From a thermodynamic point of view, couples can be photocatalytically reduced by conduction band electrons if they have redox potentials more positive than the flatband potential Vfb of the conduction band and can be 1 All standard reduction potentials given in this work are vs. When a transformation to the zerovalent state is possible, this allows the recovery of the metal by mechanical or chemical procedures, with an important economical return. Various semiconductors have been applied in the photocatalytic transformation or deposition of metals such as chromium, gold, silver, platinum, palladium, rhodium, mercury, lead, manganese, thallium, and copper, among others Serpone et al.
Other applications of heterogeneous photocatalysis related to metal ions are light energy storage, photographic imaging, prevention of semiconductor corrosion, and preparation of modified semiconductors, but these topics will not be treated here. In view of the enormous literature published on the subject, only the cases of chromium, mercury, lead, uranium, and arsenic are reviewed here. In , we published an extensive review on metal treatment by heterogeneous photocatalysis in which the early literature is mentioned Litter, In this chapter, we will remind the most important issues and update the most recent information.
Reduction potentials are taken from Bard et al. Litter couples and the thermodynamic ability of the TiO2 photocatalytic system to reduce or oxidize the corresponding species. Nevertheless, under the working conditions of ordinary photocatalytic reactions, that is, under nonintense irradiation, multielectronic reactions are rather unlikely, considering the frequency of photon absorption Grela and Colussi, Recent experiments of our research group see sections 3, 4 and 5 led to the conclusion that most photocatalytic processes on metal ions occur through successive one-electron pathways that produce unstable intermediates until the most stable species is formed.
Three types of mechanisms can be considered for the photocatalytic removal of metal ions: a direct reduction by photogenerated electrons, b indirect reduction by intermediates generated by hole or hydroxyl radical oxidation of electron donors present in the media, and c oxidative removal by holes or hydroxyl radicals Lin and Rajeshwar, , all of them represented in Figure 3. In the direct reduction a , the initial electron transfer step, reaction 11 , is usually considered as the rate determining one Mills and Valenzuela, For predicting the feasibility of the transformation, the reduction potential of the first step related to the energy of the conduction band has to be considered.
Different pathways a , b , and c are indicated see text. The diagram of energy levels is only qualitative. The process will be very dependent on the nature of the added agent: low-molecular-weight acids, alcohols, and aldehydes do not cause any effect, while easily oxidizable organics such as EDTA, salicylic acid, and citric acid provide very fast reduction rates. Indirect reduction mechanism b was suggested by Baba et al.
As no metal deposition was observed in the absence of alcohols, these authors proposed that photogenerated conduction band TiO2 electrons do not take part directly in the deposition. Concurrently, reaction 14 is hindered if HR is added at relatively high concentrations because reaction 16 will predominate. In Figure 3, a simplified diagram of this process is presented. It has been pointed out that direct reduction of metal ions to the zerovalent state by reducing radicals is rather slow, but once some metal nuclei are formed, they serve as cathodic site to facilitate further reduction Baba et al.
In mechanism c , oxidative transformation of the metal species takes place by holes or hydroxyl radicals or other reactive oxygen species, ROS attack Figure 3. CHROMIUM Chromium VI is a frequent contaminant in wastewaters arising from industrial processes such as electroplating, leather tanning, or paints due to its carcinogenic properties, its concentration in drinking waters has been regulated in many countries. The preferred treatment is reduction to the less harmful Cr III , nontoxic and less mobile.
This process is performed generally with chemical Treatment of Heavy Metals and Metalloids in Water by Heterogeneous Photocatalysis 45 reagents such as sodium thiosulfate, ferrous sulfate, sodium meta-bisulfite, or sulfur dioxide; in this way, the ion can be precipitated in neutral or alkaline solutions as Cr OH 3 and removed from the aqueous phase.
Several examples, all of them investigated before , have already been described in our previous review Litter, , including reactors for technological applications Aguado et al. Other papers appeared later Aarthi and Madras, ; Cappelletti et al. The list includes microparticles used in slurries or conveniently supported, mixed, and modified semiconductors and even nanomaterials nanoparticles, nanotubes. The photocatalytic Cr VI reduction is more feasible at low pH because the net reaction consumes protons Equations 18 and 19 , but use of neutral or alkaline conditions can be more convenient because Cr III can be precipitated as the hydroxide and immobilized, avoiding expensive separation steps; after the photocatalytic process, an adequate acid or strong basic treatment easily separates Cr III from the catalyst Lin et al.
