The effect of ultrasonication on the surface properties, ionic composition and electrophoretic mobility of an aqueous coal suspension

Colloids and Surfaces, 30 (1988) 373-385 Elsevier Science Publishers B .V ., Amsterdam - Printed in The Netherlands

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The Effect of Ultrasonication on the Surface Properties, Ionic Composition and Electrophoretic Mobility of an Aqueous Coal Suspension C .W. ANGLE", J .C . DONINI and H .A . HAMZA Edmonton Coal Research Laboratory, CANMET, EMR, P.O. Bag 1280, Devon, Alberta TOC IEo (Canada) (Received 9 June 1986; accepted in final form 7 October 1987)

ABSTRACT The effect of ultrasonication on a suspension of fine hydrophobic coal particles was investigated . It is shown that ultrasonication promotes wetting and separation of the whole coal into three fractions with different electrophoretic mobilities . These fractions were found to float, to sediment and to form a stable suspension . In this study, a suspension of fine coal in deionized water was sonicated for various time intervals . It is shown in this study that ultrasonication, one of the common methods of dispersion, has considerable effect on the sample properties both because of chemical changes and because of sampling problems . Electrophoretic mobility distributions of the coal in the middle phase were measured and found to vary cyclically with sonication time . Separated coal fractions were characterized by infrared spectroscopy, which indicated variations in clay content and oxidation state . The differences in these fractions were confirmed electrophoretically . Changes in the ionic composition of the water with sonication time were also measured . The principal anions released and readsorbed were sulphate, chloride, nitrate, but nitrite was only released ; cations included silicon, magnesium, sodium, calcium, aluminum and iron . Sonication can be very useful as a separation tool where modification of the ionic state of the coal surface is not a critical factor . Electrophoretic results during or immediately after sonication may represent an unsteady state and a modified charged particle surface . Sonication is therefore not to be recommended as a preparation technique for dispersion in electrokinetic measurements of coal.

INTRODUCTION This paper covers some of the experimental observations resulting from ultrasonication . It reports on the changes that occur in the ionic composition of the suspending medium as well as the electrophoretic mobilities of the suspended particles as a result of ultrasonication . Differences in the surface properties of sonicated coal are analyzed by Fourier Transform infrared "To whom correspondence should be addressed .

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photoacoustic spectroscopy (FTIR-PAS) technique and pH was monitored before and after sonication. A review of the literature shows that ultrasonication has been commonly used as a preparative method to effect wetting of hydrophobic particles and to promote dispersive behavior in colloidal systems for electrokinetic studies . As early as 1938 Sollner reviewed the use of ultrasonic waves in colloid chemistry and proposed the mechanism underlying ultrasonication [ 11 . Heakal and Herbillon (1976) used ultrasonication technique to promote dispersion of gypseous soils as well as aggregation with certain other calcareous soils [2] . Cserfalvi et al . (1976) dispersed sodium bentonite and agar sols by ultrasonic radiation [3] . Drever (1969) obtained disperse systems of clay minerals for continuous particle electrophoresis with ultrasonication [ 4 ] . Kratohvil and Matijevic (1982) used an ultrasonic bath to disperse carbon black powders in water . They studied the electrokinetics and stability of carbon black in the presence of surfactants and electrolytes as a function of pH [5], Celik and Somasundaran (1980) studied the effect of pretreatments, such as ultrasonication, on the flotation and electrokinetic properties of coal as a function of pH [ 6 ] . These are a few illustrations of the many uses of ultrasonication in applications of surface and colloidal systems such as coals and minerals . EXPERIMENTAL

Materials

The coal sample was obtained from a Western Canadian mine . It was ground to -325 mesh and had a proximate analysis of 21 .02% volatiles, 10 .65% ash and 68.3% fixed carbon . The mean equivalent spherical diameter was 10 .11 µm. The coal had a specific gravity of 1 .73 g cm -3 and a specific surface area of 5 .70 m 2 g-1. The buffer solutions in the study were supplied by Fisher Scientific . Water was of 18 MD resistance . A millipore (type H.A.) 0.45 pm membrane filter was used for filtering . Parafilm was used for sealing the containers during stirring . The instrumentation used in the study included a Pen Kern System 3000 electrokinetic analyzer [ 7 ] , a Dionex ion chromatograph, a Beckman inductive coupled plasma cation analyzer, a Radiometer automatic titration system, a Broker 113V Fourier Transform Infrared Analyzer using a PAScell (photoacoutic), and a Hitachi X650 SEM . Other supplies were as follows : a Branson Sonogen Sonication bath, a Fisher magnetic stirrer, a Mettler AE160 electronic balance, Kimble culture tubes (for the storage of the filtrate), and multifit disposable syringes (with needle attachment for sampling) . Methods

