Goethite surface complex

He, J Ma, W Song, W Zhao, J Qian, X Zhang, S Yu JC. Photoreaction of aromatic compounds at a-FeOOH/H20 interface in the presence of H2O2 evidence for organic-goethite surface complex formation. Water Research, 2005 39, 119-128. 

Despite their different trends in sorption with pH, both arsenate and arsenite form similar surface complexes on goethite. Surface complexes of arsenate and... 

Figure 5. Some of the possible inner-sphere complexes on goethite surfaces. Complex A is the bidentate double-comer sharing complex on the (110) surface. Complexes B and C are edge sharing complexes on the (021) surface.

In a second example, we calculate how pH affects sorption onto hydrous ferric oxide, expanding on our discussion (Section 10.4) of Dzombak and Morel s (1990) surface complexation model. We start as before, setting the dataset of surface reactions, suppressing the ferric minerals hematite (Fe203) and goethite (FeOOH), and specifying the amount of ferric oxide [represented in the calculation by Fe(OH)3 precipitate] in the system... 

Anion Binding. This discussion illustrates how valuable information on enthalpy changes of surface reactions (either from temperature dependence or from direct calorimetric measurements) are. Zeltner et al. (1986) have studied calorimetrically the surface complex formation of phosphate and salicylate on goethite. They show that these reactions are exothermic (at pH = 4) with AHadS values at low coverage ( 10 %) of ca. -24 kJ mol 1, they argue tentatively that these values indicate biden-tate surface complex formation. They also show that -AH decreases with increasing surface coverage. 

Final justification for using terms such as inner- or outer-sphere awaits direct spectroscopic confirmation. Electron Spin Resonance, Mossbauer, and Fourier Transform Infrared-Cylindrical Internal Reflection Spectroscopic techniques are being used to establish the structure of surface complexes (see, e.g., McBride, Ambe et al., and Zeltner et al., this volume). The potential for using EXAFS (extended x-ray absorption fine structure) to establish the type of surface complex for Pb + adsorbing onto goethite is currently being undertaken in our laboratory. 

Figure 6. CIR-FTIR spectra of SAL and iron SAL complex in solution and SAL on goethite. (upper) 0.1 M SAL in 1 M KC1 at pH 5.5 (middle) SAL on goethite in D.O with goethite surface groups subtracted at pD 4.5, 0.01 M KC1 and 100 g/L solid concentration (lower) aqueous iron SAL complex at pH 1.6.

The acidity of organic ligands is enhanced by coordination with the oxide surface, i.e. the surface promotes deprotonation of the functional groups (COOH or OH). Such ligands, therefore, adsorb on the surface at a pH 2-3 units lower than that at which complexation with Fe in solution would occur (Kummert and Stumm, 1980). An example of this is the deprotonation of the alcoholic OH group of tartaric acid upon adsorption on the goethite surface (Cornell and Schindler, 1980). The appropriate reaction for the acid in water is. 

For some of the most intensively studied anions, increased information has not clarified the speciation or coordination of the adsorbed complex. For example, on the basis of IR data it has been claimed that sulphate adsorbs on goethite as both a bidentate and a monodentate surface complex (Parfitt and Smart, 1977 Persson and Lovgren, 1996). Some of the conflict may arise from differences in the conditions under which the experiments were carried out. Most IR spectroscopy measurements of adsorbed sulphate on Fe oxides were made on dried samples. Hug (1997) compared... 

Fig. 11.3 Surface complex of octahedrally coordinated Cd on a) goethite and b) lepidocrocite. The Cd surface complex can be bonded to either the edge (E) or vertex of one octahedron. For each type of complex (e. g. C(ooi)), the super-...

Adsorption of ferricyanide (a contaminant of sites of former gas plants) on goethite decreases with rising pH and is dependent on ionic strength it is fully reversible and is thought to involve both inner and outer sphere complexes. Adsorption of ferrocyanide on goethite involves initial formation of inner sphere complexes followed by precipitation of a Berlin Blue-like phase on the goethite surface (Rennert and Mansfeldt, 2001). 

Most organics with double aromatic rings show maximum adsorption on goethite at low pH. The exceptions are catchecol and dihydroxynapthalene, the adsorption of both of which rises as the pH rises from 3 to 8. Under alkaline conditions, both organics form strong surface complexes which appear to involve hydrogen bonding. 

---