Volume 13, numéro 3, 2000

FR :

La migration et la spéciation de Cr, de Cu et de Pb sont étudiées sur le sol d'une colonne alimentée par les eaux usées de tanneries. La localisation des métaux est évaluée à l'aide du schéma d'extractions sélectives préconisé par Legret et al. (1988). Les résultats ont montré que l'horizon superficiel renferme des teneurs en métaux très élevées. Parmi les métaux étudiés, le Cr s'avère retenu essentiellement dans la phase résiduelle mais il se présente aussi sous des formes plus mobiles. Le Cu est principalement associé avec la phase résiduelle et organique. Le Pb est lié aux phases acido-soluble et réductible. De ce fait, l'épandage des eaux usées de tanneries, sans aucun traitement préalable, peut être une source de contamination des sols et des eaux souterraines, notamment pour le Cr.

EN :

In Morocco, the town of Marrakesh, as in other areas of the country, is subject to multiple daily aggressions generated by industrial pollution, in particular from tanneries. The tannery waste water is discharged directly into the aquatic and terrestrial ecosystems without any preliminary processing. In order to evaluate the danger that can be generated by tannery effluents in the receiving ecosystems, and to study the ecotoxicological and environmental impact resulting from that chemical treatment of these effluents, we determined, before and after treatment, the inhibitory effect of the two most polluted effluents originating from manufactured leather: liming-deliming and chromium tanning.

The purpose of our investigation was thus to evaluate the acute toxicity of the chromium tanning and epilage-coat waste waters before and after chemical treatment. The invertebrate Daphnia pulex was used for the toxicity test and the toxicity due to these industrial waste waters was evaluated by regression analysis. The processing carried out on the chromium tanning waste water consisted of precipitation with sodium carbonate at different pH values. The treatment of liming-deliming wastewaters consisted of an elimination and recovery of the sulphide compounds without carrying out their degradation.

The chromium level was determined by atomic absorption spectrophotometry with graphite furnace atomization. The results of the toxicity test showed that the investigated chromium tanning and liming-deliming effluents presented were toxic, with respective IC50-24h values of 0.15 and 3.36 (expressed as dilution ratios). This toxicity could be explained by high levels of chromium, sulphides and organic matter in these waste waters. It is however difficult to correlate the IC50 values obtained with those of the physical-chemical parameters. The tested effluents are very complex matrices. On the other hand, we noticed a notable difference between these two effluents. The mineral tanning effluent is contaminated with chromium and organic matter, whereas the second is especially rich in organic matter and sulphide.

The total chromium concentration corresponding to the IC50-24h value of the raw chromium tanning effluent is lower than that which would be expected on the basis on the known toxicity of K₂Cr₂O₇, showing the toxicity of raw chromium waste waters is due not only to chromium, but also to other chemicals used during the manufacturing process of leather.

After processing by chemical precipitation, the IC50-24h values increased from 0.15 to 26.6 and from 3.36 to 11.1 respectively, for the chromium tanning and the epilage-coat effluents. Consequently, these treated sewages can be classified as low toxicity wastes. The comparison of the results is very difficult considering that the test conditions and the physicochemical and heavy metal characteristics of the effluents vary enormously over time, without forgetting the physicochemical composition of the experimental solution used by standard OECD (fresh water) and the standard ISO (synthetic water). From where necessity to vary tests for determining well the degree of an effluent toxicity which the effects appear differently at various levels trophic.

The diminution observed in the toxicity of the effluents is linked to a reduction of suspended matter (95 %), COD (55 %), Cr (90 %) and sulphides (50 %). Hence, this treatment presents at least two advantages. The first is environmental as just mentioned. The second is economic since chromium can be recycled for reuse in leather tanning. The results suggest that the recovered chromium is similar to commercial tanning chromium. These results also show that the fixing of chromium does not depend on the nature of the treated skin. In fact, a technical and economic analysis showed that proposed treatment can be economically beneficial (3112,5 $ per 1000 tons of skins).

FR :

La répartition horizontale du phytoplancton et du zooplancton a été étudiée dans le lac-réservoir El Kansera en période de stabilité thermique, en relation avec certains paramètres abiotiques (transparence, matières en suspension, conductivité, température, oxygène dissous, pH, nutriments azotés et phosphorés).

La répartition longitudinale du phytoplancton montre un gradient décroissant, avec des maxima de biomasse (poids frais) situés au 2/3 de la longueur du réservoir, et des pics de chlorophylle a enregistrés vers l'amont de la retenue. L'abondance du zooplancton suit une évolution quasi-analogue à celle du phytoplancton avec en plus, une répartition hétérogène des Rotifères et des différents stades de développement des Crustacés. Les Rotifères et les petits stades de développement des Cladocères ou des Copépodes sont concentrés dans la zone médiane du lac, tandis que les stades adultes de ces Crustacés dominent vers l'aval. La prédation vertébrée des " Carpes Chinoises ", associée à la mauvaise qualité de la nourriture algale en période d'étiage (blooms à Microcystis), seraient à l'origine de cette répartition. Les résultats de cette étude permettent de distinguer 3 zones de la retenue : la zone d'embouchure de l'oued** Beht (station S1) agitée, turbide et peu oxygènée est la moins productive, la zone médiane (stations S2 et S3), constituant la transition entre le milieu lotique et lacustre, est la plus enrichie en peuplements algaux et la plus peuplée par le zooplancton. Enfin, la zone lacustre proprement dite (stations S4 et S5) est assez productive et constitue un site idéal pour les adultes des Crustacés.

