Saddle towers with infinitely many ends

We prove the existence of nonperiodic, properly embedded minimal surfaces in $\mathbb{R}^2\times\mathbb{S}^1$ with genus zero, infinitely many ends and one limit end (in particular, they have infinite total curvature).Comment: 16 pages, 3 figure

Elimination de l'ion chrome hexavalent dans l'eau par les celluloses greffées

L'étude concerne le chrome (VI) et son élimination dans l'eau par les celluloses greffées (échangeuse d'anions) AE2 sous forme OH¯, Cl¯, S04=.La capacité d'échange correspond à 0,5-0,6 meq g-1 de cellulose. Les cinétiques d'échanges du Cr (VI) avec les ions CL¯ OH¯ et SO4 ont été déterminées en fonction du pH du milieu, de la température, de ta force ionique, de La quantité de détergents et des formes de celluloses AE2. Les résultats expérimentaux montrent que les formes de celluloses sont efficaces dans l'ordre OH¯ > Cl¯ > SO4= vis-à-vis du Cr (VI) et que l'échange décroît pour les ions étudiés dans l'ordre suivant : CrO4= > Cr207= > HCrO4¯.Ion exchange is used extensively for water and waste water treatment for the removal of a variety of ion species. It is known that at acidic pH nome anion-exchange resins can selectively remove and recover chromate from cooling water (SENGUPTA, 1986) where chromate is used as a corrosion inhibitor. This method is employed for plating waste treatment and to recover hexavalent chromium. The waste stream is first passed through a cation exchanger to remove metal ions such as iron, zinc, nickel and trivalent chromium. The hexavalent chromium passes through as CrO4=, and is subsequently removed in a anion exchanger.Hexavalent chromium may exist in the aqueous phase in different ionic forms with total chromate concentration and pH dictating which particular chromate species will predominate. The equilibrium reactions for different Cr (VI) species are :H2CrO4 ⇌ HCrO-4 + H+HCrO-4 ⇌ CrO-4- + H+Cr2O-7- + H2O ⇌ 2HCrO-4Cr2O-7- + H2O ⇌ 2CrO-4- + 2H+In the present study a description is given of the extraction of small quantities of Cr (VI) ion in water by modified cellulose (anionic form, (AE2) counterions OH¯, Cl¯, S04= ; 0,5-0,6 meq l-1 ion exchange capacity) which is represented by the following chemical formula : (CHATELIN, 1982 ; WATTIEZ, CHATELIN, 1981).Cell - O - CH2 - CH - O - CH2 - CH -                          |                         |                        CH2                    CH2                          |                         |                         N+    CL-             N+    CL-                       /  |  \                   /  |  \             CH3  CH3  CH3         CH3  CH3  CH3Kinetics of exchange and effects of pH, competiting ions, concentration, temperature, ionic strength, surfactants and types of cellulose (Cl¯, OH¯, SO4=) have been studied.Chromate only, chromate-pH, chromate-chloride, chromate-sulfate and chromate-surfactant kinetics (20 ± 1 °C) were determined for different anionic forms of modified cellulose. Kinetics data were generated by a batch equilibration technique where a weighed amount (5 g) of modified cellulose was shaken gently for 0.5-1 h with a fixed volume of solution (500 ml) containing Cr (VI) with successively H+, Cl¯, SO4=, a surfactant of known initial composition. A the end of the equilibration solution, the composition was determined again to calculate the Cr (VI) uptake. Equilibrium was achieved in 0.5 h, but 75 % of Cr (VI) was removed in 5 min.RESULTSSome specific aspects of the ion-exchange process for chromate recovery from industriel waste have been reported.It was observed that the more efficient types of cellulose for chromium removal where OH¯ > Cl¯ > SO4= respectively and that ion exchange decreased as followsCrO4= > Cr207= > HCrO4¯

Contribution à l'étude de la validité de différents modèles, utilisés lors de l'adsorption de solutés sur charbon actif

