Reduction of hexavalent chromium by pyrite-rich andesite in different anionic solutions

Abstract.

Introduction of Cr(VI) to the environment has been of environmental concern due to its toxicity and mobility. In treatment processes for wastewater, groundwater, and soil contaminated with Cr(VI), the reduction of Cr(VI) to Cr(III) and the subsequent precipitation of insoluble Cr(III) compounds are generally employed. Finely ground, hydrothermally altered andesite containing 11.8% of pyrite reacted with Cr(VI) solutions (0 to 400 mg l^–1) in the presence of dissolved oxygen and 0.01 to 0.1 M of KCl, K₂SO₄, and KH₂PO₄. Before and after the reaction, the morphology of the pyrite was also examined with a scanning electron microscope. The altered andesite was an effective Cr reductant in KCl and K₂SO₄ solutions, but KH₂PO₄ decreased the Cr(VI) reduction capacity of the altered andesite. The higher the initial concentration of Cr(VI), the more Cr(VI) reduced to Cr(III) in KCl and K₂SO₄ solutions, but no relationship between the initial concentration of Cr(VI) and the amount of reduced Cr(VI) in KH₂PO₄ solutions was observed. The final solution pH for KCl and K₂SO₄ increased with an increasing initial concentration of Cr(VI). For KH₂PO₄, the pH was not significantly changed by changing the initial concentration of Cr(VI), but it did increase with an increasing KH₂PO₄ concentration. A concentration of aqueous ferrous iron [Fe(II)] nonstoichiometrically decreased with an increasing amount of reduced Cr. The changes in solution pH and concentrations of Fe(II) and Cr(VI) indicate that the reduction of Cr(VI), both at solid-solution interface and in solution, occurred simultaneously. Pyrite had relatively smooth surfaces and sharp edges before the Cr reduction, but showed rough surfaces, dissolution pits on surfaces, and smoothed edges after the Cr reduction.