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Some studies on the adsorption behaviour of an oxidised starch onto haematite
0892-6875/88 $3.00 + 0.00 Pergamon Press pie
Minerals Engineering, Vol. 1, No. 3, pp. 241-254, 1988 Printed in Great Britain
SOME STUDIES ON THE ADSORPTION BEHAVIOUR OF AN OXIDISED STARCH ONTO HAEMATITE S. SUBRAMANIAN Dept.of Metallurgy,
and K.A. NATARAJAN
Indian Institute of Science, (Received 1 May 1988)
Bangalore-560
012, India
ABSTRACT The interaction of an oxidised starch with natural haematite has been investigated through adsorption studies, X-ray photoelectron and FTIR spectroscopic techniques. The adsorption density is found to vary with pH, with the maximum adsorption taking place in the pH range 4 to 10. Adsorption isotherms at 30 and 50°C show that the adsorption density varies also with temperature. The infrared and X-ray photoelectron spectra of adsorbed starch on haematite indicate evidence of chemical interaction and hydrogen bonding. Further, the direct interaction of ferric ion with starch has been studied through binding in solution, S~-EDAX analysis, viscosity and conductivity measurements, and these results suggest chemical interaction between iron and starch. Based on the above findings, probable adsorption mechanisms in this system have been postulated. Ke,xword,s Adsorption studies; oxidised starch; ha.satire; X-ray photoelectron spectra; FTIR spectra; viscosity; conductivity; chemical interaction; hydrogen bonding INTRODUCTION S t a r c h e s and their d e r i v a t i v e s have found m a n y a p p l i c a t i o n s in m i n e r a l processing operations s u c h as f l o t a t i o n , flocculation and selectlve flocculation [I-8]. In such applicatlons, for flotation or flocculatlon to take place effectively, the adsorption of starch is an important prerequisite. T h e s t a r c h c o u l d a d s o r b on t h e s u r f a c e of the mineral, i m p a r t i n g it hydrophilic characteristics due to the large number of its hydroxyl groups, or act by forming bridges between mineral particles and agglomerating them. The adsorption mechanism of organic polymers such as starch is considerably more complex than that of simple ions or molecules and cannot be ascribed to any one particular phenomenon. For a given system, there may be a d o m i n a n c e of electrostatic charge effects or hydrogen bonding or chemlcal reactions at the functional groups and these depend on the characteristics of the substrate as w e l l as the a d s o r b a t e [9-14]. The a d s o r p t i o n of s y n t h e t i c p o l y m e r s has attracted much attention [15] and the 'bridging' mechanism has been suggested to predominantly govern flocculation using polymers [16,17]. La Mer and Healy have brought out the limitations of the bridging theory and indicate that the k i n e t i c s of floc f o r m a t i o n i n v o l v e s a d i f f u s i o n controlled process [18]. S p e c i f i c chemical interaction between Fe 3+ and starch constituents has been suggested recently through a conductometric and IR spectroscopic study [19]. The o b j e c t of the p r e s e n t i n v e s t i g a t i o n is to e l u c i d a t e the a d s o r p t i o n mechanism of an oxidised starch o n t o haematite, b o t h w i t h r e s p e c t to the mineral-solution i n t e r f a c e and the b u l k s o l u t i o n p r o p e r t i e s and also to correlate this information with the flocculatlon of haematite ore fines. Apart from adsorption measurements, X-ray photoelectron and infrared spectroscopic techniques have been used to c h a r a c t e r l s e the a d s o r b e d species. The bulk s o l u t i o n p r o p e r t i e s have been ascertained by the interaction of ferric ion with starch through binding in solution, S E M - E D A X analysis, v i s c o s i t y and conductivity measurements. ME =/~-f
24L
242
S. StmltAMANXASand K. A. NATARAJAN
EXPERIMENTAL Pure mineral samples of haematite from Vajra Mines, Tumkur district, Karnataka were supplied by Alminrock Indscer Fabriks, Bangalore. Mineralogical analysis showed that the haematite sample was of 99% purity with occasional traces of magnetite and quartz. The samples were first crushed in a Jaw crusher and subjected to wet grinding in an ellipsoidal ball mill at a speed of 40 rpm. The ground samples were wet screened in a double-decked vibratory screen through 325 and 400 mesh sieves. The product was reground until all the material passed through the 400 mesh sieve. This was then dewatered, dried and blended to get fine powder samples, which were then i n t i m a t e l y m i x e d by c o n i n g and q u a r t e r i n g and s t o r e d in stoppered, a i r - t i g h t c o n t a i n e r s . The BET s p e c i f i c s u r f a c e area of the haematite sample used was 12.88 m2/g. The o x i d i s e d starch, A n i l o x AP, was p r o c u r e d from Anil S t a r c h Products, A h m e d a b a d and has b e e n p r e p a r e d by the o x i d a t i o n of m a i z e s t a r c h w i t h h y p o c h l o r i t e (35-38 gpl a v a i l a b l e chlorine) at alkaline pH values at room temperature. The relative molecular weight of this starch, as determined by the gel permeation chromatography technique was 2xi06. Various m e t h o d s of p r e p a r a t i o n of s t a r c h s o l u t i o n such as c a u s t i c i z i n g , d i s s o l v i n g in b o i l i n g w a t e r and autoclaving were tried and ultimately the autoclaving procedure as outlined below was adopted. Predetermined weights of starch powder were dissolved in distilled water by heating in an autoclave at I atmosphere steam pressure for half an hour followed by cooling in a water b a t h to r o o m temperature. Fresh starch solutions were prepared each day to minimise the effects of m i c r o b i o l o g i c a l d e g r a d a t i o n . The a b o v e m e t h o d of preparation was found to give homogeneous solutions and the results obtained were consistent with the causticizing procedure. For the adsorption tests, lg samples of haematite powder was pulped to a total v o l u m e of 2 0 c m 3 in s t o p p e r e d E r l e n m e y e r flasks. The pH of the p u l p was a d j u s t e d to a d e s i r e d v a l u e u s i n g e i t h e r h y d r o c h l o r i c a c i d or s o d i u m h y d r o x i d e . This was f o l l o w e d by the addition of a predetermined amount of starch solution. The flasks were then agitated in a mechanical shaker housed in an incubator chamber set to a desired temperature, for 2h, by which time e q u i l i b r i u m was a t t a i n e d . The r e s i d u a l c o n c e n t r a t i o n of s t a r c h in t h e supernatant was then analysed, spectrophotometrically [20]. Whenever the pulp remained cloudy they were centrifuged at a speed s u f f i c i e n t to r e m o v e the s u s p e n d e d p a r t i c l e s w i t h o u t a p p r e c i a b l y settling the starch. As controls, starch solution was always centrifuged simultaneously. The possibilities of ferric ion binding with starch in the solution phase were investigated by mixing solutions of f e r r i c s u l p h a t e of different concentrations with a stock solution of starch AP. Such mixtures were agitated in a mechanical shaker for half and hour and the concentration of the residual f e r r i c ion in the supernatant solution was analysed after centrifugation of the sedimented reaction product. The above experiments were carried out at two different pH values namely 2.7 and 3.4 and the weight to weight ratio in which the ferric ions were bound to the starch was estimated and expressed in terms of percentage binding. The r e l a t i v e v i s c o s i t y of 0.4% starch (28°C) as a f u n c t i o n of pH u s i n g a immersed in a constant temperature bath. on the viscosity of starch AP was then concentration as well as pH.
AP was measured at room Cannon-Fenske capillary The effect of addition of measured as a function of
temperature viscometer ferric ions ferric ion
The specific conductance of 0.4% starch AP was measured at room temperature (28oc) in the absence and presence of varying concentrations of ferric ions using a Systronics direct reading digital conductivity meter type 304. Double distilled water of conductivity 2.3 micromhos was used in the preparation of all solutions for these tests. IR spectrograms were recorded using a FT-IR spectrometer, Model Nicolet 170SX, operating in the range of 5000 to 10 cm -I and having an ultimate resolution of 0 . 0 6 c m -I . T h i s i n s t r u m e n t p o s s e s s e d c o m p u t e r m e m o r y capabilities and the differential spectra w i t h r e s p e c t to the v a r i o u s a d s o r b e d s p e c i e s on the mineral surface were recorded and desired regions in the spectra were expanded for clarity and for precise interpretation of the data. Samples for the IR
Adsorption behaviour of starch onto haernatite
243
studies were vacuum dried and the KBr pellet technique was used to prepare the specimens. A v a c u u m generator spectrometer ESCA III Mark II (VG Scientific Ltd., U.R.) was used for the X-ray photoelectron spectroscopic studies. The e x c i t a t i o n source was AI Ka X-ray source. Scanning electron micrographs of the iron-starch reaction product were taken in a Cambridge Instruments Stereoscan Model 150 Scanning Electron Microscope. T h e E D A X [Energy Dispersive Analysis of X-rays) spectrum of the iron-starch reaction product was recorded using EDAX Model 711 a t t a c h m e n t to the a b o v e SEM. RESULTS AND DISCUSSION Before the adsorption studies of p o l y m e r i c s t a r c h f l o c c u l a n t on h a e m a t i t e c o u l d be d i s c u s s e d it m a y be p e r t i n e n t to b r i e f l y c o n s i d e r the surfacechemical as well as the physico-chemical nature of the haematite surface. The s t r u c t u r a l s c h e m e of haematlte shows that the oxygen atoms are arranged in approximately hexagonal close packing. T h e r e are two s u c c e s s i v e layers of oxygen atoms and between the two layers there are positions for cations such that each lles between six oxygen atoms. If all these positions were filled, there would be as many cations as oxygen atoms in each sheet, but only twothirds of the available positions are filled in haematite [21]. In Fe203 and other transition metal oxides, active centres of metal cations exist and they have been used to bind a number of organic compounds [22,23]. A representative IR spectrum of the haematlte sample used in this s t u d y is presented in Figure I. The spectrum exhibits bands at 3695, 3660, 3625 cm -I and a broad band near 3450 cm -I besides another band at 3360 cm -I . All these
cm -I
.....
