Near-Infrared Correlation Spectroscopy - ACS Symposium Series

Chapter 21

Near-Infrared Correlation Spectroscopy Quantifying Iron and Surface Water in a Series of Variably Cation-Exchanged Montmorillonite Clays 1,6

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Lelia M. Coyne , J. L. Bishop , T. Scattergood , A. Banin , G. Carle , and J. Orenberg 5

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San Jose State University, San Jose, CA 95192 State University of New York at Stony Brook, Stony Brook, NY 11794 Hebrew University, Rehovot 76100, Israel NASA-Ames Research Center, Moffett Field, CA 94035 San Francisco State University, San Francisco, CA 94132

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Downloaded by AUBURN UNIV on March 3, 2016 | http://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch021

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Minerals are multisite catalysts for a variety of natural reactions. Clay minerals contain most of the generic types active sites of all minerals. Determining reaction mechanisms on natural catalysts is difficult because they frequently are not monomineralic and contain multiple active centers. Near Infrared Reflectance Analysis (NIRA) is a promising method for studying determinants of chemical reactions on mineral surfaces. Using NIRA, the quantitative relationship between absorbance and both iron and hydration was examined in a series of variably Ca/Fe-exchanged montmorillonite clays prepared from crude SWy and Otay montmorillonites (naturally occurring in the Na/Ca form). We have found linear relationships in the 600-1100 nm wavelength region between spectral absorbance and total iron (as Fe O ) over a range of 1-7% (0 - 100% of the cation exchange capacity for both parents) and in the 600-1100 nm and 1100-2500 nm wavelength regions between spectral absorbance and water content resulting from variation of relative humidity over a range of 4-100%. The correlation with iron is relatively insensitive to the location (surface vs. structural) of the iron. The correlation with moisture is keenly sensitive to the environment of adsorbed water. We also have determined that, on increasing water content, absorbance is increased over most of the 600-1100 nm (iron) wavelength range in Naor Ca-clays whereas it is decreased in Fe-clays. By contrast, absorbance is 2

3

'Current address: Mail Stop 239-4, NASA-Ames Research Center, Moffett Field, CA 94035 0097-6156/90A>415-0407$06.75A) e 1990 American Chemical Society

In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, Lelia M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

408

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES


increased with increasing water for all clays in the 1100-2500 nm (hydroxyl) region. The sign reversal of the correlation with water in the region of iron absorption in the Fe-clay can be attributed to hydroxylation of the surface iron with consequent decrease of iron absorption. Clay minerals have long been recognized as multisite catalysts for a variety of organic reactions (1-7). Among the demonstrated determinants of c a t a l y t i c a c t i v i t y are structural cation substitutions and t h e i r charge compensating exchangeable cations, the number and c o n d i t i o n of the s t r u c t u r a l hydroxyls and the degree of hydration of the surface. Some examples of these determinants are: a) structural ferric iron in the mechanism o f m o n t m o r i l l o n i t e - i n d u c e d h y d r a z i n e o x i d a t i o n (SL) ; b ) e x c h a n g e a b l e iron in the oxidation of carboxylic acids (the l a b e l e d r e l e a s e e x p e r i m e n t u s e d as one o f the V i k i n g b i o l o g y experiments t o t e s t the s u r f a c e o f Mars f o r life) (JL) ; c) residual structural hydroxyls in the p o l y m e r i z a t i o n of styrene on c a l c i n e d c l a y aluminosilicates (1H, p . 1 8 1 ) ; d) d i s s o c i a b l e w a t e r i n t h e dehydration of alcohols (11). Trapped separated charge pairs stored as 0~-centers have been shown in other mineral systems (12) and p o s t u l a t e d in clay systems (13-14) t o i n f l u e n c e s u r f a c e reactivity. The a c t i v i t y of clay minerals, proven in the reactivity of t e r r e s t r i a l (15-16), and p o s t u l a t e d in Martian Q) soils, is disproportionate to t h e i r quantity, r e l a t i v e to other minerals. This is the result of several factors: small particle size, high specific surface area, B r o n s t e d and Lewis a c i d i t y , redox and other potentially c a t a l y t i c a l l y a c t i v e s i t e s common t o c l a y m i n e r a l s , a n d a limited capacity for size exclusion (which i s influenced by t h e number and v a l e n c e o f e x c h a n g e a b l e c a t i o n s ( £ ) ) . Although the surface reactivity of clays has been studied, there are several reasons why the true determinants of reactivity have not yet been cleanly characterized. These a r e : a) Clay minerals represent a broad class of n o n - s t o i c h i o m e t r i c compounds, thus t h e y o c c u r w i t h a wide range o f compositions. b) Experimentally it is demanding to select or prepare natural materials which d i f f e r i n the concentration or condition of only one type of site. Therefore, a t t r i b u t i n g the dependence of a reaction to a given s i t e i s d i f f i c u l t , because any change i n the reaction rate induced by alteration of this site may be enhanced o r d i m i n i s h e d by a d v e n t i t i o u s alteration of another. c) A putative site, i n i t s e l f , may b e l e s s i m p o r t a n t t h a n covarying factors. F o r example, a c t i v i t y may a p p e a r t o be c o r r e l a t e d w i t h i n c r e a s i n g c a t i o n i c substitution


21. COYNE ET AL»

d)


e)

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Near-Infrared Correlation Spectroscopy

i n t h e c r y s t a l s t r u c t u r e , b u t i n s t e a d may a c t u a l l y b e associated with a c i d i t y of the waters of hydration of the exchangeable cations. R e c e n t s t u d i e s h a v e shown i n t e r a c t i o n b e t w e e n several c l a y s i t e s p r e v i o u s l y c o n s i d e r e d t o be independent, i.e. interlayer hydration, the oxidation state of s t r u c t u r a l i r o n , a n d t h e O - c e n t e r p o p u l a t i o n (11). The o p t i m a l concentration of a structural entity serving in more than one role may result from a compromise between amounts m a x i m i z i n g i t s different effects. For instance, the a c i d i t y of the surface of clays i s a strong function of the degree of dryness, as Bronsted acidity is thought to result from the dissociation of water of hydration of exchangeable cations (18-20). Thus, the a c t i v i t y o f the surface i n an acid catalyzed reaction, such as styrene p o l y m e r i z a t i o n (1Û), s h o u l d b e i n c r e a s e d b y i n c r e a s i n g dryness. On t h e o t h e r h a n d , t h e m o b i l i t y o f reactants t o w a r d t h e s u r f a c e a n d p r o d u c t s away f r o m i t w i l l be i n f l u e n c e d by the presence o f a t r a n s p o r t i n g solvent, such as water. Therefore, the optimal water concentration may l i e somewhere between those maximizing acidity and mobility. In fact, the formation of peptide bonds on clay surfaces is e f f i c i e n t l y promoted only in a wet/dry cycling reaction protocol (2JL) / a common condition in geological settings and, likely, of great reactive significance.

