Photocatalytic degradation of leather dyes in aqueous solution using solar/uv illuminated TiO 2 /ZnO

Transcription

1 Proceedings of International Symposium on Environmental Pollution Control and Waste Management 7-10 January 2002, Tunis (EPCOWM 2002), p Photocatalytic degradation of leather dyes in aqueous solution using solar/uv illuminated TiO 2 /ZnO V.MURUGESAN* and S.SAKTHIVEL Department of Chemistry, Anna University, Chennai , India v_murugu@hotmail.com Tanneries are the major consumers of water and consequently one of the largest group of industries causing intense water pollution. The effluent contains wide range of chemicals including various dye stuffs. These dyes are highly structured organic compounds and are difficult to be broken down biologically. Heterogeneous photocatalysis is a promising technique which has been successfully used to eliminate toxic and bioresistant pollutants from aqueous solution through transformation into benign compounds. Photocatalytic oxidation is a relatively new development for the removal of organic impurities from wastewater. TiO 2 and ZnO are the most preferable materials for the photocatalytic process due to their high photosensitivity, non-toxic nature and large band gap. When illuminated with an appropriate light, the photocatalyst generates electron/hole pairs with free electrons produced in the nearly empty conduction band leaving positive holes in the valence band. These electron/hole pairs are capable of initiating a series of chemical reactions that eventually mineralise the organic compounds. Moreover the formation of harmless end-products represents another attractive feature of this process. Hence attempt has been made in this paper to demonstrate that photocatalytic degradation of aqueous solution of commonly used leather dyes such as acid green 16 (AG 16) and acid brown 14 (AB 14) utilising sunlight/uv irradiation and TiO 2 /ZnO catalyst is an effective method for the disposal of dye wastewater 1. Experimental techniques Commercial samples of common leather dyes, acid green 16 and acid brown 14 were used as such without any further purification. The commercially available photocatalyst such as TiO 2 (Degussa P 25, Germany, having 70% anatase and 30% rutile), ZnO, SnO 2, ZrO 2, CdS and WO 3 were used as received. All other chemicals, reagents and additives used in the investigation were of analytical grade. The metal doped semiconductors were prepared by impregnation method. In this method the appropriate metal salt solution was added to the required weight of the catalyst in such a way that they wet the catalyst completely. The slurry was stirred at ambient temperature overnight and dried in an air oven at 110ºC for 12h. The dried powder was subjected to reduction in hydrogen. The samples were heated at 400ºC in hydrogen atmosphere for 12 h and then cooled to room temperature. The catalysts were stored in a vacuum desiccator. The coupled semiconductors were also prepared by impregnation method. TiO 2 was weighed accurately and made a slurry with required amount of distilled water. Then the desired amount of α-fe 2 O 3 was added to the TiO 2 slurry. The mixture was sonicated for 20 min, stirred at ambient temperature over night and dried in an air oven at 60ºC. The dried powder was calcined at 400ºC for 4 h. Similar procedure was adopted for the preparation of all other coupled semiconductors. TiO 2 and ZnO supported on glass beads and alumina beads were prepared by thermal attachment method. Glass beads were etched with 40% hydrofluoric acid and washed thoroughly with distilled water. Alumina beads were initially washed with dil.nitric acid followed by distilled water. Then the photocatalyst slurry was prepared with distilled water and stirred over night before introducing the beads. The entire sample was mixed thoroughly and placed in an oven for 24 h at 120ºC. The samples were then washed with water and removed the free catalyst particles. The final TiO 2 and 654

2 Photocatalytic degradation ZnO contents of glass beads were 11-12% and 7-8% respectively. TiO 2 and ZnO thin films on the glass plates were prepared using cement as the additive. The quantitative decolourisation of dyes were observed by measuring the absorbance after irradiation for specific interval of time employing a UV-Visible spectrophotometer (Hitachi UV-2000, double beam). The mineralisation products such as nitrate, ammonia, sulphate and chloride were measured by ion chromatography whereas the extent of photomineralisation by HPLC and total organic carbon (TOC) analyser. The dichromate reflux method was adopted to estimate the COD in the samples. The intensity of solar light was measured using Lux meter at time intervals of one hour between 8 a.m and 5 p.m. The average measured intensities are presented in Table 1. The UV and solar photocatalytic reactors used in the investigation is depicted in Figure

