Revealing the Regularities of Sorption Binding of Cadmium Ions by Pectin Substances from Aqueous Solutions

The complex influence of sorption of cadmium ions on the state of pectin substances was examined using IR spectroscopy of polymer films according to the change of the absorption bands attributed to the stretching vibrations of С–О bond of carboxylic groups of pectates, deformation vibrations of О–Н bond of secondary hydroxyls, and stretching vibrations of carbonyl in ester groups. The isotherms of Cd2+ adsorption were analyzed at 293–308 K. The sorption process on the samples of pectin isolated from the biomass of burdock Arctium lappa L., rhubarb Rheum, citrus cultures, and model substrates of pectinate, pectate, and pectic acid based on them was adequately described using the Langmuir adsorption model. The thermodynamic parameters of adsorption that quantitatively characterize the binding of cadmium with –СООН groups according to the ionic exchange mechanisms, cationic substitution with –СООСа1/2 groups and coordination interactions with alcohol hydroxyls and carbonyl oxygen of methoxylated galacturonate units were determined.