Selective flocculation of ultrafine iron ore. 1. Mechanism of adsorption of starch onto hematite

COLLOIDS AND Colloids and Surfaces A: Physicochemicaland Engineering Aspects99 (1995) ll 27

ELSEVIER

A

SURFACES

Selective flocculation of ultrafine iron ore. 1. Mechanism of adsorption of starch onto hematite P.K. Weissenborn a,1, L.J. Warren b,., J.G. D u n n a a School of Applied Chemistry, Curtin University of Technology, GPO Box U1987, Perth, W.A. 6001, Australia b CSIRO, Division of Mineral Products, Private Bag, PO Wembley, W.A. 6014, Australia Received 15 June 1994; accepted 18 January 1995

Abstract

The mechanism of adsorption of wheat starch and its components (amylopectin and amylose) onto hematite has been investigated. The nature of the species adsorbed onto hematite and the extent of adsorption were determined by diffuse reflectance infrared Fourier transform spectroscopy. Wheat starch and amylopectin were adsorbed strongly onto hematite and the results were consistent with the formation of a surface complex, rather than adsorption by hydrogen bonding. Supporting evidence was obtained from reversibility tests and complexation of amylopectin with iron(III) in solution. Keywords: Adsorption; Hematite; Iron ore; Selective flocculation; Starch;

1. Introduction

Starch and related polysaccharides are widely applied in the mineral processing industry as depressants in froth flotation, as flocculants and as selective flocculants. All of these applications involve adsorption of the polysaccharides onto minerals. In this paper, we report on the adsorption of starch onto hematite, in relation to its proved application as a selective flocculant for ultrafine iron ore [ 1]. Starch is a complex natural polysaccharide consisting of two components - - amylose and amylopectin. Both are polymers of the monosaccharide ~-D-(+)-glucopyranose (Fig. 1). Amylose is a linear molecule containing alpha glucosidic link-

* Corresponding author. 1 Visiting scientist at the Institute of Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden. 0927-7757/95/$09.50 © 1995 Elsevier ScienceB.V. All rights reserved SSDI 0927-7757(95)03111-1

ages from carbon atom number one (C-l) to carbon atom number four (C-4) of the adjacent glucopyranose unit. Amylopectin is a branched molecule; it consists of several hundred short amylose chains cross-linked to each other by glucosidic linkages from C-1 to C-6 of the adjacent glucopyranose unit [-2]. Numerous mechanisms for the adsorption of starch and related polysaccharides onto minerals have been proposed. Hydrogen bonding between starch hydroxyl groups and mineral surface H

6CH2OH~.--O

.o-

\

-

HO~

H n

OH

Fig. 1. Structure of ~-D-( + )-glucopyranose. Hydrogen atoms attached to C-2 and C-5 are not shown.

12

P.IC Weissenborn et al./Colloids Surfaces A: Physicochem. Eng. Aspects" 99 (1995) 1 1 ~ 7

hydroxyl groups has been the favoured mechanism for many years [3-5], mainly because of the presence of a large number of hydroxyl groups on the starch and mineral surfaces. Iwasaki [-6] stated that a single hydrogen bond is relatively weak, but that the total bonding energy per starch molecule is very large because of the high number of hydroxyl groups per molecule. Electrostatic interaction appeared to play a relatively minor role in adsorption compared to hydrogen bonding, but was sufficient to cause differences in adsorption densities of starch on minerals with different surface charges [3]. Interaction between polysaccharides and hydrophobic sites on mineral surfaces (such as sulphides) is the favoured mechanism in the literature, as reviewed by Pugh [7]. Over the last decade, evidence has been obtained to suggest that a chemical interaction between the polysaccharide and mineral is likely [-8 12]. Liu and Laskowski [10] used FT-IR transmission spectroscopy to show that adsorption of dextrin (derived from starch) onto lead hydroxide results in the disappearance of the deformation absorption bands of the glucopyranose ring. Based on this evidence, it was proposed that the lead forms a five-membered ring complex via the oxygen atoms attached to C-2 and C-3 in the glucopyranose unit. Further evidence demonstrated that the adsorption of dextrin was at a maximum at the isoelectric point of the lead hydroxide and caused the solution pH to drop slightly. Based on these results, the adsorption mechanism/reaction shown in Fig. 2 was proposed. Khosla et al. [8] and Subramanian et al. [9] also proposed chemical interactions based on changes in the infrared spectrum of starch upon adsorption, but did not specify the structure of the adsorbed species. Further evidence on the complexing ability of polysaccharides has been gained from the complexation of metal ions with mono- and disaccharides such as fructose, glucose and sucrose

/ --c--o HO---P ~ / / +

[13-15]. Davis and Dellar [13] used fructose to complex iron(III) and prevent precipitation of iron hydroxide at an alkaline pH. To summarise, recent literature appears to have dismissed hydrogen bonding, electrostatic and hydrophobic interactions as significant adsorption mechanisms. Instead, chemical interaction is proposed as the mechanism of adsorption. The aim of this study was to investigate the interaction between wheat starch and its components with hematite, and to make a direct comparison between the hydrogen bonding and surface complexation adsorption mechanisms. This involved diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, reversibility of adsorption and iron(III) complexation experiments.

2. Experimental 2.1. Materials

Iron(III) oxide of purity greater than 99% was obtained from Aldrich (catalogue no. 31, 005-0). X-ray diffraction analysis gave a pattern consistent with that of synthetic hematite (e-FezO3). Details of particle size, surface area and density are given by Weissenborn et al. [16]. The mono- and polysaccharides used are listed in Table 1. Wheat starch and amylopectin solutions were prepared by adding an aqueous slurry containing 0.2500 g of wheat starch or amylopectin to approximately 200 ml of 0.10 mol 1-1 sodium hydroxide solution (prepared in Milli-Q water) at 90-95°C with stirring at low speed. The sodium hydroxide solution was deoxygenated by boiling and purging with oxygen-free nitrogen for 5 min prior to the addition of the slurry, to minimise the reduction in molecular weight of the polysaccharides [17]. The solution was cooled in an ice bath, stirred at low speed until room temperature was reached,

• - - / c -/- o . , ~_/~, cL o ..o--~Pb'/ + H3O+

--/-

-

V/C,,

Fig. 2. Adsorption mechanism for the dextrin/lead hydroxide system as presented by Liu and Laskowski [10].