The coal powder sample was characterized for surface functionality by FTIRPAS technique . The range of shapes and sizes of the particles under study was

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determined by scanning electron microscopy . Size analysis was also performed on the Coulter Counter using a 100 ,um aperture tube. In all subsequent experiments the same beaker was used in order to minimize the effects of the vessel variables [ 2 ] . The coal slurry was prepared by adding 0.20 g of coal powder into a 600 ml beaker and then adding 500 ml of deionized water . The beaker was sealed with parafilm and the mixture was stirred for 4-5 h to achieve wetting. In sonication experiments the `equilibrated' sample in the beaker was placed into a Branson Sonogen bath . The bath contained 300 ml of deionized water, which was cooled to 23 ° C with a cooling coil . The sample was sonicated at 55 KHz for various times (1, 2, 3 and 4 h) then removed and stirred for various lengths of time . During sonication 10 ml samples were removed at 15 min intervals using a multifit disposable syringe . The sample was immediately filtered through a 0 .45 pm membrane filter attached to an Amicon manifold vacuum filtration apparatus . The filtrate was analyzed for anion and cation content using a Dionex ion chromatograph and a Beckman inductive coupled plasma cation analyser. The pH was measured before and after sonication . Separate runs were carried out in order to determine the relative amounts of coals which floated under sonication for 3 h . The floated coal was skimmed off on predried and preweighed filter paper, dried at 109'C in a drying oven and reweighed . The float, the suspended coal and the sediment fractions were removed and dried for FTIR analysis . The wetted and sonicated coals in two identical beakers were photographed to illustrate the difference . Electrophoretic mobilities were performed at varying time intervals during and after sonication using the Pen Kern System 3000 Analyzer according to the method described in previous reports [1,8,12] . The slurry in deionized water was allowed to equilibrate for 4 h and then sonicated for 1 h as before . 5 ml samples were removed at varying time intervals (5, 10, 15, 30, 45 and 60 min) and the electrophoretic mobilities were measured . The sonication bath was thermostatted at 23'C . A spectrum of mobility distributions was recorded together with the average and standard deviation of the spectrum for each sample population . Average electrophoretic mobilities of the separated fractions after equilibration in the continuous medium were measured . RESULTS AND DISCUSSION

The original coal sample as characterized for size distribution and heterogeneity in shapes are shown in Figs 1 and 2. Figure 1 shows the Coulter Counter size distribution of the coal sample to have a broad distribution with a mean equivalent spherical diameter of 10 .11 µm . The scanning electron micrograph shows not only the actual range of sizes of particles under study but also the variety of shapes and cleavages which are prone to `cavitation', when sonicated [ 1 ] . Figure 3 shows a photograph of the coal after complete wetting and `equi-



376 100 r 80 z

LEGEND • Differential o Cummulative

W U

60 a = 40 Cs W

W

3 20

4 6 10 20 DIAMETER (Pm)

40

Fig. 1 . Size distribution of unoxidized coal sample ( -325 mesh) .

Fig. 2 . Scanning electron micrograph of coal sample showing heterogeneity .

libration' prior to and after sonication for 1 h . The floated fraction was about 9 .3% by weight of the total amount wetted .

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Fig. 3 . Photographs of coal slurry at equilibration before sonication (right) and after 1 h sonication (left) . Infrared spectroscopy Figure 4a shows the FTIR spectrum of the whole original coal sample . From this figure it can be clearly seen that clay bands appear around 3600 and 1092 cm-' . The shape of the peaks of the aromatic C-C stretching mode around 1600 cm-' indicates that the coal is highly unoxidized . Figure 4b shows the spectrum of the floated 9 .3% portion and, although bands due to some clays are apparent, the intensity of the clay bands are far less than the suspended fraction shown in Fig. 4c . Figure 4c also shows far more oxidation in the C=O region (1600 cm-') . The sediment is less oxidized (Fig . 4d) than the suspended particles (Fig . 4c), although it contains more intense clay bands than the floated material . Quantitative determination of minerals, as a proven technique using FTIR is well described by Shepherd et al . [ 9 ] . The characteristic bands of oxidized coals has been similarly described by Starsinic et al . [10] and more recently the functional groups of coal were discussed by Cooke et al . [ 11 ] . The above results are therefore consistent with the spectroscopic characterization of coals and minerals found in the literature . The results indicated that with sonication the more oxidized coal stays in suspension while the less oxidized cleaner coal is induced to float to the top . Thus sampling of whole coal from the mid portion of a beaker for electropho-



