Adsorption of polar organic molecules on sediments: Case-study on Callovian-Oxfordian claystone

The redox reactions of U(VI)/UO2 on Tamusu claystone: Effects of Fe2+/Fe3+ and organic matters

The claystone-based Tamusu area in the Bayingebi Basin, Inner Mongolia, is preselected as a China's high-level radioactive waste (HLRW) repository site. This study investigated the redox reactions of U(VI)/UO2 on Tamusu claystone. Five Tamusu claystone samples collected from boreholes Tzk1 and Tzk2 at different depths were used for batch experiments at pH ∼5.0, ∼7.0, and ∼9.0. These claystones contain considerable amounts of organic matters and Fe2+-containing minerals such as pyrite, fluorannite, and ankerite. Results showed that aqueous U(VI) could be partially reduced to U(IV) and/or U(V)-containing precipitates (U3O8, U4O9, etc.) by these Tamusu claystones, and the reaction is more favorable under acidic condition. We proposed that leaching of the structural Fe2+ followed by surface adsorption and interface reaction, is the primary mechanism responsible for U(VI) reduction. Under alkaline condition, organic matters might dominate the partial reduction of aqueous U(VI). Besides, the phosphorus-containing spots on Tamusu claystone surfaces are the reactive sites for U aggregation, implying the possible formation of U(VI)- and/or U(IV)-phosphate minerals. It is important to note that, due to the presence of minor Fe3+ in Tamusu claystones, the high-purity UO2 could undergo partial oxidation to U4O9 and/or U3O8. Therefore, insoluble UO2+x (0 < x ≤ 0.67) is proposed to be the most thermodynamically stable form in Tamusu claystone. This study enhances our comprehension of the essential geochemical processes of uranium in claystone surroundings, but also offers crucial information for the safety evaluation of China's HLRW repository.

Effect of solutes structure and pH on the n-octanol/water partition coefficient of ionizable organic compounds

Liquid-liquid partition coefficient is a useful tool to predict biological and environmental fate of organic compounds, for example bioaccumulation or toxicity of lipophilic contaminants. Conversely, the partitioning of ionizable compounds is poorly studied in contrast to that of neutral compounds. Yet, such topic deserves attention, since numerous organic contaminants are ionizable as well as their degradation products. Hence, the contribution of charged species has to be considered in order to model accurately the mass balance or partition of ionizable compounds. In this context, we investigated the liquid-liquid partition of 13 ionizable compounds (oxalic acid, histidine, benzimidazole, etc.), covering various classes of compounds (carboxylic acids, amino-acids, etc.). The n-octanol/water partition coefficient was measured from pH 1 up to 13, in order to fully gather the distribution of both neutral and charged species. Empirical models describing these results are reviewed and partition parameters adjusted for charged species. The study of benzoic acid derivatives (benzoic, salicylic, ortho- and iso-phthalic acids) provides insights on the influence of chemical groups on the partitioning. In the case of tryptophan, the use of acid/base microconstants allowed to estimate the partition of both the zwitterion and its neutral tautomer. Despite a major zwitterionic form (log P^Z(tryptophan) = -1.58 ± 0.30), the minor but neutral tautomer (log P^N(tryptophan) = +0.03 ± 0.30) drives the partition equilibrium. Overall, the provided data may be useful to assess the retention of contaminants, its dependency on pH and salinity variations, and thus understanding their environmental fate. Such data may also be useful as well for molecular simulation involving solvation of organic ions in aqueous and non-aqueous solvents.

Mobility of organic compounds in a soft clay-rich rock (Tégulines clay, France)

The migration of organic compounds in soils is a major concern in several environmental issues. Contaminants display distinct behaviours as regards to their specific affinities towards soils constituents. The retention mechanism of hydrophobic compounds by natural organic matter is well known. The retention of ionizable compounds is mainly related to oxides and clay minerals, even if less documented in reductive media. In this work, we investigated the migration of organic compounds in a soft clay-rich sedimentary rock (Tégulines clay, France). The aim was to determine the relative contributions of natural sorbents on retention, and eventual correlations with solutes properties. Both hydrophobic compounds (toluene, benzene, naphthalene) and hydrophilic species (adipate, oxalate, ortho-phthalate, benzoate) were investigated, using batch and diffusion experiments. The retention of neutral aromatic compounds correlates with their lipophilicity (log P_OW), confirming that absorption mechanism prevails, despite a low content of natural organic matter (≤0.5%). A low retention of ionizable compounds was quantified on Tégulines clay. The eventual discrepancies between data acquired on crushed rock and solid samples are discussed. Low effective diffusion coefficients are quantified. These values hint on the relative contributions of steric and electrostatic exclusion, despite a large pore size in such "soft" clay-rock. Overall, the dataset illustrates a general scheme for assessing the migration over a wide variety of organic compounds. This approach may be useful for predictive modelling of the fate of organic compounds in environmental media.

