THE RI ETVELD METHOD AS A THE RI ETVELD METHOD AS A TOOL FOR - - PowerPoint PPT Presentation

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THE RI ETVELD METHOD AS A THE RI ETVELD METHOD AS A TOOL FOR ASSESSI NG HEAVY TOOL FOR ASSESSI NG HEAVY-

• METAL I MMOBI LI ZATI ON I N S/ S

METAL I MMOBI LI ZATI ON I N S/ S TREATMENT I NVESTI GATI ONS TREATMENT I NVESTI GATI ONS

Dimitris Dermatas and Maria Chrysochoou Dimitris Dermatas and Maria Chrysochoou W.M. Keck Geoenvironmental Laboratory W.M. Keck Geoenvironmental Laboratory Stevens Institute of Technology Stevens Institute of Technology

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X X-

• Ray Powder Diffraction

Ray Powder Diffraction

• Determines the average mineralogical composition

Determines the average mineralogical composition

• f a solid sample
• f a solid sample
• Can only detect crystalline phases

Can only detect crystalline phases

• Each crystalline phase reflects X

Each crystalline phase reflects X-

• rays in a unique

rays in a unique way according to the morphology and composition way according to the morphology and composition

• f the crystal
• f the crystal
• X

X-

• Ray Powder Diffraction employs random

Ray Powder Diffraction employs random

• rientation of crystals in a sample, in order to
• rientation of crystals in a sample, in order to
• btain an intensity function that is the sum of the
• btain an intensity function that is the sum of the

reflections produced by all crystal planes over a reflections produced by all crystal planes over a range of diffraction angles range of diffraction angles -

• > this is the XRPD

> this is the XRPD pattern pattern

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Why use XRPD in S/ S treated Why use XRPD in S/ S treated media? media?

The success of an S/S treatment is commonly evaluated using regulatory leaching tests (Toxicity Characteristic Leaching Procedure in the U.S., EN 12457 in the E.U.) Regulatory tests alter the S/S matrix and are not designed to address the immobilization mechanisms of hazardous compounds (e.g. precipitation of heavy metals as insoluble compounds, sorption on CSH, etc.) XRPD allows for the identification of crystalline phases prior and following the S/S treatment and possibly also the speciation of hazardous species Knowing the speciation of hazardous compounds and most importantly heavy metals is critical in predicting solubility and applying geochemical reaction and fate-and-transport modeling The properties of the cementitious matrix itself are elucidated by XRPD analysis

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XRPD quantitative analysis by XRPD quantitative analysis by Whole Pattern Fitting (WPF) Whole Pattern Fitting (WPF)

The qualitative analysis of an XRPD pattern consists in matching all

• bserved peaks with crystalline phases

It is important to recognize that the observed peak intensity of each phase does not necessarily reflect its quantity in the sample

Basic principle of WPF: Basic principle of WPF: Fitting the experimentally observed diffraction pattern with a Fitting the experimentally observed diffraction pattern with a synthesized pattern using the phases identified in the synthesized pattern using the phases identified in the qualitative analysis qualitative analysis

• > the minimization of the difference between the two patterns is

> the minimization of the difference between the two patterns is performed by a mathematical algorithm performed by a mathematical algorithm H.M. Rietveld was the first to introduce this method into XRD H.M. Rietveld was the first to introduce this method into XRD analysis in 1969 analysis in 1969

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Steps for WPF application Steps for WPF application

• Perform qualitative analysis in order to match all

Perform qualitative analysis in order to match all

• bservable peaks
• bservable peaks
• Load identified phases into Whole Pattern Fitting

Load identified phases into Whole Pattern Fitting dialog window of commercially available software dialog window of commercially available software

• Run software algorithm in order to produce a

Run software algorithm in order to produce a calculated pattern calculated pattern

• Refine model parameters to minimize the difference

Refine model parameters to minimize the difference between experimental and calculated pattern between experimental and calculated pattern

• Read quantitative results based on the optimized

Read quantitative results based on the optimized set of parameters set of parameters

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Refinable parameters Refinable parameters

Global parameters (refer to entire pattern) Global parameters (refer to entire pattern)

• Background curve

Background curve

• Angular corrections

Angular corrections

• Amorphous humps

Amorphous humps

• Profile function

Profile function Phase parameters (refer to each compound loaded) Phase parameters (refer to each compound loaded)

• Lattice constants

Lattice constants

• Peak Intensity

Peak Intensity

• Peak width

Peak width

• Preferred Orientation parameters

Preferred Orientation parameters

• Position and type of atoms in the crystal

Position and type of atoms in the crystal

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Mathematical model Mathematical model

• The refinement model is a least

The refinement model is a least-

• square

square model that seeks to minimize the residual model that seeks to minimize the residual between the observed and the calculated between the observed and the calculated intensity, summed over all data points intensity, summed over all data points

( )

∑

− =

i i ci i

y y y S

2

The weight percentages are then calculated based on the integrated intensities (peak areas) and the Reference Intensity Ratios (RIR) of the loaded phases

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Criteria of fit Criteria of fit