f)

Exclusive responsibility of a single site for a given r e a c t i o n may n o t b e t h e c a s e . M o r e t h a n o n e s i t e may be i n v o l v e d , as most r e a c t i o n s p r o c e e d t h r o u g h s e v e r a l s t e p s , n o t a l l o f w h i c h may b e f a c i l i t a t e d b y t h e same s i t e . For instance, montmorillonite is a m u l t i f u n c t i o n a l c a t a l y s t i n t h e c y c l i z a t i o n o f 3 AMP to 2', 3 * - c y c l i c AMP, where i t s e r v e s as a selective adsorbent for products and reactants, and as a catalyst for two reactions within the overall t r a n s f o r m a t i o n (22). F o r a l l o f the above r e a s o n s , studies of relationships between m i n e r a l s i t e s and m i n e r a l r e a c t i v i t y would be simplified, and the confidence i n the f i n d i n g s increased, i f statistically testable methods were to be used to identify key v a r i a b l e s and to test hypotheses. Near infrared reflectance analysis (NIRA) i s s u c h a m e t h o d . It was d e v e l o p e d b y K a r l N o r r i s i n t h e I 9 6 0 ' s t o quantitate the c o n c e n t r a t i o n of key c o n s t i t u e n t s i n multicomponent m i x t u r e s s u c h a s w h e a t (22). In NIRA, a f t e r measurement o f t h e a b s o r p t i o n s p e c t r a , (determined from reflectance data) independently determined values of quantities of interest are entered i n t o a computer as c o n s t i t u e n t d a t a . A linear regression of constituent amount as a function of wavelength is performed throughout the spectrum. Analytical wavelengths s u i t a b l e for q u a n t i f y i n g the constituent of interest are selected using correlation coefficients, standard errors 1



410


and t h e s e n s i t i v i t y of the absorbance changes with change i n c o n s t i t u e n t (2i). The structural and spectral complexity of clay minerals is sufficient t o c o n s i d e r a s i n g l e m i n e r a l as a multicomponent mixture in itself. Detectible by near infrared spectroscopy are adsorbed water and s t r u c t u r a l hydroxyls (2H), exchangeable and s t r u c t u r a l transition metal cations (2 6 and this work), adsorbed species i n c l u d i n g atmospheric gases (22) / o r g a n i c m a t e r i a l s (2)/ accessory minerals (28) and, possibly, trapped hole centers (0"-centers) . Thus i t is of interest to adapt NIRA t o s t u d i e s o f m i n e r a l s u r f a c e a c t i v i t y . We h a v e d o n e t h i s by examination o f a small set of highly homologous c l a y s i n w h i c h l a b o r a t o r y c o n t r o l o f o n l y one v a r i a b l e a t a time c o u l d be accurately achieved and independently confirmed. In t h e c u r r e n t s t u d y , we t e s t e d t h e a p p l i c a b i l i t y o f near infrared spectroscopy coupled with correlation analysis to quantitation of the dependence of m o n t m o r i l l o n i t e s p e c t r a on two o f t h e s t r u c t u r a l features known t o be o f importance in clay surface chemistry hydration and exchangeable iron. These clays were d e v e l o p e d as M a r s S o i l A n a l o g M a t e r i a l s (MarSAMs) i n t h a t they satisfy the constraints of the mineralogical c o m p o s i t i o n o f Mars as d e t e r m i n e d f r o m consideration of the Martian reflectance signature, the elemental analysis and s i m u l a t i o n s o f M a r t i a n s u r f a c e c h e m i s t r y d e t e r m i n e d by V i k i n g , and the presumed g e o l o g i c a l h i s t o r y o f Mars.

Experimental Clays. The v a r i a b l y exchanged C a / F e m o n t m o r i l l o n i t e clays used for the i r o n dependence aspect of this study were p r e p a r e d a n d a n a l y z e d u s i n g m e t h o d s d e s c r i b e d i n B a n i n q£. â l . (2â) . The t o t a l i r o n c o n t e n t s o f the c l a y s p r e p a r e d from two p a r e n t materials having different amounts of structural iron are summarized i n Table I . F o r t h e adsorbed water studies, c r u d e SWy a n d i t s 100% C a a n d F e f o r m s p r e p a r e d i n 1987 b y t h e same m e t h o d w e r e u s e d . Table

I.

IRON CONTENT

O F MARSAM

MATERIAL

~ ~ ~

CRUDE SWY 0 % Fe (1985) 20 % Fe (1985) 50 % Fe (1985) 80 % Fe (1985) 100 % Fe (1985) Fe (1982) 100 % CRUDE OTAY 100 % F E O T A Y ( 1 9 8 5 )

