Speciation of the rare earth element neodymium in groundwaters of the Nevada Test Site and Yucca Mountain and implications for actinide solubility

• Published in: Applied geochemistry 1995, Vol.10 (5), p.565-572
• Main Authors: Johannesson, Kevin H., Stetzenbach, Klaus J., Hodge, Vernon F.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Freshwater; Hydrogeology; Hydrology. Hydrogeology; Pollution, environment geology
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/0883-2927(95)00028-3

Concentrations of the rare earth element (REE) Nd and major ions were measured in groundwater samples obtained from 4 wells on the Nevada Test Site and one well located 3 miles east of the proposed high-level nuclear waste repository at Yucca Mountain, adjacent to the Nevada Test Site. The speciation of dissolved Nd was modelled using a combined specific ion interaction (i.e., Pitzer Model) and ion pairing model. The model provided a means of determining the activity coefficients of the Nd 3+ ion (ym), complexing anions (γx), and Nd 3+ -anion ion pairs (γM MX). The modelling indicated that between 92% and > 99% of dissolved Nd is complexed with CO 3 2− as either carbonato (NdCO 3 +) or dicarbonato (Nd(CO 3) 2 −) complexes. An elevated Nd concentration in one of the groundwater samples was associated with higher calculated free CO 3 2− concentrations (i.e., [CO 3 2−] F, where the subscript F denotes free concentrations). Based on the relatively large stability constants for Nd CO 3 complexes as compared with stability constants for other inorganic ligands, it is proposed that elevated [CO 3 2−] F concentrations in groundwaters can result in enhanced dissolved Nd concentrations. The analogous chemical behavior of the trivalent REEs, especially Nd 3+, and the trivalent actinide series elements (Am 3+, Cm 3+, Cf 3+), suggests that these actinides will be primarily inorganically complexed with CO 3 2− in the local groundwaters and that elevated [CO 3 2−] F concentrations may increase the ability of these groundwaters to solubilize and transport these actinides.