pH-Dependent Adsorption of La³⁺ and Y³⁺ on a Graphitic Carbon Nitride-Derived Material: Insights from Potentiometric Titration

Rare earth elements (REEs) are strategic resources essential for advanced technologies, but their recovery is hindered by low concentrations in mineral ores and the environmental impacts of conventional industrial production methods. This study evaluated the adsorption of La³⁺ and Y³⁺ from an equimolar binary solution using a graphitic carbon nitride-derived material, focusing on the influence of pH. Adsorption experiments were conducted at pH 2.0, natural pH (≈4.9), and pH 10.0. Potentiometric titration revealed four surface functional groups (pKa = 5.4, 6.2, 8.7, and 10.2), demonstrating the strong pH dependence of the adsorbent surface chemistry. At pH 2.0 and 4.9, surface protonation resulted in low adsorption. At pH 10.0, complete metal removal was observed; however, the hydrolysis and precipitation of La(OH)₃ and Y(OH)₃ also contributed to this result. These findings show that removal efficiency depends on both adsorbent surface chemistry and metal ion speciation in solution.