4000-2000
....
2 0 0 0 - - 400 c m - 1
/
/~
,1 I~ C0~ <%10
I t
O~
./
I
e
t
/
//"
/ .-
e.
./" g
. ......................
"iI ~ g
4OO0
I
I
I
-/
I 1600
""
/
?
3600
t,,?
/
'~" ~
....... "%kl
,,,
...I
I
3200 I
I
I
I I 12100
I
2800 I
J
I
I
2400 I 800
I
2~I 400
W a v e n u m b e r (crn "1)
Fig.1
FTIR spectrum of haematite
bands are a t t r i b u t a b l e to s t r e t c h i n g v i b r a t i o n s of the s u r f a c e h y d r o x y l groups. The IR s p e c t r a l s t u d i e s of haematlte by Rochester and Topham [24] revealed several absorption bands arising from the stretching v i b r a t i o n s of the surface hydroxyl groups. It is also known that chemisorption of water on haematite leads to the formation of surface hydroxyl groups [25]. The narrow bands at 3695 and 3660 cm -I originate from isolated surface hydroxyl groups, while the band centred at 3450 cm -I corresponds to hydroxyl groups perturbed by lateral hydrogen bonding interactions with neighbouring groups. There are a l s o b a n d s at 2870 and 2820 cm -I assignable to strongly hydrogen bonded OH groups. The bands at 536 and 464 cm -I are characteristic of Fe-O stretching vibrations. The weak bands at 1092, 1028 and 1004 cm-1 arise from overtone and combination bands of Fe-O stretching [26]. It thus becomes important to take
244
S. SUBRAMANIANand K. A, NATARAJAN
into account the hydroxylated n a t u r e of t h e h a e m a t i t e surface, w h e n considering the adsorption of starch o n t o h a e m a t i t e . F u r t h e r , it has b e e n r e p o r t e d [27] that h y d r o x y l a t e d iron oxide can be of two structures (a) a reactive open compound with better chelating properties as compared to (b) a non-react£ve ringed structure. Several polynuclear hydroxo species are known to exist in aqueous, colloidal systems containing iron oxide sols [28]. Adsorption studies: The adsorption isotherm of starch AP on haematite shown in Figure 2 is seen to follow the general shape of the S i m h a - F r i s c h - E i r i c h p l o t [29]. This also corresponds to the H-2 (high affinity) class of the Giles classification [30]. Similar results have been reported by other workers [7,31 ]. Cursory tests were c o n d u c t e d to c h e c k w h e t h e r the a d s o r p t i o n process is reversible using the adsorption procedure detailed earlier. The desorption of the adsorbed starch from the mineral surface was attempted by treating the adsorbed mineral with hot water and autoclaving. Even after autoclaving twice only about 18% of the adsorbed starch could be removed indicating at l e a s t t h e p a r t i a l irreversibility of a d s o r p t i o n u n d e r the e x p e r i m e n t a l c o n d i t i o n s studied. K h o s l a et al [19] h a v e r e p o r t e d a hysterisis in the adsorption-desorption c u r v e s for s t a r c h on h a e m a t i t e i n d i c a t i n g partial irreversibility of adsorption. The above observations suggest the involvement of n o n electrostatic forces in the adsorption process.
Rcsiduol Concentration (molellitrc) 110 8
Fig.2
Adsorption isotherm of starch AP onto haematite
Effect of pH: The effect of pH on the adsorption density of starch AP on haematite is shown in Figure 3. In these tests, 0.Sg/l concentration of starch AP was used. A p l a t e a u r e g i o n is o b s e r v e d b e t w e e n pH 4 to about 9, where the adsorption density is relatively higher and decreases sharply at pH values lower than 4 on the acidic side and higher than about 9 to 9.5 on the alkaline side. The isoelectric point of the haematlte sample used in this study was found to be located at pH 4.1 [32]. More or less uniformly higher adsorption densities for starch AP on haematite over a w i d e pH r a n g e of 4 to a b o u t 9 i n d i c a t e an adsorption mechanism o t h e r t h a n that due to c o u l o m b i c forces. The pH dependence observed in this study has been similarly reported by Kennedy et al [33] for the a d s o r p t i o n of g l u c o s e on c o a t e d Fe203 surfaces and has been attributed to the role of hydroxyl species and of the hydrous oxide formed as a function of pH in the chemical interaction between the glucose species and the oxide surface. Thus the hydroxylated surfaces may be playing a role in the a d s o r p t i o n p r o c e s s w h i c h is pH d e p e n d e n t . At low pH (<4) there is little hydrous oxide precipitated and as the pH of the solution is increased above pH 4 the h y d r o x y l coating on the surface increases and the adsorption density also i n c r e a s e s c o r r e s p o n d i n g l y . H o w e v e r at h i g h a l k a l i n e pH levels (>9) c o m p l e t e p r e c i p i t a t i o n of the hydrous oxide occurs and the coating becomes thicker allowing more c r o s s - l i n k i n g of the h y d r o x i d e ions to occur. T h i s
Adsorption behaviour of starch onto haematite
245
results in a lower surface area to volume ratio and a consequent lowering of the adsorption density. Beyond pH 9.5 the rapid decrease in the a d s o r p t i o n d e n s i t y could also be due to increased electrostatic repulslon between the increasingly negative haematite surface and the p o l y m e r i c starch chain. As mentioned earlier, with respect to hydrated iron oxide, a reactive open chain compound is m o r e p r o b a b l e under low a l k a l i n e conditions, w h i l e the ring s t r u c t u r e will be m o r e p r e d o m i n a n t at h i g h e r alkaline pH values. It thus appears from the adsorption behaviour of starch AP on haematite, that in the m a x i m u m adsorption density region i.e. from pH 4 to 9, coulombic forces of interaction are not predominant and may be governed more by non-electrostatlc forces. It is p e r t i n e n t to point out here that the pH dependence of the a d s o r p t i o n d e n s i t y of starch c l o s e l y r e s e m b l e s the e f f e c t of pH on the v i s c o s i t y of starch (Figure 8). It thus b e c o m e s of i n t e r e s t to view the adsorption mechanism in its entirety, taking both the i n t e r f a c i a l and bulk solution properties into consideration.
37 o x
~ "
36 35
x
Fig.3
PH Adsorption density of starch AP onto haematite as a function of pH
Effect of temperature: The effect of temperature as well as time on the adsorption density of starch AP on haematite was investigated at an initial pH of 8 and at a constant ionic strength of 2 x I0-3M KNO 3. The temperatures were controlled at 30oc and 50°C and the results are illustrated in Figure 4. The amount of starch AP adsorbed, increased with temperature and the change in pH was of the order of -0.3 unit by the end of the experiment. Such an increase in a d s o r p t i o n d e n s i t y w i t h temperature indicates the possibility of chemical interaction of starch with haematite. Earlier studies on the adsorption of o c t y l h y d r o x a m a t e on ferric oxide and manganese dioxide have revealed increased adsorption density with rise in temperature [34,35]. Khosla et al [19] have also reported increased starch adsorption on haematite with increase in temperature. Many adsorption processes are characterized by an e x o t h e r m i c heat of a d s o r p t i o n and as a consequence, a d s o r p t i o n should be e x p e c t e d to d e c r e a s e w i t h increase in temperature. However, l i t e r a t u r e e v i d e n c e i n d i c a t e s that as long as the adsorption process is s t i l l spontaneous, and that the free energy of adsorption is negative, it must follow that entropic factors constitute the driving force of adsorption. Chemisorption is favoured at higher temperatures due to faster reaction kinetics. The current observations on the temperature effects on starch adsorption on haematite clearly indicate the possibillty of chemical interaction. At higher temperatures, the diffusion rate of polymeric m o l e c u l e s such as s t a r c h w o u l d be e n h a n c e d leading to h i g h e r adsorption density. Further, dissolution and aging processes would be enhanced at higher t e m p e r a t u r e s and these r e s u l t in v a r i o u s polymeric species of iron to be formed, which could interact with starch. It is generally observed that when interacting polymers are added to metal oxide suspensions in water, two major effects are possible: (a)
polymer molecules may adsorb on solids.
(b) they may influence the dissociation of the ions. Both these effects are strongly pH dependent as well as temperature dependent,
246
S. SUBRAMANIANand K. A. NATARAJAN
and hence they must be related. In the type of interactions that take place, the size, shape, ligancy protonation and metal c o m p l e x a t i o n of the p o l y m e r molecules as well as the surface characteristics of the solid adsorbent must play a role.
37o711
V"
~0
"0
~ ~.7~
_ ~
•
• 50"C
~
o 30"C
0
0
,'
4
20
"
Time(h) Fig.4
Adsorption density of starch AP onto haematite as a function of time at different temperatures
Effect of starch on the dissolution of haematite: With a view to ascertain whether the dissolution of iron from haematite could be p r o m o t e d by starch, leaching tests were carried out in the presence and absence of starch, and the results are given in Figure 5. It is evident that the dissolution r a t e of i r o n i n c r e a s e s w i t h time, a c i d i t y and s t a r c h concentration. It could also be expected that this dissolution would increase w i t h t e m p e r a t u r e . E v e n at n e a r n e u t r a l pH, s t a r c h c o u l d b r i n g about the dissolution of iron, t h o u g h to a l e s s e r e x t e n t as c o m p a r e d to a c i d i c pH levels. It is interesting to note that the dissolution rate of ferric ions from h a e m a t i t e in the p r e s e n c e of s t a r c h i n c r e a s e s w i t h d e c r e a s e in pH, w h e r e a s the a d s o r p t i o n d e n s i t y of s t a r c h AP on haematite was observed to increase with increase in pH from about 2 to 4, reaching a higher steady state value in the pH range of about 4 to 10 and decreasing rapidly thereafter. Two competing factors, namely dissolution and adsorption are thus playing a role, b o t h b e i n g pH d e p e n d e n t . It is also a p p a r e n t that w h i l e the rate of the adsorption process is quite fast, reaching a saturation coverage rapidly, the dissolution rate is much slower.
