Graphene oxide nanosheets for treatment of mine-drainage contaminated water: The effect of phosphate functionalisation on U(VI) removal

Acid mine drainage (AMD) poses significant challenges to fresh water supplies in mining regions globally. Not only is water made too acidic for consumption, the acidic conditions also lead to the mobilization of toxic elements including uranium. Nanomaterials have been shown to be effective for the treatment of such contaminated water. In this study, we investigated the effect of phosphate functionalisation on the removal of uranyl ions from mine-drainage contaminated water. Graphene oxide (GO) nanosheets functionalised through the Arbuzov reaction were therefore synthesized and characterized for structure, elemental composition and functionalisation. Adsorptive removal of uranyl [U(VI)] ions by the nanosheets was then examined in batch studies as a factor of time, pH, U(VI) concentration, and adsorbent concentration. X-ray diffraction confirmed the synthesis of GO nanosheets from graphite, while the presence of phosphate groups in GO was confirmed by FTIR. TEM images revealed a tendency for stacking, particularly for functionalised nanosheets. Adsorption of U(VI) from solution was found to be optimal at pH 4 for both GO and GO-PO4. However, adsorption by GO-PO4 attained equilibrium earlier and was >15% higher than that of GO, effects ascribed to the phosphate functionalisation. Importantly, even in mine-drainage contaminated water (pH 2.78) GO-PO4 out-performed GO by >30%. These data suggest that functionalisation of GO nanosheets increases their efficiency in U(VI) removal from mine-drainage contaminated water by GO. We discuss the proposed mechanisms for the mode of phosphate functionalisation.