Behavior of Lanthanides in Soils: Praseodymium as a Particular Case

The behavior of praseodymium in a forest soil with different quantities of organic matter was investigated. The forest soil (FS) was treated with acid and alkali solutions to separate the soluble organic matter (fulvic and humic acids) (IM-FS), and then IM-FS was treated with hydrogen peroxide (IM-FS(b)). The materials (FS, IM-FS, and IM-FS(b)) were characterized by X-ray diffraction (XRD), scanning electron microscopy(SEM)-EDS, and Fourier-transformed infrared spectroscopy (FT-IR). The points of the zero charge (PZC) and the organic carbon percentages were also determined. The effect of the organic matter, contact time, praseodymium concentration, temperature, and praseodymium desorption were evaluated. The minerals found in the materials were quartz, albite, and kaolinite; the PZC were 6.12, 7.28, and 6.11 for FS, IM-FS, and IM-FS(b), respectively, and their organic carbon percentages were 9.1 ± 0.1, 7.2 ± 0.6, and < 0.1, respectively. The adsorption kinetic data for FS and IM-FS were best adjusted to the pseudo second-order model, and the isotherms data to the Langmuir–Freundlich model indicating a chemical adsorption mechanism on heterogeneous materials, the maximum adsorption capacities were 18.53, 12.98 mg/g and virtually negligible for FS, IM-FS, and IM-FS(b) respectively. The adsorption processes were exothermic; the desorption processes showed that praseodymium is more stable in FS than IM-FS, and Na₂H₂EDTA was the best desorbent agent. The adsorption capacities and praseodymium stability depend on the organic matter content. The adsorption capacities followed the order: FS > IM-FS > IM-FS(b).