.Sglt
~
,,u
/
starch AP
PH 3.3
~~0.25g~l
starch AP eNo starch
Time (h) Fig.5
Dissolution
of
iron
from haematite
by starch
AP a s a f u n c t i o n
of
time
The tendency for metal oxide-starch interaction at the interface, could also be seen in the light of metal ion-starch interactions in the bulk solution. The tendency for starch to form surface interaction products with hydroxylated m e t a l o x i d e s is the same as that for forming interaction products with the corresponding metal ions or its hydroxy species in solution. Therefore, the interaction tendency for the starch with a metal ion or its hydroxy species in solution can also project as to what may be happening at an o x l d e - s o l u t i o n interface, where conditions exist for similar reactions to take place.
Adsorption behaviour of starch onto haematite
247
FTIR spectroscopic studies on the adsorption of starch AP on haematite: The FTIR spectrum of starch AP a d s o r b e d on h a e m a t i t e s u r f a c e is s h o w n in F i g u r e 6. T h i s is a difference spectrum and is obtained by subtracting the spectrum of the haematlte sample from the spectrum of the adsorbed species on haematite surface. This provides the spectrum of the adsorbate. The spectrum s u g g e s t s t h a t the a d s o r p t i o n of s t a r c h on h a e m a t i t e o c c u r s b y s u r f a c e r e a c t i o n s involving-CH2OH and -COH groups of the starch. A b~oad absorption near 3360 cm -~ assignable to -OH stretching of -COH and -CH2OH groups of free s t a r c h a p p e a r s d l s t i n c t l y at 3350 and 3430 cm -I in the s p e c t r u m of the adsorbed species. This suggests that the -COH group is coordinated through the -CO group with consequent increase in the -OH stretching frequency. The -CH2OH groups in the proximity are also llkely to interact with Fe on the surface or may be involved in hydrogen bonding. The spectrum demonstrates by the presence of bands near 3700 and 3600 cm -I that not all the surface hydroxyl groups on haematlte are involved in chemical interaction.
c o =
'o' 4000
,
I
3550
,
I
,
I
I
3100 Wovcnumber (cm "t )
2650
2200
l;
14 eO
I 0
I 1800
I ~00
I
I
I
1200
900
600
W(Ivenumber(cm -1)
Fig.6
Differential FTIR spectrum of absorbed starch AP onto haematite
An intense broad band at 1642 cm -I in the spectrum of starch corresponding to ring stretching of the g l u c o p y r a n o s e r i n g is split into a d o u b l e t in the s p e c t r u m of the a d s o r b e d s p e c i e s and is found at 1641 and 1626 cm -I. The latter band may be attributed to the pyranose ring stretching and this ring has -COH and - C H 2 O H groups coordinated to Fe. Another absorption near 1080 cm -1 in the spectrum of free starch appears at slightly lower wave numbers =1072 c m -I in the adsorbed species. This may be assigned to -CO stretching of the alcoholio -COH or CH2OH groups. The small shift could be attributed to the interaction of -CO group with Fe. There is a band near 1450 cm -I indicating that the C-OH group may be hydrogen bonded to oxygen of Fe-OH. Another intense b a n d a t 1 4 6 2 c m -I c o r r e s p o n d s to C - O H g r o u p of the free starch. The intensities of the 1640 and 1460 c m -1 bands show that the ratio of the bonded to free groups in starch is nearly 1:1.
S. SU~RAMANIAN and K. A, NATARAJAN
248
The absorptions at 864 and 930 cm -I due to ring d e f o r m a t i o n and ring stretching respectively of free starch show changes in the s p e c t r u m of the a d s o r b e d species. W h i l e the former b a n d is split in the spectrum of the adsorbed species, the latter band is unaltered. The intense band at 1156 cm -I in the spectrum of starch due to ring C-OH stretching remains unaffected in the spectrum of the adsorbed species indicating non-involvement of this group in hydrogen bonding. The strongest peak in the spectrum of starch at 1017 cm -i a s s i g n e d to CH 2 twisting, also r e m a i n s u n a l t e r e d in the spectrum of the adsorbed species. X-ray photoelectron
spectroscopic
studies:
The X-ray photoelectron spectra in the O(Is), C(Is) and Fe(2p) r e g i o n s are shown in Figure 7. The binding energy values are listed in Table I. The O(Is) core level for free starch is at 535.6 eV. Following the adsorption of starch on h a e m a t i t e , there is a noticeably large decrease in the peak position of O(Is} line to 531.7eV. This shift in the O(Is) binding energy indicates that a chemical reaction between haematite and starch has occurred. The peak position of C(Is) of free starch registers a similar but smaller shift towards lower e n e r g y w h e n s t a r c h is adsorbed on haematite. This observation reveals that chemical interaction between haematite and the adsorbed starch is through the oxygens of starch. The corresponding changes in the Fe (2p) levels are slight broadening and small shifts towards higher binding energies. The C(Is) core level at 285.4eV present in all the spectra is easily traced to the impurity carbon. These results are c o n s i s t e n t w i t h the c o n c l u s i o n s d r a w n from the infrared spectral studies. o I
Ft(2p)
O(ls)
C(ls) I
,¢
iv)
T
5taech Z z ¢q
/L~ I 280
I 285
F¢203+ ",-.._.~ t arc h I 290
I 295
j
/
~
:5
530
I
I
535
F¢203+ I 710
I
540
I 715
I 720
I 725
I 730
BINDING ENERGY (¢V)
Fig.7
X-ray photoelectron
spectra of adsorbed
TABLE 1
Binding Energy Values
Fe(2p) 2P3/2 ; 2 P l / 2
'Sample
C(ls)
O(ls)
Starch
288.0
535.6
285.4
530.8
712.3;
725.4
285.4; 287.0
530.8; 531.7
713.2;
726.5
Fe203
(haematite)
Fe203 + Starch
Viscosity
starch AP onto haematite
of starch AP as a function of pH:
The relative viscosity of starch AP as a function of pH is shown in Figure 8. The v i s c o s i t y was found to increase with increase in [oH from 2 to 3, then remained more or less constant in the pH range 4 to about 10, followed by a further decrease at alkaline pH values. Such viscosity variations reflect the conformation of the starch molecules in the solution at different pH values as well as the i n f l u e n c e of the added sodium ions to control the pH. Amylose
Adsorption behaviour of starch onto hacmatite
249
present in the starch is known to possess a random coll configuration in the neutral pH range, changing to one of helix at alkallne pH range [35]. Such a change in conformation from an extended chain to one of helical, would bring about a reduction in the measured viscosity. A d d i t i o n of small a m o u n t s of s o d i u m salts such as s o d i u m chlorlde, is known to reduce the viscosity of amylose by 10% or less of its original value [36]. On the acidic side of the pH range below pH 3, significant decrease of the viscosity was observed. Since the starch AP used in these studies was prepared by hypochlorite o x i d a t i o n of m a i z e s t a r c h at an a l k a l i n e pH value, it possesses an anionic character over a wide range of pH and it may be expected that some protonation could occur in highly acidic solutlons.
=-,
1.35 -
/
8 1.3~N =_a 1.2! U
0
I
2
I
I
6
I
8
I
10
|
12
"
pH
Fig.8
Relative viscosity of starch AP as a function of pH
Effect of ferric ion addition on starch viscosity: The effect of addition of Fe 3+ ions on the relative viscosity of starch AP is shown in Figure 9. Viscosity measurements in this study were carried out only at acidic pH levels, since further increase in pH to alkaline values led to precipitation and turbidity in the solution, affecting the measurements. It is apparent that Fe 3+ ions tend to increase the viscosity of starch AP. Such an increase in the observed viscosity of starch AP in the presence of Fe 3÷ ions could be attributed to starch-ferric ion interaction. The higher viscosities o b t a i n e d for the s t a r c h - f e r r l c ion interaction product may also suggest a change in the structural conformation for the starch polymer as a result of ferric ion addition; in this case, the change to a linear structure and the absence of intra-polymer chelation. The viscosity of a polymer solution is a function of polymer configuration in solutlon and so it might be profoundly affected by the addition of reagents. Although the v i s c o s i t y of s t a r c h was found to increase with increase in ferric ion addition, there was a tendency for the viscosity of starch to drop down at high metal ion concentrations and at high pH values, due to precipitation. Conductivity measurements in starch solutions: The electrical conductivity of a solution is due to the movement of all the c h a r g e d s p e c i e s in the field; all ions present in the system contributing towards the c o n d u c t i v i t y of the s o l u t i o n . If an e l e c t r o l y t e is a d d e d to a n o t h e r electrolyte, keeping the volume unaltered, the overall conductivity will be the s u m of the individual conductances of the electrolytes, in so far as t h e two e l e c t r o l y t e s do not i n t e r a c t . If I o n - i o n or o t h e r types of interactions exist or if changes in total ion concentration take place, then the overall conductivity of the solutlon will also be affected in the form of positive or negative deviations from the expected values. Thus, on adding the c o n d u c t a n c e s of two e l e c t r o l y t e s , if the o v e r a l l c o n d u c t i v i t y cannot be accounted for on the basis of the mobilities of the ions known to be present in the two electrolytes, an ion-lon interaction or complex or adduct formation is indicated. The r e s u l t s of the c o n d u c t i v i t y m e a s u r e m e n t s carried out with starch containing varying concentrations of ferric ions at different pH values illustrated in Table 2. In all the cases, positive or negative deviations the additive values of the conductances were observed indicating ferric starch interaction in solution.
AP, are from ion-
250
S. SUBRAMANIAN and K. A. NATARAJAN
1.21 I0-3
I 10-2
I 10-I
Fe 3÷ Content ration, M
Fig.9
Relative viscosity of starch AP as a function of ferric ion concentration at different pH values
TABLE 2 SI.No.
