MÃ¶ssbauer Effect Study of Victorian Brown Coal - ACS Publications

9 Mössbauer Effect Study of Victorian Brown Coal J. D . C A S H I O N and B. M A G U I R E

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch009

Department of Physics, Monash University, Clayton, Victoria, 3168, Australia L . T . KISS Herman Research Laboratories, State Electricity Commission, Richmond, Victoria, 3121, Australia

Mössbauer

spectra using

-sulfur brown

57

Fe were taken of low-rank, low

coals from the

Latrobe Valley, Victoria,

Australia, having a typical water content of 67%.

Spectra

at 78 Κ on bed-moist samples showed at least six poorly resolved quadrupole split doublets, one due to pyrite a n d / o r marcasite, and the others, which do not fit any known spectra, most probably due to different hydrolyzed iron carboxylates. D r i e d samples gave completely different spec tra with generally one intense quadrupole split doublet and one or more relaxed magnetic splittings. -1

typically moved +0.1-0.2 mm s

The isomer shifts

on drying. However, the

most dramatic change was an increase of nearly an order of magnitude in the absorption dip, showing that the water molecules are bonded intimately to the iron atoms in the bed-moist state.

' H p h e p r i n c i p a l a i m o f this i n v e s t i g a t i o n w a s t o d e t e r m i n e t h e c h e m i c a l state o r states o f n o n p y r i t i c i r o n i n v a r i o u s b r o w n coals f r o m t h e L a t r o b e V a l l e y i n V i c t o r i a , A u s t r a l i a . S a m p l e s w e r e selected to c o v e r t h e k n o w n v a r i a t i o n s f r o m the field, w i t h concentrations o f i r o n v a r y i n g f r o m 0.07 t o 1 . 8 % o n a d r y basis. T h e n a t u r e of a n y o r g a n i c a l l y b o u n d i r o n w a s of p a r t i c u l a r interest. L e f e l h o c z et a l . ( I ) t e n t a t i v e l y assigned o n e of t h e i r i r o n spectra t o o r g a n i c i r o n , b u t this w a s s u b s e q u e n t l y d e t e r m i n e d t o b e a c l a y site (2,3).

R e c e n t d e t a i l e d Môssbauer effect i n v e s t i g a t i o n s

(4,5,6,7)

on

coals f r o m t h e U . S . n o r t h e a s t e r n fields h a v e n o t s h o w n a n y e v i d e n c e of

©

0065-2393/81/0194-0209$05.00/0 1981 American Chemical Society

Stevens and Shenoy; Mössbauer Spectroscopy and Its Chemical Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

210

MOSSBAUER

SPECTROSCOPY A N D

ITS

CHEMICAL

APPLICATIONS

o r g a n i c a l l y b o u n d i r o n . S c h a f e r ( 8 ) s t u d i e d the o x i d a t i o n of carboxylates i n V i c t o r i a n b r o w n c o a l , b u t his r e p o r t does not c o n t a i n a n y d e t a i l s of the M o s s b a u e r spectra or p a r a m e t e r s o b t a i n e d . T h e coals u s e d i n this i n v e s t i g a t i o n are of m u c h l o w e r r a n k

(67%

c a r b o n ) t h a n the U . S . coals t h a t h a v e b e e n s t u d i e d . T h e V i c t o r i a n coals are m u c h h i g h e r i n p h e n o l i c a n d c a r b o x y l i c f u n c t i o n a l groups a n d h a v e a l o w s u l f u r c o n c e n t r a t i o n ( < 0 . 4 % ) , w i t h a l o w l e v e l of s u l f u r b e i n g present as p y r i t e or m a r c a s i t e , F e S . P o s s i b l y the largest difference f r o m 2

the U . S . samples is the h i g h w a t e r content of a b o u t 6 7 % as m i n e d , a n d care has b e e n t a k e n to m a i n t a i n the samples i n this c o n d i t i o n .

When

left o p e n to the a i r , the coals r a p i d l y lose m o i s t u r e d o w n to a c o n c e n t r a Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch009

t i o n of

about

15%.

Samples dried below

m o i s t u r e w h e n exposed to the a t m o s p h e r e . M o s s b a u e r spectroscopy

this v a l u e r e a d i l y

absorb

F o r the bed-moist samples,

is one of the f e w t e c h n i q u e s t h a t c a n a n a l y z e

the states o f the i r o n w i t h o u t r e q u i r i n g d r y i n g , d i s s o l u t i o n , or s o m e o t h e r c h a n g e to the s a m p l e . T h e w a t e r i n b e d - m o i s t c o a l is c o n t a i n e d i n several different forms a n d a n a p p r o x i m a t e classification i n o r d e r of d e c r e a s i n g ease of r e m o v a l is ( 9 ) :

free w a t e r i n m a c r o p o r e s a n d interstices, c a p i l l a r i e s , a n d w a t e r

o n the w a l l s of pores too s m a l l to h a v e a m e n i s c u s . T h i s last t y p e c a n b e d i v i d e d f u r t h e r i n t o m u l t i l a y e r a n d m o n o l a y e r w a t e r , w i t h the m o n o l a y e r w a t e r b e i n g b o n d e d to o x y g e n - c o n t a i n i n g h y d r o p h i l i c sites o n the c o a l b y hydrogen bonds.

