Competitive ion-exchange reactions of Pb(II) (Pb2+/PbCl+) and Ra(II) (Ra2+) on smectites: Experiments, modeling, and implication for 226Ra(II)/210Pb(II) disequilibrium in the environment

Development of L-Met functionalized cellulose nanofiber for an efficient removal of Pb2+ from aqueous solution: Characterization, adsorption behavior, DFT calculations and physiochemical mechanism

Lead-containing wastewater has been a significant challenge in the field of wastewater treatment. Cellulose surface has a large number of active sites, which is conducive to load modification. And amino acids have rich functional groups, which is a good choice for cellulose modification. In this study, the non-toxic L-methionine modified cellulose nanofiber (M-CNF) adsorbent was developed to adsorb Pb2+ in aqueous solution. M-CNF exhibited a maximum adsorption efficiency of 98.24 ± 0.04 %. The theoretical adsorption capacity could reach 113.3 ± 13.4 mg g-1. After five cycles of adsorption-desorption process, the removal efficiency of Pb2+ was still >80 %. The pseudo-second-order kinetic model (R2 = 0.9913) and Brunauer-Emmet-Teller adsorption isotherm model (R2 = 0.9916) can be used to describe the adsorption behavior of M-CNF on Pb2+. The Gibbs free energy (ΔG) and enthalpy (ΔH) of the reaction are negative, indicating that the reaction was a spontaneous exothermic process. Density functional theory calculation indicated that -COOH and -C-S-C- groups played a role in the adsorption process. The adsorption mechanism of M-CNF might be the interaction of ion exchange, electrostatic adsorption, complexation and van der Waals force. This work suggested that M-CNF played a definitive role in the adsorption and removal of Pb2+ in aqueous solutions.

Influence of clay minerals on pH and major cation concentrations in acid-leached sands: Column experiments and reactive-transport modeling

A series of laboratory experiments are conducted to simulate the acidification and subsequent recovery of a sand aquifer exploited by in situ recovery (ISR) mining. A sulfuric acid solution (pH 2) is first injected into a column packed with sand from the Zoovch Ovoo uranium roll front deposit (Mongolia). Solutions representative of local groundwater or enriched in cations (Na+, Mg2+) are then circulated through the column to simulate the inflow of aquifer water. pH and major ion concentrations (Na+, Cl-, SO42-, Ca2+, Mg2+, K+) measured at the column outlet reproduce the overall evolution of porewater chemistry observed in the field. The presence of minor quantities of swelling clay minerals (≈6 wt% smectite) is shown to exert an important influence on the behavior of inorganic cations, particularly H+, via ion-exchange reactions. Numerical models that consider ion-exchange on smectite as the sole solid-solution interaction are able to reproduce variations in pH and cation concentrations in the column experiments. This highlights the importance of clay minerals in controlling H+ mobility and demonstrates that sand from the studied aquifer can be described to a first order as an ion-exchanger. The present study confirms the key role of clay minerals in controlling water chemistry in acidic environments through ion-exchange processes. In a context of managing the long-term environmental footprint of industrial and mining activities (ISR, acid mine drainage…), this work will bring insights for modeling choices and identification of key parameters to help operators to define their production and/or remediation strategies.

Modeling subsurface contaminant transport from a former open-pit uranium mine in fractured granites (La Ribière, France): Reducing uncertainties with geophysics

The long-term management of tailings from former uranium (U) mines requires an in-depth understanding of the hydrogeological processes and water flow paths. In France, most of the legacy U mines are located in fractured crystalline (plutonic) rocks, where the intrinsic subsurface heterogeneity adds to the uncertainties about the former extraction and milling activities and the state of the mine when production was ceased. U ores were mainly processed by sulfuric acid leaching, leading to high-sulfate-content mill tailings now contained in several tailing storage facilities (TSFs). The La Ribière site, located in western central France, is a former open-pit and underground U mine, closed in 1992 and used to store mill tailings. This site is being used as a test case to establish a workflow in order to explain and predict water flow and subsurface contaminant transport. A conceptual model of water flow and sulfate transport, at the scale of the La Ribière watershed, is first developed based on available information and hydrogeochemical monitoring. Recent geophysical investigations allows refining this model. Electrical Resistivity Tomography (ERT) proves to be efficient at localizing the extent of the highly conductive sulfate plume inherited from the U-mill tailings, but also at imaging the weathering profile. Magnetic Resonance Sounding (MRS), despite the limited signal intensity due to the low porosity in crystalline rocks, gives some insight into the porosity values, the depth of the fractured layer and the location of the low-porosity ore-processing muds. Based on this conceptual model, a 3D flow and non-reactive transport model with the METIS code is developed and calibrated. This model allows predicting the evolution of the sulfate plume, but will also be used in future investigations, to build reactive transport models with simplified hydrogeology for U and other reactive contaminants.