Comparative study of UV-activated processes for the degradation of organic pollutants in

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1 Comparative study of UV-activated processes for the degradation of organic pollutants in water Italo Mazzarino Dipartimento di Scienza del Materiali e Ingegneria Chimica Politecnico di Torino c. Duca degli Abruzzi Torino, Italia italo@athena.polito. it Abstract Homogeneous and photocatalytic photo-activated processes were experimentally investigated by carrying out the degradation of various refractory organic pollutants in pilot size reactors. The efficiency of the processes was evaluated by measuring both the degradation of the primary pollutants and the decrease of the total organic carbon in the treated water. The comparison between the purification processes is performed in terms of rate of degradation of the organic pollutants and energy efficiency. 1 Introduction A wide number of organic pollutants of industrial waste waters shows a strong resistance to the conventional purification processes. These compounds are usually defined as "refractory pollutants". The study of the processes for the treatment of waters contaminated by refractory pollutants is an interesting challenge for the applied research. In last years various processes based on oxidation reactions activated by short wavelength radiations have been investigated, e.g. Matthews [1], Peyton et al. [2], Negrini et al. [3] and Oppenlander et al. [4].

2 38 Water Pollution A common feature of these processes is the radical reaction mechanism. The first step of the photoactivated processes is the production of highly reactive free radicals in the aqueous media. This phenomenon can occur by different mechanisms: A) the decomposition of the pollutant due to the direct impact of photons with the organic molecules (direct photolysis); B) the degradation of a reactive inorganic compound added to the water (hydrogen peroxide is the most commonly used); C) the adsorption of photons on a photoconductor followed by the reaction between charge carriers produced by thisfirststep (electrons and electronic vacancy) and the chemical species absorbed on the photoconductor surface (heterogeneous photocatalysis). A first exam of the basic features of these mechanisms shows that the direct photolysis is the less attractive for a practical application. In fact, a process based on this mechanism is effective only if the photons have an energy sufficient to brake the molecules of the pollutant. For many organic compounds this requires the use a very short wavelength radiation which are quite expensive to produce. Furthermore the photolysis of the primary pollutants can lead to the formation of stable intermediates. In this case the purification processes stops after the first step and no significant reduction of the organic load of the water is obtained. Hydroxyl free radicals can be easily obtained by the decomposition of hydrogen peroxide in aqueous solutions. The reaction is quite fast at room temperature in presence of UV radiations. The efficiency of the process increases with the decrease of the wavelength of the radiation and with the concentration of the hydrogen peroxide, e.g. Mazzarino et al. [5]. The photocatalytic process requires the presence of a solid photoconductor in contact with the polluted water. Titanium dioxide is commonly used as the photocatalyst because of its low cost. The band gap of Titania is quite small and the photoconductivity occurs in presence of relatively long wavelength radiations (X<360 nm). The process can be carried out by dispersing fine particles of the catalyst in the liquid media (slurry catalytic system). As an alternative it is possible to use a fixed catalyst supported on the solid surface of an inert material. If compared with the catalyst dispersion this last solution seems to be advantageous for two main reasons: i) the opacity of the catalyst suspension limits remarkably the diffusion of the activating radiation in the reacting media; ii) the separation of the solid after the photocatalytic treatment in the case of the dispersed catalyst is not easy because of the small size of the particles. The results obtained with fixed photocatalysts are very promising and the feasibility of the photocatalysis could be remarkably improved in this way, e.g. Crittenden et al. [6], Brucato et al. [7], Renzi et al. [8].

