Volume 12, Number 4, 1999

FR:

Le long de la côte méditerranéenne française du Golfe du Lion, l'étang de Thau présente des caractères assez particuliers. Il est parfois soumis à des conditions anoxiques appelées "malaigues" qui résultent de l'accumulation de matières organiques durant la période chaude liée au développement des macrophytes. Ces dépôts organiques associés aux biomasses résultant des activités conchylicoles et aux apports extérieurs contribuent en cours d'année aux échanges biogéochimiques entre la colonne d'eau et les dépôts.

Dans ce même milieu, l'analyse de la distribution et de la nature de la matière organique par des méthodes fines comme la chromatographie liquide haute performance ou la pyrolyse a permis de préciser son origine et son évolution dans la colonne d'eau et les dépôts. Durant les quatre saisons, les particularités de la matière organique ont donc été analysées en terme d'accumulation, de dégradation et de conservation. L'été constitue une période de production et de dégradation. L'automne est principalement caractérisé par des processus dégradatifs et des apports terrigènes (composés phénoliques). L'hiver correspond à une période de relative stabilité de la matière organique consécutive aux conditions froides. Le printemps enfin représente une période de reprise de l'activité biologique produisant une matière organique fraîche riche en sucres.

Sous les tables conchylicoles on observe un accroissement de la matière organique dans la colonne d'eau et les dépôts. Mais les processus actifs de dégradation réduisent considérablement la quantité de matière organique déposée. Les résultats de ces mécanismes varient selon les stations sous table et hors table.

Dans les dépôts les résultats de la dégradation dans la colonne d'eau amènent à une décroissance des composés biodégradables et à un accroissemenet des composés résistants comme les phénols et les hydrocarbures aromatiques. Ces processus de minéralisation s'accroissent vers la profondeur dans les dépôts au profit du pôle aromatique.

Les relations entre les nutriments et la matière organique qui constitue à la fois leur source et leur puits se marquent bien sous les tables conchylicoles où les sels nutritifs s'accumulent en surface.

EN:

The Thau lagoon along the French Mediterranean coast of the Gulf of Lions has unusual characteristics. It is sometimes subjected to anoxic conditions, known as "malaigues", which result from the accumulation of organic matter during the warmer period. Throughout the year this organic deposition, associated oyster farming and terrigenous inputs, contributes to biogeochemical exchanges between the water column and the underlying deposits. In this same environment, high-resolution analytical techniques (HPLC ; PY-GC-MS) were used to analyze the distribution and nature of the organic matter and to determine its origin and behaviour in the water column and sediments.

Total suspended matter (TSM) was determined by filtration of water samples pumped up from different levels of the water column and filtered onto glass fiber filters (GF/F grade) previously heated at 450 °C for 4 hours. Particulate organic carbon (POC) was determined on the same samples with a Leco CS 125 analyzer after removal of inorganic carbonates by treatment with a H₂ SO₄ (2N) solution. Dissolved organic carbon (DOC) was determined on the filtrates using a Shimadzu TOC 5000 analyzer. The determination of polysaccharides in the TSM was achieved by a colorimetric method involving a H₂ SO₄ (3N) solution and anthrone reagent (Gallali 1972).

Phenolic compounds were determined by high performance liquid chromatography (HPLC) after cupric oxide alkaline oxidation of TSM samples. The oxidized samples were acidified (HCl, 2N) and subjected to liquid-liquid extraction with ethyl acetate (Hartley & Buchan 1973; Hedges & Ertel 1982). The limit of detection is 10^-4 g and the precision of the method is about 2% for each compound. Separation and quantification of phenolic monomers was carried out by HPLC (Hartley & Buchan, 1973 ; Serve et al., 1983). Of a total of 28 identified products, eleven represent the monomers constituting lignin and are taken into account according to Hedges & Parker (1976), Hedges & Mann (1979) and Hedges & Ertel (1982). The products of oxidative hydrolysis of lignin belong to the following three series : 4-hydroxybenzyl "H" (p-hydroxybenzoic acid, p-hydroxybenzaldehyde, p-hydroxyacetophenone), 3-methoxy-4-hydroxybenzylic "V" (Vanillyl) and 3,5-methoxy-4-hydroxybenzylic "S" (Syringyl). Each of these three series presents an alkyl side chain with 1, 2 or 3 carbon atoms. The compounds in C6-C1 can be acids or aldehydes, those in C6-C2 are ketones and those in C6-C3 are acids. The latter, having a phenylpropenic structure, belong to the Cinnamyl "C" series (ferulic acid, p-coumaric acid). Separation of phenols was carried out on a Merck analytical column (250 mm long x 4 mm in diameter) with a Lichrosorb reversed phase C18 stationary phase of 5 µm granulometry, equipped with a precolumn (40 mm long) containing the same phase. Elution was achieved with ternary eluents (water, acetonitrile, acetic acid), in a high pressure binary gradient (Charrière 1991). The eluted products were determined qualitatively, by comparison of their retention times with those of commercial products (detection in UV at 275 nm), after a co-injection if necessary, and quantitatively by an internal standard method (phloroglucinol : 1,3,5-benzenetriol and p-anisic acid : p-methoxybenzoic acid).

Analysis of the major classes of organic compounds was carried out by coupled pyrolysis - gas chromatography - mass spectrometry. A CDS 1000 pyrolysis probe was directly fitted with a Perkin-Elmer 8700 gas chromatograph (GC) equipped with a TR-WAX capillary column (length: 30 m, diameter: 0.32 mm, phase thickness: 0.50 µm). Pyrolysis temperature was 700 °C for 10 s and the column temperature was programmed from 60°C to 240 °C at a rate of 6 °C/min according to Puigbo et al. (1989). Pyrolysis fragments were identified by coupling the GC to a HP 5989 mass spectrometer. Twenty three major peaks were selected on the pyrochromatograms and each selected compound was expressed as a percentage of the sum of the surface of these 23 peaks Pyrolysis products were grouped into five main families, each of them including similar molecules or closely related chemical structures: aromatic hydrocarbons, nitrogenous compounds, sugars, phenols and amino sugars.

