Goethite surface charge

Fig. XIII-9. The dependence of the flotation properties of goethite on surface charge. Upper curves are potential as a function of pH at different concentrations of sodium chloride lower curves are the flotation recovery in 10 M solutions of dodecylammo-nium chloride, sodium dodecyl sulfate, or sodium dodecyl sulfonate. (From Ref. 99.)...

Tipping, E., and D. Cooke (1982), The Effect of Adsorbed Humic Substances on the Surface Charge of Goethite in Freshwater", Geochim. Cosmochim. Acta 56, 75. 

Fig. 10.5 Surface charge density of hematite and goethite as a function of pH in an inert electrolyte of various concentrations (KCI for hematite NaNOs for goethite) as obtained by acid/base titration (hematite Atkinson et al., 1967, with permission goethite S. Glasauer, unpubl.).

Ternary adsorption of humic molecules and Ca ions modified the surface charge and the colloidal behaviour of goethite (Tipping and Cooke, 1982). Fe oxides in natural environments often form in the presence of or through the activity of, micro-organisms (see Chap. 17). The adsorption properties of the oxides may, therefore, be modified by adsorbed organic molecules and ternary Fe-OCO-M complexes (CO-org. 

Felmy, A.R. Rustad, J.R. (1998) Molecular statics calculations of proton binding to goethite surfaces Thermodynamic modeling of the surface charging and protonation of goethite in aqueous solution. Geochim. Cosmochim. Acta 62 25—31... 

Zeltner,W.A. Anderson, M.A. (1988) Surface charge development at the goethite/aqueous solution interface. Effects of CO2 adsorption. Langmuir 4 469—474... 

Using Eq. 11 -34 (Box 11.1) and the goethite surface site density, its pA i and pK values, and the ionic strength of the solution, find the intensity of surface charging of the goethite at the three pH values of interest (note that [Pg.446]

Excess Surface Charge. The reaction at the goethite surface producing charged sites by adsorption of H+ and OH" as potential determining ions can be represented as follows,... 

Potentiometric titration of an aqueous suspension of oxides in the presence of varying concentrations of indifferent electrolyte has been used successfully to determine the zero point of charge (z.p.c.) and the variation in excess surface charge with pH (I, 8). The variation in excess surface charge (rH+-r0H-) with pH and NaCl concentration is shown for goethite in Figure 4. 

The excess surface charge in the presence of specifically adsorbed ions was found by measuring the amount of selenite adsorbed and the amount of hydroxyl displaced into the solution. The quantity of OH" displaced (A) was estimated for constant pH (a hypothetical situation) from the curves for titration of goethite, goethite plus selenite, and selenite alone by the equation. 

Figure 10.3 The dependence of the flotation properties of goethite (FeO(OH)) on surface charge. Lower curves show the flotation recoveryin 10 5 M solutions of dodecylammonium chloride, sodium dodecyl sulphate, and sodium dodecyl sulphonate. From Fuerstenau [623], Copyright 1962, American Institute of Mining, Metallurgical and Petroleum Engineers.

Figure 3. The surface charge density at the goethite/electrolyte interface a function of pH in a presence and absence of cadmium ions.

The simplified mass and proton balance model determined what the surface species distribution of goethite would be in a mixed, seawater type electrolyte. This surface species distribution was used to calculate a surface charge for goethite. 

This calculated surface charge successfully predicted the titra-table surface charge, as determined by potentiometric titration, of goethite in a seawater major-ion electrolyte (Figure 7). 

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