Modeling metal ion-humic substances complexation in highly saline conditions

Abstract

International audienceBecause highly saline groundwaters are found at potential repository sites for nuclear waste, geochemical models should predict the speciation of relevant radionuclides in brines, including their complexation with substances such as humic acids (HA). In this study, available experimental radionuclide-HA complexation data in high 1:1 background electrolyte solutions (0.01 < mNaCl/NO3/ClO4mNaCl/NO3/ClO4 < 4 molal, m) are reviewed. Discrepancies in the amplitude of ionic strength effects on radionuclide-HA complexation are observed, which might depend on the nature of the interacting radionuclide or on the origin of HA. However, significant differences in the experimental conditions and calculations applied to determine conditional metal ion-HA complexation constants hamper direct comparison between these datasets. To clarify whether metal ion-HA binding in saline solutions can be described, two sophisticated humic-ion binding models (Model VII and NICA-Donnan) are presently used. This is the first time that Model VII and NICA-Donnan are applied to predict metal ion-HA binding at high ionic strength (I > 1 m). The advantage of these models, compared to more simple ones (e.g., the polyelectrolyte or the charge neutralization models), is that both electrostatic and chemical contributions to the overall metal ion-HA binding are explicitly taken into account. Model VII and NICA-Donnan are shown to produce very similar results. Trends in conditional metal ion-HA binding constants and in the maximum metal ion uptake by HA (e.g., the loading capacity) with I agree with experiments. The present data evaluation suggests that most of the apparent discrepancies between various experimental datasets arise from differences in the experimental conditions. Both Model VII and NICA-Donnan predict that the specific ion interaction theory (SIT) parameters for metal ion-HA systems, which are required for high ionic strength with more simple models, vary with pH and metal loading. Overall, Model VII and NICA-Donnan are able to account for various mechanisms involved in metal ion-HA complexation, including the metal loading effects and cation competition, and might be helpful predictive tools for performance safety assessment up to highly saline conditions

Modeling rare earth elements binding to humic acids with model VII

“…In agreement with the preliminary work of Carbonaro and Di Toro (2007), in which only three REE were studied, linear equations involving an intercept provided the best prediction of REEmonodentate ligands binding, confirming that both inner-and outer-sphere complexation contribute to REE-monodentate ligand complexation (Choppin, 1997;Carbonaro and Di Toro, 2007). Derived shown to model accurately the effect of high ionic strengths (Marsac et al, 2017), REE patterns could also be predicted in highly saline waters. Finally, Model VII can also predict REE-HA binding patterns in the presence of ligands such as carbonates, although further work is required in order to account for the formation of (i) ternary carbonate-REE-HA complexes for the whole REE group and…”

“…A thorough description of Model VII can be found in Tipping et al (2011), and its implementation in PHREEQC is described in Marsac et al (2017…”

“…where P is an adjustable parameter, z is the ion charge, Z is the net humic acid charge (eq g -1 ), I is the ionic strength (M), ψ is the surface potential (V), T is the temperature (K), F is the Faraday constant and R is the gas constant. This equation cannot be used in PHREEQC but Marsac et al (2017) showed that it is equivalent to a constant capacitance model (CCM), with a capacitance (C):…”

Rare Earth Elements Geochemistry and Nd Isotopes in the Mississippi River and Gulf of Mexico Mixing Zone

“…(2) the model predictions agree well with the results of ultrafiltration experiments; and (3) sensitivity analysis indicates that the model correctly predicts the impact of changing ionic strength and competing cations (i.e., Fe 3+ , Al 3+ ) on REE complexation with natural organic ligands (Tang and Johannesson, 2003;Pourret et al, 2007;Marsac et al, 2017).…”

Determination of complex formation constants of neptunium(V) with propionate and lactate in 0.5–2.6 m NaCl solutions at 22–60°C using a solvent extraction technique

“…Indeed, illite is the most important adsorbent for radionuclides in OPA and COx [43]. PHREEQC can perform aqueous speciation calculations under dilute to highly saline conditions with SIT, and has previously been applied in surface complexation modeling studies at various ionic strengths [44][45][46]. The 2-site protolysis non-electrostatic surface complexation and cation exchange model (2SPNE SC/CE model [47]) was used to simulate Np sorption to illite.…”