Hydrous oxides acid strength

In very acidic solutions (pH < 2.4-3) with ionic strengths below 0.1 M and at 25 °C and 1 bar pressure, scorodite has a pK of about 25.83 0.07. The pK of amorphous Fe(III) arsenate is approximately 23.0 0.3 under the same conditions (Langmuir, Mahoney and Rowson, 2006). At higher pH values, scorodite dissolves incongruently, which means that at least one of its dissolution products precipitates as a solid. The incongruent dissolution of scorodite in water leads to the formation of Fe(III) (oxy)(hydr)oxide precipitates that is, Le(III) (hydrous) oxides, (hydrous) hydroxides and (hydrous) oxyhydroxides (Chapter 3). During the formation and precipitation of the iron(III) (oxy)(hydr)oxides, As(V) probably coprecipitates with them (Chapter 3 also see Section 2.7.6.3). The dissolution rate of scorodite at 22 °C in pH 2-6 water is slow, around 10—9 —10—10 mol m-2 s-1, which explains its presence in many mining wastes (Harvey et al., 2006). 

The presence of additional oxygen attached to M increases the acid strength of the compound. Thus, in the series of M oxides the approximate order of increasing acid strength and increasing cation-exchange character for the insoluble hydrous oxides [111] can be represented as... 

The three most abundant minerals forms are Fe203 (hematite), Fe(OH)j (hydrous ferric oxide or ferrihydrite), and FeO(OH) (goethite). The chemical reactions describing their dissolution and K in slightly acidic water at 25°C, 1 atm, and the ionic strength of... 

Figure 11 The pH response of tin(IV)antimonate with various compositions during their neutralization with base. a-Sn a-hydrous tin(IV) oxide, A-SbA amorphous antimonic(V) acid, numerical number mol ratio of Sb/Sn, exchanger, 0.25 g total volume, 12.5 cm" temp., 25 C ionic strength, 0.1 M (KOH + KCl). (From M. Abe and T. Ito, Abgyo Kagaku Zassbi, 79 A40 (1967). With permission.)...

The adsorption of diphenylmercury (DPM) and phenylmercuric ion (PM) was studied on the solid phases described above (hydrous manganese oxides, amorphous iron oxides, humic acid and bentonite clay). The solid phase (5-15 mg) was added to 25-50 ml of filtered seawater yielding solid phase concentrations of approximately 100 to 400 ppm suspended matter. The concentration of seawater was also varied in order to study the variation of adsorptive behavior with changes in ionic strength. The suspension was then spiked with either DPM or PMA to yield concentrations of organometallic which varied from 0.10 to 3.5 ppm. The range in organometallic concentration used for this study was determined by the sensitivity of the detection method and the solubility of DPM and PMA in seawater. 

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