Enhanced fluoride removal from water using acid-modified red clay soil from the Loess Plateau of China

Water, as the fundamental source of various ecosystems and a prerequisite for both global economic and social development, has witnessed a concerning decline in quality over recent decades (He et al. 2020). Notably, fluoride contamination in groundwater and surface water has been as a global challenge of remarkable significance due to the over-discharge of various industrial wastewater containing high fluoride (Mohapatra et al. 2009; Bhatnagar et al. 2011; Loganathan et al. 2013; Jadhav et al. 2015; Vinati et al. 2015; Biswas et al. 2016; He et al. 2020; Wan et al. 2021; Solanki et al. 2022; Hussain et al. 2024). As one of the most active elements, fluoride is widely distributed in pedosphere, hydrosphere, atmosphere, and biosphere (Wan et al. 2021), and found predominantly present as fluoride-containing minerals such as fluorite or fluorspar (CaF₂), sellaite (MgF₂), fluorapatite (Ca₅(PO₄)₃F), and cryolite (Na₃AlF₆) (Mohapatra et al. 2009; He et al. 2020). In nature, fluoride enters groundwater through the dissolution of fluoride-containing minerals under favorable conditions (Mohapatra et al. 2009; He et al. 2020) or surface water bodies via stormwater runoff that dissolves fluoride-containing minerals during stormwater events (Wan et al. 2021). The discharge of fluoride-containing wastewater from diverse industries, including electroplating, metal processing, battery industry, electronic manufacturing, coal mining, and photovoltaic industry, poses a significant threat to water resources (Biswas et al. 2016; Wan et al. 2021; Solanki et al. 2022; Hussain et al. 2024).

Excessive fluoride exposure can cause adverse health effects and threaten human health and life (Hussain et al. 2024). According to the World Health Organization, concentrations of fluoride in drinking water exceeding 1.5 mg/L are detrimental to human health, leading to conditions such as dental and skeletal fluorosis (Mohapatra et al. 2009; He et al. 2020; Solanki et al. 2022; Hussain et al. 2024). Recent investigations have further revealed that excessive intake of fluoride can cause non-skeletal fluorosis, and affect the nervous system, endocrine glands, reproductive system, as well as organs like the kidney and liver (Mohapatra et al. 2009; Bhatnagar et al. 2011; Vinati et al. 2015; He et al. 2020; Wan et al. 2021; Solanki et al. 2022; Hussain et al. 2024). Over 260 million people worldwide mainly from approximately 25 countries in Africa, America, and Asia, particularly Tanzania, USA, Mexico, China, India, and Japan, consume drinking water with high fluoride concentrations (Mohapatra et al. 2009; He et al. 2020; Hussain et al. 2024). Hence, different countries have carried out drinking water standards and water management strategies addressing fluoride issues to ensure drinking water quality (Solanki et al. 2022). Many countries set a permissible limit for fluoride in drinking water as less than 1.5 mg/L while some countries such as China and Singapore set it as less than 1.0 mg/L (Solanki et al. 2022). However, industrialization resulted in fluoride concentrations in water bodies, particularly in groundwater, frequently higher than 30 mg/L in the United States of America, Africa, and Asia (Mohapatra et al. 2009). Therefore, the urgency to address the toxic effects of fluoride on human health has prompted a global quest for effective technologies to eliminate excess fluoride from water environments.

Global researchers have developed various techniques, including precipitation-coagulation, electrocoagulation, ion exchange, membrane processes including reverse osmosis and electrodialysis, and adsorption, to reduce fluoride levels and mitigate the adverse health effects associated with excessive fluoride exposure (Kemer et al. 2009; Mohapatra et al. 2009; Bhatnagar et al. 2011; Loganathan et al. 2013; Jadhav et al. 2015; Vinati et al. 2015; He et al. 2020; Wan et al. 2021; Solanki et al. 2022). Compared to the other technologies, adsorption has proven to be a viable approach for effectively reducing excessive fluoride levels in groundwater and drinking water (Kemer et al. 2009; Bhatnagar et al. 2011; Loganathan et al. 2013; Vinati et al. 2015; He et al. 2020; Hussain et al. 2024). Its cost-effectiveness, operational simplicity, high removal capacity, reliability, low energy consumption, and the potential for adsorbent reuse make it a practical choice (Kemer et al. 2009; Mohapatra et al. 2009; Bhatnagar et al. 2011; Loganathan et al. 2013; Vinati et al. 2015; He et al. 2020; Wan et al. 2021; Hussain et al. 2024). These attributes motivate researchers to further investigate various adsorbents for additional potential (Kemer et al. 2009; Bhatnagar et al. 2011; Loganathan et al. 2013; He et al. 2020; Wan et al. 2021; Hussain et al. 2024).