Les résultats de l'adsorption sur charbon actif en poudre de solutions aqueuses de différents composés organiques: phénol, aniline, nitrobenzène, acide salicylique, nitro-4 phénol, méthyl-2 dinitro-4,6 phénol, phénylalanine et tyrosine ont été traités à l'aide des équations de Langmuir, Elovich, Freundlich, Temkin, Fowler-Guggenheim, Hill et De Boer, Kiselev afin de déterminer divers paramètres d'équilibre: la capacité maximum d'adsorption, l'énergie d'adsorption, l'énergie d'interaction, les constantes d'équilibre adsorbat-adsorbant et les interactions (éventuelles) entre les molécules adsorbées.La relation de Temkin (3=RTt~Q In KoC permet de déterminer la variation de l'énergie d'adsorption ~Q et la constante Ko de l'équilibre (~3 est le degré de re- couvrement du charbon par le soluté, et C la concentration à l'équilibre). L'équa- tion de Fowler-Guggenheim KC=~3/(1~3) Exp (2 ~ W/RT) conduit à la déter- mination de l'énergie d'interaction W entre molécules adsorbées et à une constante d'équilibre K. Par contre, dans l'équation de Hill et de De Boer KlC=~/(1~)) Exp [~/(1~) - K2~/RTI, K2 représente une constante d'énergie d'interaction entre molécules adsorbées et, dans celle de Kiselev KIC=~3/[(1+ ~) (1 + Kn~3)]~ Kn est une constante de formation de complexe éventuel entre molécules adsorbées. On vérifie que l'application de la relation de Temkin est satisfaisante pour tous les composés étudiés et permet de les classer selon leur affinité sur le charbon mais les résultats obtenus en utilisant les équations suivantes (Fowler ...) montrent qu'il n'y aurait pas de formation de complexe ou d'interaction entre molécules adsorbées.Analysis of the results of adsorption from aqueous liquid media onto activated carbon can be carried out by different models based on thermodynamic principles. Classically the Langmuir (eq. 1), Freundlich or Elovich (eq. 4) isotherms are used, which lead to the determination of an experimental maximum capacity, qm, and a constant K, characteristic of the adsorbate-adsorbent interactions. The following equations (Table I) have been transposed from the vapour phase to the liquid phase. With the Temkin relation: [Theta]=RT/[Delta]QlnK[inf]0C (eq. 6), it is possible to determine the variation of adsorption energy, [Delta]Q, between the adsorbed molecules and the solid phase, and the equilibrium constant K[inf]0 ([Theta] is the degree of surface covering of the solid phase [Theta]=q/qm, q is the adsorption capacity). The Fowler-Guggenheim equation: KC=[[Theta]/(1-[Theta])] Exp (2[Theta]W/RT) (eq. 7) gives the interaction energy, W, between the adsorbed molecules and an equilibrium constant, K. The Hill and De Boer relation: K[inf]1C=[Theta]/(1-[Theta])] Exp [[Theta]/(1-[Theta]) -K[inf]2[Theta]/RT] (eq. 8) yields an energetic interaction constant K[inf]2 (J.mol-¹) characteristic of the interactions between the adsorbate molecules and an equilibrium constant, K[inf]1. In the Kiselev relation: K[inf]1C=([Theta]/[(1-[Theta]) (1 + K[inf]n[Theta]] (eq. 9), K[inf]n is a complex formation constant between adsorbed molecules and K[inf]1 is a constant relative to the adsorbate-adsorbent interaction. Linearization of the equations of Langmuir, and Elovich leads to qm and K values. For the Freundlich relation, if the experiments are made at constant Co and variable concentrations of adsorbent, the Freundlich relation can be transformed as relation (5): q=qm (C/Co)[sup]1/n). The value of qm and K are reported in the Table II. When the values obtained by the Elovich equation are very different from the Langmuir relation, they are not in concordance with the experimental adsorption isotherm as shown on the Figures 4, 5 and 6.A value of qm is necessary to calculate the ([Theta](=q/qm) of the Temkin, Hill-De Boer, Fowler- Guggenheim and Kiselev equations; [Theta] is calculated with the Langmuir value of qm: the linearized relations were tested for the following compounds: phenol, aniline, nitrobenzene, salicylic acid, 4-nitro phenol, 2-methyl-4,6 dinitro phenol, phenylalanine and tyrosine, studied at micromolar concentration. The results are shown in Table II. The Temkin linearization is of good quality for all the compounds; an example is given on the Figure 1. For the others (Figs. 2, 3), the linearization is not always verified (Hill-De Boer for phenylalanine: Fig. 3a) and the results are framed two times in the Table II.With the obtention of the two parameters [Delta][Theta], K K, W; K[inf]1, K[inf]2 and K[inf]1, K[inf]n, the isotherm can be recalculated. The results for some solutes are on Figures 4, 5, 6, 7, 8. Relatively poor results are obtained for Fowler-Guggenheim, Kiselev or Hill-De Boer models, where no association is present between the adsorbed molecules.The evolution of the variation of the adsorption energy ([Delta][Theta]) is reported on the Figure 9 for the different compounds. The greatest values are obtained for nitrobenzene and 4-nitro-phenol (+ 80, + 40 kJ.mol-¹ probably due to the presence of the nitro group). All the values are positive (exothermic reaction ( [Delta][Theta]=-[Delta]H)) showing the affinity of molecules for the activated carbon

