The development of teeth and bones in the human body requires an appropriate amount of fluoride (0.5–1.5 mg/L), which is one of the essential trace elements for life [1]. However, excessive fluoride intake (greater than 1.5 mg/L) can result in fluorosis, dental fluorosis, fluorosis, and even brain damage. Fluoride pollution in water bodies presents significant risks to human health. Thus, the removal of excessive fluoride from groundwater is crucial for safeguarding human life.
To effectively address the issue of fluoride pollution, various countries have developed multiple methods to treat fluorine contamination. Currently, precipitation [2], adsorption [3, 4], electrochemistry [5, 6], ion exchange [7, 8], and membrane filtration [9] are the primary approaches for fluoride removal. The adsorbents commonly employed for fluoride removal encompass a range of materials such as activated carbon, clay minerals, chitosan, ion exchange fibers, ion exchange resins, activated alumina, zirconium oxide, iron oxide, metal-organic frameworks, hydroxyapatite, and more [10–16]. It is crucial to consider that minerals like silicates undergo hydrolysis, leading to groundwater typically having a pH range of 7–9 [17]. Moreover, natural water bodies often contain numerous coexisting ions. Consequently, under conditions of intense competition from ions and alkaline pH, the selectivity of fluoride adsorbents becomes paramount. However, adsorbents like activated carbon, clay minerals, chitosan, ion exchange fibers, and ion exchange resins mainly rely on their porous structure or surface functional groups to sequester fluoride ions. These adsorbents exhibit relatively low adsorption capacity and are easily susceptible to interference from coexisting ions such as SO42−, NO3−, Cl−, and others. As a result, these commonly employed adsorbents demonstrate poor selectivity for F− in fluoride-contaminated water with high concentrations of competing ions. To enhance adsorption efficiency, it is common practice to introduce organic functional groups or multivalent metal cations into the aforementioned adsorbents. While metal oxides and hydroxides generally exhibit good fluoride adsorption capacity, most of them still fall below an adsorption capacity of 100 mg/g. In contrast, MgO stands out as a readily available and promising adsorbent. Furthermore, magnesium oxide possesses several advantageous properties, includ ingnon-toxicity, high adsorption capacity, high porosity, high ion selectivity, large surface area, segregated OH, and a high zero point charge (pHpzc) [18, 19].
In prior studies, researchers have developed several magnesium oxide adsorbents specifically tailored for fluoride removal. Xavy et al. [8] conducted experiments involving the preparation of three distinct morphologies of nano MgO for wastewater treatment using two methods: the sol-gel method and hydrothermal method. The rod and spherical-shaped nano MgO exhibited higher surface areas, which promoted the presence of increased surface defects. These defects acted as active sites, ultimately enhancing the adsorption performance. In another study, Kong et al. [18] successfully synthesized porous MgO nanosheets through a straightforward precursor calcination technique, where polyethylene glycol was incorporated as a dispersant during the preparation process. The long-chain structure of polyethylene glycol tightly enveloped the precursor basic magnesium carbonate crystal, while the branched hydroxyl groups absorbed free metal ions. This mechanism prevented collisions between the grains and hindered crystal growth, leading to the formation of porous MgO nanosheets. However, it is important to note that the preparation process described above entails the use of a reactor with high energy consumption or strong current action. This aspect can potentially result in reduced economic benefits and pose safety risks.
Among the various preparation methods, the hydrothermal method and homogeneous precipitation method offer distinct advantages. The hydrothermal method allows for the production of powders with excellent dispersion, crystallinity, low energy consumption, and controllable reactions. Additionally, the lower surface energy of the powder helps minimize aggregation [20–22]. On the other hand, the homogeneous precipitation method enables a gradual and uniform release of crystallographic ions in the solution, addressing the limitations of traditional precipitation methods, such as high instantaneous local concentrations and poor dispersion when adding precipitants [23–26]. In this study, the advantages of both the hydrothermal and homogeneous precipitation methods were combined to prepare nano magnesium oxide for the first time using a hydrothermal homogeneous precipitation method. The adsorption performance of the resulting nano magnesium oxide for fluoride was comprehensively investigated, revealing an adsorption capacity exceeding 120 mg/g at pH 6.5. The impact of pH and anions on fluoride removal was also examined, while providing insights into the underlying removal mechanism.