Numerical criteria Numerical criteria

• R

R-

• factor

factor

• Goodness of fit indicator

Goodness of fit indicator

• > these depend largely on counting statistics and

> these depend largely on counting statistics and pattern complexity and are to be used as guiding pattern complexity and are to be used as guiding numbers rather than as absolute criteria numbers rather than as absolute criteria Graphical criteria Graphical criteria

• Plot of residual pattern

Plot of residual pattern

• > particularly useful as it indicates

> particularly useful as it indicates “ “problem areas problem areas” ” e.g. unidentified peaks, poor choice of minerals, e.g. unidentified peaks, poor choice of minerals, preferred orientation phenomena preferred orientation phenomena

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Example of residual Example of residual pattern plot pattern plot

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I mportant considerations I mportant considerations

• WPF is a mathematical model

WPF is a mathematical model -

• > it

> it minimizes the residual regardless of the minimizes the residual regardless of the physical meaning of parameter values physical meaning of parameter values

Peak broadening may be a result of poor degree of crystallization - > manual restriction of FWHM (peak width) to the instrument- specific broadening is necessary to avoid overestimation of phases Significant shift in lattice constants may indicate a poor choice of mineral or have a physical meaning e.g. change in the state of hydration

The analyst must be in position to judge what adjustments to make, even though the goodness of fit may be worse, in order to avoid mathematical curve fitting

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Mass balances Mass balances

• An indirect method to check WPF results is by

An indirect method to check WPF results is by performing a mass balance performing a mass balance

1. 1.

Calculate distribution of elements in the mineral Calculate distribution of elements in the mineral phases used to conduct WPF phases used to conduct WPF

2. 2.

Multiply with the quantitative results to obtain Multiply with the quantitative results to obtain element distribution of the phase in the analyzed element distribution of the phase in the analyzed sample sample

3. 3.

Add the results for all mineral phases to obtain Add the results for all mineral phases to obtain elemental composition of sample elemental composition of sample

4. 4.

Compare with wet chemistry data Compare with wet chemistry data

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Example of WPF and mass Example of WPF and mass balance in COPR balance in COPR

Rietveld Ca (40) Fe (56) Al (27) Mg(24) CrVI (52) Brownmillerite 10.4% 3.42% 2.40% 1.16% Portlandite 15.0% 8.11% Calcite 25.8% 10.32% Periclase 1.7% 1.02% Brucite 6.9% 2.86% Quartz 9.3% Hydroandradite 6.9% 1.79% 1.67% Katoite 3.4% 1.08% 0.49% Sjoegrenite 12.7% 2.16% 2.77% Ettringite 3.4% 0.65% 0.15% CAC-14 4.4% 1.04% 0.35% 0.34% SUM 99.9% 26.41% 6.22% 2.14% 6.65% 0.34% Total analyses 25.09% 6.49% 3.15% 4.92% 0.84% Difference 1.32%

• 0.27%
• 1.01%

1.73%

• 0.50%

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Mass balance evaluation Mass balance evaluation

• Elements with higher percentage present

Elements with higher percentage present the maximum discrepancies (uncertainty the maximum discrepancies (uncertainty increases in major phases) increases in major phases)

• Isomorphic substitutions (Mg

Isomorphic substitutions (Mg2+

2+ -

• > Ca

> Ca2+

2+ ,

, Fe Fe3+

3+ ,Cr

,Cr3+

3+ -

• > Al

> Al3+

3+ ) can account for minor

) can account for minor discrepancies discrepancies

• Systematic errors may appear as consistent

Systematic errors may appear as consistent

• ver
• ver-
• or underestimation in one or more
• r underestimation in one or more

elements elements

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Applications of Rietveld Applications of Rietveld

• Is the only available method to provide info on the

Is the only available method to provide info on the average mineralogy of all types of solids average mineralogy of all types of solids -

• >

> application in S/S design and evaluation is possible application in S/S design and evaluation is possible in order to investigate the immobilization in order to investigate the immobilization mechanisms of hazardous compounds (heavy mechanisms of hazardous compounds (heavy metals) metals)

• Can provide insight into compositional questions

Can provide insight into compositional questions (solid solutions, hydration states, variation in (solid solutions, hydration states, variation in minerals) but this requires good knowledge of minerals) but this requires good knowledge of crystallography crystallography

• Can provide information on solid state chemistry

Can provide information on solid state chemistry and physics, when single and physics, when single-

• crystal X

crystal X-

• ray diffraction is

ray diffraction is applied applied

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Limitations of Rietveld Limitations of Rietveld

• XRPD detection limit (can be as low as 0.01% for

XRPD detection limit (can be as low as 0.01% for very well crystallized phases but is generally very well crystallized phases but is generally assessed between 1 and 5%) assessed between 1 and 5%)

• Identification of amorphous phases very limited

Identification of amorphous phases very limited -

• >

> problem for S/S applications with CAH/CSH gels problem for S/S applications with CAH/CSH gels

• Uncertainty in results (1

Uncertainty in results (1-

• 5% depending on

5% depending on quantity), precision in the evaluation of quantity), precision in the evaluation of immobilization is limited immobilization is limited

• Substitutions by hazardous metals in known phases

Substitutions by hazardous metals in known phases are not always detectable; despite of large number are not always detectable; despite of large number

• f
• f PDFs

PDFs, natural and engineered systems always , natural and engineered systems always present large variations in composition present large variations in composition

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