Abstract: In response to the severe fluoride contamination in mine water from coal mining areas of Northwest China and
the limitations of conventional treatment methods such as high cost and secondary pollution,this study developed Al3+ -doped
nano-hydroxyapatite (Al-nHAP,with nAl / nCa+Al = 3%) via a hydrothermal method,systematically evaluating its performance
and mechanism for fluoride removal from simulated mine water. The optimal synthesis conditions for nHAP were determined as
follows:reaction temperature of 120 ℃,time of 12 h,and Ca/ P molar ratio of 1. 67,under which the maximum adsorption capacity
of nHAP for an initial fluoride concentration of 10 mg/ L reached 3. 57 mg/ g. After Al3+ modification,the material exhibited
enhanced crystal growth along specific facets (XRD),partial substitution of surface hydroxyl groups and strengthened
phosphate coordination (FTIR),while maintaining a mesoporous structure (BET). These changes collectively contributed to a
fluoride removal efficiency of 91. 48% and an adsorption capacity of 4. 96 mg/ g under the same conditions,representing an approximately
39% improvement over unmodified nHAP. The adsorption process was better described by the pseudo-second-order
kinetic model (R2 =0. 998 3) and the Langmuir isotherm model (maximum adsorption capacity of 16. 61 mg/ g at 25 ℃),indicating
chemisorption-dominated monolayer adsorption. Interference tests confirmed that Al-nHAP maintained strong tolerance
towards common anions such as Cl^- ,NO^-₃ ,and SO^2-₄ ,although the presence of CO^2-₃ and PO^3-₄ led to a decrease in fluoride removal
by approximately 30% due to competitive adsorption. This study provides an efficient and environmentally adaptable adsorbent
for treating high-fluoride mine water,demonstrating promising application potential.

Key words: fluoride-containing water,hydroxyapatite,Al-modified nano-hydroxyapatite,mine water,fluoride removal by
adsorption