Discrepancies from additive values of conductivity for F e 3 + - starch system
Starch AP concentration
pH
Ferric ion concentration
(M)
{%)
10-1 10-2
0.4 0.4 0.4
Discrepancy from additive value +1 .31
1.8 2.4 2.6
10-3
+I .35 -12.26
Starch-ferric ion binding studies: The actual binding of the ferric ions with starch AP were determined and the results are illustrated in Table 3. The percent binding of ferric ions with starch AP corresponding to various ferric ion concentrations and two pH values are given in the table and it is evident that up to about 75% of the added ferric ions could be bound to starch AP. TABLE 3
SI.No.
Ferric
Initial concentration of Fe 3+ ion a d d e d
binding
to
starch
AP
Final concentration of free Fe 3+ ion in
Percent bound
solution (m8/l)
(m8/1 )
pH=3.4
ion
pH=2.7
pH=3.4
pH=2.7
pH=3.4
pH=2.7
1o
142.31
162.06
44.90
65.30
68.45
59.71
2.
426.94
486.19
136.35
121.70
68.06
74.97
3.
711.56
810.32
233,05
189.00
67.25
76.68
4.
996.19
1134.45
316.90
320.80
68.19
71.72
5.
1280.81
1458.58
544.35
464,75
57.50
68.14
6.
1565.43
1782.70
812.65
678.05
48.09
61.97
Adsorption behaviour of starch onto haematite
251
SEM-EDAX studies on ferric ion-starch product: A typical scanning electron micrograph together with the EDAX spectrum of the s t a r c h A P - f e r r i c ion interaction product is shown in Figure 10. The ferric i o n - s t a r c h i n t e r a c t i o n p r o d u c t a p p e a r e d y e l l o w i s h b r o w n in c o l o u r a n d crystalline in nature. The EDAX spectrum substantiates the presence of bound ferric ions in the interacted p r o d u c t and they are found d i s t r i b u t e d all through the sample and not in a random manner.
Fig.10
Typical SEM-EDAX micrograph of starch AP-ferric ion interaction product
FTIR spectral studies on the ferric ion-starch AP system: A representative FTIR spectrum of the ferric-starch AP system is depicted in F i g u r e 11. I n t e r e s t i n g l y , t h e r e are s t r i k i n g s i m i l a r i t i e s b e t w e e n this spectrum and that of starch adsorbed on haematite as shown in Table 4. This is suggestive of chemical interaction between ferric ion and starch.
,....."-'\ .,.--'e~ ;-.L. u C
/
o
,
;
\
I
s
/!
',b,': -
/
\
,
I
Fig.11
I•
~.
, I
,,
.-
""
•,-- ~
/
a
/
"T... ,.,.J,~..~
~," ~ /
r'd,v,..
~
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."¢.'- _
./ ",,
~ i ."" U_,,"
, I
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~ 800
I
I
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-
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"-
',,
,
'"..
J
1)oo
"~ ',.
o,
"~".I_, "~" "~4oo
"'~ I I "4 600 40u W a v c n u m b e r (crn - t ) Differential FTIR spectrum of ferric-starch AP system
I
1600
/
=
p~
,," ,/o f,
/
, I
';
. , .~ .,i
o ~ ~
,
1800
/
'
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,
,.
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, 2000 I 1000
",
~
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I'
ca I--
\
crn-! crn -1 cm -1
4000-2000 2000-1000 1000-400
--.-....
252
S. SUBRAMANIAN and K. A. NATARAJAN
TABLE 4
Comparison of the freqnen¢les of Fe 3+ - starch and Fe203 - starch ~ s t e B s ' Fe 3+ - s t a r c h (cm-1)
Fe203 - starch
3335 1640 1375 1235 1140 1019 923 857
3343 1641 1374 1237 1152 1017 930 860
(cm -1 )
Adsorptio D mechanisms: A composite diagram showing the adsorption of starch AP on haematite, settling rate of iron ore fines and the relative viscosity of starch as a function of pH is i l l u s t r a t e d in F i g u r e 12. T h e r e exists a p a r a l l e l i s m b e t w e e n the adsorption behaviour of starch onto haematlte, the settling rate of the iron ore s u s p e n s i o n and the r e l a t i v e viscosity of the starch flocculant. It is interesting to n o t e f r o m t h e a b o v e f i g u r e t h a t t h e p l a t e a u regions corresponding to maximum adsorption density of starch, maximum viscosity for the starch solution, as well as the maximum settling rate for the iron ore suspension, fall w i t h i n limits in the same pH region. It thus b e c o m e s important to view the flocculation mechanism of iron ore suspensions in its entirety, taking both the adsorption behaviour of starch onto the substrate, as well as the theological property of the starch solution. The s i m i l a r pH dependence of the adsorption of starch onto ha~m~tite and the settling rate of iron ore suspensions is indicative of the similarities governing a d s o r p t i o n and f l o c c u l a t i o n mechanisms. The relationships existing between theological and flocculating properties of p o l y m e r f l o c c u l a n t s h a v e b e e n found to be important by other workers also [37,38].
o-Starch AP adsorption on haernattt¢ e - Relative Viscosity ot starch AP x-Settling rate using starch AP
3e
3? Ul
i=
U
2O
32 PH
Fig.12 Composite diagram showing the adsorption of starch AP onto haematlte, settling rate of iron fines using starch AP and the relative viscosity of starch AP as a function of pH
Adsorption behaviour of starch onto haematite
253
On the basis of the present work, the following mechanisms for the adsorption of the oxidlsed starch onto haematite: (i) The dissolution represented as:
of
Fe203
2 Fe 3÷
+
3 H20
~tarch¥
haematite ÷
in
the
presence
+
OH-
÷
FeOH;
Fe(OH)~;
of
starch
could
6 OH-
be (1)
The ferric ions thus released could form neutral complexes depending on the pH of the medium viz., F e 3+
could be proposed
Fe(OH)3;
or charged
ferric
hydroxo
Fe(OH)4
(2}
The above hydroxo complexes could react with starch, both at the haematltesolution interface as well as in the bulk solutlon by hydrogen b o n d i n g and chemical interaction. (ii) The starch could d i r e c t l y surface by hydrogen bonding.
interact
with
the
hydroxylated
iron oxide
(iii) The a d s o r p t i o n process could also be brought about by the chemical interaction of starch with the ferric ions released, as per Eq.(1). CONCLUSIONS From the results drawn:
of this
investigation
the
following
conclusions
could
be
1.
Adsorption measurements indicate that the adsorption density varies with pH as well as temperature. The partlal irreversibillty of adsorption is suggestive of t h e i n v o l v e m e n t of n o n - e l e c t r o s t a t l c forces in the adsorption process.
2.
The enhancement of the dissolution of ferric ions from haematite by starch enables the direct interaction of ferric ions with starch and may serve as the precursor step in the overall adsorption mechanism proposed.
3.
FTIR spectroscopic studies reveal the presence of surface hydroxyl groups on h a e m a t i t e . The d l f f e r e n t i a l s p e c t r u m of the a d s o r b e d starch on h a e m a t l t e indicates that both hydrogen bonding and chemical interaction forces are involved in the adsorption process. The striking similaritles between the spectra of the adsorbed starch on haematlte and that of the iron-starch system lend support to the v i e w that c h e m i c a l i n t e r a c t i v e forces of adsorption are operative.
4.
The X - r a y p h o t o e l e c t r o n s p e c t r o s c o p i c results corroborate chemical interaction mechanism between haematite and starch.
5.
Viscosity measurements indicate that the structural conformation is altered with change in pH and ferric-ion addition.
6.
The discrepancies observed in the additive values of the conductances suggest ion-lon interaction or complex formation between ferric ion starch.
7.
the
of starch
and
Starch-ferric ion binding studies confirm that a significant amount of the f e r r i c ions could be b o u n d to starch; the S E M - E D A X s t u d i e s on the i n t e r a c t i o n p r o d u c t between ferric ion and starch complement the above findings. REFERENCES
1. 2. 3. 4. 5.
above
Chang C.S., Cooke S.R.B. & Huch R.O. Trans. AIME 196, 1282 (195£0. Chang C.S. Trans.AIME Ig9 922 (1954). Cooke S.R.B., Iwasaki I. & Choi H.S. Trans. AIME 214, 920 (1959). Frommer D.W. Mln. Ensng. 16 67 (1964). Iwasakl I. & Lai R.W. Trans. AIME 232, 364 (1965).
254
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.