378 0 .2558

Whole Coal

0 .1547

0 .0537 I 4000 3000 2000 WAVENUMBERS CM-1

I

1000

Fig. 4a . Infrared spectrum of whole relatively unoxidized coal . 0 .2185 -

Floated Fraction

'Y

I

0 .1304

0 .0423 4000 3000

vty

I '

2000

1000

WAVENUMBERS CM-1

Fig . 4b . Infrared spectrum of floated portion of coal after sonication .

retic measurements is likely to be biased towards the more oxidized coal with medium clay content. These are the particles which are more stable in a slurry . With sonication, a relatively less oxidized clay-free coal which is more hydrophobic will float more readily than the more hydrophilic oxidized coal . Ionic composition

Figures 5-8 show the fluctuations in the anionic composition of the suspending medium before and during sonication for 1 h . The principal ions released by the coal surface are sulphate ions (Fig . 5), chloride ions (Fig . 6), nitrate ions (Fig. 7) and nitrite ions (Fig. 8) . It is obvious that for repeated



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0.2547

Mid Fraction

0 .1480

0 .0414 I 3000 2000 4000 WAVENUMBERS CM-1

1000

Fig. 4c . Infrared spectrum of suspended portion of coal after sonication . 0 .2759 -

Sedimented Fraction

0 .1664

'Y

~- ,r ,

0 .0569 4000

3000

2000 1000 WAVENUMBERS CM-1

Fig . 4d . Infrared spectrum of sedimented fraction of coal after sonication .

runs the composition of the suspending medium was not identical either prior to or during sonication . The irreproducibility prior to sonication, is a function of sampling differences and an indication of the non-uniform nature of transport of ions from surface to solution and vice versa . This observation is in accordance with our previous observations that the coal surface on contact with an ionic medium reaches two types of equilibrium . The first, which is a short-term equilibrium, is achieved in a few hours . The second, which is a longterm equilibrium, takes weeks or even months to achieve [12] . The first or short-term equilibrium can be observed for this coal with the ionic differences at zero time . Sonication which was thought to be a means of speeding the attainment of



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1 .2

2 .0

Chloride Ion

Sulphate Ion E a 1 .0 a

_a 1 .6

z 00 .8

z 0

aIc

R

z

E

1 .2 0 .6

zz

w

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0 0 .2 _

z0

0 .4

Z

Z Q

Q

0

I . I, I I 20 40 60 TIME (min)

0

0 1 , 0 20 40 60 TIME (min)

Fig . 5 . (left) Changes in coal slurry sulphate ion concentration with time for repeated sonication runs of 1 h, (0, A, 0, • . 0) designate different runs under the same conditions . Fig . 6 . (right) Changes in coal slurry chloride ion concentration with time for repeated sonication runs of 1 h .

Nitrate Ion

Nitrite Ion

E 2 .0 a a z 1 .6 0

E 1 .6

H

I . a S r

a a

0

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1

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.

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I

20 40 TIME (min)

I -}

60

Z 0 U z 0 .4 0 _ z 4 0

:rI 0

I I,

20 40 TIME (min)

60

Fig . 7. (left) Changes in coal slurry nitrate ion concentration with time for repeated sonication runs of 1 h. Fig. 8 . (right) Changes in coal slurry nitrite ion concentration with time for repeated sonication runs of 1 h.

