INSTITUTO DE INVESTIGACIONES EN CATALISIS Y PETROQUIMICA "ING. JOSE MIGUEL PARERA"
Unidad Ejecutora - UE
capítulos de libros
Abatement of nitrate in drinking water. A comparative study of photocatalytic and conventional catalytic technologies
F. A. MARCHESINI; GUADALUPE ORTIZ DE LA PLATA; O. ALFANO; M.A. ULLA; E. MIRO; A. E. CASSANO
Advanced Oxidation Processes
Lugar: Balkema; Año: 2012;
he contamination of water due to both intensive fertilization and waste effluents from industries has produced an increase of nitrate concentration in groundwater. Nitrates are reduced to nitrites in the digestive system, affecting hemoglobin and impairing its function as oxygen-carrier, thus causing the blue baby syndrome. They are also related to several kinds of cancer, e.g. ovarian and prostate cancers. This contamination problem has led to the introduction of guidelines to establish upper limits for the concentration of nitrates and related species (nitrites and ammonia) in water for domestic use. The Drinking Water Directive- European Commission (98/83/EC) sets a maximum allowable concentration for nitrate of 50 ppm, 0.1 ppm for nitrite and 0.5 ppm for ammonia. The World Health Organization recommends a maximum nitrate concentration of 10 mg N/L in drinking water. The U.S. Environmental Protection Agency (EPA) has established a maximum contaminant level (MCL) in drinking water of 10 mg/L as nitrateN to protect infants from methemoglobinemia (Ward et al., 2005). However, the effectiveness of this regulatory limit for preventing other health risks such as cancer has not been adequately studied (De Roos et al., 2003). At present, the most widespread technologies for the removal of nitrates are biological denitrification and physico-chemical processes, namely ion exchange, reverse osmosis and electro dialysis. However, they present serious problems, i.e. bacterial processes include handling difficulties, low reaction rates, need for removal of by-products and low space velocities. Physico-chemical processes usually transform the nitrate into brine which has to be treated afterwards or disposed of. Moreover, some of these technologies can be expensive. Therefore, increasing attention has lately been paid to a novel technology, still in its development stages, catalytic denitrification (Ilinitch et al., 2000; Pintar et al., 1999), which employs solid bimetallic catalysts. In this catalytic process, nitrates are reduced to nitrogen using hydrogen (Reaction 1.1); however, undesirable products such as nitrite and ammonium are also formed (Reactions 1.2 and 1.3). 2 NO3- + 5 H2 à N2 + 2 OH- + 4 H2O (Reaction 1.1) NO3- + H2 à NO2- + H2O (Reaction 1.2) 2 NO3- + 8 H2 à 2 NH4+ + 4 OH- + 2 H2O (Reaction 1.3) It is accepted that the reaction mechanism involves the reduction of nitrate into nitrite, which is in turn reduced towards nitrogen or over-reduced to give ammonia (Pintar et al., 1999). Thus, the positive catalytic effect is related to the ability of the catalyst to selectively hydrogenate nitrite into nitrogen. Nitrite can act as a reaction intermediate, having a maximum in its concentration curve at intermediate reaction times. In general, the most active catalysts for nitrate reduction show the lowest nitrite formation because they are also active for nitrite reduction (Marchesini et al., 2008). As shown in reactions 1.1 and 1.3, nitrate reduction produces OH- ions and their local accumulation could negatively affect the catalytic activity and the selectivity to N2. Therefore, the pH of ca. 5 was maintained during the reaction time by the addition of controlled amounts of HCl (Garrón et al., 2005) in order to improve the selectivity to N2. Several solid catalysts have been presented by various authors (Deganello et al., 2000; Matatov et al., 2000; Strukul et al., 2000). The catalysts were prepared over different mesoporous supports such as massive oxides, alumina or silica, with the addition of noble metals such as Pt or Pd as the main metal, and a second metal such as Cu, Co or In as promoter metal. The possible mechanism for the catalytic reduction is through the combination of active sites in the bimetallic catalyst (Prusse et al., 2000) where the nitrate is reduced to nitrite over the bimetallic particle, and then the nitrite produced is reduced over the noble metal particle to nitrogen or ammonium depending on the site selectivity and the environmental conditions, as mentioned above. The restriction about a catalytic process for nitrate removal from drinking water is severe, i.e. the product stream must conform to drinking water standards and the energy consumption must be low for the process to be cost effective; therefore, temperature and pH cannot be adjusted freely, and no toxic substances may leach from the catalyst (Bems et al., 1999). The reactions involved in photocatalysis are mostly oxidation reactions, which utilize the positive holes produced by illuminating the semi-conductor, i.e. TiO2 to generate oxidizing radicals by reaction with adsorbed OH- or water. However, this particular application, nitrate abatement, calls for a reductive reaction. The irradiation of titanium dioxide with the appropriate wavelength gives rise to the formation of not only positive holes (vacancies) but also photoelectrons. Thus, this less widely studied reaction path might provide the necessary conditions to achieve a better controlled indirect reduction of nitrates that is the ultimate target of this work. The photocatalytic route is represented by: UV + MO → MO (h + e−) (Reaction 1.4) Here, MO stands for metal oxide, usually a semiconductor such as TiO2. The employed UV must be of the required wavelength in order to overcome the semiconductor band gap. The oxidative path is usually represented by: h+ + H2O → H+ + OH (Reaction 1.5) while the reductive reaction due to the photocatalytic effect is mostly presented as: 2e− + O2 → O2 (Reaction 1.6) However, in the absence of oxygen, another reductive reaction may occur. In the presence of a redox catalyst which uses the aforementioned electrons, the desired reduction may be written as: 2e- + (Cat) + 2 H+ à H2 (Reaction 1.7) The photoreduction of nitrates was studied by Ranjitet al. (1997); they proposed that metals with high affinity for electrons be added to TiO2. Once the electrons are trapped in this way, they can be used as reducing agents for nitrates. Unfortunately, different from what happens in oxidation practices, the reactions employed in reduction processes have not been so extensively studied. Nevertheless, the possibility of using various forms of titanium dioxide modified by metals with the ability to capture electrons has acquired new impetus and increasing importance. This possibility has been explored in studies of reductive reactions (Sakthivel et al., 2004; Shiraishi et al., 2004; Zhang et al., 2006; Cohen, 2001), hydrogen production from water (Chiarmello et al., 2011; Nada et al., 2008; Kudo, 2003), and the reduction of emissions of carbon dioxide to the atmosphere (Stock et al., 2011). In this work, Pd-In metallic couples have been selected on the grounds that In promoted catalysts have good activity for nitrate conversion and a potential high selectivity to N2, as reported elsewhere (Marchesini et al., 2012). TiO2 has been selected as support because it can perform as electron supplier in the presence of UV radiation. The aim of this preliminary work is to compare the efficiency of a catalyst formed on a titanium dioxide support, complemented with a mixture of Pd-In as metallic additives operating under conventional reaction conditions with the efficiency of the same catalyst but irradiated with UV radiation. The purpose of such a comparison is to determine if its photocatalytic activity allows a better control of the reductive function and avoids the formation of undesirable reaction products, specifically nitrites and NH4+. If the results of this exploratory study were attractive enough, possible follow-ups would include the optimization of this catalyst composition and kinetic studies to characterize all the reaction parameters.