Goethite anion adsorption

Liu F, De Cristofaro A, Violante A (2001) Effect of pH phosphate and oxalate on the adsorption/desorption of arsenate on/from goethite. Soil Sci 166 197-208 Livesey NT, Huang PM (1981) Adsorption of arsenate by soils and its relation to selected properties and anions. Soil Sci 131 88-94 Manceau A (1995) The mechanism of anion adsorption on iron oxides Evidence for the bonding of arsenate tetrahedra on free Fe(0, OH)6 edges. Geochim Cosmochim Acta 59 3647-3653. 

The surface proton adsorption which occurs after Step 2, however, complicates the determination of the heat content change resulting from anion adsorption. In order to make this correction, the heat associated with proton adsorption must be determined from the previous potentiometric-calorimetric titrations. Proton adsorption on goethite is exothermic, and Figure 1 provides an average value of -29.6 kj/mol near pH 4. This value, when multiplied by the moles of protons required to return to pH 4 after anion adsorption, allows correction for the heat associated with proton adsorption. This correction, however, is based on the assumption that the proposed two-step anion adsorption mechanism described above represents the only surface reactions which occur during anion adsorption. As such, the results obtained by this procedure are model dependent and are best used for comparative purposes. 

Adsorption of anions on oxides is usually accompanied by the uptake of protons (or the release of hydroxyl ions). The ratio between the number of protons that are coadsorbed and the level of anion adsorption is not usually stoichiometric. Studies of adsorption of oxyanions on goethite as a function of pH appear to indicate that, provided only one adsorbed species is present, the proton anion ratio is related to the mode of adsorption (Rietra et al., 1999). It follows that oxyanions which adsorb with a similar proton/anion stoichiometry, have a similar adsorption mechanism. 

Fig.n.i Effect of pH on the adsorption of various anions on goethite together with a plot of the amount adsorbed vs. pKn values of the respective acids (Hingston et al., 1968, with permission). 

Quirk, J.P. (1968) Specific adsorption of anions on goethite. Int. Congr. Soil Sci. Trans. 

Anion adsorption by goethite and gibbsite. I. The role of the proton in determining adsorption envelopes. J. Soil Sci. 23 177-192... 

Machesky, M. Andrade, W. Rose, A. (1991) Adsorption of gold(III)-chloride and gold(I)thio-sulphate anions by goethite. Geochim. Cosmochim. Acta 55 769-776... 

Farmer, V.C. (1975) Adsorption of carbon dioxide on goethite (a-FeOOH) surfaces, and its implications for anion adsorption. J. 

Zeltner,W.A. Yost, E.C. Machesky M.L. Teje-dor-Tejedor, M.I. Anderson, M.A. (1986) Characterization of anion adsorption on goethite using titration calorimetry and CIR-ETIR. In Davis, J.A. Hayes, K.E. (eds.) Geochemical processes at mineral surfaces. 

Adsorption of anions at mineral surfaces is important in soils because of the limit this process imposes on the availability of plant nutrients such as P, S, and Mo which occur naturally as anions and are added in anionic form in fertilizers. Anion adsorption is also relevant in geochemistry, ore processing, and other fields where minerals with high surface areas are brought into contact with aqueous solutions of anions. Selenite and goethite were chosen for this study because in Western Australia a selenium deficiency in pastures has been shown to be related to the incidence of white muscle disease in sheep (3), and according to workers quoted by Rosenfeld and Beath (9) selenium in soils of higher... 

Goethite Successfully applied the CD-MUSO surface complexation model to literature data for anion adsorption to goethite including As(V)-P competition. CD-MUSIC is the most promising of the SCMs for modeling complex natural systems Hiemstra and van Riemsdijk (1999)... 

An important example of equation (9.11) occurs with the specific adsorption of oxy-anions by metal oxyhydroxides. Taking M0O4- as the oxy-anion and goethite (a-FeOOH) as the metal oxyhydroxide, one can write down the reaction sequence ... 

The adsorption of oxalate ions on metal oxides is presented in Figure 5. The adsorption of oxalate ions is characteristic for anion adsorption on metal oxides, and decreases with increasing pH. The following sequence of oxalate adsorption affinity is observed goethite > titania > silica. At the silica/electrolyte interface,... 

Figure 4. Adsorption of four anions by goethite. Two samples of goethite were used and the levels of addition differed for the differing anions [56].

The following correlation log K (surf)= log K (soln) +1.5 was obtained for adsorption of five anions on goethite [615], but the K(surf) calculated from this equation for H2PO4 is lower than the observed one by two orders of magnitude. Another limited correlation between the stability of surface and solution complexes of anions has been reported by the same authors in Ref, [672]. It should be emphasized, that the literature data on stability constants of aqueous complexes involving A1 and Fe and specifically adsorbing anions is scarce, and for a few anions whose sorption behavior has been extensively studied, the stability constants of aqueous complexes with Fe and A1 are not known. 

Other publications postulate specific adsorption between the surface and the /3-plane. For example Barrow and Bowden [69] interpreted adsorption of anions on goethite in terms of the mentioned above four layer model. The four layers are (in order of increasing distance from the surface) surface layer (H" and OH ions), the layer of specifically adsorbed anions, the first layer of inert electrolyte counterions (analogous to the /3-layer in TLM). and diffuse layer. This model requires an additional adjustable parameter, namely, the capacitance between the surface and the layer of specifically adsorbed anions. Barrow and Bowden report 2.99 F m for phosphate and 60,000 F m ( ) for silicate. The fit in the four layer model was substantially better than with simpler models for fluoride adsorption, but for other anions equally good fit could be obtained without introducing the additional electrostatic plane. In another paper of this series the capacitance of 3-5 F was used in model calculations of phosphate adsorption on aluminum and iron oxides [92]. Similar approach was used by Venema et al. [93] who applied the 1-pK model to interpret the Cd binding by goethite. The ions were assumed to... 

Machesky, M.L., Bischoff, B.L., and Anderson, M.A., Calorimetric investigation of anion adsorption onto goethite. Environ. Sci. Technol., 23, 580, 1989. 

Figure 3.16. Surface charge of goethite (FeOOH) in solutions of 1.0, 0.1, and 0.01 M NaCl, estimated from the difference in proton and hydroxyl uptake (H" — OH ) by the oxide during acid-base titration. PZC, point of zero charge, (Adapted from F. J. Hingston, 1970. Specific adsorption of anions on goethite and gibbsite. Ph.D, dissertation. University of Western Australia, Perth.)...

Adsorption of Aqueous Silica onto Titania—pH and Concentration. A number of detailed experimental and modeling studies (26-32) have reported the adsorption of aqueous silicate onto oxides such as alumina and goethite and various other (often soil constituent) particulates such as fluorides. Studies of anion adsorption onto titania are not so common (33). This situation is somewhat strange given the technical importance of titania. The solubility of titania in water between pH 3 and... 

One mechanism that is consistent with the observed properties of the particles in these suspensions involves the dissolution of amorphous Si02 and adsorption of soluble silicate on the Fe(OH)3 surface. This process could occur in parallel with the heterocoagulation mentioned earlier. Soluble silicate species might then compete with Se03 or PO4 for surface sites as suggested by Goldberg (8) for the P04/silicate/goethite system. Sorption of silicate species onto Fe(OH)3 need not affect cationic adsorbates. Benjamin and Bloom (10) demonstrated that adsorption of cations is often minimally affected by anion adsorption even under conditions where anion-anion competition is severe (11). 

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