STABILIZATION OF CHROMIUM IN THE FORM OF MIXED Fe(III)-Cr(III) HYDROXIDES

Pollution by hexavalent chromium, contained in industrial waste, can cause serious environmental and health problems. Research has led to the development of quite a few stabilizing techniques, among which the most effective is the reduction of Cr(VI) by Fe(II) salts. Chromate, when reduced to the trivalent state, precipitates in the form of an amorphous Cr(III) hydroxide. When the reductant is ferrous iron the final product is a mixed Fe(III)-Cr(III) hydroxide, with increased stability compared to the simple chromium hydroxide. All previous experimental studies were carried out at 25 degrees C, and the solubility of solids was determined without applying any thermal treatment or aging procedure. During the present experimental work, Cr(OH)(3) alone and mixed Cr(OH)(3) - Fe(OH)(3) precipitates were produced at higher precipitation temperatures (40-70 degrees C) and/or submitted to aging in order to investigate the effect of these parameters on the crystallinity, the chemical characteristics and the solubility of solids. These tests showed that all solids were amorphous. The hydrothermal aging procedure applied on the Cr(III) hydroxides had a positive effect on their stability compared to freshly prepared hydroxides that lacked any aging process. The Fe(III)-Cr(III) hydroxides presented even lower solubility, with total Cr concentrations below drinking water limit at pH values greater than 4.5. However, the application of higher temperatures had no effect on the stability of mixed hydroxides.