Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments

Abstract

Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U(IV)], a form once considered largely immobile. Changing hydrologic and geochemical conditions cause U to be released into groundwater. Knowledge of the chemical form(s) of U(IV) is essential to understand the release mechanism, yet the relevant U(IV) species are poorly characterized. There is growing belief that natural organic matter (OM) binds U(IV) and mediates its fate in the subsurface. In this work, we combined nanoscale imaging (nano secondary ion mass spectrometry and scanning transmission X-ray microscopy) with a density-based fractionation approach to physically and microscopically isolate organic and mineral matter from alluvial sediments contaminated with uranium. We identified two populations of U (dominantly +IV) in anoxic sediments. Uranium was retained on OM and adsorbed to particulate organic carbon, comprising both microbial and plant material. Surprisingly, U was also adsorbed to clay minerals and OM-coated clay minerals. The dominance of OM-associated U provides a framework to understand U mobility in the shallow subsurface, and, in particular, emphasizes roles for desorption and colloid formation in its mobilization.

Diagenetic formation of uranium-silica polymers in lake sediments over 3,300 years

“…Only a few studies that characterized U speciation in natural low-temperature environments actually reported occurrences of crystalline U(IV), and a fortiori UO 2 , for alluvial sediments exposed to seasonal redox cycling (23,24,28), or subjected to laboratory incubation or biostimulation (18,21,27,65,66). In contrast, a majority of natural environments do not exhibit UO 2 , such as recent lake sediments (6, 7), U-contaminated organicrich wetlands (19,20,22,25,30), or other biostimulated soils and sediments (17,(69)(70)(71)(72). In the light of these observations, the present study supports that the formation of uraninite is inhibited by the presence of aqueous and solid-associated ligands (e.g., organic matter-bound phosphoryl and carboxyl groups, and mineral phosphate and silicate ligands) that immobilize U(IV) under noncrystalline forms (12, 13, 16, 57, 67).…”

Combined Effects of Fe(III)-Bearing Clay Minerals and Organic Ligands on U(VI) Bioreduction and U(IV) Speciation

“…Understanding these effects on bioreduction is important as U, organic ligands, and Fe(III)−clay minerals are co-present at many contaminated sites. 14 In the absence of ligands, structural Fe in clay minerals can undergo many redox cycles without significant dissolution; 53,54 however, ligands such as citrate and EDTA can dissolve a fraction of the structural Fe in nontronite to form soluble Fe−ligand complexes. 55,56 Fe−ligand complexes exhibit different sorption capacities, speciation, bioavailability, and Fe(III)/Fe(II) redox potentials, 56 all of which are expected to impact the role of Fe in mediating U redox reactions.…”

“…Uranium contamination is a growing problem because of the operation of nuclear power plants and the mining of uranium-rich ores . The mobility of uranium in aquatic and terrestrial environments is largely controlled by its complexation with organic matter and its valence state. − Uranium has two dominant oxidation states: hexavalent uranium [U­(VI)] and tetravalent uranium [U­(IV)]. − U­(VI), usually in the form of a soluble uranyl cation (UO 2 2+ ), is predominant under oxidizing conditions. , Under reducing conditions, U­(VI) can be reduced to U­(IV) (uranous ion, U 4+ ), which occurs as sparingly soluble uraninite (UO 2 ), amorphous U­(IV)-phosphate phases, monomeric U­(IV) adsorbed to metal oxide surfaces, or persistent colloids. ,− …”

“…These studies show that the formation of U­(VI)–carbonate complexes inhibits U­(VI) reduction by mineral-associated Fe­(II). ,− Although organic ligands are expected to form similar aqueous ligand–U­(VI) complexes at pH > 5, it is currently unknown whether organic ligands can affect U–Fe redox reactions involving Fe-bearing clay minerals and microbes. Understanding these effects on bioreduction is important as U, organic ligands, and Fe­(III)–clay minerals are co-present at many contaminated sites . In the absence of ligands, structural Fe in clay minerals can undergo many redox cycles without significant dissolution; , however, ligands such as citrate and EDTA can dissolve a fraction of the structural Fe in nontronite to form soluble Fe–ligand complexes. , Fe–ligand complexes exhibit different sorption capacities, speciation, bioavailability, and Fe­(III)/Fe­(II) redox potentials, all of which are expected to impact the role of Fe in mediating U redox reactions.…”

Thermodynamic properties of the uranyl carbonate minerals roubaultite, fontanite, widenmannite, grimselite, čejkaite and bayleyite

“…The uranyl carbonate [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] minerals are extraordinarily important in actinide environmental chemistry [1][2][3][13][14][15][21][22][23][24][25][26][27][28][29][30] because their dissolution/precipitation processes control the aqueous mobility of uranium 1-10,13-17,19.23-30 in circumneutral to alkaline conditions 2,8,10 and, therefore, their knowledge is essential in the research of migration of uranium from contaminated sites [31][32][33][34][35][36][37][38][39] and nuclear waste repositories (NWR). 1,…”