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: clinopyrrhotite. 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 weak acidic media, favored the removal process. For clino-pyrrhotite with a restricted grain size, the minimum required quantity of the mineral was proportional to the total quantity of Cr(VI) to be removed. Quantitatively, one cubic meter of industrial wastewater that contained similar to 1 mmol dm(-3) of Cr(VI) and had a pH value between 1 and 10 would be effectively treated after it was in contact with 220 kg of 145 +/- 28 mu m clinopyrrhotite for an hour. Furthermore, the quantity of the final solid waste byproduct was small, and the solid residue of clino-pyrrhotite could be reused after a simple rinse with water. Compared to the previous Cr(VI)-bearing wastewater treatment schemes, this method was simple, effective, economical, and environmentally sound. It has great potential for use in industrial-scale applications.