Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.3535c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 17, pp.3535-3547
Computer Science
11
Scientific paper
Pb 2+ and Zn 2+ adsorption was studied in batch experiments with material collected from a shallow, unconfined aquifer of glacial outwash sand and gravel in Falmouth, Massachusetts, USA. The aquifer solids contain primarily quartz (95% w/w), with minor amounts of alkali feldspars and ferromagnetic minerals. Pb 2+ and Zn 2+ adsorption experiments with various grain size and mineral fractions of the aquifer solids showed that (1) Zn 2+ adsorption was independent of grain size, but Pb 2+ was preferentially adsorbed by the <64 m size fraction and (2) Pb 2+ adsorption decreased after removal of the paramagnetic, Fe-bearing mineral fraction, but Zn 2+ adsorption was unaffected. Pb 2+ and Zn 2+ adsorption on mineral separates from the aquifer material compared with metal adsorption on a purified quartz powder indicated that adsorption of both metal ions was dominated by coatings on the quartz fraction of the sediment. Characterization of the coatings by AES, SEM-EDS, and TOF-SIMS demonstrated that the natural quartz grains were extensively coated with Al- and Fe-bearing minerals of variable composition. Thin sections of quartz grains examined by TEM showed that the coatings contained both polycrystalline regions and single mineral crystals. The coating thickness varied from <10 nm up to 30 m. The coatings were mostly resistant to dissolution by an extraction protocol designed to dissolve noncrystalline phases. The effect on metal adsorption of dissolving surface coatings from the sediment by chemical extraction was also measured. A hydroxylamine-HC] extraction designed to dissolve crystalline Fe oxide phases decreased Pb 2+ and Zn 2+ adsorption relative to untreated sediment (extracted Fe/Al ~ 1), but Pb 2+ and Zn 2+ adsorption were not appreciably changed after sediment was extracted with dithionite-citrate (extracted Fe/Al ~ 5). Overall, the results suggest that Pb 2+ preferred to form complexes with iron hydroxyl sites, while aluminol sites were more important for Zn 2+ adsorption. However, a definitive understanding of adsorption reactions in groundwaters will require detailed studies of the extensive coatings formed at mineral-water interfaces by chemical weathering processes.
Coston Jennifer A.
Davis James A.
Fuller Christopher C.
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