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Retention and selectivity of phosphate and fluoride from single and industrial aqueous solutions using purified and surfactant modified Tunisian clay

Wastewater coming from the manufactory of the phosphoric acid fabrication is one of the most significant environmental problems throughout the Gabes region in Tunisia. This wastewater is very loaded with fluoride (F–) and phosphate (PO4) and have an acidic pH (pH = 3.2; [F] = 1,840 mg L–1; [P] = 1,420 mg L–1). In this work, a smectitic clay mineral from the region (Jebel Haidoudi, Gabes) was tested for the removal of phosphate and fluoride ions from aqueous solutions. The clay samples are first purified and modified using cationic surfactant (hexadecylpyridinium (HDPy+)) at different concentrations (1.0 to 3.0 CEC of clay CEC – cationic exchange capacity) then characterized by X-ray diffraction and Fourier-transform infrared spectroscopy. A series of batch adsorption experiments were carried out. Three parameters were investigated including the effect of pH, contact time and initial fluoride and phosphate single concentrations. The experimental results show that most of the adsorption took place during the first 5 h and the optimum pH value ranged from 3 to 4. The pseudo-second-order kinetic model described better the (F) and (P) adsorption. Thus, the isotherm studies showed that the best linear-fit was obtained with the Langmuir isotherm. The adsorption selectivity of (P and F) ions in the wastewater solution by the treated clays show that the fluoride is more selective in the acid pH. Finally, organoclay (3 CEC) is the best adsorbent and could be considered as an efficient and cost-effective adsorbent.