Advanced oxidation technologies : photocatalytic treatment of wastewater

7.1. Summary and conclusionsThe last two decennia have shown a growing interest in the photocatalytic treatment of wastewater, and more and more research has been carried out into the various aspects of photocatalysis, varying from highly fundamental aspects to practical application. However, despite all this research, there is still much to investigate. Suggested photocatalytic mechanisms, such as those for oxidation by hydroxyl radicals and for oxidation at the surface of photocatalysts, need to be verified experimentally for various types of pollutant under various reactive conditions. The kinetic processes occurring during photocatalytic reactions have still not been fully clarified. Efficient photocatalytic treatment of pollutants in wastewater has so far been achieved only by using the powdery photocatalyst TiO ₂ in suspension. This method cannot be applied in practice unless there are means to keep or recycle the photocatalyst within the treatment unit. Optimization of the photocatalytic process requires modification of photocatalyst surfaces, the discovery of new efficient photocatalysts, and research into new combinations of photocatalysis with other methods. The present dissertation deals with photocatalytic processes and systems, their combination with photolysis and other advanced oxidation technologies, and their application for the treatment of environmental pollutants in wastewaters.Chapter I introduces advanced oxidation technologies (AOTs), which consist mainly of the use of ozone, hydrogen peroxide, ultraviolet light (UV) and non-thermal plasmas; electrohydraulic cavitation and sonolysis; electron-beam and gamma irradiation; catalytic oxidation; wet air oxidation; supercritical water oxidation; electrochemical oxidation (electrolysis); and photocatalysis. The basic principles and applications of these technologies are discussed, focusing on photocatalysis for the treatment of wastewater, and more than two hundred references are given.The general principle of the photocatalytic treatment of pollutants is as follows. When a semiconductor catalyst suspended in an aqueous solution is illuminated using light with an energy (hv) exceeding or equal to the band gap of the semiconductor (hv ≥Eg), electron-hole pairs (e <sup>-</SUP>-h <sup>+</SUP>) with a certain electric potential are generated at the catalyst surface. For the semiconductor TiO ₂ which has a band gap of 3.05 eV, this light must be near-UV with a wavelength smaller than 410 nm. At catalyst surfaces, the generated holes with a positive charge move to the anodic area, whereas the generated electrons move to the cathodic area. At the anodic area of a catalyst, an oxidative half reaction will occur, such as the oxidation of organic pollutants in wastewater to CO, At the cathodic area of a catalyst, a reductive half reaction will occur, such as the reduction of oxygen in wastewater to H ₂ O.Chapter 2 discusses the photocatalytic oxidation of methanol, ethanol and chloroform, trichloroethylene (TCE), and dichloropropionic acid (DCP) in aerated aqueous slurries using plain TiO ₂ Pd/TiO ₂ or Pt/TiO ₂ as a catalyst. Based on experimental data, various photocatalytic mechanisms are elaborated upon.In TiO ₂ suspensions, it was observed that during illumination with near-UV light (320 nm ≤ λ ≤410 nm) in the presence of oxygen the photocatalytic oxidation of methanol and ethanol was accelerated by modifying the catalyst with Pt or Pd. Pd present on TiO ₂ created a less strong effect than Pt. During not any experiment involving methanol were intermediates of methanol oxidation (such as formaldehyde and formic acid) detected in the aqueous solution using a gas chromatograph. The only product detected was CO, However, during the photocatalytic oxidation of ethanol, various types of intermediate (such as acetaldehyde and acetic acid) were detected in the aqueous solution. These intermediates varied in concentration depending on the type of catalyst. Under similar reaction conditions, the ratio between acetaldehyde and acetic acid was 30:1 with TiO ₂ and 0.23:1 with Pt/Ti021 which means that with TiO ₂ the acetaldehyde concentration was 130 times higher than with Pt/TiO ₂ . Therefore, it can be concluded that further oxidation can be achieved easiest with Pt/TiO ₂ . The rite at which alcohol was mineralized <em>(i.e.,</em> C02 was produced) was found to depend on the initial solution pH. At an acidic pH, C02 was produced immediately, whereas at an alkaline pH the mineralization process occurred much slower.When plain TiO ₂ was used as a