Influence de l'ordre d'introduction des solutés (Naphtol-2 et Calcium) sur l'adsorption des tensioactifs sur charbon actif F400

L'effet de l'ordre d'introduction des solutés Naphtol-2 et Calcium (N-2 et Ca) sur l'adsorption de tensioactifs par le charbon actif en poudre (CAP) F400 brut a été étudié en discontinu. Les mélanges étudiés sont: le tensioactif anionique dodécyl-sulfate de sodium (DSS) et le N-2; le DSS, le N-2 et le Ca; le tensioactif cationique bromure de benzyldodécyldiméthylammonium (BBDDA) et le N-2 et enfin le BBDDA et Ca. Les résultats expérimentaux montrent tout d'abord que quel que soit l'ordre d'introduction des solutés N-2 et Ca, les concentrations en DSS à l'équilibre sont sensiblement identiques. Par ailleurs, la cinétique d'adsorption du DSS et du BBDDA croît en présence des ions Ca.Cependant, la cinétique d'adsorption du N-2 en présence du DSS diminue ce qui se traduit par une inhibition de l'adsorption du N-2 par le DSS. Quant à l'adsorption du BBDDA en présence du N-2 pour différents ordres d'introduction, le tracé des isothermes de LANGMUIR montre que la capacité maximale d'adsorption Г∞ du BBDDA ne change pas tandis que la constante de LANGMUIR (K) diminue. Par contre la capacité maximale d'adsorption du N-2 ** varie légèrement et passe de 1,67 à 1,5 mmol/g pour différent ordre d'introduction du BBDDA. On note également que Г∞ du N-2 en présence de concentrations croissantes en BBDDA diminue fortement et passe de 1,67 à 0,8 mmol/g.L'ajout de N-2 après 3 heures de contact du BBDDA /CAP ne provoque pas la désorption de ce dernier. Le même comportement a été observé avec le N-2, alors que l'ajout du DSS au mélange N-2/CAP provoque la désorption de celui ci.The effect of the introduction order of naphtol-2 and calcium (N-2 and ca) on the adsorption of surfactants by raw powdered activated carbon (PAC) F 400 has been studied in a static system, studied mixtures included the anionic surfactant: sodium dodecyl-sulfate (SDS) with N-2 or with N-2 + Ca, and the cationic surfactant beniyldodecyldimethylarnmoniurn bromide (BDDAB) with N-2, or with Ca. The experimental results demonstrate that whatever the order of solute introduction, the SDS concentrations at equilibrium are almost identical. Moreover, the adsorption kinetics of SDS and BDDAB increase in the presence of calcium ions. The adsorption kinetics of N-2 decrease in the presence of SDS, suggesting inhibition of N-2 adsorption by SDS. Concerning the adsorption of BDDAB the presence of N-2, for different introduction orders, plotting of the LANGMIIIR isotherms shows that the maximal BDDAB adsorption capacity Г∞, does not change but the LANGMUIR constant (K) decreases. For naphtol-2, however the maximal adsorption capacity Г∞, does vary slightly, going from l,67 to l.5 mmol/g for different BDDAB introduction orders. In the presence of increasing BDDAB concentrations, the maximum adsorption capacity of N-2 decreases from 1.67 to 0.8 mmol/g.The addition of N-2 after 3 hours of BDDAB/PAC contact does not lead to BDDAB desorption. similar behavior is observed on addition of N-2 after 3 hours of SDS/PAC contact, even though the addition of SDS to a N-2/PAC mixture leads to N-2 desorption