EN :

The goal of this study is to assess the degree of horizontal heterogeneity of phytoplankton and zooplankton communities in the lake-reservoir El Kansera (Morocco), in order to define the principal factors responsible for the plankton distribution in the lake.

Five stations have been studied; station S1 is the most influenced by allochthonous inputs from oued Beht (a tributary), stations S2 and S3 represent a transition between the lotic and lacustrine environments, and stations S4 and S5 represent true lacustrine conditions.

The phytoplankton and zooplankton samplings were carried out during two seasons in 1996 (spring and summer), with a plankton net which consisted of a silk cone (height: 120 cm, mesh size: 40 µm). An average sample was prepared by several vertical tows. The samplings of chlorophyll and abiotic parameters were performed at different depths with a polyvinylchloride bottle with a capacity of 5 L and an automatic closing device. The phytoplankton counting was done according to the method of Legendre and Watt (1972). The algal biomass (µg.L^-1) was estimated by chlorophyll-a (method of Lorenzen 1967) and wet weight (method of Lohmann (1908). Zooplankton counting was done in a Dolfuss cell (method of Pont 1983). The biomass of Rotifers, Cladocerans or Copepods (µg.L-1) was calculated according to the method of Dumont et al. (1975) and Bottrell et al. (1976). The physico-chemical analyses were performed at LPEE (Public Laboratory of Essay and Studies). Temperature, pH and conductivity were measured in situ, with a sounding Horiba U7. Dissolved oxygen was measured by the method of Winkler (Rodier 1984). Water transparency was measured by Secchi disc (Reynolds). Suspended matter was determined according to the method of AFNOR (1994). Nitrates and total phosphorus were analysed by colorimetry, respectively after reduction on a cadmium column and mineralization in acid medium (AFNOR 1994).

The lake-reservoir El Kansera presents a longitudinal gradient (oued Beht-dyke) of biotic and abiotic parameters in relation both to morphometric and hydrologic characteristics and to the importance of allochthonous inputs and the nutrient cycles (nitrates and phosphorus). The maximum phytoplankton abundance (wet weight) is observed at 2/3 of the lake's length, and the chlorophyll-a peaks are recorded in the upper zone. In the spring, two alga species are dominant in the lake, Scenedesmus ecornis and Cyclotella ocellata, whereas in summer, one species is abundant (Microcystis aeruginosa).

Zooplankton abundance shows a similar evolution to that of the phytoplankton, with a longitudinal heterogeneity of Rotifers (Brachionus calyciflorus, Polyarthra vulgaris and Keratella tropica) and different development stages of Cladocerans (Bosmina longirostris and Diaphanosoma brachyurum) and Copepods (Acanthocyclops robustus). Rotifers and the small stages of Crustaceans are concentrated in the median zone, whereas the Crustacean adults are dominant in the deeper point of the reservoir. Vertebrate predation by the " Chinese carp " and the degraded quality of the algae (blooms of Microcystis) also seem to contribute to this distribution.

The results of the study, confirmed by the statistical Student test, indicate three zones in the lake-reservoir: the upper zone (station S1), which is located at the mouth of the oued Beht, is the least productive due to its shallow depth, low oxygenation, high turbidity and frequent mixing. The intermediate zone, between lotic and lacustrine conditions (stations S2 and S3), is strongly enriched with nutrients (nitrates and phosphorus) and characterised by a higher productivity; this zone presents dense phytoplanktonic populations. The lacustrine zone (stations S4 and S5) is of intermediate productivity, and offers good conditions for large size classes of Crustacea.

EN :

One of the problems water managers and researchers have had to deal with for a long time is that of identifying the symptoms of eutrophication, with all its multiple facets. The search for a simplified model of this complex process does not seem, so far, to have attained this objective.

In order to identify and classify the various trophic states of waters (lakes or rivers), two main types of trophic indicators have been and are still being used, those belonging to the biocenosis (biological factors) and those belonging to the biotope (physical-chemical factors).

- The aim of the biological approach to eutrophication is to measure its impact on the environment's biodiversity. Thus, several classification indices have been drawn up, for example: the Biotic Index, which is one of the oldest; the General Quality Biological Index, which has been improved to become the Normalised Global Biological Index (NGBI) method; and more recently the Trophic Diatom Index. Working with such indices requires quite complex analyses since it is necessary to identify the local fauna and flora. Furthermore, these indices only apply to rivers, not to lentic environments.

