Kinetics of selenate sorption in soil as influenced by biotic and abiotic conditions: a stirred flow-through reactor study

Influence of non-equilibrium and nonlinear sorption of 137Cs in soils. Study with stirred flow-through reactor experiments and quantification with a nonlinear equilibrium-kinetic model

This paper addresses the modelling of cesium sorption in non-equilibrium and nonlinear conditions with a two-site model. Compared to the classical K_d approach, the proposed model better reproduced the breakthrough curves observed during continuous-flow stirred tank reactor experiments conducted on two contrasted soils. Fitted parameters suggested contrasted conditions of cesium sorption between 1) equilibrium sites, with low affinity and high sorption capacity comparable to CEC and 2) non-equilibrium sites, with a fast sorption rate (half-time of 0.2-0.3 h), a slow desorption rate (half-time of 3-9 days) and a very low sorption capacity (0.02-0.04% of CEC). Comparison of EK sites densities with sorption capacities derived from the literature suggests that the EK equilibrium and kinetic sites might correspond to ion exchange and surface complexation of soil clay minerals respectively. This work stresses the limits of the K_d model to predict ¹³⁷Cs sorption in reactive transport conditions and supports an alternative non-equilibrium nonlinear approach.

Influence of non-equilibrium sorption on the vertical migration of 137Cs in forest mineral soils of Fukushima Prefecture

Sorption hypotheses and models are required for the prediction of ¹³⁷Cs migration in soils contaminated after nuclear reactor accidents and nuclear weapons tests. In assessment models, the K_d (distribution coefficient) hypothesis for sorption, which assumes that sorption is instantaneous, linear and reversible, has often been coupled with the convection-diffusion equation (CDE) to model ¹³⁷Cs migration. However, it fails to describe ¹³⁷Cs migration velocities which often decrease with time. Alternative equilibrium-kinetic (EK) hypotheses of ¹³⁷Cs sorption/desorption have been suggested by laboratory experiments but have not been fully validated in field conditions. This work addressed the influence and magnitude of non-equilibrium ¹³⁷Cs sorption in field conditions by reinterpreting, with an inverse approach, series of ¹³⁷Cs profiles measured in mineral soils of forest plots located in Fukushima Prefecture (2013-2018). Our results show that the inclusion of non-equilibrium sorption significantly improves, compared to the equilibrium hypothesis, the realism of simulated ¹³⁷Cs profiles. Fitted sorption parameters suggest a fast sorption kinetic (half-time of 1-7 h) and a pseudo-irreversible desorption rate (half-time of 3.2 × 10⁰-3.4 × 10⁶ years), whereas equilibrium sorption (4.0 × 10^-3 L kg^-1 on average) only affects a negligible portion of ¹³⁷Cs inventory. By June 2011, such EK parameters fitted on our plots realistically reproduced profiles measured in the same forest study site (Takahashi et al., 2015). Predictive modeling of ¹³⁷Cs profiles in soil suggests a strong persistence of the surface ¹³⁷Cs contamination by 2030, with exponential profiles consistent with those reported after the Chernobyl accident. This study demonstrates that hypotheses and parameters of ¹³⁷Cs sorption can be partially inferred from in situ measurements. However, further experiments in controlled conditions are required to better estimate the sorption parameters and to identify the processes behind non-equilibrium sorption.

Adsorption/Desorption Patterns of Selenium for Acid and Alkaline Soils of Xerothermic Environments

Selenium adsorption/desorption behavior was examined for eight Greek top soils with different properties, aiming to describe the geochemistry of the elements in the selected soils in terms of bioavailability and contamination risk by leaching. Four soils were acid and four alkaline, and metal oxides content greatly differed between the two groups of soils. The concentrations of Se(IV) used for the performed adsorption batch experiments ranged from 1 to 50 mg/L, while the soil to solution ratio was 1 g/0.03 L. Acid soils adsorbed significantly higher amounts of the added Se(IV) than alkaline soils. Freundlich and Langmuir equations adequately described the adsorption of Se(IV) in the studied soils, and the parameters of both isotherms significantly correlated with soil properties. In particular, both KF and qm values significantly positively correlated with ammonium oxalate extractable Fe and with dithionite extractable Al and Mn, suggesting that amorphous Fe oxides and Al and Mn oxides greatly affect exogenous Se(IV) adsorption in the eight soils. These two parameters were also significantly negatively correlated with soil electrical conductivity (EC) values, indicating that increased soluble salts concentration suppresses Se(IV) adsorption. No significant relation between adsorbed Se(IV) and soil organic content was recorded. A weak salt (0.25 M KCl) was used at the same soil to solution ratio to extract the amount of the adsorbed Se(IV) that is easily exchangeable and thus highly available in the soil ecosystem. A much higher Se(IV) desorption from alkaline soils was observed, pointing to the stronger retention of added Se(IV) by the acid soils. This result implies that in acid soils surface complexes on metal oxides may have been formed restricting Se desorption.