photocatalyst, 98% of the TCE present in the water phase at an initial concentration of 25 ppm and 75% of the chloroform present in the water phase at an initial concentration of 120 ppm were oxidized after an illumination period of one hour and three hours, respectively. About 18% of DCP present at an initial concentration of 1,430 ppm was dechlorinated and 23 % of this DCP was decarboxylated during an illumination period of three hours. Compared with the results obtained with plain TiO ₂ , no effect of Pd metallized on TiO ₂ was observed during the photocatalytic oxidation of either chloroform, TCE or DCP. The acceleration of DCP dechlorination affected by Pt/Ti02 exceeded that affected by plain Ti02 by 53%, but Pt/TiO ₂ did not affect the rate at which DCP was decarboxylated.Based on experimental data and an evaluation of possible reaction pathways, it was concluded that during aerated photo-oxidation of alcohols and organochlorides <em>(e.g.,</em> TCE, DCP) Pt accelerates the cathodic process of 0 ₂ reduction occurring at catalyst surfaces. If a ratecontrolling step is part of the anodic process occurring during a photocatalytic reaction (as is the case with the photocatalytic oxidation of organochlorides), modified Pt or Pd on TiO ₂ cannot influence the rate of this reaction unless it can affect the reactants present at the anodic area. In general, the rate-controlling step of an anodic process can be attributed chiefly to the resistance to adsorption and electron transfer offered by reactants in relation to anodic surfaces. A pathway favourable for anodic processes occurring at catalyst surfaces is direct adsorption of reactants and direct electron transfer to holes with a positive charge (i.e., a direct oxidation reaction occurring at surfaces).Chapter 3 discusses the interaction occurring between Pt or Pd and TiO ₂ . A porous micro-cell model for a system containing a powdery photocatalyst is presented. Furthermore, the kineticprocesses of the photocatalytic mineralization of alcohols occurring at the surface of metallized and native TiO ₂ in aerated systems are detailed, the adsorption of alcohols present on TiO ₂ surfaces is calculated, and the process during which oxygen is reduced at TiO ₂ surfaces is discussed.An X-ray diffraction experiment carried out to determine the crystal structures of catalyst components revealed a diffraction angle of 2Θ=40°, which deviates from those shown by the crystal structure of the metal Pt, anatase and rutile present in a sample of Pt/TiO ₂ pretreated at a high temperature. Based on the principle of strong metal-semiconductor interaction (SMSI) and diffraction data on Pt-Ti intermetallic compounds, it was supposed that it is the strongest diffraction angle of Pt-Ti intermetallic compounds.Surface element analysis of the photocatalyst Pd/Ti0 ₂ using an electron beam probe (X-ray exciting spectrum) reveals that not all Id was deposited on the outside of the Ti02 powder; therefore, part of it was probably deposited inside. Based on the results of this experiment and the experiment of XPS (X-ray Photoelectron Spectroscope) in chapter 6, a new porous microell model for a system containing a powdery photocatalyst was developed. In this model the outer surface of a particle can be illuminated, but its pores form a dark, unilluminated area, which acts as a permanent cathode.The overall photocatalytic reaction occurs at two areas of photocatalyst surfaces. Ethanol is oxidized at the anodic area, whereas oxygen is reduced at the cathodic area. Experiments revealed that the photocatalytic degradation of alcohols occurring at the anodic area is a firstorder reaction. This can be easily explained by assuming the existence of a Langmuir adsorption isotherm, i.e., alcohols are first adsorbed onto the catalyst surface and then oxidized photocatalytically. This surface mechanism can be described by the Langmuir adsorption constant (K) and the real reaction rate constant (k) applying to this reaction. Therefore, it can be assumed that, at least in a dilute aqueous solution, one of kinetic mechanisms of photocatalysis occurring during the oxidation of some organic compounds is similar to a gasphase reaction occurring at the solid surface of a heterogeneous catalyst, which can be described by the Langmuir equation. Consequently, the fact that no intermediates were detected in solution during the photocatalytic oxidation of methanol must be due to the fact that the intermediates formaldehyde and formic acid remained at the photocatalyst surface and were not desorbed into solution before being further oxidized to the final product, CO,According to a