3 Murugesan and Sakthivel Results and discussion Photocatalytic degradation of aqueous solution of acid green 16 (AG16) and acid brown 14 (AB14) dyes were carried out with TiO 2 /ZnO semiconductor in the slurry form by employing UV/solar light as irradiation source. The photocatalytic reactor employed for this purpose was a batch type reactor. Experiments were also performed in presence of light without catalyst, and in presence of catalyst but without light to ensure that the degradation of dyes is a photocatalytic phenomenon. Effect of initial concentration of dye The effect of initial concentration of dye on the percentage decolourisation and degradation, and the rate of dye degradation was performed by varying the initial concentration of dye from 1x10-4 to 1x10-3 mol lit -1 and 1x10-4 to 6x10-4 mol lit -1 for AG 16 and AB 14 respectively with constant catalyst loading. It is evident from the Table 2 that percentage decolourisation and degradation decreases with increase in initial concentration of the dye. As the initial concentration of dye increases, the path length of photons entering the solution decreases, and in low concentration the reverse effect is observed thereby increasing the number of photon absorption by the catalyst 2. Matthews 3 also observed similar results in the photocatalytic degradation of methylene blue dye with TiO 2. Hence it is concluded that as the initial concentration of dye increases, the requirement of catalyst surface needed for the degradation also increases. Since illumination time and amount of catalyst are constant, the OH and O 2-2 species attacking the dye molecules decrease with increase in dye concentration

4 Photocatalytic degradation Effect of catalysis loading In order to assess the optimum catalyst loading for effective degradation of dyes, varying amounts of catalyst were attempted in a series of experiments. The values in Table 3 demonstrate that the percentage degradation and the rate of photodegradation increase linearly with catalyst loading upto 250 mg/100ml. Above this loading, increase in turbidity of the solution reduces the light transmission through the solution. The increased loading of catalyst increases the quantity of photons absorbed and consequently the degradation rate. Further increase in catalyst loading beyond 250 mg/100ml may result in deactivation of the activated molecules due to collision with the ground state molecules. Moreover, shielding by the catalyst may also take place. Lea and Adsina 5 have manifested that the photodecomposition rate has both positive and negative orders with respect to the catalyst content. The rate may drop as a result of loss in surface area for light-harvesting occasioned by agglomeration at high solid concentration. Effect of irradiation time The relationship between photodegradation efficiency and irradiation time is presented in Table 4. It is clearly seen that the degradation increases with increasing irradiation time. The photocatalytic degradation of the dyes occur on the surface of the catalyst where OH and O 2-2 radicals are available for photocatalytic degradation. The formation of OH and O 2-2 increases with increase in irradiation time and hence the dye is completely degraded in the course of time. Effect of light intensity The degradation experiments were carried out in different periods of time with different light intensity on a sunny day. Similar results are obtained for both TiO 2 and ZnO and depicted in Figure 2. The percentage decolourisation, degradation and rate of degradation increases with increase in the light intensity. Since the catalyst powder is suspended in a stirred solution, the light intensity will affect the degree of absorption of light by catalyst surface. Mineralisation studies of dyes As the irradiation time increases, dye molecules are degraded to components of lower and lower molecular weight fractions and consequently complete mineralisation which is observed from the chromatographs taken after each irradiation time. The complete mineralisation is also confirmed from total organic carbon (TOC) analysis and quantitative estimation of inorganic ions such as NH + 4, NO - 3, Cl - and SO 2-4. Figure 3 shows the degradation of organic carbon content and decrease in COD values for both AG 16 and AB 14 using TiO 2 and ZnO as photocatalyst. Figure 3 also demonstrates the formation of inorganic ions as a function of time. From the experimental results, it is evident that 657

5 Murugesan and Sakthivel both AG 16 and AB 14 decompose into colourless fragments in the initial stages and subsequently they mineralise into harmless end products. Bandara et al 6 proposed similar reports for dye degradation. They have also reported similar trend in the TOC removal and formation of inorganic ions in orange 2 azo dye by UV/fenton system. Comparison of photocatalytic activity of commercial catalysts In order to understand the efficiency of commercial photocatalysts towards photomineralisiation of AG 16 and AB14, identical experiments were carried out with TiO 2, ZnO, SnO 2, ZrO 2, α-fe 2 O 3, WO 3 and CdS using solar light and UV light sources. The results clearly indicate that ZnO is found to be the most active in the degradation of both AG 16 and AB 14 using sunlight as the energy source. The reason for greater activity of ZnO compared to TiO 2 is not due to the absorption of more light quanta. The biggest advantage of ZnO is that it absorbs large fraction of the solar spectrum than TiO 2. It is for these reasons ZnO is more active than other photocatalysts 7. Photocatalytic studies of TiO 2 /ZnO supported systems The photocatalytic reaction of leather dyes were carried out with supported catalysts using constant dye concentration. The degradation rate of AG 16 and AB 14 is observed in the order alumina beads (AB) > glass plate (GP) > glass beads (GB) for both TiO 2 /ZnO supported systems. It can be noticed that the rate of degradation is observed high in adsorbent material such as alumina beads suggesting that the adsorbed dye molecules are involved in degradation reaction and that the adsorption is significantly effective to enhance the rate of reaction of the substrate. Photocatalytic studies of metal doped TiO 2 and ZnO The metal doped catalysts were tested for the photocatalytic degradation of leather dyes. In order to find out the optimum metal content for effective degradation, the metal content was varied for all the metal doped TiO 2 and ZnO. The percentage degradation increases with increase in the metal content upto certain level and then decreases. The optimum metal loading was found to be 0.8, 0.8, 2.0 and 0.05 wt% for Pt, Au, Ru and Pd respectively both for TiO 2 and ZnO. Higher metal loadings result in slower degradation as excess metal contents shield the catalyst surface and 658

