Abstract
Research on Ca2+ adsorption onto the mineral surface is of significant importance with regard to geochemical processes. Sverjensky (Geochim Cosmochim Acta 70(10), 2427–2453, 2006) assumed that alkaline earths form two types of surface species on oxides: tetranuclear (> SOH)2(> SO−)2_M(OH)+ and mononuclear > SO−_M(OH)+. To look into the above assumption we investigated calcium adsorption on SiO2 and Al2O3 because they are the most widespread minerals in the environment. We have determined the proton surface charge, electrokinetic potential and metal adsorption as a function of pH. The Ca2+ uptake and concentration in the system were monitored by the calcium ion-selective electrode (Ca-ISE). The Ca-ISE measurements indicated a similar affinity of Ca2+ for both materials despite their differently charged surface, negative for silica and mainly positive for alumina. This may suggest that simple electrostatic interactions are not the primary driving force for calcium adsorption, and that solvation of calcium ions at the surface may be crucial. We have analyzed our experimental data using the 2-pK triple-layer model (2-pK TLM). Three calcium complexes on the mineral surface were reported. Two of them were the same for both oxides, i.e. the tetranuclear (> SOH)2(> SO−)2_Ca2+ and mononuclear complexes > SO−_CaOH+. Additionally, minor contribution from >SOH…Ca2+ for silica was assumed. In the case of Al2O3 the hydrolyzed tetranuclear complexes (> SOH)2(> SO−)2_CaOH+ at pH > 7.5 occurred based on the modeling results. Two types of surface complexes suggested by Sverjensky allowed for the correct description of proton and calcium uptake for alumina. However, the electrokinetic data excluded hydrolyzed tetranuclear surface species for this oxide.
Highlights
In the soil and water environment calcium ions are of great importance (Stumm and Morgan 1996)
A different method of calcium determination in the silica dispersion was used by Janusz et al (2003), i.e. the radiotracer technique. Those investigations showed that the higher NaClO4 electrolyte concentration, the lower Ca2+ adsorption occurred confirming the earlier findings by Kosmulski (1994) who used the same technique for both alumina and silica in NaCl solution
The precise location of the silica PZC could not be observed since the proton surface charge density curves did not intersect at one exact point, but remained superimposed
Summary
In the soil and water environment calcium ions are of great importance (Stumm and Morgan 1996). It was shown that the latter process depends on many factors, for instance, pH, temperature, ionic strength, and the presence of organic matter (Atesok et al 1988; Ridley et al 1999; Weng et al 2008; Lee et al 2014; Allen et al 2017; Cherian et al 2018; Jia et al 2019) Minerals such as silica and alumina constitute particles widely spread in the Earth’s crust (Sparks 2003). Aluminum(III) oxide is useful in terms of catalysis, adsorption, pigmentation of paints and cosmetics as well as constituting electronic devices (Cheah et al 1998; Teaney et al 1999; Kasprzyk-Hordern 2004; Chasserio et al 2009) The particles of both oxides remaining in water are in a continuous contact with C a2+ in the soil environment. The most recent research discussed divalent (among others Ca2+) and monovalent adsorption affinity for the α-Al2O3/water interface using the vibrational sum frequency generation (vSFG) (Piontek et al 2019)
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