Application of Fourier transform ion cyclotron resonance mass spectrometry in deciphering molecular composition of soil organic matter: A review

Swiftly deciphering soil organic matter (SOM) composition is critical for research on soil degradation and restoration. Recent advances in analytical techniques (e.g., optical methods and mass spectrometry) have expanded our understanding of the composition, origin, and evolution of SOM. In particular, the use of Fourier transform ion cyclotron resonance mass spectrometers (FTICR-MS) makes it possible to interpret SOM compositions at the molecular level. In this review, we discuss extraction, enrichment, and purification methods for SOM using FTICR-MS analysis; summarize ionization techniques, FTICR-MS mechanisms, data analysis methods, and molecular compositions of SOM in different environments (providing new insights into its origin and evolution); and discuss factors affecting its molecular diversity. Our results show that digenesis, combustion, pyrolysis, and biological metabolisms jointly contribute to the molecular diversity of SOM molecules. The SOM thus formed can further undergo photodegradation during transportation from land to fresh water (and subsequently oceans), resulting in the formation of dissolved organic matter (DOM). Better understanding the molecular features of DOM therefore accelerates our understanding of SOM evolution. In addition, we assess the degradation potential of SOM in different environments to better inform soil remediation methods. Finally, we discuss the merits and drawbacks of applying FTICR-MS on the analysis of SOM molecules, along with existing gaps in knowledge, challenges, and new opportunities for research in FTICR-MS applications and SOM identification.

Organic contaminated sediments remediation with active caps: Nonlinear adsorption unveiled by combined isotherm and column transportation studies

Bench-scale column studies were performed with four cap materials (sand, apatite, organoclay and granular activated carbon) for five target compounds (toluene, naphthalene, phenanthrene, pyrene and dichlorobiphenyl) to represent a range of cap materials and pollutants (volatile & semi-volatile compounds) commonly found in sediments. Two moment-derived methods were used to model cap performance. Rough agreement was observed between the column experiments and modeling data with the fronting and tailing effects identified from certain breakthrough curves indicating a high potential of non-linear adsorption. Distribution coefficients (k_d) were experimentally determined with isotherm studies together with measurements of surface area and microporosity of the cap materials via nitrogen adsorption porisimetry. These studies unveiled the occurrence of nonlinear adsorption by Freundlich simulation. The effects of nonlinear adsorption of the cap were further explored via modeling. Results suggested better prediction of cap performance assuming nonlinear adsorption instead of linear adsorption results based upon the risk of release for a 30-year period.

Diffusion of organic anions in clay-rich media: Retardation and effect of anion exclusion

The transport of emerging organic contaminants through the geosphere is often an environmental issue. The sorption of organic compounds slows their transport in soils and porous rocks and retardation is often assessed by extrapolation of batch experiments. However, transport experiments are preferable to strengthen migration data and modelling. In this context, we evaluated the adsorption of various organic acids by means of through-diffusion experiments in a sedimentary clay-rich rock (Callovo-Oxfordian, East of Paris Basin, France). A low diffusivity of organic anions was quantified with effective diffusion coefficients, D_e, ranged between 0.5 and 7 10^-12 m² s^-1. These values indicated an organic anion exclusion. As for chloride, the porosity accessible to organic anions was lower than that of water: ε_a(organic anions) < ε(water). The partial exclusion of organic anions from rock porosity was linked to both charge and size effects. A significant retardation was observed for organic anions such as oxalate, citrate or α-isosaccharinate. Yet, retardation measured by diffusion experiments was significantly lower than expected from batch experiments on crushed samples. An empirical correction factor is proposed to account for a possible decrease of retardation with accessible porosity of diffusing solute. This feature has significant implications for the estimation of migration parameters of organic compounds in the environment.

Transport of low molecular weight organic compounds in compacted illite and kaolinite

¹⁴C-containing dissolved organic compounds may significantly contribute to the calculated annual overall dose emanated from a deep geological repository for radioactive waste. To date, there is a general lack of knowledge concerning the transport behaviour of low molecular weight organic compounds in the geosphere. The present work is aiming at a generic approach to measure weak adsorption of such compounds onto selected clay minerals. Percolation experiments were employed to sensitively measure the retardation of low molecular weight carboxylates and alcohols in compacted illite and kaolinite as a function of the ionic strength. Detection limits of ∼10^-5 m³ kg^-1 for the involved sorption distribution coefficients were attained thereby. The adsorption of alcohols on clays was near the detection limit and assumed to occur predominately via H-bonding. The adsorption of organic anions was influenced by several factors such as molecular structure, type of clay surfaces and the chemical composition of the aqueous phase. It was found that the relative position of neighbouring hydroxyl groups strongly influenced the retardation behaviour. Alpha-hydroxylated carboxylates, such as lactate, were found to be most retarded. Ligand exchange at the edge aluminol sites is the most probable explanation for the uptake of the negatively charged organic test compounds by the clay surface. The breakthrough behaviour of organic anions was additionally impacted by anion exclusion in illite. The demonstrated weak retardation of the test compounds can be robustly introduced in transport models, leading thus to a much lower contribution of ¹⁴C to the expected long-term overall dose.