An infrared study of a polyacrylate flocculant adsorbed on hematite

Hematite was flocculated at various pH values using a 100% polyacrylate of high molecular weight, and the adsorbed configuration of the polymer was determined from infrared (DRIFT) spectra. The positions of the symmetric and asymmetric carboxylate stretches for the polyacrylate sodium salt and the adsorbed flocculant, and their separation were used to determine the adsorbed configuration. The adsorption mechanism is dominated by chemisorption, except at neutral pH in the presence of calcium ions when electrostatic interactions also play a role. At pH 7 the polyacrylate adsorbs in an essentially monodentate configuration. At this pH there is a significant degree of hydrogen bonding between the C=O group of the flocculant and either the surface hydrogens of the hydroxylated hematite or the solvent (water). At higher pH (11-14) the monodentate configuration is much less evident and an asymmetric bidentate bridging structure is formed. In the presence of calcium ions at pH 7 a bidentate chelating structure occurs in addition to the monodentate structure. This chelating structure was not found at pH 13 and 14.