S. SUBRAMANIAN and K. A. NATARAJAN
Iwasakl I., Carlson W.J., Jr. & Parameter S.M. Trans. AIME 244, 88 (1969). BalaJee S.R. & lwasakl I. Trana AIME 244, 401 (1969). Subramanian S. & NataraJan K.A. Trans. Ind. Inst. Metals 32, 157 (1979). McLaren A . D . J . Phys. Chem. 58, 129 (1954). McLaren A.D., Peterson G.H. & Barshad I. Soii Scl. Soc. of Amer. 22, 239 (1958). Holmes R.M. & Toth S.J. So11 Sol. 84, 479 (1957). Michaels A.S. & Morelos O. Ind. Eng. Chem.47, 1801 (1955). Mortenson J.L. Soil Scl. Soc. of Amer. 21, 385 (1957). Packter A. S o 1 1 S c l . 83, 355 (1957). S t r o m b e r g R.R. ' A d s o r p t i o n of Polymers', in T r e a t i s e on Adheslon and Adhesives, Patrick R.L. (Ed), Dekker, New York (1967). Michaels A.S. Ind. Eng. Chem. 46, 1485 (1954). Ruehrwein R.A. & Ward D. Soil Scl. 73, 485 (1952). La Met V.K. & Healy T.W. Rev. Pure Appl. Chem. 13, 192 (1963). Khosla N.K., Bhagat R.P., Gandhi K.S. & Biswas A.K. Collolds Surfaces 8, 321 (1984) Dubois M., Gills K.A., Hamilton J.K., Rebers P.A. & Smith F. Anal. Chem. 28, 350 (1956). B r a g g L. & Claringbull G.F. Crystal Structures of Minerals, G. Bell and Sons Ltd., London (1965). Kennedy J.F. & Kay I . M . J . Chem. Soc. Perkln I 329 (1976). Graham K. & Madeley J . D . J . Appl. Chem. 12, 485 (1962). Rochester C.H. & Topham S . A . J . Chem. Soc. Faraday I 75, 1073 (1979). McCafferty E. & Zettlemoyer A.C. Dlsc. Faraday Soc. 52, 239 (1971). Ross S.D. Inorganlc Infrared and Raman Spectra, McGraw Hill Book Co. (UK) Ltd., England (1972). Zaidi S.A.B. & Mahdihassan Pakistan J. Sci. Ind. Res. 6, 103 (1963). B l a c k A.P. in Prlnciples and Applicatlons of Water Chemistry (Ed} Faust S.D. & Bunter J.V., John Wiley, (1967). Simha R., F r i s c h H. & E i r i c h F.R. J. Phys. Chem. 57, 584 (1953), J. Polymer Sci. 29, 3 (1958). Giles C.H., M a c E w a n T.H., N a k h w a S.N. & Smith D. J. Chem. Soc. 3973 (1960). Iwasaki I. & L i p p R . J . F l o c c u l a t i o n and C l a r i f i c a t i o n of M i n e r a l Suspensions, W a t e r P o l l u t i o n C o n t r o l R e s e a r c h Series, E n v i r o n m e n t a l Protection Agency, USA (1971). Subramanian S. Ph.D. Thesis, University of Mysore, (1985). Kennedy J.F., Barker S.A. & White C.A. Carb. Res. 54, I (1977). Raghavan S. & Fuerstenau D . W . J . Co11. Int. Sol. 50, 319 (1975). NataraJan R. & Fuerstenau D.W. Int. J. Mlner. Process. 11, 139 (1983). B e m i l l e r J.N. in Starch C h e m i s t r y and T e c h n o l o s y , Ed. Whistler R.L., Bemiller J.N. & Paschall E.F., Academic Press, New York (1984). Mackenzie J.M.W. Trans. AIME 231, 44 (1964). Slater R.W., Clark J.P. & Kitchener J.A. in Eishth International Mineral Processing ConEress. Leningrad, Paper C-5 (1968).
Minerals Engineering, Vol. 1, No. 3, pp. 241-254, 1988 Printed in Great Britain
SOME STUDIES ON THE ADSORPTION BEHAVIOUR OF AN OXIDISED STARCH ONTO HAEMATITE S. SUBRAMANIAN Dept.of Metallurgy,
and K.A. NATARAJAN
Indian Institute of Science, (Received 1 May 1988)
Bangalore-560
012, India
ABSTRACT The interaction of an oxidised starch with natural haematite has been investigated through adsorption studies, X-ray photoelectron and FTIR spectroscopic techniques. The adsorption density is found to vary with pH, with the maximum adsorption taking place in the pH range 4 to 10. Adsorption isotherms at 30 and 50°C show that the adsorption density varies also with temperature. The infrared and X-ray photoelectron spectra of adsorbed starch on haematite indicate evidence of chemical interaction and hydrogen bonding. Further, the direct interaction of ferric ion with starch has been studied through binding in solution, S~-EDAX analysis, viscosity and conductivity measurements, and these results suggest chemical interaction between iron and starch. Based on the above findings, probable adsorption mechanisms in this system have been postulated. Ke,xword,s Adsorption studies; oxidised starch; ha.satire; X-ray photoelectron spectra; FTIR spectra; viscosity; conductivity; chemical interaction; hydrogen bonding INTRODUCTION S t a r c h e s and their d e r i v a t i v e s have found m a n y a p p l i c a t i o n s in m i n e r a l processing operations s u c h as f l o t a t i o n , flocculation and selectlve flocculation [I-8]. In such applicatlons, for flotation or flocculatlon to take place effectively, the adsorption of starch is an important prerequisite. T h e s t a r c h c o u l d a d s o r b on t h e s u r f a c e of the mineral, i m p a r t i n g it hydrophilic characteristics due to the large number of its hydroxyl groups, or act by forming bridges between mineral particles and agglomerating them. The adsorption mechanism of organic polymers such as starch is considerably more complex than that of simple ions or molecules and cannot be ascribed to any one particular phenomenon. For a given system, there may be a d o m i n a n c e of electrostatic charge effects or hydrogen bonding or chemlcal reactions at the functional groups and these depend on the characteristics of the substrate as w e l l as the a d s o r b a t e [9-14]. The a d s o r p t i o n of s y n t h e t i c p o l y m e r s has attracted much attention [15] and the 'bridging' mechanism has been suggested to predominantly govern flocculation using polymers [16,17]. La Mer and Healy have brought out the limitations of the bridging theory and indicate that the k i n e t i c s of floc f o r m a t i o n i n v o l v e s a d i f f u s i o n controlled process [18]. S p e c i f i c chemical interaction between Fe 3+ and starch constituents has been suggested recently through a conductometric and IR spectroscopic study [19]. The o b j e c t of the p r e s e n t i n v e s t i g a t i o n is to e l u c i d a t e the a d s o r p t i o n mechanism of an oxidised starch o n t o haematite, b o t h w i t h r e s p e c t to the mineral-solution i n t e r f a c e and the b u l k s o l u t i o n p r o p e r t i e s and also to correlate this information with the flocculatlon of haematite ore fines. Apart from adsorption measurements, X-ray photoelectron and infrared spectroscopic techniques have been used to c h a r a c t e r l s e the a d s o r b e d species. The bulk s o l u t i o n p r o p e r t i e s have been ascertained by the interaction of ferric ion with starch through binding in solution, S E M - E D A X analysis, v i s c o s i t y and conductivity measurements. ME =/~-f
24L
242
S. StmltAMANXASand K. A. NATARAJAN
EXPERIMENTAL Pure mineral samples of haematite from Vajra Mines, Tumkur district, Karnataka were supplied by Alminrock Indscer Fabriks, Bangalore. Mineralogical analysis showed that the haematite sample was of 99% purity with occasional traces of magnetite and quartz. The samples were first crushed in a Jaw crusher and subjected to wet grinding in an ellipsoidal ball mill at a speed of 40 rpm. The ground samples were wet screened in a double-decked vibratory screen through 325 and 400 mesh sieves. The product was reground until all the material passed through the 400 mesh sieve. This was then dewatered, dried and blended to get fine powder samples, which were then i n t i m a t e l y m i x e d by c o n i n g and q u a r t e r i n g and s t o r e d in stoppered, a i r - t i g h t c o n t a i n e r s . The BET s p e c i f i c s u r f a c e area of the haematite sample used was 12.88 m2/g. The o x i d i s e d starch, A n i l o x AP, was p r o c u r e d from Anil S t a r c h Products, A h m e d a b a d and has b e e n p r e p a r e d by the o x i d a t i o n of m a i z e s t a r c h w i t h h y p o c h l o r i t e (35-38 gpl a v a i l a b l e chlorine) at alkaline pH values at room temperature. The relative molecular weight of this starch, as determined by the gel permeation chromatography technique was 2xi06. Various m e t h o d s of p r e p a r a t i o n of s t a r c h s o l u t i o n such as c a u s t i c i z i n g , d i s s o l v i n g in b o i l i n g w a t e r and autoclaving were tried and ultimately the autoclaving procedure as outlined below was adopted. Predetermined weights of starch powder were dissolved in distilled water by heating in an autoclave at I atmosphere steam pressure for half an hour followed by cooling in a water b a t h to r o o m temperature. Fresh starch solutions were prepared each day to minimise the effects of m i c r o b i o l o g i c a l d e g r a d a t i o n . The a b o v e m e t h o d of preparation was found to give homogeneous solutions and the results obtained were consistent with the causticizing procedure. For the adsorption tests, lg samples of haematite powder was pulped to a total v o l u m e of 2 0 c m 3 in s t o p p e r e d E r l e n m e y e r flasks. The pH of the p u l p was a d j u s t e d to a d e s i r e d v a l u e u s i n g e i t h e r h y d r o c h l o r i c a c i d or s o d i u m h y d r o x i d e . This was f o l l o w e d by the addition of a predetermined amount of starch solution. The flasks were then agitated in a mechanical shaker housed in an incubator chamber set to a desired temperature, for 2h, by which time e q u i l i b r i u m was a t t a i n e d . The r e s i d u a l c o n c e n t r a t i o n of s t a r c h in t h e supernatant was then analysed, spectrophotometrically [20]. Whenever the pulp remained cloudy they were centrifuged at a speed s u f f i c i e n t to r e m o v e the s u s p e n d e d p a r t i c l e s w i t h o u t a p p r e c i a b l y settling the starch. As controls, starch solution was always centrifuged simultaneously. The possibilities of ferric ion binding with starch in the solution phase were investigated by mixing solutions of f e r r i c s u l p h a t e of different concentrations with a stock solution of starch AP. Such mixtures were agitated in a mechanical shaker for half and hour and the concentration of the residual f e r r i c ion in the supernatant solution was analysed after centrifugation of the sedimented reaction product. The above experiments were carried out at two different pH values namely 2.7 and 3.4 and the weight to weight ratio in which the ferric ions were bound to the starch was estimated and expressed in terms of percentage binding. The r e l a t i v e v i s c o s i t y of 0.4% starch (28°C) as a f u n c t i o n of pH u s i n g a immersed in a constant temperature bath. on the viscosity of starch AP was then concentration as well as pH.
