Hydrophobic attraction may contribute to aqueousflocculation of clays

Marek Zbik *, Roger G. Horn

Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Warendi Road, Mawson Lakes, SA 5095, Australia

Abstract

SEM observations of low solid content vitrified clay suspensions reveal that clay platelets build porous three-

dimensional networks with platelets contacting each other mostly by their edges. To explain this behaviour, which must

require long range edge-to-edge (EE) attractive forces, a hydrophobic-like interaction has been proposed. This

interaction may be induced by the presence of nano-bubbles existing on the edges of clay crystals which may cause clay

particles to flocculate. The following indirect evidence for such hydrophobic behaviour is presented. First, a clay

platelet is shown attached to an oil drop by its edge; second, clay flocs were attracted by a vertically placed Teflon strip

but not to the hydrophilic mica basal surface; third, a much thicker porous sediment occurred in CO2-saturated water

solution compared with vacuum degassed water.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Kaolinite; Birdwood kaolin; Hydrophobic force; Colloid gelation

1. Introduction

Dewatering of sludge by flocculation is becom-

ing increasingly urgent in view of the growing

demand for sites for the disposal of mining

slurries, tailings and other mineral ‘‘wastes’’.

Mine tailings are often disposed of as high water

content slurries into tailing dams. Because clay

particles are extremely small, natural separation by

sedimentation governed by Stoke’s law is very

slow. To achieve fast separation of clays from

water, separate particles have to be bonded into

large and high-density aggregates, which may

sediment more rapidly. To build these aggregates

the desired structures have to be developed in clay

suspensions, which is usually achieved by floccula-

tion using long chain polymers.

Contemporary approaches to control clay sus-

pension are based on the DLVO theory of colloid

stability [1,2]. Electrostatic and van der Waals

forces competing together determine whether a

* Corresponding author. Tel.: �/61-8-8302-3688; fax: �/61-8-

8302-3683.

E-mail address: marek.zbik@unisa.edu.au (M. Zbik).

Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 323�/328

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0927-7757/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0927-7757(03)00250-4

particular colloid clay suspension will be stabilised

(in sol form) or coagulated (in gel form). On this

basis opportunities for control of floc character-

istics have been described [3] as physical displace-

ment between particles, chemical changes in

background electrolyte, and introduction of new

mineral phase into the system. All these manipula-

tions may collapse the electrical double layer and

allow particles to be brought close enough for van

der Waals forces to bond particles into larger

aggregates which significantly speed up the sedi-

mentation rate.

However, as previous authors have noted, in

real clay suspensions particles do not always form

sols, but are found contacting each other to form

gels [4]. Even the earliest SEM observations of the

clay particle disposition in dilute suspension [5]

reveal that platelets are in permanent contact with

surrounding particles forming a three-dimensional

continuous network, described [6,7] as a house-of-

cards structure. Such a structure, which has high

porosity, and therefore, causes a slow settling rate,

is associated with the presence of attractive forces

holding the platelets together.

In certain kaolinites these gels form at very low

solids loadings. Examples are kaolinite from Bird-

wood in South Australia, Canadian kaolin con-

taining natural oil [8] and Australian kaolin

associated with coal deposits (unpublished data).

It is interesting to note for the following discussion

that the latter two examples are likely to be

contaminated by hydrophobic material.

The house-of-cards structure consists of edge-

to-face (EF) and edge-to-edge (EE) contacts. EF

contacts have long been presumed to result from

attractive double-layer forces between positively-

charged platelet edges and negatively-charge faces

[7]. However, EE contacts, which recent observa-

tions [9,10] suggest may be predominant in low-

density flocs, cannot be explained in this way. In

this paper we propose an alternative idea, that the

EE attraction results from coalescence of nano-

bubbles attached to platelet edges. Nanobubble

coalescence has recently been presented as a

9partial explanation [11�/15] for strong, long-range

attractive forces that have measured between

hydrophobic surfaces. [16,17].