Elimination de colorants de l'industrie textile par des sciures de bois

L'élimination de la couleur de rejets de petites industries textiles peut être réalisée à l'aide de sciures d'essences de bois diverses dont le choix est fait après étude de leur relargage dans l'eau (couleur et DCO); les sciures de chêne, de châtaignier, de pin Douglas conduisent à des relargages de substances fortement colorées correspondant à des DCO d'environ 60 mg d'oxygène par gramme de sciures sèches et ceci en réacteur discontinu. Par contre, les sciures de hêtre, bouleau, sapin et peuplier relarguent nettement moins de produits colorés ou non et conduisent à des DCO plus faibles, excepté pour le bouleau (30 mg d'O2.g-1).Le pH de l'eau est également modifié par addition de sciures et l'on obtient des valeurs de pH d'équilibre atteint en 10 min., de 4,3; 5; 5,8 et 7 pour le bouleau, le sapin, le hêtre et le peuplier respectivement.L'élimination de colorants à 25 mg.l-1 dans l'eau par des doses de sciures de 50 g.-1 pour 2 h de temps de contact, montre qu'une meilleure décoloration (98 à 100 %) est obtenue pour le colorant cationique Basic Red 22 avec les quatre sciures sélectionnées, qu'avec les colorants anioniques (ou acides) dont les valeurs de rétention, variables selon les sciures (de 4 à 8 %) peuvent être très bonnes également.Les isothermes d'adsorption permettent de déterminer les capacités d'adsorption à saturation; à titre d'exemple, des valeurs de 0,1 à 0,2 mmol.g-1 sont obtenues sur des sciures de peuplier et de hêtre pour le colorant BR 22 et de 4 à 8.10- mmol.g-1 pour le colorant anionique Acid Blue 25, sur ces mêmes sciures. Contrairement à d'autres matériaux, on observe que les résultats obtenus ne dépendent que très peu de la température (entre 15 et 35 °C).Une étude dynamique sur colonne confirme les écarts pour les deux types de colorants, mais les résultats dépendent de la vitesse de passage : des pourcentages d'élimination de 0,4 à 1 % en masse peuvent être obtenus pour l'AB 25 le BR 22 sur la sciure de hêtre. Il faut cependant signaler que les produits minéraux et organiques contenus dans les effluents colorés industriels diminuent l'efficacité des sciures par rapport à leur efficacité sur des solutions de colorants purs.Anticipated water pollution regulations require textile dye industries to reduce substantially the amount of colour in their effluents. One possible method of colour removal is through adsorption techniques. The most commonly used adsorbent for treatment of textile effluents is activated carbon. The capability of sawdust for removing colour was recognised some time ago (POOTS et al. 1978; ABO-ELELA and EL-DIB, 1987; ASFOUR et al., 1985) as were those of peat and of charred woollen fiber treatments (PERINEAU et al., 1983). Sawdust has recently received more attention (ASFOUR et al., 1985) owing to its economic advantages when the sawmills are near the textile mills. In order to measure the real efficacy of some types of sawdust, evaluation of the released coloured products and their influence on the effluent water pH are never described. The aim of the present work is to study and evaluate the behaviour of types of sawdust from Limousin woods (released COD and pH of solutions) and the discolouring power of the selected sawdust (beech, poplar, birch and fir trees) with respect to textile dyes in dilute solutions and industrial effluents.MATERIALS AND METHODSExperiments were conducted using sawdust from beech, chestnut, poplar, oak, birch, fir and Douglas pine, in doses of 50 g.l-1 of water. The industrial sawdust particle site was between 0.5 - 1.5 mm.The dyestuffs