The influence of diesel—truck exhaust particles on the kinetics of the atmospheric oxidation of dissolved sulfur dioxide by oxygen

Abstract

The automobile exhausts are one of the major sources of particulate matter in urban areas and these particles are known to influence the atmospheric chemistry in a variety of ways. Because of this, the oxidation of dissolved sulfur dioxide by oxygen was studied in aqueous suspensions of particulates, obtained by scraping the particles deposited inside a diesel truck exhaust pipe (DEP). A variation in pH showed the rate to increase with increase in pH from 5.22 to about ∼6.3 and to decrease thereafter becoming very slow at pH = 8.2. In acetate-buffered medium, the reaction rate was higher than the rate in unbuffered medium at the same pH. Further, the rate was found to be higher in suspension than in the leachate under otherwise identical conditions. And, the reaction rate in the blank reaction was the slowest. This appears to be due to catalysis by leached metal ions in leachate and due to catalysis by leached metal ions and particulate surface both in suspensions. The kinetics of dissolved SO2 oxidation in acetate-buffered medium as well as in unbuffered medium at pH = 5.22 were defined by rate law: k obs = k 0 + k cat [DEP], where k obs and k 0 are observed rate constants in the presence and the absence of DEP and k cat is the rate constant for DEP-catalyzed pathway. At pH = 8.2, the reaction rate was strongly inhibited by DEP in buffered and unbuffered media. Results suggest that the DEP would have an inhibiting effect in those areas where rainwater pH is 7 or more. These results at high pH are of particular significance to the Indian subcontinent, because of high rainwater pH. Conversely, it indicates the DEP to retard the oxidation of dissolved SO2 and control rainwater acidification.

Inhibition of Aquated Sulfur Dioxide Autoxidation by Aliphatic, Acyclic, Aromatic, and Heterocyclic Volatile Organic Compounds

“…In the absence of any added buffer, the reaction mixture had a pH of ∼8.2. Since the uncatalyzed reaction has been shown to be very slow at low pH , we chose to study the kinetics at pH∼ 8.2 at which the reaction rate is measurable. Since the uncatalyzed reaction has been shown to be independent of [H + ] at pH ≥ 7, the experimental kinetics rate law determined at pH 8.2 shall not include a [H + ] dependence term and it would remain simpler.…”

“…The experimental procedure was same as described previously by Brodzinsky et al and subsequently used by us , and is briefly given here. The reactions were conducted in 0.15 L Erlenmeyer flasks, open to air, to allow the passage of atmospheric oxygen.…”

“…The kinetics was followed by withdrawing the aliquot samples periodically and titrating the unreacted sulfur(IV) iodometrically in slightly acidic medium using sodium thiosulfate as a titrant and starch as an indicator as described earlier . The reproducibility of the replicate measurements was generally ∼10%.…”

“…may at best qualitatively indicate the extent of inhibition, when pH ≥ 7. where k obs and k 0 are the first‐order rate constant for the oxidation of sulfur(IV) in the presence and absence of i th inhibitor, respectively, and B i is the inhibition parameter and [Inh] i is the concentration of i th VOC. It has been recently shown that the uncatalyzed sulfur(IV) autoxidation is independent of acidity at pH ≥ 7 , Hence, it is quite possible that B values determined at pH ≥ 7 for VOCs, which do not undergo change in speciation with change in pH, Eq. might be more useful in estimating extent of inhibition.…”

Co2O3 Catalyzed Oxidation of SO2 in Aqueous Solution Differing Effect of Benzamide in Alkaline Medium

“…The S(IV) autoxidation reaction is known to proceed via both radical and non-radical mechanisms [16][17][18] . An interesting feature of many radical reactions is that, the reaction rate is inhibited by organics such as acetic acid, oxalic acid 19 , alcohols 20 , carboxylic acid 21 , formic acid, isopropyl alcohol, isoamyl alcohol, aniline, benzamide, sodium benzoate [22][23][24][25][26][27] , ascorbic acid 28 , organic compounds 29 , VOC 30 and diesel truck particles 31 .…”