The survey of all these parameters showed some characteristic differences over the four seasons. Summer appears as a period when the biological production reaches maximum levels in the water column. At that time, organic matter is stratified with high levels of accumulation in the deeper layers. DOC is also abundant throughout the water column and organic compounds belonging to the class of sugars decrease according to depth. Autumn corresponds to Mediterranean storms and typical rainfalls. Terrestrial inputs increase in this season and degradative processes affect the organic matter that was produced in large quantities in the summer by the autotrophic organisms of the lagoon. DOC is recycled and reflects the degradation of autochthonous organic material. Winter, with reduced TSM levels related to low terrestrial inputs, is characterized by a homogenization of the water column and a weak biological activity. Lignin-derived phenols are abundant and correspond to a period of low biological activity. In contrast, in the spring the biological activity recovers, as indicated by the high sugar content of the DOC and by a homogenization of the water column.

Under the oyster beds, an increase of organic matter is observed in the water column as well as in the sediments. However, the active degradation processes in summer and autumn reduce considerably the amount of the settling organic matter. The results of these processes are variable according to whether the stations are under or outside of the oyster beds. Degradation in the water column leads to a decrease of biodegradable compounds in the sediments and an increase in resistant compounds like phenols and aromatic hydrocarbons. These mineralization processes increase with depth in deposits, as reflected by higher proportions of aromatic compounds. The relationship between nutrients and organic matter, the latter constituting both their source and their sink, appears in sediments under oyster beds, where the inorganic nutrients accumulate at the surface.

FR:

Les rejets urbains par temps de pluie constituent à l'heure actuelle une des causes majeures de la pollution du milieu naturel. Dans ce contexte, la modélisation est un des moyens pour comprendre, caractériser et finalement anticiper cette pollution.

L'objet de cet article est la présentation générale du modèle de simulation HORUS et les principaux résultats obtenus en phase de validation.

HORUS est un modèle de recherche événementiel de type conceptuel et a pour objectif de reproduire le fonctionnement d'un réseau d'assainissement par temps de pluie, au point de vue de l'hydraulique et de la pollution pour des réseaux pluviaux ou unitaires pouvant contenir des ouvrages particuliers. HORUS simule les différents phénomènes d'accumulation et d'érosion des solides sur les surfaces imperméables ainsi que les phénomènes de sédimentation ou d'érosion en collecteurs, en respectant un niveau de complexité homogène pour l'ensemble des étapes de calcul. Les polluants simulés sont les Matières En Suspension (MES), Demande Chimique en oxygène (DCO) et Demande Biochimique en oxygène à 5 jours (DBO5). HORUS a été calé, validé et transposé sur dix réseaux réels de caractéristiques et de localisations variées, avec une centaine d'événements pluvieux de caractéristiques très différentes. Les différents sites et mesures ont permis une large validation et l'obtention de résultats tout à fait satisfaisants pour les pollutogrammes en concentration et flux. Les résultats de validation qualitative et quantitative sur différents bassins versants sont proposés au sein de cet article.

EN:

Urban wet weather pollution represents at the present time one of the major causes of pollution of receiving waters. During a storm event, generation of pollution and erosion of the impervious area or the sewer itself are very complex processes, which respond to a large number of relevant factors. In this context, modelling is one way to study, characterize, understand and eventually anticipate this pollution.

HORUS is an event-based conceptual research model. The objective of the HORUS model is, taking into account the characteristics of the catchment and the structure of the sewerage system, to reproduce the hydrographs and TSS, COD and BOD concentrations generated by any rainfall event. The desire to develop a model with a reduced number of parameters oriented our choice towards conceptual formulations, calibrated and validated on the maximum number of real sites. HORUS simulates the principal phenomena involved in the generation and transfer of pollution in urban drainage systems, with a homogeneous level of complexity for all steps in the calculations. The algorithms have been chosen for their robustness, their simple mathematical formulation, their reduced number and their suitability for calibration and validation. HORUS consists of five main connected modules:

- a hydrological module consisting of a linear reservoir including runoff losses

- a hydraulic module based on the solution of the Muskingum equations (including particularities such as weirs, basins, sluice gates, ...)

- a module for producing and transporting solids in the catchment, which covers several phenomena. The build-up model of catchment deposits is one initially proposed by the SWMM. The washoff by rainfall is modelled by a modification of the SWMM formulation, adapted for a large range of rainfall events. The propagation of particles by runoff is modelled using a linear reservoir with a lag-time different from the runoff initially proposed by Brombach. COD and BOD are calculated through a potency factor governing the relations between these pollutants and TSS. This module includes an algorithm for gully pots; this algorithm has been programmed but is not yet activated.

- a module for producing and transporting solids in the sewer system. Sediment transport in the sewer is modelled by the Velikanov theory, with the calculation of sedimentation and erosion of solids during dry or rainy weather. Pollutographs are propagated by convection. COD is calculated through a potency factor governing the relations between this pollutant and TSS. This module includes algorithms for bed load and wash load calculations; these algorithms have been programmed but are not presently active.

- a module for numerical optimization using the Powell method and evaluation of hydraulic and pollution results for calibration and continuous simulations.

We have used ten different sites (combined, mixed or separate systems, with or without deposits in collectors), where there were reliable quantitative and qualitative measurements during main rainfall events, for the phases of calibration, validation or transposition. The urban catchments present different characteristics: imperviousness coefficients between 22 and 78%; mean catchment slopes between 0.3 and 6.5%; collector slopes between 0 and 27.4 mm/m. The rainfall events used for model development cover a wide range: antecedent dry weather periods (ADP) between 2 hours and 21 days; rainfall heights up to 40 mm; rainfall peaks over five minutes (Imax5) between 2.2 and 60 mm/h; and rainfall durations between 20 minutes and 12 hours.