'85

SAMPLE S E T

%Fe203

3.61 3.87 4.11 5.21 5.96 6.08 6.87 1.11 3.54


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Pellet

Preparation. C l a y s a m p l e s o f 1 . 0 6 g r a m s e a c h (an amount j u s t a d e q u a t e t o f i l l a s t a i n l e s s steel planchette w i t h t h e c r u d e m a t e r i a l a f t e r p r e s s i n g ) were p l a c e d i n t h e d i e o f a p e l l e t p r e s s o b t a i n e d from E n g l i s h China Clays I n t e r n a t i o n a l a n d c o v e r e d w i t h a n i n v e r t e d 2 . 5 4 cm o . d b y 2 mm d e e p t y p e 3 0 4 s t a i n l e s s steel planchette. They were p r e s s e d f o r 30 s e c o n d s a g a i n s t a g l a s s o p t i c a l f l a t using a p r e s s u r e o f 1.2 kgm/cm^ a t t h e p i s t o n h e a d i n o r d e r t o form a compact, but s o f t , pellet. As the densities of the various clays were different, some of the materials o v e r f i l l e d the planchettes. These samples were s t r u c k o f f with a stainless steel blade, re-weighed and the pellet r e - p r e s s e d u p r i g h t a g a i n s t the f l a t t o smooth the surface. The f i l l e d planchette was c e n t e r e d on a s p r i n g - l o a d e d platform i n the sample h o l d e r and c o v e r e d w i t h a t h i n quartz cover plate sealed into a screwcap cover. The r e - p r e s s e d samples appeared smooth a f t e r placement i n the sample holder, but some surface roughness may have r e s u l t e d from the s t r i k i n g - o f f p r o c e d u r e . Humidity E q u i l i b r a t i o n .

The c l a y s were h u m i d i f i e d a t 2 5 ° C in desiccators using a series of eutectic salts. Each d e s s i c a t o r was c o n t r o l l e d a t a d i f f e r e n t r e l a t i v e h u m i d i t y u s i n g o n e o f t h e s a t u r a t e d s a l t s o l u t i o n s l i s t e d i n Table II. The samples were weighed r e p e a t e d l y u n t i l equilibrium (constant weight) was r e a c h e d . After the spectra were obtained, the samples were b r o k e n i n t o t h r e e aliquots, r e w e i g h e d a n d d r i e d f o r ~24 h o u r s a t 1 0 5 ° C t o o b t a i n t h e dry weights of the clay, from which the water contents were calculated. The water contents, as percent dry w e i g h t o f t h e c l a y , a r e a l s o s h o w n i n Table I I . Table

II.

Eutectic

S a l t s and D e s i c c a n t s Humidity Study

Salt/Desiccant

CaCl

%RH

(solid)

2

CH3COOK

(satd.

)

Relative

Τ (»C) Ref. % Water Content crude CA Fe —

Q£)

2.7

4.3

1.4

20.

20.

(11)

5.1

10.0

7.7

22.5

25.

(21)

6.5 sol.

Used i n

(satd.

sol.

)

51.

24.5

(21)

10.6

17.6

12.7

NH C1 4

(satd.

sol.

)

79.

25.

(21)

14.3

18.2

14.8

CuS04

(satd.

sol.

)

98.

20.

(21)

26.9

Ca(N0 ) 3

2

26.9

Reflectance Spectroscopy. S p e c t r a w e r e t a k e n f r o m 600 2500 nm u s i n g a Mark II 6250 Diffuse Reflectance Spectrometer o b t a i n e d from P a c i f i c S c i e n t i f i c . This


412


spectrometer is a single beam instrument with 0/45° i l l u m i n a t i o n / d e t e c t i o n . A l l o f the s p e c t r a were r e f e r e n c e d to a ceramic standard, which exhibits 82 ± 2% r e f l e c t a n c e i n t h e 6 0 0 - 2 5 0 0 nm r a n g e . Measurements of spectra using different sizes of planchettes showed that planchettes of l /4" (3.18 cm) d i a m e t e r g a v e s p e c t r a e q u i v a l e n t t o t h o s e o f 1" ( 2 . 5 4 cm) planchettes. Increased noise and spectral distortion occurred using 3/4" (1.91 cm) planchettes due to i n t e r c e p t i o n o f t h e i n c i d e n t l i g h t beam b y t h e edge o f the planchette. S i n c e no d i f f e r e n c e s were found i n spectra u s i n g s a m p l e t h i c k n e s s e s f r o m 2 t o 5 mm, o n l y 2 mm t h i c k samples were u s e d . T h e i r i s o f t h e s p e c t r o m e t e r was set a t 1/4 f u l l opening to minimize the undesirable spectral contributions from n o i s e and from the Wood's anomaly of the g r a t i n g which appears at 1 5 1 0 - 1 5 2 0 nm f o r t h e NIR g r a t i n g ( b l a z e d a t 2 0 0 0 nm) a n d a t - 7 7 0 nm f o r t h e m i d - N I R g r a t i n g ( b l a z e d a t 900 n m . . In order to minimize s p e c t r a l variations resulting from sample placement, several reflectance curves were measured for each sample as a f u n c t i o n of sample r o t a t i o n . F i f t y replicate scans were made i n e a c h o f six horizontal rotations and averaged, giving a final spectrum for each clay composed of 300 s c a n s χ t h e number o f r e p l i c a t e samples.


1

Samples used f o r the i r o n d e t e r m i n a t i o n were scanned in the wavelength r e g i o n o f 6 0 0 - 1 1 0 0 nm u s i n g t h e first order of the mid-NIR grating. Samples used for the m o i s t u r e d e t e r m i n a t i o n were scanned i n wavelength region of 1 1 0 0 - 2 5 0 0 nm i n t h e f i r s t o r d e r o f t h e N I R g r a t i n g a n d from 680-1235 nm u s i n g the second order of the same grating. Second order spectra show s l i g h t anomalies a t t r i b u t a b l e to incomplete r e j e c t i o n of f i r s t order l i g h t . Data A n a l y s i s . A 10 p o i n t c u r v e s m o o t h i n g u s i n g 1 0 nm s e g m e n t s was p e r f o r m e d on t h e data in preparation for derivatization. The s m o o t h i n g makes no a p p a r e n t c h a n g e in t h e a p p e a r a n c e o f t h e o r i g i n a l s p e c t r a i n t h e 1 1 0 0 - 2 5 0 0 nm wavelength r e g i o n , and only suppresses minor n o i s e i n the 680-1235 or 600-1100 nm r e g i o n s . However, noise in unsmoothed s p e c t r a might r e s u l t i n more s i g n i f i c a n t errors in the c o r r e l a t i o n coefficients calculated from derivative spectra. T h e r e p r o d u c i b i l i t y o f c r u d e SWy c l a y s p e c t r a t a k e n i n t h e two w a v e l e n g t h regions is shown in F i g u r e s 1 & 2. First derivatives (called Dl) of those s p e c t r a t a k e n u s i n g b o t h o r d e r s o f the NIR g r a t i n g (giving 1 1 0 0 - 2 5 0 0 nm a n d 6 8 0 - 1 2 3 5 nm r e g i o n s ) were calculated u s i n g 10 nm s e g m e n t s w i t h g a p s o f 10 nm b e t w e e n s e g m e n t s . Second derivatives (called D2) of spectra using this g r a t i n g w e r e c a l c u l a t e d u s i n g 20 nm s e g m e n t s a n d n o gap between segments. F i r s t d e r i v a t i v e s o f s p e c t r a taken u s i n g the mid-NIR g r a t i n g ( 6 0 0 - 1 1 0 0 nm) w e r e c a l c u l a t e d like those of first derivatives using the NIR g r a t i n g , but second d e r i v a t i v e s w e r e c a l c u l a t e d u s i n g 2 0 nm s e g m e n t s a n d 10 nm g a p s b e t w e e n t h e m . B a s e l i n e o f f s e t s are removed by t h e f i r s t d e r i v a t i v e , and c o n s t a n t s l o p e s i n t h e


