Selective recovery of chromium ions from waste tannery solution for preparation of chromium oxide nanoparticles

Abstract

This investigation seeks to reduce the envi- ronmental impact of the tanning process by recovering Ca and Cr from tannery waste solution. The treatment process reduces both the amount of Cr discharged to the environ- ment and the amount of raw Cr extracted from natural resources. The procedure was based on the selective separation of Ca ions from the waste solution as Ca ox- alate, prior to the recovery of Cr as the hydroxide pre- cipitate at pH 6.5. The dried Cr hydroxide cake was then subjected to heating (heating rate 0.5 C min -1 ) at differ- ent temperatures up to 500 C to produce Cr oxide nanoparticles. The physicochemical characteristics of the nanoparticles were investigated using X-ray diffraction, Fourier transform infrared, scanning electron microscopy, transmission electron microscopy, surface area measure- ment and thermal analysis. Results revealed that heating of the treated Cr hydroxide at 300 C yielded amorphous Cr oxide, while with heating at 400 C, the Cr oxide started to crystallize to hexavalent, trivalent and divalent Cr species. At 500 C, a pure phase consisting of trivalent Cr oxide nanoparticles was formed that contained traces of hex- avalent Cr. A mechanism for the formation of the different forms of Cr oxide was proposed and confirmed by XRD.