AP was measured at room Cannon-Fenske capillary The effect of addition of measured as a function of
temperature viscometer ferric ions ferric ion
The specific conductance of 0.4% starch AP was measured at room temperature (28oc) in the absence and presence of varying concentrations of ferric ions using a Systronics direct reading digital conductivity meter type 304. Double distilled water of conductivity 2.3 micromhos was used in the preparation of all solutions for these tests. IR spectrograms were recorded using a FT-IR spectrometer, Model Nicolet 170SX, operating in the range of 5000 to 10 cm -I and having an ultimate resolution of 0 . 0 6 c m -I . T h i s i n s t r u m e n t p o s s e s s e d c o m p u t e r m e m o r y capabilities and the differential spectra w i t h r e s p e c t to the v a r i o u s a d s o r b e d s p e c i e s on the mineral surface were recorded and desired regions in the spectra were expanded for clarity and for precise interpretation of the data. Samples for the IR
Adsorption behaviour of starch onto haernatite
243
studies were vacuum dried and the KBr pellet technique was used to prepare the specimens. A v a c u u m generator spectrometer ESCA III Mark II (VG Scientific Ltd., U.R.) was used for the X-ray photoelectron spectroscopic studies. The e x c i t a t i o n source was AI Ka X-ray source. Scanning electron micrographs of the iron-starch reaction product were taken in a Cambridge Instruments Stereoscan Model 150 Scanning Electron Microscope. T h e E D A X [Energy Dispersive Analysis of X-rays) spectrum of the iron-starch reaction product was recorded using EDAX Model 711 a t t a c h m e n t to the a b o v e SEM. RESULTS AND DISCUSSION Before the adsorption studies of p o l y m e r i c s t a r c h f l o c c u l a n t on h a e m a t i t e c o u l d be d i s c u s s e d it m a y be p e r t i n e n t to b r i e f l y c o n s i d e r the surfacechemical as well as the physico-chemical nature of the haematite surface. The s t r u c t u r a l s c h e m e of haematlte shows that the oxygen atoms are arranged in approximately hexagonal close packing. T h e r e are two s u c c e s s i v e layers of oxygen atoms and between the two layers there are positions for cations such that each lles between six oxygen atoms. If all these positions were filled, there would be as many cations as oxygen atoms in each sheet, but only twothirds of the available positions are filled in haematite [21]. In Fe203 and other transition metal oxides, active centres of metal cations exist and they have been used to bind a number of organic compounds [22,23]. A representative IR spectrum of the haematlte sample used in this s t u d y is presented in Figure I. The spectrum exhibits bands at 3695, 3660, 3625 cm -I and a broad band near 3450 cm -I besides another band at 3360 cm -I . All these
cm -I
.....
4000-2000
....
2 0 0 0 - - 400 c m - 1
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I
2~I 400
W a v e n u m b e r (crn "1)
Fig.1
FTIR spectrum of haematite
bands are a t t r i b u t a b l e to s t r e t c h i n g v i b r a t i o n s of the s u r f a c e h y d r o x y l groups. The IR s p e c t r a l s t u d i e s of haematlte by Rochester and Topham [24] revealed several absorption bands arising from the stretching v i b r a t i o n s of the surface hydroxyl groups. It is also known that chemisorption of water on haematite leads to the formation of surface hydroxyl groups [25]. The narrow bands at 3695 and 3660 cm -I originate from isolated surface hydroxyl groups, while the band centred at 3450 cm -I corresponds to hydroxyl groups perturbed by lateral hydrogen bonding interactions with neighbouring groups. There are a l s o b a n d s at 2870 and 2820 cm -I assignable to strongly hydrogen bonded OH groups. The bands at 536 and 464 cm -I are characteristic of Fe-O stretching vibrations. The weak bands at 1092, 1028 and 1004 cm-1 arise from overtone and combination bands of Fe-O stretching [26]. It thus becomes important to take
244
S. SUBRAMANIANand K. A, NATARAJAN
into account the hydroxylated n a t u r e of t h e h a e m a t i t e surface, w h e n considering the adsorption of starch o n t o h a e m a t i t e . F u r t h e r , it has b e e n r e p o r t e d [27] that h y d r o x y l a t e d iron oxide can be of two structures (a) a reactive open compound with better chelating properties as compared to (b) a non-react£ve ringed structure. Several polynuclear hydroxo species are known to exist in aqueous, colloidal systems containing iron oxide sols [28]. Adsorption studies: The adsorption isotherm of starch AP on haematite shown in Figure 2 is seen to follow the general shape of the S i m h a - F r i s c h - E i r i c h p l o t [29]. This also corresponds to the H-2 (high affinity) class of the Giles classification [30]. Similar results have been reported by other workers [7,31 ]. Cursory tests were c o n d u c t e d to c h e c k w h e t h e r the a d s o r p t i o n process is reversible using the adsorption procedure detailed earlier. The desorption of the adsorbed starch from the mineral surface was attempted by treating the adsorbed mineral with hot water and autoclaving. Even after autoclaving twice only about 18% of the adsorbed starch could be removed indicating at l e a s t t h e p a r t i a l irreversibility of a d s o r p t i o n u n d e r the e x p e r i m e n t a l c o n d i t i o n s studied. K h o s l a et al [19] h a v e r e p o r t e d a hysterisis in the adsorption-desorption c u r v e s for s t a r c h on h a e m a t i t e i n d i c a t i n g partial irreversibility of adsorption. The above observations suggest the involvement of n o n electrostatic forces in the adsorption process.
Rcsiduol Concentration (molellitrc) 110 8
Fig.2
Adsorption isotherm of starch AP onto haematite
Effect of pH: The effect of pH on the adsorption density of starch AP on haematite is shown in Figure 3. In these tests, 0.Sg/l concentration of starch AP was used. A p l a t e a u r e g i o n is o b s e r v e d b e t w e e n pH 4 to about 9, where the adsorption density is relatively higher and decreases sharply at pH values lower than 4 on the acidic side and higher than about 9 to 9.5 on the alkaline side. The isoelectric point of the haematlte sample used in this study was found to be located at pH 4.1 [32]. More or less uniformly higher adsorption densities for starch AP on haematite over a w i d e pH r a n g e of 4 to a b o u t 9 i n d i c a t e an adsorption mechanism o t h e r t h a n that due to c o u l o m b i c forces. The pH dependence observed in this study has been similarly reported by Kennedy et al [33] for the a d s o r p t i o n of g l u c o s e on c o a t e d Fe203 surfaces and has been attributed to the role of hydroxyl species and of the hydrous oxide formed as a function of pH in the chemical interaction between the glucose species and the oxide surface. Thus the hydroxylated surfaces may be playing a role in the a d s o r p t i o n p r o c e s s w h i c h is pH d e p e n d e n t . At low pH (<4) there is little hydrous oxide precipitated and as the pH of the solution is increased above pH 4 the h y d r o x y l coating on the surface increases and the adsorption density also i n c r e a s e s c o r r e s p o n d i n g l y . H o w e v e r at h i g h a l k a l i n e pH levels (>9) c o m p l e t e p r e c i p i t a t i o n of the hydrous oxide occurs and the coating becomes thicker allowing more c r o s s - l i n k i n g of the h y d r o x i d e ions to occur. T h i s
Adsorption behaviour of starch onto haematite
245
results in a lower surface area to volume ratio and a consequent lowering of the adsorption density. Beyond pH 9.5 the rapid decrease in the a d s o r p t i o n d e n s i t y could also be due to increased electrostatic repulslon between the increasingly negative haematite surface and the p o l y m e r i c starch chain. As mentioned earlier, with respect to hydrated iron oxide, a reactive open chain compound is m o r e p r o b a b l e under low a l k a l i n e conditions, w h i l e the ring s t r u c t u r e will be m o r e p r e d o m i n a n t at h i g h e r alkaline pH values. It thus appears from the adsorption behaviour of starch AP on haematite, that in the m a x i m u m adsorption density region i.e. from pH 4 to 9, coulombic forces of interaction are not predominant and may be governed more by non-electrostatlc forces. It is p e r t i n e n t to point out here that the pH dependence of the a d s o r p t i o n d e n s i t y of starch c l o s e l y r e s e m b l e s the e f f e c t of pH on the v i s c o s i t y of starch (Figure 8). It thus b e c o m e s of i n t e r e s t to view the adsorption mechanism in its entirety, taking both the i n t e r f a c i a l and bulk solution properties into consideration.
37 o x
~ "
36 35
x
Fig.3
PH Adsorption density of starch AP onto haematite as a function of pH
Effect of temperature: The effect of temperature as well as time on the adsorption density of starch AP on haematite was investigated at an initial pH of 8 and at a constant ionic strength of 2 x I0-3M KNO 3. The temperatures were controlled at 30oc and 50°C and the results are illustrated in Figure 4. The amount of starch AP adsorbed, increased with temperature and the change in pH was of the order of -0.3 unit by the end of the experiment. Such an increase in a d s o r p t i o n d e n s i t y w i t h temperature indicates the possibility of chemical interaction of starch with haematite. Earlier studies on the adsorption of o c t y l h y d r o x a m a t e on ferric oxide and manganese dioxide have revealed increased adsorption density with rise in temperature [34,35]. Khosla et al [19] have also reported increased starch adsorption on haematite with increase in temperature. Many adsorption processes are characterized by an e x o t h e r m i c heat of a d s o r p t i o n and as a consequence, a d s o r p t i o n should be e x p e c t e d to d e c r e a s e w i t h increase in temperature. However, l i t e r a t u r e e v i d e n c e i n d i c a t e s that as long as the adsorption process is s t i l l spontaneous, and that the free energy of adsorption is negative, it must follow that entropic factors constitute the driving force of adsorption. Chemisorption is favoured at higher temperatures due to faster reaction kinetics. The current observations on the temperature effects on starch adsorption on haematite clearly indicate the possibillty of chemical interaction. At higher temperatures, the diffusion rate of polymeric m o l e c u l e s such as s t a r c h w o u l d be e n h a n c e d leading to h i g h e r adsorption density. Further, dissolution and aging processes would be enhanced at higher t e m p e r a t u r e s and these r e s u l t in v a r i o u s polymeric species of iron to be formed, which could interact with starch. It is generally observed that when interacting polymers are added to metal oxide suspensions in water, two major effects are possible: (a)
polymer molecules may adsorb on solids.
