Comparative Evaluation of Conventional Defluoridation Technologies for Fluoride Removal from Real Moroccan Phosphate Mine Waters

Abstract

Excess fluoride originating from phosphate mining activities poses a significant environmental and public health challenge, particularly in arid and semi-arid regions where water resources are limited. This study presents a comparative evaluation of four conventional defluoridation technologies for the treatment of real phosphate mine waters collected from two major Moroccan phosphate mining sites (Youssoufia and Khouribga). The investigated processes included coagulation–flocculation using aluminum sulfate and ferric chloride, chemical precipitation with calcium hydroxide and calcium chloride, adsorption on aluminum oxide (Al2O3) and zirconium oxide (ZrO2), and fluoride removal using calcined bovine bone apatite under both batch and continuous-flow conditions. Adsorption equilibrium was interpreted using Langmuir and Freundlich isotherm models, while the effects of adsorbent dosage, contact time, and water matrix composition were systematically investigated. Among the coagulation processes, aluminum sulfate achieved fluoride removal of up to approximately 82.5% in phosphate washing water and approximately 76.3% in mine drainage water, whereas ferric chloride removed about 52% of the dissolved fluoride under the reported conditions. Lime and calcium chloride exhibited moderate removal efficiencies of 66% and 61%, respectively. Aluminum oxide showed the highest equilibrium adsorption capacity (qm = 7.14 mg g−1), while zirconium oxide displayed faster fluoride uptake because of its higher surface affinity for fluoride ions. The presence of competing ions in real mine waters was associated with lower adsorption performance compared with synthetic fluoride solutions. Calcined bone apatite proved to be the most effective material, achieving approximately 83% fluoride removal within 20 min under batch conditions and maintaining good performance during continuous fixed-bed operation, producing treated water with fluoride concentrations below the World Health Organization guideline value. Overall, the results demonstrate that calcined bone apatite provides the highest fluoride-removal performance among the investigated materials under the tested conditions. Its waste-derived origin, rapid adsorption kinetics, and effective fluoride removal make it a promising material for the treatment of fluoride-rich phosphate mine waters. The comparative evaluation further indicates that integrating chemical pretreatment with adsorption may represent a promising strategy for the treatment and potential reuse of mining effluents, although the performance of such a combined treatment train should be validated experimentally.