a thesis proposal by katya bazilevskaya department of crop and soil sciences
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A Thesis Proposal by
Katya Bazilevskaya
Department of Crop and Soil SciencesThe Pennsylvania State University
November 5, 2006
Advisor: Carmen Enid Martinez
Mixed Fe-Al hydroxide nano-particles: precipitation and transformation in
solution and on quartz surface
Center of Environmental Kinetic Analysis (CEKA):
What factors control the nucleation, growth, and transformation of mixed Fe-, Al-, and Si- oxides from solution (homogeneous) and on mineral surfaces (heterogeneous)?
Low-crystalline Fe and Al hydroxides exist as colloidal (<0.1 µm size) particles in natural waters or as coatings on the soil minerals
Have strong ability to adsorb contaminants due to their high surface area and pH-dependent surface charge.
Rationale
From Penn et al. (2001)
Better understanding of crystallization pathways is needed to formulate rate models for soil environments and to improve remediation techniquesThis research is developing new approaches and techniques to study composition and transformations of mixed Fe-Al hydroxide nano-colloids
Research Hypotheses
In nano-colloidal system:
Mixed Fe-Al hydroxides have slower crystallization rate than the pure phases
Two main factors that dictate the rate of crystallization of mixed Fe-Al hydroxides are time and chemical composition (Fe/Al ratio);
Crystallization pathways can produce a variety of intermediate metastable phases
Aluminum impurity in Fe-hydroxides results in lower degree of crystalinity with sluggish kinetics of crystallization of intermediate species which are more reactive towards contaminants
Transformation of mixed Fe-Al hydroxide nano-particles upon aging
Formation of Fe-Al coatings on quartz in the lab and in the field
Synthesis of Al-substituted hematite nano-particles
#1
#2
#3
XRD: crystal structure (cell parameters)EXAFS: hematite structure (Fe-O, Fe-Fe, Fe-Al distances)TEM: particle morphology of particular crystal phase
How Al-substitution influence the crystal structure and morphology of nano-hematite? Would these properties be different depending on the precipitation pathway?
What is the structure and composition of soil coatings? How coating structure and morphology change with time? How presence of Al modify the properties of coating?
XRD: qualitative mineral compositionATR-FTIR: quantitative composition
How to quantitatively resolve mineral composition in the mixture? What is kinetics of mineral crystallization in mixture?
GIXAS: structure of the coating (Fe-O, Fe-Fe, Fe-Al distances)GISAXS: particle morphology, distribution on the surface of particular crystal phase
Project Research Questions Methods
Project overview
Homogeneous precipitation and transformation of mixed Fe-Al nano-particles using ATR-FTIR and XRD
Research hypothesis: kinetics of crystallization of minerals (goethite and/or gibbsite) from mixed Fe-Al hydroxides is much slower than that of the pure phase due to the formation of intermediate phases that are indefinitely metastable in low-temperature soil environments.
Specific objectives:
(1) determine the mineral composition (percentage of goethite and gibbsite) in mixed Fe-Al hydroxides as a function of Al-substitution and reaction time, and
(2) determine the effect Al-substitution may have on the rate of crystallization of the primary (Fe) precipitate.
Project #1
[Fe+Al] = 10-2 M; %Al: 0, 10, 25, 30, 50, 75, 100; pH = 5Slow titration rate (0.1 ml/min) Dialysis to remove salts and excess AlTime: 0, 2, 9, 23, and 54 days, aged at 50 C
Experimental conditions:
Methods: nano-particle synthesisProject #1
Fe (+Al), pH ~2
0.1 M KOH
Methods: Infrared measurements
ATR Crysta
l
Evanescent wave0.1 µm
ATR Crystal
Sample
Incident radiation Reflected radiation
Mirror
The principle of ATR-FTIR
Infrared radiation is focused onto the edge of ATR crystal, reflected through the crystal and directed to the detector.Radiation penetrate up to 0.1 µm into solution, where it is absorbed and cause the vibrations of molecule bondThe infrared spectra is obtained with unique bands for each bond type
Project #1
Methods: XRD measurements
Project #1
Brookhaven National Laboratory beamline X-16C
Project #1
Molecular dynamic modeling: calculation of vibrational OH-frequencies
100% Fe 100% Al
(a) (b) (c) (d)
FeAlOH
Goethite Al-substituted goethite
Isomorphous substitution Al-clusters
Gibbsite
Build goethite and gibbsite models based on available experimental crystallographic data
Create Al-substituted goethite and calculate new atomic coordinates
Using Vienna Ab-initio Simulation Package (VASP) obtain theoretical OH- frequencies for goethite and Al-substituted goethite
Use these frequencies as a reference to interpret Infrared data
Changes in mineral composition of mixed Fe-Al nano-particles with increasing Al-substitution. Suspensions were aged for 2 days at 50 C
0% Al
Wavenumber, cm-1
700 900 1100 3000 4000
Inte
nsity
10% Al
100% Al
75% Al
50% Al
25% Al
30% Al
Preliminary work: infrared dataProject #1
Dotted and solid lines (perpendicular to x-axis) show characteristic goethite and gibbsite band positions, respectively
0%Al 25%Al
Transformation of Fe(-Al) nano-particles upon aging at 50 C.
Project #1 Preliminary work: infrared data
Project #1
Expected results
MCR analyses of high resolution ATR FTIR data will allow to de-convolute and quantify the mineral composition in complex Fe-Al hydroxides mixtures
XRD data will be in good agreement with infrared data
The main intermediate species that control the kinetics of crystallization will be identified
Project #2
The presence of aluminum in solution will change the mineral composition and coating thickness at any given time compared to aluminum free systems;Iron and aluminum migrate through the soil profile in the form of hydroxide nano-particles that accumulate in lower profiles to form coatings on sand grains;The composition and spatial distribution of different mineral phases in the coatings may reveal the mechanism of its formation.
