Study on Photocatalytic Degradation of Nitro and Amino Group Containing Compounds and Their Mixtures, Using UV-Vis Spectroscopy and Multivariate Methods

Abstract

The treatment of water contaminated with traces of toxic organic compounds which are mainly nitro and amino groups containing compounds is a common problem throughout the world. The presence of substantial concentrations of toxic of nitro and amino group’s compounds like nitroaromatic compounds such as nitrobenzene, nitroaniline and nitrophenol in industrial effluents is inhibitory to several anaerobic biodegradative reactions which may result in treatment process failure. Unfortunately, once released into the environment many of these compounds persist for long periods of time. In recent years, application of advanced oxidation technologies involving strongly oxidising hydroxyl radical has gained increasing interest for the treatment of industrial waste waters, and contaminated ground and drinking water. In particular, heterogeneous photocatalytic degradation in the presence of a semiconductor catalyst has been shown to be a promising method for the destruction of toxic chemicals. The appeal of this process technology is the prospect of mineralization of the pollutants into environmentally harmless compounds. In recent years, interest has focused on the use of TiO2 as a photocatalyst for the destruction of polluting materials. TiO2 is an important photocatalyst due to its strong oxidizing power, non-toxicity and long-term photostability. Activation of the semiconductor catalyst is achieved through the absorption of a photon of ultraviolet band gap energy resulting in the formation of electron donor (reducing) sites an electron acceptor (oxidising) sites. The carbon containing pollutants are oxidised to carbon dioxide and water, while the other elements bonded to the organic compounds are converted to anions such as nitrate, sulphate or chloride. UV-Vis spectroscopy is widely used to determine the concentration of the compound in photocatalysis. But here in this system of nitro and amino groups containing compounds i.e o-nitroaniline and p-toluidine it failed. It was not possible to determine the concentration of individual compounds in mixture using this analysis. COD analysis gives the overall effect on mineralization. So, mathematical models are an alternative way to resolve mixtures of compounds with strong overlap peaks and have vi been successfully applied in infrared, UV-vis, and fluoresce spectral data. Four multivariate methods like multilinear multivariable (MLV), principal component analysis (PCA) and partial least square (PLS1 and PLS2) was employed in this study to determine the concentration of PT and ONA during the photocatalytic oxidation of their mixture. These methods were develop in MATLAB for datasets and testsets. The photocatalytic degradation of nitro and amino groups containing compounds like o-nitroaniline and p-toluidine binary mixture was studied using P25 Degussa TiO2 in presence of UV light source. Initial concentration of both the compounds was 25 ppm in their mixture and the amount of catalyst was 100 mg which was photocatalytically degraded for 6 h. Multivariate methods such as MLV, PCA and PLS (PLS1 and PLS2) were studied in order to determine the concentration profile for both the compounds in mixture. R2 values and the RMSE values for both the compounds were used for the selection of best model, using 32 datasets and 13 testsets. PLS1 model best fits the requirements and was used to predict the concentration of both the compounds. The effect of intermediates on the concentration of compounds was around ±5%. The kinetic behaviour of o-nitroaniline and p-toluidine mixture by photocatalysis using P25 Degussa TiO2 was determined by developing a PLS1 model. It behaviour pseudo first order kinetics. The degradation rates of both organics were decreased in the binary mixture compared to the individual degradation rates of the organics. The results showed that o-nitroaniline degrades twice faster than p-toluidine in case of their mixture. Now tertiary system was analysed. 25 ppm of Nitrobenzene was added to the above mixture. The photocatalytic degradation conditions were same. All the four methods MLR, PCR, PLS1 and PLS2 were studied. PLS1 model best fit to this system since it showed less RMSE value and R2 value nearer to 1 for both 46 datasets and 10 testsets. Around 10% effect of intermediates was seen in determining the concentration of the compounds in mixture. The system obeyed pseudo-first order kinetics for the concentration determined by PLS1 model. In this case ONA and PT degrade faster than NB. Moreover the degradation rates decreases in mixture for compounds compared to the individual degradation rates. There is a possibility of NB getting formed during the degradation of PT and ONA. vii The results for binary and tertiary system was so because of the competition for the active reaction site on the catalyst since the amount of catalyst was same, attributed to interaction between the mother compounds and interaction between the intermediates. The present work indicates that UV-Vis spectroscopy coupled with PLS calibration can be used to in situ monitor the concentration changes, providing a novel approach to determine the competitive effects of different organic pollutants during water purification and wastewater treatment by photocatalysis. Keywords: Photocatalysis, P25 Degussa TiO2, Degradation, o-nitroaniline, p-toluidine and nitrobenzene mixture, Multivariate methods.