5 research outputs found
Competitive sorption of Ni and Zn at the aluminum oxide/water interface: an XAFS study
Abstract Trace metals (e.g. Ni, Zn) leached from industrial and agricultural processes are often simultaneously present in contaminated soils and sediments. Their mobility, bioavailability, and ecotoxicity are affected by sorption and cosorption at mineral/solution interfaces. Cosorption of trace metals has been investigated at the macroscopic level, but there is not a clear understanding of the molecular-scale cosorption processes due to lack of spectroscopic information. In this study, Ni and Zn cosorption to aluminum oxides (Ī³-Al2O3) in binary-sorbate systems were compared to their sorption in single-sorbate systems as a function of pH using both macroscopic batch experiments and synchrotron-based X-ray absorption fine structure spectroscopy. At pH 6.0, Ni and Zn were sorbed as inner-sphere surface complexes and competed for the limited number of reactive sites on Ī³-Al2O3. In binary-sorbate systems, Ni had no effect on Zn sorption, owning to its lower affinity for the metal oxide surface. In contrast, Zn had a higher affinity for the metal oxide surface and reduced Ni sorption. At pH 7.5, Ni and Zn were sorbed as mixed-metal surface precipitates, including NiāAl layered double hydroxides (LDHs), ZnāAl LDHs, and likely NiāZnāAl layered triple/ternary hydroxides. Additionally, at pH 7.5, Ni and Zn do not exhibit competitive sorption effects in the binary system. Taken together, these results indicated that pH critically influenced the reaction products, and provides a crucial scientific basis to understand the potential mobility, bioavailability, and ecotoxicity of Ni and Zn in natural and contaminated geochemical environments
Barium isotopic fractionation in latosol developed from strongly weathered basalt
Weathering is a key process in the transfer of material from continents to the hydrosphere. A latosol profile in Zhanjiang, Guangdong Province, South China, formed through intense weathering of basalt, was studied to improve understanding of Ba isotopic fractionation during basalt weathering. Profile horizons were grouped into Ba-depleted and Ba-enriched layers (D- and E-layers, respectively) according to the mass fraction of Ba lost or gained from the weathered profile relative to bedrock. delta Ba-137/134 values in the soil profile ranged from -0.22% to +0.02%, lower than those of the parent basaltic rock (0.03% +/- 0.03%). In the D-layers, Ba isotopic fractionation can be explained by Rayleigh fractionation, implying that heavy Ba isotopes are preferentially leached. The Rayleigh fractionation model is not applicable to the E-layers because they preferentially acquired isotopically light Ba isotopes during weathering. Results indicate a net loss of heavy Ba isotopes during strong weathering of basalt due to the precipitation of Fe-Mn (oxyhydr) oxides and adsorption on secondary minerals. A mass-balance model indicates that the average delta Ba-137/134 value of materials leached from the weathered profile is similar to 0.08%, slightly higher than that of the bedrock. This suggests a loss of heavy Ba isotopes into the hydrosphere during weathering of basalt, consistent with the enrichment of heavy Ba isotopes in river waters. (C) 2019 Elsevier B.V. All rights reserved
Coupling Molecular-Scale Spectroscopy with Stable Isotope Analyses to Investigate the Effect of Si on the Mechanisms of ZnāAl 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 ZnāAl layered double hydroxide (LDH) precipitates were
formed in the absence of silicate or at low Si concentrations (ā¤0.4
mM), whereas the formation of ZnāAl LDH was inhibited at high
silicate concentrations (ā„0.64 mM) due to surface-induced Si
oligomerization. Significant Zn isotope fractionation (Ī66Znsorbedāaqueous = 0.63 Ā± 0.03ā°)
was determined at silicate concentrations ā„0.64 mM, larger
than that induced by sorption of Zn on Al oxide (0.47 Ā± 0.03ā°)
but closer to that caused by Zn bonding to the surface of Si oxides
(0.60ā0.94ā°), suggesting a presence of ZnāSi
bonding environment. STEM showed that the sorbed silicates had a close
spatial coupling with Ī³-Al2O3, indicating
that >SiāZn inner-sphere complexes (ā>ā
denotes
surface) likely bond to the Ī³-Al2O3 surface
to form >AlāSiāZn 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