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2 TIME (hrs ) Fig. 9 . Fluctuations of cation concentration of coal slurry during sonication for 2 h.

equilibrium, and promoting dispersion did not only initiate further disequilibrium, but also produced a final `equilibrium' state different from that without sonication. The change in ionic compositions of the medium before and after sonication confirms this observation and also indicates that the surface ion composition is modified by sonication . Sulphate, nitrate as well as chloride ions have been shown to decrease with sonication (Figs 5-7), suggesting that a change from negative anion adsorption to positive anion adsorption is taking place at the surface . Nitrite ions, on the other hand, are released from the coal surface and increase in solution with sonication (Fig . 8) . The cations Si", Mg", Na', Ca" and K+ are released from the coal surface (Fig . 9) . The principal ions were silicon and calcium . The fluctuation of silicon, magnesium, calcium, sodium, and potassium, during and after sonication, suggested that the surface lattice ionic composition was being modified. In essence the coal matrix was displaying rapid cation exchange and readsorption of solubilized anions with sonic energy . Electrophoretic mobility

After equilibration for 8 h, the average electrophoretic mobilities of the separated fractions of coal, which were described in the FTIR analysis, were found to be as follows :floatedc-6 .5E-09 m 2 V- ' a - ', the suspended fraction -1 .35E-08 m2 V -' s- ' and the sediment fraction -1 .2E-08 m2 V-' S -1 . These differences further illustrated that the negatively charged particles were more predisposed to the stable suspension . The floats were less negatively charged than either the suspension or the sediment . These results agree with the surface oxidation and degree of clay bands shown in the FTIR spectra . From Fig . 10 it can be seen that for repeated runs, the electrophoretic mobilities of the suspended coal particles as a function of sonication time appear cyclic . This is similar to the cyclic fluctuations of the cation concentration, which in this case is the counterion . However, the changing counterion and

3(S2



-0 .1

W

l l 1 , I , I -2 .1 40 80 120 160 0 TIME (min)

Fig. 10 . Electrophoretic mobility versus sonication time for mid fraction coal slurry for repeated runs. (A) Run 1 ; •) run 2, both 1 h sonication, 0 .5 h relaxation.

coion concentration are not the only factors contributing to the fluctuating mobilities . The coal populations of fine particles are also shifting . The composition of coal particulates of different oxidation states as well as the contribution of clay particles are changing and are confirmed by the infrared results. Also there is the possibility of lattice cleavages with sonic energy, thereby exposing new surfaces [ 2 ], and hence differently charged particulates should not be ignored. Figure 11 shows a detailed shift in the composition of charged particle populations in the electrophoretic spectra as a function of sonication time . After sonication it can be seen that the electrophoretic mobilities of the particles measured in the repeated runs in Figs 10-12 are closer in values . This observation is consistent with the uniformity of particulate population of the middle phase which was achieved after sonication . This was so also because time was allowed for an equilibrium between the surfaces and the continuous phase to be established . Reproducible electrophoretic results depend on both uniformity of particulate population and the existence of an `equilibrium' between the surface ions and the supporting electrolyte. This equilibrium state is emphasized by many researchers studying mineral oxide water interfaces . Iwasaki et al . in their study of goethite have stated that ions which establish equilibrium at the interface and which also determine the potential drop between the solid and the liquid phases are also responsible for the electrical charge on the solid surface [ 13 ] . Most researchers in electrokinetics would agree that the theories developed for electrophoretic measurements do not include a continuously changing charged surface as displayed by the results of Figs 10-12 . Derjaguin and Dukhin treated the equilibrium and the non-equilibrium double layers and electrokinetic phe-



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E. M.

E. M . TVne •O min .

MEAN-- .122E-007 MEDIAN-- .120E-007 ST DEV. • 1 .903E-008

Time • 30min . MEAN •- .107E-007 MEDIAN-- .100E-007 ST. DEV.- 1 .864E- 008

sonication

l O

0

E. M.

E. M. Tim MEAN-- .960E-008 MEDIAN950E-008 STDEV-. 1.823E-008 .

Time •4 5min .

• 15 min . MEAN-- .157E-007 MEDIAN • • . 10 8E •0 07 ST. DEV. • 1 .905E-008

""canon

sonicatbn

1

+ 0

F 0

Fig . 11 . Electrophoretic mobility distribution of mid fraction coal slurry during sonication at variable time intervals. E . M. Time MEAN •- .785E-008 MEDIAN •-.900E-OOS ST DEV. • 1.652E-008

-

• 1 hr. sonication

J 0

E . M, Time • 1 .5 hr. (after sonication)

MEAN-- .125E-007 MEDIAN •- .120E-007 ST DEV. • 1 .654E-006

t 0

Fig. 12 . Electrophoretic mobility distributions of mid fraction coal slurry at 1 h (top) of sonication and 0 .5 h (bottom) after sonication .

