Geochemistry and mineralogy of Paleocene-Eocene Oum El Khecheb phosphorites (Gafsa-Metlaoui Basin) Tunisia

International audienceUpper Paleocene to Lower Eocene phosphates of the Jebel Oum El Khecheb profile (Gafsa-Metlaoui Basin GMB, Tunisia) are mostly composed of brown phospharenites lithified or not, granular, with pellets, coproliths, bone clasts and fish teeth. The matrix in analyzed samples is reduced and constituted of various clay minerals (dominantly smectites, palygorskite, sepiolite and rare kaolinite), opal-CT and carbonates. Clinoptilolite is recognized along the series; very few amounts of quartz and feldspar grains are sparsely present. The phosphate mineral is a carbonate-fluorapatite (CFA) sulphated, sodic enriched in strontium and rare earth elements (REE). Statistical analyses of mineralogical and geochemical data highlight the associated elements in the main mineral lattice. In the CFA, the group is composed of CaO-P2O5-Na2O-SO3-F-Sr-Th-REE-U-Ba-Zr-Cr. In the silicates, the group contains SiO2-Al2O3-Fe2O3-MgO-K2O-Na2O-TiO2-Co-Cs-Rb-Sc-Ta-Sb-Mo-As and Cu. Sulphides are represented by Zn and Cd; iron oxides by mainly Fe and Ni. Dolomite and gypsum are present; SO3 is contained in the latter, but CFA may also have some of this radical in its structure. The Upper Continental Crust (UCC) normalized REE patterns present a variably negative Ce-anomaly, suggesting that the imprint of the marine genesis environment is very well-preserved in the main phosphatic units. Positive-Eu and negative-Sm anomalies were detected in some samples. The fractionation of light-REE to heavy-REE, represented by the La/Yb ratio of untreated values varies between 8.28 and 12.94, somewhat less than UCC value (13.64). Enrichment of HREE is due to phosphates. Both ratios reflect the marine genesis environment wherein the formation of the CFA took place in a confined and oxygen poor environment. Then the CFA removed the REE and the trace elements present from the interstitial waters, as was proposed for the various phosphatic basins of Tunisia

Geochemistry of solid effluents and phosphate ore washed from Metlaoui-Gafsa Basin, Tunisia

International audienceThe effluent and washed ore show that treatment of ore enrichment is not aggressive. The results are supported by the projection factor, which clearly show the relationships between different elements and mineralogical composition of the effluents and washed ore.The results are a high concentration of 26-35% P2O5 (fluorapatite), but it remains carbonates (calcite anddolomite) in low quantity. The apatite show significant REE contents that can be by-product of phosphoric acid. Silicates and carbonates are the major part of effluents. The sulphate and fluoride contents are of the order of 3%, and the Na2O content varies between 1 and 1.5%, we observed a good correlation between F-P2O5-Na2O-P2O5 and P2O5-SO3.The levels of Cd, Zn, Cr, V, Ni, Sc, Nb and Cu are from the laundry reject. The maximum values of these elements are in the effluent from the M'DHILLA laundry.The Cd and Zn are found in high concentrations in the solid effluent. The high Cr content is linked with hydrocyclones in chromed steel. The washed phosphate ore was strongly depleted in heavy metals, which were concentrated in the fine fraction released into the hydric network and are thus the main cause of pollution in the region

Accumulation of heavy metals in native plants growing near the phosphate treatment industry, Tunisia

International audienceThe enrichment of the phosphates by a humid process can be accompanied with rejects of mud poured in hydrographic network of the region of Gafsa-Metlaoui to Chott El Gharsa. These releases are rich in phosphate and trace elements; the average is around these values: P2O5: 10.96 %, Cd: 25.83 ppm, Zn: 260.91 ppm, Cr: 387.7 ppm, Cu: 16 ppm, Ni: 26 ppm and Sr: 717.15 ppm. Soils adjoining these rejects present total concentrations in Cd, Zn and Cr higher to the maximal contents tolerated in soils. A two step sequential extraction showed that heavy metals (Cd, Cr, Cu and Ni) were poorly labile (i.e. not soluble in diluted CaCl2), indicating that their leaching under natural conditions is probably very low. However, extraction with DTPA, CaCl2 and TEA generated significant amounts of metals (mainly Cd and Cu), suggesting that they were potentially mobilizable. However, zinc and strontium are more concentrated in the mobile phase, indicating that the Zn-Sr-solutions are possible in natural conditions. This study evaluates the potential for phytoremediation of 30 species collected from six localities, growing on a contaminated site of the study area. Plants and the associated soil samples were collected and analyzed for total metals concentrations. While total metals concentrations in soils vary from 1 to 36 ppm Cd, 1.6 to 2463 ppm Zn, 8.5 to 442 ppm Cr, 2.8 to 38.5 ppm Cu, 1 to 31.7 ppm Ni and 144.6 to 1310 ppm Sr, while those in the plants ranged from 0.11 to 82 ppm Cd, 0.5 to 567 ppm Zn, 0.04 to 174.7 ppm Cr, 0.95 to 51.73 ppm Cu, 0.17 to 6.9 ppm Ni and 6 ppm to 2858 ppm Sr. None of the plants were suitable for phytoextraction because no hyperaccumulator was identified. However, plants with a high bioconcentration factor (BCF, metal concentration ratio of plant roots to soil) and low translocation factor (TF, metal concentration ratio of plant leaves to roots) have the potential for phytostabilization. The plants most effective in the accumulation of metals in leaves are Malva aegyptiaca (TF=30.7) for Cd, Frankenia thymifolia (TF=8.55) for Zn, Peganum harmala (TF=29.14) for Cu and Citrulus sp (TF=10.42) for Sr. Anthemis stiparum was most suitable for phytostabilization of contaminated sites with Cd (BCF=23.51). Our study showed that native plant species growing on contaminated sites may have the potential for phytoremediation