**5. Conclusions**

The use of byproducts from the phosphate industry could create opportunities for the treatment of water contaminated by textile dyes. This study shows that natural phosphate, phosphogypsum, and phosphate waste rock are indeed appropriate for anionic dye removal. However, phosphate waste rock (123.4 mg.g<sup>1</sup> ) has a removal capacity that is higher than that of the natural phosphate (48.4 mg.g<sup>1</sup> ) and phosphogypsum (49.0 mg.g<sup>1</sup> ) materials.

The removal is dependent on the pH of the dye solution, with higher uptake of the dye at a lower pH in the case of the acid dye. The quantity retained increases when the pH increases for the basic dye.

The removal rate of the acid dye fits the pseudo-second-order model for all three materials. The Langmuir isotherm model more appropriately explains the experimental data while, it suggests the formation of a dye monolayer on the surface of the waste products. The Freundlich model isotherm explained appropriately the experimental data, and it suggests the heterogeneity of the surface and presumes that the adsorption takes place at sites with different adsorption energies.

n the case of acid dye, the thermodynamic parameters revealed that the removal process is spontaneous, exothermic, and occurred via chemisorption with the phosphate waste rock. However, physisorption is the proposed mechanism of removal with natural phosphate and phosphogypsum.

For the anionic dye, the regeneration of waster byproducts reveals that close to 80% of the retention dye was adsorbed after four cycles for the phosphate waste rock compound, and it was diminished to 60% after its consumption for seven cycles for all the adsorbents. A trials design was proposed, these experiences are based on the Langmuir model and the high adsorbed quantity. The sought mass of phosphate reuses to achieve a constant percentage of cationic Methylene Blue dye retention, could be readily necessary. The quantity values is linked to the phosphate reuse because their difference in their retention efficiency. Nevertheless, these reported data suggested that wastewater treatment is a potential application for the waste products of phosphate mining.