used in all the experiments are reported in table 1. The initial concentration of colouring matter was 25 mg.l-1 and determined spectrophotometrically at maximum absorbance wavelength. In a batch system the time required for equilibrium was 2 hours (or less).RESULTS AND DISCUSSIONKinetic rejections of the sawdust are shown in figure 1; four types of sawdust are suitable : beech, poplar, birch, and fir were selected for further experiments. The absorbance depends on the wavelength and becomes maximum near UV range (as shown in fig. 2 and 3). The COD of the released products is particularly important for chestnut, oak, Douglas pine (60 mg.O2.g-1; table 2) and also for birch (31 mg.O2.g-1). These data have never been mentioned in published works where the wood was not submitted to any form of pre-treatment.Sawdust contains some acidic or basic groups that modify the pH of water (fig. 6, 7) which becomes more acidic; the phenomenon can be used to neutralize textile fixing effluents loaded with carbonate ions (pH : 10). Batch results (table 3) indicate that the tour selected types of sawdust give good colour removal for the 8 dyes; particularly, the cationic dye : Basic Red 22 is discoloured with very good yield (96 to 99.5 %) on beech and on birch sawdust.A variation of temperature between 15 and 35 °C does not change significantly the adsorption results and at pH = 2, the release decreases (table 2) and the adsorption increases (table 4).Equilibrium conditions of adsorption of basic and acidic dyes on four types of dust were studied using the Langmuir equation; this equation was also used for the determination of the « ultimate capacity » Г∞ and the equilibrium constant K (STUMM and MORGAN, 1981). The resulting Г∞ and K for some dyes (AB 25, BR 22, NLB) on fir, beech, poplar, and birch sawdust are given in table 5. The best results were obtained for the cationic dye BR 22 which showed an ultimate capacity of 0.210 and 0.06 mmol.g-1 on beech and on birch sawdust respectively. [The result for other dyes is in the order of 0.005 mmol.g-1]. The results obtained by extrapolation of the linearized Langmuir equation are somewhat different from those obtained by the experimental saturation curve (fig. 8) for BR 22 on polar sawdust. For AB 25, the saturation curve shows a great increase of the adsorption capacity for concentrated solutions (2 - 2.25 g.l-1), this is probably due to a micellary process as shown on figure 9 : the critical micellary concentration is between 1.5 and 3.3 g.1-1.DYNAMIC STUDYFlow tests using small columns (3 - 7 cm of height) are used with beech sawdust for determining the breakthrough curves (rate : 1.03 m.h-1). The concentrations of AB 25, BR 22 and their mixture are 25 mg.l-1 (close to industrial waste-water concentration). The adsorption capacity deduced from the « Bed Depth Service Time » model is 0.4 % and 0.8 % for each dye respectively.Glass columns (4 cm diameter) containing 20 g or 30 g of beech sawdust were also used. The values of adsorption for a rate of 3.1 m.h-1 are 0.45 mg.g-1 (0.04 %) for AB 25 and 13 mg.g-1 (1.3 %) for BR 22. Approximately 650 litres of this dye can be treated by this sawdust (fig. 10 and 11).Industrial effluents (unknown composition) are less discoloured, probably due to the presence of many other compounds.Batch experiments adsorption of dyes on wood sawdust can lead to different measurements of the efficiency of the sawdust depending on rejection by the adsorbent of chemical species in the bulk solution. These substances modify the pH, the adsorbance of the solution and the amount of adsorbed dyes by competitive reaction

