Potentiality of low-temperature carbides from excess sludge to recover low-concentration rare earth ions: Isotherm, kinetic and thermodynamic

Abstract

A low-temperature carbonization technique conducted under limited oxygen has been used to recycle excess sludge. The obtained low-temperature carbonized sludge (LCS) has been used in adsorption experiments with low-concentration rare-earth wastewater (rare-earth ion content of 103.45 mg/L, calculated on the basis of rare-earth oxides (REO)) to identify the most suitable carbonization conditions. Moreover, a response surface method has been used to optimize the adsorption conditions, focusing on the kinetics, isotherm, and thermodynamics of the process. Finally, the rare-earth ions loaded on the LCS were desorbed to assess the regeneration performance of the material. The results showed that the maximum degree of adsorption of low-concentration rare-earth ions on LCS was 94.1% under operation conditions optimized by the response surface method. Analysis of the kinetics and isotherm of the adsorption process showed it to be controlled by both liquid film diffusion and particle diffusion, corresponding to a pseudo-first-order kinetic model and the Langmuir adsorption isotherm. Analysis of the thermodynamics of the process showed the adsorption of rare-earth ions on LCS to be a non-spontaneous, exothermic, entropy-reducing process at high temperature. The results of desorption tests showed that the optimal conditions involved the treatment of LCS (1 g) loaded with rare-earth ions with 0.5 mol/L HCl (50 mL) for 1 h. The degree of desorption remained at about 70% after six desorption and regeneration cycles. Based on characterization results, it is inferred that the adsorption is a physicochemical process involving surface adsorption supplemented by chemical complexation.