Photo-reduction and adsorption in aqueous Cr(VI) solution by titanium dioxide, carbon nanotubes and their composite

Abstract

Background: This study compared the removal of aqueous Cr(VI) by multi-walled carbon nanotubes (CNTs) modified by sulfuric acid, titanium dioxide (TiO_2) and composite of CNTs and TiO_2. Results: More than 360 h contact time was needed to completely adsorb 3 mg L^(−1) of Cr(VI) by CNTs, indicating that the rate of adsorption by CNTs alone was slow. The reaction time approaching equilibrium depended on the Cr(VI) concentration. XPS analysis of CNTs after adsorbing Cr(VI) showed that the Cr(VI) on the surface of CNTs was partially reduced to Cr(III). A 3 mg L^(−1) solution of Cr(VI) was fully photocatalyzed by commercial TiO_2 (Degussa P25) in less than 0.5 h under UV irradiation. Unlike P25, reduction by another commercial TiO_2 (Hombikat UV100) took 4 h and more than 2 h were necessary for reduction by the composite. Thus the efficiency of Cr(VI) photo-reduction by the composite was lower than by TiO_2, but higher than that by CNTs. XPS analysis of TiO_2 and composite showed the existence of both Cr(VI) and Cr(III) on their surfaces. Conclusion: In contrast to TiO_2, the reduction rate of aqueous Cr(VI) using CNTs as adsorbent was slow. P25 had a markedly higher photocatalytic efficiency than the composite or UV100 alone. Using P25 to reduce aqueous Cr(VI) has a higher potential for practical application. The diameters of TiO_2 and CNTs and the ratio of TiO_2/CNTs are key problems in the preparation of TiO_2/CNTs composite.