ML20027C051
| ML20027C051 | |
| Person / Time | |
|---|---|
| Issue date: | 08/30/1982 |
| From: | Brooks D, Corrado J NRC |
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| ML20027C049 | List: |
| References | |
| REF-WM-1 NUDOCS 8210120353 | |
| Download: ML20027C051 (38) | |
Text
s 9
MAFTl:
I DRAFT TECHNICAL POSITION ON RADIONUCLIDE SPECIATION AND SOLUBILITY DETERMINATIONS m
WPt _1 PDR s
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ABSTRACT Construction authorization and the licensing of a high-level waste repository involve assessments of~the rate of nuclide migration from the repository and the accumulation of radionuclides at the accessible environment.
Knowledge of the nature and solubility of radionuclide compounds likely to form under existing groundwater conditions at a repository site, and under conditions of elevated temperature, is essential for 4
assessing radionuclide release rates to the accessible environment. To obtain this information, it will be necessary to identify precipitates that form under the expected groundwater conditions, and either to measure their solubilities or demonstrate that sufficient verified thermochemical data are available to confidently calculate the information. This document provides guidance for the documentation of radionuclide solubilities which might be used in support of site characterization, construction authori. tion and licensing of a high level waste repository.
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NOTICE
- This document is being issued in order to provide guidance that the NRC believes should be followed to partially meet the requirements of 10 CFR 60. This document is not a substitute for the regulations, and compliance is_ not a requirement.
However, an approach or method different from the guidance contained herein will be accepted only if the substitute approach or method provides a basis for determining that the above cited regulatory requirements have been met to identify elements of an acceptable program of data acquisition and quality assurance in the areas of radionut:ide speciation and solubility.
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' ACKNOWLEDGE 11ENT i
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r Section 2.4 of this document is modified from a~ draft report by Lawrence Berkeley Laboratories under'NRC Technical Assistance Contract B-3109 (Task 1), Geotechnical Sciences i
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Table of Contents Page
1.0 INTRODUCTION
6.-12
1.1 BACKGROUND
6 1.2 REGULATORY FRAMEWORK - - - - -
- - - ~ ~ ~ 9 2.0 DISCUSSION
- - - - - - - - - - - - - - - - - - - - - - - - - - - 1 3 -3 3 2.1 G EN E RAL - - - - - - - - - - - - - - - - - - - - - - - - - -
- 13 2.2 I S S U ES - - - - - - - - - - - - - - - - - - - - - - - 14 2.2.1 Speciation
15 2.2.2 S o l u b i l i ty - - - - - - - - - - - - - - - - -
4 15 2.3.
INFORMATION NEEDS 16 2.3.1 Basic Elements of Data Acquisition Program -- - - - - 16 2.4 METHODS - ----
.17 2.4.1 Approach for Characterization of Groundwater _-- -- - - -17 2.4.1.1 Chemical Methods for Determining Groundwater
- - - 19 Composition 2.4.1.2 Redox Measurements - - - -- - - -- - - - - 21 2.4.2 Approach for Characterization of Solubility - - -- - 24 2.4.2.1 Solubility Measurements - - - -
28 2.4.2.1.1 Separation of Solid and Solution Phases -- - '29 2.4.2.1.2 Characterization of Solid Phase -- - - - '
2.4.2.1.3 Characterization of Aqueous Phase-- - - -
32 3.0 '
SUMMARY
AND CONCLUSION - -
33 - 34 4.0. REFERENCES 35 - 38
e, 6
1.0 -INTRODUCTION
1.1 BACKGROUND
The long-term performance of a high-level waste repository involves geochemical interactions within the. geologic repository candidate area among the waste form, bi2ckfili, host rock, and groundwater.
The rate at which radionuclides may'be transported to the accessible environment is determined by:
1) the solubility of each radionuclide present in the waste package (i.e., initial concentration of each radionuclide insthegroundwater),
2) the retardation (reactions of the radionuclides with -
minerals in the backfill and in fractures in the host.
rock),and f
L 3) the rate and pa h of gr'ouhdwater movement! _ '.