Fractionnement et caractérisation de la matière organique des lixiviats de décharges d'ordures ménagères

Les lixiviats de décharge constituent une source de nuisance qui vient s'ajouter aux nombreux problèmes de contamination du milieu environnant s'ils ne sont pas traités avant leur rejet. La matière organique, principale composante de ces effluents, doit retenir une grande attention dans la mesure où il est difficile d'éviter la propagation et la diffusion de cette pollution dans les sols et vers les nappes phréatiques. Cette étude vise à fractionner et à caractériser la matière organique présente dans des lixiviats de décharges d'ordures ménagères afin de prévoir et d'orienter le choix des traitements les plus adaptés compte tenu de leur biodégradabilité.La méthode de fractionnement utilisée comprend une adsorption spécifique sur résines macroporeuses Amberlite XAD (combinaison de XAD-7 et XAD-4) pour séparer les composés hydrophobes et hydrophiles qui sont ensuite extraits à la soude (composés acides) et au dichlorométhane (composés neutres). Le fractionnement de la matière organique par filtration sur résines XAD-7 et XAD-4, après une première étape de précipitation en milieu acide (pH=1), a permis de répartir l'ensemble des composés organiques du lixiviat dans six fractions de spécificités différentes fonction de la taille et/ou du caractère hydrophile ou hydrophobe des molécules. Les résultats montrent que ce protocole expérimental permet d'extraire au moins 98 % de la matière organique totale (pourcentage relatif aux teneurs de la demande chimique en oxygène ou DCO), dont la plus grande proportion est constituée des substances humiques (76 % à 90 % en DCO). Diverses méthodes analytiques ont été proposées en vue de caractériser les fractions isolées telles que l'analyse élémentaire, la spectrophotométrie infrarouge, la résonanoe magnétique nucléaire C13 (RMN Cl3) et la chromatographie CG/FID et CG/SM.Landfill leachates represent an obvious source of pollution for the environment and many studies have attempted to analyze organic pollutants found in leachates. A number of methods have been described in the literature for the isolation and concentration of dissolved organic matter from landfill leachate samples. Membrane ultrafiltration, gel permeation and high performance liquid chromatography are commonly used because these techniques can be easily adapted to separate soluble organic substances from large volumes of leachate. The objective of this study was to fractionate and characterize dissolved organic matter found in leachates collected from sanitary landfills.The discharges are defined with regard to the geological context from which they are situated and the nature of the waste. The discharges are classified in three categories, based upon the value of the permeability coefficient K, the substratum and its continuity.- class 1; impermeable site (K 10-[sup]6 ms-¹). The studied leachates come from landfill of class 2:- Crézin (Haute-Vienne) of compacted type: it was used for household rubbish and assimilated ordinary wastes. - Foussais-Payre (Vendée): leachate coming from the fermentation area of a composting plant and from the compost refuse. Fractionation of dissolved organic matter was applied on three leachates samples. The first sample was the raw leachate collected from the Crézin landfill and the two others came from Foussais-Payre (a raw sample and a sample treated in an aerated lagoon for 52 days). Because of the wide variety of organic compounds that can be found in such leachates, we classified and isolated the different groups of organic constituents using an XAD resin adsorption procedure. The experimental method consisted of acidifying samples to pH 1 to isolate the first fraction (fig. 1) and then treating the remaining supernatant with XAD-7 and XAD-4 resins. The adsorption on XAD resins allowed the isolation of the other organic fractions (figs. 2 to 6). Various analytical techniques were applied to characterize the isolated fractions such as elementary analysis infrared spectroscopy, ¹3C nuclear magnetic resonance (¹3C CP/MAS NMR), gas chromatography (GC/FID), and gas chromatography coupled to mass spectrometry (GC/MS). Results showed that more then 90% of the total organic carbon (TOC) in leachates can he recovered by the isolation procedure. Most of the isolated compounds corresponded to humic substances (76% to 90% of the chemical oxygen demand). Hydrophobic and hydrophilic neutral compounds were found only in small concentrations

Adsorption du pentachlorophenol sur divers matériaux : Influence de co-adsorbats organiques (Substances humiques et lindane)