P.K. Weissenbornet aL/ColloidsSurfacesA: Physicochem.Eng. Aspects99 (1995) 11~7

13

Table 1 Mono- and polysaccharides used in the adsorption and complexation experiments Sample

Source

Starch

N.B. Love Starches Australia Fluka Chemie AG Sigma Chemical Co. BDH BDH

Amylopectin Amylose D-( -- )-Fructose D-(+ )-Glucose

Catalogue number

10120 A-0512 28437 10117

Botanical origin

Molecular weight (as labelled)

Wheat

NA"

Maize Potato

10 6 10 7

NA 180 180

a Not available. made up to 250ml and stored for 12 h. The resultant wheat starch and amylopectin concentrations were 1000mg 1-1. Solutions were discarded after 24 h. The amylose solution was prepared in a similar way, but was slurried initially with a minimum of ethanol. The use of water rather than ethanol resulted in poor wetting and the formation of insoluble amylose globules, consistent with the observations of Street and Close

[183. Glucose and fructose solutions, used for iron(III) complexation experiments, were prepared by dissolving 9.00 g in 400 ml of Milli-Q water at room temperature. The solution was made up to 500 ml to give a concentration of 0.1 mol 1-1 (18 g 1-1). Iron solutions of concentrations 5.6 × 10 5 and 0.001 mol 1 1 were prepared in deionised water using iron(III) nitrate nonahydrate from BDH. All reagents/solutions were of analytical reagent quality. All experiments were performed using deionised water.

2.2. Methods Preparation of samples for DRIFTspectroscopy The hematite samples with adsorbed polysaccharide were prepared using the adsorption/flocculation procedure described by Weissenborn et al. [16]. In brief, the procedure involved addition of a polysaccharide solution to a hematite slurry at pH 10.5, stirring for 10 rain and solid/liquid separation by natural settling. The hematite was dried at 60°C for 12 h. D R I F T spectra were also obtained for the dissolved polysaccharides, to enable a comparison

with the spectra of the undissolved (as received) polysaccharides. The polysaccharide solutions were adjusted from p H 13 (0.1 mol 1- I sodium hydroxide) to pH 10.5 using hydrochloric acid and gentle stirring. An aliquot of the p H 10.5 solution was dried at 60°C for 12 h.

Quantitative analysis of adsorbed polysaccharide on hematite by DRIFTspectroscopy A procedure was devised to compare quantitatively the amounts of polysaccharide adsorbed onto hematite under different conditions. The peak area of the band at approximately 1155 cm -1 in the spectrum of adsorbed polysaccharide was used as a measure of the amount adsorbed onto hematite. The D R I F T technique was chosen because of its superior sensitivity to surface (adsorbed) species over attenuated total reflection and transmission techniques [ 19,20-[. Accurately weighed amounts of hematite coated with polysaccharide and spectroscopic grade potassium bromide (Fluka Chemie AG) were mixed by gentle grinding using an agate pestle and mortar, and sieved to less than 150 gm. 2% hematite in potassium bromide was found to be the optimum mixture. The sample mixture was loosely packed in a diffuse reflectance sample cup of diameter 12 m m and depth 2 ram, and levelled with a spatula. A constant sample mass of 0.31 _+ 0.01 g ensured that the packing density was similar for all samples. The sample cup was loaded into a Perkin-Elmer diffuse reflectance accessory and the infrared energy reaching the detector was adjusted. A spectrum was obtained under the following operating conditions: instrument, Perkin-

14

P.K Weissenborn et al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11-27

Elmer 1720 Fourier transform-infrared spectrometer; detector, deuterated triglycinesulphate; number of scans, 200, giving maximum peak intensity; resolution, 4 c m - 1; apodisation, normal; computer software, Perkin-Elmer infrared data manager version 3.33. A spectrum of hematite (blank) was obtained using the same procedure as for the hematite samples coated with polysaccharide. The hematite blank was prepared using the adsorption/flocculation method, ensuring that any potential alteration of the hematite surface due to wetting, adjustment of solution pH and drying was identical to that experienced by the hematite coated with polysaccharide. All spectra were ratioed (1:1) against a background spectrum of potassium bromide ground to less than 150 ~tm. The infrared signal reflected from the hematite coated with polysaccharide was scaled (adjusted) to 90% of the signal reflected from the background sample. The signal reflected from the hematite blank was scaled to 95%. A similar scaling procedure was adopted by Brimmer and Griffiths [-21] to avoid obtaining a reflectance exceeding 100% (or negative absorbance). All reflectance spectra were converted to Kubelka-Munk units. The spectrum of adsorbed polysaccharide was obtained by subtraction of the hematite blank spectrum from the spectrum of hematite coated with polysaccharide. The band at 915 cm 1 in the spectrum of hematite was used for spectral subtraction. The subtraction factor was determined using interactive subtraction software and was typically between 1.0 and 1.5. Comparison of the amounts of polysaccharide adsorbed onto hematite under different conditions involved measuring the peak area of a band characteristic of adsorbed polysaccharide. The dominant bands for all adsorbed polysaccharides were in the region 1180 1000cm 1. The band at approximately 1155 cm -1 was preferred for quantitative analysis, because its area, shape and position were more reproducible than those of other bands.

Reversibility and prevention of adsorption of the polysaccharides onto hematite The reversibility of adsorption was tested by washing the hematite coated with polysaccharide in deionised water, and performing ultrasonication

using a C o l ~ P a r m e r 8851 ultrasonic bath and a Heat Systems-Ultrasonics Sonicator W-380 probe. Sub-samples of hematite were taken during each test and the amount of polysaccharide adsorbed measured by the quantitative D R I F T spectroscopy method. Adsorption experiments were also carried out in 85% dimethylsulphoxide (DMSO) and 5 mol 1-1 urea, with the aim of preventing adsorption of the polysaccharides. The amount of polysaccharide adsorbed onto the hematite was measured by quantitative D R I F T spectroscopy.

Iron complexation The ability of the polysaccharides to complex iron(III) in solution was assessed by mixing 50 ml of 5.6 x 10 -5 mol 1-1 iron(III) with 50ml of 1000 mg 1-1 amylopectin, and 50 ml of 0.001 mol 1-1 iron(III) with 50ml of either 0.1 mol 1 1 fructose or 0.1 mol 1-1 glucose. All solutions were adjusted to pH 3.5 before mixing. The solution concentrations were calculated such that there were 100 monomer units (as glucose or fructose) per iron(III) atom (a ratio of 100:1). For amylopectin, this ratio was an approximation because of the uncertainty in its molecular weight. The excess number of monomer units ensured a sufficient driving force for iron complexation. After mixing, the pH was slowly adjusted to 8.5 with gentle stirring. A stable complex with iron(III) formed if no or minimal precipitation of iron(III) hydroxide occurred. The extent of iron(III) complexation was determined by measuring the iron(III) concentration in the supernatant after allowing one week for precipitation. A Varian AA-10 atomic absorption spectrometer was used for the analysis of iron(III).

