Ion-exchange equilibria between solid aluminium pectinates and Ca, MnII, CuII and FeIII ions in aqueous solution

Abstract

The ion-exchange equilibria between solid aluminium pectinates and aqueous solutions of calcium, manganese, zinc, copper and iron(III) nitrates have been studied at 298 K. The equilibria are generically represented by the equation: (z–i) Al(OH)₂P(s)+ M(OH)^{(z– i)+}_i(aq)⇌ M(OH)_i P_(z–i)(s)+(z–_i) Al(OH)⁺₂(aq) where P is the active site of pectinates containing (z–i) carboxy groups and their molecular surroundings, and M(OH)^{(z– 1)+}_i represents Ca(OH)⁺, Mn(OH)⁺, Zn(OH)⁺, Cu(OH)⁺, Fe(OH)⁺ and Fe(OH)²⁺. The equilibrium constants were determined using pectins with different degrees of methylation of the carboxy groups. The results are discussed in terms of interactions occurring in the ionic exchange for each of the metallic nitrates. Together with data from the literature it is possible to establish the following order of preference in bonding with pectins: Fe^III > Al > Cu > Pb > Cd > Zn > Ni > Co > Sr ≈ Ca ≈ Mn > Mg.