Binding of lead and chromium(III) cations to pectin

Abstract

Binding of Pb2+ ions to magnesium pectinates was studied by dialyzation technique in relation to their esterification degree (E) in the interval E = 0 to 84,7%, at an ionic strength I = 0.30. It has been found that the binding of Pb2+ ions to pectinates proceeds according to the theory of multiple equilibria. The values of stability constant K and selectivity coefficient KPbMg evidence the high selectivity of pectinates towards Pb2+ ions. Both those constants are, in contrast with value corresponding to other bivalent cations (e.g.Ca2+, Sr2+), independent on the esterification degree of pectin in the E 50% range. A strong binding of Pb2+ to carboxyl groups takes place also at a high esterification degree of pectin. The exchange of Cr3+ 3 K+ was investigated with a model preparation of potassium pectate crosslinked with epichlorhydrin by a batch method at various pH of equilibrium solutions (I = 0.15). Potassium pectate was shown to be a highly selective ion exchanger towards Cr3+. The binding of Cr3+ to pectate also occured in a sufficient extent at a low pH of equilibrium solutions (pH 3). Due to a very strong binding of Cr3+ to pectate a substantial shift of dissociation equilibrium of carboxyl groups in favour of further binding of Cr3+ ions occured.