3. Results

3.1. Quantitative DRIFTspectroscopy Fig. 3 shows the spectra of hematite (blank) and amylopectin adsorbed onto hematite. The band at 915 cm-1 in the spectrum of hematite was used for spectral subtraction, such that all hematite bands in the region 1300-700 cm 1 were removed. This range was very infrared active for the polysaccha-

P.K. Weissenbornet al./ColloidsSurfaces A: Physicochem. Eng. Aspects 99 (1995) 11 27 0.04

i

i

i

i

i

i

i

i

I

i

15 i

i

915

-

0.03

r~ ,

0.02

I tt~

I

/,J

0.01

I I

# \

0.00 4000

i

i ~ ~ ~,""

3750

I

3500

i

I

3250

i

i

3000

2750

2500

2250

2000

1750

1500

1250

I

,

1000

Wavenumbers(cm-t) Fig. 3. DRIFT spectra of (a) hematite and (b) amylopectin adsorbed onto hematite. 50 mg 1 1 amylopectin with 1% w/v hematite at pH 10.5, dried at 60°C, 2% w/w in KBr. Scale for Kubelka-Munk (KM) function applies to (a) and (b).

rides and, hence, subtraction was performed to optimise the removal of hematite absorption in this region. Below 7 0 0 c m - 1 , the hematite absorbed very strongly and spectral subtraction was unable to remove reliably the hematite component from the adsorbed polysaccharide spectra. Fig. 4 shows the effect of wheat starch concentration on the D R I F T difference spectrum of wheat starch adsorbed onto hematite. A plot of peak area versus wheat starch concentration showed a linear increase up to approximately 175mg 1-~ (corresponding to a peak area of approximately 0.15), after which adsorption reached saturation. Attempts to quantify the peak area in mg g-~ of polysaccharide by reference to standard mixtures of polysaccharide and hematite were unsuccessful, owing to apparent differences in peak areas of adsorbed and non-adsorbed polysaccharide. A cross-check of the reliability of the amount of wheat starch adsorbed onto hematite measured as peak area was made with the amount adsorbed measured in mg g - 1 by thermogravimetric analysis [ 16]. The effect of wheat starch concentration was investigated, and very good agreement in terms of

changes in amount adsorbed and concentration at which adsorption reached a plateau was obtained. The D R I F T procedure gave peak areas reproducible to within + 10%. This was only possible by a combination of careful and reproducible particle sizing, sample packing, scaling, subtraction and peak area integration. Of the many variables affecting peak area, scaling was the most important.

3.2. DRIFTspectra of the polysaccharides The spectra of wheat starch before and after dissolution are shown in Fig. 5. Dissolution and drying shifted and changed the shape of some bands. The spectra of amylopectin and amylose are similar to that of wheat starch. Absorbance band positions for all spectra are compared in Table 2.

3.3. DRIFTspectra of the adsorbed polysaccharide species The spectra of non-adsorbed (dissolved/dried) and adsorbed wheat starch, amylopectin and amy-

P.K. Weissenborn et al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11 27

16 0.030

I

I

•

I

I

5oo 0.025

.~

0.020

~

(I.015

0.010

0.005

0.000

,

I

1200

,

t

1150

I

,

1100

,

1050 Wavenumbers (cm-1)

I

,

I

1000

950

900

Fig. 4. D R I F T difference spectra for wheat starch on hematite showing the effect of wheat starch concentration (in mg 1 ~) on peak area. Wheat starch with 1% w/v hematite at pH 10.5, dried at 60°C, 2% w/w in KBr. Scale for K M function applies to all spectra•

0.06

J

i

i

i

0.05

--,tl

o',

I I

I

~

~= 0.04

I| i m

r'

0.03

;~

\

J

]

1018

e-

io

er~

I 0

~

II F

I

I

0.02

I

\

e--,

o~-

f~

:/

lee

!

A

~o0

~

||1

it

I

i

I

0

r

,I,I

0.01

0.00 -~ ---~ ~ --~. 4000 3750

l

3500

,

I

3250

,

I

3000

,

i

2750

,

i~-~---i

~

,

"r'- ~.

2500 2250 2000 Wavenumbers (cm -I)

I

1750

I500

,

i

1250

1000

I

750

i

500

Fig. 5. D R I F T spectrum of wheat starch. (a) As received, 1.1% w/w in KBr. (b) Dissolved, pH 10.5, dried at 60°C, 0.2% w/w in KBr. Scale for K M function applies to (b), (a) × 4.5.

P.I~ Weissenbornet al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11-27

17

Table 2 Peak positions and peak shifts for polysaccharides before and after dissolution Wheat starch

Amylopectin

Before dissolution

After dissolution

Peak shift

3460 3294 2934 2900 2086 1646 1456 1418 1361 1339

ND a 3338 2930 2898 ND 1650 1456 1419 1367 1343

1243 1206 1149

1235 1203 1152

- 8 - 3 +3

1077 1018 931 ND 862 ND 765 710 675

1080 1025 937 880 ND 842 765 702 677

+3 +7 +6

600 576 534

606 577 522

+6 +1 - 12

+44 -4 --2 +4 0 +1 +6 +4

0 -8 +2

Amylose

Before dissolution

After dissolution

3486 3264 2934 2901 2086 1650 1456 1418 1360 1339 1265 1244 1205 1150 1103 1077 1006 930

ND 3322 2934 2884 ND 1654 1456 1420 1366 1341 1300 1249 1207 1153

862 ND 766 712 669 649 ND 608 575 534

ND 847 763 707 671 648 615 ND 575 532

1081 1025 933

Peak shift

Before dissolution

After dissolution

+ 58 0 - 17

3318 2932 ND 2087 1649 1455 1414 1369 1339 1297 1239 1205 1150 1102 1079 1027 935

3325 2931 2898 ND 1651 1455 1415 1364 1338 1300 1251 1206 1154 1106 1081 1024 931

862 845 760 706 669 649 617 607 574 529

ND 847 764 715 669 635 617 ND 579 534

+4 0 +2 +6 +2 +5 +5 +2 +3 +4 + 19 +3

- 3 --5 +2 - 1

0 -2

Peak shift

+7 - 1

+2 0 +1 --5 - 1 +3 + 12 +1 +4 +4 +2 - 3 -4

+2 +4 +9 0 - 14 0 +5 +5

a Not detected.