- For the physical-chemical approach, the aim is to quantify the trophic state of an aquatic environment by measuring a number of physical-chemical parameters. This approach is easier to implement and the results provided are those currently used by water managers.

It is obvious that the two approaches are linked, since the biodiversity of an aquatic environment is conditioned by the physical-chemical quality of its water. However, the study of the eutrophication process in surface waters faces two main difficulties, which are inherent in the very nature of the system and the phenomena under study:

1. Since aquatic systems naturally differ one from the other, it is obviously difficult if not impossible to establish a reference state to determine the trophic level of a water with absolute accuracy.

2. The second difficulty lies in the choice that must be made among the analytical parameters, to select those that are the most appropriate to describe the phenomenon. Although it is currently admitted that the nitrogen, phosphorus and phytoplankton parameters, among others, cannot be ignored, their intrinsic values are not sufficient to completely describe the process of eutrophication. Indeed, the values of most of these parameters are linked both to the causes and the effects of eutrophication, and as a consequence they cannot be interpreted unambiguously. The same restriction applies to correlations that are established between these parameters and the plant biomass. Because of the originality of the lake system with which we are dealing, the present study offers the opportunity to better understand the reasons for both these problems.

The ten lakes we have studied (number 1 to 10) present the rare advantage of being supplied by the same streams, running across a restricted geographical zone that is geologically and climatically similar. However, the trophic characteristics of these waters have been altered by their passage through different agricultural and urban zones. Thus, lakes 1 to 4 are located in an area of low urban density and are colonised by phytoplankton, the density of which decreases from Lake 1 to Lake 4; macrophytes are not present in these lakes.

Lake 5, located in the centre of the town, receives domestic waste water. This lake was, over a long period of time, entirely covered with water hyacinths (Eichhornia crassipes), very invasive floating macrophytes, and with lotuses (Nelumbo nucifera) and rooted macrophytes. During the study period, following the manual removal of the macrophytes in July 1995, the water was strongly colonised by algae, as can be seen from the high concentration of chlorophyll-a, close to 200 µg/l.

Lake 6 presents a similar situation; it was almost entirely covered by Eichhornia crassipes until July 1997, and was then manually cleared. As the elimination of the water hyacinths profoundly modified the characteristics of this lake, we will show the two periods of time separately.

Lake 7 receives the waste water from a densely populated area. After being cleared in July 1997 of a thick layer of various plants that had covered it for several years, the lake was invaded by water lettuce (Pistia stratiotes), a floating macrophyte, which has dominated the lake since December 1997.

Finally lakes 9 and 10 are almost completely covered by lotuses (Nelubo nucifera), which are rooted macrophytes, along with a few Pistia stratiotes, while Lake 8 is periodically colonised by water lilies (Nymphea lotus) and algae.

To follow changes in the water quality of the 10 lakes studied, 21 sampling stations were chosen, usually at the entrance and exit of each lake. The mean values for the physical-chemical parameters for any lake gave a reasonable representation of the lake water under study. Sample collection in the ten lakes lasted two hours (between 8 and 10 a.m.). The sample collection, and the analysis of the 19 physical-chemical parameters taken into account, was carried out between April 1996 and April 1998 (twice a month in the rainy season and once a month otherwise), which represented about 15 000 measurements. At each location, a litre of water was taken, 50 cm below the surface, with a polyethylene bottle fixed on a 5 m bamboo pole. Portions of 250 ml were then transferred into a brown glass bottle, for later analysis of chlorophyll. After the in-situ analyses, the bottles were kept in the dark in a cooler.

Because of the unique sampling situation, it might have been anticipated that the comparison of the different trophic states on the basis of the physical-chemical variables would be possible, but this was not the case. On the contrary, this study tended to show that the physical-chemical characteristics of these waters are as much influenced by the feedback effect of eutrophication as by the external factors, so that the contribution of each of the variables to the various types of eutrophication cannot be clearly determined. This is particularly true in the case of parameters characteristic of nutrients or those reflecting the water response.

Although it may appear trivial, this "feedback action" can naturally be generalised to all lake systems, in temperate or tropical climates, whatever the kind of biomass that colonises them. The trophic level of these waters must therefore be evaluated on the basis of new criteria, which will take into account all the relations that link the various parameters.

FR :

L'étude des phénomènes d'eutrophisation se heurte essentiellement à deux difficultés qui sont inhérentes d'une part, à la diversité spatio-temporelle des eaux superficielles, et d'autre part, à l'action qu'exercent en retour les phénomènes d'eutrophisation sur ces eaux.

- La diversité spatio-temporelle des systèmes aquatiques s'accompagne d'une diversité des phénomènes d'eutrophisation qui en sont le siège et a pour conséquence de rendre difficile sinon impossible l'établissement de critères de comparaisons entre les différents systèmes aquatiques. Dans ces conditions il est peu aisé de faire le choix objectif d'un état de référence universel qui seul permettrait de déterminer de manière absolue le niveau trophique d'une eau.

