids and DNA. In the case of DNA damage, produced by pyrimidine dimmers formation by the generation of covalent bonds among the bases in the same DNA chain (Sichel *et al*., 2009), it generates mutations that may lead to loss of functional capability and death of cell (Malato *et al*., 2009). At the same time, when microorganisms are exposed to ultraviolet radiation (UV, λ ≤ 400 nm) during the photocatalytic reaction, DNA damage occurred directly through the radiation absorption by cell chromophores, which absorb radiation and produce heat. This interaction leads to an increase in ATP and RNA synthesis, jointly with the increase of ROS production. Microorganisms receiving a sub-lethal dose of UV radiation may become resistant to induced oxidative stress, partially recover their defense mechanisms and adapt to oxidative stress generated by exposure to UV radiation alone (tanning effect) (Bandala et al., 2011b).
