Association of uranium with colloids of natural organic matter in subsurface aquatic environment

Speciation of uranium with natural organic matter (NOM), such as humic and fulvic acids in ground water is studied by using a size fractionation technique. NOM is a poly-functional, poly-electrolytic, heterogeneous amalgam of organic molecules of varying molecular weight and size. The NOM constitute an important pool of ligands for complexing metals. The size fractionation of dissolved NOM is achieved based on their nominal molecular weight cut-off limit {NMWL} by using ultra-filtration technique. Samples of ground water were collected from five different locations. After filtering through a 0.45 μm filter by using suction filtration, various physicochemical characteristics of the water were measured by using ion chromatography. Dissolved organic carbon (DOC) in ground waters were measured by a total organic carbon (TOC) analyzer. The levels of total DOC in the ground water were in the range of 10.42.17.83 ppm. The solution obtained from filtration through < 0.45 μm, was sequentially passed through the ultra-filtration membranes having pore diameter of 14 nm (300k NMWL), 3.1 nm (50k NMWL), 2.2 nm (30k NMWL), 1.6 nm (10k NMWL) and 1.1 nm (0.5k NMWL) by using stirred ultra-filtration cells, operated in concentration mode. The dissolved organic carbon (DOC) that passed through 10,000 NMWL filter, i.e., approximately 1.6 nm filter, consisted mainly of fulvic acid and accounts for 70% DOC. The concentration of uranium in various fractions was measured by a laser fluorimeter in which a pulsed nitrogen laser is used to excite uranyl species at 337.1 nm. Maximum natural uranium in the ground aquatic environment is found to be bound on low molecular weight humus colloids, i.e., 30 k NMWL followed by 10 k and 0.5 k NMWL. (author)