Prairie and Stange and Wang et al. While the fate of Cr V and Cr IV is not clear, it is hypothesized that they are probably reduced by conduction band electrons or can suffer disproportionation to other species. On the contrary, Cr VI removal in the presence of the donors is total in a very short time, as the short-circuiting process is hindered. Our research group also reached to very important conclusions concerning the role of dissolved molecular oxygen in the photocatalytic reduction of Cr VI , which was the object of controversy for many years.
In fact, it can be possible to think that O2 inhibits Cr VI reduction, given the likelihood that conduction band electrons are consumed via reaction 5. There is in this respect important evidence supporting this lack of effect of oxygen: 1. There is no difference in the Cr VI photocatalytic reduction efficiency while working under nitrogen or under oxygen or air Siemon et al. There is no variation in Cr VI photocatalytic reduction efficiency while measured over pure or platinized TiO2 Siemon et al.
Experimental results suggest that electron transfer to metallic ion from the conduction band or from Pt is rapid and there is no oxygen mediation requirement. Spectroscopic evidences an absorption band at nm of the formation of a charge-transfer complex between Cr VI and TiO2 nanoparticles, showing a strong interaction of the metal ion with the semiconductor surface, were reported in a recent paper Di Iorio et al.
The behavior of Cr VI contrasts strongly with that of other metal ions such as Hg II see section 4 , whose reduction is greatly inhibited by dissolved oxygen. In this sense, it is a unique system and the fact that its photocatalytic reduction can be made in air represents an important technological advantage. The photocatalytic Cr VI reduction has been reported to take place also under visible irradiation.
Kyung et al. A similar behavior was 48 Marta I. Litter observed in the presence of the nonionic surfactant Brij Cho et al. Sun et al. In the presence of dyes, it is widely accepted that the photocatalytic process under visible light is different from that under UV light. However, the reducing power remains intact, making possible Cr VI reduction. Another dye, hydroxoaluminiumtricarboxymonoamide phthalocyanine AlTCPc , adsorbed on TiO2 particles at different loadings was tested for Cr VI photocatalytic reduction under visible irradiation in the presence of 4-CP as sacrificial donor.
In conclusion, much experimental work has been done on Cr VI photocatalytic reduction since The advances are related to the elucidation of mechanistic pathways, detection of intermediary species, kinetic calculations, role of dissolved oxygen, and potential use of visible light. However, several interesting points are still worthy of investigation with the aim of optimizing the technology for real use in wastewaters; the presence of synergetic organic compounds such as carboxylic acids or phenols, common constituents of real wastes, makes the process even more attractive.
The World Health Organization and national environmental agencies recommend a limit of 0. The health hazards due to the toxic effect of mercury at Minamata, Japan, and Iraq are very well known Bockris, The major use of mercury compounds is as agricultural pesticides. It is also used in the chlorine-alkali industry, in paints, as a catalyst in chemical and petrochemical industries, in electrical apparatus, cosmetics, thermometers, gauges, batteries, and dental materials. For this reason, it is a very common pollutant in wastewaters.
Removal of mercuric species in aqueous solutions is difficult because they are hard to be bio- or chemically degraded. At high concentrations, mercury can be removed from the solution by membrane filtration, precipitation with chemicals, ion exchange, adsorption, and reduction Botta et al. Mercury transformation by heterogeneous photocatalysis with semiconductors including electrodes, micro- and nanoparticles such as ZnO, TiO2, WO3 under UV, visible irradiation, and even solar light has been reported in a series of papers.
Some of them have been detailed in our previous review Aguado, et al. Litter Lau et al. It was concluded that the removal efficiency depends strongly on pH, that the reaction is inhibited by oxygen and that there is an enhancement by organic donors. Depending on the conditions, different products are formed on the photocatalyst surface: Hg 0 , HgO, or calomel.
Time profiles of Hg II concentration with time were characterized by a relatively rapid initial conversion followed by a decrease or an arrest of the rate. Three pH values 3, 7, and 11 were tested, finding that the faster transformation takes place at pH 11 for all salts. Inhibition by oxygen was observed in acid and neutral media but not at basic pH. In line with the fact that photocatalytic reactions under nonintense photon fluxes take place through monoelectronic steps, a direct reductive mechanism pathway a in Figure 3 involving successive one-electron chargetransfer reactions was postulated Botta et al.
