Uranyl Adsorption onto Hydrous Ferric Oxide-A Re-Evaluation for the Diffuse Layer Model Database

The diffuse layer model (DLM) database of Dzombak and Morel was developed to quantify the adsorption of dissolved species onto the hydrous ferric oxide (HFO) surface, and contained numerous surface complexation reactions, including surface complexation reactions for uranyl (UO2+2) consisting of Hfo_sOUO(2)(+) and Hfo_wOUO(2)(+). However, these constants were not based upon experimentally obtained data, but rather were derived from linear free energy relationships (LFER) using log K-MOH values. When compared to experimental data, the LIFER-derived constants for uranyl were shown to overestimate adsorption by a factor of 10 in some cases. At least 14 uranyl HFO data sets have been previously published and were used to re-estimate constants by coupling the geochemical computer code PHREEQC with UCODE_2005, an automated parameter optimization program. Five uranyl-bearing surface complexation reactions were initially evaluated; the constants were optimized by allowing UCODE to incrementally vary selected log K-x(int) values until the best fit to the experimental data was obtained. Assumptions consistent with the original DLM were retained. Changes to the K-1(int) and K-2(int) constants, and addition of uranyl monocarbonate and uranyl dicarbonate surface complexes, will update and correct the uranyl sorption reactions in this widely used database.