During the calibration stage, the different parameters were adjusted through numerical optimization using the Powell method. This method allowed the choice of one group of parameters for all the catchments and rainfalls used in this stage. The validation phase was carried out in two stages. The first stage was completed with 47 other rainfall events (with different dry weather periods, rainfall heights, rainfall peaks,...) on the same sites that were used for the calibration. The second stage was carried out on three other sites (Aix-Zup, Budron and Mantes la Ville). In these two stages, the simulations were made without changes to the calibrated parameters. The single adjusted parameter was the initial mass of particles present in the catchment and in the sewer after the preceding rain. The qualitative validation showed satisfactory results for TSS, COD and BOD pollutographs expressed in concentrations, with respect to the shape of the pollutographs, their maximum, and the mass transported in the system.

The quantitative validation step revealed that the most significant errors are for the TSS concentrations lower than 50 mg/l for the separate catchment and 100 mg/l for the mixed and combined sewers. Thus, for measurements higher than 50 mg/l for the separate catchment, the mean relatives errors and standard deviation are of 0.15% and 40% and for measurements higher than 100 mg/l for the mixed and combined sewer, the mean relative errors and standard deviation are of 13% and 60%. Moreover, for 75% of the simulated rainfalls the calculation of the transited mass of TSS ranged between -20 and +20%.

FR:

Très souvent, les services techniques ne disposent que du plan du réseau d'assainissement sans le profil en long. Or les cotes radiers sont nécessaires aux simulations hydrauliques effectuées, par exemple, lors des études diagnostic. Pour pallier ce manque, les bureaux d'étude effectuent généralement un relevé sommaire et interpolent linéairement les cotes radier manquantes. Cette interpolation linéaire peut être la source d'erreurs importantes. Nous proposons donc dans cet article une nouvelle méthode d'interpolation permettant de minimiser ces erreurs. Cette méthode utilise trois types d'informations : les données connues, les contraintes et les critères. Les données connues correspondent aux informations disponibles quant au réseau. Les contraintes sont les règles constructives auxquelles tout réseau d'assainissement doit se conformer. Les critères sont les règles d'optimisation construites à partir d'observations sur des réseaux réels. Pour résoudre ce problème d'optimisation sous contraintes, nous utilisons des algorithmes génétiques parce que ces derniers sont capables de travailler avec un grand nombre de variables, des nombres réels et des fonctions non-linéaires.

Des tests ont été effectués sur les réseaux des villes d'Annequin, de Bapaume et de Lyon. Dans tous les cas (tronçons longs ou courts, pente forte ou faible), les résultats obtenus avec notre méthode sont meilleurs que ceux obtenus avec la classique interpolation linéaire. Il est même possible de déterminer la présence de contre-pente.

EN:

In order to realise hydraulic simulations of urban drainage networks, data describing the networks are needed. Very often, some of the invert levels are missing. Usually, design departments carry out an incomplete survey of the network invert levels and then linearly interpolate the missing ones. Our objective is to propose a more precise method to interpolate unknown invert levels. The method we propose in this paper involves computing (interpolating) the missing invert levels by using three types of information: (network) known data, constraints and criteria. Known data correspond to the (assumed) available information. Constraints are construction rules that any urban drainage network must respect. Criteria are optimisation rules based on observations of real urban drainage networks. To solve this problem, we use genetic algorithms because these algorithms are able to work with many variables, real numbers and non-linear functions.

Tests have been carried out on urban drainage networks in Annequin, Bapaume and Lyon. The results obtained are quite good for long or short reaches, and for steep or nearly flat slopes. It is even possible to identify the presence of negative slopes. In all cases, the proposed method of interpolation gives better results than the linear interpolation.

FR:

Les travaux réalisés dans cette étude avaient pour objectif l'application de techniques membranaires, dialyse de Donnan et électrodialyse, au traitement d'eaux dont la teneur en fluorure est supérieure à la valeur maximale admissible. Ces deux techniques qui mettent en jeu des membranes échangeuses d'ions, se distinguent par la nature des forces motrices de transfert. Trois eaux modèles relatives à trois pays différents (Maghreb, Sénégal et France) ont été préparées et traitées sur pilotes pré-industriels. Dans tous les cas étudiés, bien que divers anions (Cl^-, HCO₃^-, SO₄^2-) et cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) soient présents initialement dans les solutions à traiter, une concentration en fluorure conforme aux normes a pu être atteinte avec toutes les membranes testées.

L'électrodialyse qui abaisse la teneur de tous les ions présents dans l'eau, anions et cations, entraîne une déminéralisation partielle et par conséquent un adoucissement de la solution traitée. Par contre, la dialyse de Donnan, du fait de la diffusion du sel du compartiment receveur vers la solution traitée, augmente légèrement la minéralisation initiale. Cette technique qui, du point de vue énergétique, est un procédé plus économique que l'électrodialyse, semble donc plus adaptée au traitement d'eaux fluorurées à faible minéralisation.

EN:

The purpose of this work was to apply Donnan dialysis (DD) and electrodialysis (ED) for removing fluoride ion from waters where the concentration exceeds acceptable values. The techniques both use ion-exchange membranes but involve different driving forces: the difference in the electrochemical potential on both sides of the membrane for DD and the difference in the electric potential in ED.

Both techniques were applied to treat model waters, the compositions of which were very close to those of natural waters contaminated by fluorides. Three standard waters related to three different countries (Maghreb, Senegal and France) were prepared and treated with pre-industrial pilots. The active membrane area was 1760 cm² for Donnan dialysis, 552 and 2000 cm² for electrodialysis. Two anion exchange membranes, DSV from Asahi Glass and AFX from Tokuyama Soda, were tested in Donnan dialysis. Three electrodialysis stacks equipped with different anion and cation exchange membranes, AMV-AM1/CMV from Asahi Glass, AMX/CMX and ACS/CMS from Tokuyama Soda, were used. Conductivity, pH and the concentrations of each ionic species were monitored during membrane treatment. The initial fluoride concentration were 9.5, 6.08 and 2.66 mg L^-1 in each standard water, respectively.

In all cases, despite the presence of different anions (Cl^-, HCO₃^-, SO₄^2-) and cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) generally present in ground waters, a fluoride concentration in agreement to the norms (< 1.5 mg L^-1) could be achieved regardless of the composition of the treated waters or the nature of the tested membranes.