lose o n h e m a t i t e f o r s e l e c t e d r e g i o n s o f t h e i n f r a r e d s p e c t r u m a r e s h o w n i n Figs. 6 - 8 . T h e s p e c t r a of adsorbed amylopectin and amylose over the region 4000-1000 cm -1 were similar to that of adsorbed w h e a t s t a r c h ( F i g s . 6 ( A ) a n d 6(B)). S i g n i f i c a n t b a n d shifts b e t w e e n n o n - a d s o r b e d a n d a d s o r b e d polysaccharides were most apparent in the region b e l o w 1000 c m 1 ( T a b l e 3, a n d Figs. 6 ( C ) , 7 a n d 8).

3.4. Reversibility and prevention of adsorption of polysaccharides onto hematite The results from reversibility tests are shown in T a b l e 4. W a s h i n g t h e h e m a t i t e h a d n o effect o n t h e

desorption of amylopectin. Partial removal was possible using ultrasonic waves generated by an u l t r a s o n i c b a t h , o r in t h e c a s e o f a m y l o s e b y washing in 85% DMSO. A significant decrease in the peak areas of the adsorbed wheat starch, amylopectin and amylose was only possible using severe ultrasonication, which also gave relatively stable suspensions (no flocculation or coagulation was observed). Attempts to prevent the adsorption of polys a c c h a r i d e s w e r e u n s u c c e s s f u l ( T a b l e 4). T h e p r e s e n c e o f 8 5 % D M S O o r 5 t o o l 1-1 u r e a d u r i n g adsorption and flocculation did not significantly affect t h e a m o u n t o f p o l y s a c c h a r i d e a d s o r b e d o r

18

P.K. Weissenborn et aL/Colloids Surfaces A, Physicochem. Eng. Aspects 99 (1995) 11-27 I

'

I

,

I

,

I

,

I

'

I

'

I

'

I

,

~~)

0.015

I

%

0.010

0.005

0.000 400{

3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250

(A)

Wavenumbers (cml) '

I

'

I

'

t

'

I

,

I

,

0.015

0.010

0.005

0.000 1150

1200

1100 1050 1000 Wavenumbers (cm-l)

(B)

950

900

0.007 0.006

,,.~'~,'%

0.005

"

,

°

\

,,,,~

,

/'J \

0.004

,

i:"

z

/\

~l

/

\

A

\ I' \ ~

0.003

'J-)

0.002 0.001 1000

I I i

I

950

(c) Fig. 6. (Caption opposite.)

,

I

I

I

i

I

900 850 800 Wavenumbers (cm-l)

I

I

750

i

I

700

P.K. Weissenborn et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11-27 0.010 0.009

'

i

i

i

i

i

; I

0.007

i

i

0008 =

19

/

(a) ~, _ // ]

.-

\

.,I - ~

r\

~

~ !

i

']

~. ----" ]

'k

0.006 0.005

\ 0.004

,

i

1000

,

950

i

,

900

i

,

i

850

,

i~

800

750

Wavenumbers (cm-l) Fig. 7. 1000-700 c m - 1 D R I F T difference spectrum of amylopectin adsorbed onto hematite with hematite subtracted (a), compared to the D R I F T spectrum of non-adsorbed amylopectin (b). 50 mg 1-a amylopectin with 1% w/v hematite at pH 10.5, dried at 60°C, 2% w/w in KBr. Scale for K M function applies to (a), (b) x 1.5.

0.007

i

i

i

t

i

(b)

0.006

r~

~

'

0.005 0.004

\

~ //

~

,

0.003 X--

0.002

X X A X

0.00 l

r~. ~

E

0.000 1000

i

I

950

i

I

I

900 850 Wavenumbers (cm-l)

I

800

~,_ _

L-

750

Fig. 8. 1000-700 c m - 1 D R I F T difference spectrum of amylose adsorbed onto hematite with hematite subtracted (a), compared to the D R I F T spectrum of non-adsorbed amylose (b). 50 m g 1 1 amylose with 1% w/v hematite at pH 10.5, dried at 60°C, 2% w/w in KBr. Scale for K M function applies to (a), (b) x 1.2.

Fig. 6. (A) 4000-1200cm 1 D R I F T difference spectrum of wheat starch adsorbed onto hematite with hematite subtracted (a), compared to the D R I F T spectrum of non-adsorbed wheat starch (b). 50 mg 1 1 wheat starch with 1% w/v hematite at pH 10.5, dried at 60°C, 2% w/w in KBr. Scale for K M function applies to (a), (b) x 1.3. (B) 1200-900 cm 1 D R I F T difference spectrum of wheat starch adsorbed onto hematite with hematite subtracted (a), compared to the D R I F T spectrum of non-adsorbed wheat starch (b). Scale for K M function applies to (a), (b) x 2.35. (C) 1000-700 cm-1 D R I F T difference spectrum of wheat starch adsorbed onto hematite with hematite subtracted (a), compared to the D R I F T spectrum of non-adsorbed wheat starch (b). Scale for K M function applies to (a), (b) x 1.8.

P.K. Weissenborn et aL/Colloids Surfaces A." Physicochem. Eng. Aspects 99 (1995) 11-27

20

Table 3 Peak positions and shifts in cm 1 for non-adsorbed and adsorbed polysaccharides on hematite for the range 980-700 c m Wheat starch Before adsorption

937 880

Amylopectin After adsorption 957 934

Peak shift

Before adsorption

After adsorption

Peak shift

Before adsorption

After adsorption

-3

934

942

+8

932

957 936

+4

+21

865 848 763

863 845 770

-2 -3 +7

865 848 764

893 856 775

+28 +8 + 11

863 842 764 703

785 739 716

Amylose Peak shift

+ 13

Table 4 Reversibility and prevention of adsorption for the adsorption of polysaccharides onto hematite Test

Adsorption/flocculation using 200 ml deionised water at pH 10.5, 50 mg 1-1 polysaccharide. Hematite flocs sampled immediately after separation from supernatant, no washing. - - hematite sampled after one washing in 200 ml deionised water at pH 10.5. - - two washings. - - three washings. four washings. - - after four washings, stirring at 2000 rev m i n - 1 in baffled beaker in 200 ml deionised water at pH 10.5 plus two further washings. - - after four washings ultrasonicate in ultrasonic bath for 10 rain plus two further washings. - - after two washings, ultrasonicate using ultrasonic probe on m a x i m u m setting in 60 ml deionised water at pH 10.5 for 10 min, temperature increased to 60 70°C, plus two further washings.

