A modified method to calculate dual-isotope slopes for the natural attenuation of organic pollutants in the environment.

ABSTRACT

Two-dimensional compound-specific isotope analysis has become a powerful tool for distinguishing reaction mechanisms. Lambda (Λ), an essential and important parameter for processing two-dimensional isotope fractionation data, exhibits values specific to a reaction mechanism. In the present article, we modified the existing algorithms for calculation of lambdas based on a review of current methods. Specifically, by regressing [(1000+δE0,2)*(n1*x2)*ΔδEbulk,1] versus [(1000+δE0,1)*(n2*x1)*ΔδEbulk,2] by the York method, a novel method was developed to calculate Λs. The improved method eliminates both the influence of the nonreacting position and the initial isotope signatures. Furthermore, this method retains the advantages of a two-dimensional isotope plot, which eliminates contributions from commitment to catalysis, does not require determination of the fraction of remaining substrate, and can be constructed even from field data. Additionally, the one-sample t test is applied to generate a 95% confidence interval of the dataset of Λris for various reaction mechanisms. The ranges of 5.67-24.8, 8.54-9.80, 0.51-8.35, 25.2-36.8, and 7.09-21.9 are applicable for the oxidation of C-H bonds (ZC=1, ZH=3; ZC and ZH are the number of indistinguishable carbon and hydrogen atoms in intramolecular competition, respectively), oxidation of C-H bonds (ZC=1, ZH=4), aerobic biodegradation of benzene (ZC=6, ZH=6), methanogenic or sulfate-reducing biodegradation of benzene (ZC=6, ZH=6), and nitrate-reducing biodegradation of benzene (ZC=6, ZH=6). The accumulation and correction of these values will make the data measured in the field easier to interpret.