Modeling and electrokinetic evidences on the processes of the Al(III) sorption continuum in SiO 2( s) suspension

Abstract

Reactions of Al(III) at the interface between SiO 2( s) and aqueous solution were characteristically and quantitatively studied using electrophoretic methods and applying a surface complexation/precipitation model (SCM/SPM). The surface and bulk properties of Al(III)/SiO 2 suspensions were determined as functions of pH and initial Al(III) concentration. Simulated modeling results indicate that the SCM, accounting for the adsorption mechanism, predicts sorption data for low surface coverage only reasonably well. Al(III) hydrolysis and surface hydroxide precipitation must be invoked as the Al(III) concentration and/or pH progressively increase. Accordingly, the three processes in the Al(III) sorption continuum, from adsorption through hydrolysis to surface precipitation, could be identified by the divergence between the SCM/SPM predictions and the experimental data. SiO 2( s) suspensions with low Al(III) concentrations (1×10 −4 and 1×10 −5 M) exhibit electrophoretic behavior similar to that of a pure SiO 2( s) system. In Al(III)/SiO 2 systems with high Al concentrations of 1×10 −3, 5×10 −3 and 1×10 −2 M, three charge reversals (CR) are observed, separately representing, in order of increasing pH, the point of zero charge (PZC) on the SiO 2 substrate (CR1), the onset of the surface precipitation of Al hydroxide (CR2), and at a high pH, the PZC of the Al(OH) 3 coating (CR3). Furthermore, in the 1×10 −3 M Al(III)/SiO 2( s) system, CR2 is consistent with the modeling results of SCM/SPM and provides evidence that Al(III) forms a surface precipitate on SiO 2( s) at pH above 4. SiO 2( s) dissolution was slightly inhibited when Al(III) was adsorbed onto the surface of SiO 2( s) , as compared to the dissolution that occurs in a pure SiO 2( s) suspension system. Al hydroxide surface precipitation dramatically reduced the dissolution of SiO 2( s) because the Al hydroxide passive film inhibited the corrosion of the SiO 2( s) surface by OH − ions.