calculation of the surface coverage by alcohols on photocatalyst particles, at similar molar concentrations the coverage of ethanol was twice as high as dig of methanol. The maximum rate and efficiency of the photocatalytic oxidation of methanol and ethanol at catalyst surfaces can be estimated based on the results obtained. To explain the surface mechanisms taking place during the photocatalytic oxidation of alcohols, it is suggested that at catalyst surfaces end-group adsorption of methanol and ethanol occurs.Chapter 4 details lab-scale experiments during which various photochemical methods for the elimination of pollutants from wastewater were employed in combination under aerated conditions. These pollutants included pure phenol in aqueous solutions (initial concentration = 25 ppm), and substituted phenols and COD (Chemical Oxygen Demand) in industrial wastewaters, such as those from the production of phenolic resins, oil refinement, the dry distillation of shale oil, and the production of naphthenic acid. The combinations applied included illumination with UV light (using a 200-W high-pressure mercury lamp, wavelength 313 ≤ λ ≤456 nm), the use of magnetite or aluminum oxide as a photocatalyst, H ₂ O ₂ and iron compounds (FeCl ₃ and Fe(NH ₄ ) ₂ (SO ₄ ) ₂ as Fenton reagents). Using magnetite simultaneously as a photocatalyst and a solidified Fenton reagent proved to be a sophisticated method for combining photocatalysis with the photo Fenton reaction.The experimental results presented in this chapter show clearly the separate and combined effects of UV light, a photocatalyst, ferric compounds, and H ₂ O ₂ . For example, magnetite or aluminum oxide combined with UV light decreased the COD of certain toxic industrial wastewaters by around 60- 70% in 1-4 hours. The separate use of UV light and magnetite had little effect on the degradation of COD and phenol.The rate of phenol photolysis during illumination with UV light was found to depend on the solution pH and was highest at an initial pH of 3.5. Hydrogen peroxide or ferric ions (Fe <sup>3+</SUP>/Fe <sup>2+</SUP>) in an aqueous solution of phenol greatly increased the rate of phenol elimination, when only UV photolysis was applied and no catalyst was present. The photo Fenton reaction system of UV/H ₂ O ₂ /Fe-compounds showed the highest rate of photochemical elimination of phenol and other organic compounds in the above-mentioned combinations.An improved photochemical method, using a calcium compound as a chemical promoter, clearly made a ferric and an aluminium photocatalyst more effective in the treatment of toxic compounds (particularly phenols in various wastewaters). According to the experimental results, the combined photochemical method employing UV light, hydrogen peroxide and a photocatalyst (used simultaneously as a solidified Fenton-reagent) is a very promising method of photochemical wastewater treatment.Chapter 5 discusses the photocatalyzed deposition and concentration of uranium(VI) (uranyl) in wastewaters, using suspended TiO ₂ or Pt/TiO ₂ as a catalyst. As a result of pretreatment, uranium- containing wastewaters usually contain several types of chelating reagent, such as EDTA (ethylenediaminetetraacetic acid). In deaerated U(VI)/EDTA solutions, reductive deposition of uranium and release of CO ₂ from EDTA occurred as a result of photocatalysis at TiO ₂ or Pt/TiO ₂ surfaces during illumination with near-UV light. In the experimental set-up chosen, at most 50-60% of the uranium(VI) present in the solution was deposited. Pt promoted this reductive deposition of uranium(VI) on TiO ₂ only to a slight degree, although it accelerated the oxidative deposition of other metals on TiO ₂ during other experiments. The ultimate amount of C02 released during experiments was equivalent to a single decarboxylation of EDTA and was independent of illumination time. This explains die limited reductive deposition of uranyl (around 50-60%). Furthermore, it means that the tricarboxylic acid resulting fromEDTA oxidation cannot be easily oxidized further, and then it renders any further reductive process of uranyl on TiO ₂ surfaces.In aerated solutions, no uranium was deposited but a lot of CO, was released, probably as a result of EDTA mineralization on photocatalyst surfaces. Nearly 100% of the deposited, reduced product of uranium(VI) present at TiO ₂ or Pt/TiO ₂ surfaces was reoxidized reversibly and desorbed into solution to form dissolved uranium(VI), simply by exposing the solution to air after illumination. During one experiment, the reductive deposition process, during which around 55% of the uranium(VI), in solution could