At pH 11, however, calomel was not observed, as it disproportionates to Hg 0 and HgO. In these cases, mechanism a or b can take place, according to Figure 3. Complexation with citrate also proved to be a very good alternative for Hg II removal from water through metallic mercury deposition Tennakone and Ketipearachchi, Formic acid also enhanced Hg II removal, the effect increasing with the organic donor concentration until a limiting value Wang et al.
An interesting application of Hg II photocatalysis is the use of an activated carbon developed from municipal sewage sludge using ZnCl2 as chemical activation reagent combined with TiO2. Hg II was first photoreduced to Hg 0 and then adsorbed on the carbon and TiO2 surfaces, with a final recovery of the metal on a silver trap by heating Zhang et al.
The process begins by acid attack of the solid wastes and treatment of the acid solution under UV irradiation in the presence of TiO2 and citric acid. The selective precipitation of reduced mercury took place, while the other metal compounds remained in the solution. It was claimed that the final effluents reached a quality close to that of the standards imposed by international environmental agencies Bussi et al. The toxicity of organic mercury compounds, for example, methyl- or phenylmercury, is considerably higher than that of the inorganic species.
For example, the massive case of poisoning in Japan, the Minamata Bay incident, was attributed to industrial discharge of organomercurials, and declining bird populations in Sweden was blamed on the use of phenyl- and methylmercurial pesticides as seed dressings Baughman et al. TiO2-photocatalytic treatment of methylmercury was tested. It was found that metallic mercury can be deposited only in the presence of methanol and absence of oxygen, according to an indirect photocatalytic reduction Serpone et al.
Interesting applications for treatment of dicyanomercury II and tetracyanomercurate II ion, high-toxic pollutants coming from precious metal cyanidation processes, were also studied Rader et al. Litter Complete mineralization of the dye mercurochrome merbromin by TiO2 photocatalytic oxidation was found to occur in oxygenated solutions in the presence of citrate, with the corresponding deposition of metallic mercury Tennakone et al.
Previous work of Prairie et al. The reaction was faster at pH 11, with formation of mixtures of Hg and HgO. Oxygen inhibited the reaction. It was found that, fortunately, no dangerous methyl- or ethylmercury species were formed in the case of PMA. Calomel formation from PMC under nitrogen reinforces the two successive one-electron transfer reactions, as in the case of inorganic salts.
However, it is not possible to distinguish between simultaneous or consecutive steps. In closing, important advances have been performed after on Hg II photocatalysis, especially concerning the highly toxic organomercuric compounds. However, and as said in a previous paper Botta et al. Thus, very sensitive analytical tools must be used to control the concentration of species in the solution.
The physicochemical properties of the products derived from the treatment also introduce serious difficulties. Although zerovalent mercury can be carefully distilled off by mild heating, trapped and recondensed, or it can be dissolved with nitric Treatment of Heavy Metals and Metalloids in Water by Heterogeneous Photocatalysis 53 acid or aqua regia for confinement or further treatment of smaller volumes of the effluent, metallic Hg is volatile and somewhat water soluble, HgO is also fairly water soluble, and Hg I and Hg II nitrates and perchlorates are water soluble.
In spite of these potential complexities, it shall be emphasized that it is always better to have the pollutant immobilized as metallic deposit, treating it later on the solid residue as a hazardous species. It must be also reminded that calomel, if formed, is a less toxic species than HgCl2 with all this leading to a less hazardous chemical system. Lead pollution is mainly anthropogenic and originates in municipal sewages, mining, refining of Pb-bearing ores, chemical manufacture, and other sources. It is a component of insecticides, batteries, water pipes, paints, alloys, food containers, and so on.
It has been extensively used as a gasoline additive tetramethyl- or other alkyl-lead compounds. Although this application has been fortunately forbidden or reduced in most countries, some dangerous wastes could still be present, and they have to be treated. Lead may also be present naturally in groundwater, soils, plants, and animal tissues Vohra and Davis, The World Health Organization and national agencies recommend a maximum of 0.
