Fluoride ion removal from mine water via nucleation crystallization pelleting process

Objective The mine water associated with coal mining tends to be rich in fluoride ions. If discharged directly without effective treatment, such water will cause severe pollution to regional ecology, affecting the quality of water resources and the stability of the ecosystem. MethodsThis study focuses on the challenging treatment of the fluoride pollution caused by coal mining-associated mine water. To overcome the bottlenecks including low efficiency and weak anti-interference of traditional methods for fluoride removal, this study designed a setup for fluoride removal using the nucleation crystallization pelleting (NCP) processing and proposed a novel fluoride removal method—NCP chemical precipitation. The fluoride pollution of mine water poses great environmental risks since the resulting fluoride mass concentration in surface water might exceed relevant standards by 8‒15 times. The deep fluoride removal in a complex water quality environment is challenging in the prevention and control of fluoride pollution. This study developed a coordinated regulating mechanism integrating multi-phase reactions: chemical precipitation, nucleation induction, and porous adsorption. Process optimization experiments on a laboratory scale were conducted using a continuous flow chemistry system with a hydraulic retention time (HRT) of 45 min and an upward flow rate of 1.8 m/h. Then, this study systematically determined the dynamic process of fluorine migration and transformation using advanced characterization techniques such as X-ray photoelectron spectroscopy (XPS) and in-situ Fourier transform infrared (FTIR) spectroscopy. Results and ConclusionsThe results indicate that the mass concentration of fluoride dropped from 12.6 mg/L to 7.6 mg/L (removal rate: 39.8%) after only a single stage of processing under the optimized conditions (i.e., a nucleation inducer (CaCl2) dosage of 1 200 mg/L and a seed loading ratio of 1∶50), with efficiency being 2.3 times higher than that of conventional coagulating sedimentation. X-ray diffraction (XRD) corroborated that thermodynamically stable aragonite and vaterite crystals were generated from reactions between Ca2+ and F−. Notably, coexisting carbonates enhanced fluoride removal by forming CaCO3·CaF2 composite precipitates (FTIR reveals a characteristic peak at 1 080 cm−1) or porous calcite carriers (SEM images indicate a porosity increase of 62.76%). This study revealed the regulation pattern of interfacial reactions in a carbonate system. Energy dispersive spectroscopy (EDS) confirmed the gradient distribution of fluorine elements in the cross section of formed particles, revealing the progressive removal mechanism from surface adsorption to lattice fixation. The NCP technique can effectively remove fluoride ions in mine water with complex water quality and can deal with the complex chemical composition in mine water. The results of this study will lay a foundation for the engineering application of the fluoride removal technology based on the NCP process while also providing a feasible technical route for solving the environmental pollution caused by fluoride-bearing mine water.