Mechanistic Thermodynamic Model for Zn ²⁺ Uptake on Saponite

Zn uptake by the synthetic trioctahedral clay mineral saponite was investigated in batch sorption experiments over a wide range of conditions (4.7 < pH < 9.6; 2.3 × 10–7 < [Zninit.] < 3 × 10–3 M) in 0.1 M NaCl. The pH- and concentration-dependent Zn2+ uptake follows the typical behavior previously reported for dioctahedral clay minerals and can be formally described by sorption on two types of surface sites. The sorption data were reproduced using the 2SPNE SC/CE model, which includes high-affinity/low-capacity “strong” sites and low-affinity/high-capacity “weak” sites. The protolysis constants of amphoteric oxygen sites were fixed based on the model available for montmorillonite, and only the site capacities and Zn complexation constants were optimized. Ab initio calculations, polarized EXAFS, and chemical constraints from solution-phase elemental analyses indicate that Zn is preferentially located on the edge surface attached to the octahedral sheet of saponite. The pronounced polarization dependency of the EXAFS spectra corroborates octahedral sheet-controlled Zn binding and rules out sorption on tetrahedral sites. The data sets consistently indicate a loading-dependent evolution of Zn coordination. At low surface loadings (<5 mmol/kg), Zn is predominantly taken up as a strongly bound inner-sphere complex at the octahedral edge sites, with 5-fold coordination by Mg. With increasing Zn concentrations, two additional Zn environments are identified and become dominant: (i) a weaker bound species with one-fold Mg coordination, consistent with the “weak” site complex predicted by the surface complexation model, and (ii) another strongly bound species with a 3-fold Mg coordination. The progressive consumption of dissolved Si and Mg with increasing Zn concentrations suggests the onset of an additional reaction pathway consistent with Zn–Mg phyllosilicate formation and agrees with the emergence of the highly coordinated species. Overall, the results show that, beyond the “strong” and “weak” surface complexes captured by the 2SPNE SC/CE model, an epitaxial Zn–Mg phyllosilicate phase likely forms at higher Zn concentrations.