Wheat starch peak area

0.077

" Amylose pH 10.

Amylose peak area

0.085

-

0.07

-

0.081 0.082 0.086 0.076

0.055 -

0.054 0.019

Adsorption/flocculation using 200 ml of 5 mol 1-1 urea at pH 10.5, 50 m g I 1 polysaccharide. Hematite flocs separated from supernatant and washed three times in deionised water at pH 10.5. Adsorption/flocculation using 200 ml 85% D M S O at pH 10.5, 50 m g 1 1 polysaccharide. Hematite flocs separated from supernatant and washed twice in deionised water at pH 10.5. a stir for 10 h in 200 ml 85% DMSO, two further washings in deionised water at p H 10.57

Amylopectin peak area

0.017

0.024

0.070

0.077

0.077

0.048

0.076

0.087

0.022

P.K. Weissenborn et at~Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11 27

21

4. Discussion

Reggiani [24,25] for amylose and its oligomers. The similarity of the amylose spectrum to that of amylopectin and wheat starch suggests that the assignments apply equally to amylopectin and wheat starch. Branching appears to have no significant effect on the infrared spectra of amylopectin and wheat starch. Colombo and Rule [26] also observed similar spectra for a variety of polysaccharides. The assignments are consistent with those of Barker and co-workers [27,28], Cael et al. [29], Nakanishi and Solomon [30], Tipson and Parker [31], Wilson and Belton [32], and Wells and Atalla [33]. Precise assignment of the bands in the region 1450 1000 cm- t is not possible, as all the vibrations in this region are highly coupled. Wells and Atalla [33] attempted to describe some of the various ring vibrations in a study on the vibrational spectra of glucose, galactose and mannose.

4.1. Assignment of infrared absorption bandsfor polysaccharides

4.2. Comparison between infrared spectra of polysaccharides before and after dissolution~drying

The assignments of infrared absorption bands (Table 6) are based mainly on those of Casu and

The dissolution of starch by heating a slurry above its gelatinisation temperature (60°C for

the recovery of hematite. Washing for 10 h in 85% DMSO was not sufficient to remove wheat starch or amylopectin. However, approximately half of the amylose was desorbed under the same conditions. Both 85% DMSO and urea are very good solvents of starch, and are strong hydrogen-bond acceptors [22,23].

3.5. Iron(III ) complexation tests The ability of fructose, glucose and amylopectin solutions to prevent the precipitation of iron hydroxide was used to measure their relative complexing abilities with iron(III) ions. The results are shown in Table 5.

Table 5 Iron(III) complexation tests using fructose, glucose and amylopectin Iron(III) concentration (mol 1 1)

Test solution concentration (mol 1 1)

Ratio of monomer to iron(III)

Observations

5 x 10 -4

Blank

5 x 10 -4

Fructose 0.05

100:1

No precipitation of Fe(OH)3. Yellow solution due to iron(III).

5 x 10 4

Glucose 0.05

100:1

Precipitate formed within 12 h. Colourless solution.

5 x 10 -4

Amylopectin 5x10 9a

5.6 : 1b

Small amount of precipitate formed within 12 h. Faint yellow solution.

2.8 x 10 -5

Blank

2.8 x 10 5

Amylopectin 5x10 9.

Precipitate formed within 2 min. Colourless solution.

Precipitate formed with 12 h. Colourless solution. 100:1 b

No precipitation of Fe(OH)3. Faint yellow solution similar to blank at pH 3.5.

Percent Fe(III) held in solution at pH 8.5 0 92 0.2 72

0

100

a Estimated based on molecular weight of 1 x 108. b Represents number of glucopyranose monomer units. It is estimated that one amylopectin molecule contains 560000 glucopyranose units.

22

P.K. Weissenbornet al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11 27

Table 6 Assignment of infrared adsorption bands for all polyaccharidesbased on Casu and Reggiani [-24,25] Approximate band position (cm 1)

Vibrational assignment

Approximate band position (cm-1)

Vibrational assignment

3470

O-H stretch, intramolecularly hydrogen bonded between hydroxyl groups attached to C-2 and C-Y from adjacent ~-glycopyranoseunits.

935

Ring vibration

3350

O-H stretch, intermolecularly hydrogen bonded

890-845

deformation. Dependent on conformation. Equatorial (chair} hydrogen at about 850 cm-', axial (boat) hydrogen at about 890 cm- '.

2930, 2900

C H and C - H 2 stretching

765

Ring breathing

2085

Unknown

710

Ring vibration

1650

H-O H deformation for water

670-650

O H out-of-plane-deformation

C1-H

1455

C-H2 deformation

605

Ring vibration

1420-1200

O-H in-plane deformation coupled with C H deformation

575

Ring vibration

1150

C-O-C glucosidiclinkage stretch coupled with C-OH stretch and O H deformation

530

Ring vibration

1080 1000

C-O stretch coupled with C C stretch and O-H deformation

wheat starch) involves disruption of starch granule integrity and a loss in crystallinity. Subsequent drying results in partial recrystallisation, but the original integrity and crystallinity of the starch granule are not regained [2,34,35]. This was apparent from the differences in the spectra of the polysaccharides before and after dissolution/drying. The most significant difference between the infrared spectra of the polysaccharides before and after dissolution/drying occurred for the O - H stretching frequencies (Fig. 5 and Table2). The O - H stretching frequency at approximately 3300 cm 1 shifted to approximately 3330 cm -1 and became less broad. The shoulder due to intramolecular hydrogen bonding (approximately 3470 c m - i ) disappeared. The O H deformation frequency at 1360cm -1 for all polysaccharides became less broad after drying, but did not shift significantly. These changes agree with the observations of Casu

and Reggiani [24]. They attributed the differences between the spectra of crystalline and amorphous amylose to a loss in crystallinity and reduction in the number of intra- and intermolecular hydrogen bonds in the amorphous amylose. Hence, the changes in hydrogen-bonding stretching and deformation frequencies are consistent with a decrease in the crystallinity of the polysaccharides upon gelatinisation, retrogradation and drying. Subtle changes in the C O H stretching frequencies, C1-H deformation and ring vibrations were also apparent, and are attributed to the decrease in crystallinity.