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Predicting radionuclide transport through geologic systems is,
,e complicated by the heterogeneous and complex nature of.- >
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7 hy Many radionuclides form insoluble compounds as well as solutinn complexes with components of groundwaters. Therefore, precipitation of stable solid phases could lower the concentration of radionuclides in the groundwater. Conversely, the formation of aqueous complexes could tend to increase the radionuclide concentration in groundwater. Thus, knowledge of the solubility-(item 1, page 6, section 1.1., paragraph 1) of waste form JClide Compounds should be determined in order to assess the amounts and release rate of radionuclides from an underground facility. Also, while solubilities may vary with specific conditions at a site, the underlying reactions will be operative and x
s at any given time which makes these data important for performance assessmentsr Finally, knowledge of solution species and solubility limits are..needed to define the sorptive properties (item 2, page 6, section 1.1, paragraph 1) of the host rock and engineered barriers.
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Geochemical' interactions that control the residence' times of radionuclides in the aqueous phase from the waste canister to the
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. accessible'snvironment include at least the following reaction
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. Precipitation / dissolution (solubility) 7
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Diffusion (including. dead-endporeandmatrix)
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>iEach lof these mechanisms contributes a component of retardation to t^l the overall chemical retardation of waste nuclides from the waste' package to the accessible environment, and in order to characterize retardation'erch of these mechanisms must be evaluated.- The degree
'to which each reaction / mechanism is studied in characterising
-retardation' depends upon'its relative contribution to the-retardation process.
i.
This document is restricted to defining the basic elements of an adequate and acceptable program for determining radionuclide species and solubilities (item'1,'page 7, section 1.1, paragraph 1).
It addresses any and all'pDssible radi,onuclide species (solutes) and compounds-(precipitates) that might exist within expected ranges of.
repository conditions between the outer edge of the waste form
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1 1.2 REGULATORY FRAMEWORK DOE / EPA /NRC/ Responsibilities The Department of Energy (D0E) has the responsibility for finding and implementing a solution to the high-level waste disposal problem by developing sites into working repositories for the-disposal of high-level waste. Also, DOE is responsible for designing, constructing, and operating the disposal facility. This includes conducting technical development and site characterization programs in geochemistry.
Other federal agencies that have statutory responsibilities are the Environmental Protection Agency (EPA) and the Nuclear Regulatory Commission (NRC).
EPA's responsiblity is to set overall radiological standards for the release of radionuclides into the environment. The NRC is respor,sible for licensing a repository in a way that assures public health and safety.
Site characterization, construction authorization and licensing will essentially involve demonstrating in a rigorous fashion, short and-long-term compliance with an EPA standard (which is currently being drafted). NRC will assess DOE's siting proposals and major aspects of their design.
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10 Solubility and speciation are related to the following sections of 1
a)
Section 60.11(a) requires assessment of the site characterization program with respect to investigation activities which address the ability of the site to host a repository and isolate radioactive waste; b)
Section 60.21(c) requires that the Safety Analysis Report contain an analysis of the geochemical aspects of the site which bear significantly on its suitability for disposal of radioactive waste; c)
Section 60.21(a) indicates that the Commission must determine whether the DOE had adequately described the 1
On July 8,1981, NRC published proposed technical criteria, and other conforming provisions, for incorporation into 10 CFR Part 60 (46 FE 35280). The adoption of these provisions as final rules was assumed-fcr purposes of preparing this guide.
The guide will be modified, as appropriate, to take into account any changes that may i.
be made in the final technical creteria.
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geochemical characteristics of the proposed site prior to f
construction authorization; f
i d). Section 60.111(b) requires that the geologic repository and each of its components satisfy specific requirements ~
I related to radionuclide release rates;
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Section 60.122 specifies geochemical con'ditions at the i
I site which may be considered favorable in their effects on t
the ability of the site to meet performance objectives; i
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Section 60,123 specifies geochemical conditions which i
would have potentially adverse affects on the ability of the site to meet performance objectives; and d
d' g)
Section 60,132 specifies additional design requirements.
' fo'r the urderground facility which will provide control of I
I radionuclide releases and migration.
- i The amount'of geochemical'research that can be done in support of
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b high-level radioactive waste isolation is enormous. However,-the.
- i work becomes'nore tractable if research and experiments such_ as.
those concerning solubility.and speciation~are bounded and conducted withinLexpected site-specific ranges of chemical / environmental 4^
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-cond tions. -Therefore it is essential that there be agreement concerning:
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ISSUES What are the specific geochemical issues that are important in terms of overall repository performance?
2)
INFORMATION NEEDS What geochemical mechanisms must-be considered in the licensing process and what level of certainty can be placed on them to retard radionuclide migration?
3)
METHODS I
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What tests, methods, and investigative strategies will be sufficient and appropriate in gathering and interpreting geochemical data?
