The rate constants for a step inhibitor + sulphate radical â organic radical + ... The rate constant for the isoprene + sulphate radical step was of the order of 108 ...
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Transformations of Atmospheric Constituents and Pollutants Induced by S(IV) Autoxidation — Chemistry and Kinetics A contribution to subproject CMD/APP
Wanda Pasiuk-Bronikowska, Tadeusz Bronikowski, Krzysztof J. Rudzinski, and Józef Ziajka Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka str. 44/52, PL 01-224 Warsaw, Poland
Summary Laboratory studies on the inhibition of S(IV) autoxidation by organic compounds representing abundant components of the atmosphere were continued. The selected objects were: cis-verbenol, myrtenol, sobrerol, phenol, catechol, gallic acid, caffeic acid and isoprene. These, when at trace amounts, occurred efficient regular inhibitors primarily scavenging sulphate radicals. At higher concentrations the unstable products of their transformations imprinted on the kinetics of S(IV) autoxidation, accelerating chain initiation and generating rate oscillations. Such oscillations were disclosed in prolonged experiments with gallic acid, similarly as in the case of sobrerol reported previously. The double face of phenol was shown, the pollutant behaving as an inhibitor, but also as an additional initiator (positive synergism). The rate constants for a step inhibitor + sulphate radical → organic radical + sulphate anion, derived from quasi-stationary experiments, were of the order of 108 M-1s-1 for myrtenol and sobrerol, 109 M-1s-1 for catechol and gallic acid, and 1010 M-1s-1 for caffeic acid, the latter value evidently diffusion limited. At a sufficient amount of Fe(II) the autoxidation of S(IV) was terminated by the reaction of peroxymonosulphate radicals with Fe(II). Kinetics of non-stationary autoxidation of S(IV), both uninhibited and inhibited by isoprene, was described using the composite rate equations, as well as a first-guess chemical mechanism. The kinetics differed upon the autoxidation initiator used, K2S2O8 or Mn(III). The rate constant for the isoprene + sulphate radical step was of the order of 108 M-1s-1. Aims of the research Studies of the influence of selected organic substances (inhibitors) on the formation of acidity by the autoxidation of aqueous sulphur dioxide were continued. The emphasis was on quantification of the inhibiting effect of hydroxylated organic compounds of a ring structure, monoterpenic and aromatic. Moreover, isoprene inhibition was studied to compare it with that by the compounds with an OH group in the vicinity of a double bond. The aim not less important was an attempt to determine the appropriate rate constants for steps involving the inhibitors from the chemical-mechanistic model of the overall chain reaction leading to S(VI). Activities during the year Laboratory experiments were performed using various techniques to follow the extent of S(IV) autoxidation in the presence of an inhibitor: conductometric measurement of the concentration of S(VI) produced in a heterogeneous reactor with a planar gas-liquid interface and well-mixed bulk of CaSO3.1/2H2O suspension, and measurement of the concentration of consumed oxygen with the Clark-type electrode in perfectly mixed homogeneous reactors of different design. Chemical-kinetic modelling of the inhibited reaction was carried on by applying the general treatment of chain reactions modified accordingly. Specific rate constants were determined from the models and compared with the data available in literature. Six contributions presented at the EUROTRAC-2 and EUROTRAC-2/CMD conferences should also be mentioned.
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3.0 ∆[S(VI)] x104 , M
myrtenol
4
[O2] x10 , M
2.0
1.0 caffeic acid
0.0
ol er r b so
2.0
ol ech cat
1.0
0
0
300
600
900
0
time, s Fig. 1. Typical results for the homogeneous reactor (S(IV) autoxidation inhibited by myrtenol 4.6x10-5 M at pH 3, S(IV) 2,0x10-3 M and Fe 1x10-5 M, and by caffeic acid 5.0x10-7 M at pH 5, S(IV) 1.5x10-3 M and Mn 3x10-5 M).
10
20 30 time, min
40
Fig. 2. Typical results for the heterogeneous reactor (S(IV) autoxidation inhibited by sobrerol 4.0x10-6 M and by catechol 1.2x10-7 M, in both cases at pH 7 and S(IV) 5.6x10-4 M. No catalyst was added.
