Surface proton adsorption goethite

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. 

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.)...

For the study of adsorption to general oxide surfaces, the ideal ionic force-field would be consistent with a molecular mechanics model for dissociable water. Such a forcefield has indeed been developed and has been applied to the study of silicate (Rustad Hay, 1995) and iron(III) (Rustad et al., 1995) hydrolysis in solution, to bulk iron oxyhydroxide structures (Rustad et al., 1996a) and to the protonation of goethite surfaces (Rustad et al., 1996b). 

Kinetics of Selenium Adsorption. Zhang and Sparks 4G) examined selenate and selenite adsorption and desorption on goethite using pressure jump relaxation techniques. Selenate produced a single relaxation, that was interpreted as outer-sphere complexation with surface protonation based on fitting to the triple layer model. The forward rate constant was 10 L mol s Selenite adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of inner-sphere complexes of both HSeOj and SeOj, based on optimized fits using the triple layer model. The model optimized fit for the pK, of the surface species was approximately 8.7. Forward rate constants for the first step were on the order of 10 L -mor -s for HSeOj and 10 L -mor -s for SeOj. Forward rate constants for the formation of the inner-sphere complexes were 100 and 13 s respectively for HSeOj and SeOj. Agreement between the equilibrium constant obtained from batch and kinetic studies was taken as confirmation of the proposed reactions. 

Acid-base constants/proton-affinity constants These parameters cannot, at present, be established experimentally. Because there are probably several distinct sites and electrostatic effects, the overall surface-charge curves do not give any hint as to the nature and reactivity of these sites. Experimental back-titration data by Schulthess and Sparks [103], which indicate steps in the proton adsorption isotherm (i.e., the surface-charge density versus pH curve), were interpreted by these authors to be indicative of individual sites with individual reactivities. However, such data are not reproduced by the bulk of research groups. In the previous subsection, the back-titration technique has been discussed in some detail and, in particular, the results on goethite obtained with base titrations at low pH were found to show significant scatter compared to data obtained with the coulometric approach. 

The examples shown is Section D indicate that the shape of calculated uptake curves (slope, ionic strength effect) can be to some degree adjusted by the choice of the model of specific adsorption (electrostatic position of the specifically adsorbed species and the number of protons released per one adsorbed cation or coadsorbed with one adsorbed anion) on the one hand, and by the choice of the model of primary surface charging on the other. Indeed, in some systems, models with one surface species involving only the surface site(s) and the specifically adsorbed ion successfully explain the experimental results. For example, Rietra et al. [103] interpreted uptake, proton stoichiometry and electrokinetic data for sulfate sorption on goethite in terms of one surface species, Monodentate character of this species is supported by the spectroscopic data and by the best-fit charge distribution (/si0,18, vide infra). 

It has been found that adsorption/desorption of anions such as Cl and CIO4 on soil constituents is very rapid. In fact, reequilibrium is too rapid to be observed using p-jump relaxation. Fortunately, the electric-field pulse technique can be used for such systems. This method was employed by Sasaki et al. (1983) to study CI and CIO4 adsorption on goethite. Two relaxations on the order of microseconds were observed in acidified aqueous suspensions of a-FeOOH with either NaCl or NaClO4. The fast relaxation was dependent on the applied electric field intensity and was attributed to a physical diffusion phenomenon. The slower relaxation was independent of the applied electric field intensity and was interpreted in terms of the association/dis-sociation reaction of counter ions with protonated surface hydroxyl groups as ion pairs... 

Zhang, J.S., Stanforth, R., and Pehkonen, S.O., Proton-arsenic adsorption ratios and zeta potential measurements Implication for protonation of hydroxyls on the goethite surface, 7. Colloid Interf. Sci., 315, 13, 2007. 

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