Removal of lead from water is performed generally by precipitation as carbonate or hydroxide with or without coagulation. However, most of these treatments are expensive, and some other ways of lead elimination from wastewater are necessary to be developed. Heterogeneous photocatalysis of Pb II systems has received scarce attention. In our previous review Litter, , we cited a few early papers Inoue et al. The mechanisms of transformation of lead II in water by UV-TiO2 are especially attractive because they depend very much on the reaction conditions, related to the nature of the photocatalyst, the effect of oxygen, and the presence of electron donors.
Litter Recent results of our group Murruni et al. As in previous reported cases Chen and Ray, ; Kobayashi et al. All these results can be explained by the occurrence of oxidative or reductive pathways. Removal is poor over pure TiO2 in O2 or air due to the high overpotential of this reaction. In contrast, the reaction is remarkably rapid over platinized TiO2, because Pt facilitates the reaction Chenthamarakshan et al. In agreement, the concentration of dissolved O2 was found to decrease on illumination Tanaka et al.
Thus, the oxidative route seems to be the preferred photocatalytic pathway in the absence of electron donors. Accordingly, Pb shows essentially no or a very weak tendency to accept photogenerated electrons from TiO2, as said in Section 2. This reduction pathway was not observed even using platinized TiO2 under nitrogen.
Chenthamarakshan et al. In fact, direct reduction of Pb II to Pb 0 by a bielectronic process has been reported under laser irradiation, where due to the high photonic frequency, accumulation of electrons may allow multielectronic injection Rajh et al. Methanol, ethanol, 2-propanol, 1-butanol, t-butanol, and citric acid and formic acid were tested successfully under nitrogen and over pure TiO2 Chenthamarakshan et al.
However, formic acid is considered a better additive because it can be used at lower concentrations, and it does not introduce toxic degradation products in the system Murruni et al. In those cases, no lead deposits on the photocatalyst are obtained, but stains were observed on the lamp surface, composed mainly by colloidal zerovalent Pb, as demonstrated by X-ray diffraction XRD and X-ray photoelectron spectroscopy XPS analysis. Reactions in the presence of electron donors are inhibited by oxygen or air, because of the competition of O2 with Pb II for the reducing species Kabra et al.
In these conditions, the oxidative mechanism leading to Pb IV is not feasible, considering that the donor competes with Pb II for the oxidant, leading to a decrease more than to an enhancement of Pb II removal. This approach avoids expensive platinization of the catalyst or use of ozone. In addition, reactions can be conducted under air, and lead II and organic scavengers alcohols, carboxylates may be present together in Treatment of Heavy Metals and Metalloids in Water by Heterogeneous Photocatalysis 57 industrial wastes, with this approach having the potential of providing economical methods for the removal of the very toxic lead-related water pollutants.
Natural uranium is a mixture of three radionuclides, U, U, and U, with higher proportion of the last one. Uranium is present in the nature in granite and other mineral deposits. The main applications of uranium are as fuel for nuclear power plants, in catalysts, and in pigments.
Pollution sources are lixiviation of deposits, emissions from the nuclear industry, combustion of coal or other fuels, and U-containing phosphate fertilizers WHO, In human beings, uranium can provoke nephritis, and it is considered carcinogen, causing bone cancer. These consequences are even more noxious than radiological risks Katsoyiannis, The World Health Organization and national regulatory agencies recommend no more than 0. Removal methods of uranium from water are ionic exchange, ultrafiltration, adsorption on granular ferric hydroxide, iron oxides ferrihydrite, hematite, magnetite, and goethite , activated carbon or TiO2; evaporation, biorreduction, and zerovalent iron have been also tested Behrends and van Cappellen, ; Cantrell et al.
There are very few reports in the literature concerning heterogeneous photocatalysis for uranium treatment in water. In our previous review, only one case of photocatalytic reaction on uranium salts was reported Amadelli et al. For example, uranyl 58 Marta I. Accordingly, Amadelli et al.
Complexation of uranyl with the hole scavengers was found to play an essential role in the photoredox process, and, in addition, this indicates the predominance of pathway b of Figure 3. Similar results are presented in by Chen et al. No uranium deposition in the presence of air was observed; however, in the presence of EDTA and absence of oxygen, a reductive deposition took place.
Further exposure to air reoxidizes and redissolves the uranium species. Platinization of TiO2 enhanced the reaction only slightly, confirming the predominance of a reductive process. This method was proposed for recovering uranyl from aqueous solutions of dilute uranium VI -EDTA species, which are usually present in wastewaters of nuclear power stations. Another work indicates the possibility of photocatalytic treatment of the wastes of nuclear fuels with separation of Np, Pu, and U Boxall et al. While mechanistic studies on the photocatalytic removal of uraniumrelated species are scarce and merit further research, additional investigation is mandatory to clarify major reaction engineering issues following this approach.
