Disequilibrium state natural water

Nearly all natural waters contain and in a state of disequilibrium (i.e., (234u/238u) 1) Generally > 1, and in some cases as high as 30 (Osmond... 

Many natural waters, including most waters at low temperature, do not achieve redox equilibrium (e.g., Lindberg and Runnells, 1984 see Chapter 7). In this case, no single value of pe or Eh can be used to represent the redox state. Instead, there is a distinct value for each redox couple in the system. Applying the Nernst equation to Reaction 3.46 gives a pe or Eh representing the hydrolysis of water. Under disequilibrium conditions, this value differs from those calculated from reactions such as,... 

Models of natural waters calculated assuming redox disequilibrium generally require more input data than equilibrium models, in which a single variable constrains the system s oxidation state. The modeler can decouple as many redox pairs as can be independently constrained. A completely decoupled model, therefore, would require analytical data for each element in each of its redox states. Unfortunately, analytical data of this completeness are seldom collected. 

Let us now consider the problem from the standpoint of calcite precipitation kinetics. At saturation states encountered in most natural waters, the calcite reaction rate is controlled by surface reaction kinetics, not diffusion. In a relatively chemically pure system the rate of precipitation can be approximated by a third order reaction with respect to disequilibrium [( 2-l)3, see Chapter 2]. This high order means that the change in reaction rate is not simply proportional to the extent of disequilibrium. For example, if a water is initially in equilibrium with aragonite ( 2c=1.5) when it enters a rock body, and is close to equilibrium with respect to calcite ( 2C = 1.01), when it exits, the difference in precipitation rates between the two points will be over a factor of 100,000 The extent of cement or porosity formation across the length of the carbonate rock body will directly reflect these... 

Aquatic microorganisms supply electrons through transplasmamembrane reductases to external solutes, enzymatically catalyze a variety of redox and other reactions on the cell surface, and are a source of dissolved extracellular enzymes. Both bound and dissolved extracellular enzymes are probably significant in maintaining a state of disequilibrium for some redox processes in natural waters and in accelerating some thermodynamically favorable reactions. In addition, as described for nickel and nitrogen in the urease example, these enzymes may also render the chemistry of the various components of aquatic systems highly interdependent. 

As noted, we have previously demonstrated the presence of Mn2+ in other Kansas stream samples at pH 8-8.5. This leads us to believe that the presence of the disequilibrium state of Mn2+ in natural water is much more common than presently suspected. The divalent manganese, as ESR spectroscopy indicates, is of special interest because it is the most chemically reactive form in natural water systems. 

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