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Title: Effects of pH, temperature, and aqueous organic material on the dissolution kinetics of meta-autunite minerals, (Na, Ca)2-1[(UO2)(PO4)]2 • 3H2O

Abstract

Autunite minerals have been frequently identified in contaminated sediments as the long-term controlling phase of uranium. Under these conditions the mobility of uranium in subsurface pore waters is limited by the rate of dissolution of autunite and meta-autunite group minerals, [(UO2)(PO4)]2 ? xH2O. Single-pass flow-through (SPFT) tests were conducted to quantify the dissolution kinetics of natural calcium meta-autunite, Ca[(UO2)2(PO4)2]2 ? 3H2O, and synthetic sodium meta-autunite, Na2[(UO2)(PO4)]2 ? 3H2O, as a function of pH (7 -10) and temperature (5 ? 70 C) in the presence and absence of aqueous organic material. The data indicate that release of uranium and phosphorus are non-stoichiometric over the range of experimental conditions investigated. In a 0.1 M NH4OH buffer solution, acquisition of valid dissolution rate data was limited by uramphite solubility, NH4[(UO2)(PO4)]2 ? xH2O. Dissolution rates obtained in a 0.01 M TRIS [tris (hydroxymethyl) aminomethane] buffered solution increased by a factor of {approx}100X over the pH interval of 7 to 10 (? = 0.90?0.08), irrespective of temperature. At constant pH the rate data showed a minor increase with temperature. Data from experiments using a more concentrated 0.05 M TRIS buffer exhibited a {approx}35-fold increase in rates compared to those in a 0.01 M TRIS buffermore » at constant temperature and pH. The difference in rate between interlayer cation (Na+ or Ca2+) and uranium release is {approx}10,000 in neutral solutions; however, the difference diminishes to {approx}10 at higher pH values. The combination of structural dissolution and ion exchange explain these trends in interlayer cation behavior. Data presented here illustrate the significance of pH and dissolved organic material on the dissolution of autunite minerals.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
876829
Report Number(s):
PNNL-SA-42231
Journal ID: ISSN 0003-004X; AMMIAY; KP1301020; TRN: US0601449
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: American Mineralogist; Journal Volume: 91; Journal Issue: 1
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; AUTUNITE; DISSOLUTION; ION EXCHANGE; KINETICS; PH VALUE; SEDIMENTS; SOLUBILITY; URANYL COMPOUNDS; ENVIRONMENTAL TRANSPORT; CALCIUM COMPOUNDS; SODIUM COMPOUNDS; alkaline; dissolution; autunite; uranium; phosphate; SPFT