2. Materials and methods

The material used for this study was commer-

cially available K15GM kaolin from Birdwood in

South Australia (Commercial Minerals Ltd, South

Australia). Dry kaolin was prepared as a high solid

content slurry in deionised water by shaking in a

SPEX mill for 5 min. This slurry was then diluted

to a final solid concentration of 4 wt.% and stirred

for 2 h using a magnetic stirrer. In sedimentation

experiments one water sample was degassed in

vacuum for 30 min and another was saturated by

CO2 from a gas cylinder then left for about 30 min

on the bench top to release bubbles. Suspensions

were prepared from CO2-saturated and vacuum

degassed water by mixing kaolin in a SPEX mill

for 5 min and dilution to 4 wt.% suspension in 100

ml glassy cylinders. The pH of the CO2-saturated

water was 3.5, and the vacuum degassed water was

reduced to the same pH using HCl. Prior to

settling measurements cylinders were inverted ten

times.For the microstructural study of the suspen-

sions, a freeze-drying method was used to avoidstructural rearrangement caused by surface ten-sion during oven or room temperature drying. Themethod is described in [18]. Fow freeze-fracturemethod, samples from the syringe, were placed incopper rivets and plunged into a dewar of liquidpropane that was cooled by submersion in liquidnitrogen. When vitrified (i.e. frozen rapidly with-out formation of crystalline ice), samples weretransferred to liquid nitrogen for short-term sto-rage.

The vitrified sample was kept in a frozen stateand placed onto the liquid nitrogen cooled speci-men stage of the Field Emission Scanning ElectronMicroscope (FESEM) with cathode voltage 5 kV.The samples were fractured under vacuum bybreaking the glued rivets after which a smallamount of vitrified H2O was sublimed by raisingthe temperature of the FESEM specimen stage to�/90 8C. After a short period at this temperaturethe stage temperature was lowered to approxi-mately �/180 8C to stop any further sublimation.After sublimation the samples which now havecrystal grains protruding from the vitrified ice,were coated with gold and palladium to a thick-ness of 3 nm using a high resolution sputter coater.

M. Zbik, R.G. Horn / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 323�/328324

3. Results and discussion

Fig. 1(A) shows freeze-fracture micrographs of

the freshly prepared Birdwood kaolinite suspen-

sion. Also included (Fig. 1(B)) is a freeze-drying

micrograph of the Australian coal-associated kao-

lin [9]. Clearly, many EE contacts are present,

which is difficult to explain on the basis of DLVO

theory because positively charged edges should

repel each other, whilst being attracted to nega-

tively-charged faces. Furthermore, the van der

Waals force, which is the normal cause of coagula-

Fig. 1. Kaolinite platelets in dilute aqueous suspension shows mostly edge to edge orientation. (A) CM kaolin separate platelets in

freeze-fracture micrograph. (B) EE oriented kaolinite platelets in freeze-drying coal-associated kaolin.

M. Zbik, R.G. Horn / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 323�/328 325

tion, does not favour EE contacts because these

would have weaker attraction than EF or face-to-

face contacts. As an explanation of the predomi-

nance of EE contacts we propose the existence of

an effect similar to that which has been invoked to

explain the so-called hydrophobic force.

Direct measurements of surface forces have

shown that an unexpected attraction causes hydro-

phobic surfaces in aqueous solutions to jump into

contact from a comparatively large distance

[16,17]. This attraction has been attributed to the

presence of nano-bubbles with radii estimated to

be of order 10�/100 nm, attached to hydrophobic

surfaces and able to bridge between two surfaces

and pull them together [11�/15]. Recent AFM

images [19,20] have confirmed the presence of

nano-bubbles on hydrophobic surfaces.

In another study, forces between a hydrophobic

oil droplet and a hydrophilic flat mica surface were

investigated in 0.1 wt.% NaCl electrolyte using an

AFM [21] and no attachment to the hydrophilic

mica basal surface was observed. However, attach-

ment does occur between the oil and the edge of a

mica flake, as shown in Fig. 2. Clearly it is the edge

of the mica and not the face that is attached to the

drop. This suggests a hydrophobic-type interac-

tion occurring with the edge, and since mica is a

layered aluminosilicate related to many clays, we

suggest that a hydrophobic-type interaction may

likewise occur between edges of kaolin platelets.

