Removal of Cr(VI) and Cr(III) from Aqueous Solutions and Industrial Wastewaters by Natural Clino-pyrrhotite

This paper introduced a simple method of treating Cr(VI)-bearing toxic wastewaters using a natural mineral:  clino-pyrrhotite. Laboratory bench-scale mixing experiments were carried out in both Cr(VI)-bearing artificial solutions and industrial wastewaters under controlled conditions. The effects of solution pH, Cr(VI) concentration, mineral grain size, mineral/solution ratio, and reaction time on the Cr(VI) removal were studied. Chromium was effectively removed from the solutions and wastewaters. After the treatment, the liquid was clean enough to be discharged directly into the natural environment. The Cr(VI) removal process involved sequentially the adsorption of Cr(VI), in the form of Cr2O72- or CrO42-, onto the mineral surface, the reduction of the adsorbed Cr(VI) to Cr(III), catalyzed at the vacant Fe sites of the mineral, and finally the precipitation of Cr(III) as Cr2S3, Cr2O3, and Cr(OH)3 solid phases. Conditions such as a fine mineral grain size, an excessive quantity of clino-pyrrhotite and a w...