Effet auto-épurateur de la lithologie des affleurements géologiques dans un climat semi-aride: cas du bassin versant de l'Oued Mellègue (Nord-Ouest de la Tunisie)

Résumé

Le Nord-Ouest de la Tunisie renferme plus que 81% des eaux de surface et 95% des mines et gîtes miniers. Le bassin versant de l'Oued Mellègue, situé dans cette zone, englobe 13 mines de plomb, zinc, fer, strontium, fluor, barium et phosphates. L'impact des rejets de ces mines sur les eaux de surface a été apprécié grâce à plusieurs compagnes d’échantillonnage. Les résultats d'analyse par ICP/MS de ces échantillons montrent qu'ils renferment des quantités non négligeables d'Al, As, B, Ba, Br, Cd, Co, Cr, Cu, Hg, Mo, Ni, Sr, U et de Zn. L'analyse des ces éléments dans les eaux du Mellègue, de ses affluents et dans la retenue de Nebeur montre que leurs concentrations sont inférieures aux normes d'eau potable. Ce comportement pourrait être dû à la nature des affleurements géologiques formés par des carbonates des argiles et des évaporites, à l'absence de cours d'eau drainant directement les rejets, à la formation de minéraux secondaires caractérisés par une importante rétention des métaux et à l’étendue des bassins versants non pollués.     

Stratigraphie et évolution structurale paléozoïque d''un segment de la Meseta orientale marocaine (Monts du Sud-Est d''Oujda): rôle des décrochements dans la formation de l''olistostrome intraviséen et le plutonisme tardi-hercynien

In the Paleozoic basement of the southeast Oujda mountains, the Lower Ordovician, Silurian-Devonian and Devonian-Dinantian are dated for the first time through palynology. This paper shows the autochtonous character and continuous stratigraphy of the formations of the Lower Ordovician to Devonian or probably Dinantian. The Intra-Visean Olistostrome is interpreted as a tectono-sedimentary breccia associated with strike-slip faults. The structural Variscan evolution is characterized by two major phases, each of them being divided in three stages. The ante-Upper Visean early phase is characterized in chronological order by (1) submeridian folds, (2) northeast-southwest folds and (3) extensional, oblique-slip, east-west trending faults. The second phase post-dates the Westphalian C and is marked by open east-northeast - west-southwest trending folds cut by reverse faults with the same trend and by a set of north-south sinistral and east-west dextral strike-slip faults. These later faults have allowed the emplacement of late Hercynian granitoïds.A palaeogeographical and structural reconstruction comparable to that established in the rest of the Moroccan Meseta is proposed.     

Pauperizing the pastoral periphery: the marginalization of herding communities in the world’s dry lands

Environmental degradation and other socio-economic problems are too often discerned in contemporary pastoral systems in the wake of global economic change. This paper identifies cases where pastoral people respond to external pressures and opportunities in positive ways and adapt to changes. We hope that by doing this a framework of pastoral adaptations can be developed. The first part of this paper focuses on impacts of global economic change, which are mostly negative in nature. Through an extensive literature search in geography, anthropology, range management, and development field, the second part introduces cases that we categorize as positive adaptations.     

Generation of isoprenoid compounds,notably prist-1-ene,via photo- and autoxidative degradation of vitamin E

In order to mimic the photodegradation of vitamin E during phytoplankton senescence and study the behaviour of the resulting photoproducts during cell lysis, vitamin E dispersed in seawater was irradiated with solar light in the presence of hematoporphyrin as sensitizer. Under these conditions, singlet oxygen-mediated photooxidation and free radical oxidation (autoxidation) acted simultaneously on the substrate, affording 4,8,12-trimethyltridecanal, 4,8,12-trimethyltridecanoic acid, 6,10,14-trimethylpentadecan-2-one, α-tocopherylquinone, 4,8,12,16-tetramethylheptadecan-4-olide, 2,3-epoxy-α-tocopherylquinone and 5,6-epoxy-α-tocopherylquinone as minor products. Different mechanisms were proposed to explain the formation of these different isoprenoids. The main products (85% of the degraded substrate) appeared to be previously described diastereoisomeric trimeric oxidation products of vitamin E, whose pyrolysis during gas chromatography–mass spectrometry (GC–MS) afforded 2,6,10,14-tetramethylpentadec-1-ene (prist-1-ene). On the basis of their greater ability to produce prist-1-ene during pyrolysis than intact vitamin E and their highly favoured production in phytoplanktonic cells, these trimers are proposed as likely sources of this isoprenoid alkene, previously identified in many pyrolysates from immature kerogens.     

Suffusion evaluation of coarse-graded soils from Rhine dikes

Acta Geotechnica - An experimental study on internal erosion of coarse-grained soils collected from the Rhine River is presented. The tests performed on laboratory column aim to assess the...     

Two-stage reverse flotation process for removal of carbonates and silicates from phosphate ore using anionic and cationic collectors

Phosphate rock contains various gangue minerals including silicates and carbonates which need to be reduced in content in order to meet the requirements of the phosphate industry. Froth flotation has become an integral part of phosphate concentration process. In this study, double reverse flotation was applied to recover apatite from phosphate ore. H₃PO₄ and CaO were used as phosphate depressants, in acidic and alkaline conditions. Fatty acids and amines were added as carbonate and silicate collectors respectively. An experimental protocol devised to optimize the grade and recovery of phosphate using anionic–cationic method was found effective. Consequently, a required high quality of phosphate concentrate containing 30.1% P₂O₅ was obtained, with a recovery of 94%. X-ray diffraction and optical microscopy studies were performed to define the main minerals.     

Carbon and oxygen isotope analysis of calcite or dolomite associated with organic matter

A combined 5% sodium hypochlorite (<24 hr.) followed by silver phosphate (15–20 mn) pre-treatment process has been developed for the isotopic analysis of calcite- or dolomite-bearing samples associated with organic matter and sulphide minerals. This treatment purifies the CO₂ gas of contaminant sulphur-bearing organic species and H₂S which are generated during the reaction of the sample with phosphoric acid.