Comparison of Approaches to Calibrate a Surface Complexation Model for U(VI) Sorption to Weathered Saprolite

Abstract

A surface complexation model describing the sorption of uranyl ions and uranyl carbonate on weak and strong sites was used to analyze experiments conducted on pH-dependent U(VI) sorption to weathered shale/limestone saprolite. Sorption data were collected at two different solid to solution ratios. Various methods of estimating equilibrium reaction coefficients and site densities were investigated. As a first approximation, extractable iron oxides were assumed to behave as ferrihydrite with reaction coefficients as reported by Waite (Geochim Cosmochim Acta 58:5465-5478, 1994). A generalized composite (GC) approach was then employed with coefficients estimated by an inverse modeling method applied both in a stepwise fashion and simultaneously to whole data set. Uncertainty in model parameters and predictions was lowest using the simultaneous inverse method, but results from the stepwise method were very similar. The generalized reaction network accurately described pH-dependent U(VI) sorption on weathered saprolite between pH 4 and 9.

Kinetic analysis and modeling of oleate and ethanol stimulated uranium (VI) bio-reduction in contaminated sediments under sulfate reduction conditions

“…Previously, we determined the values for log K e1 , log K e2 , log K e3 , and log K e4 to be −2.64, −6.84, −13.68, −17.28, respectively, and strong and weak sorbing site densities to be 0.0018 mole sites/mole Fe(III) and 0.8732 mole sites/mole Fe(III), respectively, for the sediment from the studied site [41]. In addition, a surface complexation model describing sulfate sorption [42] was introduced to model.…”

Prediction of uranium and technetium sorption during titration of contaminated acidic groundwater

“…In the investigated groundwater system, dissolved Fe concentrations are low relative to Al. However, because other areas of the study site have high Fe levels which are expected to play an important role in sorption of uranium [16], Fe is included in the model in this study. Model M1 considers aqueous complexation reactions of carbonate, sulfate, nitrate, chloride, K, Ca, Mg, Mn, Fe, Al, Si [26] and U [27] (details are provided in Table A.1), and precipitation/dissolution reactions (Table 1).…”

“…Sorption of TcO 4 − on Mg/Al layered hydroxides was found to be a stepwise ion exchange process [15]. Sorption of U(VI) onto Fe hydroxides has been extensively modeled [16,17]. However, reliable thermodynamic data are not available to simulate sorption of U(VI) by Al hydroxides, or sorption of Tc by Fe or Al hydroxides.…”

Mathematical Formulation Requirements and Specifications for the Process Models

“…Based on a measurement of the relative amounts or surface areas of each mineral present in the soil or sediment, sorption by the mixture of phases can be predicted by an equilibrium calculation, without any fitting of experimental data for the mixture. CA model predictions are sometimes made by assuming that one mineral component dominates sorption (Barnett et al, 2002;Davis et al, 2004;Payne et al, 2004;Zhang et al, 2009), allowing a straightforward equilibrium calculation, if the exposed surface area of that mineral component in the soil or sediment can be quantified.…”