Oxidation of nauseous sulfur compounds by photocatalysis or photosensitization

Reduced sulfur compounds such as methanethiol (MSH), dimethylsulfide (DMS) and dimethydisulfide (DMDS) are nauseous by-products produced by a great number of industrial processes. Oxidation of these reduced sulfur compounds in polluted atmospheres and hence the decrease of their harmful and malodorous effects is thus a matter of concern in numerous industrial and water treatment plants. Photocatalytic treatment of gaseous flow polluted by these sulfur compounds has been actively investigated for the last few years. The first part of the paper is devoted to a literature review on the different TiO2-based photocatalytic processes designed for the oxidation of these gaseous compounds. The comparison of their efficiency is done according to the process parameters: batch or flow reactors, photocatalytic materials, residence time, gas flow, pollutant nature and concentration, relative humidity, …. Special attention is paid to the poisoning of the photocatalytic material and to its possible recycling. In the second part of the paper, alternative materials based on aromatic photosensitizers (9,10-dicyanoanthracene, 9,10-anthraquinone) deposited or grafted on silica matrices are then presented and their efficiency compared to more conventional TiO2-based materials. It is demonstrated that the oxidation products are totally different from those obtained with TiO2. With the photosensitizing materials, singlet oxygen addition is shown to be the major pathway, leading to sulfoxide and sulfone starting from DMS and to methyl methanethiosulfonate starting from DMDS. With TiO2-based materials, in the absence of water and hence of hydroxyl radicals, products arising of Csingle bondS and Ssingle bondS bond cleavage are mainly obtained: disulfide from DMS and CH3SSSCH3 together with CH3SCH2SSCH3 from DMDS. These latter products may be accounted for by electron transfer from sulfide or disulfide to photogenerated holes, leading to radical mechanisms. Mineralization to CO2 and H2O is also shown to occur with DMS, but is not favoured under these conditions, due to the absence of water and of a too fast gas hourly space velocity (GHSV).