Principal results Inhibition of S(IV) autoxidation under quasi-stationary conditions. These conditions were fulfilled in two ways: the rate of oxygen consumption due to S(IV) autoxidation was adjusted so to ensure the negligible change of the initial concentration of S(IV) in short experiments (Pasiuk-Bronikowska et al., 2001, Ziajka and Pasiuk-Bronikowska, 2000; see also Fig. 1) or the concentration of S(IV) was maintained constant due to the dissolution equilibrium of CaSO3.1/2H2O in long runs (Pasiuk-Bronikowska et al., 2001 and 2000; see also Fig. 2). In all experiments the overall reaction studied was zero order with respect to oxygen. Influence of the inhibitor concentration on the rate of this reaction is shown in Fig. 3 and Fig. 4.
myrtenol
cis -verbenol CH3
sobrerol
phenol
CH3
OH
CH2OH HO OH H3C COOH
OH
HO
CH3 OH HC
OH
OH
OH
catechol
gallic acid
COOH
C H
OH
H2C
CH3 C H
CH2
OH caffeic acid
isoprene
To quantify the inhibiting ability of particular organic pollutants we applied a general steady-state treatment of chain reactions, which required the rate of initiation was equal to the rate of termination. As the rate of initiation was not affected by an inhibitor, and the rate of termination increased due to SO4•─ scavenging by this inhibitor, it was possible to derive the relatively simple rate equation for the overall reaction of the inhibited autoxidation of S(IV). Two main cases were distinguished:
(i) Straight line 1/rS(VI) versus [ I ] plots were obtained from the experimental data. In terms of the chain reaction mechanism it means that the termination via the self-reaction of SO5•─ is replaced by the termination via scavenging of SO4•─ by an organic inhibitor I. (ii) Straight line rS(VI) versus [ 1/I ] plots were obtained from the experimental data. Mechanistically it means two comparably fast terminating steps coexist, one involving SO5•─ scavenged by Fe(II) and another one involving SO4•─ scavenged by an organic inhibitor I.
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1000
9.0 6.0
2
rO x 107, Ms-1
rS(VI) x 10 8, M/s
verbenol myrtenol
12.0
3.0 0.0
gallic acid catechol
100
10
1
0.0
1.0
2.0
3.0
10
20
30
40
[inhibitor] x107, M
[ inhibitor ] x 104, M Fig. 3. Rate of oxygen consumption in the autoxidation of S(IV) inhibited by cis- verbenol and myrtenol at varied concentrations (S(IV) 2.0×10–3 M, pH = 3.0).
Rate constants calculated from slopes of such plots are given in Table 1. We also applied this approach to determine rate constants for scavenging sulphoxy radicals by selenium species (Bronikowski et al., 2000). We were not able, as yet, to derive the rate constant for phenol, due to a strong synergic effect observed in the case of this inhibitor (Pasiuk -Bronikowska et al., 2000). This was attributed to the formation of phenoxyl radicals acting as one-electron oxidants of S(IV).
0
Fig. 4. Rate of S(VI) formation in the autoxidation of S(IV) inhibited by gallic acid and catechol at varied concentrations (S(IV) 5.6×10–4 M, pH = 7.3).
Table 1. Rate constants determined from the quasi-stationary experiments for the reaction: scavenger + SO4•─ (23-25oC) Scavenger Rate constant Comments cis-verbenol myrtenol sobrerol catechol gallic acid caffeic acid
1.3x109 1/(Ms) 8.6x108 1/(Ms) 3.7x108 1/(Ms) 3.8x108 1/(Ms) 2.0x108 1/(Ms) 4.3x109 1/(Ms) 2.9x109 1/(Ms) 4.7x1010 1/(Ms)
Fe catalyst, pH 3 Fe catalyst, pH 3 no catalyst, pH 7 Fe catalyst, pH 7 Co catalyst, pH 7 no catalyst, pH 7.5 no catalyst, pH 7.5 Mn catalyst, pH 5
Inhibition of S(IV) autoxidation under non-stationary conditions. The kinetics of inhibited non-stationary autoxidation of S(IV) was studied for isoprene inhibitor and K2S2O8 or Mn(III) initiator. Results were approximated with overall rate equations, first or zero order with respect to O2 (or S(IV), by stoichiometry) for K2S2O8 or Mn(III) initiator, respectively. Both composite rate constants were inversely proportional to the initial concentration of isoprene (Rudzinski and Pasiuk-Bronikowska, 2001; Rudzinski et al., 2000). A two-step mechanism of inhibition (addition of SO4•─ to isoprene and to the so formed radical) was suggested. For the first step the rate constant of 1.25x108 M-1s-1 was estimated by modelling (Rudzinski et al., 2000, see Fig.5). The non-stationary behaviour was also observed for gallic acid in long experiments, initially quasi-stationary (Fig. 6). The oscillations, similar to those found previously for sobrerol, were due to the overlapping effects of the products of inhibitor transformation, including organic initiators like one formed from α-pinene (Ziajka and Pasiuk-Bronikowska, 2000). Main conclusions The inhibiting effect of the compounds studied in this work implies that: terpenoids (including isoprene) and their derivatives may be responsible for the long-distance transport of unreacted S(IV), particularly at night-time; phenolic compounds, if being an independent
CMD Annual Report ‘00
K2S2O8 initiator Mn(III) initiator model simulation
2.0
inhibition induced oscillations
8
[O2]/[O2]o
1.0
rS(VI) x10 , M/s
126
0
0.5
0.02
0.0235 mM isoprene
1.0
0.0
0.0 0
2000
4000
time, s Fig. 5. Inhibition of S(IV) autoxidation by isoprene at 24.9 oC and pH = 8.5÷8.9. Initial concentrations: Na2SO3 – 1.02 mM, O2 – 0.22÷0.24 mM, K2S2O8 – 0.1mM or MnSO4 – 0.01mM, isoprene – on the plot.
