Characteristics of fluoride adsorption in different soil types: Potential factors and implications for environmental risk assessment.

Fluorine is the 13th most abundant element in the earth’s crust, mainly existing in anionic form as fluoride (F^?) in soils (Dehbandi et al., 2017). Although F is a toxic inorganic agent, an appropriate amount is vital for essential processes in the human body (Liu et al., 2020). However, excessive F is harmful to both humans and animal/plant life, and the risk of F pollution in aquatic and terrestrial ecosystems is recognized as a major environmental and health problem (Sarinana-Ruiz et al., 2017). Soil fluoride is the main source of F in the food, drinking water, and groundwater in most parts of China (Zhu et al., 2016). The rapid expansion of China’s industry has caused increasingly severe levels of fluoride contamination in the environment (Gan et al., 2021). In particular, the widespread use of phosphate fertilizer has resulted in fluoride accumulation in topsoil, posing a potential risk of soil fluoride pollution (Huang et al., 2023). Furthermore, China lies in a global fluoride belt (Liu et al., 2020), and is among the countries with generally high fluoride concentrations in its soil and groundwater. Recent research has found that the total F content of Bijie city’s soil reached 2780.60 mg kg^?1 (Li et al., 2023); in Cangzhou Plain, up to 5 and 7 mg L^?1 of F^? were found in its shallow and deep groundwater, respectively (Liu et al., 2024). Fluoride is a non-essential nutrient element for the growth and metabolism of crops; however, soil fluoride can be absorbed by plants, and adversely affect their growth (Ahmad et al., 2022), which could pose a threat to regional food security. Accordingly, understanding the environmental behavior of fluoride in soils is imperative for assessing the risk of fluoride accumulation.

Adsorption and desorption are countervailing geochemical processes of fluoride enrichment and release that widely occur in water–soil systems (Shi et al., 2019). When fluoride is adsorbed by soil, it is sequestered and immobilized in the soil matrix by physical or chemical action, while desorption behavior concerns the release of fluoride from soil. Hence, studying its adsorption-desorption dynamics is important for better understanding how fluoride behaves in the environment. Fluoride is mainly found in the colloidal or clay fraction of soil, and its movement is affected by soil type and the latter’s adsorption capacity (Muthu Prabhu et al., 2023). In general, the type of soil adsorbent (i.e., carbon content, clay content, salinity), soil pH, total amount of F^? deposition, organic matter, and aluminum hydroxides are all pertinent factors influencing the solubility or retention of F in soil. Under specific environmental conditions, soluble fluoride-containing minerals could dissolve rapidly in soil, which will increase its fluoride concentration (Wang et al., 2022a). The water-soluble fluoride undergoes plant uptake via roots and its content is determined by adsorption and desorption dynamics in soil. Some research suggests that contaminants adsorbed to the solid phase components of soil may not get uptaken, rendering them non-toxic to organisms (An et al., 2012; An et al., 2015). Research by D’Alessandro et al. (2012) confirmed the importance of fluoride adsorption by soil for protecting groundwater. How fluoride adsorbs onto metal oxides (Huang et al., 2011), clay minerals (Agarwal et al., 2003; Thakre et al., 2010), and soil (Zhu et al., 2007; Wehr et al., 2022; Iriel et al., 2018) has been investigated; however, previous studies have mainly focused on the simple characteristics of fluoride adsorption. There is limited information available regarding the in-depth investigation of adsorption mechanisms of fluoride in soil via characterization techniques. Furthermore, systematic studies that assess environmental risk according to characteristics of fluoride adsorption in soil are lacking. Moreover, soil is an intricate environmental matrix in comparison to other adsorption materials that have well-known structures, and fluoride’s environmental behavior in soils is arguably more complex, and may also differ among soil types. Therefore, identifying the potential factors influencing fluoride adsorption is of great significance for assessing the behavior and fate of various fluorides in soil environments. This knowledge is crucial to not only control regional fluoride pollution but also protect the drinking water supply and ensure food security.

In this study, we hypothesized that the fluoride adsorption capacity of soil is affected by its properties, thereby affecting the bioavailability of fluoride. To test this hypothesis, we selected three representative agricultural soil types from China’s Shaanxi Province, to investigate their adsorption-desorption characteristics for fluoride. An array of established characterization techniques (SEM, EDX, BET, XRD, XPS, and FTIR) were used to analyze differences in the properties of three soils and the adsorption mechanism of fluoride in soil. Through these analyses, this study’s findings contribute to a more comprehensive understanding of the environmental behavior of fluoride in different kinds of soils.

ABSTRACT ONLINE AT https://www.sciencedirect.com/science/article/abs/pii/S0269749124022541