Oxides, concentrations sites

Reactions between Fe(ll) in contaminated groundwater (5.8 mg/L) and oxic sediment also affected As mobility. Ferrous iron was oxidized by manganese oxides to ferric iron which precipitated as hydrous ferric oxide, creating additional sorption sites. Evidence for this reaction included an increase in ferric oxide concentrations in reacted column sediments and manganese concentrations in leachate that were greater than in the initial eluent. 

Electron transfer reactions of metal ion complexes in homogeneous solution are understood in considerable detail, in part because spectroscopic methods and other techniques can be used to monitor reactant, intermediate, and product concentrations. Unfavorable characteristics of oxide/water interfaces often restrict or complicate the application of these techniques as a result, fewer direct measurements have been made at oxide/water interfaces. Available evidence indicates that metal ion complexes and metal oxide surface sites share many chemical characteristics, but differ in several important respects. These similarities and differences are used in the following discussions to construct a molecular description of reductive dissolution reactions. 

An international expert panel has issued an air quality criteria document for photochemical oxidants and related hydrocarbons that builds on the U.S. Department of Health, Education, and Welfare (dhew) air quality criteria document for photochemical oxidants. It discusses oxidant concentration patterns in the context of the same tabular material presented earlier. New information is added for the city of Delft the monthly means of daily maximums of hourly ozone concentrations are shown in Table 4-3, and the monthly average ozone concentrations are shown in Table 4-4. As in other cities, the worst month seems to be August, with a mean daily maximum (of hourly concentrations) of 0.071 ppm (140 Mg/m ). Table 4-5 compares the number of days in May through July 1971 when the ozone concentration at one or more sites reached or exceeded the hourly average of 200 Mg/m or of 100 Mg/m (from NATO data °). A comparison is made between Delft and five other monitoring sites in the Netherlands. Amsterdam had a peak value of 0.18 ppm in March 1971. 

FIGURE 4 36 Trend in oxidant concentration by year at camp sites. Reprinted with permission from AltshuUer. ... 

TABLE 4>11 Summary of Total Oxidant Concentrations Recorded at camp Sites, 1964-1972 ... 

TABLE 4-12 Number of Days and Percentage of Days with Corrected Oxidant Concentration over 0.08 ppm at CAMP Sites, 1964-1973 ... 

The bioassay technique was developed to reduce the uncertainties associated with the use of native vegetation or cultivated crops. Plants can be started under controlled conditions and exposed under standardized conditions. Species and cultivars can be selected for oxidant sensitivity and symptom characteristics. The two studies just noted were the most closely controlled. Similar work has not been repeated. However, many investigators have grown plants under known cultural conditions and then transplanted them to field sites where they received special care. These plants can then be read for foliar symptoms throughout a given period, and the symptoms related to oxidant concentrations. The lack of apparent correlation in the two early studies could be due to the lack of specificity for the monitored oxidants, the presence of different concentrations of interacting oxidants at different times, or variations in cultural conditions between exposure times. 

By following the reaction scheme proposed by dos Santos Afonso and Stumm (22) for the reductive dissolution of hematite surface sites (Scheme 1), we were able to explain perfectly the observed pH pattern of the oxidation rate of H2S. The rate is proportional to the concentration of inner-sphere surface complexes of HS" formed with either the neutral (>FeOH) or the protonated (>FeOH2+) ferric oxide surface sites. 

To summarize the qualitative findings, the methanol synthesis activity in the binary Cu/ZnO catalysts appears to be linked to sites that also irreversibly chemisorb CO and not to sites that adsorb CO reversibly. Since irreversible adsorption of CO follows linearly the concentration of amorphous copper in zinc oxide, these sites are likely to be that part of the copper solute that is present on the zinc oxide surface. No correlation of the catalyst activity and the copper metal surface area, titrated by reversible form of CO or by oxygen, could be found in the binary Cu/ZnO catalysts (43). In contrast with this result, it has been claimed that the synthesis activity is proportional to copper metal area in copper-chromia (47), copper-zinc aluminate (27), and copper-zinc oxide-alumina (46) catalysts. In these latter communications (27,46,47), the amount of amorphous copper has not been determined, and obviously there is much room for further research to confirm one or another set of results and interpretations. However, in view of the lack of activity of pure copper metal quoted earlier, it is unlikely that the synthesis activity is simply proportional to the copper metal surface area in any of the low-temperature methanol-synthesis catalysts. 

