Phosphate functionalised titania for heavy metal removal from acidic sulfate solutions

Adsorbent materials based on titania and phosphate are ideal for treatment of solutions contaminated with heavy metals under acidic conditions, due to their inherent chemical stability and low pKa. Herein, phosphate functionalised titania has been investigated for the first time for removal of heavy metals (Cr, Fe, Cu, Eu, U) under conditions relevant to acid mine drainage (pH 2-5 sulfuric acid). Successful functionalisation was found to depend on the phase of titania used, with anatase preferred according to computational results from density functional theory. The effect of phosphate ligand structure was explored, revealing that the phosphate ethyl ester maximised heavy metal removal. The presence and concentration of counterions (sulfate, nitrate, ammonium) also impacted the speciation and binding of heavy metal cations, demonstrating the importance of adsorbent testing under realistic conditions. Increasing the porosity of the titania framework enhanced heavy metal removal, while maintaining selectivity for the toxic heavy metals over non-toxic cations Na and K. As such, phosphate functionalised titania shows great promise for heavy metal remediation in acidic sulfate environments.