Sulfate formation rate

Model calculations indicate that metal-catalyzed autoxidations may contribute significantly to the overall sulfate formation rate in atmospheric droplets, particularly in the range of Fe and Mn concentrations observed in urban fog (, 23,, 37-39)... 

Sulfate formation by pathways (c)was studied by Penkett (1972) and Penkett and Garland (1974). Their laboratory experiment with bulk water and water drops demonstrated that 03 oxidizes bisulfite ions very rapidly and effectively. These authors found that at 10 C, with an ozone concentration of 0.05 ppm, the sulfate formation rate is given by the following equation ... 

The above discussion is summarized in Fig. 42. In this figure, due to Beilke and Gravenhorst (1978), three curves are represented. The dashed line shows the uncatalyzed sulfate formation rate as a function of pH as reported by Beilke et al. (1975) for 10 °C and for a S02 concentration of 1 ppb. The solid line indicates the... 

Sulfate formation rate in the droplet phase as a function of pH of the liquid for three different S02 oxidation mechanisms (Beilke and Gravenhorst, 1978). Conditions S02 1 ppb 03 40 ppb T= 10 C. (By courtesy of Atmospheric Emironment)... 

The sulfate formation rate d[S(VI)]/dt is simply the sum of the reaction rates that produce sulfate in this system, namely, that of the H202-S(IV) and 03-S(IV) reactions. The value of S(VI) at time f is then used to calculate [HSOJ] and [SO4-] at time f. These concentrations are then substituted into the electroneutrality equation to obtain the new [H+] and the concentrations of other dissolved species. This process is then just repeated over the total time of interest. 

Table 5.26 SO2 oxidation (percentage of pathway ) and sulfate formation rates (in % h ) for central European conditions data from Moller (1995a), Moller and Mauersberger (1992).

Nicklin and Holland, I963A). The effect of increased alkalinity on thiosulfate formation is rather significant above a pH of 8.8. This effect is more pronounced in the reactions of hydrosulfide with dissolved oxygen. For instance, at an oxygen partial pressure of 7 psia in the sour gas, a rise in pH from 8.3 to 8.8 triples the amount of sulfur converted to thiosulfate (from 3 to 9%) (Nicklin and Holland, l%3B). Likewise, increasing the concentration of dissolved solids in the liquor from 7 to 20% (by weight), doubles the sulfate formation rate. 

The temperature dependency of the sulfate reduction rate for single sulfate-reducing bacteria is high, corresponding to a temperature coefficient, a, of about 1.13, i.e., a change in the rate with a factor Q10 = 3.4 per 10°C of temperature increase. Because diffusion of substrate into biofilms or sediments is typically limiting sulfide formation, the temperature coefficient is reduced to about... 

Studying the oxidation of S(IV) by 02 in pure water without traces of catalysts or inhibitors has proven extremely difficult. Based on a compilation of many studies, Radojevic (1984) has recommended that the uncatalyzed rate of oxidation (in terms of the rate of sulfate formation) is given by... 

Relative rates of sulfate reduction and methanogenesis in lakes of varying trophic status are claimed to indicate that sulfate reduction rates are limited by the supply of sulfate (4, 5, 13). According to this hypothesis, at high rates of carbon sedimentation, rates of sulfate reduction are limited by rates of sulfate diffusion into sediments, and methanogenesis exceeds sulfate reduction. In less productive lakes, rates of sulfate diffusion should more nearly equal rates of formation of low-molecular-weight substrates, and sulfate reduction should account for a larger proportion of anaerobic carbon oxidation. Field data do not support this hypothesis (Table II). There is no relationship between trophic status, an index of carbon availability, and rates of anaerobic... 

Oxidation of Reduced S. Indirect evidence suggests that microbial oxidation of sulfide is important in sediments. If it is assumed that loss of organic S from sediments occurs via formation of H2S and subsequent oxidation of sulfide to sulfate (with the exception of pyrite, no intermediate oxidation states accumulate in sediments cf. 120, 121), the stated estimates of organic S mineralization suggest that sulfide production and oxidation rates of 3.6-124 mmol/m2 per year occur in lake sediments. Similar estimates were made by Cook and Schindler (1.5 mmol/m2 per year 122) and Nriagu (11 mmol/m2 per year 25). A comparison of sulfate reduction rates (Table I) and rates of reduced S accumulation in sediments (Table III) indicates that most sulfide produced by sulfate reduction also must be reoxidized but at rates of 716-8700 mmol/m2 per year. Comparison of abiotic and microbial oxidation rates suggests that such high rates of sulfide oxidation are possible only via microbial mediation. 

The formation of pyrite in sediments depends on the availability of three parameters iron, sulfate, and organic matter (48). Although organic matter content controls the formation rate under marine sulfate-rich conditions, sulfate concentration is usually regarded as the limiting factor under fresh-... 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

The bulk solution becomes impoverished with respect to oxygen. However, the sulfate concentration remains constant as long as the recycling rate of reduced sulfur to sulfate is higher than the sulfate reduction rate (denoted SRR in Figure 6). In the opposite case the sulfate concentration of the bulk solution also decreases, and H2S is slowly enriched (point 6). The requirements for FeS precipitation and subsequent pyrite formation are then fulfilled (point 7). 

Adewuyi and Carmichael (73) observed the reaction of CS2 with OH- to form a dithiocarbonate complex as the rate-determining step for the overall reaction 74 above. The formation of sulfate was found to be dependent on the rate of hydrolysis and to be preceded by long induction periods. As shown in Figure 3, the formation rate of sulfate was also found to be pH dependent and to increase exponentially with time. 

The relative importance of these two sulfate formation pathways will vary with season and climatic factors. The chemical form of airborne sulfur can play a major role in determining the nature and rate of any adverse effects, as discussed below. 

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