Coupling Molecular-Scale Spectroscopy with Stable Isotope Analyses to Investigate the Effect of Si on the Mechanisms of Zn-Al LDH Formation on Al Oxide.

While silicate has been known to affect metal sorption on mineral surfaces, the mechanisms remain poorly understood. We investigated the effects of silicate on Zn sorption onto Al oxide at pH 7.5 and elucidated the mechanisms using a combination of X-ray absorption fine structure (XAFS) spectroscopy, Zn stable isotope analysis, and scanning transmission electron microscopy (STEM). XAFS analysis revealed that Zn-Al layered double hydroxide (LDH) precipitates were formed in the absence of silicate or at low Si concentrations (≤0.4 mM), whereas the formation of Zn-Al LDH was inhibited at high silicate concentrations (≥0.64 mM) due to surface-induced Si oligomerization. Significant Zn isotope fractionation (Δ66Znsorbed-aqueous = 0.63 ± 0.03‰) was determined at silicate concentrations ≥0.64 mM, larger than that induced by sorption of Zn on Al oxide (0.47 ± 0.03‰) but closer to that caused by Zn bonding to the surface of Si oxides (0.60-0.94‰), suggesting a presence of Zn-Si bonding environment. STEM showed that the sorbed silicates had a close spatial coupling with γ-Al2O3, indicating that >Si-Zn inner-sphere complexes (">" denotes surface) likely bond to the γ-Al2O3 surface to form >Al-Si-Zn ternary inner-sphere complexes. This study not only demonstrates that dissolved silicate in the natural environment plays an important role in the fate and bioavailability of Zn but also highlights the potential of coupled spectroscopic and isotopic methods in probing complex environmental processes.