The influence of pH and temperature on the aqueous geochemistry of neodymium in near surface conditions

Abstract

Geochemical calculations were employed for the solubility and speciation of neodymium in a model soil solution as a function of pH. The calculations were based on the recently determined stability constants for Nd and solubility product for the Nd end-member of mineral monazite (NdPO(4)). Simulations were carried out at near neutral pH (pH 6.0 to pH 7.5) and 25 degrees C at the atmospheric CO(2) partial pressure. Additional calculations were also performed to assess Nd mobility at the extreme temperature conditions (300 degrees C) at neutral pH. Our results suggest that relatively dilute (Ionic Strength, I = 0.1), low-temperature waters may transport very small quantities of rare earth elements and actinides to the surficial environment at near neutral pH conditions. Evidently, higher temperature or extreme fluid composition may have a greater potential for mobilization of these elements. The results are pertinent to researchers interested in engineering applications for the precipitation of Nd and surrogate actinides from aqueous nuclear wastes.

Biogeochemical Mobility of Contaminants from a Replica Radioactive Waste Trench in Response to Rainfall-Induced Redox Oscillations

“…However, both the adsorption of Nd complexes trench minerals and precipitation of secondary Nd-containing solids were likely to be important contributors. The rapid removal immediately after injection aligns with research, which has shown that Nd (and contaminant counterparts Pu and Am) strongly sorb to a variety of mineral phases including phosphates and carbonates 28,81 as well as iron and manganese oxides. 82,83 Although the system is unlikely to be at thermodynamic equilibrium, such calculations may provide insight into the processes responsible for the Nd concentrations within the trench.…”

“…In addition to sorptive processes, precipitation of secondary Nd phases, particularly Nd phosphate and to a lesser extent Nd carbonate, could also account for the lower concentrations of dissolved Nd measured in the trench (relative to Sr and Cs). Thermodynamic calculations predicted that even in the presence of only minor phosphate concentrations (Table S3.1), Nd almost entirely partitioned as the oversaturated solid-phase NdPO 4 (Figure S3.6), a mineral known to readily precipitate in near-surface environments …”

“…Although this process likely resulted in limited immediate mixing and dispersion of contaminants, the procedure attempted to recreate a waste container breach. Trivalent lanthanides (e.g., Nd) are appropriate analogues for the actinides present in the LFLS legacy waste trenches (Pu and Am), due to their similar size and oxidation states. ,, The analogues were added in somewhat elevated concentrations given the uncertainty of their untested environmental behavior. The concentrations at the point of release (i.e., 2.8 m b.g.l.)…”

Review and Assessment of Organic and Inorganic Rare Earth Element Complexation in Soil, Surface Water, and Groundwater

“…Cteiner [42] , oxalate, and acetate on monazite solubility. At pH levels ranging from 6.0 to 6.5, Nd (oxalate) was the dominant species, followed by Nd 3+ and NdSO 4 + .…”

Lanthanide Soil Chemistry and Its Importance in Understanding Soil Pathways: Mobility, Plant Uptake, and Soil Health

“…Cteiner [27] , oxalate and acetate on solubility. At pH levels ranging from 6.0 to 6.5 Nd(oxalate) was the dominant species, followed by Nd 3+ and NdSO 4 + .…”