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Heterogeneous reaction kinetics of sulphur dioxide and airborne limestone particles

University of British Columbia

The literature on acid rain provides evidence that large (>5 μm) alkaline particles in the atmosphere, which derived from surface soil, play an important role in mitigating the effect of acid rain. Not only do they help to neutralize acidity but they also are a source of nutrients to the terrestrial and aquatic environments. While ground-up limestone has been added to the forests and the lakes of regions deficient in alkalinity, this mode of application is prohibitively expensive. An economic alternative method of distribution would utilize the advective and turbulent diffusive processes within the troposhere in order to supply limestone particles of approximately 10 μm diameter to the terrestrial and the aquatic environments. However, while the particles are being transported through the atmosphere they will also interact to some degree with the atmospheric pollutants (SO₂, NOx, photochemical oxidants, etc.). While the two-phase chemical interaction of water droplets and gaseous pollutants has been extensively studied, little is known about the reaction between limestone particles and gaseous pollutants under ambient conditions. As a first step in understanding these processes laboratory experiments were conducted in order to measure the rate of the heterogeneous reaction between limestone particles and sulphur dioxide (SO₂) gas in clean humid air. Calcitic limestone (200-270 mesh Tyler) from Texada Island, B.C., was reacted with 30-80 ppb SO₂ at room temperature (17-22°C) over a humidity range of 70-100%. Ancillary experiments were also conducted to determine the maximum dissolution rate of these limestone particles in an acidic, aqueous environment. The overall rate (r) for the SO₂ reaction with Texada Island limestone was determined to be approximately first-order with respect to the SO₂ concentration and to be strongly dependent upon the relative humidity: [Formula Omitted] Limited experiments with precipitated dolomite indicated that it reacts with SO₂ somewhat faster than does the calcitic limestone. A study of the individual mass transfer and reaction processes indicated that the rate limiting step for the overall reaction was the aqueous phase oxidation of the bisulphite ion. Limestone dissolution (determined experimentally), and estimated gas and aqueous phase diffusive processes, were not rate limiting. In a clean humid atmosphere, free of photochemical oxidants, a limestone aerosol would react very slowly with SO₂ (SO₂ removal rate of about 3x10⁻³% per hour). However, in a humid polluted atmosphere rich in photochemical oxidants, similar to that responsible for acid fog and acid rain, the aqueous phase oxidation of dissolved SO₂ is not expected to be rate limiting. Under such conditions bisulphite dissociation: [Formula Omitted] Therefore in humid, polluted atmospheres limestone aerosol will act as a "sink." for SO₂ for soluble photochemical oxidants, and probably also for HNO₃ (nitric acid). Since the oxidants are considered to be phytotoxic while the other pollutants are responsible for creating acid rain we conclude that the deployment of a limestone aerosol may have a positive impact on the atmospheric environment, besides being beneficial to the aquatic and terrestrial environments. It is strongly recommended that further research be done in this area in order to better quantify the rate processes, and perhaps also to identify practical methods for increasing the atmospheric concentration of alkaline aerosol in those geographic regions which are deficient.

Text

2010-09-09

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http://hdl.handle.net/2429/28379

Chemical and Biological Engineering

University of British Columbia

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