Schafer (10)

showed that carboxylic a c i d groups,

p a r t i c u l a r l y i n t h e salt f o r m , are the most significant h y d r o p h i l i c site, w i t h the F e

salt b e i n g the m o s t efficient, f o l l o w e d b y M g , C a , C u ,

2 +

2 +

B a , A l , F e , N a , and K 2 +

3 +

3 +

+

+

2 +

2 +

i n o r d e r . T h e p h e n o l i c h y d r o x y l g r o u p s are

less efficient t h a n the c a r b o x y l i c groups.

Sample Preparation and Experimental Details T h e b e d - m o i s t samples w e r e r e c e i v e d i n a i r - t i g h t containers. A b s o r b ers w e r e m a d e b y q u i c k l y s c r a p i n g off the outer, s l i g h t l y o x i d i z e d a n d b l a c k e n e d layers u n t i l a u n i f o r m b r o w n r e g i o n w a s o b t a i n e d . T h i s w a s s c r a p e d into L u c i t e containers to a thickness of 1//* ( w h e r e p is the a t o m i c a b s o r p t i o n coefficient) of t y p i c a l l y 3 - 4 m m , a n d t h e n sealed. T h e w h o l e o p e r a t i o n took a p p r o x i m a t e l y 2 - 3 m i n , a n d n o b l a c k e n i n g of t h e c o a l w a s o b s e r v e d i n this t i m e . Separate absorbers w e r e also m a d e of some of the outer b l a c k e n e d layers. O n e sample was received i n an already oxidized form a n d two samples h a d b e e n c r u s h e d , d r i e d , a n d c o m p r e s s e d i n t o b r i q u e t t e s . T h e s e samples w e r e a l l o w e d to r e m a i n i n e q u i l i b r i u m w i t h t h e a t m o s p h e r e . A l l the M o s s b a u e r r u n s w e r e t a k e n w i t h a C o R h source u s i n g a conventional constant-acceleration drive w i t h data collection into either a 1024 or 512 c h a n n e l a n a l y z e r . A l l the samples w e r e r u n at 78 K , w i t h some r u n s also b e i n g m a d e at r o o m t e m p e r a t u r e a n d at 4.2 K . C o u n t i n g t i m e for the b e d - m o i s t samples w a s u s u a l l y o n the o r d e r of 1 w e e k w i t h over 1 0 counts p e r c h a n n e l . H o w e v e r , for most of these spectra t h i s w a s 5 7

7


9.

CASHION

E T

Victorian

AL.

Brown

211

Coal

i n a d e q u a t e to resolve t h e m a n y s m a l l o v e r l a p p i n g lines, so t h e n u m b e r of d a t a p o i n t s w a s r e d u c e d to 170 b y s u m m i n g the d a t a i n groups of three adjacent c h a n n e l s . T h i s g a v e u p to 4 X 1 0 counts p e r c h a n n e l , c o r r e s p o n d i n g to a s t a n d a r d d e v i a t i o n of less t h a n 0 . 0 2 % . C u r v e fitting w a s t h e n c a r r i e d out b y s t a n d a r d least-squares t e c h n i q u e s u s i n g s i n g l e L o r e n t z i a n l i n e s , or i n some cases, m a g n e t i c - or q u a d r u p o l e - s p l i t spectra, w h e r e t h e y c o u l d be u s e d r e a s o n a b l y . 7

Results A list of the samples s t u d i e d together w i t h s o m e of t h e i r r e l e v a n t d a t a are g i v e n i n T a b l e I . C h e m i c a l analysis i n d i c a t e d t h a t o n l y three of t h e samples c o n t a i n e d d e t e c t a b l e a m o u n t s of F e S t h a t c o u l d b e i n t h e Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch009

2

f o r m of e i t h e r p y r i t e or m a r c a s i t e . parameters that corresponded

O u r s p e c t r a o n these samples g a v e

to p y r i t e , b u t w i t h t h e p o o r r e s o l u t i o n

o b t a i n e d w e w o u l d not r u l e o u t the p o s s i b i l i t y of s o m e m a r c a s i t e as w e l l . C o n s e q u e n t l y , w e s h a l l refer to t h e F e S

2

s o m e w h a t loosely as p y r i t e i n

this c h a p t e r , b u t w i t h t h e i n t e r p r e t a t i o n t h a t i t i n c l u d e s the p o s s i b i l i t y of b o t h p y r i t e a n d m a r c a s i t e . T h e t o t a l i r o n concentrations o n a d r y basis r a n g e d f r o m 0.07

to

1 . 7 7 % , b u t w e note t h a t w i t h a 6 7 % w a t e r content, t h e effective i r o n c o n c e n t r a t i o n is m u c h less. T h e p H of most of t h e d r i e d samples w a s not measured.

