Photocatalytic degradation of Chromium (VI) from wastewater using nanomaterials like TiO2, ZnO, and CdS

K. M. Joshi 0 V. S. Shrivastava 0 0 K. M. Joshi (&) V. S. Shrivastava Nanochemistry Research Laboratory, G.T.P. College , Nandurbar 425412, India The photocatalytic degradation of Cr(VI) from wastewater by using nanomaterials TiO2, ZnO, and CdS. All the experiments were carried out in the batch process. The wastewater obtained from various industries. The amount of chromium was removed using photocatalyst with UV light and in the dark at different pH range. The maximum removal of Cr(VI) was observed at pH 2; out of these photocatalyst TiO2 showed highest capacity for Cr(VI) removal than TiO2 thin film. The removal of chromium has been studied by considering influent concentration, loading of photocatalyst, pH, and contact time as operating variables. The degradation was characterized by FTIR, XRD, SEM, and EDX analysis before and after application of photocatalysts. - The pollution of hazardous metals is increasing with extensive of industrial developments (Bhatkhande et al. 2001). Biological interest in chromium arises from its prominent role in industrial pollution and its toxicity to microbes, plants, and animals (Mills et al. 1996). Large amounts of chromium are introduced into the environment through various industries like dyeing and printing in textile industries, chemical manufacture, leather tannery, metal plating, and processing industrial effluents affected living cells (Lozano et al. 1992; Suksabye et al. 2007). Chromium exists in two oxidation states, Cr(III) and Cr(VI). The hexavalent form is 500 times more toxic than the trivalent form (Kowalski 1994; Gimenez et al. 1996). Several studies have shown that the trace amount of chromium (VI) is considered to be essential for normal metabolic process (Barkat et al. 2004). Chromium discharged into the sewage system causes serious environmental impact. Chromium occurs mainly in Cr(III) form, which is oxidized into chromium (VI) due to the presence of organic compounds (Sarin and Pant 2006). The maximum level of Cr(VI) permitted in wastewater is 0.05 mg L-1 (Acer and Malkoc 2004). However, higher levels of chromium metals have been found to be toxic mainly to the kidney and liver (Kajitvichyanukul et al. 2005; Shrivastava et al. 1996). World health organization (WHO) has determined that chromium (VI) is carcinogenic (Suksabyea et al. 2009). The human toxicity includes lung cancer as well as liver and gastric damage (Acer and Malkoc 2004; Kabra et al. 2008). Nowadays, most of the industries are facing the difficult problem of disposal of chromium (VI) in wastewater produced in huge quantity. Hexavalent chromium form chromate (CrO4-2) is considerably more soluble in water than trivalent chromium Cr(III) (Butler and Davis 1993). The chromium removal treatment includes precipitation, ion exchange, photocatalysis, reverse osmosis, and adsorption process (Lin et al. 1993). Most of these methods require high capital and recurring expenditure and consequently they are not suitable for small-scale industries (Lee et al. 2006; Chenthamarakshan et al. 2000). Among all the aforementioned methods, photocatalysis is a highly effective and cheap process than the other methods. Photocatalysis is one of the potential techniques to either oxidize or reduce hazardous pollutants. In the present investigation, photocatalytic degradation method is used for the removal of chromium (VI) from industrial wastewater using TiO2, ZnO, and CdS as photocatalysts. In addition, a comparative study using nanosized TiO2 powder and Solgel thin film of TiO2 was also studied under UV irradiation. Besides the abovedetailed effect of pH, contact time, photocatalyst dosage, and kinetic study were also investigated. The valence band hole is strongly oxidizing, and the conduction band electron is strongly reducing. At the external surface, the excited electron and the hole can take part in redox reactions with adsorbed species such as water, hydroxide ion (OH-), organic compounds, or oxygen. The charges can react directly with adsorbed pollutants, but reactions with water are far more likely since the water molecules are far more populous than contaminant molecules. Oxidation of water or OH- by the hole produces the hydroxyl radical (OH*), an extremely powerful and indiscriminant oxidant (Fig. 1). Materials and methods All the chemicals used in this experiment were all of analytical grade. Accurately weighted potassium chromate treated with diphenyl carbazide (DPC) working as masking agent produces reddish purple colour in acid solution, and was neutralized by addition of NaOH. Nanocrystalline titanium dioxide was prepared via solgel hydrolysis and condensation of ethanol solutions (Merck Chemicals) of titanium isopropoxide (Ti (OC3H7)); diethyl glycol (DEG) was obtained from Merck Chemicals. Deionized water was used throughout the experiment. The absorbance was measured using a UVVis spectrophotometer obtained maximum at kmax = 540 nm. Batch experiments were Fig. 1 Simplified mechanism for the photocatalysis of a semiconductor catalyst carried out with different doses (15 g/L) of with TiO2, ZnO, and CdS. Preparation of nanosized photocatalysts Nanosized semiconductors such as TiO2, ZnO, and CdS are some of the most basic functional materials. The anatase TiO2 was prepared by using TiCl4 (2030% HCl) solution and ammonia. The pH was adjusted by NH3 between 10 and 12. The TiO2 particles are formed in the beaker, by the hydrolysis of TiCl4. Surface morphology of TiO2 nano particles was analyzed using XRD. Nanosized ZnO semiconductor was prepared using Zinc sulfate and sodium hydroxide solution. During the reaction, ZnO was precipitated and it was annealed in the air at 250 for half an hour. After annealing ZnO nanosized particles are formed. Cadmium sulfide can be prepared by the precipitation from soluble cadmium(II) salts with sulfide ion by washing with H2SO4 and precipitation of CdS is formed. After washing the precipitate, soluble cadmium salts were followed by calcinations at 260 C to convert it in to the hexagonal nanosized particles. These materials are used for photocatalysis for removal of Cr(VI). After adsorption of Cr(VI)ions the size of the photocatalysts particles are increased (Table 1). Preparation of thin film Titanium isopropoxide solution was added slowly to 0.1 M nitric acid solution dropwise under vigorous stirring; during the addition, a white precipitate was formed. The solution was then heated at 70 C for 6 h for peptization. In this way, white nanoparticles of TiO2 colloids were obtained as indicated by the appearance of turbidity. Finally, aqueous polyethylene glycol with molecular weight of 20,000 solution, with 40% by weight of TiO2 was added to get the viscous dispersion of TiO2 colloidal solution. Microscopic glass slides were used as substrates. Prior to coating, the substrates were cleaned thoroughly. First, the substrates were cleaned in water with liquid detergent. Then, they were ultrasonically cleaned in acetone (...truncated)