be deposited, was repeated three times. This is therefore a promising photocatalytic method for the recovery and concentration of uraniurn(VI) in uranium-containing wastewaters.Chapter 6 deals with the photocatalytic dehydrogenation of ethanol in the presence of metallized US <em>(i.e.,</em> M/CdS with one or two metals) in a deaerated system using visible light for illumination. The effect of various M/CdS photocatalysts, present in solution in an optimal amount, on the dehydrogenation of ethanol was investigated. This process results in the release of H ₂ and the production of acetaldehyde. A study was also conducted of the effect of pretreating M/CdS at a high temperature on the distribution of I'd on US surfaces, the role of Pd or Pt present on CdS, and the role of a photocatalytic promoter, <em>i.e.,</em> a second metal component <em>(e.g.,</em> Cu, Ag, Rh), present on CdS. In addition, a complex catalysis mechanism for the photocatalytic dehydrogenation of alcohols was suggested.The optimum amount of photocatalyst was found to be 2.5 g/l, which was accompanied by a hydrogen production of 300 ml/h.l. Pretreatment at a high temperature reduced the size of deposited Pd particles by 10% from 8.8 nm to 7.9 nm, and increased the number of Pd particles present on US particles (with a diameter of 33.8 nm) by 41 %. This resulted in a 100% increase in photocatalytic activity of the photocatalyst Pd/CdS during ethanol dehydrogenation. The deaerated dehydrogenation of ethanol was found to be a first-order reaction at an ethanol concentration below 50%. The reaction activity of the photocatalysts tested decreased in the following order: 5% wt. Pd/CdS (prepared by chemical deposition, pretreated at a high temperature)>5 % wt. Pt/CdS (physical mixing or chemical deposition with or without pretreatment)>I % wt. Rh ₂ O ₃ /CdS (physical mixing, no pretreatment)>>PdO, Rh, Ag, Cu, CuO/CdS. Plain US showed a negligible photocatalytic activity during ethanol dehydrogenation. The optimal Pt content in a Pt/CdS system was found to be around 5%. From the experimental results, it was concluded that the second metallic component on US possibly acts as a promoter during dehydrogenation. 5 % wt. Pt + 5 % wt. Cu/CdS was found to be the optimum combination in a system containing these two metallic components under the conditions used during our experiments. I'd or Pt loaded on a US surface acts as an electron pump that transfers photogenerated electrons e <sup>-</SUP>_CB from the valence band of US to Pd. Pd acts as a proton-bonding adsorption centre.If at Pd/CdS surfaces electrons can flow from Id to US and Pd <sup>0</SUP>can be oxidized to Pd <sup>2+</SUP>during illumination, the photocatalytic oxidation of alcohols at such surfaces may also occur according to a heterogeneous catalytic mechanism called a complexing catalysis mechanism, because after a Pd/CdS catalyst has been pretreated at a high temperature, a strong metal and semiconductor interaction (SMSI) (e.g., chemical bonds between either Pd and Cd or Pd and S) can occur at the Pd-CdS junction. This interaction can increase the flow of electrons from Pd to CdS. In view of the palladium standard redox potential, the holes (h <sup>+</SUP>_VB ) with a positive charge at US surfaces have sufficient electric potential to oxidize Pd <sup>0</SUP>to Pd <sup>2+</SUP>.From the investigations into photocatalysis applied to wastewater treatment detailed in this dissertation, the following general conclusions can be drawn.<ul><li>Experiments carried out to study the photocatalytic oxidation of methanol and ethanol have resulted in an increased insight into and better understanding of the mechanisms occurring during the photocatalytic treatment of wastewaters.<LI>During this study, more insight has been gained into the practical application of photocatalysis and the problems that still need to be solved regarding the treatment of certain industrial wastewaters.<LI>Organochlorides can also be easily degraded using a photocatalyst; this process may be a promising application.<LI>Combined photochemical methods that include the use of a photocatalyst may be attractive for the treatment of phenol-containing industrial wastewaters.<LI>Experimental results of the photocatalytic deposition and concentration of soluble uranium show that photocatalytic treatment is an interesting method for the recovery of radioactive heavy metals.<LI>Metallization is a popular method for raising the activity of photocatalysts consisting of a semiconductor. The functions of metallization were revealed as a result of experiments involving the use of metallized TiO ₂ and CdS.</ul>