Research hypotheses
Specific objectives
Formation of Fe-Al coatings on a quartz substrate: laboratory and field investigations
(1) Study the formation and development of coatings in situ by placing a quartz wafer into a Spodosol profile (field experiment);
(2) Determine structure of the coatings as a function of Al-substitution and reaction time (lab experiment)
Project #2Podzolization mechanisms
Organic acids
Organo-Al -(Fe) complexesSi, Al, Fe Al-Fe-Si
inorganic sols
Fe-oxide, allophane, imogolite
Microbial activity
supersaturation precipitation
Addition of Al and Fe
adsorption
Microbial degradation of organic ligands
Release of Fe and Al
Fe-oxide, allophane, imogolite
Flocuulation by cations: K+, Mg2+
O
E
Bh
Bs
C
pH = 4
pH = 5
Si
Adsorption/precipitation theoryProto-imogolite theoryBiodegradation theory
State College
Philipsburg
Milesburg
504
I-80
220322
BellefonteBlack Moshannon Lake X
100 cm 100 cm 100 cm 100 cm4 m
Time1=6 months Time 2 =12 months Time 3 =24 months
Time 4=36 months
4 m
5 cm
Project #2 Methods: field experiment
Field layout well-polished quartz wafersemi-polished quartz waferssilica gelsand
Project #2
Methods: lab experiment
[Fe2+] = 10-4 M pH ~ 5
[Al] = 0.2x10-4 M [Fe2+] = 10-4 M
pH ~ 5
Reaction time, hours
0.5 1 1.5 2
Methods: GIXAFS measurementsProject #2
< 0.18 degree
I1I0
Energy Dispersive Detector – 30 element Ge
X-ray fluorescence
Analysis within the first nanometers from the surface Polished surface is required 30 element Ge-detector in fluorescence mode Grazing incidence angle, θ = 0.18o
Info about oxide structure: Fe coordination number ID of Fe neighboring atoms (Fe, Al or C):differentiate among Fe-O-Fe, Fe-O-Al and Fe-O-C local bonding environments
Methods: GISAXS measurements
a non-destructive structural probe does not require a conducting surface or sample preparation (in-situ characterization possible) yields excellent sampling statistics (averages over macroscopic regions to provide information on nanometer scale) provides information on particle geometry, size distributions, spatial correlations
Grazing Incidence Small angle scattering of x-ray
Project #2
irradiate a sample with a well-collimated X-ray beam measure the resulting intensity as a function of angle between the incoming beam and scattered beamdetermine the structure that caused the observed pattern
Expected resultsProject #2
Statistically significant and steady increase in coating thickness in three years will allow to estimate the rate of coating formation in the field; Spectroscopic analysis (GIXAS) will give reasonable information of the coating structure, i.e. we should be able to distinguish between Fe-organic (Fe-O-C) bonding and Fe-O-Fe(Al) bonding in the organic-rich and organic-free horizons, respectively;Spectroscopic data will be similar for the laboratory and field samples (Bx horizon); We will be able to infer mineral composition on the coatings from our spectra; Coating structure and morphology will be different in the presence of aluminum compared to Fe-only experiments.
In the field: quartz wafer placed in spodosol Bx horizon and recovered after 1 year
AFM image
Fe K-edge
EXAFS spectrum (GIXAFS)
In the lab: fused silicon after 2.5 hours reaction with Fe(II) 10-4 M solution under oxidizing conditions
AFM image
Fe K-edge
EXAFS spectrum (GIXAFS)
Project #2 Preliminary work
Project #3
Synthesis and characterization of Al-substituted hematite nano-particles
The effect of Al-substitution in micron-size hematite particles (decrease in crystallinity and particle size) is also true for nano-size Al-substituted hematite particles;This effect is even more pronounced for nano-particles due to the higher surface to bulk ratios encountered in nano-sized particles,.The amount of aluminum incorporation into the hematite structure depends on the pathway of nano-hematite formation
Research hypotheses
Specific objectives
Synthesize Al-substituted nano-hematite particles (less than 30 nm) following different synthesis procedures
Compare particles properties (size, shape and structure) among different sets using XRD, TEM, EXAFS and chemical analyses.
a. Procedure #1
Fe Al Fe+Al Fe
Al
b. Procedure #2 c. Procedure #3
Methods: nano-hematite synthesisProject #3
mol%Al [Fe], M [Al], MTotal [Al+Fe],
M Fe:Al0 0.01 0 0.01 010 0.009 0.001 0.01 0.120 0.008 0.002 0.01 0.330 0.007 0.003 0.01 0.4
EXAFS: method
Fe:Al = 1:1
Fe-O Fe-FeFe-Al
Project #3
0
2
4
6
8
10
12
14
16
18
20
0 20 40 60 80 100Size, nm
Vol
ume
%
0% Al10% Al20% Al30% Al
Preliminary work
Project #3 Al incorporation in nano-hematite particles
Al/(Al+Fe) in starting solution, mol%0 5 10 15 20 25 30 35
Al(A
l+Fe
) in
hem
atite
, mol
%
0
2
4
6
8
10
nHm-30
nHm-20
nHm-10
nHm-0
Particle size of hematite suspensions with different initial aluminum mol %.
TEM dataXRD data
Project #3
Preliminary work
Expected results
Project #3
The percent of Al-substitution will be different depending on the synthesis procedure, and it will be, probably, the highest using procedure #3 (addition of Fe-chloride dropwise to Al-solution)
As Al-percentage in initial solution increases, hematite particle sizes are expected to decrease; particle morphology will change from round to elongated shapes
Research timetable
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