- La deuxième difficulté, et sans doute la plus grande, est liée à l'action qu'exercent les phénomènes d'eutrophisation sur les variables analytiques du système. Cette action a pour conséquence de rendre les valeurs intrinsèques de ces variables impropres à l'étude de ces phénomènes et de leur évolution. Ces variables étant liées simultanément aux causes et aux conséquences des diverses manifestations des phénomènes d'eutrophisation.

Les 10 lacs que nous avons étudié ici, présentent l'avantage peu courant d'être alimentés par les mêmes eaux traversant une zone géographique restreinte et par conséquent géologique et climatique identique. Ils sont soumis à l'influence d'activités agricoles ou urbaines différentes mais bien identifiées, entraînant pour ces eaux des caractéristiques trophiques différentes. Dans un tel cas, il aurait pu sembler a priori possible d'établir une classification des différents états trophiques des lacs. Cependant, même dans une situation aussi idéale, et essentiellement à cause des deux difficultés mentionnées précédemment, nous tentons de montrer ici qu'il n'en est rien. Cela amène les commentaires d'ordre général suivants:

- Si pour le système lacustre étudié ici, probablement parmi les plus idéaux, il est difficile à partir des critères généralement retenus dans la plupart des études, (nutriments azotés, phosphorés , chlorophylle, oxygène dissous, etc.) d'établir un classement trophique, il semble a fortiori peu probable que cela puisse être réalisé sur des systèmes moins idéaux.

- La littérature actuelle fait clairement apparaître ce problème majeur. Cependant pour y remédier, elle propose généralement de faire le choix d'autres indicateurs physico-chimiques ou même biologiques qui, pour les mêmes raisons, semblent également voués à l'échec.

En conclusion de cette étude, nous proposons donc d'abandonner de tels indicateurs liés au milieu et de choisir de nouveaux critères qui prendraient en compte les perturbations apportées par le phénomène aux interactions liant les variables analytiques: les différents états trophiques d'une eau étant probablement mieux caractérisés par les différents états des relations liants les variables analytiques que par les valeurs intrinsèques de ces variables.

FR :

Les applications potentielles des cultures de microalgues et cyanobactéries en dépollution d'effluents dans des photobioréacteurs à membrane souffrent de performances limitées par un colmatage de l'élément filtrant dû en grande partie aux exopolysaccharides sécrétés par ces micro-organismes. Cette étude du laboratoire quantifie les effets de l'adsorption de ces polysaccharides sur des membranes organiques d'ultra et microfiltration tangentielle de matériaux et charges de surface différents. L'alginate de sodium est utilisé comme adsorbat modèle. Les membranes propres sont d'abord testées par une mesure de flux à l'eau pure, puis mises en contact avec une solution d'alginate durant un temps choisi. Le flux à l'eau pure des membranes après adsorption est ensuite à nouveau mesuré.

La réduction relative du rayon de pore (ZEMAN, 1983) met en évidence l'effet de la mouillabilité et des charges superficielles. L'étude comparée de membranes d'ultra et microfiltration montre que cette réduction relative du rayon de pore augmente avec le seuil de coupure ou le diamètre de pore. L'effet de la concentration révèle aussi que la résistance hydraulique d'adsorption (MATTHIASSON, 1983) à l'équilibre évolue selon l'isotherme de LANGMUIR. Le modèle cinétique traduisant l'évolution de la résistance d'une membrane d'ultrafiltration proposé par BAKLOUTI et al.(1984), amélioré par AIMAR et al. (1988) puis discuté par RUIZ-BEVIÁ et al. (1997), est complété par un nouvel exposant agissant sur le facteur temps.

La comparaison des résistances à l'écoulement de membranes de microfiltration avec celle d'une membrane d'ultrafiltration hydrophile neutre permet de dégager des critères de choix pour l'optimisation du fonctionnement d'un photobioréacteur à membrane utilisable en dépollution d'effluents.

EN :

Potential applications of microalgae and cyanobacteria for treatment of wastewater effluents using membrane-photobioreactors suffer from limited performance due to fouling effects, mainly attributable to exocellular polysaccharides secreted by these micro-organisms. A membrane photobioreactor is defined as a process associating the culture of photosynthetic micro-organisms with a continuous separation by membrane filtration of the biomass and the water treated. The goal of the present laboratory-scale study was to quantify polysaccharide adsorption effects on organic membranes (ultra and microfiltration) characterised by different materials and surface charges. Sodium alginate was used as the "model adsorbate".

Seven plane organic membranes were tested. The influence of membrane cut-off (or of pore diameters) as well as that of the material polyethersuphone (PES), polyacrylonitrile (PAN), polyvinilidene fluoride (PVDF) and of its properties (hydrophobicity, surface charges, …) were assessed. The study consisted of two parts :

1. the first part was concerned with the kinetics of alginate adsorption and the influence contact time and solute concentrations on the reduction of pore diameter (ZEMAN, 1983) or on the increase of hydraulic resistance (MATTHIASSON, 1983);

2. the second part dealt with adsorption equilibrium (formulations of LANGMUIR and FREUNDLICH).

