Impact of cementitious leachate on sorption of various iodine species to sediments

Abstract

Cementitious materials are commonly used for the long-term disposal of low-level radioactive waste (LLW) in subsurface engineered disposal facilities. Iodine-129 is a key-risk driver, along with ⁹⁹Tc and ¹³⁷Cs, in proposed low- and high-level waste disposal facilities in the United States. The distribution coefficient (K_d; concentration ratio of I_solid/I_liquid) is one of the key parameters used for risk assessments to estimate the risk posed by radioactive waste disposal facilities. The objective of this study is to examine iodine (I) K_d values in sandy and clayey sediment environments impacted by cementitious leachate. Batch sorption experiments were conducted using subsurface sediments from Savannah River Site (SRS) for three iodine species (iodide (I⁻), iodate (IO₃⁻) and organo-iodine (org-I)). Besides the SRS groundwater (pH = 5.5), three background solutions were used to simulate the cementitious leachate in different stages of cement aging, including cementitious leachate from a freshly prepared grout (Stage I of cement aging), Ca(OH)₂-saturated solutions (Stage II, moderately aged grout leachate), and CaCO₃-saturated solutions (Stage III, aged grout leachate). Sorption of iodide (the most common iodine species) to the sandy and clayey sediments was generally undetected in most solutions, except in groundwater (0.9–5.6 L/kg). Iodate K_d values ranged from 2.1 to 14 L/kg with sandy sediment and from 3.9 to 54 L/kg with clayey sediment, with the same background solution rankings for both sediments: Stage I < Stage III < Stage II < groundwater. Org-I demonstrated the strongest sorption to both sediments, varying from 20 to 149 L/kg to sandy sediments and from 17 to 117 L/kg on clayey sediments. Compared to groundwater conditions, the three iodine species generally demonstrated decreased sorption in the presence of cementitious leachate-impacted sediments. An important exception was org-I, which had previously been shown to comprise 45 % of the total iodine released from grout waste forms. It generally had enhanced sediment sorption in the presence of Stage II Ca(OH)₂ and Stage III CaCO₃ simulant. The fraction of iodine sorbed in the presence of cementitious leachate simulant with respect to the concentration sorbed under background groundwater conditions (calculated as K_d-CementLeach/K_d-GW) was 0.02–0.23 for iodide, 0.07–0.88 for iodate, and 0.14–5.29 for org-I. These results provided novel insight into the importance of accounting for the change in water chemistry in nuclear waste disposal systems containing cementitious engineered barriers. Neglecting to do so may result in the underestimation of the long-term risk posed by radioiodine to the environment and human health. Additionally, these results underscored the divergent geochemical behavior of the three dominant iodine species in waste disposal environments.