Recovery of hexavalent chromium from water using photoactive TiO2-montmorillonite under sunlight

Abstract

Hexavalent chromium was removed from water under sunlight using a synthesized TiO₂-montmorillonite (TiO₂-M) employing tartaric acid as a hole scavenger. Cr(VI) species was then reduced to Cr(III) species by electrons arising from TiO₂ particles. After that, the produced Cr(III) species  was transferred to montmorillonite  due to electrostatic attractions leading to  set free TiO₂ particles for a further Cr(VI) species reduction. Furthermore, produced Cr(III), after Cr(VI) reduction, does not  penetrate into the solution. The results indicate that no dark adsorption of Cr(VI) species on TiO₂-M is present, however, the reduction of Cr(VI) species under sunlight increased strongly as a function of tartaric acid concentration up to 60 ppm, for which the extent of reduction is maximum within 3 h. On the other hand, the reduction extent of Cr(VI) species is maximum with an initial concentration of Cr(VI) species lower than 30 ppm by the use of 0.2 g/L of TiO₂-M. Nevertheless, the increase of the Cr(VI) initial concentration led to increase the amount of Cr(VI) species reduced (capacity of reduction) until a Cr(VI) concentration of 75 and 100 ppm, for which  it remained constant at around 221 mg/g. For comparison, the increase of Cr(VI) species concentration in the case of the commercial TiO₂ P25 under the same conditions exhibited its deactivation when the reduced amount decreased from 198.1 to 157.6 mg/g as the concentration increased from 75 to 100 ppm.