Adsorption and photocatalytic degradation of cysteine in presence of TiO2

Amino-acids are elemental constituents of microorganisms, e.g. bacteria, fungi, virus, mold, among others and are at the origin of the formation of odorous compounds after chlorination treatment due to the filtration of these microorganisms. Our study aims at studying the photocatalytic degradation of cysteine chosen as model of amino acid for understanding better the photocatalytic chemical pathway of microorganisms.The kinetics of adsorption in the dark and under UV light, the kinetics of photocatalytic oxidation, the effect of pH and the main initial pathways of cysteine degradation have been investigated in aerated TiO2 Degussa P-25 aqueous suspensions illuminated at λ > 340 nm. Langmuir and Langmuir–Hinshelwood kinetics were observed respectively in the dark and under UV light. The coverage of TiO2 surface under UV revealed a coverage of TiO2 equivalent to that in the dark, indicating no modification of adsorption sites under irradiation. This behaviour is in agreement with a geometrical adsorption of cysteine as proved by using the molecular surface of cysteine and by taking into account the accessibility of water molecules.The effect of pH on adsorption and photocatalytic degradation showed a good correlation between dark adsorption and photocatalytic degradation.The mineralisation of organic carbon and nitrogen shows the presence of traces of organics hardly mineralisable but also the formation of similar amount of NH4+ and NO3− ions from the beginning of the reaction contrarily to what was observed for other amino-acids. This behaviour is explained by the different ionisation states of cysteine. Taking into account the evolution of Total Organic Carbon, sulphate, nitrate and ammonium, an initial reaction pathway has been proposed.

A good correlation is observed between coverage rate determined by Langmuir model and the number of cysteine adsorbed considering the accessible solvent (water) area.Figure optionsDownload as PowerPoint slideHighlights
► Correlation between geometrical cysteine and Langmuir coverage rate.
► Initial disappearance of cysteine correlated with the dark adsorption.
► Similar NH4+ and NO3− amount formed due to H-bond between un-ionized SH and uncharged NH2.
► Three initial main reactions: decarboxylation; sulphur atom removal; deazotation.