The effect of monovalent anion species on the aggregation and charging of allophane clay nanoparticles

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Abstract

Allophane is a kind of clay mineral which has amorphous hollow spherical structure. To clarify the dispersion, aggregation and charging behaviour of allophane in the presence of various monovalent anions, F, Cl, Br, I, BrO3, IO3, and SCN, the stability ratio and electrophoretic mobility of allophane were investigated. The stability ratio was obtained from the temporal change in the average hydrodynamic diameter of allophane measured by dynamic light scattering. The charging behaviour was evaluated from the measurement of electrophoretic mobility (EPM) of allophane. These experiments were performed as a function of electrolyte concentration at pH 5, where the net charge of allophane is positive. The experimental results demonstrated that the stability ratio decreased with increasing the salt concentration and finally it became unity independent of the salt concentration. Consequently, we observed slow aggregation regime, fast aggregation regime, and critical coagulation concentration (CCC). Therefore, the stability ratio follow the classical theory of Derjaguin, Landau, Verwey, Overbeek (DLVO) at least qualitatively. However, the CCC of allophane showed difference for each anion species; the CCC followed the order F < IO3 < Cl < SCN < BrO3 < Br < I. We also confirmed the difference in EPM among anion species; the reduction of EPM magnitude was significant for well-hydrated ions. We presume that the CCC and EPM of allophane depend on the affinity of anions to the allophane surface characterized by the degree of hydration of each ion. That is, well-hydrated anions can adsorb to the hydrophilic surface of allophane and effectively decrease the positive charge of allophane. Besides this, the fluoride ion induced much lower CCC and charge reversal. These results are due to the strong affinity of fluoride ion for the allophane surface. Meanwhile the dependence of CCC on the zeta potential followed the DLVO prediction. Although the zeta potential of allophane is affected by anion species, the CCC behaviors can be understood by the DLVO theory through the evaluation of zeta potential of allophane. This means that the measurement of stability ratio and electrophoretic mobility accompanied with the DLVO theory are valuable tools for the understanding of the aggregation-dispersion of charge-stabilized natural clays.

Introduction

To understand the dispersion and aggregation behaviors of soil colloids such as clay particles and humic substances is important not only in soil science but also in considering environmental problems [[1], [2], [3], [4], [5]]. Typical clay minerals in young volcanic ash soil are allophane and imogolite. It is thus valuable to understand the aggregation and dispersion of allophane and imogolite when considering the environmental problems in young volcanic ash soils.

Allophane and imogolite are alumino-silicates and have unique features. Allophane is an amorphous hollow spherical particle with an outer diameter of 3–5 nm and the thickness of its wall is about 0.6–1.0 nm [[6], [7], [8]]. SiO2/Al2O3 ratio of allophane is reported to vary between 1.0 and 2.0 [7]. The outer and inner surface of the hollow sphere are alumina-like and silica-like, respectively. The aggregation of allophane occurs around its isoelectric point around pH 6–7 [9]. Generally, allophane particles in aquatic suspension exist as irreversible aggregates with diameters of 100–500 nm determined by electron microscopy and particle size measurement in water (Fig. 1). Meanwhile, imogolite has nano-tubular shape and positive zeta potential over a wide range of pH; the isoelectric point of imogolite is over 9 [10]. Classical sedimentation experiments revealed that the imogolite aggregates at pH > 6 [11]. On the one hand, the allophane aggregates only around its isoelectric point around pH 6 [9,11,12]. Although imogolite has larger external surface area than allophane, the amount of reactive AlOH is less than allophane because of fewer defect edge sites on imogolite as compared with allophane [13,14]. These features imply that the charging and aggregation of allophane are more sensitive to the change in solution chemistry. Therefore, this study has focused on the aggregation and charging of allophane.