4.3. Comparison between infrared spectra of adsorbed and non-adsorbed polysaccharides Owing to the spectral differences between "as received" and dissolved/dried polysaccharides, the

P.K. Weissenbornet al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11~7

spectra of the polysaccharides adsorbed onto hematite should clearly be compared with the spectra of the dissolved/dried polysaccharide samples. The spectra of all the adsorbed polysaccharides had a broad O - H stretching band centred at approximately 3370 cm-a (Fig 6(A)). The position of this band increased by approximately 40 c m - 1 and the bandwidth decreased significantly relative to the non-adsorbed polysaccharides. The O - H in-plane deformation band at approximately 1360 cm-x did not shift significantly, but decreased in intensity. The presence of the H - O - H deformation due to water (1650 c m - ' ) suggests that water is associated with both adsorbed and non-adsorbed polysaccharide. The most intense bands for the adsorbed polysaccharide spectra occurred at approximately 1155, 1080 and 1030cm - t . None of these bands shifted significantly from their position in the non-adsorbed polysaccharide spectra, although the shape of the 1030 cm -1 band did change (Fig. 6(B)). The major differences between the non-adsorbed and adsorbed polysaccharide spectra occurred in the range 1000-700 cm -1 (Table 3) and included the following. (i) The appearance of a weak band at 957 c m - 1 for adsorbed wheat starch and amylose. (ii) Minor shifts in the band at approximately 935cm-1 (ring vibration) for all adsorbed polysaccharides. (iii) Appearance of a band at 893 cm -1 for adsorbed amylose, assigned to the axial C I - H deformation. This band appears to have shifted from 848 cm -~, assigned to the equatorial C I - H deformation, which suggests a change from equatorial ( C , - H ) to boat (axial C1-H) conformation upon the adsorption of amylose. The weak band at 8 5 6 c m - ' suggests that some portions of the amylose molecule remain in the chair conformation. (iv) The band at 863 c m - ' for adsorbed wheat starch shifted from 842 cm -1, suggesting that a slight change in conformation occurred upon adsorption onto hematite. Amylopectin exhibited less obvious changes in this region, but the weak band at 863 c m - ' is consistent with a slight change in conformation. The extent of the change is not

23

as great as for amylose, which is to be expected because the branched structure of the amylopectin molecule is less amenable to conformational change than the linear amylose molecule. (v) The band at 765 cm ' for the non-adsorbed polysaccharides is assigned to a glucopyranose ring breathing vibration. Upon adsorption, this band shifted to 785 cm -1 (+21 cm -1) for wheat starch, to 770cm ' ( + 7 c m - ' ) for amylopectin and to 775 cm -a (+ 11 cm -1) for amylose. (vi) Adsorbed wheat starch has an additional band at 739 cm -1, and a band at 716 cm -1 which shifted from 703 cm 1 (ring vibration). Neither of these bands were detected in the spectra of the adsorbed amylopectin and adsorbed amylose, possibly because of the strong adsorption of hematite below 750 c m - 1. 4.4. Adsorption mechanism - - surface complex formation

The changes observed in the glucopyranose ring vibrations and C I - H deformation when wheat starch adsorbs onto hematite suggest that a surface complex similar to that of Liu and Laskowski [ 10] may have formed (Fig. 2). The complex proposed by Liu and Laskowski [ 10] necessitated a planar or near-planar configuration of the lead, C-2, C-3, 0 - 2 and 0-3 atoms. The consequent strain placed on the glucopyranose ring was cited as the reason for the disappearance of the dextrin ring vibrations at 930 and 850 cm -1, and the equatorial C1-H deformation at 850 cm 1. No new bands or band shifts were detected. To account for the new bands and band shifts observed in this study, an alternative surface complex in which complexation occurs via the hydroxyl groups attached to the C-2 and C-3' atoms from adjacent glucopyranose rings is proposed (Fig. 9). The appearance of the weak bands at 957 c m - 1 for adsorbed amylose and at 7 3 9 c m - ' for adsorbed wheat starch, along with the band shifts shown in Table 3, are most probably due to the formation of new chemical bonds between polysaccharide hydroxyl groups and surface iron atoms. The change in the equatorial C1-H deformation band upon adsorption suggests that the conformation of the glucopyranose rings is distorted relative

24

P.K. Weissenborn et al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11-27

.o/\o/\o/\o/\oFig. 9. Proposedsurfacecomplexbetweenthe hydroxylgroups attached to C-2 and C-3' atoms and surfaceiron atoms fromhematite. to the stable chair form. For amylose, a change to the boat conformation appears to have occurred. Such conformational changes would be consistent with the formation of a surface complex across the C - O - C glucosidic linkage. The formation of the surface complex shown in Fig. 9 is also supported by the ability of the hydroxyl groups attached to C-2 and C-3' to form an intramolecular hydrogen bond [25,36] by rotation of the C - O - C glucosidic linkage or the C - O H bonds. Intramolecular hydrogen bonds between other hydroxyl groups within individual glucopyranose units are not possible because of unfavourable bond angles and distances. Thus, the ability of the C-2 and C-3' hydroxyl groups to form an intramolecular hydrogen bond suggests that these hydroxyl groups would be equally able to complex with surface iron atoms. Further, the excess electrons provided by the C - O - C glucosidic oxygen, in addition to those from the 0-2 and 0-3' atoms, serve to stabilise the eight-membered cyclic complex. In the context of the overall DRIFT spectra, the changes in the ring vibrations and C~-H deformation upon adsorption of the polysaccharides appear almost insignificant. However, dramatic changes in the overall spectra of the adsorbed polysaccharides cannot be expected if, for example, only two adjacent glucopyranose units in a chain of 20 or more are complexed with the iron. Obviously, the spectrum would be dominated by the absorbance of the non-complexed glucopyranose rings. This would explain why no changes were observed for the C - O stretching vibration upon complexation with iron. Thus, not every C-2 and C-3' hydroxyl group pair along the polysaccharide molecule need be involved in surface complex formation. Attachment to the hematite surface may only be