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13 2.0 DISCUSSI0fl 2.1 GENERAL After breach of waste containment radionuclides will enter the local groundwater system. The radionuclides will react with various components of the groundwater, and possibly the host rock, to form relatively insoluble compounds and solution species which can provide major controls on the solution concentrations and migration rates of the radionuclides. For this reason, knowledge of speciation of the radionuclides and the solubilities of compounds which they may form is of primary importance in characterizing
- geochemical retardation.
In order to assess radionuclide speciation, it is necessary to identify 1) the source radionuclides and 2) the ligands (inorganic and organic) in the groundwater.
Identification of~ ligands can be carried out through an adequate plan for characterization of groundwater (including the groundwater prior to construction and waste emplacement, the likely groundwater that will eventually come in contact with the waste package, and the likely groundwater that will exist after contact and dissolution.of the waste package).
An assessment of reactions between source radionuclides and groundwater constituents makes possible anticipation of all the
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14 possible-species. The species which must be considered can be limited by the use of predominance diagram codes which predict which species will dominate in a particular groundwater system. A program for determining redionuclide speciation must conclude with an adequate plan for quality assurance.
The elements of an acceptable program for acquisition of data on the solubility of radionuclide species must involve the compilation of solubility product data for dominant species (e.g., Rai, Serne, fdi al-1980, Rai, Strickert di al-1981 ).
Assessment of solubility f
limits for dominant species should be undertaken for the expected range in physical and chemical conditions (e.g., Wood and Rai-1980, Deutsch di al-1981, Rai and Serne-1978). Also, solubility data 3
acquisition should include an adequate plan for quality assurance as well as laboratory validation of solubility limit data (including technology for characterization of the solid phase, separation of solid and solution phases, and analysis of the aqueous phase).
2.2 ISSUES The central issue in.this discussion is.which radionuclide srecies will accumulate at the accessible environment that yield elemental concentrations which exceed the EPA (or other adopted) standards.
i-15 2.2.1 Speciation
.There exist several possibilities for radionuclides entering the groundwater system for the first time following -loss of containment. The' radionuclides may enter the groundwater system in elemental form or they may form complexes or "speciate" with ions present in the waste form, in the waste canister, or in the groundwater and the host rock.
2.2.2 Solubility Concentrations of radionuclide species in solution can increase until they reach an upper limit, "a solubility limit." Above-this concentration, a radionuclide species begins to form a precipitate so that further solution concentration increases do not. occur (provided that the rate of precipitation is sufficiently rapid).
In this way, solubility of particular-species provides controls on solution concentrations and migration rates of radionuclides.
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16 2.3 INFORMATION NEEDS 2.3.1
. Basic elements of data acquisition program _
In order to identify radionuclide species, it is necessary to know which nuclides are part of the waste inventory and what ligands (inorganic and organic) are in tiie groundwater. The latter requirement can be fulfilled by carrying out an adequate plan for charact'erization of groundwater that will include technically acceptable methods for sampling groundwater, chemical analyses, redox couple measurements, data reduction, statistical analyses, and interpretation.
Reactions may be written for speciation of radioelements at appropriate valence states with available ligands. The number of anticipated species may be limited by identifying dominant species using an Eh-pH dominance diagram computer code (such as SOLUPLOT).
In the assessment of solubility for dominant species, the effect of changing pH, Eh, temperatures, chemical and mineralogical regimes that extend from the waste canister to
.the accessible environment, must be considered. Thermodynamic
. calculations using solubility product and equilibrium. constant data can be used to determine the solubilities of dominant species of radioelements under a range of conditions.
Laboratory validation of solubility. data should include.the
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1 17 ll interlaboratory comparison of data using technically acceptable methods for characterization of the solid phase, separation of solid and solution phases, and analysis of the aqueous phase.
Therefore, basic elements of an acceptable program for generating speciation and solubility data will include the characterization of initial groundwater conditions followed by interlaboratory comparisons of groundwater chemistry and t
determination of expected groundwater chemistry conditions, determination of speciation and solubility, and assessment of speciation and solubility data through peer review and validated computer models.
2.4 METHODS 2.4.1 Approach For Characterization Of Groundwater In order to predict the nature of possible solution species and precipitates of radionuclides likely to form in ~groundwaters, it is necessary to know the chemical composition, pH and redox properties of representative and uncontaminated groundwater samples. A detailed chemical analysis of major and trace components is necessary. An analysis of particulates in groundwater must also be made to ensure that radionuclides will not be transported by particulates present in the water.