T h e l a b e l i n g of the first six samples refers to t h e b o r e

holes f r o m w h i c h t h e y w e r e t a k e n . Bed-moist Samples.

A t t e m p t s to t a k e s p e c t r a of these samples a t

room temperature resulted i n very small absorption dips from w h i c h it w a s difficult to d i s c e r n a n y s t r u c t u r e . C o n s e q u e n t l y the m a j o r effort w a s p u t i n t o l i q u i d n i t r o g e n spectra, a l l of w h i c h gave at least five p o o r l y Table I.

Details of the Coal Samples

Form

M2275 LY1280 H1317 N3372 M2276 C92 Top-oxidized #15 #19

Bed-moist Bed-moist Bed-moist Bed-moist Bed-moist Partially dried Dried Briquette Briquette

0

Fe as FeS (Dry Basis (%))

Total Fe (Dry Basis (%))

pH

15.7-16.0 24.6-25.0 52.0-52.8 54.7-55.0 44.3-44.6 —

0.21 0.00 0.00 0.00 0.00 0.09

0.58 0.07 0.22 0.34 1.77 0.15

3.6 3.4 5.1 4.5 4.6 4.2

— —

0.36 0.00

0.42 0.80

2

Sample

Examined

Depth

(m)

* Dashes indicate that the information was neither known nor measured. concentrations are quoted on a dry basis.


Iroi

212

MOSSBAUER

Table II.

SPECTROSCOPY A N D

ITS

CHEMICAL

APPLICATIONS

Parameters of the Doublets Observed for the Bed-moist Samples at 78 K a

jg Site

Qg

(mms- )

(mms' )

1

1 2 3 4 5 6

Percentage M2275

1

+0.15(4) +0.16(6) +0.21(6) +0.22(2) +0.27(9) +0.38(5)

0.51(4) 1.20(6) 0.91(6) 0.00(4) 1.25(9) 0.61(5)

LY1280

Absorption

H1S17

NSS72

M2276

C92

60 — 11 22 — 7

67 — — 25 8 —

62 — 9 21 — —

37 19 19 19 — —

33 — — 18 16 —

43 10 18 14 — 8

— — — — —

— — — — —

— — — 7 —

7 — — — —

— 17 8 4 5

— — — 7 —


Single lines 7 8 9 10 11

-0.76(6) -0.21(6) -0.04(6) +1.02(6) +1.37(6)

° The single lines probably belong to doublets whose other member has not been positively identified. Isomer shifts are relative to iron metal at room temperature and the quadrupole splitting is l/2e qQ. The numbers in parentheses indicate the error in the last figure. 2

r e s o l v e d a n d o v e r l a p p i n g lines i n t h e r e g i o n b e t w e e n —0.04 a n d + 0 . 0 9 m m s"

1

w i t h respect to i r o n m e t a l . T h e d o m i n a n t a b s o r p t i o n p e a k for

e a c h s a m p l e v a r i e d b e t w e e n 0.1 a n d 0 . 4 % , d o w n to 0 . 0 3 % .

a n d peaks were identified

T y p i c a l l i n e w i d t h s ( F W H M ) w e r e a b o u t 0.32-0.35 m m

s" , w i t h o n l y a f e w lines b e i n g s i g n i f i c a n t l y n a r r o w e r t h a n this. 1

T w o sets o f peaks w e r e o b s e r v e d for a l l of the samples a n d d o m i n a t e d m o s t o f the s p e c t r a — a q u a d r u p o l e - s p l i t d o u b l e t w i t h a n i s o m e r shift of +0.15

m m s"

1

( a l l shifts w i l l b e q u o t e d w i t h respect to i r o n at r o o m

t e m p e r a t u r e ) a n d a s p l i t t i n g of 0.51 m m s' , a n d a s i n g l e - l i n e o r p o s s i b l y 1

a n u n r e s o l v e d q u a d r u p o l e - s p l i t d o u b l e t at + 0 . 2 2 m m s" . A t o t a l of 1

five

q u a d r u p o l e - s p l i t d o u b l e t s w e r e i d e n t i f i e d , a n d t h e i r p a r a m e t e r s are g i v e n i n T a b l e I I together w i t h details of t h e samples f o r w h i c h t h e y

were

o b s e r v e d . T h e p e r c e n t a g e a b s o r p t i o n values g i v e n are the f r a c t i o n of the t o t a l a b s o r p t i o n p r e s e n t i n t h a t resonance.