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.349 Γ

.063 1100 1300

413

ROTATIONAL DIFFERENCES REPLICATE DIFFERENCES SMOOTHED AVERAGE

1500 1700 1900 2100 WAVELENGTH, nm

2300 2500

Figure 1. Reproducibility of absorbance spectra ( 1 1 0 0 - 2 5 0 0 nm) o f i r o n - c o n t a i n i n g m o n t m o r i l l o n i t e s a s a f u n c t i o n o f sample r o t a t i o n . Shown a r e s p e c t r a o f two r e p l i c a t e samples demonstrating maximal divergence from each other, s p e c t r a o f one o f t h e r e p l i c a t e s taken in two positions showing maximal divergence from each other, and the smoothed average o f t h r e e r e p l i c a t e s in the six positions described in the text, i.e., an a v e r a g e o f 18x50 s p e c t r a .


414



ROTATIONS REPLICATES SMOOTHED AVERAGE

-.00187 600 671 743 814 886 957 1029 1100 WAVELENGTH, nm Figure 2. Reproducibility of absorbance spectra ( 6 0 0 - 1 1 0 0 nm) o f i r o n - c o n t a i n i n g m o n t m o r i l l o n i t e s a s a f u n c t i o n o f sample r o t a t i o n . A s i n F i g u r e 1, b u t the smoothed average is of two replicates, not three replicates in the six positions, i.e., an average of 12x50 s p e c t r a , b) f i r s t d e r i v a t i v e t r a n s f o r m , c) 2nd derivative transform.


21. COYNE ET AL.


415

b a s e l i n e are removed by the second d e r i v a t i v e . Positions of peaks, s h o u l d e r s and v a l l e y s a r e e s t i m a t e d most easily by e x a m i n a t i o n o f t h e p r i m a r y a b s o r b a n c e d a t a (log R /R) but q u a n t i t a t i o n of a b s o r b i n g c o n s t i t u e n t s i s improved by using second d e r i v a t i v e data.


Q

The Pacific Scientific (PSCO) simple regression analysis, b a s e d o n D r a p e r a n d S m i t h (33) was a p p l i e d to the absorbance data i t s e l f and to the 1st and 2nd derivative data using as constituent data total iron determined by XRF, and water by g r a v i m e t r i c measurement. R e g r e s s i o n s r e p o r t e d here were p e r f o r m e d at t h e wavelength s e l e c t e d by t h e computer as g i v i n g t h e b e s t correlation and a l s o a t a number o f a d d i t i o n a l w a v e l e n g t h s associated with prominent s p e c t r a l features.

Results Dependence of the Spectra on Iron C o n t e n t i n a R e g i o n Dominated by Iron A b s o r p t i o n . Spectra of the five homologous iron exchanged clays prepared from SWy m o n t o m o r i l l o n i t e i n 1 9 8 5 a r e s h o w n i n F i g u r e 3a. Those of c r u d e SWy a n d O t a y c l a y s a n d o f t h e 100% F e f o r m s of SWy p r e p a r e d i n 1 9 8 2 a n d O t a y a r e s h o w n i n F i g u r e 3b. Plots of the correlation coefficients obtained from the absorbance and i t s first and second d e r i v a t i v e s (Dl a n d D2) a r e s h o w n s u p e r i m p o s e d o n t h e a b s o r b a n c e spectrum o f t h e 100% F e SWy i n F i g u r e 4a. C o r r e l a t i o n coefficients for the f u l l set, s u p e r i m p o s e d on t h e a b s o r b a n c e spectrum for 100% F e O t a y s a m p l e , are shown i n F i g u r e 4b. The q u a l i t y of the c o r r e l a t i o n i s s i g n i f i c a n t l y improved using 1st d e r i v a t i v e data and improved s t i l l f u r t h e r u s i n g 2nd derivative data. T h i s c a n b e s e e n f r o m Table I I I w h e r e a summary o f the correlation coefficients obtained from absorbance and f i r s t and second d e r i v a t i v e s o f absorbance at s e v e r a l prominent wavelengths, i n c l u d i n g the computer optimum, a r e d i s p l a y e d . Table III. CORRELATION C O E F F I C I E N T S F O R IRON ABSORPTION OF T H E HOMOLOGOUS S E T A T S E V E R A L WAVELENGTHS C A L C U L A T E D FROM PRIMARY A B S O R B A N C E D A T A AND FROM 1ST AND 2ND DERIVATIVE TRANSFORMS

DATA

TYPE

ABSORBANCE Dl D2

iron

Lines of content

WAVELENGTH 600nm .987 ±.19 -.991 ±.15 -.996 ±.10

best f i t computed

970nm .967 ±.30 .989 ±.17 -.985 ±.20

H051nm> .962 ±.32 -.930 -.492" ±1.03

COMPUTER O P T .987 ( 600 NM) ±.19 .997 ( 900 NM) ±.09 ( 1 0 0 0 NM) 1.000 ±.02

f o r D2 o f c a l c u l a t e d v s . m e a s u r e d from r e g r e s s i o n a n a l y s i s at the


