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Title:A revised monod-type rate law predicting variable sulfur isotope fractionation factors as a function of microbial sulfate reduction rate
Author(s):Giannetta, Max Greene
Advisor(s):Druhan, Jennifer L
Contributor(s):Sanford, Robert A.
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Microbial sulfate reduction is associated with characteristic sulfur isotope partitioning, which can serve as a proxy for the rate of this reactivity in a wide variety of reducing environments. We demonstrate a new model for this functional relationship through the use of a modified Monod-type rate expression constrained by a novel set of experiments. A series of batch reactors containing an identical amount Desulfovibrio vulgaris, a strain of sulfate reducing bacteria (SRB), were subjected to differing continuous mass addition rates of formate to minimize growth and control the rate of sulfate reduction via electron donor limitation. The isotopic composition ( 34S) of the unreacted sulfate pool was measured through time for each experiment. This approach resulted in five steady state reduction rates of 2.06, 1.22, 0.83, 0.52 and 0.28 μmol*hr-1 that enriched the unreacted sulfate in 34S, where each rate was associated with a characteristic enrichment factor ( obs) of 0.9976, 0.9962, 0.9938, 0.9924 and 0.9903, respectively. This relationship was used to calibrate a coupled set of isotope-specific Monod rate laws that were modified to incorporate (1) both minimum ( 1 = 0.998) and maximum ( 2 = 0.930) fractionation factors and (2) a rate-controlling electron donor factor (DF). These parameters constrain a model which produced realistic predicted shifts in the apparent fractionation factor ( obs) as a function of reduction rate in an electron donor limited system. Application of these parameters in the updated model accurately reproduced our data and thus offers a means to predict the relationship between obs and sulfate reduction rate. We argue that this approach offers a reasonable approximation to more detailed microbial reactive network models, while still maintaining sufficient simplicity and versatility to allow incorporation into multi-component reactive transport simulations. Thus, the current study provides a foundation for accurate simulation of rate-dependent fractionation in open, transient, and through-flowing systems.
Issue Date:2018-04-19
Rights Information:Copyright 2018 Max Giannetta
Date Available in IDEALS:2018-09-04
Date Deposited:2018-05

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