Electrodialysis decreased the anion and cation concentrations and induced a partial demineralization (about 70%) and consequently a softening of the treated water. On the contrary in Donnan dialysis, due to the electrolyte diffusion from the receiving compartment to the treated solution, the mineralization of the treated water increased slightly (about 10%). In this latter process, the anion concentration declined while no changes were observed in the cation concentration, except for sodium because of the electrolyte leakage. The DSV membrane was the most effective anion exchange membrane to use in DD. In ED, the AMV-AM1/CMV stack was selected on the basis of the demineralization and softening ratio, and the energy consumption.

Donnan dialysis, which from an energy consumption point of view is more economical than ED, thus seems more adapted to the treatment of low mineralization waters.

FR:

Cette étude trace un profil des diverses technologies utilisées et en développement pour la séparation et/ou la récupération des métaux dans les effluents industriels. Les principes de fonctionnement de ces technologies sont abordés, ainsi que leurs avantages et limites d'utilisation. Les procédés d'enlèvement et de récupération des métaux comprennent les techniques de précipitation (formation d'hydroxydes, de carbonates, de sulfures, etc.) et coprécipitation (sels de fer et d'aluminium, etc.), d'adsorption (sable, cellulose, charbon activé, pyrite, ciment, lignite, mousse de tourbe, sciure de bois, etc.) et de biosorption (bactéries, levures, moisissures, algues marines et d'eaux douces), d'électrodéposition et d'électrocoagulation, de cémentation, de séparation par membranes (osmose inverse et électrodialyse), d'extraction par solvant (acides carboxyliques, amines aliphatiques ou aromatiques, acides aminés, composés phénoliques, phosphates alkyl, etc.), et d'échange d'ions (résines naturelles et synthétiques). La précipitation ou la coprécipitation représentent les procédés les plus largement utilisés et étudiés pour l'enlèvement des métaux des effluents industriels, suivis des techniques d'adsorption. Les procédés plus sophistiqués tels que l'électrodéposition, l'extraction par solvant, la séparation par membranes et l'échange d'ions, bien que largement utilisés dans les procédés métallurgiques, sont relativement peu employés et examinés pour le traitement des effluents industriels. La biosorption a fait l'objet de plusieurs travaux de recherche au cours des dernières années et représente une option intéressante pour le traitement de divers types d'effluents contenant de faibles concentrations en métaux. Finalement, le recyclage et la gestion optimale des effluents constitue une avenue de plus en plus suivie par les industries soucieuses de satisfaire aux nouvelles réglementations et législations.

EN:

This study is dedicated to the review of the different technologies used and evaluated for the removal and/or recovery of metals from industrial effluents. The principles involved in these technologies are discussed, as well as the advantages and limits associated with these processes. The metal removal and recovery processes include the following techniques: precipitation, adsorption and biosorption, electrowinning and electrocoagulation, cementation, membrane separations, solvent extraction and ion exchange.

Precipitation and coprecipitation are the most used and studied methods for metal removal from industrial waste waters. The method of precipitation used most often to remove metals from waste water consists of precipitating them in the form of hydroxides. The usual procedure involves the addition of chemicals such as lime (CaO or Ca(OH)₂), Mg(OH)₂, NaHCO₃, Na₂ CO₃, (NH₄)₂ CO₃, NaOH or NH₄ OH. The precipitation of metals by carbonates or sulphides is an effective alternative to hydroxide precipitation. The use of carbonates allows the precipitation of metals to occur at pH values lower than those necessary with the hydroxides. Moreover, the precipitates thus formed are denser and have better characteristics of solid-liquid separation. Precipitation by sulphides is normally carried out with reagents such as: Na₂ S, NaHS, H₂ S or FeS. In acidic media, the lower solubility of metal sulphides (Cd, Co, Cu, Cr, Ni, Mn, Zn, etc.), makes it possible to reach concentrations lower than those obtained by precipitation as hydroxides. Coprecipitation with aluminum and iron salts is also an effective means for the removal of metals from effluents.

Adsorption methods are also widely applied and examined for this purpose. However, in most cases the use of adsorbents requires an effluent neutralization step. Indeed, the neutralization of acid effluents must take place to allow their disposal in sewerage systems. A wide variety of adsorbents can be employed, both organic and inorganic: aluminum or iron oxides, sand, activated carbon, mixtures of coal and pyrite, iron particles, gravel or crushed brick, cement, etc. Studies have demonstrated the possibility of eliminating metals by adsorption on vegetable matter: peat moss, sawdust and wood bark, etc. Chitin and chitosan, two natural polymers that are abundant in the cell walls of fungi and shellfish, also have excellent properties of metal fixation. The utilization of different agricultural by-products (peanut skins, coconuts, corn cobs, onions skins, tea leaves, coffee powder, canola meal, etc.) for metal adsorption has also been proposed.

Biosorption has been intensively studied in recent years as an economical treatment for metal recovery from dilute industrial effluents. Biosorption implies the use of live or dead biomass and/or their derivatives, which adsorb the metal ions with the ligands or functional groups located on the external surface of the microbial cells. Capacities for metal adsorption on various types of biomass (bacteria, yeasts, fungi, marine and freshwater algae) have been evaluated. The microorganisms used for the metal adsorption step must usually be immobilized in a matrix or in an easily recoverable support. The immobilizing agents or matrices most usually employed are alginate, polyacrylamine, polysulphone, silica gel, cellulose and glutaraldehyde.

Electrowinning is a well-established technology that is widely employed in the mining and metallurgical industries (heap leaching, acid mine drainage, etc.), in metal transformation industries (wastes from plating and metal finishing), and in the electronics and electrical industries for the removal and/or the recovery of metals in solution. Many metals (Ag, Au, Cd, Co, Cr, Cu, Ni, Pb, Sn and Zn) present in the effluents can be recovered by electrodeposition using insoluble anodes.