Arsenic pollution can be originated in anthropic activities mining, use of biocides, wood preservers. However, most pollution is natural, coming from mineral dissolution in surface or groundwaters Bundschuh et al. The mobility of arsenical forms in waters is very dependent on pH, Eh conditions, and presence of different chemical species Smedley et al.
Consequently, removal methods Treatment of Heavy Metals and Metalloids in Water by Heterogeneous Photocatalysis 59 must take into account these physicochemical properties. However, reduction of As III or As V by TiO2 conduction band electrons, which could lead to the less mobile elemental As, seems to be not feasible, as judged from the recent stopped-flow experiments developed by our research team Meichtry et al.
The values for the subsequent monoelectronic couples until As 0 are not known. The reaction rate did not depend on pH, at least between 5 and 9. Litter of TiO2, increased the oxidation rate. The reaction was very fast in the presence of oxygen but not completely inhibited in its absence. Zhang and Itoh described a low-cost, environmentally friendly adsorbent for As III photocatalytic removal, formed by a mixture of TiO2 and slag-iron oxide obtained from an incinerator of solid wastes.
Arsenite is first oxidized to arsenate in a fast process, followed by a slow adsorption of arsenate, although the material shows an adsorbent capacity higher than that of pure anatase. Ferguson and Hering reported a method to oxidize As III in a fixed-bed, flow-through reactor with TiO2 immobilized on glass beads. The reactor residence time, the influent As III concentration, the number of TiO2 coatings on the beads, the solution matrix, and the light source were varied to characterize the reaction and determine its feasibility for water treatment. A reactive transport model with rate constants proportional to the incident light at each bead layer fitted reasonably the experimental data.
The reaction was also effective under natural sunlight. It was found that dissolved oxygen concentration was one of the most important factors for the reproducibility of the experiments. In a recent work Morgada de Boggio et al. Before or after irradiation, nongalvanized packing wire was added. Fostier et al. As already said, As V or As III transformation by a direct reductive pathway driven by TiO2 conduction band electrons is not thermodynamically possible.
However, complete As V removal in the presence of methanol under N2 at pH 3 was successful, indicating the participation of an indirect reductive mechanism Yang et al. As an evidence, XPS measurements revealed the presence of elemental As deposited on the photocatalyst.
As 0 with only changing pH and methanol addition. Summarizing, although the photocatalytic mechanisms for As removal have been analyzed, it is felt that application of photocatalysis is still in an early stage and more research studies for the possible application are necessary. The reductive mechanism is promissory in this sense. An overview of the literature on the subject for chromium, mercury, lead, uranium, and arsenic indicates that, in general, much important fundamental and applied research is still missing.
The usual mechanism is direct reduction by conduction band electrons, which is very slow in the absence of electron donors but can be accelerated by organic electron donors. In many cases, the organic compounds are present simultaneously with Cr VI in wastewaters as a result of different industrial processes. Because Cr VI photocatalytic reduction is not inhibited by oxygen, this represents an additional advantage for the application.
Recently, some photocatalytic reactions under visible light with good yields were observed, making possible 62 d e f g Marta I. Litter the potential use of costless solar light. Mechanistically, several questions remain unanswered, and investigations merit to be continued. For mercury II photocatalysis, fewer examples are reported. The reaction occurs through direct reduction driven by conduction band electrons; electron donors cause an enhancement of the reaction rate. The features of the reaction are very dependent on the conditions of the medium and the nature of the starting mercuric compound; the final products can be metallic Hg, HgO, or calomel.
Interesting and encouraging results have been found in the case of the extremely toxic organomercurial species. Lead II is a very motivating photocatalytic system. Although removal from water can take place by an oxidative pathway, over pure TiO2 this route is poor and can be enhanced only by platinization or use of ozone, both expensive methodologies. On the other hand, the direct reductive route is unfeasible because reduction of Pb II to the monovalent state is highly energetic. The indirect route, driven by reducing species formed in the presence of alcohols or carboxylates is a viable route.
