Effective removal of cesium from wastewater solutions using an innovative low-cost adsorbent developed from sewage sludge molten slag

This study investigates the effective removal of cesium (Cs) from aqueous solution using sewage sludge molten (SSM) slag that has undergone the surface modification with alkali (NaOH) hydrothermal treatment. The raw and modified slags were characterised systematically using the BET method, the FESEM, the XRF, the XRD spectroscopy and the CEC analysis to understand the physicochemical changes of the materials, and its sensitivity to Cs ions adsorption. Batch adsorption experiments were carried out to investigate the effects of adsorbent dose, contact time, solution pH, different initial Cs concentrations, temperature and the effect of competitive ions on Cs adsorption. The adsorption isotherm, kinetic and thermodynamic studies were also evaluated based on the experimental results. A higher Cs removal efficiency of almost 100% (for 20-100 mg/L of initial concentration) was achieved by the modified SSM slag, and the maximum adsorption capacity was found to be 52.36 mg/g. Several types of synthetic zeolites such as zeolite X, zeolite Y, zeolite A, and sodalite were formed on surface of the modified slag through the modification process which might be enhanced the Cs adsorption capacity. Kinetic parameters were fitted by the pseudo-second order model. The adsorption isotherms data of modified slag were well-fitted to the Langmuir (R-2 = 0.989) and Freundlich isotherms (R-2 = 0.988). The thermodynamic studies indicated that the adsorption process by the modified slag was spontaneous and exothermic. In the competitive ions effect, the modified slag effectively captured the Cs ion in the presence of Na+ and K+, especially at their lower concentrations. Moreover, the modified slag was reused for several cycles after the successful elution process with an appropriate eluting agent (0.5 M H2SO4), without deterioration of its original performance. Therefore, the SSM modified slag could be effectively used as a low-cost potential adsorbent for high Cs adsorption from wastewater.