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 PZ(tryptophan) = -1.58 ± 0.30), the minor but neutral tautomer (log PN(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 POW), 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.

The design and fabrication of beta sensor system for in situ diffusion tests in mudstone in France.

Long term in situ diffusion experiments in the Callovian-Oxfordian mudstone (France) are designed in the context of nuclear waste management. ß-emitters HTO and 36Cl radiotracers are representative of neutral and anionic species in these experiments. Studies have been carried out to design an in situ beta monitoring system in order to quantify tracer migration in the rock pore water over time. The development, design and calibration of ß-emitter radiation sensors were performed. An optimised geometry was calculated for the detection chamber of 36Cl in solution (Ø×L=30×2mm2) via Monte-Carlo N-Particle transport simulation tools. A better SNR (Signal to Noise Ratio) was obtained with YSO cylindrical crystal (Ø×L=6.5×0.5mm2). A sensitivity of 0.21cpm/Bq/mL was measured using 36Cl standards. The 36Cl detection limits were 18Bq/mL and 14.2Bq/mL after 1h and 24h of accumulation time respectively. Such beta sensors, placed 350mm from the radiotracer injection borehole, could successfully discriminate 36Cl anisotropic diffusion in various directions within 4 years. It will be tested in situ and can be adapted to other in situ experience.