Fast adsorptive and photocatalytic purification of air from acetone and dimethyl methylphosphonate by TiO2 aerosol

A high concentration (1.5 x 10(6) cm(-3)) TiO2 aerosol of the average particle size 0.5 mu m was generated by a sonic method inside 0.1 m(3) Plexiglas chamber and applied for the adsorptive and adsorptive-photocatalytic Purification of air from vapors of acetone and chemical agents' model dimethyl methylphosphonate (DMMP). The adsorptive capture of acetone over the TiO2 aerosol results in establishing equilibrium adsorption state and is limited by the rate of the aerosol admission into the chamber. A model derived from the Langmuir isotherm describes well the acetone concentration vs. aerosol mass Curve and allows obtaining the adsorption constant and monolayer coverage of acetone in a 10 min experiment. The UV irradiation of TiO2 aerosol accelerates dramatically the purification from acetone at the high relative humidity (RH)of the air. Increased RH of air decreases the rate of the acetone adsorption but has a little positive effect on the rate of photocatalytic oxidation of acetone over aerosol particles. The DMMP adsorption over TiO2 aerosol is accompanied by the immediate (tau < 10 s) and irreversible hydrolysis of DMMP with the formation of gas phase methanol and adsorbed methyl methylphosphonic acid. The irreversible reactive adsorption results in the very fast air purification (tau = 20-40 s)due to very small diffusion distances of substrate to the TiO2 surface in aerosol. The increase of the air RH from 4 to 37% (296 K) decreases the rate of adsorption but accelerates significantly the rate of photocatalytic oxidation. The complete air purification from organic Compounds within 10 min is possible only with the photocatalytic oxidation because the adsorption alone does not remove methanol. The time needed for the air purification over the nanosized TiO2 aerosol is directly determined by the rate of the aerosol generation which allows a further optimization of the TiO2 aerosol air purification. The obtained results approve experimentally a suggestion that the photocatalytic oxidation over solid atmospheric aerosols actually takes part in the Earth atmosphere and serves as an important sink for airborne organics. (C) 2009 Elsevier B.V. All rights reserved.