Kinetics and mechanism of oxidation of apple pectin by Cr^VI in aqueous acid medium

Abstract

Selective oxidation of galacturonic acid residues of apple pectin by CrVI affords CO2/HCO2H, oxidized pectin, and CrIII as final redox products. The reaction shows first‐order kinetics in [pectin], [CrVI], and [H+], at fixed ionic strength and temperature. Kinetic studies show that the redox reaction proceeds through a mechanism combining CrVI → CrIV → CrII and CrVI → CrIV → CrIII pathways. The mechanism is supported by the observation of free radicals, CrO (the formation of which implies involvement of CrII and CrIV) and CrV (formed in monoelectronic redox processes) as reaction intermediates. The reduction of CrIV and CrV by pectin was independently studied and found to occur more than 103 times faster than pectin + CrVI reaction, in acid medium. At pH 3–5, apple pectin + CrVI redox reaction is slow, oxo‐chromate(V)–pectin species stabilize and remain in solution during several hours. The present results show that these abundant and ubiquitous components of the cell walls of all land plants are able to reduce CrIV–VI or stabilize high‐valent chromium depending on pH. Copyright © 2008 John Wiley & Sons, Ltd.

A promising methyl ester of pectic acid polysaccharide in biomedicine and pharmaceutics applications: Oxidation of methyl ester of pectic acid by alkaline hexacyanoferrate(III). A kinetic and mechanistic orientation

“…Apple pectin (BDH) was used without further purification. Different concentrations of PEC were prepared as described elsewhere . This process was performed by stepwise addition of the sample reagent to deionized water while rapidly stirring the solution to avoid the formation of lumps, which swell with difficulty.…”

“…Different concentrations of PEC were prepared as described elsewhere. [19][20][21][22][23][24][25][26] This process was performed by stepwise addition of the sample reagent to deionized water while rapidly stirring the solution to avoid the formation of lumps, which swell with difficulty. The inherent and reduced viscosities were measured using an Ubbelhode viscometer for 4% pectin solution in water (w/v) and found to be 0.62 and 2.16 dL g −1 at 20 • C, respectively.…”

“…The isokinetic relationship for the second-order rate constants in the oxidation of some polysaccharides by alkaline hexacyanoferrate(III) permanganate, 24 and acidic chromate 25 were decreased in the order alkaline permanganate > acidic permanganate > acidic chromate, respectively. The reactive species in the ratedetermining steps play the main role in such magnitude where the attraction forces between the opposite charged reactants does not need much energy to overcome the energy barrier to form the transition or intermediate complexes than that of the reactants of the same sign which need much energy to overcome the energy barrier.…”

“…On the other hand, the kinetics of oxidation of PEC by various oxidants such as alkaline permanganate, acidic permanganate, and chromic acid was found to be quite different depending on the nature of the oxidant and the type and pH of the medium used. But, the oxidation products were found to be the same.…”

Thermodynamic and Dynamic of Chromium Biosorption by Pectic and Lignocellulocic Biowastes

“…Chromium and its compounds are widely employed in industry [1] and Cr(III) is used as a tanning agent, resulting in severe groundwater contamination around tanneries [2,3]. Oxidation of polyuronic acids and pectin by hipervalent Chromium showed the presence of Cr(V), Cr(IV) and Cr(III)-saccharide complexes, with Cr(III)pectin as an insoluble compound in the reaction media [4,5]. Following the treatment of Cr(VI) solution ([Cr(VI)] 0 = 0.0176 M) with 1.0 g of D-polygalacturonic acid, only 10% of the total Cr (Cr T = Cr(III) + Cr(VI)) remained in solution [5].…”