Heat-treated polyacrylonitrile nanofibers: A new material for efficient photo-assisted reduction of Cr(VI)

Abstract

► Heat-treated poly(acrylonitrile) was found to efficiently photoreduce Cr(VI). ► Activity was by far higher than similar potentially active materials. ► Reduction takes place under either visible or UV light. ► The phenomenon is explained by electron delocalization along the polymer chain. Electrospinning was used to prepare polyacrylonitrile (PAN)-based nanofibrous mats. The mats were processed at an elevated temperature. It was found that the heat-treated polyacrylonitrile was able, under UV light, to reduce toxic Cr(VI) species to the less toxic Cr(III) very efficiently. The reduction rates were found to depend strongly on the temperature of the heat-treatment process, with hardly any correlation with the specific surface area of the polymer. Optimal performance was obtained with mats that had been heated in air to 250 °C and then were held under nitrogen at 350 °C. This photoinduced activity was significantly superior to similar means of Cr(VI) removal. Good correlation was found between the ability to adsorb the Cr(VI) species and reduction kinetics. The results are explained by a mechanism comprising of charge injection from the polymer to photo-excited Cr(VI) species. This injection is, in particular, efficient whenever the preparation conditions are such that in-chain charge delocalization and good coupling between the polymer and the Cr(VI) ions are formed. The authors do not negate the possibility that the observed phenomena, most likely originating from intra-chain charge conduction may be utilized in other areas, in particular photovoltaics.