Sulfate stratification

Water chemistry (pH, DOC, sulfate, TSS, chlorophyll, temperature, ANC, color, nutrients, DO, stratification status) Intensive and cluster Quarterly A, B... 

In the Lake Vechten sediments referred to above, stratification of the two groups of orgcmisms appeared to be due Isurgely to the sensitivity of methane producers to HjS which became inhibitory above about 0.1 mM (Cappen-berg, 1975). In many sediments, however, H2S does not accumulate to a significant extent due to its fixation as iron sulfides and its diffusion from the sediment. Winfrey emd Zeikus (1977), for example, reported that, in Lake Mendota sediments, the amount of sulfide which could be added before free HjS appeared in the pore waters was about 25 times the concentration of sulfate required to inhibit methanogenesis. 

There are then several possible explanations for the stratification of sulfate reduction and methane accumulation in nature and indeed many factors may contribute to this phenomenon and different mechanisms may apply in different environments. One complicating factor is that methane is produced not only by COj reduction but also by fermentations (Mechalas, 1974 Mah et ah, 1977) and the conditions required for these reactions are not necessarily the same. Nevertheless, the stratification of methane and sulfide serves to further illustrate the close interdependence of organisms with different metabolic specialities. 

An example of sulfate reduction in a shallow fjord estuary is the upper basin of the Pettaquamscutt River (Rhode Island, U.S.A.) studied by Orr and Gaines (1973). It is normally well-stratified water but, every few years, there is overturn which mixes near-surface oxygenated water with the bottom water containing biogenic sulfide. In a time span of just over one month after an overturn in November 1971, 80% of the sulfide was oxidised. After stratification was re-established, the sulfide remained below 4 m depth and accumulated at a calculated rate of 48 g S m" y. ... 

Stratification in estuaries is in some respects similar to stratification in lakes, although in estuaries the density difference is primarily due to the difference in salinity between freshwater and ocean water, instead of being primarily due to temperature differences, as in most lakes. Freshwater has a density of approximately 1.00 g/cm3, whereas ocean water has a density of approximately 1.03 g/cm3 due to dissolved salts [primarily sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulfate (SO4 ). This is a much larger density difference than that which occurs due to temperature differences in surface waters hence, the stratification may be very strong. Whatever its cause, stratification always inhibits the vertical transfer of dissolved chemicals from layer to layer. 

Figure 14. Stratification of horizontal foam film from 0.1 mol/L sodium dodecyl sulfate surfactant solution.

An example of such an analysis is given in Fig. 13.19. In a biofilm, profiles of O2, N02, NO3, and H2S were measured. From these profiles, the aerobic respiration, nitrate consumption (denitrification), and sulfate reduction rates could be calculated. The analysis shows nicely the stratification of processes, that is, the sequence of used e-acceptors ( Fig. 13.20). 

The microporosity is also important for this apphcation, in order not to allow shorts through the backweb during battery hfe. Bottom shorts are avoided by a mud room of sufficient dimensions, and side shorts by plastic edge protectors on the frames of the negative electrode. Some manufacturers have switched to using sleeves of polyethylene separator material, rendering an edge protection superfluous. The use of three-side-sealed separator pocket in traction batteries should be avoided, because experience has shown this can lead to increased acid stratification, subsequent sulfation, and thus capacity loss. 

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