3 Water Pollution 39 In the present work the following processes have been examined: a) homogeneous photo-oxidation in presence of hydrogen peroxide; b) heterogeneous photocatalytic oxidation by a suspended catalyst; c) heterogeneous photocatalytic degradation by a fixed supported catalyst; d) the combination of (a) and (c) processes. 2 Experimental The experiments were carried out in a tubular continuous photochemical reactor equipped with an axial UV lamp. The reactor was 750 mm long and its internal diameter was 33 mm. The length of the lamp was the same of the reactor and its external diameter was 25 mm. The power of the UV source was 40 W and the photonic efficiency was about 40%. The emitted radiation was almost monochromatic with a wavelength of 254nm. The liquid phase consisting of an aqueous solution of the tested pollutant was fed at the bottom of the reactor and flowed upward in an annular section between the surface of the lamp and the reactor shell. The experimental equipment could also operate in biphasic flow regime by mixing a gas phase with the liquid before to feed it to the reactor. The temperature, the pressure and the flow rates of the fluid phases were controlled during the experiments. The tests with the dispersed catalyst were performed by using a suspension of a commercial Titania powder (Degussa P25) in the pollutant solution. A stable and uniform dispersion was obtained by sonication of the liquid-solid system with a 20 khz horn. Both the lamp and the reactor were carefully cleaned of the deposited solid particles after each test in order to restore the same initial conditions. The supported photocatalyst consisted of emicylindrical stainless steel bodies (each 125 mm long) coated by a thin film of Titania. These bodies were placed in the reactor coaxial with the UV lamp. The coating on the steel surface was performed by a physical vapour deposition process. After the deposition the Titanium oxides were converted into the photocatalytically active material (Anatase) by a thermal treatment at 500 C. 3 Results The first class of pollutants examined in this work was the aliphatic acids group. These compounds can be found, at relatively high concentration ( mg/l) in waste waters produced by the cleaning of boilers in power plants. Various acids characterised by different degree of molecular complexity have been tested. The most simple among these is the formic acid. With this pollutant the deep oxidation to water and carbon dioxide is easily achieved by all the tested photoactivated process. In other words no intermediate can be found at a detectable concentration in the treated water.

4 40 Water Pollution With other aliphatic acids (glycolic, citric and propanoic) the conversion is sensibly slower and various partial oxidation products are detected. This fact leads to a significant difference between the conversions of the primary pollutants (the acids) and the organic load (TOC). The figure 1 shows the ratio between the TOC and the acid relative conversions observed in the case of photocatalytic oxidation by the supported catalyst. The trend is almost the same for all the photo-activated processes tested in this work T (mm) Figure 1. Ratio between the TOC and the primary reactant conversion vs. the residence time for various acids (initial concentration 20 mg/l) The figure 2 shows a comparison between the various processes in the case of the degradation of the Formic acid. As far as the photocatalytic oxidation is concerned the supported catalyst (Cat S) proves to be is sensibly more efficient than the dispersed one (Cat D). The better performance of the fixed system is probably due to a better irradiation of the catalytic surface. The best results are obtained by the homogeneous oxidation process in the presence of the hydrogen peroxide. The experimental tests were carried out at various concentrations of the oxidising reactant. The data reported in the figure 2 were obtained using a large excess of the hydrogen peroxide (2 times the stoichiometric amount required to achieve the full conversion the organic pollutant to water and CO2). At lower concentration of H2 2 the conversion rate decreases sensibly and with a stoichiometric amount it is very close to that performed by the supported photocatalyst If compared with the homogeneous oxidation the combined process (Catalyst + H2O2) does not lead to a significant increase of the purification efficiency.

5 Water Pollution 41 The contribution of the catalytic mechanism to global of degradation is negligible probably because most of the radiation is absorbed in the liquid by the decomposition of the Peroxide and can not reach the catalyst surface. C/C. # A T CatS HA Cat + HA CatD i (min) Figure 2. TOC conversion vs. residence time in the degradation of Formic acid (initial concentration 200 mg/l) C/C T (min) Figure 3. TOC conversion vs. residence time in the degradation of Phenol (initial concentration 20 mg/l) With Phenol and Toluen di-isocianate (Figures 3 and 4) the conversion of the TOC is slower than in the case of aliphatic acids, but the relative efficiency of

6 42 Water Pollution the various degradation processes is very similar. The best performance is obtained by the homogeneous photo-oxidation and no significant improvement of the abatement efficiency is achieved by the combination of the catalytic and the homogeneous processes. C/C CatS A Cat + HO T (min) Figure 4. TOC conversion vs. residence time in the degradation of Toluen di-isocianate TDI (initial concentration 20 mg/l) C/C T (min) Figure 5. TOC conversion vs. residence time in the degradation of a dyes mixture (initial concentration 5 mg/l of TOC) The figure 5 shows the conversion of the total organic carbon in a solution of various organic dyes. In this case the exact composition of the solution was

