Various causes behind the desorption hysteresis of carboxylic acids on mudstones

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.

Accumulated Gibbs free energy as a quantitative measure of desorption hysteresis associated with the formation of metastable states

The persistence of metastable states was proposed in the literature as one explanation for sorption-desorption hysteresis (SDH) of organic compounds on soils and sediments. When such metastable states freely exchange sorbate molecules with the surroundings and there is no spontaneous exit of a whole system from that state, it is possible to determine the extra Gibbs free energy (ΔG^ext) accumulated in a system due to the persistence of metastable states. A novel contribution of this paper is the characterization of SDH, in which the sorption isotherm (SI) and desorption isotherm (DI) do not close a loop, in terms of free energy needed to create "frozen", metastable states. To that end, liquid phase sorption of non-ionized sorbates is considered and by integrating over the sorption-desorption sequence, ΔG^ext and an integral hysteresis index (IHI) were obtained. Experimental data collected from the literature on aqueous sorption and desorption of polyaromatic hydrocarbons, triazines and ureas were examined on soils, sediments, organic matter-rich sorbents, montmorillonites and fullerene. Positive ΔG^ext values were obtained to quantify the thermodynamic potential for spontaneous exit from a metastable state that is not implemented due to the kinetic barriers. Relating the ΔG^ext values to sorbate molecular structure and sorbent properties may allow the prediction of SDH for various chemicals on sorbents in which the sorbate-induced perturbation of a sorbent matrix is believed to be a cause for the formation of persistent metastable states and the appearance of a non-closed sorption-desorption sequence.

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.

Effects of wind-wave disturbances on adsorption and desorption of tetracycline and sulfadimidine in water-sediment systems

Wind-wave disturbances frequently disperse sediment particles into overlying water, which facilitates the adsorption and desorption of contaminants in aquatic ecosystems. Tetracycline (TC) and sulfadimidine (SM2) are common antibiotics that are frequently found in aquatic environments. This study utilized microcosms, comprising sediment and water from Lake Taihu, China, to examine the adsorption and desorption of TC and SM2 under different wind-wave disturbances in a shallow lake environment. The adsorption experiments were conducted with three different concentrations (1, 5, 10 mg/L) of TC and SM2 in the overlying water, and two different (background and strong) wind-wave conditions for 72 h. Subsequently, four microcosms were employed in a 12-h desorption study. Analysis of adsorption progress showed that TC concentration in the overlying water decreased quickly, while SM2 remained almost constant. In the desorption experiments, SM2 released to the overlying water was an order of magnitude greater than TC. These results indicate that sediment particles strongly adsorb TC but weakly adsorb SM2. Compared to background conditions, the strong wind-wave conditions resulted in higher concentrations of TC and SM2 in sediment and facilitated their migration to deeper sediment during adsorption, correspondingly promoting greater release of TC and SM2 from sediment particles into the overlying water during desorption.

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

The release and transport of anthropogenic organic matter through the geosphere is often an environmental criterion of safety. Sedimentary rocks are widely studied in this context as geological barriers for waste management. It is the case of Callovian-Oxfordian claystone (COx), for which several studies report adsorption of anthropogenic organic molecules. In this study, we evaluated and reviewed adsorption data of polar organic molecules on COx claystone. Experiments were performed on raw claystone, decarbonated and clay fractions. Adsorption isotherms were measured with adsorbates of various polarities: adipate, benzoate, ortho-phthalate, succinate, gluconate, oxalate, EDTA, citrate. A significant adsorption was observed for multidentate polycarboxylic acids as evidenced with phthalate, succinate, oxalate, gluconate, EDTA and citrate (R_d = 1.53, 3.52, 8.4, 8.8, 12.4, 54.7 L kg^-1 respectively). Multiple linear regression were performed as a statistical analysis to determine the predictors from these adsorption data. A linear correlation between adsorption data (R_d) and dipole moment (μ) of adsorbates was evidenced (R² = 0.91). Molecules with a high dipole moment, μ(D) > 2.5, displayed a significant adsorption, R_d≫1 L kg^-1. A qualitative correlation can be easily estimated using the water/octanol partition coefficient, P_ow, of adsorbates (R² = 0.77). In this case, two opposite trends were distinguished for polar and apolar molecules. The use of organic carbon content in sediments is relevant for predicting adsorption of apolar compounds, log (P_ow)>+1. The oxides/clays contents may be relevant regarding polar molecules, log (^apparentP_ow)<-1. The proposed scheme offers a general methodology for investigation of geo-barriers towards heterogeneous organic plumes.