Nous avons réalisé, pour de faibles concentrations, l'étude de l'adsoprtion d'un pesticide le pentachlorophénol, sur plusieurs matériaux adsorbants : le charbon actif, la kaolinite et la bentone. Les résultats montrent que l'adsorption du pentachlorophénol sur la kaolinite reste négligeable tandis que la bentone SD-3 (argile traitée) permet d'obtenir des résultats intéressants avec toutefois une capacité maximale d'adsorption 10 à 100 fois plus faible que celle du charbon actif. Le charbon actif reste le meilleur matériau adsorbant pour le pentachlorophénol. La solubilité de ce composé étant très variable en fonction du pH du milieu on constate que l'adsorption du pentachlorophénol-pentachlorophénate (5 mg.l-1) diminue pour des valeurs de pH supérieures au pKa du couple acide-base que nous avons déterminé expérimentalement, égal à 5,9 ± 0,1. L'adsorption du pentachlorophénol n'est pas affectée par la présence d'acides humiques (obtenus à partir de substances commerciales) utilisés comme coadsorbats (10 mg.l-1) alors que les acides fulviques (20 mg.l-1) extraits de sols semblent provoquer une promotion de l'adsorption. On observe d'autre part que le pentachlorophénol n'affecte pas l'adsorption des acides humiques mais augmente légèrement celle des acides fulviques. Ces remarques suggèrent la possible formation d'un complexe acides fulviques-pentachlorophénol. Lorsqu'on utilise le lindane, pesticide hydrophobe comme coadsorbat (165 mg.l-1), on observe encore d'une façon systématique pour tous les nombreux essais une légère promotion de l'adsorption.Adsorption studies for the removal of the pesticide pentachlorophenol found in a number of water supplies (BELAMIE and GIROUD, 1986) were carried out using various materials including kaolinite, bentone SD-3 and powdered activated carbon (WEBER and GOULD, 1966; WEBER and JODELHAH, 1985). It was found that adsorption on kaolinite was negligible, whereas bentone SD-3 presented and adsorption efficiency from 10 to 100-fold less than equivalent quantities of powdered activated carbon (LOTSE et al., 1968; SHAROM et al., 1980). The effect of the pH on the removal of pentachlorophenol by activated carbon was studied. The removal efficiency of pentachlorophenol by activated carbon is better in acidic media. A clear dependence of adsorption on the pH appeared to be the result of a marked variation of the pesticide solubility as a function of the pH (fig. 3). Adsorption of pentachlorophenol/phenate (5mg.l-1) diminishes markedly at pH values above the pKa of this weak acid (that we found equal to 5,9 ± 0,1) when the pentachlorophenol exists almost entirely in ionic form in aqueous solution, and is enhanced at low pH when the percentage of molecular species (whose concentration can be determined from pKa value) becomes significant (WARD and GETZEN, 1970). These remarks and the adsorptive capacities (163 mg.g-1= 0,6 mmol.g-1 at pH = 5,2 and 79 mg.g-1 0,3 mmol.g-1 at pH =12,7), suggest a П-П interaction between pentachlorophenol and activated carbon which seems to be confirmed by the results with bentone SD-3 (tables 1 to 4), and the values of the electrokinetic potential of these materials. This study emphasizes the effect of organic coadsorbates (e.g., dissolved humic substances and the pesticide lindane) on the adsorption capacity of activated carbon for pentachlorophenol. Two different natural organic matters were studied as coadsorbates : purified humic acids from a commercial source (at 10 mg.l-1) and fulvic acids extracted from a top soil horizon (et 20 mg.l-1) (SCHNITZER and SKINNER, 1963; THURMAN and MALCOLM, 1981). Pentachlorophenol adsorption was not affected by humic acids, whereas an increase of adsorption seemed to be observed in the presence of fulvic acids (fig. 6). Pentachlorophenol does not affect the adsorption of humic acids, but improves slightly the removal of fulvic acids. This suggests an association between the two kinds of organic compounds (WERSNAW et al., 1969; KHAN, 1972; OGNER and SCHNITZER, 1970), the resulting « complex », fulvic acids/pentachlorophenol, being more adsorbed than the compounds themselves. The coadsorbate lindane (0,65 mg.l-1) which is easily adsorbed by activated carton (GOMELLA and BELLE, 1975...) seemed also to Improve slightly the removal efficiency of pentachlorophenol by activated carton (fig. 7)

Magnetic structures of Mn3-xFexSn2: an experimental and theoretical study

We investigate the magnetic structure of Mn3-xFexSn2 using neutron powder diffraction experiments and electronic structure calculations. These alloys crystallize in the orthorhombic Ni3Sn2 type of structure (Pnma) and comprise two inequivalent sites for the transition metal atoms (4c and 8d) and two Sn sites (4c and 4c). The neutron data show that the substituting Fe atoms predominantly occupy the 4c transition metal site and carry a lower magnetic moment than Mn atoms. Four kinds of magnetic structures are encountered as a function of temperature and composition: two simple ferromagnetic structures (with the magnetic moments pointing along the b or c axis) and two canted ferromagnetic arrangements (with the ferromagnetic component pointing along the b or c axis). Electronic structure calculations results agree well with the low-temperature experimental magnetic moments and canting angles throughout the series. Comparisons between collinear and non-collinear computations show that the canted state is stabilized by a band mechanism through the opening of a hybridization gap. Synchrotron powder diffraction experiments on Mn3Sn2 reveal a weak monoclinic distortion at low temperature (90.08 deg at 175 K). This lowering of symmetry could explain the stabilization of the c-axis canted ferromagnetic structure, which mixes two orthorhombic magnetic space groups, a circumstance that would otherwise require unusually large high-order terms in the spin Hamiltonian.Comment: 11 pages, 13 figure