Sample inhomogeneities i n

e v i t a b l e i n the s m a l l samples u s e d i n M o s s b a u e r spectroscopy c a n alter these n u m b e r s c o n s i d e r a b l y so t h a t t h e i r a c c u r a c y is c e r t a i n l y n o t better than

±10%. I n a d d i t i o n , T a b l e I I lists several o t h e r lines t h a t are u n d o u b t e d l y

t h e u n m a t c h e d halves o f f u r t h e r d o u b l e t s , b u t w h o s e o t h e r h a l f is e i t h e r s u p e r i m p o s e d o n a stronger l i n e o r h i d d e n i n the w i n g s . A p p a r e n t l y there are n o s t r o n g p e a k s at l a r g e r v e l o c i t i e s as w o u l d b e the case if, for example, high-spin F e

2 +

w e r e present, b u t some s p e c t r a d i d s h o w e i t h e r

v e r y w e a k p e a k s or r e l a x a t i o n effects.

F u r t h e r s p e c t r a , p r e f e r a b l y at 4.2

K , are r e q u i r e d to resolve this.


9.

CASHION

E T

Victorian

AL.

Brown

213

Coal

F i g u r e 1 shows spectra of f o u r of t h e samples, a n d the d i f f e r i n g a n d g e n e r a l l y p o o r r e s o l u t i o n c a n b e seen c l e a r l y . T h e n u m b e r o f c h a n n e l s after f o l d i n g has b e e n r e d u c e d to 170 f o r a l l of these spectra. I n F i g u r e s l a a n d b w e see t w o of the c h a r a c t e r i s t i c spectra f r o m M 2 2 7 5 a n d L Y 1 2 8 0 d o m i n a t e d b y t h e t h r e e p e a k s m e n t i o n e d e a r l i e r . T h e s p e c t r u m of H 1 3 1 7 is s i m i l a r to these t w o .

F i g u r e l c shows t h e s p e c t r u m of N 3 3 7 2 w h i c h

h a d t h e poorest r e s o l u t i o n of a l l the samples s t u d i e d , w h i l e F i g u r e I d shows t h e s p e c t r u m of M 2 2 7 6 w h i c h contains m o r e t h a n three t i m e s t h e


i r o n c o n c e n t r a t i o n of a n y of the other samples. T h i s l a r g e r i r o n c o n c e n -

_l -12

I

I

I

I

I

-08

-04

0

04

08

L 12

VELOCITY (mm s-1)

Figure 1. Mossbauer spectra of bed-moist samples of (a) M227S (b) LY1280, (c) N3372, and (d) M2276 taken at 78 K (note the different vertical scales for each spectrum)


214

MOSSBAUER

SPECTROSCOPY A N D

ITS

CHEMICAL

APPLICATIONS

t r a t i o n is k n o w n to alter the types of i r o n c o m p o u n d s f o r m e d as w i l l b e d i s c u s s e d i n the next section. D r i e d Samples.

S a m p l e C 9 2 s h o w e d a s p e c t r u m s i m i l a r t o those of

t h e b e d - m o i s t samples a n d is i n c l u d e d w i t h t h e m i n T a b l e I I . Its spec t r u m is i n t e r m e d i a t e b e t w e e n those s h o w n i n F i g u r e s l b a n d c.

However,

t h e top o x i d i z e d s a m p l e ( F i g u r e 2 ) s h o w e d a c o m p l e t e l y different spec t r u m w i t h a m u c h l a r g e r recoilless f r a c t i o n . T h i s w a s c h a r a c t e r i s t i c of most of the d r i e d samples w h i c h g e n e r a l l y h a d a n i n t e n s e c e n t r a l d o u b l e t a n d a w e a k e r c o n t r i b u t i o n f r o m one or m o r e h y p e r f i n e fields, the l a t t e r u s u a l l y s h o w i n g signs of e l e c t r o n i c r e l a x a t i o n . I n a n effort to s h e d some l i g h t o n the m a r k e d difference b e t w e e n the Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch009

spectra of t h e b e d - m o i s t a n d d r i e d samples, w e r a n a n a b s o r b e r m a d e f r o m the outer b l a c k e n e d section of the M 2 2 7 6 s a m p l e w h i c h h a d s h o w n t h e largest p r e v i o u s d i p of 0 . 4 % .

T h e difference

between

this s p e c t r u m

( F i g u r e 3) a n d the bed-moist spectrum ( F i g u r e I d ) was startling.

The

m a x i m u m p e r c e n t a g e effect h a d i n c r e a s e d b y a l m o s t a f a c t o r of t e n to 3.5%

a n d t h e outer w i n g s n o w s h o w e d s t r o n g e v i d e n c e

relaxation

effects.