Electrocoagulation is another electrochemical approach, which uses an electrical current to remove several metals in solution. In fact, the electrocoagulation systems can be effective in removing suspended solids, dissolved metals, tannins and dyes. The contaminants present in waste water are maintained in solution by electrical charges. When these ions and the other charged particles are neutralized with ions of opposite electric charge, provided by a electrocoagulation system, they become destabilized and precipitate in a form that is usually very stable.

Cementation is a type of precipitation method implying an electrochemical mechanism. In this process, a metal having a higher oxidation potential passes into solution (e.g., oxidation of metallic iron, Fe(0), to ferrous iron, Fe(II)) to replace a metal having a lower oxidation potential. Copper is the metal most frequently separated by cementation. However, the noble metals (Ag, Au and Pd), as well as As, Cd, Ga, Pb, Sb and Sn, can also be recovered in this manner.

Reverse osmosis and electrodialysis are two processes using semipermeable membranes applicable to the recovery of metal ions. In electrodialysis, selective membranes (alternation of cation and anion membranes) fit between the electrodes in electrolytic cells. A continuous electrical current and the associated ion migrations, allow the recovery of metals. The techniques of membrane separation are very efficient for the treatment of dilute waste waters.

The metallurgical industry has used solvent extraction for many years for a broad range of separations. This technique is employed today for the removal of soluble metals (Cd, Cr, Co, Cu, Ni, Mo, U, V, Zn, etc.) from waste water. Separation is carried out in contact with an immiscible organic phase to form salts or complex compounds, which give a favorable solubility distribution between the aqueous and organic phases. Various types of reagents can be used for the extraction: carboxylic acids, aliphatic or aromatic amines, amino acids, alkyl phosphates, phenolic compounds. The non-selective removal of metal contaminants in aqueous solutions can be obtained with a whole range of organic reagents. Promising new reagents have been proposed recently for the selective extraction of metals, such as Cd, Co, Cr and Zn.

Ion exchangers are insoluble substances having in their molecular structure acidic or basic groups able to exchange, without modification of their physical structure, the positive or negative ions fixed at these groups. The first ion exchangers used were natural substances containing aluminosilicates (zeolites, clays, etc). Nowadays, the most-used ion exchangers are mainly organic in nature (resins). For the extraction of metals, the removal of cations in solution is usually done with the sulphonic acid group (-SO₃^- H⁺) of a polystyrene resin, or, with a chelating resin containing iminodiacetate functional groups. Ion exchange has recently received considerable attention for the separation and concentration of metals from waste water. These developments are especially applicable to the plating and metal transformation industries, for the removal of Cr, Co, Cu, Cd, Ni, Fe and Zn.

The more sophisticated processes, such as electrowinning, solvent extraction, membrane separations and ion exchange, although frequently used in metallurgical processes, are less popular for wastewater treatment than are precipitation methods. Finally, recycling and optimal management of effluents constitutes an approach more and more widely applied by industries to satisfy new environmental regulations and laws.

FR:

Cette étude a eu pour objectif de comparer les vitesses de décomposition du peroxyde d'hydrogène et d'oxydation de l'atrazine par les systèmes catalytiques Fe(III)/H₂O₂, Cu(II)/H₂O₂, et Fe(III)/Cu(II)/H₂O₂. Les expériences ont été réalisées à pH 3,0, à une température de 25,0 (± 0,2) °C, en milieu perchlorate, en présence et en absence d'oxygène dissous. L'étude comparative a confirmé que les vitesses de décomposition de H₂O₂ et d'oxydation de l'atrazine sont beaucoup plus lentes en présence de Cu(II) qu'en présence de Fe(III) et l'addition de Cu(II) augmente l'efficacité du système Fe(III)/H₂O₂. Pour nos conditions expérimentales ([composé organique]_o < 1 µM) les expériences de cinétique compétitive, réalisées avec des solutions aqueuses contenant trois composés organiques (atrazine, 1,2,4-trichlorobenzène, 2,5-dichloronitrobenzène), ont montré que le radical hydroxyle représente la principale espèce responsable de l'oxydation des composés organiques. Les résultats ont également mis en évidence la formation très rapide d'un composé entre Cu(II) et H₂O₂ (étude spectrophotométrique) et ont montré l'importance de la concentration en oxygène dissous sur les vitesses globales de décomposition de H₂O₂ et de l'atrazine par les systèmes Cu(II)/H₂O₂ et Fe(III)/Cu(II)/H₂O₂.

EN:

Toxic and refractory organic pollutants in industrial wastewater can be degraded by advanced oxidation processes (AOPs) alone, or in combination with physico-chemical and biological processes. Of these oxidation methods, Fenton's reagent (Fe(II)/H₂O₂) and Fenton-like reagents (Fe(III)/H₂O₂, Mⁿ⁺ or Mⁿ⁺¹ /H₂O₂) are effective oxidants of large variety of organic pollutants.

The mechanism of decomposition of H₂O₂ and of oxidation of organic solutes by Fenton's and Fenton-like reactions has been the subject of numerous studies. However, there are still many uncertainties as to the nature of the oxidant species formed and the rate constants of elementary reactions (Table 1).

Our recent studies carried out in HClO₄ /NaClO₄ solutions and in the presence of very low concentrations of organic solutes (atrazine, 1,2,4-trichlorobenzene; concentration < 3 µM) have shown that the reaction of Fe(II) with H₂O₂ leads to the formation of two intermediates and that the overall initiation step (reaction 1, Table 1) at pH < 3.5 leads to the formation of OH radical (Gallard et al., 1998a). Other work with different organic compounds and higher concentrations of organic solutes indicates that the intermediates (Fe(II)-hydroperoxy complexes, ferrous ion) might also oxidize organic compounds. Ferric ion can also catalyze the decomposition of H₂O₂. The mechanism is initiated by the formation of two Fe(III)-peroxy complexes at pH < 3.5 (reaction 2a, Table 1) followed by their slow decomposition into Fe(II) and HO₂^·/O₂^·- (reaction 2b, Table 1) (Gallard et al., 1999; De Laat and Gallard, 1999; Gallard and De Laat, 1999).

