Chromium(VI) adsorption-reduction using a fibrous amidoxime-grafted adsorbent

To comply with regulations and mitigate environmental pollution, efficient technologies are required for treating chromium-contaminated industrial wastewater. To develop a safe and simple method for the reduction of toxic chromium(VI) to the less harmful chromium(III), we proposed using radiation-induced graft polymerization. This method allows for any functional group to be introduced into conventional polymeric materials, which enables the synthesis of solid adsorbents with good handling properties. Herein, we synthesized a fibrous adsorbent with amidoxime functional groups that have a high affinity for chromium, and investigated the changes in the chromium valence state over time in the amidoxime-grafted adsorbent. Fourier transform infrared analysis suggested that the amide groups were oxidized by chromium(VI) and converted to nitro groups. Furthermore, in situ X-ray absorption fine structure analysis revealed that most of the chromium(VI) was reduced to chromium (III) relatively early during the adsorption process, followed by gradual changes in the bonding structure of the amidoxime-chromium complex. Although more than one day was required to completely stabilize the structure of chromium(III), this approach provides a safe method for chromium(VI) removal. Furthermore, because wastewater can be treated via simple circulation with this method, it is easier to operate than coagulation sedimentation or electrochemical methods. Thus, it is expected to contribute to the development of practical technologies for wastewater treatment.