18 Because many radionuclide metal ions fom insoluble compounds and solution complexes with the anions of typical groundwater (for example, hydroxide, sulphide, sulphate, carbonate, phosphate, silicate, chloride, flouride and nitrate anions),
the identities and concentrations of groundwater anions are needed. The radionuclide anions selenide and iodide can form insoluble compounds and aqueous complexes with several of the heavy metals (e.g., Fe, Mn, Ni, Co, Cu, Ag, Hg, and Pb) found at trace levels in groundwaters. Therefore, trace element concentrations should be determined.
In addition, mixed compounds containing radionuclides and major groundwater cations may form, e.g. (Ca (U0 )2 (H SiO )2); thus 2
2 4
concentration of the major cations should be measured.
The oxidation states of the waste radionuclides will determine the nature of the precipitates likely to form and the solubilities of these compounds will in part determine solution concentrations.
In order to predict these oxidation states, knowledge of the redox properties of the groundwater / host rock system is required. The pH must be known because solubility and adsorption are functions of pH.
Experiments to detemine the likely pH in the thermally disturbed zone should also be undertaken.
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19 A complete characterization of groundwater will be required.
Sampling and analyses should be coordinated to meet the data needs of both the hydrological and the geochemical issues being i
addressed at a site.
i Also, if compositionally different groundwaters are to be present in the repository and/or within the waste package (backfill / canister / waste form) due to chemical changes induced by the engineered system at the time of loss of containment (canister integrity), then these waters will have to be taken into account when determing source-term speciation/ solubility.
2.4.1.1 Chemical Methods for Determining Groundwater Composition A number of analytical methods are available for determining the concentration of major and trace elements, as well as anions, in water samples. They include neutron activation analysis, fluoromet"y, emission spectroscopy (particularly inductively coupled plasma), atomic absorption spectroscopy (both flame and non-flame), atomic fluorescence spectroscopy, electrometry, and ion-exchange chronatography. Since the utility, detection limits and reliability of the various methods differ for different elements, no single method can be recommended for a complete chemical analysis of a water sample.
The choice of methods usually depends on the instruments that 9;
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20 are available and on the components to be measured. The analytical methods listed above are well established and the procedures, advantages, limitations and precautions have been
-discussed in a number of publications (e.g., Winefordner,1976; Pinta,1978; Wood 1976;1970;Ellis1968).
Precision and sensitivities of analytical techniques have increased to such an extent in recent years that the weakest links in the analysis are the procurement of uncontaminated samples, and the subsequent handling and preparation of samples. Therefore, certain precautions should be observed and certain procedures followed to insure reliable results (e.g.,
3 Wood, 1976; Brown, 1970; Ellis 1968).
A sufficient number of replicate samples should be analyzed to establish the consistency of sample preparation and the i
accuracy of measurements. Because discrepancies between different analytical methods for some elements at low concentrations arise from differences in sample preparation techniques, it is desirable to analyze for some elements by two or more alternative methods..
Checks should be made to test the internal consistency of the analyses. These checks should include tests of chenical and electrical balance, reconciliation of alkalinityititrations E
21 with C0 measurements and pH, and reconciliation of solution 2
composition with the nature of the contacting geologic media.
U However, these tests provide only approximate indications of the validity of the analyses.
Large deviations can indicate a large error in one or more of the determinations, but consistency is not conclusive proof that each determination is accurate.
2.4.1.2 Redox Measurements An important parameter which must be considered in the study of multivalent radionuclides, e.g., Tc, U, Np, and Pu, is the oxidation-reduction potential (Eh) of a groundwater.
Eh controls the chemical reactions in which elements undergo a loss or gain of orbital electrons and thus change their valence state. Eh may be considered to be a direct indicator of the redox state of a rock-water system and to be the master variable analogous to pH. However, the ability of Eh measurements to accurately describe the redox properties of natural waters has been questioned. -(Thorstenson,1982; Morris,1967;Stumm,1966). Oxidation-reduction related reacticns in natural systems are frequently irreversible and not at equilibrium, so that measured Eh is-often a mixed potential resulting from several redox reactions which may not readily couple with each-other. A meaningful Eh cannot be I
22 defined for such nonequilibrium systems. However, although true equilibrium may not be achieved, partial equilibrium may be approached in many cases, and a metastable system may exist for which a working redox potential can be related to certain redox couples.