possible for selected hydroxyl groups, owing to conformational constraints along the backbone of the glucopyranose units. Evidence for the requirement of a large number of glucopyranose rings (or C-2 and C-3' complexation sites) per iron atom was given by the solution complexation tests. Table 5 showed that for amylopectin, a glucopyranose to iron ratio of approximately 6:1 was insufficient to complex all of the iron(III). The extent of complexation was 30% less than that at a ratio of 100:1. Hence, it appears that there are relatively few iron complexing sites on an amylopectin molecule, and that not every C-2 and C-3' hydroxyl group pair is involved in complexation. This is consistent with the explanation of a lack of detection of a change in the C - O vibration in the infrared spectrum of the adsorbed polysaccharides. Table 5 clearly shows that fructose and amylopectin were capable of complexing with iron(III). Fructose is a strong complexing agent for iron [ 13,14,37]. The results suggest that the complexing ability of amylopectin is equal to that of fructose. Glucose has almost no ability to complex iron. In order to translate the complexation of iron(III) in solution to the complexation of iron(III) atoms on the surface of hematite, conformational effects need to be considered and may restrict complexation on the surface. Nevertheless, the fact that amylopectin reacts so strongly with iron(III) in solution is good evidence that amylopectin will react with iron atoms on the hematite surface. Adsorption tests using DMSO and urea as solvents showed that neither was capable of reversing or preventing the adsorption of wheat starch or amylopectin onto hematite; only severe ultrasonication decreased the amount adsorbed (Table 4). Mostafa [38] reported that ultrasonica-

P.K. Weissenbornet al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11-27

tion is capable of breaking C-C bonds in long chain molecules. The decrease in the amount adsorbed may be due to breakage of polysaccharide chains, consistent with the loss in flocculating ability of wheat starch and amylopectin, and the observed stability of the suspension. Ultrasonication may also have caused some attritioning of the hematite particles. This could account for some of the increased stability, but if attritioning was the only effect of ultrasonication, the very significant decease (75% for wheat starch) in peak area would not have been observed. Therefore, the polysaccharides appear to be strongly adsorbed, consistent with the formation of (a) chemical bond(s) between the polysaccharide and hematite surface. 4.5. Adsorption mechanism - - hydrogen bonding

Most arguments in the literature justifying hydrogen bonding as an adsorption mechanism are based on the fact that both starch molecules and mineral oxide surfaces have hydroxyl groups capable of hydrogen bonding. However, it is very difficult to prove experimentally that hydrogen bonds are formed between starch hydroxyl groups and mineral surface hydroxyl groups, primarily because intra- and intermolecular hydrogen bonds are already present in starch molecules. The presence of water and surface hydroxyl groups of minerals further complicates the detection of hydrogen bonds between starch and the mineral surface. The DRIFT spectroscopy results provide the most detailed data published on spectral changes that could be associated with hydrogen bonding between starch and a mineral surface. The increase in the O-H stretching frequency relative to the non-adsorbed polysaccharides suggests that the hydrogen bonds have become weaker or longer upon adsorption, and the decrease in bandwidth suggests a reduction in the number of hydrogen bonds [39,40]. Hence, the starch molecules appear to be less strongly hydrogen bonded to each other after adsorption onto the hematite surface and/or the starch molecules are less strongly hydrogen bonded to the hematite than they were to each other before adsorption.

25

The changes to the infrared bands in the region 1080-1000 cm 1, associated with the C-O stretch coupled to the C-C stretch and O-H deformation (Table 6), were not significant and therefore do not support strong hydrogen bonding of starch to the hematite surface. Strong hydrogen bonding would be expected to affect the C-O stretch, even though it is further removed from the hydrogen bond itself. The relatively small changes in the hydrogen bonding of starch molecules on adsorption would not be expected to cause any changes in the glucopyranose ring vibrations and the C1-H deformation. However, it was these vibrations that were most affected by adsorption (Table 3). The adsorption experiments in DMSO and urea were designed to test for the presence of hydrogen bonds. DMSO is a strong hydrogen-bond acceptor [23] and has a strong association with molecules containing hydroxyl groups. As a consequence, DMSO is capable of affecting the conformation and reactivity of the molecules [41]. The main reason for this is believed to be the prevention of intermolecular hydrogen bonding between amylose molecules by preferential formation of hydrogen bonds between DMSO and amylose [22]. DMSO does not appear to affect the intramolecular hydrogen bonding between C-2 and C-3' hydroxyl groups [36]. Urea is also a strong hydrogen-bond acceptor and has been used to confirm the presence of discrete amylopectin molecules in solution rather than hydrogen-bonded aggregates [42]. Thus, the presence of DMSO and urea would be expected to decrease or prevent the adsorption of the polysaccharides onto hematite. The results in Table 4 clearly show that DMSO and urea did not significantly effect the adsorption of the polysaccharides, and strongly suggest that hydrogen bonding is not the major driving force for adsorption. Adsorbed amylose was partly desorbed by washing in 85% DMSO for 10h (Table4), evidence that it is not as strongly adsorbed as amylopectin. Hence, hydrogen bonding may play a minor role in the adsorption mechanism. The above evidence does not clearly disprove the existence of hydrogen bonding as an adsorption mechanism, but merely suggests that some weak hydrogen bonding to the surface may have occurred in the regions of free or unattached

26

P.K. Weissenborn et al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 1 1 ~ 7

iron atoms. The reported ability of hydroxyl groups attached to C-2 and C-3' to form an intramolecular hydrogen bond supports this interpretation. Minor shifts in O-H stretching frequencies suggested additional weak hydrogen bonding between hydroxyl groups from the polysaccharides and hydroxyl groups from surface iron atoms. Adsorption experiments in DMSO and urea were unsuccessful in preventing or reversing adsorption, and supported a strong association between the polysaccharides and hematite, rather than hydrogen bonding. The ability of amylopectin to act as a complexing agent was demonstrated by mixing soluble iron(III) with an amylopectin solution. Making the solution more alkaline did not

glucopyranose units/hydroxyl groups unable to participate in surface complexation. A reaction scheme consistent with such weak hydrogen bonding is shown in Fig. 10.

5. Conclusions

The major changes in the infrared spectra of wheat starch, amylopectin and amylose upon adsorption onto hematite were shifts in the glucopyranose ring vibrations and the C,-H deformation. The shifts were consistent with the formation of a surface complex between polysaccharide oxygen atoms attached to C-2 and C-3' and surface CH2oH/O~

CH2oH/O~

CH2oH/O~

+

HO\ /O-H H0\ /OH HO\ / O H ......

Fe . . . . . . . . .

Fe . . . . . . .

HO\ /OH

Fe . . . . . . . . .

Fe . . . . . . .

.o/\o /

....

Fe . . . . . . . . .

Fe . . . . . . .

Fe . . . . . . . . .

