namely, thermodynamic modeling of potentiometric and ad-The surface speciation of orthophosphate ions on goethite has sorption data (5,6,8) and IR spectroscopic investigations been studied as a function of pH, time, total phosphate concentra- (10)(11)(12)(13)(14)(15)(16). Regardless of the technique, the results are usually tion, and ionic medium by means of diffuse reflectance FTIR specinterpreted in terms of surface complexation theory (17). troscopy. The samples were prepared in accordance with a distri-The main objective of the thermodynamic work is to formubution diagram of surface species as calculated from thermodylate a set of equilibria that explains the observed changes namic data. In agreement with the thermodynamic model three in the aqueous phase. This includes the reaction between dominating surface complexes could be distinguished with IR phosphate ions and surface metal ions, and commonly also spectroscopy, and the relative distribution of the species was shown protonation reactions of the adsorbed phosphate. There is to be primarily a function of pH. The IR characteristics of the individual surface complexes were indicative of molecular sym-good agreement between the models, based on thermodymetries of the PO 4 unit of C 3v , C 2v , and C 3v , respectively. This namic data, presented by various authors (5,6,8). The was concluded to be incompatible with the bidentate, bridging dominating surface complexes derived are three species mostructural model previously suggested. Instead, the IR data are in nodentately coordinated to the surfaces, namely, GMe{Ogood agreement with a monodentate coordination of phosphate to PO 3 H 2 , GMe{OPO 3 H 0 , and GMe{OPO 20 3 . In Ref.
(5) the surfaces, where the three surface complexes only differ in two minor bridging complexes have also been included in the the degree of protonation. A comparison between the adsorption model. Since protonated complexes are involved the surface behavior of phosphate on goethite and hematite was also made.
speciation is strongly pH dependent. The direct bond be-Here the importance of the aqueous stability of the adsorbent on tween the surface metal ions and phosphate implies that the adsorption mechanism was shown.