Factors affecting the precipitation of chromium(III) in jarosite-type compounds

Jarosite precipitation is a useful means of stabilizing toxic species, and accordingly, the factors affecting the precipitation of chromium(III) in jarosite-type compounds was systematically investigated in a series of laboratory experiments. Although end-member Cr(III) analogues of jarosite-type compounds could not be precipitated at temperatures <100 °C, several percent Cr(III) substitution for Fe(III) in potassium jarosite and sodium jarosite was observed. However, at temperatures >200 °C, the Cr(III) analogue of potassium jarosite (KCr₃(SO₄)₂(OH)₆) is readily precipitated. The Cr(III) analogue has the R $\bar 3$ m structure characteristic of jarosite-type compounds, with a=7.23±0.02Å and c=17.02±0.02 Å. The well-crystallized material typically contains (wt pct): ～7K, ～25Cr, and ～41SO₄. The composition suggests the partial substitution of hydronium ion for potassium and some chromium vacancies in the structure. The formation of the Cr(III) analogue is promoted by increasing temperatures, retention times, and Cr(III) concentrations. Increasing acid concentrations reduce the amount of product formed but suppress the undesirable precipitation of amorphous phases. Although increasing K₂SO₄ concentrations result in a greater mass of precipitate, the products formed from concentrated K₂SO₄ solutions are contaminated with an amorphous phase. In fact, the overall results suggest that an amorphous phase precipitates initially and that the Cr(III) analogue of potassium jarosite forms by the recrystallization, or the dissolution-reprecipitation, of the amorphous phase.