The Eh of water samples is frequently measured with a noble metal (usually platinum) electrode and a reference electrode system using a sensitive voltmeter. _This type of electrode measurement of Eh for geologic purposes is beset with a number of experimental problems and limitations.(e.g...Langmuir,1971; Stumm1970;Benson,1980; Morris,1967). Direct measurement of
- Eh values for natural waters involves complex theoretical and practical problems in spite of the apparent simplicity of the electrochemical techniques. Detailed quantitative interpretation is unjustified in most cases. The results of r
most experiments with electrode measurements of Eh values described in the literature lead to the conclusion that the measurement provides at best, qualitative results (e.g., Wook, 1967; Morris, 1967;'Stumm, 1966; Thorstenson,'1979and 1982).
Therefore, Eh measurements by electrodes alone should not, in general, be relied upon to predict the oxidation states of radionuclides that can exhibit multiple valence states under groundwater conditions.
23 In theory it is possible to evaluate the redox potential in natural waters by determining the relative concentrations of members of some or all of the redox couples in the system.
Normally, on13 a few elements are major participants in redox reactions in r.atural waters, i.e., C, 0, N, S, Fe, and Mn.
The potential generated by a couple is related to the activites of the components through the Nernst equation, i
o, Rj".
(exidi:ed state) nF (recucec statal where E is the standard potential of the half-cell reaction, R is the gas constant, T is the absolute temperature, F is the Faraday constant and N is the number of electrons involved in the half-cell reaction.
After measuring the concentrations of appropriate components of the couples that are controlling the solution redox properties, e.g.,
en4-cO, H O - 0, NHk - NO,
2 2
2 3
2
+
4*
s: SO, Fe Fe
, and Mn I Mn an Eh value can 4
be calculated for each couple.
Thus, Eh values calculated from the concentrations of the components of the major couples can be used estimate the redox level of water samples and for the identification of the tantrolling redox reactions.
- However,
-the reliability of this method depends on the particular system and will require verification.
Finally these methods for determining the redox potential of groundwater are not sufficiently reliable and accurate for the
24 prediction of the oxidation states of multivalent radionuclides in the groundwater system. Therefore, it may be necessary to determine the oxidation states by direct measurement, either in situ or in very closely related laboratory experiments. This process can be difficult and time-consuming but there may be no viable alternative. However, for the initial characterization f
an attempt should be made to obtain the necessary data on the groundwater composition to calculate Eh values for the major redox couples. If these values are reasonably self consistent, it may only be necessary to verify the results by direct measurement of the oxidation states of a few selected multivalent radienuclides.
2.4.2 Apprcach For Characterization of Solubility Two different approaches can be taken for determining the effect of formation of insoluble compounds on solution concentrations of radionuclides. One method is to calculate solution concentrations from available thermochemical data on solubility product constants and solution complexation constants. The other method is direct measurement.
In order to predict solution concentrations of radionuclides in a repository / groundwater system, the identities and solubilities of the solid phases and identities of the solution
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25 species likely to form under specific groundwater / geologic conditions are needed.
Solubility product constants alone are
)
usually not sufficient to predict solution concentrations because the formation of hydrolysis products and other complexes increases apparent solubilities.
For example, one can predict the concentration of UO + in solution in 2
equilibrium with solid UO2 (OH)2 H O from the solubility 2
- constant, UO[+HO
' UO (OH) 4 H
=
2 2
2 UO +,2H c
^
Cc (OH)
+ 2H 2
2 2U0,2+.'2H O (UO ) (OH)'* + 2H
^
=
2 2
3U0 +
4"2U IUU 3 IU"
- 4" 23 2
300 + + 5H o
^
(UO )3(OH)5
+ SH
=
y 2
2 3U0
- 78 0 IUU 3 IU"'7 2
2 23 IUU I IU"I +
- 7" 4UO
+ 7H O 2
24 7
All of these equations must be solved simultaneously in order to calculate the concentrations of the individual solution species since they can occur together in solution.
In addition, if other solution complexes are present they must be included.
For example, if carbonate is.present in the uranium system, reactions involing the carbonate complexes of uranyl must be included, i.e.,
UOj, + CO'_
UO CQ
=
g 2 3 2
UO,++2cO;= UO(c3)$~
2 UOj+-2cOj= UO (cO 3 4-2 3
T
s 26 The formation of insoluble phases and their equilibria with solution species are much more complicated processes in natural systems than in the homogeneous solutions normally encountered in laboratory measurements. Since the waste radionuclides would most likely be present at low concentrations in the contaminated groundwaters it is reasonable to expect that they would behave in a manner similar to other trace elements.
