Concentration (opération chimique)

Résumé

En chimie, concentrer signifie augmenter le rapport entre la quantité d'une substance et le volume du milieu qu'elle occupe. Plus spécifiquement, la concentration est la mesure de la quantité d'une matière donnée dissoute dans un volume d'une autre substance. Le terme concentration s'applique à toutes sortes de mélanges, mais est surtout utilisé lorsqu’il s'agit de solution. Il s'agit alors de la quantité de soluté dissoute dans un solvant. Pour concentrer une solution, il faut rajouter du soluté ou réduire la quantité de solvant, par exemple par évaporation sélective. Concentrer est l'opération inverse de diluer. Pour diluer une solution, a contrario, il faut rajouter du solvant ou diminuer la quantité de soluté. Il existe une concentration au-delà de laquelle il n'est plus possible de dissoudre du soluté dans la solution : la solution est alors dite saturée. Ce point de saturation (la solubilité de ce soluté dans ce solvant) dépend de nombreuses variables telles que la températu

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Further results on latent discourse models and word embeddings

Youssef Allouah

We discuss some properties of generative models for word embeddings. Namely, (Arora et al., 2016) proposed a latent discourse model implying the concentration of the partition function of the word vectors. This concentration phenomenon led to an asymptotic linear relation between the pointwise mutual information (PMI) of pairs of words and the scalar product of their vectors. Here, we first revisit this concentration phenomenon and prove it under slightly weaker assumptions, for a set of random vectors symmetrically distributed around the origin. Second, we empirically evaluate the relation between PMI and scalar products of word vectors satisfying the concentration property. Our empirical results indicate that, in practice, this relation does not hold with arbitrarily small error. This observation is further supported by two theoretical results: (i) the error cannot be exactly zero because the corresponding shifted PMI matrix cannot be positive semidefinite; (ii) under mild assumptions, there exist pairs of words for which the error cannot be close to zero. We deduce that either natural language does not follow the assumptions of the considered generative model, or the current word vector generation methods do not allow the construction of the hypothesized word embeddings.

MICROTOME PUBL2021

Chargement

Cadmium (Cd) is a non‐essential trace element which is toxic at high concentrations. Cd is added to New Zealand soils through superphosphate fertilizer. The Cd is then absorbed by vegetables and may exceed the limit of 0.1 mg/kg value, thus leading to economic and health risks. Previous studies have shown that lignite can significantly reduce the absorption of Cd by pasture, and that this effect is dependent on soil type. In addition, other studies have shown that compost can reduce the absorption of some metals. The effects of these amendments were tested on Cd uptake by vegetables, as they are more sensitive to the toxic effects of Cd than pasture grass. We studied the absorption of Cd in vegetables grown in two New Zealand soils with and without Cd spiking. We also studied the effect of an amendment with compost and two amendments with lignite. The results show that the amendment with compost reduced the absorption of Cd in all plants studied. The decrease of the absorbed concentration was up to 60% in some cases. Amendments with lignite showed significant decreases and one significant increase in the concentration of Cd in vegetables. The decreases of Cd observed in the various amendments were not typically related to the extractable Cd concentration or to the soil pH. Finally, we observed no clear pattern concerning the absorption of other elements. Because of the highly variable composition of compost, a comprehensive study with different types of compost is recommended to better understand the effects of compost on Cd uptake by vegetables. In addition, a study on the persistence of compost into the soil should also be performed.

2013

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The continuous accumulation of micropollutants in water is a growing problem for our water resources. Currently, most conventional wastewater treatment plants (WWTP) are not able to remove them efficiently. Several pharmaceuticals, for which the measured concentrations in German surface water might have an effect on some living organisms, are included in this pollutant class, as for example sulfamethoxazole (SMX), carbamazepine (CBZ) and diclofenac (DFC). Phytoremediation is known as an emerging and advantageous technique for soil and water remediation. The potential of using horseradish hairy root cultures (HR) as a plant model for the study of SMX phytoremediation has already been pointed out in a previous work and was tested again in this work, with the addition of a temporal scale (1, 2 and 6 days of contamination). The experiment confirmed the potential of using horseradish HR for SMX phytoremediation, with a disappearance rate from the medium around 90% already after 1 day and close to 100% after 6 days. A novel metabolite (SMX-Glycoside) was also detected in the medium as well as in the HR (concentration measurement with LCMS). Moreover, an enhancement of detoxifying enzymes and no enzymatic stress symptoms could be observed (enzymatic activity measurements with a spectrophotometer). Given that, in real life, SMX is not the only pharmaceutical present in WWTP influents, a second experiment was carried out with a mix contamination (SMX, CBZ and DFC). Although some differences of the SMX fate and of the enzymatic activity were observed, the potential for SMX phytoremediation could be confirmed also in this case (disappearance rate higher than 80% after 1 day and close to 100% after 6 days). DFC was also taken up and metabolized in the analyzed time period, whereas a lower disappearance rate was observed for CBZ. This experiment permitted thus to confirm the potential of using horseradish HR for SMX phytoremediation and to extend it also to a mix contamination (SMX and DFC).

2014

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