The role of natural organic matter in suspension stability: 1. Electrokinetic–rheology relationships

doi:10.1016/j.colsurfa.2006.08.028

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 295, Issues 1–3, 1 March 2007, Pages 38-48

https://doi.org/10.1016/j.colsurfa.2006.08.028 Get rights and content

Abstract

The electrokinetics and rheology of a model alumina suspension has been investigated in the presence of a series of natural organic matter (NOM) fractions from soil and aquatic sources. The role of natural organic matter in changing both the dispersed state of the solids and the rheology of concentrated suspensions has been investigated. The data shows that at high surface coverage there is a good correlation between the nature of the adsorption layer and the electrokinetic and rheology responses. The data highlights key differences between the different fractions in aiding dispersion. At intermediate surface coverage, the correlation is less robust particularly for the more hydrophilic NOM fractions. The measured value of the electrokinetic potential is no longer a good indicator of colloidal stability as indicated by a DLVO type analysis. The reason may be due to the adsorbed layer showing a different response to particle movement in electrokinetic measurements than it does in static rheology measurements. In the latter case, the behaviour is dominated by particle interaction.

Introduction

Clean drinking water is essential to community health but water sources such as streams or dam type catchments are often high in turbidity and colour. The turbidity is due to suspended solids such as colloidal clay particles. The colour is caused by dissolved organic species such as natural organic matter (NOM), which often gives the water a yellow-brown tea colour. Invariably the particulates are also coated by less soluble forms of NOM. Residual NOM in drinking water causes aesthetic, odour, taste and health problems, the latter as a result of disinfection by-products if certain fractions are not removed through processing [1]. Therefore, in order to obtain high quality drinking water, it is desirable to remove the suspended solids and colour. The NOM will be made up of a variety of components that depend on source, environment and ageing. Understanding the role of these components and how they affect the treatment process is of ongoing concern to water treatment plant operators.

Generally, coagulants such as ferric or alum and flocculants are added to the water to remove particulate and dissolved contaminants. These additives destabilise the suspension causing the material to flocculate. The resultant flocs are removed from the water by filtration or settling to produce potable water and a separate slurry containing the waste solids. In some instances, the slurry is disposed of directly into the sewer but in most cases, the solids are dewatered by a variety of methods prior to disposal [2], [3].

Therefore, the post coagulation fate of most NOM is as a key component of an alum or ferric rich sludge. It is becoming more common that sludges of this type are dewatered through filtration and centrifugation [4], [5], [6], [7], [8] and although there is substantial interest in the chemical and physical makeup of NOM [9], [10] the role of these components in changing the rheology and processing of these sludges has been given less attention. As a result, although the relationship between the nature of the NOM and its removal from solution through coagulation and ion exchange is reasonably well understood [11], the case for sludge processing is less clear. It is postulated that different fractions of NOM will have dissimilar effects. This would certainly be expected on the basis of the role of different dispersants in changing the surface property rheology relationships for concentrated suspensions [12], [13], [14], [15], [16], [17], [18].

The ubiquitous negative charge found on colloids in natural aqueous environments is testament to the effect of adsorbed NOM on the charge properties of surfaces [19]. These changes to the charging characteristics of the surface affect the double layer interaction energy and hence, particle–particle interactions. The conclusion from previous studies on changes to surface charge caused by NOM adsorption is that NOM adsorbs specifically and adds negative charge to the surfaces, thus lowering the iep [20]. In most cases this gives the colloidal particles significant negative charge, thus stabilising them in aqueous media. One notable exception was a study by Jekel [21] which reported that whilst NOM did adsorb to silica, there was little change in the surface charge. He attributed extra stabilisation to steric effects caused by adsorbed neutral NOM molecules. It is perhaps worth comment here that as silica is highly charged across most of the pH range of interest to dispersion in aquatic environs, it is hardly surprising that NOM did not greatly affect the measured charge density of this surface.

