Filtration of ozone by carbonaceous materials: Removal efficiencies and reactions with volatile organic compounds.

Abstract

Activated carbon (AC) and other materials have been used widely in air filtration applications, including occupational respirators, room air and ventilation system filters, and vapor recovery systems. While carbonaceous materials are known to remove gases and vapors including ozone (O₃) and volatile organic compounds (VOCs), the removal of O₃ at environmental concentrations has not been well characterized. This research investigated whether removal efficiencies at low O₃ concentrations could be predicted using tests conducted at high O₃ concentrations, whether VOC loadings on filters affected O₃ removal efficiency, and whether reactions between O₃ and VOCs occurred heterogeneously on AC and produced emission products. Additionally, removal of O₃ by diesel particulate matter (DPM), a significant contributor to the carbon aerosol in urban atmospheres, was evaluated. AC and DPM samples were exposed to O₃ in a flow-through experimental system, and filter effluents were monitored for O₃, VOCs and other parameters. For virgin AC filters, O₃ removal rates varied by O₃ concentration, bed mass and exposure time, and O3 removal capacities ranged from 2 to 43 wt % at 65 hr. Of four chemisorption models tested, the Elovich equation provided the best fit to breakthrough data. VOC-loaded filters differed in breakthrough behavior and were 5 to 25% less efficient at removing O₃ than unloaded filters, attributed to poisoning by toluene and pseudo-poisoning by <italic>d</italic>-limonene. Gas- and condensed-phase compounds formed in homogeneous reactions between O₃ and <italic>d</italic>-limonene included formic acid (molar yield of 37%), 4-acetyl-1-methylcyclohexene (4%), propanoic acid (3%) and limonene oxides (3%). Heterogeneous experiments using limonene-loaded AC filters showed formation of 4-acetyl-1-methylcyclohexene and limonene oxides, but only small quantities were emitted in filter effluent, and the bulk of the limonene remained unreacted on the AC even after exposure to a stoichiometric excess of O₃. DPM-loaded filters removed 6 +/- 2 wt % of O₃, much less than that found for AC, and DPM is expected to remove only a small fraction of O₃ from urban/tropospheric or indoor air. Overall, this research provides quantitative estimates of O₃ removal by AC and DPM, information that can be used in the design and evaluation of air cleaning systems.