Clearly

the water molecules

of

electronic

h a d been intimately

b o n d e d to the i r o n atoms to p r o d u c e s u c h a l a r g e c h a n g e , a n d t h i s w i l l b e discussed f u r t h e r i n the next section. T h e b r i q u e t t e d samples s h o w e d s p e c t r a s i m i l a r to e a c h other, c o n s i s t i n g of a v e r y intense d o u b l e t ( 4 %

d i p ) , a n d r e l a x a t i o n effects c o r r e

s p o n d i n g to a m a g n e t i c h y p e r f i n e s p l i t t i n g .

S a m p l e 15 also h a d a l a r g e

100

£

9 8 -

96 -

-12

-4

0

4

1

1

12

mm/s

Figure 2.

Mossbauer

spectrum of the top-oxidized sample taken

at78K



9.

CASHION

Figure

3.

E T A L .

Victorian

Brown

215

Coal

Mossbauer spectrum taken at 78 K of a sample of which had been slightly oxidized on exposure to the air

M2276

a d d i t i o n a l l i n e of i n t e n s i t y 1 % a t + 2 . 7 m m s" c l e a r l y associated w i t h 1

another line near zero velocity superimposed o n the m a i n doublet. T h e o b s e r v e d p a r a m e t e r s f o r a l l o f these samples are g i v e n i n T a b l e III. Table III.

Parai

O b s e r v e d f o r t h e D r i e d C o a l Samples*

Temp (K)

IS (mm s' )

QS (mm s' )

#15

300

0.45(4) 2.8(1)

#29

300

0.34(4) 1.3(1) 0.3(2) 0.35(4) 0.3(2) 0.51(4) 0.4(2) 0.6(3) 0.57(3) 0.4(2) 0.5(1) 0.3(3) 0.20(4) 0.31(4) 0.46(2) 0.48(3)

Sample

Dried M2276

78

Top-oxidized

78

Partially dried cleaned coal

300

1

1

0.60(4) 0.78(4)

0.32(3) -0.5(2) -0.5(1) 0.40(4) 0.62(4) 0.89(3) 0.96(5)

Hyperfine Field (T) 0 0 probably ~ 3 3 0 34(4) 0 43(2) ) 55(2) i 0 33(4) 48(1) 55(3) 0 0 0 0

Absorption (%) 54 8 38 30 70 30 70 /U

36 9 35 20

• Values marked (*) could not be determined but are not necessarily zero. F o r other explanations, see Table II.


216

MOSSBAUER

SPECTROSCOPY A N D

ITS

CHEMICAL

APPLICATIONS

Interpretations and Discussion C h e m i c a l m e a s u r e m e n t s o n L a t r o b e V a l l e y coals h a v e s h o w n

(8)

that, at l o w c o n c e n t r a t i o n s , the n o n p y r i t i c i r o n is u s u a l l y present as l o w s p i n i r o n ( I I ) b o n d e d to c a r b o x y l a t e s , b u t at h i g h e r c o n c e n t r a t i o n s , o x y h y d r o x i d e s m a y start f o r m i n g . I t w a s e x p e c t e d t h a t S a m p l e M 2 2 7 6 w a s t h e o n l y one to b e i n this r e g i m e . I t w a s a s s u m e d i n i t i a l l y t h a t the s p e c t r a of o u r coals s h o u l d c l o s e l y r e s e m b l e those f r o m t h e U . S . coals, so a s s i g n m e n t of t h e p e a k s menced b y considering b o t h the parameters obtained there a n d d a t a o n oxides a n d h y d r o x i d e s (11)

a n d clays ( 1 2 ) .

com

(4,5,6,7)

T h e o n l y site

i n T a b l e I I t h a t d e f i n i t e l y c o u l d b e a s s i g n e d o n this basis w a s Site 6, Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch009

w h i c h is c l e a r l y a t t r i b u t a b l e to p y r i t e . Its o b s e r v a t i o n i n S a m p l e s M 2 2 7 5 a n d C 9 2 c o r r e l a t e d w i t h t h e i n i t i a l c h e m i c a l analyses c a r r i e d out at t h e H e r m a n R e s e a r c h L a b o r a t o r i e s , a l t h o u g h o u r q u a n t i t a t i v e v a l u e is m u c h lower presumably because of a n inhomogeneous distribution. Sites 1-5 c o u l d not b e a s s i g n e d to a n y of m o r e t h a n 50

compounds

c o n s i d e r e d , a n d this i n d i c a t e d either n e w types of i r o n c o m p o u n d s

such

as o r g a n i c a l l y b o n d e d i r o n o r a l t e r a t i o n of t h e p a r a m e t e r s f r o m k n o w n c o m p o u n d s r e s u l t i n g f r o m t h e h i g h w a t e r content. S i t e 4 c o u l d c o r r e s p o n d to F e