Trace elements usually occur in nature as mixed cationic compounds, as solid solutions with mineral phases formed by more abundant elements, and as co-precipitates with the precipitates formed by more abundant elements. Unfortunately, complete thermodynamic information on such reactions does not exist for any of the waste radionuclides in natural systems.
Therefore, it is not possible at present to calculate the course of such complicated reactions for these radionuclides.
t In situ experiments with the waste nuclides to obtain the necessary thermodynamic data would be extremely difficult to i
carry out.
Therefore, it is highly unlikely that the necessary thermodynamic information for waste radionuclides can or will become available in the near future. What can be done more readily is to consider the solubilities of simple or pure compounds of the waste elements that can form with groundwater components under conditions of temperature, Eh and pressure
27 P
characteristic of the repository environment. This is a reasonable approach because from thermodynamic considerations, compounds or minerals of higher free energy tend to convert to compounds or minerals of lower free energy, i.e., solid phases i
will continuously change to phases having lower solubilities.
Thus, in principle, upper limits of radionuclide concentrations in solution could be estimated.
The second approach for determining the effects of the formation of insoluble compounds on solution concentrations of i
radionuclides is direct measurement of the solution concentrations as functions of the initial radionuclide concentration over the range of solution compositions, temperature, Eh, pH and other parameters expected for the repository groundwater. The pro!'aio aitn this approach is that fundamental thermodynamic data would not be obtained to extrapolate to possible future conditions of the repository system beyond the range of parameters studied. However, if available thermodynamic data were used to predict solubilities and the direct measurement were used to test these predictions, then the careful characterization of the solid phases and solution' species produced in such solubility measurements would greatly enhance the utility of the information.
28 2.4.2.1 Solubility Measurement Methods Measurement of the solubility of a compound in aqueous solution involves basically the following steps: formation or preparation of the solid phase, characterization of the solid phase, separation of the solid and aqueous phases and analysis of the aqueous phase for the dissolved species. The most commonly used method for determining solubility is first to prepare the compound by some standard pr'ocedure. The material is characterized and then an excess of the solid is placed in i
contact with an aqueous solution of the appropriate composition. Some of the solid phases dissolve and the system is allowed to come to equilibrium before the aqueous phase is analyzed for the concentration of the element of interest and the precipitating counter-ion.
From the concentrations, a solubility product can be calculated.
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A second, but less commonly used method involves the preparation of an aqueous solution containing both the element of interest and the precipitating counter-ion at concentrations j
that produce a supersaturated solution with respect to precipitation of.the compound. There may or may not be pre-prepared solid phases present. Again, the system is allowed to come to equilibrium and the aqueous phase analyzed.
29 For reliable results, it should be demonstrated that equilibrium has been achieved in solubility measurments. The most rigorous method of demonstrating that a solubility experiment has reached equilibrium is to approach equilibrium from both undersaturated and supersaturated conditions, i.e., a combination of the two methods discussed above.
2.4.2.1.1 Separation of Solid and Solution Phases l
The compounds of many of the long-lived radionuclides that are likely to form in natural systems are very insoluble and very low solution concentrations of the radionuclides would be expected, e.g., 10-8 to 10-12M. However, these radionuclides can form colloidal suspensions which, if included in the analysis of the solution phase could lead to large errors in-the solubility measurement. The separation of solid and solution phases is an important step in the analysis.
Techniques often employed for separating the solution and solid phases in solubility studies include (1) gravitational settling, (2) centrifugation, (3) filtration, or a combination of all three methods.
In the first nethod the solid phase is simply allowed to. settle for an extended period of time before
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a portion of the aqueous phase is withdrawn for analysis.
This-method could allow suspended or colloidal material to be
30 withdrawn as well. Contrifugation induces and enhances gravitational settling.
However there is little information on the minimum time or revolution speed needed to achieve adequate separation. Filtration can be used as a final or single separation step. The use of two or three filters with decreasing pore sizes in the range of 0.4 to 0.15 micron to l
filter the same sample would be needed and care should be taken since there is some evidence that that filters themselves may l
at times adsorb soluble species from solution and that i
l different materials and different filter constructions behave differently in this respect (Polansky,1977).
- Finally, i
whatever method or combination of methods is used, verification of the effectiveness of the separation will be needed.
2.4.2.1.2 Characterization of Solid Phase Characterization of the solid phase used in making a solubility 1
determination is of utmost importance. When using a compound 1
prepared under one set of solution conditions to measure solubility under a different set of solution conditions, it cannot be automatically assumed that the prepared compound is the controlling solid phase without confirmation.
