Graphitic carbon nitride based photocatalysis for redox conversion of arsenic(III) and chromium(VI) in acid aqueous solution

In this study, the synergistic redox conversion of As(III) and Cr(VI) in acidic media was efficiently achieved with a graphitic carbon nitride (g-C3N4) based photocatalyst material (with pyromellitic diimide doping) under visible light irradiation. The non-adsorptive nature of As(V) on the g-C3N4 based photocatalyst made it very steady in recycle treatment of both pollutants compared to traditional titanium dioxide based photocatalysts (P25). While the adsorption of few Cr(III) on the photocatalyst would activated photogenerated hydrogen peroxide (H2O2) to yield hydroxyl radical. The As(III)/Cr(VI) conversion was effective at initial pH from 3.0 to 6.0 and concentrations of As(III) and Cr(VI) from 10 to 1000 μM by the photocatalyst material, and the presence of other ion salts such as KCl, Na2SO4 and Mg(NO3)2 had no any inhibitory effect on their conversion. The photogenerated species such as hole coupled with superoxide radical and photogenerated electron coupled with H2O2 were responsible for the oxidation of As(III) and the reduction of Cr(VI), respectively. Besides, the Cr(V)/H2O2 Fenton-like system that was established during the photocatalytic treatment by g-C3N4 based photocatalyst, had synergistic impact towards the simultaneous conversion of As(III) and Cr(VI), i.e., reduction of Cr(VI) to Cr(V) as well as generation of hydroxyl radicals for As(III) oxidation.