Effects of various aqueous environmental conditions on the removal of fluoride in fixed bed column reactors packed with calcium hydroxyapatite pellets

Hydroxyapatite sorption is considered to be a cost effective treatment process for removing excess fluoride from drinking water. It is a calcium phosphate crystalline mineral of significant importance to biological applications in dentistry and bone implants, as it comprises the majority of bone and tooth enamel. It also plays a direct role in the large-scale storage and transport of phosphate in marine and geological environments. Hence, many of its properties have been studied extensively. Yet, despite many advancements in understanding them, hydroxyapatite proves to exhibit a high degree of complexity. It is highly tolerant of substitutions of other ions and exhibits a highly variable morphology. The author of this work made an effort to synthesize the work of others in a way that helps make sense of how hydroxyapatite interacts with and removes fluoride from water. Understanding the mechanisms of defluoridation by hydroxyapatite is fundamental to establishing a way to move forward in the development of more effective fluoride adsorbents. Fixed-bed column tests were conducted using hydroxyapatite pellets as the adsorbent material. The nature of its interaction with water quality parameters including pH and the concentrations of fluoride, carbonate, calcium and phosphate was observed in order to gain a foundational understanding of the mechanisms of fluoride removal at work. These studies were substantiated with analytical surface and bulk analysis techniques: X-ray photoelectron spectroscopy (XPS) and X-ray diffraction, as well as surface area analysis. The hydroxyapatite used in this work was found to have the following composition Ca10.12(PO4)5.4(CO3)1.12(OH)1.80. Approximately 1.2 % of this material is calcium carbonate by wt. A number of interesting findings resulted from this work. First, it was observed that increasing calcium and phosphate concentrations in solution results in an increasing role that precipitation plays in the removal of fluoride. The removal capacity of this material improved by 30% solely from the contributions made by calcium and phosphate. Second, it was proven through the collective findings from defluoridation experiments and XPS measurements that fluoride is able to travel through the interstitial pores within hydroxyapatite pellets, and that a significant amount of fluoride removal can occur this way, although kinetically slower than by other removal processes. Third, the defluoridation data suggests that only surface sites of the hydroxyapatite material, not the bulk, are responsible for fluoride removal