Citation Formats

Wellman, Dawn M, Icenhower, Jonathan P, Gamerdinger, Amy P, and Forrester, Steven W. Effects of pH, temperature, and aqueous organic material on the dissolution kinetics of meta-autunite minerals, (Na, Ca)2-1[(UO2)(PO4)]2 • 3H2O. United States: N. p., 2006. Web. doi:10.2138/am.2006.1807.
Wellman, Dawn M, Icenhower, Jonathan P, Gamerdinger, Amy P, & Forrester, Steven W. Effects of pH, temperature, and aqueous organic material on the dissolution kinetics of meta-autunite minerals, (Na, Ca)2-1[(UO2)(PO4)]2 • 3H2O. United States. doi:10.2138/am.2006.1807.
Wellman, Dawn M, Icenhower, Jonathan P, Gamerdinger, Amy P, and Forrester, Steven W. Sun . "Effects of pH, temperature, and aqueous organic material on the dissolution kinetics of meta-autunite minerals, (Na, Ca)2-1[(UO2)(PO4)]2 • 3H2O". United States. doi:10.2138/am.2006.1807.
@article{osti_876829,
title = {Effects of pH, temperature, and aqueous organic material on the dissolution kinetics of meta-autunite minerals, (Na, Ca)2-1[(UO2)(PO4)]2 • 3H2O},
author = {Wellman, Dawn M and Icenhower, Jonathan P and Gamerdinger, Amy P and Forrester, Steven W},
abstractNote = {Autunite minerals have been frequently identified in contaminated sediments as the long-term controlling phase of uranium. Under these conditions the mobility of uranium in subsurface pore waters is limited by the rate of dissolution of autunite and meta-autunite group minerals, [(UO2)(PO4)]2 ? xH2O. Single-pass flow-through (SPFT) tests were conducted to quantify the dissolution kinetics of natural calcium meta-autunite, Ca[(UO2)2(PO4)2]2 ? 3H2O, and synthetic sodium meta-autunite, Na2[(UO2)(PO4)]2 ? 3H2O, as a function of pH (7 -10) and temperature (5 ? 70 C) in the presence and absence of aqueous organic material. The data indicate that release of uranium and phosphorus are non-stoichiometric over the range of experimental conditions investigated. In a 0.1 M NH4OH buffer solution, acquisition of valid dissolution rate data was limited by uramphite solubility, NH4[(UO2)(PO4)]2 ? xH2O. Dissolution rates obtained in a 0.01 M TRIS [tris (hydroxymethyl) aminomethane] buffered solution increased by a factor of {approx}100X over the pH interval of 7 to 10 (? = 0.90?0.08), irrespective of temperature. At constant pH the rate data showed a minor increase with temperature. Data from experiments using a more concentrated 0.05 M TRIS buffer exhibited a {approx}35-fold increase in rates compared to those in a 0.01 M TRIS buffer at constant temperature and pH. The difference in rate between interlayer cation (Na+ or Ca2+) and uranium release is {approx}10,000 in neutral solutions; however, the difference diminishes to {approx}10 at higher pH values. The combination of structural dissolution and ion exchange explain these trends in interlayer cation behavior. Data presented here illustrate the significance of pH and dissolved organic material on the dissolution of autunite minerals.},
doi = {10.2138/am.2006.1807},
journal = {American Mineralogist},
number = 1,
volume = 91,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • In-situ precipitation of autunite minerals has been proposed as a method for remediating groundwaters, deep within the subsurface, faced with uranium contamination. Recently, it has been to utilizes long-chain sodium polyphosphate compounds as a ''time-released'' source of phosphate for precipitation of uranium-phosphate minerals. Elevated sodium concentrations presented by this technique enhance formation of sodium autunite relative to the more common calcium autunite mineral phase. The goal of forming autunite minerals in-situ to remediate uranium contaminated groundwater requires a thorough understanding of the properties of sodium autunite minerals to evaluate the longevity and efficacy of a uranium-phosphate barrier. Research presented heremore » is part of a larger effort to quantify the solubility and dissolution properties uranium-phosphate minerals, that may form due to remediation efforts, under environmentally relevant conditions. This paper focuses on the development of a direct synthesis route for precipitating sodium autunite, and a comparative analysis of the structural properties and differences presented in the direct versus previous indirect methods of precipitation using extended X-ray absorption fine structure (EXAFS) spectroscopy, chemical digestion followed by inductively-coupled plasma-optical emission spectroscopy and inductively-coupled plasma-mass spectroscopy for elemental analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and multi point Brunauer-Emmett-Teller (BET) analyses.« less
  • Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO 2 2+) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exert a strong influence on uranium transport. Autunite group minerals, X 3-n (n)+ [(UO 2)(PO 4)] 2 · xH 20, have been identified as the long-term uranium controlling phases in many systems of geochemical interest. Anthropogenic operations related to uranium mining operations have created acidic environments, exposing uranyl phosphate minerals to low pH groundwaters. Investigations regarding the dissolution behavior of autunite group minerals undermore » acidic conditions have not been reported; consequently, knowledge of the longevity of uranium controlling solids is incomplete. The purpose of this investigation was to: 1) quantify the dissolution kinetics of natural calcium and synthetic sodium meta-autunite, Ca[(UO 2) 2(PO 4) 2] · 3H 2O, under acidic conditions, 2) measure the effect of temperature and pH on meta-autunite mineral dissolution, and 3) investigate the formation of secondary uranyl phosphate phases as long-term controls on uranium migration. Single-pass flow-through (SPFT) dissolution tests were conducted over the pH range of 2 to 5 and from 5° to 70°C. Results presented here illustrate meta-autunite dissolution kinetics are strongly dependent on pH, but are relatively insensitive to temperature variations. In addition, the formation of secondary uranyl-phosphate phases such as, uranyl phosphate, (UO 2) 3(PO 4) 2 · x H 2O, may serve as a secondary phase limiting the migration of uranium in the environment.« less
  • To evaluate the release of uranium from natural ore deposits, spent nuclear fuel repositories, and REDOX permeable reactive barriers, knowledge of the fundamental reaction kinetics associated with the dissolution of uranium dioxide is necessary. Dissolution of crystalline uranium (IV) dioxide under environmental conditions has been studied for four decades but a cardinal gap in the published literature is the effect of pH and solution saturation state on UO2(cr) dissolution. To resolve these inconsistencies, UO2 dissolution experiments have been conducted under oxic conditions using the single-pass flow-through system. Experiments were conducted as a function of total dissolved carbonate ([CO3-2]T) from 0.001more » to 0.1 M; pH from 7.5 to 11.1; ratio of flow-through rate (q) to specific surface area (S), constant ionic strength (I) = 0.1 M, and temperatures (T) from 23 to 60 C utilizing both powder and monolithic specimens. The results show that UO2 dissolution varies as a function of the ratio q/S and temperature. At values of log10 q/S > -7.0, UO2 dissolution becomes invariant with respect to q/S, which can be interpreted as evidence for dissolution at the forward rate of reaction. The data collected in these experiments show the rate of UO2 dissolution increased by an order of magnitude with a 30? increase in temperature. The results also show the overall dissolution rate will increase with an increase in pH and decrease as the dissolved uranium concentration approaches saturation with respect to secondary reaction products. Thus, as the value of the reaction quotient, Q, approaches equilibrium, K, (with respect to a potential secondary phase) the dissolution rate decreases. This decrease in dissolution rate was also observed when comparing measured UO2 dissolution rates from static tests where r = 1.7 ?0.14 ? 10-8 mol m 2 s-1 to the rate for flow-through reactors where r = 3.1 ?1.2 ? 10-7 mol m-2 s-1. Thus, using traditional static test methods can result in an underestimation of the true forward rate of UO2(cr) dissolution. These results illustrate the release of uranium from UO2 in the natural environment will be controlled by pH, solution saturation state, and the concentration of dissolved carbonate.« less