Influence of Boom Clay organic matter on the adsorption of Eu³⁺by illite – geochemical modelling using the component additivity approach

Abstract

Europium / Sorption modelling / Natural organic matter / Boom Clay / Complexation / Component additivitySummary. The solid-liquid distribution of europium (Eu) between an adsorptive surface and a solution phase containing a competitive colloid is the result of a delicate balance between several individual chemical reactions. In this study, adsorption isotherms of Eu in presence of dissolved Boom Clay natural organic matter were experimentally determined under conditions relevant for a geological repository (trace Eu concentrations, anoxic conditions, ∼ 0.014 mol l −1 NaHCO 3 background electrolyte). It was found that both the concentration and size distribution (or operational cut-off used to discriminate between "mobile" and "immobile" colloids) of natural organic matter has a strong influence on the observed solid-liquid distribution.The experimental data were subsequently modelled using a component additive approach with two well-established sorption/interaction models: the 2 SPNE SC/CE model for describing Eu adsorption on illite, and Humic Ion-Binding Model VI for describing Eu complexation to natural organic matter. Model parameters were gathered from dedicated measurements in batch systems containing only Eu and the interacting phase under study, under similar conditions as in the ternary isotherm experiments. Mutual interactions between illite and natural organic matter were studied and quantified. Under the experimental conditions of this study, it was found that these interactions were only of minor importance.The two models were subsequently combined to blind predict the Eu solid-liquid distribution in the ternary batch experiments. Within an error margin of 0.5 log K d units, the additivity approach succeeded well in predicting Eu uptake in all experimental systems studied. A sensitivity analysis was performed to select the most important model parameters influencing the Eu uptake, and the robustness of the model. This study has shown that the component additivity approach for describing and predicting uptake of trivalent lanthanides/actinides under Boom Clay conditions, is promising, and may help in unraveling the complex behaviour of these radionuclides witnessed in migration experiments.

Mineral–Water Interface Reactions of Actinides

“…Naturally occurring organic carboxylic acids such as humic and fulvic acids (or respective model ligands) also have strong complexing properties toward trivalent actinides and as a result influence sorption when present. ,,− Depending on the ratio/amount of organic matter sorbed at the mineral surface and in solution, its overall effect on metal ion sorption strongly depends on pH. At low pH, where significant amounts of organic matter are bound to the surface, an increase in metal ion sorption is generally observed.…”

Modelling of a large-scalein-situmigration experiment with14C-labelled natural organic matter in Boom Clay

“…These batch sorption data confirm the results from the migration experiments. Recent NOM sorption experiments on illite (major component in Boom Clay) confirmed a small but significant linear sorption, K D (NOM) ∼ 3-30 l kg −1 [18].…”

Sn(IV) Sorption onto Illite and Boom Clay: Effect of Carbonate and Dissolved Organic Matter

“…The so-called component additivity approach (CAA), or bottom-up approach, allows the use of data obtained on pure clay systems to be transferred to a range of conditions and host rock compositions, saving resources otherwise needed for studying each individual system. The CAA is a predictive modelling which combines, in an additive way, the sorption properties of the individual minerals constituting the natural system accounting for their relative concentrations [6,7,15]. Over the various models available in literature to describe the adsorption of metals and radionuclides on clay minerals, the quasi-mechanistic 2-site protolysis non-electrostatic surface complexation and cation exchange model (2SPNE SC/CE) developed by Bradbury and Baeyens [15] is largely used in the context of nuclear waste disposal.…”

“…The CAA is a predictive modelling which combines, in an additive way, the sorption properties of the individual minerals constituting the natural system accounting for their relative concentrations [6,7,15]. Over the various models available in literature to describe the adsorption of metals and radionuclides on clay minerals, the quasi-mechanistic 2-site protolysis non-electrostatic surface complexation and cation exchange model (2SPNE SC/CE) developed by Bradbury and Baeyens [15] is largely used in the context of nuclear waste disposal. For clay-rich host rocks, the CAA approach, based on the 2SPNE SC/CE model, was successfully applied to predict the sorption of several elements on the Hungarian Boda claystone [7,16], the French Callovo-Oxfordian (COx) claystone [8,17] and the Swiss Opalinus Clay [7,16].…”