In situ ATR-FTIR spectroscopic analysis of the co-adsorption of orthophosphate and Cd(II) onto hematite

We used in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to study the impact of Cd(II) on the coordination of orthophosphate to the surface of hematite in the pH range 4.5–9.0, and at aqueous reactant concentrations below saturation with respect to Cd(II)–phosphate precipitates. In the absence of Cd(II), the orthophosphate surface speciation was pH dependent and dominated by two surface species assigned as monodentate monoprotonated complexes dominating at alkaline pH and additional formation of bidentate monoprotonated complexes at pH <8.0. Addition of aqueous Cd(II) raised the amount of orthophosphate adsorbed across the pH range, with promotive effects increasing with increasing pH. We observe the formation of two structurally distinct ternary Cd(II)–orthophosphate surface complexes which change proportion with pH. The IR spectra suggest stronger distortion of the orthophosphate tetrahedra involved in the ternary complexes formed at low pH relative to those formed at high pH, indicating differences in protonation state, surface coordination, and/or coordination to surface Cd(II) between the two ternary complexes. Over most of the pH range covered, the two ternary complexes are present simultaneously at the hematite surface, and co-exist with the two binary orthophosphate surface species, with the relative proportions of the various complexes varying with pH. The presence of Cd(II) thus not only raises the extent of orthophosphate adsorption but also the level of complexity of the orthophosphate surface speciation. These results imply that the simultaneous presence of divalent metals and orthophosphate significantly influences the solubility and speciation of these compounds in environmental settings even under conditions where precipitation of metal–phosphates does not occur.