3 +

i n a symmetric electronic environment or perhaps chalcopyrite

( C u , F e ) S , a l t h o u g h this c o m p o u n d c o m m o n l y p r o d u c e s a m a g n e t i c a l l y 2

s p l i t s p e c t r u m at 78 K . O n e o r g a n i c a l l y b o n d e d c o m p o u n d of the f o r m e r t y p e t h a t has the same p a r a m e t e r s is h y d r a t e d f e r r i c oxalate, Fe

2

(C 0 )3 2

4

•5 H 0 . 2

T h e r e m a i n i n g sites d o n o t c o r r e l a t e w i t h k n o w n p a r a m e t e r s f o r o t h e r c a r b o x y l a t e s , a l t h o u g h most h y d r o l y z e d i r o n ( I I I ) c a r b o x y l a t e s h a v e i s o m e r shifts of 0.2-0.5 m m s"* a n d q u a d r u p o l e s p l i t t i n g s of 0.2-1.0 m m s" 1

1

(see,

e.g., R e f s . 13-20).

T h e p a r a m e t e r s f o r these sites also c o v e r t h e r e g i o n of

low-spin iron (II)

compounds.

W e b e l i e v e t h a t a l l of these sites c o r r e

s p o n d to o r g a n i c a l l y b o n d e d i r o n a n d t h a t the h i g h m o i s t u r e content a n d m u l t i p l i c i t y of different l i g a n d s are a l t e r i n g p a r a m e t e r s f r o m the p r e v i o u s l y o b s e r v e d v a l u e s f o r these c o m p o u n d s .

T h i s b e l i e f is s u p p o r t e d b y

m e a s u r e m e n t s t h a t w e h a v e c a r r i e d o u t o n a separate p r o j e c t i n v o l v i n g c a t a l y t i c h y d r o g e n a t i o n of s o m e L a t r o b e V a l l e y coals t h a t h a v e

been

d r i e d , c r u s h e d to —60 m e s h , a n d floated w i t h C C 1 to r e m o v e most of t h e 4

m i n e r a l m a t t e r (21). + 0 . 3 1 ( 4 ) m m s"

1

I n a d d i t i o n to t h e p y r i t e p e a k s at a n i s o m e r shift of

a n d a q u a d r u p o l e s p l i t t i n g of 0 . 6 2 ( 4 ) m m s" , these 1

coals s h o w e d three o t h e r d o u b l e t s w h i c h are also l i s t e d at t h e b o t t o m of T a b l e I I I a n d no e v i d e n c e of Sites 1-5 i n T a b l e I I . T y p i c a l a b s o r p t i o n dips were about 3 % , but the linewidths w e r e broader than the bed-moist samples, b e i n g b e t w e e n 0.35-0.50 m m s' . 1

T h e c h a r a c t e r i s t i c s i x - c o o r d i n a t e d b o n d i n g of i r o n p r o b a b l y a l l o w s considerable competition between water a n d carboxylate ligands.


The

9.

CASHION

E T

Victorian

AL.

changes o b s e r v e d

Brown

217

Coal

b e t w e e n the spectra of b e d - m o i s t a n d d r i e d M 2 2 7 6

also i n d i c a t e t h a t t h e w a t e r m u s t b e i n t i m a t e l y b o n d e d to t h e i r o n .

The

c h a n g e of a n o r d e r of m a g n i t u d e i n the D e b y e - W a l l e r f a c t o r for a 1 4 - k e V g a m m a r a y is t o t a l l y u n e x p e c t e d a n d shows t h a t the i r o n e n v i r o n m e n t i n the b e d - m o i s t coals m u s t b e d e c i d e d l y " s l o p p y . " T h e q u a d r u p o l e - s p l i t d o u b l e t s of the d r i e d samples are a l l s h i f t e d to m o r e p o s i t i v e i s o m e r shifts t h a n those of t h e b e d - m o i s t samples. t r e n d is also e v i d e n t f r o m t h e other p r o j e c t measurements.

This

However,

a s s i g n i n g the peaks is a g a i n difficult w i t h o r g a n i c a l l y b o n d e d i r o n p r o b a b l y p r o d u c i n g the intense c e n t r a l d o u b l e t s i n the s p e c t r a f r o m S a m p l e s 15 a n d 29. T h e d o u b l e t w i t h the s p l i t t i n g of 2.8 m m s" i n S a m p l e 15 is 1


