Characterisation of solid residues obtained on removal of Cr from waste water

This investigation was designed to characterise the solid residues obtained when waste water heavily contaminated with Cr3+ is purified by the 'ferrite precipitation method' and to evaluate the efficiency of the process using different starting Fe2+/Cr3+ ratios. In the case of Cr(VI) contamination, to avoid altering the proportions, the Cr6+ was reduced to Cr3+ in a preliminary step, and the same procedure applied. The solid compounds obtained were subjected. to X-ray fluorescence and chemical analysis (inductively coupled plasma and potentiometric titrations). Findings suggest CrxFe2+Fe2-x3+O4 as the most probable stoichiometry of the products, where 0.09less than or equal toxless than or equal to0.59. X-ray diffractometry, differential scanning calorimetry and Fourier transform infrared patterns indicated a 'normal spinel' structure corresponding to 'chromite-type' chromium iron oxides. This structure differs considerably from that of divalent metal-bearing ferrites ('inverse spinel') and constitutes a new hydrothermal pathway for the generation of 'non-stoichiometric chromites'. The following is proposed as the general oxide formation reaction: a Fe2+ + x Cr3+ + b OH- + c O-2 --> CrxFe3-xO4 + Fe2O3.nH(2)O. (C) 2002 Elsevier Science B.V. All rights reserved.