These species can be present together with Pb II in wastewaters, rendering the technology appropriate for Pb II removal. Photocatalytic removal of uranium salts from water was scarcely studied. Photoreduction was possible only in the presence of hole scavengers in the absence of oxygen with the formation of uranium oxides; however, a rapid reoxidation and redissolution took place after exposure to air. Comprehensive studies should, however, be performed to fully demonstrate the possible application of the technology.
The reaction of arsenic species in photocatalytic systems is also interesting because both oxidative and reductive mechanisms may lead to less toxic or solid phases. Although the oxidative system has been studied rather well, the reductive pathway must be also the goal of new research in order to find a best of application of this alternative route for As removal. Aguado, M. Energy 49, 47 Baba, R. Bahnemann, D. Photocatalytic Treatment of Waters in G. Helz, R. Zepp, and D. Crosby Eds. Lewis Publ. Bang, S.
Chemosphere 60, Bard, A. Baughman, G. Behrends, T. Bissen, M. Chemosphere 44, Bockris, J. Plenum Press, New York Botta, S.https://emacocriacheck.cf
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Ferguson, M. Fostier, A. Chemospere 72, Fu, H. A , 81 Chimie 9, Goeringer, S. Grela, M. Grenthe, I. Elsevier, North-Holland imprint Gu, B. Guan, X. Haque, N. Microchemical J. Hidalgo, M. Today , Hodak, J. Colloid Polym. Ingallinella, A. Inoue, T. Jayaweera, P.
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Litter Ed. ISBN Low-cost technologies based on heterogeneous photocatalysis and zerovalent iron for arsenic removal in the Chacopampean plain, Argentina. Bundschuh, M. Armienta, 66 Marta I. Litter P. Bhattacharya, J. Matschullat, P. Birkle, and A. Mukherjee Eds. Balkema Publishers, Taylor and Francis Publishers, pp. ISBN hardback: alk. Murruni, L. B 84, Naftz, D.
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Academic Press, San Diego B 16, Noubactep, C. Papadam, T. Prairie, M. AIChe Symp. Rader, W. Solar Energy Eng. Rajeshwar, K. Rajh, T. Solids , a. Rengaraj, S. B 77, Ryu, J. Sancha, A. Medio Amb. Schrank, S. Selbin, J. Selli, E. Energy 39, Kluwer Academic Publishers, Dordrecht , p. Shim, E. Hydrogen Energy, 33 19 , — Siemon, U.
Skubal, L. Smedley, P. Sun, B. Tanaka, K. Energy 36, Tennakone, K. Energy Mater. B 5, A 70, Testa, J. Langmuir 17, Thurnauer, M. Acta Chem. Torres, J. Tuprakay, S. B , 53 Tzou, Y. A , 15 Vaaramaa, K. Vohra, M. Wang, L. Wang, S. B 1, Wang, X. Acta 49, Wang, Z.
A 75, World Health Organization. Xu, X. Chemosphere 63, Xu, Y. Yang, H. Yang, J. Yoneyama, H. Zhang, F. Chemosphere 65, Zhang, J. Zheng, S. Introduction 1. Conclusions Recommendations List of Symbols References 92 92 94 1. Photocatalysis: a promising low-cost alternative Every day, manufacturing industries produce water and gas effluent streams with significant amounts of organic and inorganic pollutants. Organic compounds in wastestreams include textile dyes, herbicides and pesticides, alkanes, haloalkanes, aliphatic alcohols, carboxylic acids, aromatics, surfactants, among many others Guillard, ; Malato et al.
Inorganic compounds include complexes of metal ions such as mercury, cadmium, silver, nickel, lead, and other equally harmful species Chen and Ray, ; Huang et al. Many of them are well known for their toxic effects on the environment and on human health. The elimination of these pollutants requires processes able to completely mineralize the organic pollutants and to convert the metallic contaminants into less harmful forms. Heterogeneous photocatalysis has been proven to be a potential process to eliminate many of these hazardous organic pollutants present in air and water wastestreams and has therefore been the subject of extensive research over the last decades Bahnemann, ; de Lasa et al.
Photocatalytic PC processes, albeit advantageous for completely mineralizing complex harmful contaminants at relatively low cost i. This has prompted the search of new means to improve their performance to tackle more efficiently the largely spread problem of polluted wastestreams. Owing to the particular characteristic of photocatalysis to produce nonselective hydroxyl radicals HO , chemical species with high oxidative power, it was applied to environmental engineering for pollutant decontamination.