416


.158

.134

.110

SAMPLE DESCRIPTION % EXCHANGEABLE IRON, NOMINAL 0 \ Λ 20 \ \ 50 \\ 80 \ \ \ — 100 970

1028 1051


793 .086 643 .061

~ .037 < .227 ω oc

8 ω

SAMPLE DESCRIPTION 100% Fe, FROM 1982 PREPARATION SWy CRUDE SWy CRUDE, REPLICATE SAMPLE OTAY CRUDE — OTAY 100% Fe-EXCHANGED

.190h

< .154h

.117

.081

.044 600

671

743 814 886 957 WAVELENGTH, nm

1029

1100

Figure 3. Absorbance spectra of the 1985 set of MarSAMs. a) absorbance spectra of five variably Ca/Fe-exchanged materials prepared from SWy montmorillonite of nominal iron substitutions o f 0, 20, 50, 80 and 100 % of the exchange capacity. b) absorbance spectra of two replicate samples of the c r u d e p a r e n t SWy, a 100% e x c h a n g e d f o r m o f SWy p r e p a r e d i n 1 9 8 2 , t h e c r u d e p a r e n t O t a y , a n d a 100% F e - e x c h a n g e d form o f Otay.


21. COYNE ET AL.

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ABSORBANCE SPECTRUM REGRESOFABS REGRESOFD1 REGRES OF D2

CC


CC

σ> Ο

Ο Ζ

0 . 9 5 o v e r m o s t o f t h e wavelength range for the homologous set, it is clear that the s p e c t r a l absorbance i s l i n e a r with i n c r e a s i n g i r o n content over the full range of i r o n concentrations examined. As was shown i n Table III, q u a n t i t a t i o n i s improved at 'appropriately selected analytical wavelengths using d e r i v a t i v e transforms of the data. T h i s improvement is demonstrated by i n c r e a s e s i n the c o r r e l a t i o n coefficient and by d e c r e a s e s i n t h e s t a n d a r d d e v i a t i o n o f t h e average of the r e s i d u a l s of the measured values from the computed c a l i b r a t i o n c u r v e . With r e g a r d to the word ' a p p r o p r i a t e , it s h o u l d be n o t e d t h a t t h e a p p a r e n t c o r r e l a t i o n o f the peak at 1051 nm i n t h e homologous set disappears when second d e r i v a t i v e data are used and i s n o n - e x i s t e n t for the the f u l l set. The a p p a r e n t c o r r e l a t i o n i n t h e p r i m a r y absorbance d a t a f o r the homologous set i s an a r t i f a c t of t h e u n d e r l y i n g b r o a d 970 nm p e a k . The o v e r a l l 1

1



COYNE ET AL.


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FULL MARSAM SAMPLE SET: CRUDE-SWy + 0-100% Fe-SWy + CRUDE-OTA Y + 100% Fe-OTAY

b

Figure 5. Regression lines relating absorbance to measured i r o n at the 970 peak u s i n g 2nd derivative data, a) h o m o l o g o u s s e r i e s , b) f u l l s a m p l e set.


420


1906

.528

.432

(2404)


.336

.240 h

£

.144 h

ο oc " .048 LU Ο

+ 9 w

W

COMPUTER CALCULATED OPTIMUM OVER ENTIRE SPECTRUM

( ) = COMPUTER CALCULATED OPTIMA WITHIN THE REGION OF THE INDICATED SPECTRAL FEATURE. SPECTRAL FEATURE IDENTIFICATIONS ACCORDING TO CAR I AT I (1983).


21. COYNE ET AL.


423

CRUDE-SWy


932 nm

F i g u r e 8. R e g r e s s i o n l i n e s r e l a t i n g D2 o f a b s o r b a n c e to measured water content in a wavelength region d o m i n a t e d b y F e a b s o r p t i o n , a) c r u d e SWy. b ) 100% C a SWy. c ) 100% F e S w y .