(b) they may influence the dissociation of the ions. Both these effects are strongly pH dependent as well as temperature dependent,
246
S. SUBRAMANIANand K. A. NATARAJAN
and hence they must be related. In the type of interactions that take place, the size, shape, ligancy protonation and metal c o m p l e x a t i o n of the p o l y m e r molecules as well as the surface characteristics of the solid adsorbent must play a role.
37o711
V"
~0
"0
~ ~.7~
_ ~
•
• 50"C
~
o 30"C
0
0
,'
4
20
"
Time(h) Fig.4
Adsorption density of starch AP onto haematite as a function of time at different temperatures
Effect of starch on the dissolution of haematite: With a view to ascertain whether the dissolution of iron from haematite could be p r o m o t e d by starch, leaching tests were carried out in the presence and absence of starch, and the results are given in Figure 5. It is evident that the dissolution r a t e of i r o n i n c r e a s e s w i t h time, a c i d i t y and s t a r c h concentration. It could also be expected that this dissolution would increase w i t h t e m p e r a t u r e . E v e n at n e a r n e u t r a l pH, s t a r c h c o u l d b r i n g about the dissolution of iron, t h o u g h to a l e s s e r e x t e n t as c o m p a r e d to a c i d i c pH levels. It is interesting to note that the dissolution rate of ferric ions from h a e m a t i t e in the p r e s e n c e of s t a r c h i n c r e a s e s w i t h d e c r e a s e in pH, w h e r e a s the a d s o r p t i o n d e n s i t y of s t a r c h AP on haematite was observed to increase with increase in pH from about 2 to 4, reaching a higher steady state value in the pH range of about 4 to 10 and decreasing rapidly thereafter. Two competing factors, namely dissolution and adsorption are thus playing a role, b o t h b e i n g pH d e p e n d e n t . It is also a p p a r e n t that w h i l e the rate of the adsorption process is quite fast, reaching a saturation coverage rapidly, the dissolution rate is much slower.
.Sglt
~
,,u
/
starch AP
PH 3.3
~~0.25g~l
starch AP eNo starch
Time (h) Fig.5
Dissolution
of
iron
from haematite
by starch
AP a s a f u n c t i o n
of
time
The tendency for metal oxide-starch interaction at the interface, could also be seen in the light of metal ion-starch interactions in the bulk solution. The tendency for starch to form surface interaction products with hydroxylated m e t a l o x i d e s is the same as that for forming interaction products with the corresponding metal ions or its hydroxy species in solution. Therefore, the interaction tendency for the starch with a metal ion or its hydroxy species in solution can also project as to what may be happening at an o x l d e - s o l u t i o n interface, where conditions exist for similar reactions to take place.
Adsorption behaviour of starch onto haematite
247
FTIR spectroscopic studies on the adsorption of starch AP on haematite: The FTIR spectrum of starch AP a d s o r b e d on h a e m a t i t e s u r f a c e is s h o w n in F i g u r e 6. T h i s is a difference spectrum and is obtained by subtracting the spectrum of the haematlte sample from the spectrum of the adsorbed species on haematite surface. This provides the spectrum of the adsorbate. The spectrum s u g g e s t s t h a t the a d s o r p t i o n of s t a r c h on h a e m a t i t e o c c u r s b y s u r f a c e r e a c t i o n s involving-CH2OH and -COH groups of the starch. A b~oad absorption near 3360 cm -~ assignable to -OH stretching of -COH and -CH2OH groups of free s t a r c h a p p e a r s d l s t i n c t l y at 3350 and 3430 cm -I in the s p e c t r u m of the adsorbed species. This suggests that the -COH group is coordinated through the -CO group with consequent increase in the -OH stretching frequency. The -CH2OH groups in the proximity are also llkely to interact with Fe on the surface or may be involved in hydrogen bonding. The spectrum demonstrates by the presence of bands near 3700 and 3600 cm -I that not all the surface hydroxyl groups on haematlte are involved in chemical interaction.
c o =
'o' 4000
,
I
3550
,
I
,
I
I
3100 Wovcnumber (cm "t )
2650
2200
l;
14 eO
I 0
I 1800
I ~00
I
I
I
1200
900
600
W(Ivenumber(cm -1)
Fig.6
Differential FTIR spectrum of absorbed starch AP onto haematite
An intense broad band at 1642 cm -I in the spectrum of starch corresponding to ring stretching of the g l u c o p y r a n o s e r i n g is split into a d o u b l e t in the s p e c t r u m of the a d s o r b e d s p e c i e s and is found at 1641 and 1626 cm -I. The latter band may be attributed to the pyranose ring stretching and this ring has -COH and - C H 2 O H groups coordinated to Fe. Another absorption near 1080 cm -1 in the spectrum of free starch appears at slightly lower wave numbers =1072 c m -I in the adsorbed species. This may be assigned to -CO stretching of the alcoholio -COH or CH2OH groups. The small shift could be attributed to the interaction of -CO group with Fe. There is a band near 1450 cm -I indicating that the C-OH group may be hydrogen bonded to oxygen of Fe-OH. Another intense b a n d a t 1 4 6 2 c m -I c o r r e s p o n d s to C - O H g r o u p of the free starch. The intensities of the 1640 and 1460 c m -1 bands show that the ratio of the bonded to free groups in starch is nearly 1:1.
S. SU~RAMANIAN and K. A, NATARAJAN
248
The absorptions at 864 and 930 cm -I due to ring d e f o r m a t i o n and ring stretching respectively of free starch show changes in the s p e c t r u m of the a d s o r b e d species. W h i l e the former b a n d is split in the spectrum of the adsorbed species, the latter band is unaltered. The intense band at 1156 cm -I in the spectrum of starch due to ring C-OH stretching remains unaffected in the spectrum of the adsorbed species indicating non-involvement of this group in hydrogen bonding. The strongest peak in the spectrum of starch at 1017 cm -i a s s i g n e d to CH 2 twisting, also r e m a i n s u n a l t e r e d in the spectrum of the adsorbed species. X-ray photoelectron
spectroscopic
studies:
The X-ray photoelectron spectra in the O(Is), C(Is) and Fe(2p) r e g i o n s are shown in Figure 7. The binding energy values are listed in Table I. The O(Is) core level for free starch is at 535.6 eV. Following the adsorption of starch on h a e m a t i t e , there is a noticeably large decrease in the peak position of O(Is} line to 531.7eV. This shift in the O(Is) binding energy indicates that a chemical reaction between haematite and starch has occurred. The peak position of C(Is) of free starch registers a similar but smaller shift towards lower e n e r g y w h e n s t a r c h is adsorbed on haematite. This observation reveals that chemical interaction between haematite and the adsorbed starch is through the oxygens of starch. The corresponding changes in the Fe (2p) levels are slight broadening and small shifts towards higher binding energies. The C(Is) core level at 285.4eV present in all the spectra is easily traced to the impurity carbon. These results are c o n s i s t e n t w i t h the c o n c l u s i o n s d r a w n from the infrared spectral studies. o I
Ft(2p)
O(ls)
C(ls) I
,¢
iv)
T
5taech Z z ¢q
/L~ I 280
I 285
F¢203+ ",-.._.~ t arc h I 290
I 295
j
/
~
:5
530
I
I
535
F¢203+ I 710
I
540
I 715
I 720
I 725
I 730
BINDING ENERGY (¢V)
Fig.7
X-ray photoelectron
spectra of adsorbed
TABLE 1
Binding Energy Values
Fe(2p) 2P3/2 ; 2 P l / 2
'Sample
C(ls)
O(ls)
Starch
288.0
535.6
285.4
530.8
712.3;
725.4
285.4; 287.0
530.8; 531.7
713.2;
726.5
Fe203
(haematite)
Fe203 + Starch
Viscosity
starch AP onto haematite
of starch AP as a function of pH:
The relative viscosity of starch AP as a function of pH is shown in Figure 8. The v i s c o s i t y was found to increase with increase in [oH from 2 to 3, then remained more or less constant in the pH range 4 to about 10, followed by a further decrease at alkaline pH values. Such viscosity variations reflect the conformation of the starch molecules in the solution at different pH values as well as the i n f l u e n c e of the added sodium ions to control the pH. Amylose
Adsorption behaviour of starch onto hacmatite
249
present in the starch is known to possess a random coll configuration in the neutral pH range, changing to one of helix at alkallne pH range [35]. Such a change in conformation from an extended chain to one of helical, would bring about a reduction in the measured viscosity. A d d i t i o n of small a m o u n t s of s o d i u m salts such as s o d i u m chlorlde, is known to reduce the viscosity of amylose by 10% or less of its original value [36]. On the acidic side of the pH range below pH 3, significant decrease of the viscosity was observed. Since the starch AP used in these studies was prepared by hypochlorite o x i d a t i o n of m a i z e s t a r c h at an a l k a l i n e pH value, it possesses an anionic character over a wide range of pH and it may be expected that some protonation could occur in highly acidic solutlons.