Photocatalysis then emerged as a new process that could provide a solution to complete mineralization of organic contaminants and reduction of harmful inorganic metal ions. Their toxic effects on human health are well documented, being related to severe illnesses such as leukemia McDonald et al. They are highly toxic and refractory pollutants not easily removed in biological wastewater treatment plants Goi and Trapido, Hence, PC processes need to be improved to provide highly efficient solutions for heavily contaminated wastestreams and to minimize human exposure to species such as those mentioned above.
Moving toward an improved process: minimizing inefficiencies An important research approach to improve the PC process performance is the use of metallic complexes as additives in PC reactions. Some metallic compounds, such as Hg II Aguado et al. Most of those metallic compounds tested so far are equally harmful or even more so than the targeted organic pollutants, preventing them from being used as reaction enhancers.
These doping techniques reportedly improve catalyst activity in some cases, leading to higher mineralization rates than the untreated photocatalysts. The preparation techniques, however, often involve complex procedures that call for expensive reactants as metallic sources and high temperatures for calcination steps, therefore hindering their usage and production for large-scale applications. Additional equipment must also be added to the process for catalyst recovery if the catalyst cost is a factor or to produce catalyst-free water if it is intended for human consumption. These additional costs could eliminate one of the greatest advantages of PC processes: the low catalyst cost and low operating costs.
Thus, the search for means to improve the rate of mineralization of hazardous pollutants has veered off to look for inexpensive techniques that can enhance the PC processes using metallic complexes and yet be environmental friendly. Extensive research has shown that of all PC materials, TiO2 is the most active for oxidation reactions. Its higher catalytic activity, along with its low chemical and biological activity, low cost, and high stability has made it the best option for PC reactions Fox, ; Fujishima et al.
These charges can either recombine dissipating the absorbed energy or promote different reduction—oxidation redox reactions Herrmann, The redox reactions to take place depend on the chemical species present in the vicinity of the catalyst surface or in the bulk of the solution Fujishima et al. Organic oxidation and inorganic reduction Since photocatalysis was discovered in the early s, more than 6, papers related to this process have been published.
Most of the work on this subject has focused on showing that organic molecules can be oxidized in PC reactors. So far, more than organic molecules have been tested for oxidation in PC reactions Blake, In most cases, the tested organic molecules were converted to CO2, water, and mineral acids. Therefore, it can be definitely concluded that photocatalysis works for oxidation of organic molecules. The rate of oxidation depends on several factors that will be addressed in the upcoming section.
More recently, during the last decade, it was found that some metal cations in water could be reduced using photocatalysis. It was proposed that the photogenerated electron could be used for reducing inorganic metal ions. Additionally, Ag I was also reduced to Ag 0 Huang et al. Therefore, it was concluded that photocatalysis could be applied for metal cations reduction.
Figure 1 reports the reduction potential of different metals compared with both the valence band potential and the conduction band potential. All metals located above the conduction band can theoretically be reduced Chen and Ray, This fact, as will be discussed later, would reduce the recombination of the generated charges, thus increasing the energy efficiency of the system Colon et al. It was also found that there exists a synergic effect in the organic oxidation and inorganic reduction.
Some studies show that the presence of some metal ions can affect the rate of oxidation of organic molecules. For instance, the rate of oxidation of phenol can be affected by the presence of silver Huang et al. The presence of Cr VI affects the rate of oxidation of salicylic acid Colon et al. Likewise, the presence of organic molecules can accelerate or decelerate the rate of reduction of metal ions. The presence of a dye in the photoreduction of Cr IV increases the rate of reduction compared to when there is no dye present Li Puma and Lock Yue, With regard to the oxidation reactions of organic compounds, although there is still controversy over the actual oxidation mechanism, there is general consensus that hydroxyl HO radicals are the primary oxidizing species in a PC reaction.
The prevailing mechanism greatly depends on the substrate and on the catalyst surface characteristics Carraway et al. In spite of these observations, it is still difficult to distinguish between these two mechanisms as both, in many cases, lead to the same reaction products Grela et al. Iron Fe ions in photocatalytic processes As shown above, some metals can be reduced in their oxidation states in a PC process. Of the metals shown in Figure 1, Fe is the most benign and is actually needed for a proper human metabolism.
