Sulfite oxidation model

S Nakamura. Initiation of sulfite oxidation by spinach ferredoxin-NADP reductase and ferredoxin system a model experiment of the superoxide anion radical production by metalloflavoproteins. Biochem Biophys Res Commun 41 177-183, 1970. 

Compared with the more common model reaction of CO2 absorption, sulfite oxidation is moderated by the low solubility of O2. It can be used when gas-phase conversion is very fast, as in a stirred tank. The results are typical of ionic solutions, whose properties may differ from those of other process liquids. 

The sulfite oxidation rate in hold tanks of antipollution scrubbers is central to flue gas desulfurization technology. The accurate description of the rate of disappearance of sulfite slurry particles (from the scrubber liquor) bears upon both process selection and economics. This article will describe a mathematical model for a semi-batch, stirred tank reactor in which S(IV) anions, sulfite and bisulfite, are reacted with dissolved oxygen gas at saturation. Experimental work to secure several physical parameters and to verify the... 

The current model is a step closer toward a reliable working description of the sulfite oxidation rate in scrubber slurries. By incorporating a boundary layer description of the film around each particle, this model predicts the conditions at the particle surface which drive the mass transfer. The interfacial area for mass transfer was discovered to be more closely represented by a sphere than by a plate-like shape. From the model, using highly catalyzed experiments, a mass transfer coefficient of 0.015 cm sec-1 was found - quite close to literature correlation predictions. 

Table VI summarizes the results of experiments with adipic, sulfosuccinic, sulfopropionic, succinic, glycolic and hydroxy-propionic acids. The results are reported as the sulfite oxidation rate (M/hr) and the degradation rate constant, kd(M ). Analytical accuracy generally allowed determination of kd within -0.1 M l. In experiments with little or no sulfite oxidation, (All, HP3, HP5, HA1, HA3, HA4), there was no measurable degradation of the carboxylic acid. Experiments with 5, 10, and 20 mM adipic acid (A21, A14, A18, A15, A23) showed that the degradation reaction is first order in the carboxylic acid concentration, as expected and modelled by kd-...

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

The oxo-transfer chemistry of molybdenum in sulfite oxidase is probably the best characterized, in terms of synthetic models, structural and mechanistic data, of all the elements we have described up till now. The reaction cycle (Figure 17.5) involves binding of sulfite to the oxidized MoVI, two-electron reduction of the Mo centre and release of sulfate. The Movl centre is restored by successive one-electron transfers from a cytochrome (bs in mammals). The primary oxo-transfer reaction ... 

Holm and coworkers have very recently described advanced structural models for the Mo(VI) and Mo(V) states of sulfite oxidase. Thus, the reaction sequence shown in Scheme 8 yields dioxo mono(difhiolene) complexes bearing either silyloxo or thiolate ligands. The thiolate complex, [Mo02(SC6H2 Pr3-2,4,6)(bdt)]-, exhibits a square-pyramidal (pseudotetrahedral) stracture closely related to that of oxidized sulfite oxidase (Figure 11). 

In sulfite oxidase and related enzymes, transfer of the oxygen atom from Mo to substrate and its replacement from H2O both appear to occur in the Mo(VI) to Mo(IV) transformation regeneration of the Mo(VI) state is achieved via two one-electron oxidations which are mediated by the internal electron transfer chain and which generate transient Mo(V) states (Scheme 3) (7, 233, 234). A number of model catalytic systems have been reported in which O2 and/or H2O are key participants, but the role of water has not been clarified (196,198,201, 202, 235, 236). 

With the exception of the recently reported DMSO reductases from bacteria (71,72), all of the enzymes of Table I contain additional redox active prosthetic groups besides Mo-co. Substrate oxidation (reduction) occurs at the molybdenum center, and electrons are removed (added) via one of the other prosthetic groups. These two processes are coupled by intramolecular electron transfer between the molybdenum center and the other redox centers of the enzyme. Results for xanthine oxidase and sulfite oxidase and approaches to modeling the coupling in sulfite oxidase are summarized below. 

Thus the oxidation of aqueous Na2S03 solutions with C0SO4 as a catalyst proves to be a convenient model reaction for determining interfacial area in gas-liquid reactors. The kinetics of the reaction is not simple many variables influence the reaction rate but, provided the range of cobalt and sulfite concentrations, pH values, and temperatures previously indicated is satisfied, the reaction is zero-order in sulfite, first-order in cobalt, and second-order in O2. The specific rate of absorption is... 

The two pathways that are used to model S2Oa2 formation cannot out-compete the reaction of H2S2 with 02. This fact results in an underprediction of [S2032-], as can be seen in Figure 11. Sulfite, on the other hand, is modeled rather well. This situation can also be seen in Figure 12, where the profiles of [S(—II)] and its oxidation products are presented for pH 10.6. Nevertheless,... 

V.Gaspar and K.Showalter, A Simple Model for the Oscillatory lodate Oxidation of sulfite and Ferrocyanide, The Journal of Physical Chemistry, 94, 4973-4979(1990). 

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