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Science  22 Apr 2005:
Vol. 308, Issue 5721, pp. 469b
DOI: 10.1126/science.308.5721.469b

Chromate ions (CrO4), such as those in industrial waste streams, are highly toxic, and a better understanding of their transport properties in groundwater would be useful. In the outdoors, the flow of chromate ions can be influenced by myriad chemical and microbial interactions, which researchers usually lump together into measured retardation factors.

Al-Abadleh et al.have used a model system to probe the molecular origins and details of retardation in silica-rich soils. They prepared monolayers of carboxylic acid- and ester-terminated alkyl chains, which were attached via siloxanes to a fused quartz substrate, and used second harmonic generation spectroscopy to monitor the reversible binding to these surfaces of aqueous chromate. In comparison to bare silica, the organic acid layers, which are analogous to the humic acids in soil, nearly tripled the retardation factor, whereas esters increased it by 50%. Moreover, the binding energy of chromate to acid increased with chromate concentration, and an analysis of this cooperative behavior quantified the lateral intermolecular forces in a hydrogen-bonded network of acids, perturbed by metal ions. — JSY

J. Phys. Chem. B 10.1021/jp050782o (2005).

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