Fe is also one of the most abundant metals in the earth only after aluminum. Thus, because it is naturally present in large amounts and it possesses some PC properties, it represents an excellent candidate for the enhancement of the PC process. There are some interesting contributions confirming that the PC properties of iron can be exploited in photoreactions. When the solution is irradiated, the rate of hydroxyl radical HO formation is accelerated by the decomposition of H2O2 with radiation of less than nm Pignatello et al. Moreover, hydroxyl radicals can also be trapped by excess of ferrous ions reaction 7.
Thus, despite the advantages such as commercial availability of the oxidant, no mass transfer problems, and formation of hydroxyl radicals from H2O2, this process presents several serious drawbacks. One of the most important one is that H2O2 has to be continuously added in controlled amounts as a source of hydroxyl radicals Domenech et al.
Fe, on the other hand, has been used directly in PC processes as a dopant in semiconductors, in particular for TiO2. The results seem to be somewhat contradictory nonetheless. These doped catalysts have been tested in the PC reactions of short-chain carboxylic acids such as maleic, formic, and oxalic acids, among others.
They found 76 Aaron Ortiz-Gomez et al. However, for acetic and acrylic acids, all Fe-doped TiO2 catalysts showed lower activities than the Fe-free TiO2 samples. The doping procedures for both methods wet impregnation and sol-gel require elaborate steps of impregnation and calcination at K. This suggests that one has to be very cautious while preparing doped photocatalysts to prevent the metal from leaching out of the catalyst structure.
Another study on the use of Fe showed that the oxidation rate of acetaldehyde was improved with TiO2 catalysts doped with Fe and Si synthesized by thermal plasma Oh et al. The catalyst preparation technique involved a complex procedure using a plasma torch, with all this likely leading to an expensive photocatalyst of mild prospects for large-scale applications. From the studies above, one can observe that the use of Fe has been somewhat limited and that there are still many areas to explore for better utilization of Fe in PC reactions. More specifically, there is a need for the development of new inexpensive techniques or procedures to increase the photocatalyst activity.
These new procedures should make the process more efficient without the economic burden that photocatalyst doping brings about. Also, one can notice that most studies have focused on carboxylic acid species containing less than four carbons. It is of utmost importance to explore this area with more refractory molecules such as phenol and other hydroxylated aromatics to determine whether Fe can truly be applied to enhance the PC mineralization.
In this regard, the PC oxidation of phenol produces similar hydroxylated aromatics as reaction intermediates since the oxidation occurs via hydroxyl radical attack. These intermediates might be equally harmful than the parent species. These intermediate species were detected in experiments performed over a wide range of conditions and in different reaction setups.
Therefore, the formation and concentration of reaction intermediates greatly depend on the conditions at which the reaction takes place. The pH of the solution plays a key role in the formation of oxidation intermediates. Salaices et al. Also, 1,2,4-THB and 1,4-BQ were identified in most experiments with their concentrations not varying significantly from run to run. Additionally to phenolic intermediates, upon the aromatic ring opening, a series of carboxylic acids can be formed. Maleic acid, for instance, has been detected in the oxidation of byphenyls Bouquet-Somrani et al.
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Similarly, muconic acid has been reported as an intermediate of byphenyls oxidation Bouquet-Sormani et al. It is therefore expected that these or similar acids be formed during phenol oxidation. The presence of Fe ions in PC reactions could have an important effect on the formation and distribution of all reaction intermediates.
The use of Fe ions in the PC oxidation of phenol as reaction enhancer may not only affect the rate of oxidation, but also promote different reaction pathways leading to changes in the intermediate species distribution or to the formation of new ones. A complete identification and quantification of aromatics and carboxylic acids, during the oxidation of phenol for both unpromoted PC reaction no iron present and PC reaction coupled with Fe ions, will allow the formulation of a comprehensive reaction network for both systems and thus, a systematic comparison between them.
Also, this will permit the development of a more detailed kinetic model to incorporate most of the oxidation intermediates for both systems and will definitely help determine the role of Fe ions in PC reactions. Regarding the kinetic modeling, few contributions propose kinetic models for the PC oxidation of phenol and other aromatics Chen and Ray, , ; Li et al.
Such models fail to account for the formation of the different reaction intermediates, which may play an important role in the overall mineralization rate. More recently, Salaices et al. Solomonov, B. Antipin, I. Vavilova, , no.
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