In complex aqueous solutions such as groundwaters, a second, more stable solid phase may form that could control the solubility of the element of interest. Also, when preparing a sparingly soluble -
31 compound according to an established procedure, the exact composition and structure of the solid will vary depending on a number of factors, including kinetics, temperature, solution concentrations and the age of the precipitate. Furthermore,
the solid phases may change from an " active" to an " inactive" form during the course of the experiment, i.e., a free energy change is associated with the conversion of an amorphous or finely divided crystal precipitate into a well crystallized solid.
The active form of a precipitate consists of very fine crystals with disordered crystal lattices. Such a precipitate may persist in metastable equilibrium with the solution and may be converted only slowly into a more stable and inactive form (Stumm, 1970; Feitknecht, 1963). Measurements of the solubility of " active" forms give solubility products that are higher than those of the inactive forms (Zimmerman,1952; Baes, 1976), and provide data relevant to a worst-case analysis.
Finally, the particle size of the solid phase is a factor in determining.the solubility of a compound. The equilibrium concentrations of dissolved products decrease as the average I
particle size increases so special precautions on this point are sometimes needed (Zimmerman,1952; Baes,1976).
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a
-=
tc 32 X-ray diffraction is recommended for the characterization of the solid. Crystal structure and stoichiometry can usually be determined by this method if a sufficiently large single crystal is available.
Frequently, only a finely divided crystalline precipitate is available and characterization is
- made by comparison of powder patterns with those of known compounds of chemically related elements. Through this method, conclusions can be drawn as to the chemical activity or degree of_ crystallization of the precipitate from broadening of the lines-(Feitknecht,1963). Amorphous precipitates cannot be analyzed by these means and characterization must be made through elemental analysis of the solid. While this type of analysis often can 1(Intify the~ nature and stoichiometry of the solid phase, it provides no structural information and, thus, no information on the active form of the solid.
Elemental e
. analysis usually requires milligram amounts of matcrial that 1
1 may not be available and techniques for the characterization of s
small amounts of precipitates absorbed on surfaces, as might be
- the-case in natural systems, are not gc.;erally available.
V
. 2.4.2.1.3 Characterization _of the Aqueous Phase A number of analytical methods are available. for determining-i the concentrations of-the. dissolved species. The 'particular method to be'used depends on the element and its expected
"{-
4 e
n
,,--m
-,,n
--+ < - - -
33 concentration. Since the radionuclide concentrations may be l
quite low, rather sensitive methods need to be employed.
In the past, the most comonly used method included radiochemical techniques, potentiometry, polarography, colorimetry, and atomic absorption spectroscopy. Generally speaking, these methods are adequate for concentrations in the range of parts per million. More recently, neutron activation analysis, emission spectroscopy and flourescence spectroscopy have lowered the sensitivity range to parts per billion. All of these methods are well established and their advantages and limitations have been discussed in detail (Winefordner, 1976:
Pinta,1978).
3.0
SUMMARY
AND CONCLUSION
[
]
The rate at which radionuclides are transported to the accessible environment will be determined by their solubility (ccncentration in groundwater), the rate and path of groundwater movement and the reactions (retardation) of the radionuclides with minerals in the backfill and in fractures in the host rock. Theoretical analyses of likely solution species and solubilities (under reducing conditions) suggest that relevant radionuclides are likely to be in solution at levels that will control their release and concentration at the
. accessible environment to' permissible levels established by the Environmental Protection Agency. Therefore, the determination of f.
34 t
I "
radionuclide aqueous speciation,'radionuclit.e solubilities and supporting thermodynamic data, is a logical first step in assessing the importance of subsequent geochemica'i. interactions for' controlling radionuclide migration, The basic elements of a program for determkning'radionuclide.
speciation and solubility limits include (1) groundwater characterization and (2) the determination of radionucliike-solubilities. An adequate plan for characterization of groundwater includes technically acceptable methods for groundwater sampling, chemical analyses, redox couple measurements, data reduction, statistical analyses, and interpretation.
Subsequent elements of a program for determining radionuclide speciation include describing reactions for speciation of radionuclides at appropriate valence states with available ligands and limiting th'e number of anticipated species that must be considered. A possible approach for this would involve the use of Eh-pH dominance diagram computer ~ codes. The basic elements of a program for determining the solubility of
- radionuclides consist of compilation of thermodynamic data for dominant species, assessment of solubility limits for dominant species, and laboratory validation of solubility-limit data (including technology. for characterization of the solid phase, speciation of solid and solution phase, and analysis of the aqueous phase).
q' l;[ i f
n h
s 3,
,.v;;..