Although researchers have developed numerous adsorbents including carbon-based materials such as activated carbon, biochar and bone char, and carbon nanotubes, chitosan and chitosan-modified materials, metal materials, polymers and resins, metal-organic frameworks, layered double hydroxides, biomaterials (Bhatnagar et al. 2011; Loganathan et al. 2013; He et al. 2020; Wan et al. 2021), and waste mud (Kemer et al. 2009), it is essential to explore clay or clay minerals based alternatives for fluoride removal in water (Vinati et al. 2015). Previous studies have confirmed that clays demonstrate significant potential for adsorbing fluoride in water (Tomar & Kumar 2013; Vinati et al. 2015). The utilization of clay or clay minerals as adsorbents for fluoride removal in water has notable advantages over other adsorbents including abundance, renewability, and environmental friendliness, low cost, high adsorption performance, and ion exchange potential (Vinati et al. 2015).

Clay soil is a globally prevalent soil type and exhibits varying physical and chemical characteristics across different regions due to factors such as volcanic regions, sedimentary areas, climate variations, vegetation types, and geological differences (Vinati et al. 2015; Hamdi et al. 2024). The red clay soil investigated in this study is extensively distributed in the Loess Plateau region of China. The formation of red clay soil in this region can be traced back to the late Miocene to early Pleistocene loess period, during which these areas experienced prolonged weathering and erosion. Red clay soil finally formed as the result of volcanic ash mixed with loess deposits through the functions of volcanic eruptions and tectonic uplift (China Soil Science Data Center 2016). Despite different previous studies exploring the efficacy of clay and clay minerals in fluoride removal (Bhatnagar et al. 2011; Vinati et al. 2015; He et al. 2020; Wan et al. 2021), as far as the knowledge of using red clay soil or modified red clay soil from the Loess Plateau region to remove fluoride is still scarce in the current literature.

Therefore, the specific objectives of this study were to: (1) investigate and compare fluoride adsorption removal in aqueous solutions using unmodified red clay soil, as well as acid-modified, alkali-modified, and thermally modified red clay soil; (2) analyze the fluoride adsorption behavior of unmodified and acid-modified red clay soil; and (3) elucidate the mechanisms underlying fluoride adsorption onto both unmodified and acid-modified red clay soil. The ultimate purpose was to offer an eco-friendly, cost-effective, and sustainable solution for mitigating fluoride contamination in diverse water bodies. Moreover, this study aimed to provide valuable insights to researchers for facilitating the further development of red clay soil-based adsorbents to reduce fluoride pollution and protect human health.

Red clay soil samples were collected from an agricultural land situated in Changzhi, Shanxi Province, China (latitude 36°50′16″ N and longitude 112°51′08″ E). The bulk density, pH, electrical conductivity, and total organic carbon of the red clay soil were assessed following the procedures outlined by Li et al. (2022). Additionally, the pH at the point of zero charge (pH_pzc) was determined utilizing the methodology described by Daifullah et al. (2007). The collected samples experienced a series of preparatory steps, starting with washing using ultrapure water, followed by oven drying at 105 °C until reaching a constant weight. Once cooled to room temperature, the red clay soil samples were ground and sieved. The fraction of soil particles with a size less than 0.1 mm was selected for further experimentation. To examine different modifications, three methods were employed: acid treatment, alkali treatment, and thermal treatment. Acid and alkali modifications involved adding sulfuric acid of 0.5, 1.0, 2.0, 2.5, and 5.0 mol/L, and sodium hydroxide of 0.5, 1.0, 2.0, 2.5, and 5.0 mol/L, respectively, using a solid–liquid ratio of 1 g:20 mL. The mixtures were shaken at 25 °C for 4 h, followed by centrifugation and multiple rinses with ultrapure water until achieving a neutral pH value. For thermal modification, the pretreated soil samples were subjected to a 2-h heating process in a muffle furnace at temperatures of 200, 300, 400, 500, 600, 700, and 800 °C. The modified red clay soil samples were then air-dried and prepared for subsequent experiments.