424


n o n - s p e c i f i c i t y of the c o r r e l a t i o n with wavelength is not s u r p r i s i n g , considering the broadness of the iron-dependent features. I t s h o u l d be r e c o g n i z e d t h a t i r o n i n c l a y s o c c u r s in both f e r r o u s and f e r r i c o x i d a t i o n s t a t e s and i n m u l t i p l e f o r m s , as e x c h a n g e a b l e c a t i o n s on t h e s u r f a c e , and a l s o i n octahedral and, p o s s i b l y , t e t r a h e d r a l environments i n the structure. Fe (II or III) ions may b e isolated or clustered in any of these locations. Although more wavelength specific, when 2nd derivative data are examined, i t i s c l e a r that the c o r r e l a t i o n remains robust for the full set (the full set contains, in addition to the homologous set, c r u d e a n d 100% F e - f o r m s o f a c l a y of l o w s t r u c t u r a l i r o n , t w o c r u d e SWy s a m p l e s a n d a s a m p l e o f 100% F e - S W y p r e p a r e d e a r l i e r ) , as shown i n Figure 4. That the c o r r e l a t i o n i s not s e r i o u s l y d e t e r i o r a t e d when t h e c r u d e p a r e n t s , a s w e l l a s 100% F e O t a y c l a y , a r e a d d e d t o t h e h o m o l o g o u s s a m p l e s e t o f Swy c l a y s i n d i c a t e s that, in the wavelength region considered, structural and surface iron are not readily distinguished. This i n s e n s i t i v i t y i s not too s u r p r i s i n g c o n s i d e r i n g t h a t iron, b o t h i n t h e s t r u c t u r e a n d on t h e s u r f a c e , is coordinated to oxygen or h y d r o x y l groups and i s expected, for most p a r t , t o be i n o c t a h e d r a l s i t e s and i n t h e F e ( I I I ) state. T h a t t h e s p e c t r a o f t h e 1985 a n d 1982 f o r m s o f t h e W y o m i n g c l a y are only s l i g h t l y d i f f e r e n t from each other i s taken to mean that the preparation method yields similar m a t e r i a l s and t h a t t h e r e were not s i g n i f i c a n t changes in the spectra over time. The individual spectra of SWy a n d O t a y c l a y s are s i m i l a r e x c e p t t h a t t h e - 9 7 0 nm p e a k i n t h e O t a y f o r m s is c o n s i d e r a b l y l e s s i n t e n s e a n d t h e s m a l l p e a k a t ~ 1 0 5 1 nm i s more i n t e n s e i n t h e Otay t h a n i n the SWy m a t e r i a l s . Otay clay contains about 1/3 of the structural iron c o n t e n t o f t h e W y o m i n g f o r m , e x p l a i n i n g t h e d i m i n i s h e d 970 nm p e a k i n O t a y . I t i s a l s o known t h a t O t a y c l a y absorbs m o r e w a t e r t h a n d o e s SWy a n d t h a t t h e c r u d e f o r m a b s o r b s more water than the iron-exchanged form (Banin, unpublished data). T h e 1 0 5 1 nm p e a k , w h i c h , a s mentioned a b o v e , s h o w s n o c o r r e l a t i o n w i t h i r o n , may b e a t t r i b u t a b l e to water itself, characteristic of the edge sites on w h i c h t h e a d d i t i o n a l water i s thought t o be a d s o r b e d , or t o some o t h e r d i s t i n g u i s h i n g a t t r i b u t e . S i n c e t h e 1 0 5 1 nm p e a k o c c u r s w e a k l y i n t h e s p e c t r u m o f t h e SWy c l a y , too, its intensity, but probably not i t s presence, may p r o v e t o distinguish various types of montmorillonites. Whether s h i f t s i n the positions o f t h e two p e a k s w i l l be revealed when a b r o a d e r s e l e c t i o n of clays and other materials containing trace iron are considered is of interest. Since the Fe(III)/Fe(II) r a t i o has not been determined f o r t h e s e clays, a t t r i b u t i o n o f t h e 970 nm p e a k t o e i t h e r F e ( I I ) or Fe(III) can not be a s c e r t a i n e d . S t u c k i , et a l . (25.) have shown t h a t i t i s p o s s i b l e t o r e d u c e a s u b s t a n t i a l fraction of the ferric iron in clays, so determination of the o x i d a t i o n s t a t e s h o u l d be possible.


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Dependence o f the Spectra on Water Content i n t h e Region of Water and S t r u c t u r a l Hydroxyl A b s o r p t i o n . To i n t e r p r e t the l i n e a r i t y of the absorbance with water content/ it is necessary to consider the assignments of the numerous o v e r l a p p i n g b a n d s i n t h e r e g i o n f r o m 1100 - 2500 nm. In t h e v i c i n i t y o f 1 4 0 0 nm a p e a k w i t h t r a i l i n g s h o u l d e r is observed. By h e a t i n g and d e u t e r a t i o n s t u d i e s , the peak at 1 4 0 9 nm ( s e e n h e r e b e t w e e n 1 4 0 8 a n d 1 4 1 4 n m , d e p e n d i n g o n the exchangeable ion and moisture content) has been a s s i g n e d by C a r i a t i et a l . (25) t o be an o v e r t o n e o f the structural hydroxyl stretching vibration in the clay i t s e l f , with some contribution from an unspecified t r a n s i t i o n of adsorbed water. The s h o u l d e r , seen here at 1458 nm, h a s b e e n a s s i g n e d to the c o m b i n a t i o n o f t h e OH stretch and a bending overtone of weakly self-hydrogen bonded water. Sposito, et a l . (2£.), have a s s i g n e d the peak ( u n s p e c i f i e d by C a r i a t i ) to weakly, and shoulder to strongly (as did Cariati), self-hydrogen bonded water. Sposito, however d i d not c o n s i d e r a s t r u c t u r a l component of the 1410 nm b a n d . T h e d a t a o f F i g u r e 7 a,b s h o w a p o o r , but r e a l c o r r e l a t i o n between water and the intensity of t h e 1 4 1 0 nm p e a k a n d a n e x c e l l e n t o n e w i t h t h e 1 4 5 8 nm adsorbed water shoulder in the crude SWy c l a y . The correlation at 1410 nm may be due to the weakly s e l f - i n t e r a c t i n g water, or, in part, to overlap with the 1458 nm s h o u l d e r p r o d u c e d b y s t r o n g l y self-interacting water. Sposito, et a l . like Cariati, et a l . (2JL) have a s s i g n e d t h e p e a k , s e e n i n Figure 6 a t 1906 - 1908 nm, to a c o m b i n a t i o n OH s t r e t c h a n d b e n d o f w e a k l y self-hydrogen bonded water. A l l assign the shoulder, seen here at I960 nm, to strongly self-hydrogen-bonded water. The adsorption isotherm studies of Banin et a l . (23.) have shown t h a t for the SWy c l a y , monolayer adsorption sites are saturated at -10% w a t e r content, which occurs at r o u g h l y 20% r e l a t i v e humidity i n our p r e p a r a t i o n . The data in F i g u r e 7c s h o w a d i s t i n c t tendency toward saturation at 10% w a t e r content, suggesting that the weakly self-interacting (monolayer) water may n o t be w e l l - c o r r e l a t e d with water adsorption over a broad range of relative humidities. The c o r r e l a t i o n s shown b y our d a t a i n F i g u r e 7a-d a r e b e t t e r at the 1452 a n d 1 9 5 6 nm s h o u l d e r s t h a n a t t h e 1 4 1 0 a n d 1 9 0 6 nm p e a k s , suggesting that over the full humidity range the correlation is better with strongly self-interacting water. Clearly t h e r e i s n o c o r r e l a t i o n w i t h w a t e r i n t h e 2 2 0 0 nm r e g i o n , F i g u r e 7e, a s s o c i a t e d with absorption of structural hydroxyls and free of close l y i n g water bands. Good c o r r e l a t i o n is again obtained i n the region of the rising w a t e r a n d s t r u c t u r a l h y d r o x y l f u n d a m e n t a l s , Figure 7f. In summary, the correlations in the three water absorption regions are e x c e l l e n t over the e n t i r e range of r e l a t i v e h u m i d i t i e s and a s s o c i a t e d water contents. The locations of the best correlations of moisture content with the reflectance are i n s i m i l a r regions, but found at


