The present paper to study the immobilization of chromium ions Cr(VI) in an aqueous solution by adsorption on a natural phosphate (PN) and activated (PA).The natural phosphate whose main component is fluorapatite Ca10(PO4)6F2. The study of the retention of chromium (VI) ions on natural and activated flourapatite is conditioned by several parameters namely the pH. The initial concentration of metals and the contact time. Results obtained showed that after 40 minutes of contact. The results obtained showed that after 40 minutes of contact, the elimination of chromium (VI) was maximum with a yield of 67.4% for the natural flouraptite and 75.9% for the activated phosphate. Increasing the adsorbent dose (0.5 to 1 g/l) makes it possible to improve the chromium removal efficiency with an initial concentration (10 mg/l) / l. The efficiency of removing chromium (VI) by each adsorbent decreases with the increases of the initial chromium content (2 to 10 mg / l). PH has a significant effect on the retention rate of chromium (VI). The efficiency of removing Cr (VI) is interesting for acid pH. Moreover, the adsorption of this compound we described by the kinetic models of Langmuir and Freundlich.

The organic matter of rock phosphate has been the subject of numerous studies. The valuation and resolu-tion of some problems, such as fouling, encountered in the industrial production of phosphoric acid are of great interest, and enable better production yields of phosphoric acid. The solid deposits formed in the production of wet process phosphoric acid were characterized and revealed the malladrite, gypsum, trace metals, and the presence of organic matter. The identification of the organic matter was carried out in the fouling samples using FT-IR and Raman spectroscopies. The quantitative determination of the total organic matter content of the fouling samples was performed using the Walkley-Black and calcination methods. The knowledge of the organic matter in the fouling layers allows a better understanding of phenomena in processes.

The solubility of hexafluoridosilicate salts and acid aqueous solutions {yH 2 [SiF 6 ] + (1 − y)M 2 [SiF 6 ]}(aq) with Na + , K + , and NH 4 + was experimentally determined at various ionic strength fractions (y) with y = 0.20, 0.50, and 0.80 at the temperature 353.15 K. The precipitated solid phases obtained by filtration were characterized by X-ray diffraction, which show only the presence of a single crystalline form for each ternary system at various (y) values, Na 2 [SiF 6 ], K 2 [SiF 6 ], or (NH 4 ) 2 [SiF 6 ], respectively. To find the solubility, the thermodynamic properties of these ternary systems are required, based on the measured activity of water. Then, the water activity measurements for the mixed systems were done at various (y) values with y = 0.20, 0.50, and 0.80 from diluted to the saturated solutions, using the hygrometric method at T = 353.15 K. The related thermodynamic properties of hexafluoridosilicate salts and acid aqueous solutions were also determined using Pitzer's ion-interaction approach. The new ion-interaction parameters θ (H,M) and ψ (H,M,[SiF 6 ]) , with M = Na + , K + , or NH 4 + , of the ternary systems were evaluated and employed with the interaction binary parameters to estimate the solute activity and the osmotic coefficients of water for these aqueous solutions at T = 353.15 K. The calculated solubility of mixed systems is compared with that obtained experimentally.

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