4.0 REFERENCES
- j-(;4 j
Benson, L.V., Carnahan, C.L. and Che, M. (1980) A study of Rock-Water-Nuclear Waste Interactions in the Pasco Basin, 1
t[~
,. ashington,.Part II. Preliminary equilibrium - step simulations of W
v basalt diagenesis: LBL-9677, Lawrence Berkeley Laboratory, Berkeley, California.
Brown,'R., Skoug Stad, M.W., and Fishman, M.J., (1970) Methods for Collection and Analysis of Water Samples for Dissolved Minerals and Gases, in Techniques of Water Resource Investigations of the U.S.
Geological Survey, Book 5. Chapt. A-1, U. S. Geological Survey, Reston, VA.
Deutsch,W.J.,Jenne,E.A.,andKrupka,K.M.,(1981) Solubility Equilibria in Basalt Aquifers: The Columbia Plateau, Eastern Washington, Drart PNL Report, 30 pp.
Ellis, A.J., Mahon, W.A.J., and Richie, J. A., (1968) Methods of Collection and analysis of geothermal fluids, 2nd Ed. Chem. Div.
Dept. of Scientific and Industrial Res., New Zealand, Report No. CD 2103, 51_pp.
_f'
.w.,
36
~
Langmuir, D. (1971) Eh-pH determinations, Procedures in Sed. Pet.,
R.W. Carnen, ed., Chapt. 26, John Wiley and Sons, New York pp.
597-634.
Morris, J.C., and Stumm, W. (196?) Redox equilibrium and
-measurements of potentials in the aquatic environment, equililbrium l
concepts in natural water systems, Advan. Chem. Series, 67, p.
270-285.
Pinta, M. (1978) Modern Methods for trace element analysis, Ann Arbor Science, Ann Arbor, Michagan, 492 pp.
Rai, D., and Serne, R.J.,(1978), Solid Phases and Solution Species of Different Elements in Geologic Environments, PNL-2651, 129 pp.
Rai, D., and Serne, R.J., and Swanson, J.L., (1980), Solution Species of Plutonium in the Environment, in J. Environ. Qual., Vol.
9, no. 3, pp. 417-420.
' Rai, D., Strickert, R.G., Moore, D. A., and Serne, R.J.. (1981),
Influence of An Americium Solid Phase on Americium Concentrations In Solutions, in Geochim. et Cosmochim. Acta Vol. 45, pp. 2257-2265.
Stumm, W. (1966) Redox potential as an environmental parameter; concepted significance.and operational limitation, 3rd Int. Conf. on I
.a.
_a_________-.._
37 Water Pollution Research, Sec 1, No.13, Water Pollution Control
' Federation, Washington, DC., p 1-16.
. Stumm.W., Morgan J.J., (1970) Aquatic Chemistry by, Wiley -
Interscience, New York, 583 pp.
Thorstenson, D.C. (1982) The Concept of Electron Activity and the Redox Potential of Aqueous Solutions, ACS/ Division of Nuclear Chemistry and Technology.
Thorstenson, D.C. and Fisher D.W., (1979) The geochemistry of the Fox Hills - Basal Hill Creek aquifer in southern North Dakota and northwestern South Dakota, Water Resources Research, Vol. 15, No. 6, pp. 1479-1498, Winefordner, J.D. (1976) Trace analysis, spectroscopic methods for elements, Vol. 46, John Wiley and Sons, New York, 484 pp.
t Wood, B.J., and Rai, D., (1980), Nuclear Waste Isolation:~ Actinide.
Containment in Geologic Repositories, PNL Draft Report, 31 pp.
Wood, B.J, and D. Rai, (1981)' Nuclear Waste Isolation: Actinide Containment In Geologic Repositories.
9
- ^
.-_m.____.__.___
1
'ic 38 Wood, M.I. (1980) Groundwater geochemistry and interaction with basalt at Hanford, ONWI-212, Office of Nuclear Waste Isolation, Columbus, Ohio.
Wood, W.W. (1976) Guidelines for collection and field analysis of
- groundwater samples for selected unstable constituents, Techniques of Water-Resources Investigations of the U.S. Geological Survey, Book 2, U.S. Geological Survey, Reston, VA.
i d
0 4
t m.,-
m-,..
.