Selective removal of uranium from wastewater using sludge collected from refinery wastewater treatment: Equilibrium, thermodynamic and kinetics studies

•Refinery wastewater sludge was collected, treated and activated with different acidic media then used for adsorption of UO22+.•The highest adsorption capacity was found for the activated sludge by phosphoric acid then washed by Na3PO4.•The equilibrium and kinetic studies for adsorption processes were carried out.•The activated sludge with phosphoric acid showed high selectivity for uranyl ion.