Pathways of aqueous Cr(VI) attenuation in a slightly alkaline oxic subsurface.

Column experiments combined with geochemical modeling, microscopic inspections, spectroscopic interrogations, and wet chemical extractions were used to study sediment-dependent Cr(VI) desorption, physical location, mineral association, and attenuation mechanism(s) in four freshly or naturally aged contaminated sediments exposed to concentrated Cr(VI) waste fluids. Results showed that majority of Cr(VI) mass was easily removed from the sediments (equilibrium site K(d) varied from 0 to 0.33 mL g(-1) and equilibrium site fraction was greater than 95%). Long tailings of time-dependent Cr(VI) concentrations above maximum contaminant level of 1.9 micromol L(-1) were also observed (kinetically controlled fraction K(d) and desorption reaction half-lives varied from 0 to 45 mL g(-1), and from 76.1 to 126 h, respectively). Microscopic and spectroscopic measurements confirmed that Cr was concentrated within fine-grained coatings in small areas mainly rich in phyllosilicates that contained both Cr(III) and Cr(VI). However, Cr(VI) reduction was neither significant nor complete. Under slightly alkaline and oxic conditions, contaminant Cr in the sediments occurred: (i) In the highly mobile pool (over 95% of total Cr); (ii) In the slow and time-dependent releasing pool, which served as long-term source of contamination; (iii) As reduced Cr(III) which most likely formed during Cr(VI) reaction with aqueous, sorbed, or structural Fe(II).

Aluminum effect on dissolution and precipitation under hyperalkaline conditions: I. Liquid phase transformations.

Substantial amounts of self-boiling, Al-rich, hyperalkaline, and saline high-level waste fluids (HLWF) were deposited to the vadose zone at the Hanford Site, in Washington State. The objective of this study was to investigate the effects of similar fluids on the extent of dissolution and precipitation in the sediments. Metal- and glass-free systems were used to conduct batch experiments at 323 K under CO2 and O2 free conditions. Base-induced dissolution of the soil minerals was rapid in the first 48 h as indicated by immediate releases of Si and Fe into the soil solution. Potassium release lagged behind and dissolution of K-bearing minerals (mica and K-feldspar) proceeded faster only after 2 to 3 d of the experiment. Silicon and Fe release exhibited high dependence on aqueous [Al] (rate orders <-1), because Al decreased free OH concentration in the contact solution and probably inhibited soil mineral dissolution. Initial K release exhibited low dependence on [Al] (fractional rate orders). Initial dissolution rates calculated based on Si release varied with aqueous [Al] from 29.47 to 4.35 x 10(-12) mol m(-2) s(-1). Aluminum participated in the formation of the secondary phases (precipitation rates of 10(-8) mol s(-1)) but the overall precipitation rate of alumino-silicate secondary phases was probably controlled by aqueous [Si] (rates of 10(-9), and rate constants between 0.0054 and 0.0084 h(-1)). The changes in the soil solution chemistry (release of K, Si, Fe, and other elements) may play a significant role in the fate of radionuclides and contaminants like Cs, Sr, Cr, and U in the Hanford sediments.

Aluminum effect on dissolution and precipitation under hyperalkaline conditions: II. Solid phase transformations.

The high-level radioactive, Al-rich, concentrated alkaline and saline waste fluids stored in underground tanks have accidentally leaked into the vadose zone at the Hanford Site in Washington State. In addition to dissolution, precipitation is likely to occur when these waste fluids contact the sediments. The objective of this study was to investigate the solid phase transformations caused by dissolution and precipitation in the sediments treated with solutions similar to the waste fluids. Batch experiments at 323 K were conducted in metal- and glass-free systems under CO2 and O2 free conditions. Results from X-ray diffraction (XRD), quantitative X-ray diffraction (QXRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and energy dispersive X-ray fluorescence spectroscopy (EDXRF) indicated that significant solid phase transformations occurred in the sediments contacted with Al-rich, hyperalkaline, and saline solutions. The XRD and QXRD analyses confirmed that smectite and most likely biotite underwent dissolution. The SEM and the qualitative EDS analyses confirmed the formation of alumino-silicates in the groups of cancrinite and probably sodalite. The morphology of the alumino-silicates secondary phases changed in response to changes in the Si/Al aqueous molar ratio. The transformations in the sediments triggered by dissolution (weathering of soil minerals) and precipitation (formation of secondary phases with high specific surface area and probably high sorption capacities) may play a significant role in the immobilization and ultimate fate of radionuclides and contaminants such as Cs, Sr, and U in the Hanford vadose zone.

Europium uptake and partitioning in oat (Avena sativa) roots as studied by laser-induced fluorescence spectroscopy and confocal microscopy profiling technique.

The uptake of Eu3+ by elongating oat roots was studied by fluorescence spectroscopy, fluorescence lifetime measurement, and a laser excitation time-resolved confocal fluorescence profiling technique. The results of this work indicated that initial uptake of Eu3+ was highest within the undifferentiated cells of the root tip just behind the root cap, a region of maximal cell growth and differentiation and with incomplete formation of the Casparian strip around the central vascular cylinder. Distribution of assimilated Eu3+ within the root's differentiation and elongation zone was nonuniform. Higher concentrations of Eu3+ were observed within the vascular cylinder, specifically in the phloem and developing xylem parenchyma. Elevated levels of the metal were also observed in the root hairs of the mature root zone. Fluorescence spectroscopic characteristics of the assimilated Eu3+ suggested that the Eu3+ exists as inner-sphere mononuclear complexes inside the root. This work also demonstrated the effectiveness of a time-resolved Eu3+ fluorescence spectroscopy and confocal fluorescence profiling techniques for the in vivo, real-time study of metal [Eu3+] accumulation by a functioning intact plant root. This approach can prove valuable for basic and applied studies in plant nutrition and environmental uptake of actinide radionuclides.

Effect of coupled dissolution and redox reactions on Cr(VI)aq attenuation during transport in the sediments under hyperalkaline conditions.

Aluminum-rich, hyperalkaline (pH > 13.5) and saline high-level nuclear waste (HLW) fluids at elevated temperatures (>50 degrees C), that possibly contained as much as 0.41 mol L(-1) Cr(VI), accidentally leaked to the sediments at the Hanford Site, WA. These extreme conditions promote base-induced dissolution of soil minerals which may affect Cr(VI)aq mobility. Our objective was to investigate Cr(VI)aq transport in sediments leached with HLW simulants at 50 degrees C, under CO2 and O2 free conditions. Results demonstrated that Cr(VI)aq fate was closely related to dissolution, and Cr(VI)aq mass loss was negligible in the first pore volumes but increased significantly thereafter. Similar to dissolution, Cr(VI)aq attenuation increased with increasing fluid residence time and NaOH concentration but decreased with Al concentrations in the leaching solutions. Aqueous Cr(VI) removal rate half-lives varied from 1.2 to 230 h with the fastest at the highest base concentration, lowest Al concentration, greatest reaction time, and lowest Cr(VI) concentration in the leaching solution. The rate of Cr(VI) removal (normalized to 1 kg of solution) varied from 0.83 x 10(-9) (+/-0.44 x 10(-9)) to 9.16 x 10(-9) (+/-1.10 x 10(-9)) mol s(-1). The predominant mechanism responsible for removing Cr(VI) from the aqueous phase appears to be homogeneous Cr(VI) reduction to Cr(III) by Fe(II) released during mineral dissolution. Cr(VI)aq removal was time-limited probably because it was controlled by the rate of Fe(II) release into the soil solution upon mineral dissolution, which was also a time-limited process, and other processes that may act to lower Fe(II)aq activity.