100 200 300 400 500 time, min
Fig. 6. Oscillations of the rate of S(IV) formation in the autoxidaton of S(IV) inhibited by gallic acid (2.1x10-3 M, initially) at constant concentration of S(IV) 5.6x10-4 M and pH 7.
source of phenoxyl radicals, are capable to initiate the S(IV) autoxidation chain; the primary inhibitors transform into new ones, even more efficient; rate oscillations in S(IV) autoxidation were shown to be a more general phenomenon. Aims for the next year Studies of the inhibition of S(IV) autoxidation will be continued to widen the experimental basis for chemical-kinetic modelling, in particular including further work with isoprene and with next terpenoids. Moreover, the kinetics of S(IV) autoxidation in high-salt media and at low temperatures will be investigated, as related to evaporating clouds. Acknowledgements This work was carried out as the Polish contribution to the project EUROTRAC-2, supported by the State Committee for Scientific Research in Poland. References Bronikowski T., W. Pasiuk-Bronikowska, M. Ulejczyk, Inhibition of S(IV) autoxidation by selenium, Works & Studies of the Institute of Environmental Engineering PAS, no.54 (2000), 28-34. Pasiuk-Bronikowska W., T. Bronikowski and M. Ulejczyk; Study of the kinetics of S(IV) autoxidation inhibited by phenolics, in: M.J. Rossi et al. (eds.), Proc. EC/EUROTRAC-2 Joint Workshop 2000, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne-Ecublens (2000) - in press. Pasiuk-Bronikowska W., M. Krajewska, T. Bronikowski and M. Ulejczyk; Interaction between secondary organic compounds and sulphoxy radicals, in: P.M. Midgley, M. Reuther, M. Williams (eds), Transport and transformations of pollutants in the troposphere, Proc. EUROTRAC-2 Symposium 2000, Springer Verlag Berlin, Heidelberg (2001) - in press. Rudzinski K.J., W. Pasiuk-Bronikowska, Isoprene inhibition of S(IV) autoxidation initiated by peroxydisulphate, in: P.M. Midgley, M. Reuther, M. Williams (eds), Transport and transformations of pollutants in the troposphere, Proc. EUROTRAC-2 Symposium 2000, Springer Verlag Berlin, Heidelberg (2001) - in press. Rudzinski K.J., W. Pasiuk-Bronikowska, J. Krolik, Mechanistic study of isoprene inhibition of S(IV) autoxidation in aqueous phase, in: M.J. Rossi et al. (eds.), Proc. EC/EUROTRAC-2 Joint Workshop 2000, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne-Ecublens (2000) - in press. Ziajka, J., W. Pasiuk-Bronikowska; Myrtenol as an inhibitor of S(IV) autoxidation, in: M.J. Rossi et al. (eds.), Proc. EC/EUROTRAC-2 Joint Workshop 2000, Ecole Polytechnique Federale de Lausanne (EPFL), LausanneEcublens (2000) - in press. Ziajka, J., W. Pasiuk-Bronikowska; α-Pinene radicals as the side initiator of S(IV) autoxidation, in: P.M. Midgley, M. Reuther, M. Williams (eds), Transport and transformations of pollutants in the troposphere, Proc. EUROTRAC-2 Symposium 2000, Springer Verlag Berlin, Heidelberg (2001) - in press.