426


somewhat d i f f e r e n t w a v e l e n g t h s d e p e n d i n g on t h e i o n i c f o r m of the c l a y . They are keenly s e n s i t i v e t o the adsorption environment, which i s , i n t u r n , dependent on t h e d e g r e e o f surface hydration.

o f S p e c t r a o n water c o n t e n t in a Region D o m i n a t e d by I r o n A b s o r p t i o n . A correlation with water content was also sought i n the 6 8 0 - 1 2 3 5 nm wavelength region (scanned u s i n g the 2nd o r d e r o f the NIR g r a t i n g ) . Some w a v e l e n g t h s h i f t s a n d d i f f e r e n c e s o f the peak shapes were n o t e d between s p e c t r a measured w i t h t h i s g r a t i n g and those shown i n F i g u r e s 3 a,b scanned using the more suitable (as explained earlier) grating covering the 600-1100 nm r e g i o n , but the spectra were comparable. S e v e r a l weak o v e r t o n e o r c o m b i n a t i o n b a n d s o f w a t e r have been r e p o r t e d i n t h i s region (2£) / a l t h o u g h n o clearly i d e n t i f i a b l e water peaks are seen here. The s p e c t r a l e f f e c t s of v a r y i n g the water content of the c l a y were s t r i k i n g , i n s p i t e o f the absence o f water peaks. Plots of D2 absorbance y_s.. H2O at the iron a b s o r b a n c e p e a k a r e shown i n F i g u r e 8 f o r t h e c r u d e , 100% C a - a n d 100% F e - e x c h a n g e d c l a y s . The r e l a t i o n s h i p s are linear throughout the moisture range studied, but of o p p o s i t e s i g n f o r the crude and C a - c l a y s compared w i t h the Fe-clay. The sign reversal is not simply due to differences i n the wavelength o f t h e a b s o r p t i o n maximum for t h e N a - and C a - forms o f t h e c l a y as compared t o the Fe-form, as t h e w a v e l e n g t h s were t h e same. Rather, the e n t i r e f a m i l y o f s p e c t r a l c u r v e s f o r t h e f o r m e r two clays increased i n absorbance with i n c r e a s i n g water content as expected, whereas the family of spectral curves for the iron clay decreased. Unpublished data of Huguenin revealed the same trend. He attributed it to hydroxylation of surface iron (Fe^ and F e 3 ) , the result of which is strongly diminished extinction coefficients for i r o n a b s o r p t i o n accompanied by s l i g h t red shifts of the peaks. Our r e s u l t s suggest a wavelength range in which changes i n atmospheric water content will produce opposite effects on a l k a l i and a l k a l i n e earth metal-containing, in contrast to iron-containing, minerals.


Dependence

+

+

When t h e composite set containing crude, C a - and F e - c l a y s i s c o n s i d e r e d as w e l l , rather than those of the i n d i v i d u a l members s e p a r a t e l y , the c o r r e l a t i o n of spectral absorbance with water contents seen f o r the individual clays is only s l i g h t l y deteriorated i n the hydroxyl region (1100-2500 nm), b u t i s totally lost i n the iron region (600-1100 nm). T h e l o s s i n t h e 6 0 0 - 1 1 0 0 nm ( i r o n ) region i s a t t r i b u t a b l e t o the s i g n change d i s c u s s e d above. The d e t e r i o r a t i o n i n the 1 1 0 0 - 2 5 0 0 nm ( h y d r o x y l ) region can not be a t t r i b u t e d t o s h i f t s i n the p o s i t i o n s of the peaks and shoulders (cf. Figure 6), nor yet to the differences in adsorption isotherms of the clays (29), as the p l o t s of D2 a b s o r b a n c e w e r e made y^.. w a t e r c o n t e n t and not R . H . However, comparison o f r a t i o s o f peak h e i g h t s t o a



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baseline envelope, drawn by c o n n e c t i n g t h e bottoms o f the peaks for the crude, C a - and Fe-forms o f SWy, show a d i s t i n c t d i m i n u t i o n i n the peak h e i g h t s f o r the i r o n clays r e l a t i v e to the others. This diminution, comparable to anomalies i n the water dependence of the absorbance by iron clays i n the 6 0 0 - 1 1 0 0 nm r e g i o n (Figure 8 a-c), indicate l e s s absorbance by i r o n r e l a t i v e to the other c l a y s i n the h y d r o x y l as w e l l as i r o n regions. I f as p o s t u l a t e d , iron-substituted materials are surface hydroxylated, there may b e multiple environments for adsorbed water, extending the region of water absorption, thus diminishing the contrast of absorption features for iron clays. T h i s o b s e r v a t i o n o f a d i f f e r e n c e between the behaviors of iron from the sodium and c a l c i u m c l a y s is under further i n v e s t i g a t i o n , a s i t may b e a f e a t u r e of importance t o remote s e n s i n g o f h y d r a t e d m i n e r a l s u r f a c e s and to remote estimation of mineral-bound water. In addition, it is of potential importance to catalytic a c t i v i t y , b e c a u s e s u r f a c e a c i d i t y i s known t o b e strongly i n f l u e n c e d by the degree o f h y d r a t i o n o f s u r f a c e s , and i s thought t o be a s s o c i a t e d w i t h i r o n sites.

