Oligo and polyuronic acids interactions with hypervalent chromium

Abstract

Selective oxidation of galacturonic residues of oligo and polyuronic acids by Cr-VI affords CO2/HCO2H, oxidized uronic acid, and Cr-III as final redox products. Kinetic studies show that the redox reaction proceeds through a mechanism combining Cr-VI -> Cr-IV -> Cr-II and Cr-VI -> Cr-IV -> Cr-III pathways. The mechanism is supported by the observation of free radicals, CrO22+ and Cr-V as reaction intermediates. The EPR spectra show that five- and six coordinated oxo-Cr-V intermediates are formed. Penta-coordinated oxo-Cr-V species are present at any [H+], whereas hexa-coordinated ones are only observed at pH <1. At low pH Cr-V predominating species are coordinated by carboxylate groups and O-ring (g(iso) = 1.9783/5). At pH 7.5, the predominating ones are those coordinated by alcoholate groups of the ligand (g(iso) = 1.9800). Polygal can reduce Cr-VI and efficiently trap Cr-III. This behaviour represents an interesting model for the study of biomaterials, which possess a high proportion of polygal, in order to remove chromium from polluted waters.