## 1 Introduction  
Fluoride is the most widely distributed non-metal element globally due to its strong reactivity and electronegativity [1]. Within concentrations between 0.4 and 1.0 mg/L, fluoride is beneficial in preventing tooth decay and promoting bone growth and development [2]. However, extended consumption of drinking water containing fluoride levels exceeding 1.5 mg/L can result in various health issues, including impaired growth, poor intellectual development, skeletal fluorosis, osteoporosis, and arthritis [3]. In order to safeguard against the potential hazards of consuming water contaminated with fluoride, the WHO has set a cap on the allowable concentration of fluoride in drinking water, ensuring it remains below 1.5 mg/L [4,5]. Fluoride pollution is a significant environmental concern, affecting more than 300 million individuals globally, particularly in developing countries [6]. The prevalence of fluorosis, a related disease, is observed in over 25 nations spanning different continents, with China and India experiencing the most substantial impact [7]. Additionally, industrial processes, such as semiconductor manufacturing, contribute to high fluoride levels in wastewater, necessitating effective treatment methods [8, 9, 10, 11]. Various defluorination methods, like precipitation, coagulation, ion exchange, and adsorption, have been widely applied to reduce fluoride contamination [12]. However, precipitation and coagulation processes are hindered by slow processing rates, and they produce large amounts of sludge [13]. Furthermore, while the ion exchange process exhibits a high removal efficiency of up to 95%, the relatively high cost of ion exchange resins and their reduced efficiency at high ion concentrations limit their application in wastewater treatment [14]. In contrast, adsorption stands out as an environmentally friendly, cost-effective, and efficient method, characterized by easy maintenance, simple design, and low energy consumption [1, 3, 15, 16, 17]. This approach employs adsorbents with a large surface area to remove contaminants in water through physical or chemical adsorption. Despite its potential, many reported adsorbents encounter various practical challenges, such as high cost, insufficient adsorption capacity, and complex preparation methods [18]. Thus, selecting a highly efficient, low-cost adsorbent is crucial for widescale applicability in large-scale wastewater treatment [16, 19]. Some studies have explored the use of low-cost adsorbents, such as bone charcoal [20], peanut shells [21], rice husks [22], and mussel shells [23], for removing fluoride. Among these materials, bone char is frequently employed as an alternative to commercially available activated alumina [14]. However, its use is constrained by concerns related to religious beliefs and its unpleasant taste and odor [24]. Studies have also explored the use of metals-doped adsorbents (e.g., those containing Fe or Al) to increase fluorine removal efficiency. However, metal doping increases the cost of the adsorbent and requires additional processing facilities, time, and energy [25]. As a solution, green adsorbents derived from agricultural or lignocellulosic biomass are gaining traction [26]. The adoption of such renewable feedstocks aligns with the pursuit of the United Nations Sustainable Development Goals (SDGs), like SDG-3 (health and well-being) and SDG-6 (safe and clean water and sanitation) [27]. Wood, a renewable resource considered vital for developing carbon-neutral strategies by 2050, is widely available and heavily relied upon for energy in developing countries [28]. Unlike bone char, woody biomass is readily available in all countries and is estimated to have an annual production of 10-50 billion tons [6]. Despite wood-based energy production generating approximately 6-10% ash, a considerable amount (70%) of the resultant wood ash (WA) is currently disposed of in landfills [29]. Efficient resource utilization and addressing landfill-related environmental issues encourage the valorization of WA.