a t t r i b u t a b l e to s z o m o l n o k i t e , F e S 0

4

• H 0 a r i s i n g f r o m o x i d a t i o n of the 2

sulfide. N o t e t h a t t h e p r o m i n e n t d o u b l e t i n t h e o x i d i z e d M 2 2 7 6 s p e c t r u m does not c o r r e s p o n d to a n y of the d o u b l e t s i n the o r i g i n a l sample. F o u r values of h y p e r f i n e field c o u l d b e d i s c e r n e d i n the r e l a x e d w i n g s of these samples, w i t h o n l y the t o p - o x i d i z e d s a m p l e g i v i n g a n y c l e a n s p l i t t i n g s . I n S a m p l e 15, t h e d i p w a s so w e a k a n d r e l a x a t i o n sufficiently fast t h a t the field v a l u e is o n l y a n estimate, b u t S a m p l e 29 s h o w e d

a

m a x i m u m v a l u e of 34 T . T h e o x i d i z e d M 2 2 7 6 s h o w e d s p l i t t i n g s o u t to 5 5 T w h i l e the t o p o x i d i z e d s a m p l e s h o w e d three fields of 33 T , 48 T , a n d 55 T , w i t h the 4 8 - T field b e i n g w e l l r e s o l v e d ( F i g u r e 2 ) .

We

believe

that the 4 8 - T field results f r o m geothite, a - F e O O H , w i t h a s m a l l p a r t i c l e size, s i m i l a r to t h a t o b s e r v e d b y G o o d m a n a n d B e r r o w ( 2 2 ) i n t h e i r s t u d y of S c o t t i s h peats. T h e c r i t i c a l size for s u p e r p a r a m a g n e t i s m i n « - F e O O H is a b o u t 4 n m , a n d x - r a y d i f f r a c t i o n measurements are consistent w i t h t h e conjecture of s m a l l p a r t i c l e sizes. T h e 5 5 - T field is p r o b a b l y a t t r i b u t a b l e to h e m a t i t e , a - F e 0 , a l t h o u g h l i q u i d h e l i u m experiments are r e q u i r e d 2

3

to r u l e o u t m a g n e t i t e , F e 0 . 3

4

T h e 3 3 - T field p r o b a b l y results f r o m t r i -

v a l e n t i r o n b o n d e d to o r g a n i c m a t e r i a l , w i t h the i r o n - i r o n s e p a r a t i o n b e i n g the r i g h t d i s t a n c e to g i v e r e l a x a t i o n effects at l i q u i d n i t r o g e n t e m peratures. S u r p r i s i n g l y , a s p e c t r u m of the o x i d i z e d M 2 2 7 6 s a m p l e t a k e n at 4.2 K f a i l e d to s h o w a n y s h a r p e n i n g of the r e l a x a t i o n . Conclusions T h e i n i t i a l a i m of this p r o j e c t w a s to i n v e s t i g a t e t h e usefulness

of

M o s s b a u e r s p e c t r o s c o p y for c h a r a c t e r i z i n g coals i n t h e b e d - m o i s t c o n d i t i o n . T h e results s h o w a c l e a r l y o b s e r v a b l e difference b e t w e e n t h e s a m p l e e x p e c t e d to c o n t a i n o x y h y d r o x i d e s a n d those l o w e r i n i r o n w h i c h o n l y c o n t a i n carboxylates.

T h e s p e c t r a o b s e r v e d are m a r k e d l y different f r o m

those o b t a i n e d o n h i g h e r - r a n k U . S . coals for w h i c h o n l y i n f o r m a t i o n o n m i n e r a l s a n d clays has b e e n o b t a i n e d . T h e d r y i n g of the c o a l a l t e r e d t h e o b s e r v e d completely,

Mossbauer

spectrum

i n c r e a s i n g b o t h the D e b y e - W a l l e r factor a n d the


isomer

218

MOSSBAUER

SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS

shift. T h e s e changes are consistent w i t h a c h a n g e f r o m a l a b i l e , loosely b o u n d e n v i r o n m e n t t o a t i g h t l y c o n s t r a i n e d site. T h e e n o r m i t y of t h e c h a n g e b o t h i n t h e shape o f t h e s p e c t r u m a n d t h e size of t h e D e b y e - W a l l e r f a c t o r w h e n t h e c o a l is d r i e d opens u p t h e p o s s i b i l i t y t o s t u d y o n a m i c r o s c o p i c scale t h e p o o r l y u n d e r s t o o d process of t h e d r y i n g o f c o a l .