The study constituted the first step of a research program aimed at developing membrane photobioreactors for the treatment of specific industrial effluents.

The fluid used to test the membranes was quality II pure water (ISO 3696 norm). Tangential velocities were set to 2.5 m.s-1, corresponding to a Reynolds number of 2500. To represent exopolysaccharides, we used alginic acid at concentrations of 1, 10 and 50 g, neutralised with sodium hydroxide at pH 9. New (or clean) membranes were first characterised through pure water flux measurements. J₀, the flux of pure water for a new membrane, was obtained (flowrate / unit of surface area), and then the membrane was kept in contact, for a definite duration, with the alginate solution. After adsorption and rinsing, the pure water flux was measured again. Ja, the pure water flux, was measured through the membrane after adsorption.

Adsorption model at equilibrium:

The effect of adsorption is quantified under the form of the relative pore size reduction as described by ZEMAN (1983) and included in the relation : ∆r / r=1 - (Ja / Jo)^1/4. A variation of this quantification is that of the MATTHIASSON model (1983) applied to the pure water flux, based on DARCY's law expressing the relative value of the hydraulic resistance of the adsorbed layer Ra in relation to the intrinsic resistance of the membrane Rm : Ra / Rm=(Jo / Ja) - 1.

To express adsorption phenomena at the solid/liquid interface of membranes, we used LANGMUIR's law together with MATTHIASSON's experimental observation (1983): the relative resistance Ra / Rm due to adsorbed compounds is proportional to the mass "x" of solute adsorbed per unit of membrane surface area, x=Kx.Ra. If one assumes that the mass m of a homogeneous plane membrane per unit of membrane surface area is proportional to its adsorbing surface area Ω per unit of membrane surface area (m=Km.Ω), and if one combines the flux equations expressed by DARCY's and POISEUILLE's laws, then the result is m=K'm.Rm in a homogeneous membrane. Substituting x and m in LANGMUIR's law results in the equilibrium model Rae / Rm=(Jo / Ja) - 1=a.c / (1 + bc) in which c=concentration of adsorbing solute; a and b are coefficients; and Rae is the resistance due to compounds adsorbed at equilibrium.

Kinetic model:

To show the evolution of membrane resistance with time, we suggest the introduction of an empirical exponent j over the time parameter in the AIMAR et al. model (1988).

Results:

The effect of changing the alginate concentration reveals that the hydraulic resistance of adsorption, at equilibrium, (MATTHIASSON, 1983) evolves according to LANGMUIR's isotherm. The relative decrease of pore radius ∆r / r in the presence of l g.l^-1 of sodium alginate shows that a quasi-plateau is obtained after two hours using the most hydrophobic membrane.

The curves ∆r / r=f (t) for five membranes made of different materials, monitored during the transition phase before the plateau with common 1 g.l^-1 concentrations, reveal similar adsorption behaviour, characterised by the limiting common value ∆r / r=0.06 ± 0.005. However, the uncharged hydrophilic membrane PAN 3038 stands out owing to a much lower ∆r / r value of 0.09. This peculiar behaviour can also be observed in the influence of the alginate concentration: hydrophobic and charged hydrophilic membranes display a saturation effect with ∆r / r little affected by the increase of alginate concentration, whereas the uncharged hydrophilic membrane PAN 3038 displays a ∆r / r value three to six times lower with great sensitivity to concentration effects at concentrations below 10 g.l^-1. The model Rae / Rm=(Jo / Ja) - 1=a.c / (1 + bc) is in agreement with the experimental results obtained with hydrophobic and hydrophilic membranes.

The proposed kinetic model shows that time dependence of R (t) does not seem to be linked to the nature of membranes. However, compared with concentration, R (c) is very sensitive to the nature of membranes. A comparative study of ultra and microfiltration membranes shows that the reduction in ∆r / r values increases with molecular weight cut-off (or pore diameter).

Criteria for the choice of membranes:

A comparative study of three polyacrylonitrile membranes reveals that membrane 3038 PAN (neutral) displays a very interesting, peculiar behaviour: its adsorption, expressed by ∆r / r or Rae/(Rae+Rm) is four to six times weaker than that of the other two. The surface charge of membranes seems to influence the intensity of adsorption in a significant way.

Wetability also has a strong influence on adsorption. The sum of resistances Rae + Rm of ultrafiltration membrane 3038 PAN is only four times as great as those of hydrophobic microfiltration membranes. Experimentation already showed that, in the presence of microparticles, interactions between the layer of adsorbed alginate and microparticles will increase the likelihood of fouling of microfiltration membranes, decreasing their resistance down to the level of very little adsorbing ultrafiltration membrane IRIS 3038 (ROSSIGNOL et al., 1999).

