Hyper sorption capacity of raw and oxidized biochars from various feedstocks for U(VI)

Abstract

• Increased surface area and sorption capacity of biochars pyrolysed above 700 °C. • U(VI) to U(IV) reduction by non-oxidized biochars. • Oxidation of biochars eliminates reductive surface moieties. • Sorption of U(VI) on oxidized biochars by surface carboxylic moieties. The removal of uranium from aqueous solutions by biochars produced from different feedstocks at increased temperatures (850 °C), such as malt spent rootlets, coffee espresso residue, and olive kernels has been investigated by means of batch-type experiments. The sorption experiments were performed prior and after oxidation of the biochars to compare their removal capacities and the materials were characterized by Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy and X-ray diffraction (XRD) measurements to identify surface species and solid phases, respectively. In addition, surface oxidation increases further the sorption capacity due to the formation of carboxylic moieties presenting increased affinity for the U(VI) cations. However, due to oxidation the reducing moieties on the biochar surface are eliminated and the reduction of U(VI) to U(IV), which precipitates, decreases resulting in an apparent decreased sorption capacity.