Multistep Removal System of Gaseous Toluene: Adsorption, Electrochemical, and Photolytic Treatments

A large amount of atmospheric emissions result from various anthropogenic activities worldwide. Given the complexity of volatile organic compounds (VOCs) and their different adsorption capacities, redox potentials, and photolytic properties, an air purification system for the removal of VOCs that combines multiple physical processes was proposed in this study using toluene as an example. These processes include, in the first step, an adsorption treatment (AT) with activated carbon (AC), where toluene adsorption results from the insertion of aromatic rings (nonpolar groups) between the graphitic carbon planes, as demonstrated by the Raman spectroscopy; in the second step, electrochemical treatment (ECT) using TiO_2,nt|Ti||SS-304 electrodes applying an electric field to accelerate the oxidation of toluene through the production of free radicals (⋅OH), hydroperoxyl radicals and benzyl groups, followed by the rupture of aromatic rings to generate aliphatic compounds and the consequent mineralization to CO, CO₂, and H₂O; in the third step, photolytic treatment (PT) with a 254-nm UV lamp for toluene degradation is used, which is influenced by the addition of radicals, such as ⋅OH or ⋅O^- _2ad, to transform toluene into either benzene or phenol. The multistep system integrating AT, ECT and PT was more efficient overall (99.58 %) than the individual treatments (AT=50.29 %, ECT=44.38 %, and PT=52.71 %) as evaluated by gas chromatography with a BID detector; it showed enhanced efficiency enabled by the synergistic effects of combining multiple technologies to enhance the overall toluene degradation efficiency and flexibility. The multistage systems can be adapted to specific contamination conditions and process requirements with the generation of residual toluene, phenol, and aliphatic compounds and possible mineralization to molecules such as CO₂, CO, and H₂O. This small and portable multistep system provides a novel approach for treating outdoor and/or indoor air contaminated with toluene.