Yield stress and zeta potential of nanoparticulate silica dispersions under the influence of adsorbed hydrolysis products of metal ions—Cu(II), Al(III) and Th(IV)

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Abstract

The effects of hydrolysable Cu2+, Al3+ and Th4+ ions on the zeta potential and yield stress behaviour of silica dispersions were evaluated as a function of pH and metal ions concentration. Silica dispersion remained dispersed at its point of zero charge (pzc) of pH ∼2.0 (CR1). Adsorbed hydrolysis products of Cu2+ and Al3+ caused the dispersion to display two further points of charge reversal (CR2 and CR3) at moderate ions concentration. CR2 occurred near the pH for the formation of the first hydrolysis product. This pH is about 2.8 for Al3+ and 5.0 for Cu2+. For all three metal ions, CR3 approached the pzc of the metal hydroxides at complete surface coverage. At CR3, the dispersions displayed a maximum yield stress. As many as three type of attractive forces; bridging, charged patch and van der Waals, may account for the maximum yield stress at low surface coverage. At complete coverage, only the van der Waals force is in play—the adsorbed hydrolysis products must have increased significantly the effective Hamaker constant of silica. With Al3+ the yield stress was absent at CR2 because particle bridging and charged patch attraction are unimportant as the silica surface charge is near zero. Adsorption of strongly hydrolysed Th4+ ions at pH < 2.0 caused the dispersion to display only one pzc (CR3).

Introduction

Metal ions hydrolysis products have a range of industrial applications and can be used to alleviate environmental concerns of heavy metal pollution. Hydrolysis products of Al(III) and Fe(III) ions are often used as coagulant and flocculant in wastewater treatment. Coating of TiO2 paint pigment with silica and alumina is normally produced by calcination of the TiO2 particles containing the adsorbed metal ions hydrolysis products. The strong affinity of the metal ions hydrolysis products to adsorb is a common mean of removing heavy metal ions from wastewater.

James and Healy [1], [2] measured the adsorption behaviour of a range of cations, Ca2+, Co2+, Fe3+, La3+ and Th4+, on silica in dispersion. The effects of these adsorbed ions on the electrokinetic properties of silica dispersion were also determined. For each metal ions there is a critical pH where adsorption increased from 0 to 100% occurring over very narrow pH range of ∼1 pH unit. This critical pH is usually below the pH of the formation of the first hydrolysis product [1]. Adsorption of Co(II) gave rise to three points of charge reversal (CR1, CR2 and CR3) [2]. CR1 is the isoelectric point of the silica substrate. CR3, the positive to negative charge reversal, is the point-of-zero charge of the metal hydroxide at complete surface coverage. CR2, the negative to positive charge reversal occurred at pH just before the pH of bulk hydroxide precipitation. James and Healy [2] proposed a model of surface-induced metal hydroxide precipitation to account for the metal ions adsorption at pH below bulk precipitation of hydrolysis product.

Adsorption of the hydrolysable metal ions was also found to affect coagulation rate [3], [4], [5], [6] and flotation recovery [7]. Like adsorption, there is a narrow pH range where flotation recovery passes from zero to 100% [8], [9]. Clearly the nature and strength of the interparticle forces between particles containing adsorbed hydrolysis products will have an important effect on coagulation and flotation behaviour. However no work on interparticle force arising from hydrolysable cations characterisation has been done so far. This paper aims to address this issue.

The effects of anions and cations on the yield stress or interparticle forces of concentrated oxide dispersions were only recently investigated. Franks et al. [10] studied the effects of IO3, BrO3, Cl, NO3, and ClO4 on yield stress of α-Al2O3 suspensions at high ions concentration. Structure maker ions (IO3 and BrO3) produce a stronger particle network compared with structure breaker ions (Cl, NO3, and ClO4). The van der Waals force alone is unable to account for the high yield stress displayed in the presence of IO3. The presence of an additional attractive force was suggested.

