Adsorption of dissolved aluminum on sapphire-c and kaolinite: implications for points of zero charge of clay minerals

Abstract

We have studied the impact of dissolved aluminum on interfacial properties of two aluminum bearing minerals, corundum and kaolinite. The effect of intentionally adding dissolved aluminum on electrokinetic potential of basal plane surfaces of sapphire was studied by streaming potential measurements as a function of pH and was complemented by a second harmonic generation (SHG) study at pH 6. The electrokinetic data show a similar trend as the SHG data, suggesting that the SHG electric field correlates to zeta-potential. A comparable study was carried out on kaolinite particles. In this case electrophoretic mobility was measured as a function of pH. In both systems the addition of dissolved aluminum caused significant changes in the charging behavior. The isoelectric point consistently shifted to higher pH values, the extent of the shift depending on the amount of aluminum present or added. The experimental results imply that published isoelectric points of clay minerals may have been affected by this phenomenon. The presence of dissolved aluminum in experimental studies may be caused by particular pre-treatment methods (such as washing in acids and subsequent adsorption of dissolved aluminum) or even simply by starting a series of measurements from extreme pH (causing dissolution), and subsequently varying the pH in the very same batch. This results in interactions of dissolved aluminum with the target surface.A possible interpretation of the experimental results could be that at low aluminum concentrations adatoms of aluminum (we will refer to adsorbed mineral constituents as adatoms) can form at the sapphire basal plane, which can be rather easily removed. Simultaneously, once the surface has been exposed to sufficiently high aluminum concentration, a visible change of the surface is seen by AFM which is attributed to a surface precipitate that cannot be removed under the conditions employed in the current study.In conclusion, whenever pre-treatment or the starting point of an experiment favor the dissolution of aluminum, dissolved Al may remain in the experimental system and interact with the target surfaces. The systems are then no longer pristine and points of zero charge or sorption data are those of aluminum-bearing systems.

Surface-charge-induced orientation of interfacial water suppresses heterogeneous ice nucleation on <i>α</i>-alumina (0001)

“…In terms of thermodynamics, a charged surface orients OH dipoles of the interfacial water molecules parallel to one another and therefore reduces the entropy and raises the free energy of the growing ice embryo (Fletcher, 1959;Marcolli et al, 2016). This explains the reduction of the freezing efficiencies for charged interfaces by a charge-induced surface templating, which impedes ice nucleation, irrespective of the sign of the surface charge.…”

Adsorption and precipitation of myo‐inositol hexakisphosphate onto kaolinite

“…Although limited Al of kaolinite still dissolves at pH 9.0 (Figure c) without IHP, dissolved Al(III) mainly existed as Al(OH) 4 − species (Lützenkirchen et al, ). At pH 9.0, IHP was sorbed onto the kaolinite surface primarily via the formation of inner‐sphere complexes, which was supported by the results of NMR (Figure S3 in File S1) and ζ potential (Figure S2b in File S1).…”

Calcium and sodium recovery from microwave‐pretreated red mud with added solid ammonium chloride

“…The solubility of Al‐bearing minerals (e.g. boehmite, gibbsite and amorphous Al(OH) 3 ) in the red mud is lower in the neutral condition, whereas it is higher in the acidic or alkaline condition . The addition of solid NH 4 Cl could decrease the pH of the leachate, leading to lower leached Al concentration.…”

“…boehmite, gibbsite and amorphous Al(OH) 3 ) in the red mud is lower in the neutral condition, whereas it is higher in the acidic or alkaline condition. 38 The addition of solid NH 4 Cl could decrease the pH of the leachate, leading to lower leached Al concentration. In addition, the microwave pretreatment might cause transformation of soluble Al-bearing minerals to insoluble forms, leading to the lower leached Al concentration.…”