Studies of the surface charge of amorphous aluminosilicates using surface complexation models

Synthetic noncrystalline aluminosilicates with variable charge, similar to allophanes present naturally in volcanic soils, were studied. The surface charge behavior was determined by zero point charge (ZPC) measured by electrophoretic mobility (isoelectric points, IEP) and determined by potentiometric titration (point of zero salt effect, PZSE). The ZPC calculated by Parks model (ZPCc), compared with IEP values, showed that the aluminosilicate (AlSi) surface was slightly enriched by AlOH (34% Al2O3 and 66% SiO2) compared with the bulk composition (29% Al2O3 and 71% SiO2). For aluminosilicate coated with iron oxide (AlSiFe) the ZPCc (4.4) was lower than the IEP (8.46), showing that the surface composition is formed mainly from iron oxide. The PZSE values for AlSi and AlSiFe were 6.2 and 4.8, respectively. The differences between the IEP and PZSE are attributed to the formation of SiOFe or SiOAl bonds; therefore, the reactivity of Fe and Al atoms was modified on the surface. Two mechanistic models, the constant capacitance model (CCM) and the triple layer model (TLM), using the program FITEQL 3.2 were able to describe the surface behavior of both synthetic aluminosilicates. The acidity constants determined using both models for the aluminosilicates showed differences with respect to pure oxide, mainly attributed to the presence of SiOH sites on the internal surfaces. The ionic strength showed a good relation with the parameters obtained using the CCM (pKa1int, pKa2int and capacitance values) and the TLM (pKa1int, pKa2int, pKCl−int, pKK+int, and inner capacitance) for both aluminosilicates. However, the TLM was able to describe the acidity and complexation constants better since it considered the formation of the outer sphere complex between the background electrolyte and the surface. Then, the TLM makes it possible to describe real systems.