Franks [11] also reported the yield stress and zeta potential behaviour of silica dispersions under the influence of concentrated Li+, Na+, K+, and Cs+ ions with Cl as the co-ion. Adsorption of the poorly hydrated ions, K+, and Cs+, is larger and bring about charge reversal at high electrolyte concentrations. The yield stress at high pH was found to increase in the order Li+ < Na+ < K+ < Cs+. The least hydrated ions produced the largest yield stress. The yield stress at high pH and salt concentrations cannot be accounted by van der Waals force alone. An ion–ion correlation force was attributed as the additional attractive force.

Mpofu et al. [12] investigated the effect of hydrolysable Mn(II) and Ca(II) ions adsorption on the surface chemistry, particle interactions, flocculation, and dewatering behaviour of kaolinite dispersions at pH 7.5 and 10.5. Metal ion adsorption was strongly cation type- and pH-dependent and significantly influenced yield stress, settling rate, and consolidation of kaolinite slurries. The presence of Mn(II) and Ca(II) ions alone led to a systematic reduction in zeta potential as a result of specific adsorption of positively charged metal hydrolysis products at the kaolinite–water interface. This led to a lower dispersion yield stress and improved clarification but was found to have no effect on sediment consolidation.

In this study, the effects of hydrolysable Cu(II), Al(III) and Th(IV) ions on the yield stress of silica dispersions were evaluated. A range of hydrolysis products are formed by these cations. For each metal ions, the nature of the hydrolysis products is very pH-dependent [13].

Section snippets

Materials and methods

Fumed silica produced by Sigma was used. This amorphous oxide has a primary particle size of 14 nm, a surface area of 200±50m2/g and a density of 2.3 g/cm3. Acoustosizer measurements gave a median particle size of ∼50 nm. AR grade nitrate salts of copper(II), aluminum(III) and thorium(IV) were used.

In the dispersion preparation a solution of accurately known concentration of metal ions was first prepared by dissolving an appropriate amount of the metal nitrate in acidified Milli-Q water of

Copper ions

The yield stress versus pH plot for 6.8 wt% (ϕs=0.032) silica dispersions containing 0.49, 1.01 and 3.3 dwb% (g/100 g SiO2) Cu2+ are shown in Fig. 1. All dispersions exhibit no yield stress at pH less than 5.0. Two of the dispersions containing 0.49 and 1.01 dwb% Cu2+, display a maximum in the yield stress at pH 6.0 with a value of 150 and 200 Pa, respectively. These dispersions are completely dispersed again at pH above 9.0 and 10.0 as indicated by the absence of a yield stress. The dispersion

Discussion

The hydrolysis products of Cu2+, Al3+ and Th4+ formed at various pH levels are shown in Fig. 10, Fig. 11, Fig. 12. These data were obtained for 0.1 molal metal ions solutions prepared with an ionic strength of 1.0 and taken from Baes and Mesmer [13]. The cation concentration and ionic strength are very close to that used in this study.

Fig. 10 shows that Cu2+ formed four main hydrolysis products, Cu2(OH)22+, Cu(OH)2, Cu(OH)42 and Cu(OH)3. The concentrations of these products were highly

Conclusion

For Cu2+ and Al3+ ions, the second point of charge reversal, CR2, corresponds to the appearance of the first hydrolysis (positively charged) product. With Al3+ ions the absence of a yield stress at CR2 of 2.8 is due to particle bridging and charge patch attraction being unimportant because the zeta potential of the silica particle at pH 2.8 is almost zero. Moreover, the amount of hydrolysis product adsorbed is too low to have a significant effect on the particle Hamaker constant in water.

Acknowledgements

This work was supported by the Australian Research Council through Advanced Mineral Products Centre. The author wishes to acknowledge Prof. D.V. Boger for reading the manuscript and Prof. T.W. Healy for his input on this project.

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