Influence de l'origine et de la nature des substances humiques sur l'adsorption de l'atrazine sur charbon actif en poudre

Les essais d'adsorption sur charbon actif en poudre, de sept acides humiques (H) ou fulviques (F) et pour trois pH (5,6 ; 7,0 ; 8,8) marquent que ces acides ont un comportement différent suivant leur origine.Lorsque le pH de la suspension de charbon est basique, l'adsorption des différentes fractions humiques diminue, ce qui est en relation avec l'augmentation de leur solubilité. Pour un même pH de la suspension, les acides humiques et fulviques d'origine aqueuse s'adsorbent mieux que ceux d'origine commerciale ou que ceux extraits de sol.La capacité maximale d'adsorption de l'atrazine sur charbon actif en poudre : 260 mg.g-1 (Langmuir), est plus fortement diminuée en présence d'acides fulviques que d'acides humiques d'une part, et plus inhibée (jusqu'à 40 %) par les acides fulviques extraits d'eau (F1, F2) que par ceux extraits de sol (F3) d'autre part.Par contre, les acides humiques provenant de substances commerciales (H4, H5) influencent moins l'adsorption de l'atrazine.L'ensemble des essais montre que l'origine et la nature des substances humiques joue un rôle prépondérant sur l'adsorption de l'atrazine sur charbon actif, le pH ayant une influence limitée.Enfin l'étude de l'influence de l'atrazine sur l'adsorption des substances humiques sur charbon actif en poudre indique également une différence de comportement entre les acides fulviques, pour lesquels on note peu de variation, et les acides humiques, pour lesquels on note à la fois une promotion de l'adsorption et une meilleure affinité d'adsorption sur le charbon actif en poudre en présence d'atrazine.Ce travail met en évidence l'importance du choix des substances humiques pour des essais d'évaluation des interactions substances humiques - pesticides lors de l'adsorption sur charbon actif en poudre.Though the subject was copiously and oldly studied, this work is bringing a new contribution to the study of the influence of the origin and the nature upon their properties and in particular with atrazine, the more used herbicide these fast years.In order to study this influence of the origin and nature of various humic substances upon the removal of atrazine by adsorption onto powdered activated carbon (PAC), humic substances were obtained from different sources :- Natural aquatic fulvic (F1) and humic (H1) acids isolated from Beaune Lake (Limoges, France).- Natural aquatic fulvic (F2) and humic (H2) acids extracted from hydroxyde sludges of the water treatment plant of La Bastide (Limoges, France).- Soil fulvic acids (F3) extracted from a canadian top soil horizon.- Commercial humic acids (Aldrich. H4 and Jansen H5) obtained from commercial sodium humates.The results of adsorption test of humic acids, fulvic acids and atrazine (alone or together) onto powdered activated carbon (PAC) at three different pH (5,6 ; 7,0 ; 8,8) were expressed by Freundlich's and Langmuir’s models and permitted us to note that :- According to the increasing of water solubility of humic substances with basic pH, their adsorption was better in acidic medium.- Natural aquatic humic substances (H1, H2, F1, F2) were more adsorbed onto PAC than humic substances tram soil (H3) or commercial origines (H4, H5).- Atrazine adsorption capacity was 260 mg.g-1 according to Langmuir's model.- Atrazine adsorption decreased with F1 for all pH ; the same result was also observed with the other natural aquatic humic substances.- Commercial humic acids didn’t importantly affect atrazine adsorption. These differences appeared to be derived from the nature and the origin of fulvic and humic acids. The rose of atrazine on the humic substances adsorption showed a difference between humic and fulvic acids whose adsorption was not influenced by atrazine even humic acids only presented an increasing of adsorption.All these results showed the importance of the selection of humic substances to evaluate the interaction SH-Pesticides during adsorption onto PAC and let us think to a competitive adsorption between fulvic acids and atrazine onto PAC