Effect of Calcium on the Bioavailability of Dissolved Uranium(VI) in Plant Roots under Circumneutral pH

Abstract

We integrated field measurements, hydroponic experiments, microscopy, and spectroscopy to investigate the effect of Ca(II) on dissolved U(VI) uptake by plants in 1 mM HCO3− solutions at circumneutral pH. The accumulation of U in plants (3.1–21.3 mg kg−1) from the stream bank of the Rio Paguate, Jackpile Mine, New Mexico served as a motivation for this study. Brassica juncea was the model plant used for the laboratory experiments conducted over a range of U (30–700 μg L−1) and Ca (0–240 mg L−1) concentrations. The initial U uptake followed pseudo-second-order kinetics. The initial U uptake rate (V0) ranged from 4.4 to 62 μg g−1 h−1 in experiments with no added Ca and from 0.73 to 2.07 μg g−1 h−1 in experiments with 12 mg L−1 Ca. No measurable U uptake over time was detected for experiments with 240 mg L−1 Ca. Ternary Ca–U–CO3 complexes may affect the decrease in U bioavailability observed in this study. Elemental X-ray mapping using scanning transmission electron microscopy–energy-dispersive spectrometry detected U–P-bearing precipitates within root cell walls in water free of Ca. These results suggest that root interactions with Ca and carbonate in solution affect the bioavailability of U in plants. This study contributes relevant information to applications related to U transport and remediation of contaminated sites.

Organic Functional Group Chemistry in Mineralized Deposits Containing U(IV) and U(VI) from the Jackpile Mine in New Mexico

“…71 Understanding changes in organic functional chemistry at pH 7 and 2 is environmentally relevant due to their influence on chemical reactions that could impact U mobility and transport from the Jackpile Mine to surface waters and plants. 1,36 For example, circumneutral pH conditions are characteristic of surface waters adjacent to the Jackpile Mine, while a lower pH is commonly observed in sites affected by acid mine drainage. 1,45,47 Future research is necessary to identify specific mechanisms through which functional groups from the NOM found in our study control U reactivity at sites with complex mineralogy such as the Jackpile Mine, particularly the influence of NOM on U(IV) stabilization under oxic conditions.…”

“…U in soils, surface water, groundwater, and uptake in plants. 36 Although several methods have been used to understand the mineralogy of sandstone formations and U mineralized deposits, the analysis of particulate NOM is challenging. 6 The application of X-ray photoelectron spectroscopy (XPS) to observe changes in organic functional groups in U mineralized deposits as a result of pH changes is not well documented.…”

Calcium in Carbonate Water Facilitates the Transport of U(VI) in Brassica juncea Roots and Enables Root-to-Shoot Translocation

“…10,12 The presence of a relatively high calcium (Ca) concentration in carbonate water can inhibit U accumulation in plants likely as a result of interactions with neutrally charged calcium uranyl carbonate complexes (Ca–U–CO 3 ). 13 The complexation of U(VI) with carbonate and citrate enhances U root-to-shoot translocation. 14–17 Such an increase in U translocation in planta is generally attributed in the literature to the following electrostatic interactions.…”

Plant cell (Brassica napus) response to europium(III) and uranium(VI) exposure

“…In a related study, El Hayek et al ( 2018 ) employed scanning transmission electron microscopy/energy-dispersive X-ray spectrometry to confirm the predominant binding of uranium in the cell walls of Brassica juncea . Nevertheless, it must be noted that for our investigation, the detection of U(VI) in the cytosol fraction indicates that a certain amount of U(VI) was taken up by the cells.…”