Fate of selenium in soil: A case study in a maize (Zea mays L.) field under two irrigation regimes and fertilized with sodium selenite

Selenium (Se) is a trace element necessary for both human and livestock nutrition. To increase Se human intake, soil Se fertilizations were performed but the fate of the added Se remains unclear. The present research aims to: (1) determine the influence of Se fertilization on the fractionation of Se in soil; (2) assess the influence of water availability on the distribution of soil Se chemical fractions; and (3) monitor the Se content in soil, leachates and plants. To reach these goals, 200 g Se ha^-1 was applied to soil as sodium selenite in maize crops under two irrigation regimes, and the Se content in plant, soil chemical fractions and leachates were analyzed. Se application increased the total Se content of the soil, specifically it increased the Se content of the soluble, exchangeable and organic fractions with more pronounced effect in the soils with higher water availability. These differences disappeared over time likely due to the Se loss through volatilization. The hypothesis of Se volatilization is confirmed by the absence of both leachates during the maize growing season and differences among the treatments of Se content in sub-soil samples. Also, although the Se treated plants showed higher Se content than the untreated ones, overall <1% of the added Se was assimilated by plants. Hence, this study demonstrated that the addition of selenite to the soil increased the Se contents of the plants, but the Se does not accumulate in the soil because it is likely lost via volatilization. Further, leaching of Se into groundwater is avoided due to its association with both the soil organic matter and positively charged binding sites of soil, and due to its loss via volatilization. Therefore, soil Se fertilization could increase the nutritional value of plants without consequences on the environment.

Extended Kd distributions for freshwater environment

Many of the freshwater K_d values required for quantifying radionuclide transfer in the environment (e.g. ERICA Tool, Symbiose modelling platform) are either poorly reported in the literature or not available. To partially address this deficiency, Working Group 4 of the IAEA program MODARIA (2012-2015) has completed an update of the freshwater K_d databases and K_d distributions given in TRS 472 (IAEA, 2010). Over 2300 new values for 27 new elements were added to the dataset and 270 new K_d values were added for the 25 elements already included in TRS 472 (IAEA, 2010). For 49 chemical elements, the K_d values have been classified according to three solid-liquid exchange conditions (adsorption, desorption and field) as was previously carried out in TRS 472. Additionally, the K_d values were classified into two environmental components (suspended and deposited sediments). Each combination (radionuclide x component x condition) was associated with log-normal distributions when there was at least ten K_d values in the dataset and to a geometric mean when there was less than ten values. The enhanced K_d dataset shows that K_d values for suspended sediments are significantly higher than for deposited sediments and that the variability of K_d distributions are higher for deposited than for suspended sediments. For suspended sediments in field conditions, the variability of K_d distributions can be significantly reduced as a function of the suspended load that explains more than 50% of the variability of the K_d datasets of U, Si, Mo, Pb, S, Se, Cd, Ca, B, K, Ra and Po. The distinction between adsorption and desorption conditions is justified for deterministic calculations because the geometric means are systematically greater in desorption conditions. Conversely, this distinction is less relevant for probabilistic calculations due to systematic overlapping between the K_d distributions of these two conditions.

Identifiability of sorption parameters in stirred flow-through reactor experiments and their identification with a Bayesian approach

This paper addresses the methodological conditions -particularly experimental design and statistical inference- ensuring the identifiability of sorption parameters from breakthrough curves measured during stirred flow-through reactor experiments also known as continuous flow stirred-tank reactor (CSTR) experiments. The equilibrium-kinetic (EK) sorption model was selected as nonequilibrium parameterization embedding the K_d approach. Parameter identifiability was studied formally on the equations governing outlet concentrations. It was also studied numerically on 6 simulated CSTR experiments on a soil with known equilibrium-kinetic sorption parameters. EK sorption parameters can not be identified from a single breakthrough curve of a CSTR experiment, because K_d,1 and k^- were diagnosed collinear. For pairs of CSTR experiments, Bayesian inference allowed to select the correct models of sorption and error among sorption alternatives. Bayesian inference was conducted with SAMCAT software (Sensitivity Analysis and Markov Chain simulations Applied to Transfer models) which launched the simulations through the embedded simulation engine GNU-MCSim, and automated their configuration and post-processing. Experimental designs consisting in varying flow rates between experiments reaching equilibrium at contamination stage were found optimal, because they simultaneously gave accurate sorption parameters and predictions. Bayesian results were comparable to maximum likehood method but they avoided convergence problems, the marginal likelihood allowed to compare all models, and credible interval gave directly the uncertainty of sorption parameters θ. Although these findings are limited to the specific conditions studied here, in particular the considered sorption model, the chosen parameter values and error structure, they help in the conception and analysis of future CSTR experiments with radionuclides whose kinetic behaviour is suspected.