A culture system of marine microalgae in a membrane photobioreactor using ultrafiltration membrane IRIS 3038 PAN displayed a stable permeation flux during 6 weeks and easy regeneration, which meant adsorption was almost nil. The ability of some microalgae to assimilate ammonia nitrogen, nitrates and phosphates contained in waste water with excellent efficiencies (e.g., Phormidium bohneri: SYLVESTRE et al., 1996) allows one to consider using membrane photobioreactors in the treatment of home or industrial effluents. Other microalgae such as Chlorella salina (GARNHAM et al., 1992) are capable of fixing large amounts of heavy metals (Co, Mn, Zn, etc…); grown in membrane photobioreactors, they could depollute industrial effluents.

FR :

L'ultrafiltration assistée par les micelles (UFAM) a été utilisée pour la récupération des ions plomb (II) en milieu acide (Phosphorique et perchlorique) à la température 318K. Le dodécylsulfate de sodium (SDS) a été choisi comme agent tensioactif. Des taux de rétention de 99% ont été obtenus dans des conditions optimales de pression, de concentration en SDS, de vitesse de circulation du fluide et de pourcentage du volume filtré. L'équilibre d'échange ionique qui s'effectue entre le contre ion de la micelle et le cation métallique ajouté (Pb²⁺), a été caractérisé par une contante Ke(Na⁺/Pb²⁺)=1,39. Afin d'étudier la performance de l'UFAM dans les milieux acides, l'étude a été effectuée à concentration de Pb(II) constante et en variant le pH. À des pH supérieurs à 1,8 la rétention est supérieure à 95%.

EN :

The increasing contamination of urban and industrial waste water by toxic metal ions is a worrying environmental problem. These inorganic micro-pollutants are of considerable concern because they are non-biodegradable, highly toxic and in some cases have a probable carcinogenic effect. If directly discharged into the sewage system they may seriously affect the operation of biological treatment systems and render the activated sludge unsuitable for application to agricultural land.

The traditional techniques for the removal of metal ions from aqueous effluents are incapable of reducing concentrations to the levels required by law (reduction or lime precipitation) or prohibitively expensive (ion exchange, activated carbon adsorption or electrolytic removal). The use of membrane separation processes to treat waste water containing toxic metal ions is today an attractive and suitable technique, and it can easily be included in the process, which is the reason why membrane separations are being used more and more frequently. Separations can be carried at room temperature, the modular membrane surface can be easily adjusted to the wastewater flows, and various industrial membranes are now available. In order to retain metallic ions, reverse osmosis (or at least nanofiltration) can be used, due to the size of the ions in aqueous solutions. However, the usual permeate fluxes of reverse osmosis membranes are limited and require high transmembrane pressure, which makes the process expensive.

During the last decade, there has been a constantly increasing level of interest and research efforts in order to improve the performances of surfactant-based separation processes. In the present study an attempt is made to remove lead(II) ions from synthetic aqueous solutions by micellar-enhanced ultrafiltration (MEUF) using the anionic surfactant sodium dodecylsulfate (SDS). The study has been carried out at a temperature of 318 ºK and on a laboratory scale.

Ultrafiltration experiments were carried out with a tangential cell system. The inlet flux was held constant (up to 0.5 m.s^-1) and the drop in pressure was varied from 1 to 3 bars by restricting the outlet tube. Polysulfone membranes with a molecular weight cut-off (MWCO) of 10,000 Da and an effective filtration area of 30 cm2 were used. The influence of the operating parameters on permeate flux and lead rejection was studied. Rejection coefficients of 99% are reached under optimal conditions of pressure, feed concentration in SDS, tangential velocity of the feed, and percent filtered volume.

An ionic exchange model has been used to study the interaction between the lead cation (Pb²⁺) and sodium dodecylsulfate micelles. The model used to fit the experimental data is an ionic micelle in which the electric double layer is divided into a diffuse outside layer and a Stern layer inside the shear surface. Assuming that divalent cations are strongly attached to the micellar surface and located specifically in the Stern layer of the micelle, it has been found that adsorption in the Stern layer is well described by a Langmuir isotherm. From this isotherm, ion exchange constants for Pb²⁺ with the Na⁺ counter-ion have been determined Ke (Na⁺ /Cd²⁺) -=1.39, and have been compared with those obtained for other cations (Cd²⁺, Mg²⁺) in the same media.

In order to determine the performance of the MEUF process in acidic streams, studies were performed at constant cation concentrations and various pH values. The pH variation was obtained by the addition of HClO₄ or H₃PO₄. Rejection remains higher than 95% provided the pH is maintained higher than 1.8.