The formation of intermediates (complexes, cupryl ion) has also been postulated for the catalytic decomposition of H₂O₂ by Cu(II). Depending on the experimental conditions (nature and concentrations of organic solutes, pH,…), the degradation of organic compounds might be attributed to the hydroxyl radical (reaction 1, Table 1) or to other species like the cupryl ion (Cu(III)). Production of Cu(III) by reaction of OH^· with Cu(II) has also been demonstrated by pulse radiolysis experiments. Kinetic data indicate that the rate of decomposition of H₂O₂ and the rate of oxidation of organic compounds are faster with Fe(III)/H₂O₂ than with Cu(II)/H₂O₂ and that Cu(II) can improve the efficiency of the Fe(III)/H₂O₂ process.

The present study has been undertaken in order to compare the rates of decomposition of H₂O₂ and the rates of oxidation of atrazine by Fe(III)/H₂O₂, Cu(II)/H₂O₂ and Fe(III)/Cu(II)/H₂O₂ under identical conditions. These conditions (pH 3.0, I=0.1 M, [Atrazine]_o < 1 µM) were the same as those used in previous studies of the Fe(II)/H₂O₂ and Fe(III)/H₂O₂ systems.

Experiments were carried out in MilliQ water, in the dark, at 25.0 (± 0.2) °C, pH 3.0, ionic strength (I) of 0.1 M, in the presence and in the absence of dissolved oxygen. pH and I were adjusted with perchloric acid and sodium perchlorate. The concentrations of hydrogen peroxide ([H₂O₂]_o ≤ 10 mM) and of atrazine ([atrazine]_o ≤ 1 µM) were determined iodometrically and by HPLC, respectively.

In the absence of organic solutes, experimental results have shown that the rate of decomposition of H₂O₂ is faster with Fe(III) than with Cu(II) (Figure 2). In agreement with previous data (De Laat and Gallard, 1999), the initial rate of decomposition of H₂O₂ by Fe(III) can be described by a pseudo first-order kinetic law with respect to H₂O₂, and dissolved oxygen (0-1 mM) has no effect on the rate of decomposition. For the Cu(II)/H₂O₂ system, our spectrophotometric data (Figure 1) gave evidence that the decomposition of H₂O₂ by Cu(II) goes through the formation of an intermediate which might be a Cu(II)-hydroperoxy complex and which absorbs in the region 350-600 nm. Furthermore, the rate of decomposition of H₂O₂ by Cu(II) does not follow a first-order kinetic law and is affected by the concentration of dissolved oxygen (Figures 2 et 3).

As far as the oxidation of atrazine is concerned, a preliminary study of the oxidation of solutions containing atrazine, 1,2,4 trichlorobenzene and 2,5 dichloronitrobenzene in very dilute aqueous solutions ([organic solutes]_o < 3 µM) has been conducted at pH 3.0. Experimental results showed that the relative rates of decomposition of organic solutes by Fe(III)/H₂O₂, Fe(II)/H₂O₂ and Cu(II)/H₂O₂ were identical and could be described by the competitive kinetic expression (Figure 4). These data suggest that the oxidation of the organic solutes by the three systems of oxidation tested can be attributed to a unique oxidant species, the hydroxyl radical, under our experimental conditions.

The rate of oxidation of atrazine by Cu(II)/H₂O₂ was found to be much slower than by Fe(III)/H₂O₂ (Figure 5), to be dependent on the concentrations of reactants ([Cu(II)]_o, [H₂O₂]_o Figure 6) and to decrease in the presence of dissolved oxygen (Figure 7). These data confirm that the rate of decomposition of H₂O₂ by Cu(II), and as a consequence, the rate of production of OH radicals by Cu(II)/H₂O₂, are much slower than by Fe(III)/H₂O₂. In addition, a fraction of Cu(I) may be oxidized by dissolved oxygen and this reaction, which competes with the reaction of Cu(I) with H₂O₂, may also decrease the rate of formation of OH radical.

For the Fe(III)/Cu(II)/H₂O₂ system, experimental data have demonstrated that the addition of Cu(II) increases the rate of decomposition of H₂O₂ (Figure 8a) and atrazine (Figure 8b) by Fe(III)/H₂O₂ and that these increases in reaction rates depend on the concentration of dissolved oxygen. This catalytic effect of Cu(II) has been attributed to a fast regeneration of Fe(II) (which is the major source of OH radical) by the reaction of Cu(I) with Fe(III). Since this reaction competes with oxidation of Cu(I) by O₂ and H₂O₂, the catalytic properties of Fe(III) and Cu(II) mixtures will depend on the experimental conditions, such as the relative concentrations of reactants.

In conclusion, this comparative study has confirmed that the rates of decomposition of H₂O₂ and atrazine, in dilute aqueous solution, by Fe(III)/Cu(II)/H₂O₂ are faster than by Fe(III)/H₂O₂ and Cu(II)/H₂O₂. This study has also demonstrated that dissolved oxygen has a significant effect on the reaction rates in the Cu(II)/H₂O₂ and Fe(III)/Cu(II)/H₂O₂.oxidation systems. The effects of dissolved oxygen and of the addition of Cu(II) on the efficiency of the Fe(III)/H₂O₂ system could be explained by assuming that the OH radical is the major oxidant species under our experimental conditions. However, additional research is needed in order to better understand the mechanism of decomposition of H₂O₂ by Cu(II) and Cu(I) and to determine the rate constants of individual reactions involved in the Cu(II)/H₂O₂ and Cu(I)/H₂O₂ systems.

