11 Mössbauer Spectroscopic Analysis of Iron
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Oxides in Soil L A W R E N C E H . B O W E N and S T E R L I N G
B. W E E D
Departments of Chemistry and Soil Science, North Carolina State University, Raleigh, N C 27650
Iron oxides in well-oxidized soils exist primarily in the form of aluminum-substituted goethite (α-Fe O ). 2
3
(α-FeOOH)
and hematite
These microcrystalline oxides often occur in low
abundance and may be concentrated for analysis by boiling with 5 M N a O H .
This chapter is a
Mössbauer
study of the
consequences of this treatment in a variety of soil clays containing both goethite and hematite.
The iron oxide is
concentrated appreciably, while aluminum incorporation is not markedly affected.
The
Mössbauer
results for alumi
num substitution in goethite generally agree with aluminum content chemically determined after treatment with base. Hematite samples give better agreement before treatment. Detection of hematite and goethite by x-ray diffraction agrees qualitatively with the
Mössbauer
data.
T r o n oxides a n d o x y h y d r o x i d e s a r e w i d e l y o c c u r r i n g i n s o i l clays
(1).
A l t h o u g h often d i l u t e , t h e y s i g n i f i c a n t l y influence t h e p h y s i c a l a n d c h e m i c a l p r o p e r t i e s of t h e soils because of t h e i r m o d e of o c c u r r e n c e as finely
d i v i d e d p a r t i c l e s o r p o s s i b l y as coatings o n s i l i c a t e m i n e r a l s . B e
cause of this m o d e of o c c u r r e n c e , s t a n d a r d t e c h n i q u e s of analysis s u c h as x - r a y d i f f r a c t i o n are of l i m i t e d use. M o s s b a u e r spectroscopy gives u n i q u e i n f o r m a t i o n a b o u t t h e i r o n o x i d e constituents of soils ( 2 , 3 ) , a n d is p a r t i c u l a r l y u s e f u l i n d i s t i n g u i s h i n g the t w o m a j o r n a t u r a l forms of h e m a t i t e (e*-Fe 0 ) 2
3
a n d goethite
(a-FeOOH)
from
their differing
magnetic
h y p e r f i n e i n t e r a c t i o n s . T w o effects c o m p l i c a t e s p e c t r a l a n a l y s i s : s m a l l p a r t i c l e size p r o d u c e s s u p e r p a r a m a g n e t i c r e l a x a t i o n (4) a n d s u b s t i t u t i o n of n o n m a g n e t i c ions, e s p e c i a l l y A l , i n t o t h e o x i d e l a t t i c e , reduces t h e 3 +
m a g n i t u d e , a n d b r o a d e n s t h e d i s t r i b u t i o n of m a g n e t i c fields ( 5 ) . T h e s e
©
0065-2393/81/0194-0247$05.00/0 1981 American Chemical Society
248
MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS
effects are i n t e r r e l a t e d , since a l u m i n u m s u b s t i t u t i o n also reduces p a r t i c l e size ( 5 ) .
M o s s b a u e r s p e c t r o s c o p y has t h e p o t e n t i a l f o r a n a l y z i n g n o t
o n l y the c r y s t a l l i n e forms of the i r o n oxides, b u t t h e i r d e g r e e of a l u m i n u m s u b s t i t u t i o n a n d p a r t i c l e size as w e l l . T h e first step i n s t u d y i n g the i r o n o x i d e f r a c t i o n of soils is often b o i l i n g w i t h 5 M N a O H to r e m o v e p r e f e r e n t i a l l y silicates a n d a l u m i n u m oxides (6).
H o w e v e r , s u c h d r a s t i c treatment m i g h t w e l l affect the n a t u r e
of the r e m a i n i n g oxides, i n p a r t i c u l a r t h e i r a l u m i n u m s u b s t i t u t i o n a n d Mössbauer Spectroscopy and Its Chemical Applications Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/16/16. For personal use only.
p a r t i c l e size. T h e present s t u d y w a s i n i t i a t e d to d e t e r m i n e these effects f r o m the r e s u l t i n g changes i n the M o s s b a u e r spectra. T h e c l a y f r a c t i o n s , i n w h i c h the i r o n oxides are most a b u n d a n t , f r o m a v a r i e t y of soils w e r e s t u d i e d before a n d after c h e m i c a l c o n c e n t r a t i o n . M o s s b a u e r s p e c t r a at r o o m t e m p e r a t u r e a n d at 78 K w e r e o b t a i n e d , a n d results f r o m
these
spectra w e r e c o m p a r e d w i t h c h e m i c a l a n d x-ray d i f f r a c t i o n analyses.
Experimental T h e methods used were the same as those reported b y Bigham et al. ( 3 ) . T h e soils (Table I) were fractionated by standard sedimentation procedures ( 7 ) . In most cases only the clay ( < 2/xm) fraction was studied. Samples A 6 0 6 and A 6 0 8 were used as silt plus clay (
