Effect of Reaction Pathway on the Extent and Mechanism of Uranium(VI) Immobilization with Calcium and Phosphate.

Phosphate addition to subsurface environments contaminated with uranium can be used as an in situ remediation approach. Batch experiments were conducted to evaluate the dependence of the extent and mechanism of uranium uptake on the pathway for reaction with calcium phosphates. At pH 4.0 and 6.0 uranium uptake from solution occurred via autunite (Ca(UO2)2(PO4)2) precipitation irrespective of the starting forms of calcium and phosphate. At pH 7.5, a condition at which calcium phosphate solids could form, the uptake mechanism depended on the nature of the calcium and phosphate as determined by X-ray absorption spectroscopy and laser-induced fluorescence spectroscopy. When dissolved uranium, calcium, and phosphate were added simultaneously, uranium was structurally incorporated into a newly formed amorphous calcium phosphate solid. Adsorption was the dominant removal mechanism for uranium contacted with preformed amorphous calcium phosphate solids. When U(VI) was added to a suspension containing amorphous calcium phosphate solids as well as dissolved calcium and phosphate, then removal occurred through precipitation (57 ± 4%) of autunite and adsorption (43 ± 4%) onto calcium phosphate. Dissolved uranium, calcium, and phosphate concentrations with saturation index calculations helped identify removal mechanisms and determine thermodynamically favorable solid phases.

Transport of U(VI) through sediments amended with phosphate to induce in situ uranium immobilization.

Phosphate amendments can be added to U(VI)-contaminated subsurface environments to promote in situ remediation. The primary objective of this study was to evaluate the impacts of phosphate addition on the transport of U(VI) through contaminated sediments. In batch experiments using sediments (<2 mm size fraction) from a site in Rifle, Colorado, U(VI) only weakly adsorbed due to the dominance of the aqueous speciation by Ca-U(VI)-carbonate complexes. Column experiments with these sediments were performed with flow rates that correspond to a groundwater velocity of 1.1 m/day. In the absence of phosphate, the sediments took up 1.68-1.98 µg U/g of sediments when the synthetic groundwater influent contained 4 µM U(VI). When U(VI)-free influents were then introduced with and without phosphate, substantially more uranium was retained within the column when phosphate was present in the influent. Sequential extractions of sediments from the columns revealed that uranium was uniformly distributed along the length of the columns and was primarily in forms that could be extracted by ion exchange and contact with a weak acid. Laser induced fluorescence spectroscopy (LIFS) analysis along with sequential extraction results suggest adsorption as the dominant uranium uptake mechanism. The response of dissolved uranium concentrations to stopped-flow events and the comparison of experimental data with simulations from a simple reactive transport model indicated that uranium adsorption to and desorption from the sediments was not always at local equilibrium.

Phytochemical divergence in 45 accessions of Terminalia ferdinandiana (Kakadu plum).

This study investigated the variations in the levels of phenolic compounds, vitamin C, sugars and antioxidant capacities of 45 newly collected accessions of Terminalia ferdinandiana (Kakadu plum), a native Australian fruit utilised in dietary supplement industry. Pattern recognition tools, principal component analysis (PCA) and agglomerative hierarchical clustering (AHC) were applied to understand interrelationships between the antioxidant capacities [Ferric reducing antioxidant power (FRAP) and Oxygen radical absorbance capacity (ORAC)] and antioxidant groups: phenolic compounds and vitamin C. On the basis of these parameters AHC classified samples into three main groups, with accessions 2, 8, 15, 6, 3 and 5 from the Northern Territory, Australia, representing superior quality fruits combining high levels of total phenolics (505.2 to 376.1 mg GA E/g DW), vitamin C (322.2 to 173.5mg/g DW), with pronounced antioxidant capacities (FRAP: 5030.5 to 4244.9 µmol Fe(2+)/g DW; ORAC: 3861.5 to 2985.6 µmol Trolox E/g DW). Hydrolysable tannins and ellagic acid were identified as the major phenolic compounds. The levels of ellagic acid varied from 140.2 to 30.5 mg/g DW, which places Kakadu plum as a unique edible source of this compound. The levels of sugars varied from 619.0 to 130.0 mg Glu E/g DW. This study for the first time revealed a unique phytochemical profile and significant variability in phytochemical composition of Kakadu plum. These features create opportunities for selection of sources with different characteristics addressing the needs of the nutraceutical industry, food processors and the consumers of fresh fruit.