Interactions of organic and inorganic chromium species with Ca-polygalacturonate

This study investigated the interactions of CrIII and CrVI present as free metal ions or as organic CrIII (Cr-L) with a Ca-polygalacturonate (Ca-PG) network. Ca-PG networks were treated with solutions of chromium (CrIII and CrVI) at different concentrations at three different pH values. The Cr-L complex were obtained by water extraction of hydrolysed leather. The CrVI had almost no interaction with the Ca-PG networks, whereas CrIII as a free metallic ion was strongly retained by the network. The calculated Langmuir adsorption isotherms showed a twofold increase in maximum CrIII adsorption but a decrease in the affinity of the metal with the Ca-PG networks. This would indicate that the different chromium species, i.e. CrIII and its hydrolytic species, interact in a different way with the polygalacturonate chains. In addition, thermal analysis showed that CrIII sorption modified the structure of the network. These modifications were reduced, and thermal stability increased if Ca-PG adsorbed Cr-L or L. Furthermore, the treatment of Ca-PG with hydrolysed leather water extract-reduced CrIII sorption by 94%. The hydrolysed leather fraction immobilized onto the Ca-PG networks probably consisted of peptides and could therefore represent an important source of organic N.