7 Water Pollution 43 unknown because a real waste water was used. The results obtained by the photocatalytic oxidation with both the dispersed and the supported catalyst are not very good, but a satisfactory conversion of the pollutants can be obtained by the homogeneous photo-oxidation. Similar results have been obtained for the degradation of Na-dodecylsulfate (NaDS). With this pollutant a satisfactory reduction of the organic content of the solution can be achieved in a reasonable time only by the homogeneous process (Figure 6). C/C T (min) Figure 6. TOC conversion vs. residence time in the degradation of Na-dodecylsulfate (initial concentration lomg/l) The energy efficiency of a photoactivated water purification process can be defined as the ratio between the amount of photons emitted by the radiation source and the molar conversion of the organic carbon in the liquid phase. In our case the lamp has an effective light power of 16.5 W. Assuming a monochromatic radiation at 254 nm the photonic emission rate is "^ mole/s. The values of the energy efficiency calculated on the basis of the experimental results of the TOC conversion are summarised in the figures 7-9. The efficiency is relatively high in the case of light organic compounds like the formic acid, but it decreases sensibly with the increases the complexity of the molecular structure. The efficiency also depends on the pollutant concentration. The influence of the concentration is high in the case of the photocatalytic process while it is moderate for the homogeneous oxidation. This might be due to fact that the electrons and the vacancies created on the catalyst surface can easily recombine

8 44 Water Pollution with no photocatalytic effect if the concentration of the adsorbed species is very low. n p - I Supported Catalyst Dispersed catalyst Formic Glycolic Citric Propanoic Figure 7. Photonic efficiency in the degradation of various organic acids (initial concentration 200 mg/l) I I Supported Catalyst Dispersed catalyst UV ppm ppm ppm Figure 8. Photonic efficiency in the degradation of formic acid at different initial concentrations

9 Water Pollution 45 I Supported Catalyst b%# Dispersed catalyst UV+HO, Phenol 20 ppm TDI 15 ppm Dyes 5ppmC NaDS 10 ppm Figure 9. Photonic efficiency in the degradation of various organic pollutants 4 Conclusions Both the homogeneous photo-oxidation in presence of hydrogen peroxide and the photocatalytic mechanism proved to be effective in the degradation of a wide class of refractory organic pollutants. The supported fixed catalyst shows a better performance if compared with the dispersed solid system. The homogeneous process seems to be more effective than the heterogeneous photocatalysis, particularly in the case of compounds with a complex molecular structure and at very low concentration of the pollutants. On the other hand a good efficiency of the homogeneous photo-oxidation is achieved only by using a large excess of hydrogen peroxide. This fact might limit the feasibility of the process because of the secondary pollution due to the presence of a high concentration of the peroxide in the treated water. References 1. Matthews, R.W. Purification of Water with Near-UV Illuminated Suspensions of Titanium Dioxide, Water Res,, 24 pp , Peyton, G.R., Huang, F.Y., Burleson, J.L. and Glaze, W.H., Destruction of Pollutants in Water with Ozone in Combination with UV Radiation, Environ. Sci. TechnoL, 16, pp , Legrini, O., Oliveros, E., Braun A.M., Oxidation of Pollutants in Water by UV-Hydrogen Peroxide and UV-Ozone Processes, Chem Rev., 93, pp , 1993

10 46 Water Pollution 4. Oppenlander, T. and Baum, G., Bin Modularer Excimer-Durchflussreaktor zur Reinigung belasteter Abwasser durch Vakuum-UV/UV-Doppelbestrahlung ohne Oxidationmittelzusatz, Chem. Ing. Tech., 66, pp , Mazzarino, I., Piccinini, P. and Spinelli, L., Degradation of Organic Pollutants in Water by Photochemical Reactors, Proc. of the II Int. Sysmposium on Catalysis in Multiphase Reactors.Toulouse, France, Crittenden, J.C., Zhang, Y., Hand, D.W., Perram, L.D. and Marchand, E.G., Solar Detoxification of Contaminated Groundwaters Using Fixed-Bed Photocatalysts 7. Brucato, A., Mannino, V., Rizzuti, L., Sclafani, A., Photocatalytic Degradation of Phenol in a Glass-Beads Diluted Fixed-Bed Reactor, Chemical Industry and Environment II, Eds. N. Piccinini and R. Delorenzo, Torino, pp , Renzi, C., Mazzarino, I., and Baldi, G., Heterogeneous Photocatalytic Oxidation of Toxic Organic Waste Waters with Supported Semiconductors, Proc. of the V World Congress of Chemical Engineering, San Diego, USA, 1996