Synthesis and characterization of poorly-crystaline Fe-Al nano-

hydroxides

Crop and Soil Sciences Department & Center of Environmental Kinetics Analysis

The Pennsylvania State University

18th World Congress of Soil Science July 11, 2006

Katya Bazilevskaya, Douglas Archibald, Carmen Enid Martínez

Why we study hydroxide nano-particles?

Form coatings: meta-stable mixtures of Fe-, Al- and Si oxides and organics, which:

Affect mobility of nutrients and contaminants

Reflect soil-forming processes

Affect mineral dissolution rates

Have large reactive surface area Create pH-dependent charge

Slow titration rate (0.1 ml/min)

Dyalisis to remove salts and excess Al

Fe (+Al) nitrate sol-n, pH ~2

0.1 M KOH

Experimental conditions:[Fe+Al] = 10 mM;%Al: 0, 10, 25, 30, 50, 75, 100; pH = 5

Measured by light scattering (ZetaPALS, Brookhaven Instruments)

0% Al 30% Al

AFM images of Fe-Al particles

What methods to use?

X-ray diffraction (XRD): long range order information synchrotron XRD for poorly crystalline hydroxides only crystalline phase

Extended X-ray Absorption Fine structure spectroscopy (EXAFS) short range order (atom’s second neighbor information – bond distance, type of bond)) both crystalline and amorphous Can only probe for Fe, not Al

Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR FTIR)

short range order (bonding and reactivity of protons) both crystalline and amorphous distinct iron and Al hydroxide OH-vibrations ATR technique provide precise technique: measures nano-particles coatings on the sensor (do not

need to separate nano-particles by centrifugation and freeze-drying)

Structure of end-membersGoethite

Gibbsite

(OH)

stretching

(OH)

bending

(OH)

deformation

Goethite 3450, 3206 1687, 1643 888, 798Gibbsite 3620, 3525 3455, 3390 1024, 975 -

2600280030003200340036003800

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

Frequency, wavenumbers

-Log

(I/I

o)%Al

increases

600800100012001400160018000

0.005

0.01

0.015

0.02

0.025

0.03

Frequency, wavenumbers

-Log

(I/I

o)

%Al increases

Infrared spectra of Fe-Al hydroxides

with different %Al

0% Al

100% Al

10001500200025003000350040000

0.005

0.01

0.015

0.02

0.025

Frequency, wavenumbers

-Log

(I/I

o)

10001500200025003000350040000

0.005

0.01

0.015

0.02

0.025

0.03

Frequency, wavenumbers

-Log

(I/I

o)

Change of infrared spectra of Fe-Al

hydroxides with time

0%Al

25%Al

Time = 2 days @ 50 C

Time = 0 days

Preliminary Fe-EXAFS data (aged for 2 weeks)

Radial distribution functions for oxide suspensions of various Fe:Al ratios

Fe-Fe distances for • goethite3.0 – 3.4 Å(2.5-2.9 in R)

Fe-Al distances for • Fe3+-gibbsite2.9 Å (2.4 in R)• Epidote 2.95 Å (2.5 in R)• LiAlO2

with 1wt.%Fe3+

3.15 Å (2.6 in R)

From the Literature

Fouri

er

Tra

nsf

orm

ed X

AFS

, χ

(R)

Fe-O Fe-Fe

Fe-Al

Fe-only

Fe:Al=1:1

Distance to neighboring atom R, Å

Connection with real life….Spodosols

Top-view

Profile-view

To monitor formation of iron coatings on quartz in situ:

placed quartz in the soil profile

recovered after 1 year

Acknowledgments

Funding: Center of Environmental Kinetics analysis (CEKA)

XRD and EXAFS: beamlines X-16C, X-18B, X-10C, National Synchrotron Light Source, Brookhaven National Laboratory

….