Reduction and stabilization of hypervalent chromium by apple pectin

BELLÚ, SEBASTIÁN; GONZÁLEZ, JUAN C.; GARCÍA, SILVIA; SIGNORELLA, SANDRA; SALA, LUIS F.

Los cocos

Congreso; 9th Latin American Conference on Physical Organic Chemistry; 2007

UNC

Chromium(VI) is a widespread environmental contaminant and a strong oxidant. In addition to being highly toxic, Cr(VI) is mobile, and has a long residence time in surface water and groundwater; it poses health risk to humans and animals and impairs the development and growth of plants. Many interactions of hipervalent chromium with low-mass reductants have been studied but a few with biopolymers. Pectins are a group of polysaccharides present in all plant primary cell walls. The dominant structural feature of pectin is a linear chain of poly-a-(1®4)-D-galacturonic acid with varying degree of methylation of the carboxylic acid residues. This backbone is occasionally interrupted by stretches with side chains, which are rich in neutral sugars, mainly arabinose, galactose, and rhamnose. The objective of this work was to study the ability of pectins to reduce and/or stabilize hypervalent chromium in order to ascertain the potential role of this plant component in the transport of Cr. Kinetics and spectroscopic studies (UV-vis, EPR) of the reaction of Cr(VI) and apple pectin revealed that the reaction proceeded through a mechanism combining CrVI ® CrIV ® CrII  and CrVI ® CrIV ® CrIII pathways. Intermediacy of CrII in this reaction was demonstrated by detection of CrO22+ when working in O2-saturated solutions and low [Cr(VI)]. The existence of this species is indicative of the presence of Cr(II) and Cr(IV) intermediates that can contribute to the oxidative damage in plant cells. Free radicals formed in the CrIV ® CrIII path were detected by the induced polymerization of acrylonitrile. Cr(V) species formed by the Cr(VI) + Cr(II) or Cr(VI) + organic radical reactions were characterized by EPR spectroscopy. The Cr(V)-pectin species showed giso values around 1.9785 typical of oxo-Cr(V) bound to two carboxilato and two alcoholato donor groups. These species were stable for at least 6 hours at pH = 4.66, suggesting that the pectin can stabilize this oxidation state of chromium. HCOOH and CO2 were found as oxidation products of apple pectin when treated with Cr(VI), showing that carboxilate residues are the reactive sites of pectin.