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

Adsorption of uranium as uranyl ion UO₂²⁺ was investigated on the surface of pretreated refinery wastewater sludge. The sludge was chemically activated using different acids (HCl, HNO₃, H₂SO₄, H₃PO₄), and its uranium uptake capacity from aqueous solution was checked after each activation process. The highest uptake capacity was obtained for sludge treated by phosphoric acid followed by washing with Na₃PO₄ (0.2 M). The effect of pH, point of zero charge pH pzc, uranyl ion concentration and adsorbent dose on adsorption processes were studied. The adsorption is high in the pH range of 3.5–7.2 with optimum value at pH 5.3. The point of zero charge was found to be at pH 3.95. The adsorption capacity increases with increasing uranyl ion concentration but decreases with increasing sludge dose. The equilibrium studies showed that the adsorption fits Langmuir adsorption isotherm. Also, the kinetic studies showed that the adsorption is pseudo-second order with activation energy 31.15 kJ/mol, indicating that the process is chemical in nature. The activated sludge showed high selectivity for UO₂²⁺ among Zr(IV), Fe(III), Cr(III), Gd(II), Sr(II), Co(II), Ca(II), Cs(I) and Na(I) ions.