C h e m i c a l measurements u s u a l l y d e t e r m i n e t o t a l

w a t e r content o r changes i n w a t e r content, a n d as p o i n t e d o u t b y A l l a r d i c e a n d E v a n s ( 9 ) , c a r e m u s t b e t a k e n n o t t o c o n f u s e t h e r m a l release o f chemically combined water w i t h accelerated diffusion o f water o u t of m i c r o p o r e s because o f a n increase i n t e m p e r a t u r e . W i t h i r o n carboxylates b e i n g t h e most a c t i v e h y d r o p h i l i c site, i t appears t h a t M o s s b a u e r s p e c Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch009

troscopy

is w e l l p l a c e d to i n v e s t i g a t e selectively a t least o n e o f t h e

different varieties o f b o u n d w a t e r . F u r t h e r e x p e r i m e n t s at l i q u i d h e l i u m t e m p e r a t u r e s a n d i n a p p l i e d m a g n e t i c fields are r e q u i r e d to a i d i n t h e i n t e r p r e t a t i o n o f t h e b e d - m o i s t spectra a n d to c o n f i r m t h e assignments o f t h e m a g n e t i c a l l y s p l i t spectra. A n o t h e r possible extension of this w o r k w o u l d b e a s t u d y of t h e changes i n t h e c o a l d u r i n g t h e b r i q u e t t i n g process.

Acknowledgments T h i s w o r k w a s c a r r i e d o u t u n d e r c o n t r a c t t o t h e V i c t o r i a n State Electricity Commission a n d w a s supported b y the Australian Research Grants Committee.

T h e c o a l samples w e r e s u p p l i e d b y courtesy o f t h e

Victorian B r o w n C o a l Council. W e wish to thank P . E . Clark, F . P . L a r k i n s , a n d K . S. M u r r a y f o r h e l p f u l discussions.

W e are grateful to

G . P . H u f f m a n f o r f o r w a r d i n g a c o p y o f his r e v i e w b e f o r e p u b l i c a t i o n . Literature

Cited

1. Lefelhocz, J. F . ; Friedel, R. A . ; Kohman, T . P. Geochim. Cosmochim. Acta 1967, 31, 2261—2273. 2. Kohman, T. P.; Ulmer, J. D. U S A E C Report NYO-844-81, Sec.II.A.4, 1970, pp. 31-36. 3. Kohman, T . P.; Karol, P. J.; Kamarchik, P. U S A E C Report COO-3236-3, Sec.II.A.1, 1971, pp. 5-16. 4. Huffman, G . P.; Huggins, F . E. Fuel 1978, 57, 592-604. 5. Huggins, F . E.; Huffman, G . P. "Analytical Methods for Coal and Coal Products"; Karr, C., Jr., E d . ; Academic: New York, 1979; Vol. 3, pp. 371—423. 6. Jacobs, I. S. Levinson, L . M . ; Hart, H. R., Jr. J. Appl. Phys. 1978, 49, 1775-1780. 7. Russell, P. E.; Montano, P. A. J. Appl. Phys. 1978, 49, 4615-4617. 8. Schafer, H . N . S. Fuel 1977, 56, 45-46. 9. Allardice, D . J.; Evans, D . G . "Analytical Methods for Coal and Coal Products"; Karr, C., Jr., E d . ; Academic: New York, 1978, Vol. 1, pp. 247—262. 10. Schafer, H. N. S. Fuel 1972, 51, 4-9. 11. Bowen, L . H. Mössbauer Eff. Ref. Data J. 1979, 3, 76-94. ;



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CASHION

E T

A L .

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Brown

Coal

219

12. Ericsson, T . ; Wappling, R.; Punakivi, K . Geol Foeren. Stockholm Foerh. 1977, 99, 229—244. 13. Duncan, J. F . ; Kanekar, C . R.; Mok, K. F. J. Chem. Soc. 1969, A , 480-482. 14. Malathi, N . ; Puri, S. P. J. Phys. Soc. Jpn. 1970, 29, 108-111. 15. Takano, M . J. Phys. Soc. Jpn. 1972, 33, 1312-1317. 16. Lupu, D . ; Barb, D . ; Filoti, G . ; Morariu, M . ; Tarina, D . J. Inorg. Nucl. Chem. 1972, 34, 2803-2810. 17. Rumbold, B. D . ; Wilson, G . V . H. J. Phys. Chem. Solids 1973, 34, 18871891. 18. Long, G . J.; Robinson, W . T . ; Tappmeyer, W . P.; Bridges, D . L. J. Chem. Soc. Dalton 1973, 573-579. 19. Dezsi, I.; Pardavi Hordath, M . ; Molnar, B. Chem. Phys. Lett. 1973, 22, 384-386. 20. Steger, H . F . J. Inorg. Nucl. Chem. 1975, 37, 39-43. 21. Cashion, J. D . ; Clark, P. E.; Cook, P. S.; Larkins, F . P.; Marshall, M . In "Nuclear and Electron Resonance Spectroscopies Applied to Materials Science"; Kaufmann, E . N . ; Shenoy, G . K., Eds.; North-Holland: New York, 1981; in press. 22. Goodman, B. A . ; Berrow, M . L. J. Phys. (Paris) 1976, 37(C6), 849-855. R E C E I V E D June 27, 1980.


MÃ¶ssbauer Effect Study of Victorian Brown Coal - ACS Publications

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