Heme does, however, not seem to be required for protein S-nitrosylation to occur. The precise chemistry is unsettled Figure 11.7a gives one hypothetic reaction scheme. Here, S-nitrosylation generates one equivalent of superoxide, which in turn (and particularly so at high concentrations of NO) may react with another molecule of NO to generate peroxynitrite. Peroxynitrite is very reactive and may oxidize other sites in proteins, or it may give rise to 0-nitrosy-lation of protein tyrosine side chains. 

In Figure 8 we have plotted the lanthanum oxide, cerium oxide, and thorium oxide concentrations for sherds excavated in the Dominican Republic and Venezuela and sherds from the Metro excavations using a computer system developed for this purpose at Brookhaven National Laboratory (8). On the basis of these three oxides there is a distinct separation between the sherds from the Dominican Republic and Venezuela and those from Mexico City. Unlike the sherds from the Dominican Republic and Venezuela, the sherds from Mexico City appear not to have originated in Spain, at least at that specific source. There is further evidence of this distinction between the two sets of sherds. X-ray diffraction analysis of the samples from Jerez and from the New World showed that the sherds from Jerez, the Dominican Republic, and Venezuela had intense quartz peaks whereas the sherds from Mexico City did not. This constitutes additional evidence that the majolica from Mexico City came from a different source than the majolica from the Caribbean sites. 

Another approach involves the preparation and use of well-defined supported oxide catalysts. Because the method of preparation, such as simple impregnation, leads to active sites which may be isolated from one another, particularly at low concentrations of the supported oxide, the sites become relatively easy to identify and characterize at the molecular level (7). Im-... 

An X-ray photoelectron spectroscopic study of Ni DPG)2l showed no evidence of trapped valence or any appreciable change in the charge on the metal upon oxidation. The site of partial oxidation and hence the electron transport mechanism is still unclear but one explanation of the relatively low conductivity is that the conduction pathway is metal centred and that the M—M distances are too long for effective orbital overlap. Electron transport could be via a phonon-assisted hopping mechanism or, in the Epstein—Conwell description, involve weakly localized electronic states, a band gap (2A) and an activated carrier concentration. ... 

The electrochemical response of this system will depend on the timescale of the involved electrochemical experiment. Thus, if the charge transport rate is signili-cantly faster than the experimental timescale, the oxidized/reduced site concentration ratio, [ Ox ] [Rcd -nM j, will be uniform throughout the microporous layer and in thermodynamic equilibrium with the applied potential. Thus, the concentration profiles (see Figure 2.5) for the oxidized and reduced forms of the electroactive... 

The existence of strong electrophilic sites on the silica-alumina catalyst is strikingly confirmed by the spectrum of adsorbed perylene vapor (Fig. 17). The sharp absorption peakf at 534 is the same that has been observed at 580 m/i for perylene in a strong oxidant (concentrated H2SO4), and identified with the cation radical, perylenet 109, 116). 

The notion that AO molecules reduced to different degrees may exhibit distinct reactivities can be ruled out because experiments starting with the fully oxidized enzyme and ending with less than 5% oxidized Tl sites yielded essentially identical intramolecular ET rates (33). Also, no difference in these internal ET rate constants was observed with enzymes activated or pulsed" by turning over 1 mM ascorbate in the presence of 0.25 mM O2 prior to the determination of the intramolecular ET rates. In contrast to these experiments, which were all performed under anaerobic conditions, a more recent pulse-radiolytic study employed AO solutions containing small and controlled concentrations of O2 (15-65 fxm), and conspicuous differences in the kinetic behavior were observed. A new and faster intramolecular ET phase was discovered which... 

Since the catalyst has no steady-state activity at SOX towards reactions involving NO, as shown by our previous studies [8], the steady-state values for NO and N2O outlet concentrations are the inlet ones. Notwithstanding, it can be observed the transient appearance of N2O in the gas phase, after an initial period in which no nitrogen containing species are present in the outlet gas mixture. We have explained the observed phenomena as follows. In the first period, NO is irreversibly adsorbed on the reduced catalyst surface and re-oxidizes copper sites, according to the following stoichiometry ... 

Brown and Anson [16] assumed that the degree of interaction between the redox sites depends on the concentrations of the reduced (ca) and oxidized (cp) sites, and so the activity coefficients can be expressed as follows ... 

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