Pb2+ and Zn2+ adsorption by a natural aluminum- and iron-bearing surface coating on an aquifer sand

Geochimica et Cosmochimica Acta
By: , and 

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Abstract

Pb2+ and Zn2+ 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. Pb2+ and Zn2+ adsorption experiments with various grain size and mineral fractions of the aquifer solids showed that (1) Zn2+ adsorption was independent of grain size, but Pb2+ was preferentially adsorbed by the <64 μm size fraction and (2) Pb2+adsorption decreased after removal of the paramagnetic, Fe-bearing mineral fraction, but Zn2+ adsorption was unaffected. Pb2+ and Zn2+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 μm. 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 Pb2+ and Zn2+adsorption relative to untreated sediment (extracted Fe/Al ∼ 1), but Pb2+and Zn2+ adsorption were not appreciably changed after sediment was extracted with dithionite-citrate (extracted Fe/Al ∼ 5). Overall, the results suggest that Pb2+ preferred to form complexes with iron hydroxyl sites, while aluminol sites were more important for Zn2+ 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.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Pb2+ and Zn2+ adsorption by a natural aluminum- and iron-bearing surface coating on an aquifer sand
Series title Geochimica et Cosmochimica Acta
DOI 10.1016/0016-7037(95)00231-N
Volume 59
Issue 17
Year Published 1995
Language English
Publisher Elsevier
Contributing office(s) Toxic Substances Hydrology Program
Description 13 p.
First page 3535
Last page 3547