The dispersion-aggregation of colloidal particles has been discussed on the basis of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The DLVO theory claims that the interaction potential energy between two charged particles are given by the sum of electric double layer potential and the van der Waals potential. According to the DLVO theory, the aggregation of colloidal particles is induced by the increased electrolyte concentration and/or the reduced surface electric potential. As a result, with increasing the electrolyte concentration, the rate of aggregation increases in so-called slow aggregation regime and reaches to a constant maximum in so-called fast aggregation regime. The critical coagulation concentration (CCC) is defined as the boundary between these two regimes. It is widely accepted that the DLVO theory suggests that CCC depends on the ionic valence in electrolyte solutions. The DLVO theory has explained the aggregation and dispersion of many natural and synthetic colloidal particles, except for silica nanoparticles and imogolite [12,[15], [16], [17], [18], [19], [20], [21]].

Systematic studies on the aggregation-dispersion of colloidal particles have demonstrated that the CCC is affected by not only ionic valence but also ionic species. Such effect of ionic species is known as Hofmesister series [[22], [23], [24]] and is not considered by the DLVO theory. It is reported that the CCCs of positively charged particles depend on the hydration degree of anions with the same valence [[22], [23], [24]]. Nevertheless, the experimental relationship between CCC and surface charge of latex particles follows the DLVO prediction, once the electrokinetically determined surface charge densities, which depend on ion species, are obtained [22].

For the aqueous suspension of allophane particles, specific anion effects on colloidal properties have been studied by measuring the electrophoretic mobility (EPM) and the viscosity of allophane particles and suspension [25,26]. So far, however, systematic simultaneous measurements of the influence of anion species on the aggregation and charging behaviors of allophane are lacking. Therefore, the applicability of the DLVO theory to the aggregation-dispersion of allophane has not been fully examined.

To test the applicability of the DLVO theory to the aggregation-dispersion of allophane particles with various types of ions, we have measured the stability ratio of allophane particles with positive charge at pH 5 in the presence of various anions with different degree of hydration using dynamic light scattering, accompanied with the measurement of electrophoretic light scattering for the evaluation of zeta potential. The dependence of CCC on the zeta potential at CCC is analyzed on the basis of the DLVO prediction. Such analysis has never been applied to allophane. Therefore, this study provides a novel insight about whether the DLVO works for the aggregation and dispersion of allophane natural clay, once one can evaluate the zeta potential affected by ionic species. Our work can stimulate future researches on the prediction of zeta potential of natural colloidal particles.

Section snippets

Materials and method

Aggregation and charging behaviors of colloidal/nano particles have been investigated by time-resolved dynamic light scattering and electrophoretic light scattering techniques [9,[18], [19], [20],22,[27], [28], [29], [30], [31], [32], [33]]. Here, we applied these techniques to allophane suspensions with different anion species as a function of the concentration of monovalent electrolyte solution at pH 5, where the net charge of allophane is positive.

The effect of anion species on the stability ratio of allophane suspension

While the hydrodynamic diameter dh of allophane in the suspension was constant at low electrolyte concentration, it increased with time t due to aggregation at higher electrolyte concentrations (Fig. 2). The initial slope of ddh/dt increased with the salt concentration and then reached a constant (ddh/dt)f. Taking the (ddh/dt)f as a reference, we obtained the stability ratio W as W = (ddh/dt)f/(ddh/dt).

The stability ratio of allophane is plotted against the salt concentration in Fig. 3. The

Conclusion

We measured the stability ratio and electrophoretic mobility (EPM) of allophane at pH 5 as a function of the concentration of sodium salts of different monovalent anions F, Cl, Br, I, BrO3, IO3, and SCN. Our results of stability ratio showed a difference in critical coagulation concentration (CCC) for each anion. The dependence of EPM of allophane on the salt concentration was also affected by anionic species. The reduction degree of EPM magnitude of allophane in this study was

Conflict of interest

The authors declare that they have no conflict of interest associated with this article.

Acknowledgement

The authors are thankful to the financial support from JSPS KAKENHI (15H04563, 16H06382, and 19H03070).

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