Abstract
The oxygen-18 enrichment in sulfates formed at high temperatures (475–500°C) by platinum-catalyzed air oxidation of SO 2 to SO 3 in humidified air, was found to be several parts per thousand higher than in the air oxygen, SO 2, or water vapor from which the sulfates were formed. Variation of the δ 18O in the sulfates showed little dependence on variation of the δ 18O in the water vapor. The mechanism of sulfate formation involved isotopic exchange between the air oxygen and water vapor, isotopic exchange between the water vapor and SO 2, and formation of the hydrate, H 2SO 4· 3H 2O. When Fe 2O 3 or V 2O 5 was heated in mixtures of air, water vapor, and SO 2, some of the SO 2 was analytically oxidized (via SO 3 formation) to sulfate of relatively high δ 18O and the remainder to chemisorbed sulfate of relatively low δ 18O. Charcoal and fly ash (containing unburned carbon and basic oxides) reacted with the SO 2 to yield chemisorbed sulfates of relatively low δ 18O.
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