FR :

La prévention du risque d'inondation nécessite une connaissance détaillée du régime hydrologique en crue du bassin étudié. Dans ce but, l'approche débit-durée-fréquence (QdF), développée depuis déjà plusieurs années, a permis de définir un modèle statistique décrivant les crues observées en fonction de leur débit, de leur durée et de leur fréquence. Un récent travail a revisité cette approche. Grâce à son nombre réduit de paramètres, le modèle proposé, appelé modèle local convergent, peut être facilement ajusté pour chaque bassin. Dans l'ancienne approche, que nous appelons approche " bassin de référence ", l'ajustement local avait été effectué sur seulement trois bassins, dits de référence, et réputés être chacun représentatif d'une typologie d'écoulement différente. Ces trois paramétrisations types ont ensuite donné lieu à trois modèles adimensionnels, capables de caractériser la majorité des régimes observés. Le modèle adimensionnel correspondant au régime du bassin étudié devait être dénormé par deux caractéristiques locales du bassin : le débit instantané maximal de crue décennale et une durée caractéristique de crue. Une comparaison du nouveau modèle, appelé modèle local convergent, et de l'approche type " bassin de référence " a été effectuée sur une cinquantaine de bassins jaugés. Elle met en évidence la robustesse du modèle convergent et permet de discuter du choix du modèle relatif à l'approche " bassin de référence ". Le modèle local convergent autorise d'envisager le développement d'un modèle QdF régional, s'inspirant de différentes méthodes de régionalisation. Ceci permettra alors une application à des bassins peu ou non observés.

EN :

Flood risk mitigation requires a good knowledge of hydrological flood regime, which can be described by a flow-duration-frequency (QdF) approach. New developments of this approach are presented and compared to the former method.

Usually, flood frequency analysis deals only with the maximum flood peak distribution or the maximum daily discharge distribution. The QdF approach analyses maximum average flows over different durations d (d =1, 3, …, N days). Similar to intensity-duration-frequency curves, each of the QdF curves represents the flood frequency distribution, for the duration d. QdF modelling aims to express QdF curves by a Q(d,T) function (d : the duration; T : the return period).

Before this present work, QdF modelling was associated with the "reference basin" approach. In this approach, QdF curves (plotted as a function of d, for fixed T) of many studied basins are converted into a dimensionless form. The two characteristics used are the 10-year peak flood, Q(d=0, T=10 years), and a characteristic flood duration (D) of the studied catchment, calculated from different flood hydrographs. Then, three different families are determined, grouping basins with similar dimensionless QdF curves. For each of these families, one reference basin is chosen. Their dimensionless curves are parameterised, in order to obtain a continuous formulation, as a function on T and d. By denormalising one of these dimensionless QdF models with the local parameters Q(0,10) and D, it is possible to obtain the continuous Q(d,T) formulation for the studied basin. The choice of the correct dimensionless model is made via a choice criterion. It involves Q(0,10), D and shape parameters of local maximal rainfall distributions (a Gumbel law is assumed), for different durations, d. These distributions are obtained according to the intensity-duration-frequency approach. If the studied basin is ungauged, local parameters Q(0,10) and D are estimated by regional formulas, involving significant variables such as catchment area and rainfall.

Recent work has improved this "reference basin" approach. A new QdF model, called convergent local, has been developed. For fixed T, the model assumes that the Q(d,T) is described by a hyperbolic form, as a function of d. This choice of the hyperbolic form is based on the observation of many catchments (about one hundred). It has also been observed that QdF curves, plotted for fixed d as a function of T, converge toward the same point, when T decreases. Using these observations as assumptions, the model is then able to calculate Q(d,T) for any return period T and any duration d.

If a two-parameter statistical law (such as the exponential law) is adopted, the model contains only 4 parameters. The first parameter is the limit of Q(d,T), when d tends to infinity. It is estimated by calculating the average value over the entire observed period of the Q(t) discharge time series. The second one gives the hyperbolas curvatures and is ∆. The ∆ parameter has a time dimension and is consequently a characteristic duration of the studied basin. The final two parameters are the location and shape parameters, x ₀ (0) and a _q (0), of the exponential maximal flood distribution for d=0. x ₀ (0), a _q (0) and ∆ parameters are directly adjusted on observed QdF curves of the studied basin.

The comparison between the convergent local model and the "reference basin" approach has been carried out on about 50 basins, drawn from different regions of France. For each basin, the two approaches have been tested. First, the two characteristic durations D and∆, defined respectively by the "reference basins" approach and the convergent local model, are compared. As mentioned earlier, ∆ characteristic duration is an adjusted parameter and its calculation does not depend on D. In spite of their different definitions, a strong correlation between these two parameters is observed. This shows a good coherence between the two tested approaches. Second, in order to compare results, a relative mean error between calculated and observed values is determined for each basin and each model. Only the observed domain (T ≤ 20 years) has been considered, because the extrapolations cannot been validated with observed data.

Concerning the "reference basin" approach, the three reference basin models are studied, and the choice criterion is applied. Results show that this choice criterion is not relevant. Concerning the convergent local model, the observed mean relative error is lower than in the "reference basin" approach. These good results are confirmed by a very small error dispersion. Consequently, the convergent local model is robust.