nomena thoroughly in two treatises [ 14,15] . This dynamic response although accentuated in coal with sonication may also be the reason for the differences in particulate populations . Since pH remained constant throughout these experiments, the existence of ions other than H+ and OH - that contribute to the zeta potential may be inferred. The release of various surface ions with pH changes is exhibited if H + /OH - ions occupy exchangeable sites . However, since pH remained constant in these experiments it became obvious to conclude that the ions were not competitive with potential determining ions such

384

as H+/OH - . Anion adsorption was contributing to the more electronegative population of particles . CONCLUSION

(1) Sonication is not a suitable technique for preparing a disperse sample of coal for electrokinetic measurements . (2) The characteristics of the microelectrophoretic technique, or any other electrokinetic technique which deals with suspended particles, are such that only the suspended material are easily sampled . Since the suspended material is not representative of the whole, the data would be erroneous if we refer to the sample as the whole . (3) Ion exchange, driven by ultrasound energy, leads to positive adsorption of anions and cyclic release of major cations such as Si" and Ca" . The coal suspension is left far from the equilibrium that it would acquire without ultrasound . Such ion exchange would definitely affect the surface charge properties . (4) The increasing presence of nitrite ions points to a reduction process occurring with sonic energy . This would alter the surface properties of the coal . Nitrite ions have not been previously reported as present in the leachates of coal in aqueous media . It appears that the energy of the ultrasound activates some chemical reaction not otherwise observed. (5) Above all else selective flotation takes place with sonic energy . The FTIR spectra proved that the floated coal was less oxidized and contained less clay than the suspended and sedimented coal . ACKNOWLEDGEMENT

The authors wish to thank Shelby Kyca (University of Victoria, Cooperative student) for technical assistance . ©Minister of Supply and Services Canada, 1988

REFERENCES 1 2 3

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K . Sollner, Ultrasonic waves in colloid chemistry, J . Phys . Chem ., 42 (1938) 1071-1078 . M .S. Heakal and A .J. Herbillon, Use of flocculated fraction of calcareous soil suspensions after ultrasonic treatment for mineralogical study, Clay Miner ., 11 (1976) 101 . T . Cserfalvi, T. Meisel and E . Pungor, Determination of the electrokinetic potential of dispersed and macromolecular colloids by means of streaming potential measurements, J . Electroanal . Chem ., 74 (1976) 377-388 . Janier I . Drevor, The separation of clay minerals by continuous particle electrophoreses, Am. Mineral., 54 (1969) 937-942 . S . Kratohvil and E . Matijevic, Stability of carbon suspensions, Colloids Surfaces, 5 (1982) 179-186.

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M.S . Celik and P . Somasundaran, Effect of pretreatments ons flotation and electrokinetic properties of coal, Colloids Surfaces, 1 (1980) 121-124 . C .W . Angle and H .A . Hamza, The evaluation of an automated electrokinetic analyser, ERP/ERL 81-18 (TR) . C .W . Angle and H.A . Harms, Electrokinetic behavior of a relatively unoxidized coal, presented at 57th Colloid and Interface Science Symposium, Toronto, Canada, 14-18 June 1983 . R .A. Shepherd, W .S . Kiefer and W .R .M . Graham, Characterization of circle cliff tar sands1 . Application of FTIR technique to mineral matter, Fuel, 65 (1986) 1261 . M . Starsinic, Y . Otake, P .L. Walker and P .C. Painter, Application of FTIR spectroscopy to the determination of COOH groups in coal, Fuel, 63 (1984) 1082 . N .E . Cooke, 0. Maynard Foller and Rajendra P . Gaskwad, FTIR spectroscopic analysis of coals and coal extracts, Fuel, 65 (1984) 1254 . C .W. Angle and H .A. Hamza, Exploratory tests in sample preparation technique for the electrophoresis of an unoxidized coal, ERP/CRL 84-49 (TR) . I . Iwasaki, S .R .B . Cooke and A .F . Colombo, Flotation characteristics of goethite, U .S . Department of Interior, Bureau of Mines Report 5593 . B .V. Derjaguin and S .S . Dukhin, Nonequilibrium double layer and electrokinetic phenomena, in E . Matijevic (Ed .), Surface and Colloid Sciencne, Vol . 7, Wiley, New York, 1974, Ch. 3. B .V . Derjaguin and S .S . Dukhin, Equilibrium double layer and electrokinetic phenomena, in E . Matijevi5 (Ed .), Surface and Colloid Science, Vol. 7, Wiley, New York, 1974, Ch . 2.