maxima of these species should have been found near the oxic/anoxic interface, which is around 80-m depth. The relatively low concentrations of sulfite and thiosulfate near the surface suggest that other factors, such as bacterial activities, may be important in controlling their distribution, and that both sulfite and thiosulfate are turned over rapidly in these waters.The reasons for the gradual increase in concentrations of sulfite and thiosulfate with depth (down to ~2000 m) are not clear. It is possible that some sulfite could have formed by air oxidation of sulfide during sampling, since there was a brief exposure to air (<30 s) just prior to derivatization. However, from kinetic considerations, this brief exposure was not likely to have given rise to the observed high concentrations in the deep samples ( 14). Furthermore, the sulfite depth profile closely parallels that of thiosulfate, which has a much slower formation rate than that of sulfite during air oxidation of sulfide in Black Sea water ( 14). An earlier work reported more than 20 µ concentrations of thiosulfate in the Black Sea waters deeper than ~300 m and attributed bacterial processes for its formation ( 15). The similarity of sulfite and thiosulfate profiles to that of sulfide suggests a common origin of these species in bacterial sulfate reduction. In fact, studies by Millet using radiolabeled sulfate revealed that sulfite was formed as an intermediate in the conversion of sulfate to sulfide (16). Since sulfite as well as thiosulfate are also formed as intermediates in the oxidation of sulfide to sulfate (17,18), it is likely that the reverse reactions occur during sulfate reduction. Our results imply that both sulfite and thiosulfate are probably intermediates in bacterial sulfate reduction.
Metal uptake by kraft lignin, hereafter referred to as lignin, occurs by displacement of protons or bound metals with equilibrium constants K(ex)H and K(ex), respectively. Values calculated for wide ranges of initial concentrations are reasonably constant, thereby demonstrating the validity of these displacement processes and proving that uptake in these systems is not simple adsorption. It was found that the stoichiometry for Sr and Cd uptake by Ca-loaded lignin is 1 mol of metal for 1 mol of Ca released. This observation for metals of very different binding strengths is difficult to rationalize with the biotic ligand model as generally applied but is in complete agreement with an ion-exchange process. Binding strengths to lignin, which contains only oxygen ligands, follow the order Pb > Cu > Zn > Cd > Ca (strongest to weakest). For proton displacement, only more tightly bound metals such as Pb, Cu, Zn, and Cd can compete with protons for anion-binding sites at low pH, but at high pH, uptake of Ca, Sr, and Li can occur. An observed logarithmic decrease of K(ex)H with pH can be explained by having only weaker acids available for proton displacement under more basic conditions. The advantages and disadvantages of using adsorption and biotic ligand models for an ion-exchange process are discussed.
The interaction between added metal ions and acid sites of two biosorbents, peat moss and the alga Vaucheria, was studied. Results were interpreted in terms of two model substances, alginic acid, a copolymer of guluronic and mannuronic acids present in marine algae, and humic acid in peat moss. For peat moss and Vaucheria at pH 4-6, two protons were displaced per Cd sorbed, after correction for sorbed metals also displaced by the heavy metal. The frequent neglect of exchange of heavy metals for metals either sorbed on the native material or added for pH adjustment leads to erroneous conclusions about proton displacement stoichiometry. Proton displacement constants K ex H decreased logarithmically with pH and had similar slopes for alginic acid and biosorbents. This pH effect was interpreted as an electrostatic effect of increasing anionic charge making proton removal less favorable. The maximum number of exchangeable acid sites (capacity C H ) decreased with pH for alginic acid but increased with pH for biosorbents. Consistent with titration behavior, this difference was explained in terms of more weak acid sites in the biosorbents.
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