The NEA Sorption Projecta multinational cooperative program to advance

the use of Thermodynamic Sorption Models

Mark Fuhrmann

U.S. NRC

Office of Nuclear Regulatory Research

Background•Adsorption is a primary process that controls concentrations of trace elements in groundwater.

•Adsorption is accumulation of ions from solution onto surfaces of minerals or onto/into organic matter.

•It is due to the electric charge on solid surfaces attracting oppositely charged ions in solution.

•Minerals have different abilities to adsorb elements.

•Ions from solution attach to surfaces by forming several types of bonds with atoms on the solid.

Contaminants in/on Solids

(Charlet and Manceau, 1993)

Definition of Kd

• Adsorption is quantified as the Partition Coefficient (Kd). For contaminant transport: •Kd = [x] on solid phase / [x] in liquid phase

• Units of mL/g or L/kg

•This relationship defines how much a contaminant moves relative to the flow of water. Rf = 1 +(rb / Φ) Kd

Where: rb = bulk density of soil (g/cc) (e.g. 1.2 – 2.0)Φ = bulk porosity (e.g. 0.30)Rf = retardation coefficient

Kd = 0

Kd = 1

Kd = 10

From: Freeze and Cherry

Effect of Adsorption on Contaminant Transport

Kd can be determined experimentally

ASTM C-1733-10 Distribution Coefficients of Inorganic Species by the Batch Method

HOWEVER! Kd can change with many factors: •Water chemistry, especially pH•Mineral surfaces

Subtle changes in chemistry may resultin big changes in sorption.

0.0

20.0

40.0

60.0

80.0

100.0

120.0

3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

pH

U s

orp

tion

(%

)

Uranium on ferrihydrite, Payne 1999

Thermodynamic Sorption Models (TSMs)

•TSMs are a developing technique that is an alternative to experimentally defined Kd

•Treat adsorption like aqueous speciation and can describe adsorption under variable chemical conditions >SOH0 + M2+ ↔ SOM+ + H+

•A TSM coupled within a reactive transport model can be used to simulate the spatial and temporal character of the Kd distribution that is due to evolving chemical conditions.

Thermodynamic Sorption Models require

•Assumption of chemical equilibrium•Definition of the chemical components•Identification of all species that can be formed•Mass balance •Mass Action Law for each species•Thermodynamic Data Base•Activity Coefficient calculations

Requires thermodynamic data specific to the system. Parameter acquisition is needed.

Objective of the NEA Sorption Project

To clarify and demonstrate how TSMs canserve to improve confidence in Kd values

used to represent radionuclide sorption inPerformance Assessment models.

Project had three phases

1. 1997-1999 illustrated advances and diversity of TSMs

2. International comparative modeling exercise of 7 well characterized Test Cases. 20 teams participated. Result was a critical evaluation of modeling approaches.

3. “ A Guideline Document”

NEA Sorption Project phase II

•Teams chose many combinations of modeling approaches:

• Model designs (1 or 2 site surfaces, surface site densities, radionuclide surface species)

• Key model components• Choice of fitting procedures

•Modeling results sometimes clustered around the data and sometimes it did not.

•Main finding was that variability was due to choices (sometimes poor ones) made by teams.

Comparison of model results to data for 8 teamsNi sorption on Na-montmorillonite

NEA Sorption Project phase III

WhichElectrostatic

Model ?

DistinguishStrong & Weak

Sites ?

How ManySurface Site

Types ?

NonelectrostaticModel

Constant Capacitance

Model

Diffuse Double Layer Model

START

Others ....

YES NO

1 2 ...

Final Model

Model Parameter Selection

3

RelevantSurface

Complexes ?

A B ...C

Guideline document does not giveinstructions but provides discussionsof effects that modeling choices have on the results.

Contains practical guidance and observations.

TSM Guideline DocumentM. Ochs, T.E. Payne and V. Brendler

Objective: to provide recommendations and guidelines for TSM development and application to PAs and to educate modelers and the ‘safety case’ community about appropriate and useful TSM applications.

NEA Sorption Project phase III

• THEORETICAL BASIS OF TSMS AND OPTIONS IN MODEL DEVELOPMENT

• DETERMINATION OF PARAMETERS FOR THERMODYNAMIC SORPTION MODELS

• APPROACHES FOR APPLYING TSMS TO INTACT AND COMPLEX MATERIALS

• GENERAL GUIDELINES AND RECOMMENDATIONS

Applying TSMs to Intact and Complex Materials

• Two approaches: – Component Additivity: TSMs are determined on

single minerals and then results are added together in proportion to minerals in a natural material.

– Generalized Composite: adsorption on the whole substrate is described by mass laws written with generic surface functional groups. The stoichiometries and formation constants for each mass law are determined by fitting experimental sorption data (macroscopic dependence of adsorption as a function of pH and other relevant conditions) for the mineral assemblage as a whole.

Guideline DocumentTopics addressing TSMs of natural materials

• Approaches for applying TSMs to intact and Complex Materials• Real substrates (What makes them complex?)

– Complex mineralogy– Complex solution chemistry– Intact/compacted state

• Determination and estimation of TSM parameters in real systems• Modeling adsorption on mineral assemblages: CA vs GC• Sensitivity analysis for complex systems: CA vs GC• Sample characterization• Surface site density and surface area• Surface protolysis• Radionuclide surface equilibria• Surface charge behavior and EDL correction terms

Normalized sensitivity of adsorption to different model parameters

4 5 6 7 80.0

0.1

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0.3

0.4

0.5

0.6

0.7

0.8

Se

nsi

tivity

Ra

nki

ng

pH

SSA [XNa]

logK =SOCo+

logK =SOCoOH logK XNa logK (X)

2Co

Generalized Composite Model

Figure 4.2: Normalized ranking of the sensitivity of input parameters for the CA approach test case as a function of pH

4 5 6 7 80.0

0.1

0.2

0.3

0.4

0.5

0.6 Kaolinite content Montmorillonite content

logK =Si-O-Co+ [Kao] logK (=Al-O)

2-Co [Kao]

logK =Al-O-CoOH [Gib]

logK =Fe-O-Co+ [Goe] logK =Fe-O-CoOH [Goe]

logK X(s)-O-Co+ [Momo]

Se

nsi

tivity

Ra

nki

ng

pH

Component Additivity Model

Co adsorption on a South Carolina soil containing kaolinite, gibbsite, goethite and hematite, and 2:1 layer silicates

Test case 7 from Phase II. Complex system, only a few teams tried this.The generalized Composite models clustered a bit better around the data.

Summary• Phase II of the NEA Sorption Project identified

many approaches to Thermodynamic Sorption Modeling but many practical problems exist in implementing these complex models.

• Phase III has tried to address these problems by providing practical, direct guidance to modelers.

• Report is in the camera ready form and should be out very soon.