Adsorption of U(VI) on a chitosan/polyaniline composite in the presence of Ca/Mg-U(VI)-CO3 complexes

Abstract

Ca/Mg-U(VI)-CO3 complexes have been recently proposed as dominated U(VI) species in seawater. Herein, we used a bio/conducting polymeric composite material, chitosan/polyaniline (CTS/PANI), as an efficient solid-phase extractant for adsorbing U(VI) from bicarbonate-buffer solutions containing Ca2+ and Mg2+. Batch experiments indicated that the U(VI) adsorption capacity (qe) was significantly improved after the preparation of the CTS/PANI composite, and capacities calculated from the Langmuir isotherm were approximately 204.1mg/g and 212.8mg/g in the presence of Ca-U(VI)-CO3 or Mg-U(VI)-CO3 complexes, respectively. However, the increase in initial pH, total dissolved carbonate and calcium or magnesium ion concentrations inhibit uranium adsorption. In particular, Ca/Mg-U(VI)-CO3 formation is unfavorable for uranium adsorption, whereas U(VI) adsorption behavior appears less effected by Mg2+. Besides, OH−/HCO3– showed greater effect than that of Ca/Mg-U(VI)-CO3 species. The adsorption affinity was noted in the sequence of U(VI)-CO3 complexes>Ca-U(VI)-CO3 complexes>Mg-U(VI)-CO3 complexes. The kinetic data correlates well with the pseudo-second-order kinetic model, and both the Langmuir and Freundlich isotherms describe the adsorption process. The proposed mechanism involves the simultaneous adsorption of U(VI) onto amino/hydroxyl and imine/amine functional groups that are present in the composite. Additionally, 0.1mol/L HCl solution was found to be the most effective desorption agent. The findings reported in this study highlight the effects of Ca/Mg-U(VI)-CO3 complexes when designing efficient adsorbents used for uranium resource recovery from salt lake brines and seawater, and it offers a new approach for exploring relevant U(VI) adsorption behaviors.