6 prevent the light from activating the photocatalyst 8. Photocatalytic degradation Photocatalytic activity of α-fe 2 O 3, WO 3 and CdS loaded TiO 2 and ZnO catalysts. The photocatalytic degradation of leather dyes were carried out with pure TiO 2 /ZnO and α- Fe 2 O 3, WO 3 and CdS loaded TiO 2 /ZnO in order to compare the photocatalytic activity. The results indicate that the loaded samples are more photoactive than the pure TiO 2 or ZnO especially with optimum loading of α- Fe 2 O 3 (0.05 wt%), WO 3 (0.02 wt%) and CdS (0.2 wt%). Above the optimum loading the photocatalytic activity slowly decreases in all the loaded TiO 2 or ZnO. The photocatalytic activity of α- Fe 2 O 3, WO 3 and CdS loaded TiO 2 or ZnO is in the order WO 3 > α- Fe 2 O 3 > CdS and α-fe 2 O 3 >WO 3 > CdS for AG 16 and AB 14 respectively. The reason for this enhancement of activity in the case of WO 3 loading may be the formation of W 5+ species on the surface of TiO 2 or ZnO by means of transfer of photoinduced electrons from TiO 2 to W 6+ which is confirmed by the DRS spectra of WO 3 /TiO 9 2. Based on the reports of Martin et al 9 and also our experimental results, it is concluded that TiO 2 or ZnO surface adsorbed α- Fe 2 O 3, WO 3 and CdS particles play an important role in accepting the photogenerated electrons from TiO 2 or ZnO and these electrons are transferred to oxygen adsorbed on the surface of TiO 2 or ZnO particles. Conclusion Photocatalytic degradation of aqueous solution of AG 16 and AB 14 utilising solar /UV irradiation and TiO 2 / ZnO catalyst is found to be an effective method for the disposal of wastewater containing these leather dyes. The dye concentration, catalyst loading, ph of the medium, irradiation time and light intensity in addition to the nature and chemical composition of the dyes play important role in the degradation efficiency. Among the commercial catalysts, ZnO is the most active in the degradation of AG 16 and AB 14 using sunlight as the energy source. The photodegradation of dyes employing solar energy could emerge as a viable technique because of its cost effectiveness and non-hazardous nature. The photocatalytic degradation is observed high in adsorbent material such as alumina beads suggesting that the dye molecules are adsorbed on the alumina support to make high concentration environment around the supported catalyst. The photocatalytic activity of TiO 2 and ZnO towards the degradation of leather dyes can be greatly improved by loading of low band gap semiconductor on the surface of TiO 2 and ZnO catalysts. Interparticle electron transfer is found to be a crucial step in coupled semiconductor system for effective dye degradation. Thus this technique will open up new vistas in the treatment of wastewater containing toxic chemicals. REFERENCES Sakthivel S. Ph.D thesis, Anna University, Chennai, India, Davis R.J, Gainer J.L, Neal G.O and Wenwu I. Water Environ. Res. 1994, 66, 50. Matthews R.W. J.Chem. Soc. Faraday. Trans , 85, Rideh L, Wehrer A, Ronze D and Zoulalian A. J. Photochem. Photobiol. A: Chem. 1997, 36, Lea J and Adesina A. J. Photochem. Photobiol. A: Chem. 1998, 118, 111. Bandara J, Nadtochenko V, Kiwi J and Pulgarin C. Wat. Sci. Tech. 1997, 35, 87. Sakthivel S, Neppolian B, Palanichamy M, Banumathi Arabindoo and Murugesan V. Indian J. Chem. Tech. 1999, 6, 161. Crittenden J.C, Liu J, Hand D.W and Perram D.L. Wat. Res. 1997, 31, 429. Martin C, Solana G, Rives V, Marci G, Polmisano L, and Sclafani A. Catalysis letters 1997, 49,