Schwertmannite solubility

Chang, H.-W. (1999) Apparent solubilities of schwertmannite and ferrihydrite in natural stream waters polluted by mine drainage. Geochim. Cosmochim. Acta 63 3407—3416... 

A complete series between schwertmannite in the pure sulfate system and akaganeite in the pure chloride system has been synthesized from Fe ksolutions with various C17S0 ratios (Bigham et al. 1990). An ion activity product (lAP) for the solubility of schwertmannite, log lAP = 18.0 2.5, was calculated from the chemical composition of a number of mine drainage solutions by Bigham et al. (1996). 

Fig. 3. Concentration of total dissolved iron as a function of pH in the pit lakes of the IPB. The mineral phases controlling the solubility of Fe(III) at different pH ranges are indicated (Jar, jarosite Schw, schwertmannite Fer, ferrihydrite). The ionic speciation of Fe(III) (calculated with PHREEQC 2.7) is also indicated for the two end-members of the compositional range (Corta Atalaya and Los Frailes). Data taken from Ref. 45.

The concentration and/or activity of dissolved Fe(III) is thus controlled by different mineral phases depending on the pH. This is illustrated in Fig. 3, which plots the pH-dependent variation in the concentration and speciation of dissolved iron in different mine pit lakes of the IPB. Among the 15 pit lakes studied, jarosite was only observed to precipitate in Corta Atalaya, so that this mineral controls the apparent equilibrium which seems to exist between dissolved and particulate Fe(ni) in this lake. Conversely, ferrihydrite is the mineral form under which Fe(III) precipitates in Los Frailes pit lake. Most lakes, however, seem to be at or near equilibrium with respect to schwertmannite, which not only controls the solubility of Fe(III), but also buffers the systems at pH 2.5-3.5 (through reaction (5)), and sorbs toxic trace elements like As [5-14, 24—28]. 

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