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

This investigation seeks to reduce the environmental 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 environment 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 oxalate, prior to the recovery of Cr as the hydroxide precipitate at pH 6.5. The dried Cr hydroxide cake was then subjected to heating (heating rate 0.5 °C min^-1) at different 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 measurement 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 hexavalent Cr. A mechanism for the formation of the different forms of Cr oxide was proposed and confirmed by XRD.

Keywords
Nanomaterials; Secondary resources; Selective separation; Chromium oxide nanoparticles