In Situ FTIR-ATR Examination of Poly(acrylic acid) Adsorbed onto Hematite at Low pH

The adsorption of water-soluble polymers (e.g., flocculants, coagulants) onto mineral particles is typically characterized by “trains” of adsorbed polymer segments with “loops” and “tails” of unadsorbed polymer segments that extend into the solution. Ex situ spectroscopic studies carried out in the past have been complicated by the way in which the unadsorbed segments of the polymer interact with the surface of the substrate upon drying. In this study, an in situ Fourier transform infrared attenuated total reflection technique was used to examine the interaction of poly(acrylic acid) and hematite at pH 2. A hematite colloid was deposited onto a ZnSe crystal, and the poly(acrylic acid) solution was subsequently pumped across the coated crystal at a known rate. Polymer adsorption was irreversible and could be described by a Langmuir isotherm, the rate of curvature of which suggested that the adsorption was weak. The mode of adsorption was shown to be bidentate chelate complexation by the carboxylate functional group to a surface ferric ion. Pimelic acid, a simple model compound effectively representing a polymer consisting of only two monomer functionalities, adsorbed onto hematite in the same manner at pH 2 and gave no infrared spectral peaks associated with unadsorbed carboxylate groups, thereby supporting the proposed adsorption mechanism. The technique could also discriminate between the adsorbed and the unadsorbed segments of the adsorbed polymer molecule, however; the fraction of adsorbed segments was difficult to quantify. When a number of assumptions were made, it was found that at most 9% of the carboxylate functional groups of the polymer were adsorbed onto the hematite.