The flat basal surfaces of clay minerals are

believed to be hydrophilic and wetted well by

water due to strong negative charge (illite, smec-

tite, mica) or hydrogen bonding to hydroxyl

groups (kaolinite). On the edges, broken bonds

are common and may attract and adsorb gases

dissolved in water or dissociated water molecules.

These edges may accommodate nano-size gas

bubbles and became non-wetting (hydrophobic).

Due to long range hydrophobic attraction clay

mineral edges may attract other particle edges and

form three-dimensional structures based on EE

contacts maintained by bridging forces. It is also

possible that hydrocarbon impurities that com-

monly contaminate kaolinite crystals may accom-

modate micro and nano-bubbles, which may cause

this type of attraction.

To test this idea, hydrophobic (teflon) and

hydrophilic (mica) surfaces were introduced to a

dilute aqueous kaolinite suspension. As seen in

Fig. 3 a hydrophobic surface attracts the kaolinite

particles and causes massive adhesion of the

platelets to a vertically placed teflon strip. Also

the air bubbles attached to teflon have been

observed to attract clay particles in the same way

as teflon itself. There was no adhesion of the

kaolinite platelets onto the hydrophilic mica sur-

face (Fig. 3).

Fig. 2. A 2 mm in diameter oil drop in 17 mM NaCl solution

attaches a mica flake by its edge.

Fig. 3. Teflon attracts the kaolinite particles towards its surface

while mica remains relatively clear.

M. Zbik, R.G. Horn / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 323�/328326

A further piece of evidence indicating a role of

nanobubbles in formation of kaolinite flocs is

presented in Fig. 4, which shows the results of

sedimentation tests comparing the flocculation of

kaolinite suspended in vacuum-degassed water

and CO2-saturated water. Evidently the latter

forms a much lower-density sediment, which is

consistent with the formation of flocs having

edge�/edge contacts stabilised by gas bubbles*/in

this case CO2. With a reduction in quantity and/or

size of nanobubbles on clay edges in the degassed

water, the floc structure is not so open and the

sediment is denser.

The sediment heights as a function of settling

time for the two suspensions is shown in Fig. 5.

Usually a more open structure with particles

arranged in EE contacts gives larger porosity

values than the more compact EF and face-to-

face arrangement. This experiment shows that

presence of gases in aqueous solution can be

favourable to a more porous type of coagulation

with majority of EE particle arrangements. Such

results support our idea that suspension stability

depends not only on the liquid�/solid interface

properties but also the gaseous phase plays an

important role in the first step of coagulation

bringing particles together due to long range

hydrophobic attraction. Electrostatic interaction

may be prominent in the second stage of coagula-

tion when particles are close enough to each other

and could play a major role in structure rearrange-

ment in sediment.

Fig. 4. Photograph shows sediment thickness of the 4 wt.%

kaolin suspension after 15 min of sedimentation, (A) CO2 over-

saturated deionised water, (B) deionised water degassed in

vacuum during 30 min.

Fig. 5. Kaolin sediment thickness vs. time of sedimentation in gassed and de-gassed deionised water.

M. Zbik, R.G. Horn / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 323�/328 327

4. Conclusion

We have proposed that the attractive forces

required to account for EE contacts in the early

stages of flocculation of kaolinite suspensions are

caused by nanobubbles attached to the edges of

clay platelets. The nanobubbles are able to bridge

between nearby edges, resulting in a strong attrac-

tive force similar to that measured betweenextended hydrophobic surfaces in aqueous solu-

tions. Indirect evidence for this idea is provided

from observations of (1) attraction between an oil

drop and the edge (but not the face) of a mica

sheet; (2) attachment of kaolinite particles to

hydrophobic teflon but not to hydrophilic mica

faces, and (3) a more open floc structure obtained

when the suspending aqueous electrolyte is satu-rated with gas compared with a degassed solution.

The flocs that result from EE attractions are of

very low density, which impedes kaolinite sedi-

mentation. However, there is evidence [10] that EF

and FF contacts gradually develop and the open

structure evolves to a more compact one.

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