Equilibrium distribution of small organic molecules in natural waters

Details Equilibrium distribution of small organic molecules in natural waters Equilibrium distribution of small organic molecules in natural waters

Jul 1970

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1970gecoa..34..745t&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 34, Issue 7, pp.745-770

Mathematics

5

Scientific paper

The equilibrium activities of 39 dissolved species in the system C-N-S-H 2 O are computed as a function of pe - and pH at 25°C. For pe-pH conditions encountered in natural waters the predominant dissolved species are CH 4 , H 2 CO 3 , HCO 3 - , CO 3 2- , NH 4 OH, NH 4 + , N 2 , NO 3 - , H 2 S, HS - and SO 4 2- . Organic compounds of higher molecular weight, dissolved or particulate, are thermodynamically unstable in most natural environments. The distribution of dissolved species for the decomposition reactions of carbohydrate (CH 2 O) and alanine (C 3 H 7 O 2 N) in sea water are computed, assuming equilibrium among the decomposition products. The decomposition, in a closed system, of 0.1 gram-atom of organic carbon per liter of solution produces: a. (CH 2 O)-- m CH 4 ~ 10 -2 , m HCO 3 - ~ 10 -1 , m HS - ~ 10 -2.5 , m NH 4 + ~ 10 -3 , pH ~ 6.5; b. (C 3 H 7 O 2 N)-- m CH 4 ~ 10 -2 , m HCO 3 - ~ 10 -1 , m HS - ~ 10 -1.5 , m HN 4 + ~ 10 -1.5 , pH ~ 7.3. In the course of these reactions, the oxidation potential changes by only 50-60 mV after consumption of the initial oxygen. A mathematical expression for the homogeneous pe buffer capacity, E , is derived. Data from five reducing marine environments show that the predominant dissolved species tend to approach equilibrium, with the exception that the concentration of dissolved methane in certain environments is considerably higher than the predicted equilibrium concentration. Values of E range from 10 -3 to 1.2 for the environments studied.

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