FR:

Au cours de 2 crues survenues le 24 mai 1992 et le 15 mai 1993 sur 2 bassins versants, nous avons étudié la composition isotopique et chimique des précipitations (pluies et pluviolessivats) ainsi que leurs variations temporelle et spatiale. Les bassins étudiés (d'environ 1,5 ha) sont situés près de la ville de Sinnamary (Guyane Française) et sont proches l'un de l'autre (200 m). Un des bassins (bassin B) est recouvert par une forêt primaire, tandis que le second (bassin A) a été défriché et transformé en prairie (Digitaria swazilendensis, programme ÉCÉREX Orstom-CTFT). Le dispositif expérimental est composé de 31 pluviomètres sur le bassin B et de 3 pluviomètres sur le bassin A. Les hauteurs d'eau précipitées lors des événements étudiés sont importantes (environ 60 mm sur le bassin A). La hauteur d'eau précipitée est homogène spatialement sur le bassin A, alors qu'elle est très hétérogène sur le bassin B. La teneur instantanée des précipitations en¹⁸O est très variable temporellement, mais reste homogène spatialement, sur les 2 bassins. L'interception de la pluie par la canopée déstructure donc la hauteur d'eau précipitée sous forêt, mais pas sa signature isotopique. Le 24 mai 1992, nous avons observé une dilution de la composition chimique de la pluie et une diminution de son pH au cours du temps. Les pluviolessivats sont généralement plus concentrés que la pluie et leur pH est plus tamponné. Nous n'avons pas observé de corrélation entre la composition chimique de la pluie ou des pluviolessivats et l'intensité des précipitations. La variabilité spatiale de la composition chimique des pluviolessivats, étudiée lors de l'averse principale du 24 mai 1992, est très importante et 31 pluviomètres semblent insuffisants pour estimer précisément les apports au sol. L'effet de masse est respecté le 24 mai 1992, mais n'est pas visible le 15 mai 1993. La comparaison de l'évolution des teneurs intégrées en Cl^- et en¹⁸O montre que l'événement pluvieux du 24 mai 1992 est issu d'une masse d'air unique, alors que celui du 15 mai 1993 est issu de plusieurs masses d'air différentes. On remarque également que la teneur intégrée en¹⁸O des pluviolessivats est légèrement supérieure à celle de la pluie en milieu ouvert. En l'absence d'évaporation (la composition isotopique des pluviolessivats est alignée sur la droite locale des eaux météoriques), cela s'explique par un mélange entre la pluie directe et de l'eau de pluie plus ancienne, retenue sur la canopée et de composition isotopique différente.

EN:

Geochemical hydrograph separation methods are frequently employed because they allow one to determine the origin (spatial or temporal) of water that contributes to creating floods. This approach, based on mass balance equations, requires a good knowledge of the geochemical (isotopic and chemical) signals of the reservoirs that contribute to the flood. However, geochemical signals in precipitation, an obvious reservoir, may vary strongly over time. In forested watersheds, throughfall - and not direct rain - make up the input signal. The geochemical signal of throughfall may be different from that of rain and it may vary temporally and spatially. In order to clarify the use of geochemical tracers for hydrograph separation, we studied the isotopic (δ¹⁸O, δ²H) and chemical composition of precipitation (rain and throughfall) in two watersheds, as well as the spatial and temporal variations of this precipitation during two runoff events that occurred on May 24, 1992 and on May 15, 1993. The studied watersheds are located near the city of Sinnamary (French Guyana), 120 km south-west of Cayenne. They are small in size (1,5 ha) and close to each another (200 m). One basin (hereafter named B basin) is covered by primary forest, whereas the other (hereafter named A basin) was cleared and turned into grassland (Digitaria swazilendensis, ÉCÉREX program, supported by Orstom-CTFT). The climate is tropical-humid, characterised by high mean annual temperatures (26°C), which slightly varied from month to month, and high mean annual precipitation (3500 to 3900 mm.yr^-1). Precipitation occurred primarily during the main wet season, centred around May and June, and during the secondary wet season from December to January. Given the small distance between the watersheds, the differences noted between the rain collected in the A basin and the throughfall collected in the B basin (amount, geochemical signal) were attributed to the forest cover (leaching, interception,...). The monitoring equipment consisted of 31 rain gauges in the B basin and 3 rain gauges in the A basin. Rainfall was important for the two studied rain events (about 60 mm in basin A). Average rainfall in the A basin is characterised by low coefficients of variation, whereas average water inputs in the B basin showed high coefficients of variation. Thus, the amount of incoming water was spatially homogeneous in basin A, but heterogeneous in basin B. In both basins, the instantaneous δ¹⁸O value for precipitation varied considerably over time, but it was still spatially homogeneous in both watersheds with the average δ¹⁸O value showing a low coefficient of variation. This result means that the interception of the rain by the canopy destroyed the structure of the precipitation amounts under the forest, but not the structure of its isotopic signal. On May 24, 1992, we noted a dilution of the chemical content of the rain and a decrease in its pH over the course of the event. The chemical contents of the throughfall were on the whole more concentrated than in the incident rain and the pH more buffered. We did not note any correlation between the chemical content of rain or throughfall and the intensity of precipitation. The chemical composition of throughfall, studied during the main shower on May 24, 1992, exhibited considerable spatial variation and 31 rain gauges did not seem to be enough to precisely estimate the amount reaching the soil. A continuous depletion in heavy isotopes (¹⁸O,²H) and some chemical species (e.g., Cl^-) was noted for the first episode (May 24, 1992) but not for the second (May, 15, 1993). This depletion may be explained by water vapour condensation outside the Rayleigh distillation, or by mixing of different air masses. The comparison between the evolution of integrated values of δ¹⁸O and the integrated Cl^- content versus the amount of accumulated precipitation proved that the rain event of May 24, 1992, was generated by a single air mass whereas the event of May 15, 1993 was generated by several air masses. We also noted that the integrated value of δ¹⁸O for throughfall was slightly more concentrated than the content of rain. In the absence of evaporation (the isotopic composition of the throughfall corresponded to the local meteoric line), this enrichment suggests that direct rain mixed with older water that was stored in the canopy and had a different isotopic composition.

This study showed that the intensity and the geochemical signal of precipitation (rain and throughfall) vary greatly on a temporal scale in a tropical environment. It also showed that the amount of incoming water varied spatially under a forest cover, as did its geochemical (isotopic and chemical) signal. In order to achieve a stream hydrograph geochemical separation, it is necessary to collect the precipitation (rain and throughfall) with a short time step. It is also necessary to collect the throughfall across a concentrated network of rain gauges.

