Oxidation of Organic Matter and Sulfate Reduction

There are several groups of organic substances in any sediment. Each group has its own decay rate (Westrich and Berner 1984 see also Middleburg 1989 Tarutis 1993). Decay rates, whether oxic or anoxic, obey first-order kinetics. For each group, G dGJdt = -kfi,. For Gr, the total decomposable organic material present. 

Integration of the initial-rate equation gives Gj as a function of time 

In their study of modem marine sediments, Westrich and Berner (1984) found Goi = 50%, Gq2 =16%, and G r = 34% of TOC. For oxic decay their empirical rate constants were k i = 18/y (ti/2 = 0.039 y = 14 days), and k2 = 2.3/y (f,/2 = 0.3 y = 110 days). For anoxic decay of the same organic matter, which follows the same rate equations, they obtained = 4.4/y, and k2 = 0.84/y. All of this, of course, assumes G is inert. 

If similar rates apply to natural water/rock systems, which wilt generally have much longer residence times than considered in this study, we can expect that organic matter similar to G, will have disappeared (fi = 14 days). G2 kinetics will still be important. However, rates of decomposition of more recalcitrant organics (G r), because they include perhaps most of toxic organics, will still be important. 

If sulfate reduction is organic-carbon limited in the sediment described by Westrich and Berner (1984), then the same rates of organic carbon oxidation apply. For sulfate reduction, assuming that the organic carbon may be generalized as CH2O, the reaction may be written 

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Integration yields n = /j exp(-/ +/), where n and are the total number of atoms present at t and r = 0. The half-time of radioactive decay is defined by r 2 = 0.693/A+, where n = 0.5, and n =. Other examples of first-order reactions (see Section 2.7) are the oxidation of organic matter and sulfate reduction, gypsum (CaS04 2H2O) dissolution, and the oxidation of pyrite and marcasite (FeS2). 

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