Influence of Sulfate and Nitrate for Lanthanum (III) Adsorption on Bentonite: Implications for Rare Earth Wastewater Disposal

Abstract

The direct discharge of rare earth wastewater causes the waste of resources and heavy metal pollution. This paper compared the adsorption behaviors of lanthanide ions on bentonite under sulfate and nitrate systems by examining the factors affecting the adsorption, such as adsorption time, pH, background electrolyte concentration, and initial rare earth ion concentration. It was shown that the sulfate system was more favorable for the adsorption of rare earth ions on the bentonite surface. The maximum adsorption capacity in the sulfate system was about 1.7 times that in the nitrate system. In contrast, the adsorption under the nitrate system was more sensitive to the changes in pH and background electrolyte concentration. The adsorption processes under both systems are spontaneous physical adsorption processes (ΔGθ are from −27.64 to −31.48 kJ/mol), and both are endothermic (ΔHθ are 10.38 kJ/mol for the nitrate and 7.53 kJ/mol for the sulfate) and entropy-increasing (ΔSθ are 61.54 J/mol for the nitrate and 76.24 J∙mol−1 for the sulfate) processes. This study helps to provide information about the optimizing process parameters for the adsorption treatment of rare earth wastewater using bentonite.