Stability and aggregation studies show that the presence of NOM can increase the stability of oxide dispersions significantly. The addition of NOM vastly changes the electrical double layer interaction energy, however, in many studies an “extra stabilisation” is observed that cannot be explained by application of simple DLVO theory. As is the case in the study by Jekel [21], this extra stabilisation is mainly attributed to non-DLVO steric repulsion or even electro-steric interactions. As some NOM molecules are relatively large in size (molecular weight estimates range from 100 to 100,000) it seems highly likely that they would also add a steric component to the interaction profile. A clear problem in quantifying the effect of adsorbed species at surfaces (not just NOM species) is that for most of the studies presented in the literature, it has been difficult to distinguish between electro-steric and steric stabilisation mechanisms [22].

To probe the role of NOM in the stabilisation of colloids, it is proposed here to use the relationship between particle surface charge and suspension rheology as a guide to the mechanism of action of the additive. Using a model substrate, in this case, a colloidal α-alumina, it is postulated that any additives that change the surface properties of the alumina will affect the state of flocculation of the suspension and therefore change the shear yield stress. This concept was first put forward by Firth et al. [23], [24] They showed the existence of a linear relationship between the Bingham yield stress of a suspension and the square of the zeta potential of the particulate surface. The information has been exploited in our own group using the suspension yield stress measured using a vane technique and electrokinetics of the same suspension at a slightly lower concentration. The yield stress parameter has been related to the summation of the contribution of individual particle interactions. Work has included a range of model systems and the measurement of particle electrokinetics across a broad pH range. The role of additives both at and away from the iso-electric point (iep) of the suspension as compared to the suspension in the absence of additive has also been studied [18], [25], [26].

The aim of this work is to investigate the effect of a range of NOM species and fractions of NOM on the behaviour of particulate suspensions. A second aim is to demonstrate the benefits of using rheological techniques to monitor the inter-particle interactions in concentrated particulate suspensions by correlating the rheological properties with surface charge and molecular orientation.

Section snippets

Materials

High purity water from a Milli-Q ion exchange unit (resistivity 18.2 MΩ cm at 25 °C) was used in all experiments. All salts and commercial chemicals were AR grade, unless otherwise stated.

A well characterised, high purity α-alumina (AKP-30, Sumitomo Chemical Co. Ltd., Japan) was used as the particulate solid. It has a mean particle diameter of 0.3 μm, a BET surface area of 6.5–7.5 m² g⁻¹ and a density of 3.97 g m⁻³ [27]. It was used as received.

Recognising the highly variable nature of NOM in terms of

Methods

The shear yield stress of concentrated alumina suspensions was measured by the vane method [32] using a Haake rheometer comprised of a Haake M10 torque head and Haake RV20 controller. The samples were sensitive to recent shear history so the pH was measured after the torque measurement to avoid any disturbance which may cause erroneous results.

Suspensions were prepared at 25% solids volume fraction by dispersing the appropriate quantity of alumina in 60.00 ml of aqueous 10⁻² M NaCl. The pH of the

Results and discussion

The electrokinetic data for alumina in the presence and absence of NOM is shown in the following figures, with the amount added quoted as a % TOC, as described in the above section. Fig. 2, Fig. 3 show the ζ-potential versus pH behaviour of AKP-30 alumina as a function of purified Aldrich humic acid (AHA) and Armadale fulvic acid (AFA) concentrations, respectively. These two NOM samples are soil or peat based and characteristically more hydrophobic than the aquatic sources of NOM. The closed

Conclusions

The relationship between the electrokinetic and rheological behaviour of alumina suspensions in the presence of both aquatic and soil based NOM samples has been examined. All samples and fractions from these samples were observed to adsorb to the alumina and reduce the iep. The shift in iep at high additive concentrations was such that the particles were well dispersed under neutral pH conditions. The stabilisation mechanism here is primarily electrostatic although the shape of the

Acknowledgements

Funding of this work was through the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council, United Utilities PLC, UK and Yorkshire Water, UK. Discussions with Peter Hillis and Martin Tillotson of these companies are acknowledged.