General

Conclusions. Linear relationships over broad ranges of c o n s t i t u e n t v a r i a t i o n are found between s p e c t r a l absorbance of v a r i a b l y cation-exchanged montmorillonites and amounts o f two c o n s t i t u e n t s , h y d r a t i o n and t o t a l iron, which affect t h e i r surface chemistry. Quantitation is, in all cases, improved by u s i n g d e r i v a t i v e transforms o f the primary absorbance data. These correlations are insensitive to the location of iron (structural, or surface) i n t h e c l a y but appear t o be k e e n l y s e n s i t i v e to the molecular environment of the hydroxyl group of the water ( w e a k l y , ys.. s t r o n g l y s e l f - i n t e r a c t i n g ) . Increasing water content produces i n c r e a s i n g absorbance (decreasing reflectance) f o r a l l c l a y s i n t h e 1 1 0 0 - 2 5 0 0 nm r e g i o n , a n d decreasing absorbance for the i r o n c l a y i n p a r t s of the 6 8 0 - 1 2 3 5 nm r e g i o n . I t i s c o n c l u d e d t h a t NIRA represents a p r o m i s i n g method f o r s e p a r a t i n g the i n f l u e n c e s o f t h e s e and other constituents on reflectance properties and surface reactivity of clays. Establishing quantitative r e l a t i o n s h i p s between p a r t i c u l a r s t r u c t u r a l f e a t u r e s and chemical r e a c t i v i t y , independent of covarying f a c t o r s , is necessary for accurate determination of the chemical mechanisms on m i n e r a l s u r f a c e s and f o r remote s e n s i n g of p l a n e t a r y surface m i n e r a l composition and h y d r a t i o n . Acknowledgements T h i s w o r k was s u p p o r t e d b y t h e N a t i o n a l A e r o n a u t i c s a n d Space Administration jointly through the Exobiology Program of the Life Sciences and the P l a n e t a r y Geology Program o f the S o l a r System E x p l o r a t i o n D i v i s i o n s .


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Literature Cited

1. Theng, B. K. G. The Chemistry of Clay Organic Reactions; John Wiley & Sons: New York, Toronto, 1974; Chapter 7. 2. Solomon, D. H.; Hawthorne, D.G. Chemistry of Pigments and Fillers; John Wiley & Sons: New York, Toronto, 1983. 3. Fripiat, J. J.; Cruz-Cumplido, M. I. Ann. Rev. Earth Planetary Science 1974, 2, 239-256. 4. Tennakoon, D. T. B.; Thomas, J. M.; Tricker, M. J.; Williams, J. O. J. Chem. Soc., Dalton Trans. 1974 20, 2207-11. 5. Tennakoon, D. T. B.; Thomas, J. M.; Tricker, M. J. J. Chem. Soc., Dalton Trans. 1974 20, 2211-2215. 6. Pinnavaia, T. Science 1983 220, 365-371. 7. Laszlo, P. Science 1987 235, 1473-1476. 8. Russell, J. D.; Goodman, B. Α.; Fraser, A. R. Clay Minerals 1979 27, 63-71. 9. Banin, Α.; Rishpon, J. J. Mol. Evol. 1979 14, 133-152. 10. Solomon D. H.; Hawthorne, D. G. op. cit. Chapter 5, 181-184. 11. Solomon D. H.; Hawthorne, D. G. ibid 195-196. 12. Che, M.; Tench, A. J. Advances in Catalysis 1982 31, 77-133. 13. Coyne, L.; Sweeney, M.; Hovatter, W. Journal of Luminescence 1983 28, 395-409. 14. Coyne, L. Origins of Life 1985 15, 161-206. 15. Greenland, D. J.; Hayes, M. H. B. The Chemistry of Soil Constituents; John Wiley & Sons: New York, 1978; p 8. 16. Sposito, G. The Surface Chemistry of Soils; Oxford University Press: New York, 1984. 17. Coyne, L.; Costanzo, P.; Theng, B. K. G. in press, Clay Minerals. 18. Mortland, M. M. Trans. 9th Cong. Int. Soil. Sci. Soc. Adelaide; Angus and Robertson: Syndey, 1968; I 691-699. 19. Mortland, M. M.; Raman, Κ. V. Clays Clay Miner. 1968 16, 393. 20. Touillaux, P.; Salvador, C.; Vandermeersche, C.; Fripiat, J. J. Isr. J. Chem 1968 6, 337. 21. Lahav, N.; White, D; Chang, S. Science 1978 201, 67-69. 22. Ferris, J. P.; Huang, C.- H.; Hagan, W. J. Origins of Life 1988 18, 121-123. 23. Norris, K.; Hart, J. R. In Humidity and Moisture, P. N. Win, ed.; Van Nostrand: Princeton, N. J., 1965; p 24. Hirschfeld T.; Stark, E. In Near Infrared Reflectance Analysis of Foodstuffs, Chapter on Analysis of Foods and Beverages; Academic Press: New York, 1984; 505-550. 25. Cariati, F.; Erre, L.; Micera, G.; Piu, P.; Gessa, C. Clays and Clay Minerals 1981 29, 157-159.



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26. In Infrared and Raman Spectroscopy of Lunar and Terrestrial Minerals Karr, C., Jr., ed.; Academic Press: New York, San Francisco, London, 1975. 27. Mc Cord, T. B; Clark, R. N.; Singer, R. B. J. Geophysical Research 1982 87, 3021-3032. 28. Hunt, G. R.; Ashley, R. P. Economic Geology 1979 74, 1613-1629. 29. Banin, Α.; Carle, G.; Chang, S.; Coyne, L.; Orenberg, J.; Scattergood, T. Origins of Life 1988 18, 239-265. 30. Bower, J. G. Bur, Standard J. Research 1934 12, 241. 31. In C.R.C. Handbook of Chemistry and Physics, 69th Edition; Weast, R. C., ed.; C.R.C. Press, Inc: Florida, 1988. 32. In Lange's Handbook of Chemistry 12th Ed. Dean, J. Α., ed.; Mc Graw Hill Book, Co.: New York, 1979. 33. Draper, N. R.; Smith, H. Applied Regression Analysis; John Wiley and Sons: New York, London, Sydney, 1966. 34. Sherman, D. M.; Waite, T. D. American Mineralogist 198570,1262-1269. 35. Stucki, J. W.; K omadel, P.; Wilkenson, H. T. Soil Science Soc. Am. J. 1987 51, 1663-1665. 36. Hunt, G. R.; Salisbury, J. W. Modern Geology 1970 1, 284-300. 37. Sposito, G.; Prost, R.; Gaultier, J.-P. Clays and Clay Minerals 1983 31, 9-16. RECEIVED September 21,

1989


Near-Infrared Correlation Spectroscopy - ACS Symposium Series

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