Kinetic of adsorption and of photocatalytic degradation of phenylalanine effect of pH and light intensity

Phenylalanine (Phe) was chosen to study the TiO2 photocatalytic degradation of amino acids, which are at the origin of the formation of odorous compounds after chlorination. The photocatalytic degradation has been investigated in aqueous solutions containing TiO2 suspensions as photocatalyst, in order to assess the influence of various parameters, such as adsorption, initial concentration, pH and radiant flux on the photocatalytic process. Results showed no correlation between dark adsorption and photocatalytic degradation. A multilayer kinetic was observed in the dark with a monolayer corresponding to less that 1% of OH covered, whereas Langmuir–Hinshelwood model seems to modelize the photocatalytic disappearance of Phe. However, even if the form of the curve is similar to L–H model, the degradation of phenylalanine is not a kinetic of L–H as we could plan it by considering the adsorption of the phenylalanine in the dark. The study of the mineralization of carbon and nitrogen showed that nitrogen atoms were predominantly photoconverted into NH4+ and a total mineralization of nitrogen and carbon seems occur. The identification of the by-products by LC–MS reveal mono- and di-hydroxylation and nitrogen–carbon (N–C) cleavage.The effect of pH showed an increase of adsorption under acid pH but a decrease of disappearance rate. The more efficient degradation was found at basic pH. The evolution of hydroxylated compounds of phenylalanine as a function of conversion revealed the presence of more hydroxylated compounds at natural pH and at basic pH compared to acid pH suggesting a modification of mechanism with solution pH. The effect of the radiant flux evaluated under different initial concentration of phenylalanine allowed us to determine that k increases by increasing the radiant flux, whereas K decreases or remains constant from about a value of 3.5 mW/cm2. The disappearance rate as a function of radiant flux has been showed to reach a maximal value corresponding to a maximal quantum yield of 1.6%.

Amino acids, simpler molecules constituting microorganisms, are found behind filtration process and are at the origin of odorous compounds after chlorination. Our objective is to contribute to a better understanding of the efficiency of photocatalytic treatment for removing them by studying their kinetics of adsorption on TiO2 and of photocatalytic degradation under different pH and light intensities and the intermediate products. TOC disappeared (a) and evolution of nitrate and ammonium (b) as a function of irradiation time.Figure optionsDownload high-quality image (36 K)Download as PowerPoint slide