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Publication# Speciation-dependent kinetics of uranium(VI) bioreduction

Abstract

The kinetics of uranium(VI) reduction by Shewanella oneidensis strain MR-1 was studied for varied pH and concentrations of dissolved inorganic carbon (DIC) and calcium. These are key variables affecting U(VI) speciation in aqueous systems. For all conditions studied, a nearly log-linear decrease of [U(VI)] suggested pseudo-first-order kinetics with respect to U(VI). The reduction rate constants (k) decreased with increasing DIC and calcium concentration, and were sensitive to pH. A positive correlation was found between k and the logarithm of the total concentration of U(VI)-hydroxyl and U(VI)-organic complexes. Linear correlations of the rate constant with the redox potential (E-H) of U(VI) reduction and the associated Gibbs free energy of reaction (Delta G(r)) were found for both Ca-free and Ca-containing systems. Both E-H and Delta G(r) are strong functions of aqueous U(VI) speciation. Because the range in Delta G(r) among the experimental conditions was small, the differences in k are more likely to be due to differences in EH or to differences in individual rate constants of U(VI) species. Calculation of conditional reduction rate constants for the major groups of U(VI) complexes revealed highest constants for the combined groups of U(VI)-hydroxyl and U(VI)-organic species, lower rate constants for the U(VI)-carbonate group, and much lower constants for the Ca-U(VI)-carbonate group. Mechanistic explanations for these findings are discussed.

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Reaction rate constant

In chemical kinetics, a reaction rate constant or reaction rate coefficient (k) is a proportionality constant which quantifies the rate and direction of a chemical reaction by relating it with the concentration of reactants. For a reaction between reactants A and B to form a product C, where A and B are reactants C is a product a, b, and c are stoichiometric coefficients, the reaction rate is often found to have the form: Here k is the reaction rate constant that depends on temperature, and [A] and [B] are the molar concentrations of substances A and B in moles per unit volume of solution, assuming the reaction is taking place throughout the volume of the solution.

Correlation

In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics it usually refers to the degree to which a pair of variables are linearly related. Familiar examples of dependent phenomena include the correlation between the height of parents and their offspring, and the correlation between the price of a good and the quantity the consumers are willing to purchase, as it is depicted in the so-called demand curve.

Pearson correlation coefficient

In statistics, the Pearson correlation coefficient (PCC) is a correlation coefficient that measures linear correlation between two sets of data. It is the ratio between the covariance of two variables and the product of their standard deviations; thus, it is essentially a normalized measurement of the covariance, such that the result always has a value between −1 and 1. As with covariance itself, the measure can only reflect a linear correlation of variables, and ignores many other types of relationships or correlations.

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