Redox chemistry pore water

While all of these reactions are favored thermodynamically, they are almost always enzymatically catalyzed by bacteria. It has been observed from the study of pore waters in deepsea sediments (e.g., Froelich et al., 1979) and anoxic basins (e.g., Reeburgh, 1980) that there is an ordered sequence of redox reactions in which the most energetically favorable reactions occur first and the active electron acceptors do not overlap significantly. Bacteria are energy opportunists. Using estimates of the stoichiometry of the diagenesis reactions (Table 2) one can sketch the order and shape of reactant profiles actually observed in sediment pore-water chemistry... 

Figure 6.5 Electron transfer reactions for sediments (Ruddy, 1993). CHjO.N.P (organic matter) is transformed by bacterial decomposition reactions to bicarbonate in the pore-waters. This is the primary source of electrons (and therefore energy) for the remainder of the sediment redox chemistry. Most of the primary flux of electrons may pass through the sulphur, iron and manganese cycles, but will eventually react with oxygen. Only a small part of the total electron flux will ultimately be buried as reduced minerals.

The theoretical prediction of whether a particular pollutant will be soluble (and hence potentially mobile) or insoluble (and hence potentially immobile) using pore-water chemistry data is more problematical. This limitation arises because it is impossible to derive reliable estimates of in situ concentrations for several key constituents using the techniques described. In particular, reliable data for in situ pH and redox conditions Eh) cannot be acquired. This is particularly true when pore-waters cannot be extracted directly and compositions must instead be estimated from leachate data. Although there are limitations in the use of pore-water data to... 

Cerium, the only lanthanide with redox transformations at ambient oceanic conditions, has been exploited by geochemists to learn more about redox-controlled reactions in seawater. Background on the redox chemistry of Ce is presented in sect. 2. To emphasize a previous point, the Ce anomaly allows one to focus on the redox-controlled reactions of Ce by comparing its concentration to those of near neighbors (La and Nd in most cases cited next) which undergo no redox transformations. This section will focus on cerium and the Ce anomaly in open ocean seawater. The dynamic nature of the Ce redox chemistry in anoxic basins and pore waters will be discussed in the next section. 

---