As a conclusion, this paper presents new developments of the QdF approach: the convergent local continuous model. This model, locally adjusted, yields very satisfactory results. The next step is to apply it on ungauged basins, as is possible in the "reference basins" approach. This could be done by adapting regional methods, such as the index flood method.

FR :

Les résidus de transformation du bois tels les écorces et la sciure de bois ont été largement étudiés depuis quelques années pour leur propriété d'adsorption et d'enlèvement des métaux toxiques contenus dans les effluents contaminés. En ce qui concerne la sciure de bois, les recherches répertoriées ont porté principalement sur l'utilisation du sapin rouge, du manga, du tilleul, de l'épinette, du pin, du cèdre, du teck, de l'akamatsu et du buna. Pour ce qui est des écorces de bois, plusieurs espèces ont été étudiées, notamment les écorces de pin, de chêne et d'épinette. La présente revue fait le point sur les performances de ces différents adsorbants peu coûteux pour l'adsorption des principaux métaux contaminants (Cd, Cr, Cu, Hg, Ni, Pb et Zn). Les points discutés portent sur les méthodes de préparation (lavage, séchage et tamisage) et de traitement chimique de l'adsorbant (traitement acide ou basique, traitement à la formaldéhyde, phosphatation, carboxylation, sulfoéthylation, carboxyméthylation, etc.), les conditions opératoires utilisées lors de l'adsorption, les modèles thermodynamiques, cinétiques et autres applicables au couple adsorbant-adsorbat, l'effet des principaux paramètres opératoires (temps de contact, pH de traitement, température, concentration d'adsorbant, taille des particules, etc.), les principes et les mécanismes impliqués dans l'élimination des contaminants métalliques par les adsorbants présentés.

EN :

Wood industry by-products such as barks and sawdusts have been widely studied in recent years for their property of metal adsorption and metal removal from contaminated effluents. Concerning the utilization of sawdusts, many researchers have studied metal adsorption on material from species such as red fir, mango, lime, pine, cedar, teak, Japanese red pine and Japanese beech. As regards wood barks, several species were studied, in particular pine, oak and spruce. The present review gives a progress report on the efficiency of these various inexpensive materials for the adsorption of different metals (Cd, Cr, Cu, Hg, Ni, Pb and Zn). The points discussed relate to the preparation methods (washing, drying, screening) and the chemical treatments of the adsorbents (acid or base treatment, formaldehyde treatment, phosphatation, carboxylation, sulfoethylation, carboxymethylation, etc.). We also consider the operating conditions used during adsorption, the thermodynamic, kinetic and other models applicable to the adsorbent-adsorbate couple, the effect of the operational parameters (time of contact, pH, temperature, adsorbent concentration, particle size, etc.), as well as the principles and mechanisms involved in metal removal by the adsorbents.

The accumulation of organic or inorganic matter at the solid-liquid interface is the basis of almost all surface reactions. Adsorption is often a process described in terms of isotherms, which represent the relationship between the concentration of a solute in solution and the quantity adsorbed at the surface at constant temperature. The isotherms are often used to establish the maximum adsorption capacity of a given adsorbent for metals. Langmuir and Freundlich isotherms are the most frequently used and their models are presented in this review.

Knowledge of adsorption parameters is essential for understanding the adsorption mechanisms involved. Usually, the maximum adsorption capacity for sawdusts and barks is reached after one hour. The pH of the ambient water is a very important parameter because it affects the metal adsorption capacities. For most of the metals studied, the adsorption capacity increases when the pH increases. The opposite effect is observed for metals involved in an anionic complex (Cr, Se, Pt, Au). Metal adsorption efficiency also improves with increases in substrate concentration because there are more available adsorption sites. Metal adsorption is affected by an another important factor, the particle size. In effect, a smaller size particle increases the specific surface and improves the adsorption capacity. The presence of anions in the effluent doesn't appear to have a great effect on adsorption results with sawdusts. However, some anions were reported to have an influence on the metal adsorption capacities of barks. In the case of a metal mixture, the presence of one metal may influence, compete or exclude the recovery of another metal from the solution.

Sawdusts contain lignin, cellulose, tannin and protein. Wood tannin likely serves as a primary adsorption site for divalent cations. The application of chemical treatments on sawdusts could modify the lignin functional groups. Other studies regarding the participation of major components of barks (lignin, carbohydrate and protein) in the adsorption process revealed the involvement of amine and carboxyl functional groups. The proposed mechanism involves an ion-exchange process. This phenomenon suggests that cationic exchange is the active mechanism for some wood species, in agreement with the work of some researchers.

Currently, only a few industries use plant biomass to eliminate metals in wastewater. To encourage industries to use this biotechnology, research has to be oriented towards the cheapest and most competitive process rather than the current and conventional process. Forest waste products are produced in large quantities in several countries. They constitute easily-available resources of low cost. Future research in this field should be focused on cheap new chemical treatments to apply to by-products to improve their adsorption capacities.