=-,
1.35 -
/
8 1.3~N =_a 1.2! U
0
I
2
I
I
6
I
8
I
10
|
12
"
pH
Fig.8
Relative viscosity of starch AP as a function of pH
Effect of ferric ion addition on starch viscosity: The effect of addition of Fe 3+ ions on the relative viscosity of starch AP is shown in Figure 9. Viscosity measurements in this study were carried out only at acidic pH levels, since further increase in pH to alkaline values led to precipitation and turbidity in the solution, affecting the measurements. It is apparent that Fe 3+ ions tend to increase the viscosity of starch AP. Such an increase in the observed viscosity of starch AP in the presence of Fe 3÷ ions could be attributed to starch-ferric ion interaction. The higher viscosities o b t a i n e d for the s t a r c h - f e r r l c ion interaction product may also suggest a change in the structural conformation for the starch polymer as a result of ferric ion addition; in this case, the change to a linear structure and the absence of intra-polymer chelation. The viscosity of a polymer solution is a function of polymer configuration in solutlon and so it might be profoundly affected by the addition of reagents. Although the v i s c o s i t y of s t a r c h was found to increase with increase in ferric ion addition, there was a tendency for the viscosity of starch to drop down at high metal ion concentrations and at high pH values, due to precipitation. Conductivity measurements in starch solutions: The electrical conductivity of a solution is due to the movement of all the c h a r g e d s p e c i e s in the field; all ions present in the system contributing towards the c o n d u c t i v i t y of the s o l u t i o n . If an e l e c t r o l y t e is a d d e d to a n o t h e r electrolyte, keeping the volume unaltered, the overall conductivity will be the s u m of the individual conductances of the electrolytes, in so far as t h e two e l e c t r o l y t e s do not i n t e r a c t . If I o n - i o n or o t h e r types of interactions exist or if changes in total ion concentration take place, then the overall conductivity of the solutlon will also be affected in the form of positive or negative deviations from the expected values. Thus, on adding the c o n d u c t a n c e s of two e l e c t r o l y t e s , if the o v e r a l l c o n d u c t i v i t y cannot be accounted for on the basis of the mobilities of the ions known to be present in the two electrolytes, an ion-lon interaction or complex or adduct formation is indicated. The r e s u l t s of the c o n d u c t i v i t y m e a s u r e m e n t s carried out with starch containing varying concentrations of ferric ions at different pH values illustrated in Table 2. In all the cases, positive or negative deviations the additive values of the conductances were observed indicating ferric starch interaction in solution.
AP, are from ion-
250
S. SUBRAMANIAN and K. A. NATARAJAN
1.21 I0-3
I 10-2
I 10-I
Fe 3÷ Content ration, M
Fig.9
Relative viscosity of starch AP as a function of ferric ion concentration at different pH values
TABLE 2 SI.No.
Discrepancies from additive values of conductivity for F e 3 + - starch system
Starch AP concentration
pH
Ferric ion concentration
(M)
{%)
10-1 10-2
0.4 0.4 0.4
Discrepancy from additive value +1 .31
1.8 2.4 2.6
10-3
+I .35 -12.26
Starch-ferric ion binding studies: The actual binding of the ferric ions with starch AP were determined and the results are illustrated in Table 3. The percent binding of ferric ions with starch AP corresponding to various ferric ion concentrations and two pH values are given in the table and it is evident that up to about 75% of the added ferric ions could be bound to starch AP. TABLE 3
SI.No.
Ferric
Initial concentration of Fe 3+ ion a d d e d
binding
to
starch
AP
Final concentration of free Fe 3+ ion in
Percent bound
solution (m8/l)
(m8/1 )
pH=3.4
ion
pH=2.7
pH=3.4
pH=2.7
pH=3.4
pH=2.7
1o
142.31
162.06
44.90
65.30
68.45
59.71
2.
426.94
486.19
136.35
121.70
68.06
74.97
3.
711.56
810.32
233,05
189.00
67.25
76.68
4.
996.19
1134.45
316.90
320.80
68.19
71.72
5.
1280.81
1458.58
544.35
464,75
57.50
68.14
6.
1565.43
1782.70
812.65
678.05
48.09
61.97
Adsorption behaviour of starch onto haematite
251
SEM-EDAX studies on ferric ion-starch product: A typical scanning electron micrograph together with the EDAX spectrum of the s t a r c h A P - f e r r i c ion interaction product is shown in Figure 10. The ferric i o n - s t a r c h i n t e r a c t i o n p r o d u c t a p p e a r e d y e l l o w i s h b r o w n in c o l o u r a n d crystalline in nature. The EDAX spectrum substantiates the presence of bound ferric ions in the interacted p r o d u c t and they are found d i s t r i b u t e d all through the sample and not in a random manner.
Fig.10
Typical SEM-EDAX micrograph of starch AP-ferric ion interaction product
FTIR spectral studies on the ferric ion-starch AP system: A representative FTIR spectrum of the ferric-starch AP system is depicted in F i g u r e 11. I n t e r e s t i n g l y , t h e r e are s t r i k i n g s i m i l a r i t i e s b e t w e e n this spectrum and that of starch adsorbed on haematite as shown in Table 4. This is suggestive of chemical interaction between ferric ion and starch.
,....."-'\ .,.--'e~ ;-.L. u C
/
o
,
;
\
I
s
/!
',b,': -
/
\
,
I
Fig.11
I•
~.
, I
,,
.-
""
•,-- ~
/
a
/
"T... ,.,.J,~..~
~," ~ /
r'd,v,..
~
', ',
."¢.'- _
./ ",,
~ i ."" U_,,"
, I
~t~_
";
~ 800
I
I
1400
-
-.j; -,
"-
',,
,
'"..
J
1)oo
"~ ',.
o,
"~".I_, "~" "~4oo
"'~ I I "4 600 40u W a v c n u m b e r (crn - t ) Differential FTIR spectrum of ferric-starch AP system
I
1600
/
=
p~
,," ,/o f,
/
, I
';
. , .~ .,i
o ~ ~
,
1800
/
'
\\
~ ~
,
,.
"'*.
\
, 2000 I 1000
",
~
~
'
~
- / //*
k
\
\
I'
ca I--
\
crn-! crn -1 cm -1
4000-2000 2000-1000 1000-400
--.-....
252
S. SUBRAMANIAN and K. A. NATARAJAN
TABLE 4
Comparison of the freqnen¢les of Fe 3+ - starch and Fe203 - starch ~ s t e B s ' Fe 3+ - s t a r c h (cm-1)
Fe203 - starch
3335 1640 1375 1235 1140 1019 923 857
3343 1641 1374 1237 1152 1017 930 860
(cm -1 )
Adsorptio D mechanisms: A composite diagram showing the adsorption of starch AP on haematite, settling rate of iron ore fines and the relative viscosity of starch as a function of pH is i l l u s t r a t e d in F i g u r e 12. T h e r e exists a p a r a l l e l i s m b e t w e e n the adsorption behaviour of starch onto haematlte, the settling rate of the iron ore s u s p e n s i o n and the r e l a t i v e viscosity of the starch flocculant. It is interesting to n o t e f r o m t h e a b o v e f i g u r e t h a t t h e p l a t e a u regions corresponding to maximum adsorption density of starch, maximum viscosity for the starch solution, as well as the maximum settling rate for the iron ore suspension, fall w i t h i n limits in the same pH region. It thus b e c o m e s important to view the flocculation mechanism of iron ore suspensions in its entirety, taking both the adsorption behaviour of starch onto the substrate, as well as the theological property of the starch solution. The s i m i l a r pH dependence of the adsorption of starch onto ha~m~tite and the settling rate of iron ore suspensions is indicative of the similarities governing a d s o r p t i o n and f l o c c u l a t i o n mechanisms. The relationships existing between theological and flocculating properties of p o l y m e r f l o c c u l a n t s h a v e b e e n found to be important by other workers also [37,38].
o-Starch AP adsorption on haernattt¢ e - Relative Viscosity ot starch AP x-Settling rate using starch AP
3e
3? Ul
i=
U
2O
32 PH
Fig.12 Composite diagram showing the adsorption of starch AP onto haematlte, settling rate of iron fines using starch AP and the relative viscosity of starch AP as a function of pH
Adsorption behaviour of starch onto haematite
253
On the basis of the present work, the following mechanisms for the adsorption of the oxidlsed starch onto haematite: (i) The dissolution represented as:
of
Fe203
2 Fe 3÷
+
3 H20
~tarch¥
haematite ÷
in
the
presence
+
OH-
÷
FeOH;
Fe(OH)~;
of
starch
could
6 OH-
be (1)
The ferric ions thus released could form neutral complexes depending on the pH of the medium viz., F e 3+
could be proposed
Fe(OH)3;
or charged
ferric
hydroxo
Fe(OH)4
(2}
The above hydroxo complexes could react with starch, both at the haematltesolution interface as well as in the bulk solutlon by hydrogen b o n d i n g and chemical interaction. (ii) The starch could d i r e c t l y surface by hydrogen bonding.
interact
with
the
hydroxylated
iron oxide
(iii) The a d s o r p t i o n process could also be brought about by the chemical interaction of starch with the ferric ions released, as per Eq.(1). CONCLUSIONS From the results drawn:
of this
investigation
the
following
conclusions
could
be
1.
Adsorption measurements indicate that the adsorption density varies with pH as well as temperature. The partlal irreversibillty of adsorption is suggestive of t h e i n v o l v e m e n t of n o n - e l e c t r o s t a t l c forces in the adsorption process.
2.
The enhancement of the dissolution of ferric ions from haematite by starch enables the direct interaction of ferric ions with starch and may serve as the precursor step in the overall adsorption mechanism proposed.
3.
FTIR spectroscopic studies reveal the presence of surface hydroxyl groups on h a e m a t i t e . The d l f f e r e n t i a l s p e c t r u m of the a d s o r b e d starch on h a e m a t l t e indicates that both hydrogen bonding and chemical interaction forces are involved in the adsorption process. The striking similaritles between the spectra of the adsorbed starch on haematlte and that of the iron-starch system lend support to the v i e w that c h e m i c a l i n t e r a c t i v e forces of adsorption are operative.
4.
The X - r a y p h o t o e l e c t r o n s p e c t r o s c o p i c results corroborate chemical interaction mechanism between haematite and starch.
5.
Viscosity measurements indicate that the structural conformation is altered with change in pH and ferric-ion addition.
6.
The discrepancies observed in the additive values of the conductances suggest ion-lon interaction or complex formation between ferric ion starch.
7.
the
of starch
and
Starch-ferric ion binding studies confirm that a significant amount of the f e r r i c ions could be b o u n d to starch; the S E M - E D A X s t u d i e s on the i n t e r a c t i o n p r o d u c t between ferric ion and starch complement the above findings. REFERENCES
1. 2. 3. 4. 5.
above
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