Fe . . . . . . . . .

.o/ o / 42H20 F i g . 10. R e a c t i o n s c h e m e f o r s u r f a c e c o m p l e x f o r m a t i o n a n d h y d r o g e n - b o n d i n g

mechanisms.

P.K. Weissenborn et al./Colloids Surfaces A: Physicochem. Eng. Aspects 99 (1995) 11-27 precipitate iron hydroxide, thus indicating that a stable c o m p l e x h a d b e e n f o r m e d . T h i s result c o n firms t h a t s t r o n g b o n d i n g b e t w e e n i r o n ( I I I ) a n d a m y l o p e c t i n is possible.

Acknowledgements The authors thank BHP Iron Ore (Goldsworthy) L i m i t e d , a n d the M i n e r a l s a n d E n e r g y R e s e a r c h I n s t i t u t e of W e s t e r n A u s t r a l i a ( M E R I W A ) for financial support.

References [1] P.K. Weissenborn, L.J. Warren and J.G. Dunn, Int. J. Miner. Process., in press. [2] C. Sterling, J. Texture Stud., 9 (1978) 225. [3] S.R. Balajee and I. Iwasaki, Trans. Soc. Min. Eng., Am. Inst. Min. Eng., 244 (1969) 401. [4] I. Iwasaki and R.J. Lipp, in B. Yarar and D.J. Spottiswood (Eds.), Interfacial Phenomena in Mineral Processing, Engineering Foundation, New York, 1982, p. 321. [5] E. Steenberg and P.J. Harris, S. Afr. J. Chem., 37 (1984) 85. [6] I. Iwasaki, in P. Somasundaran and N. Arbiter (Eds.), Beneficiation of Mineral Fines: Problems and Research Needs, NSF Workshop, New York, 1979, p. 257. [7] R.J. Pugh, Int. J. Miner. Process., 25 (1989) 101. [8] N.K. Khosla, R.P. Bhagat, K.S. Gandhi and A.K. Biswas, Colloids Surfaces, 8 (1984) 321. [9] S. Subramanian, K.A. Natarajan and D.N. Sathyanarayana, Miner. Metallurgical Process., 6 (1989) 152. [10] Q. Liu and J.S. Laskowski, J. Colloid Interface Sci., 130 (1989) 101. [11] Q. Liu and J.S. Laskowski, Int. J. Miner. Process., 26 (1989) 297. [12] J.S. Laskowski, Q. Liu and N.J. Bolin, Int. J. Miner. Process., 33 (1991) 223. [13] P.S. Davis and DJ. Dellar, Nature, 212 (1966) 404. [14] H. Cross, T. Pepper, M.W. Kearsley and G.G. Birch, Die Stfirke, 37 (1985) 132. [15] H.A. Tajmir-Riahi, Carbohydr. Res., 183 (1988) 35. [16] P.K. Weissenborn, L.J. Warren and J.G. Dunn, Colloids Surfaces A: Physicochem. Eng~ Aspects, submitted for publication.(COL3112?)

27

[17] W. Banks, C.T. Greenwood, D.J. Hourston and A.R. Procter, Polymer, 12 (1971) 452. [18] H.V. Street and J.R. Close, Clin. Chim. Acta, 1 (1956) 256. [19] P.R. Griffiths and J.A. de Haseth, Fourier Transform Infrared Spectroscopy, Wiley-Interscience, New York, 1986, p. 544. [20] J.A. Mielczarski, J.M. Cases, E. Bouquet, O. Barres and J.F. Delon, Langmuir, 9 (1993) 2370. [21] P.J. Brimmer and P.R. Griffiths, Anal. Chem., 58 (1986) 2179. [22] D. French, in R.L. Whistler, J.N. BeMiller and E.F. Paschall (Eds.), Starch: Chemistry and Technology, 2nd edn., Academic Press, FL, 1984, p. 184. [23] S. Oae, Organic Sulphur Chemistry: Structure And Mechanism, Vol. 1, Chemical Rubber Company, Florida, 1991. [24] B. Casu and M. Reggiani, J. Polym. Sci., Part C, 7 (1964) 171. [25] B. Casu and M. Reggiani, Die St~.rke, 18 (1966) 218. [26] A.F. Colombo and A.R. Rule, Bureau of Mines Report of Investigations, RI 7306, United States Department of the Interior, Washington, DC, 1969. [27] S.A. Barker, E.J. Bourne, M. Stacey and D.H. Whiffen, J. Chem. Soc., (1954) 171. [28] S.A. Barker, R.H. Moore, M. Stacey and D.H. Whiffen, Nature, 186 (1960) 307. [29] J.J. Cael, J.L. Koenig and J. Blackwell, Biopolymers, 14 (1975) 1885. [30] K. Nakanishi and P.H. Solomon, Infrared Absorption Spectroscopy, 2nd edn., Holden-Day, Oakland, 1977. [31] R.S. Tipson and F.S. Parker, in W. Pigman, D. Horton and J.D. Wander (Eds.) The Carbohydrates - - Chemistry and Biochemistry, Vol. 1B, 2nd edn., Academic Press, New York, 1980, p. 1394. [32] R.H. Wilson and P.S. Belton, Carbohydr. Res., 180 (1988) 339. 1-33] H.A. Wells and R.H. Atalla, J. Mol. Struct., 224 (1990) 385. [34] M.J. Miles, V.J. Morris, P.D. Orford and S.G. Ring, Carbohydr. Res., 135 (1985) 271. [35] S.C. Ring, Food Hydrocolloids, 1 (1987) 449. [36] M. St-Jacques, P.R. Sundararajan, K.J. Taylor and R.H. Marchessault, J. Am. Chem. Soc., 98 (1976) 4386. [37] G. Bates, J. Hegenauer, J. Renner, P. Saltman and T.G. Spiro, Bioinorg. Chem., 2 (1973) 311. [38] M.A.K. Mostafa, J. Polym. Sci., 33 (1958) 323. [-39] W.C. Hamilton and J.A. Ibers, Hydrogen Bonding in Solids: Methods of Molecular Structure Determination, Benjamin, New York, 1968. [40] H. Ratajczak and W.J. Orville-Thomas, J. Mol. Struct., 1 (1967/1968) 449. [41] B. Casu, M. Reggiani, G.G. Gallo and A. Vigevani, Tetrahedron, 22 (1966) 3061. [42] W. Banks, R. Geddes, C.T. Greenwood and I.G. Jones, Die St~irke, 24 (1972) 245.