Jarosite precipitation is a useful means of stabilizing toxic species, and accordingly, the behaviour of chromium(Ⅲ) during jarosite precipitation was systematically investigated in a series of laboratory experiments. Although 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 was possible. However, at temperatures ＞ 200°C, the Cr(Ⅲ) analogue of potassium jarosite (KCr_3(SO_4)_2(OH)_6) is readily precipitated. 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 precipitation of amorphous phases. Although increasing K_2SO_4 concentrations result in a greater mass of precipitate, the products formed from concentrated K_2SO_4 solutions are contaminated with an amorphous phase. In fact, the overall results suggest that an amorphous phase precipitates rapidly and that the Cr(Ⅲ) analogue of potassium jarosite forms by the recrystallization, or the dissolution-reprecipitation, of the amorphous phase.
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