Sulfate Retention by Allophane: Adsorption or Precipitation?.

Sulfur K-edge XANES microspectroscopy revealed that adsorbed and precipitated sulfate coexist on a micrometer scale in Andosols subject to high volcanogenic S input. Precipitated sulfate is most likely present as aluminum hydroxy sulfate (AHS). We hypothesized that - compared to primary minerals - allophanes represent preferential sites for the formation of AHS in Andosols as sulfate may be adsorbed with a subsequent shift from adsorption to precipitation with increasing sulfate load. To test the hypotheses we (i) exposed synthetic allophane and glass for 18 months in soils close to Masaya and Poás Volcanoes and (ii) conducted sulfate sorption experiments in the lab using synthetic allophane. Experimental products were analyzed by wet-chemical methods, scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX) and spatially resolved sulfur K-edge XANES. Adsorbed ester sulfate was distinguished from inorganic precipitated sulfate by differential thermal analysis with SO₂ mass spectrometry (DTA-MS). Sulfur concentrations in allophane samples from the field experiment reached up to 15 g kg^-1 while on glass samples no sulfur could be detected by bulk elemental or SEM-EDX analysis. In allophane samples no morphologically distinct sulfur-containing phase could be identified by SEM-EDX. With XANES we determined an average of 30% of the retained sulfur being precipitated on bulk allophane samples while sulfur hot-spots analyzed by spatially resolved XANES contained up to 60% precipitated sulfate. DTA-MS analysis revealed the co-existence of organically bound and adsorbed and precipitated inorganic sulfate in allophane samples from the field experiment. XANES analysis of the products of laboratory sulfate sorption isotherms show an increasing relevance of sulfate precipitation over adsorption with increasing sulfate load at pH 4 while the percentage of precipitated sulfate is independent of the sulfate load at pH 4.5. The results are consistent with thermodynamic equilibrium calculations including different possible AHS phases.