Changes in physicochemical properties of organic matter by uranium irradiation: A case study from the Ordos Basin in China.

Abstract

Carbonaceous debris (CD) are exclusively disseminated in sandstones of uranium-bearing strata from the Dongsheng uranium ore field of the northern Ordos Basin, northern China. Physicochemical properties of CD were investigated through a series of tests including element analyses, Fourier transform infrared spectroscopy (i.e., FTIR) and organic carbon isotope (i.e., δC13) as well as specific surface area. The results show that δC13 of the samples from uranium-bearing strata range from -22.86‰ to -25.82‰, indicating that the CD is from the same origin. The average values of C, H, O and N are 54.54%, 4.55%, 39.15% and 0.49%, respectively, and H/C as well as O/C atomic ratios vary from 0.77 to 1.26 and 0.36 to 1.72, respectively, implying that CD is mostly composed of C and O, followed by H, and is grouped into Type Ⅲ kerogen at an immature to low mature stage. Compared with the samples without uranium enrichment, C, H, N and δ13C of the uranium-bearing samples increase by 13.66%, 5.32%, 12.50% and 0.98‰, respectively, while decrease by 18.50%, 7.84% and 33.33%, respectively, for O, H/C as well as O/C atomic ratios. As the intensity of uranium enrichment increases, H/C atomic ratios and aliphaticity decrease, and aromaticity increases. Besides, uranium-bearing samples are of smaller specific surface area than those without uranium enrichment. Alpha irradiation energy released by uranium and its daughters could cause the crosslinking in n-alkanes and long-chain polymers through a free-radical mechanism, which is supported by a decrease in O, aliphaticity and an increase in aromaticity. Hence, it results in polycondensation of molecular structure with a decrease in side chain length, function groups as well as specific surface area, and changes in physicochemical properties of organic matter, which is instructive in prospecting.