FR:

Une nappe d'eaux fossiles à grande profondeur (800 à 2 700 mètres) a été mise en exploitation dans le Sud-Tunisien pour alimenter une usine d'osmose inverse située à Gabès ayant une production de 15 000 m³ /jour, afin de lutter contre la désertification par irrigation et d'assurer le chauffage de serres pour la production de primeurs. La grande dureté (TH de l'ordre de 100 à 140 °F) de ces eaux géothermales a pour conséquence le colmatage rapide des conduites de distribution : 40 à 50 tonnes de tartre par forage, constitué essentiellement de carbonate de calcium, précipitent chaque année. Ce tartre est constitué d'aragonite comme le montrent la microscopie électronique à balayage et la diffraction des rayons X. Une technique électrochimique, la chronoélectrogravimétrie, permet d'étudier l'inhibition de l'entartrage par des composés de la famille des phosphates inorganiques, des phosphonates organiques et des polycarboxylates. La concentration efficace de chacun de ces inhibiteurs agissant par effet de seuil a été déterminée : elle est de l'ordre de 1,1 à 1,5 mg.l^-1 pour l'eau du forage de EL HAMMA. Un essai sur le site de EL MANSOURA a été effectué en privilégiant un inhibiteur produit industriellement dans le Sud-Tunisien, le triphosphate de sodium. A la concentration de 1 mg.l^-1 il évite l'entartrage du système de refroidissement de type cascade - piscines et des conduites de distribution.

EN:

Deep fossil waters are used in Southern Tunisia (Gabès, Kébili, Tozeur) for the Gabès reverse osmosis plant, which delivers a flow rate of 15 000 m³ /day for irrigation and for heating greenhouses used for the production of early fruits and vegetables. Drilling depths vary between 800 and 2 700 meters. Water emerges under a pressure of ca. 20 bars and has a temperature between 50 and 73 °C. Mean flow rate is 7 800 m³ /day.

Intake water at the Gabès plant has a salinity of 3.3 g.l^-1 ; after reverse osmosis the salinity is less than 0.1 g.l^-1. Water used for irrigation has to be cooled. Geothermal waters are characterized by high concentrations of calcium, magnesium, sulphate and chloride. Bicarbonate anions are present at limited concentrations (approx. 2.10^-3 mol.l^-1) that are, however, sufficient for the formation of large quantities of scale - 40 to 50 tons per year for each drilling. At the outlet of the drill hole, pressure decreases strongly, liberating carbon dioxide to the atmosphere. Water pH increases and the following equilibrium is displaced to the right, with scale precipitation : Ca²⁺ + 2HCO₃^-

CO₂ (g) + CaCO₃(s) + H₂O

Scale precipitation has two consequences :

- the plugging of distribution pipes: a 85% reduction of the pipe has been observed, after four years, for an initial diameter of 15 cm;

- water cooling installations such as cooling towers or pool systems are blocked by large quantities of scale, which have to be removed regularly.

Scales have been analysed through inductively coupled plasma spectroscopy and thermogravimetry: calcium carbonate may represent, depending on the origin of the drilling water, 60 to 95% by weight of the solid. Iron oxides, silica, calcium phosphate and aluminum are present. Scanning electron microscopy and X-ray diffraction show that calcium carbonate precipitates in the form of aragonite. This is due to two reasons: the temperature at the drilling outlet is greater than 60 °C and the high magnesium concentration favours aragonite formation. Scale inhibition is possible through the use of certain chemicals such as phosphates, organic phosphonates and polycarboxylates.

Chronoelectrogravimetry was used as the experimental method to determine the inhibitor concentration able to suppress scale precipitation. Dissolved oxygen is electrochemically reduced on a gold electrode; hydroxide anions are produced in the vicinity of the electrode and calcium carbonate precipitates according to:

4Ca²⁺ + 4HCO₃^- + O₂ + 4e -> 4CaCO₃(s) + 2H₂O

The gold electrode is deposited on the quartz disk of a recording microbalance. The electrode is polarized at - 1 V/SCE with a three-electrode potentiostatic device and the weight of CaCO₃ deposited is recorded versus time.

Four inhibitors have been studied :

- PERMATREAT 191, which is the sodium salt of aminotris(methylenephosphonic) acid N(CH₂COONa)₃;

- a proprietary organic phosphonate with high resistance to chlorine oxidation (DEQUEST 6004) ;

- phosphonobutanetricarboxylic acid (DEQUEST 7000, BAYHIBIT-AM) ;

- a copolymer of acrylic acid and acrylamidopropanesulfonic acid (FERROPHOS 5248).

Breakthrough effects are obtained in the case of EL HAMMA water for the following concentrations: PERMATREAT 191: 1.1 mg.l^-1 ; DEQUEST 6004: 1.5 mg.l^-1 ; DEQUEST 7000: 1.3 mg.l^-1 ; FERROPHOS 5248: 1.4 mg.l^-1. These concentrations are low and, as a consequence, these inhibitors can be used for antiscale action even with high water flow rates.

A field experiment was carried out on the EL MANSOURA drilling where water is cooled in three pools (input water: 5 184 m³ /day, temperature: 60 °C). For economical reasons the chosen inhibitor was sodium triphosphate Na₅ P₃ O₁₀, which is produced industrially in Southern Tunisia. By chronoelectrogravimetry it has been shown that, with EL MANSOURA water, a breakthrough effect is obtained with a sodium triphosphate concentration of 0.75 mg.l^-1.

A dosing pump was used to inject sodium triphosphate in such a way that the inhibitor concentration would be 1 mg.l^-1 in one of three pools, the two others not being treated. After four months a scale deposit of 23 cm was obtained in the untreated pools and the pipe diameter was reduced by 39%. In the treated pool scale deposit was not observed and the pipe diameter remained unchanged. In the untreated basins, examination of scale with electron scanning microscopy revealed that it was aragonite; in the treated basin, the precipitate was amorphous and an X-ray diffraction pattern with no characteristic bands was obtained.

Some algal development was observed in the pool due to phosphate addition but this development was not a nuisance after the four month period. However, it could be suppressed by the use of an organic phosphonate or polycarboxylate as scale inhibitor.