The X-ray diffraction (XRD) patterns of unmodified and modified soil samples were obtained in the range of 5°–90° (2θ) using the PANalytical X'Pert PRO powder diffractometer (Empyrean, Malvern Panalytical Ltd, United Kingdom). Fourier transform infrared spectroscopy (FTIR) analysis was conducted on unmodified and modified soil samples before and after fluoride adsorption. This analysis was carried out using the Nicolet iS20 FTIR spectrometer (Thermo Fisher Scientific Inc., USA) to explore the variations in functional groups within the spectral range of 4,000–400 cm⁻¹. Micromeritics ASAP 2460 surface area and porosity analyzer (Micromeritics, USA) was used to measure specific surface area, pore volume, and pore diameter of the unmodified soil samples. X-ray fluorescence (XRF) spectroscopy analysis was performed to elucidate the elemental composition of the acid-modified red clay soil, utilizing the Panalytical Axios instrument (Malvern Panalytical Ltd, Netherlands).

In this study, laboratory analytical grade sodium fluoride (NaF) was employed to freshly prepare different concentrations of fluoride solutions. The analysis of fluoride within the solutions was carried out utilizing an ion chromatography (IC) instrument, specifically the Dionex ICS-210 model (Thermo Fisher Scientific Inc., USA).

Batch experiments were conducted using both unmodified and modified red clay soil samples as the adsorbents to investigate the fluoride adsorption behavior. The fluoride adsorption study contained five important impacting factors including adsorbent dosage, pH, initial fluoride concentration (C₀), adsorption time, and temperature. To ensure consistency, a constant shaking speed of 200 rpm was maintained throughout the experiments. The total solution volume was 100 mL in each adsorption experiment unless otherwise specified. At predetermined time intervals, solution samples were collected for fluoride analysis. Prior to fluoride analysis, the solution samples were centrifuged to separate the solid and liquid phases. Subsequently, the solution samples were filtered through a syringe filter. The fluoride concentrations in the filtrates were then accurately determined using an IC instrument. To enhance the precision, all adsorption experiments were replicated three times.

In this study, analysis of variance (ANOVA) or t-tests were utilized to assess differences among multiple groups of data or between two groups of data. A significance threshold of a P-value less than 0.05 was established to determine statistical significance.

The bulk density of unmodified red clay soil was 1.325 g/cm³ and the particle size was less than 0.1 mm. The pH and electrical conductivity of unmodified red clay soil, determined at a soil-to-solution ratio of 1:2 (m/v), were found to be 8.26 and 0.1884 mS/cm, respectively. Additionally, the total organic carbon content in unmodified red clay soil was 21.46 g/kg.

The results highlighted a significant improvement in fluoride adsorption removal by red clay soil after acid modification. Despite the decrease in the specific surface area of the red clay soil due to acid modification, the increase in total pore volume and average pore diameter resulting from the acid modification may have contributed to the enhanced fluoride adsorption. Moreover, one-way ANOVA analysis demonstrated the significant impact of acid concentration used in the acid modification of red clay soil on fluoride adsorption removal (P-value = 9.63 × 10⁻⁸). Specifically, the red clay soil modified with 2.0-mol/L sulfuric acid exhibited significantly higher fluoride adsorption removal compared to the red clay soil modified with other concentrations of sulfuric acid (P-value < 0.05). Consequently, unmodified red clay soil and 2.0-mol/L sulfuric acid-modified red clay soil were chosen as the adsorbents for subsequent adsorption experiments.

The adsorption of fluoride onto adsorbents involves both non-specific and specific mechanisms, particularly regarding the impact of pH on fluoride adsorption (Sujana et al. 2009). Non-specific fluoride adsorption primarily involves coulombic forces and depends on the pH at the point of zero charge (pH_PZC) (Sujana et al. 2009; Loganathan et al. 2013; Vinati et al. 2015; Sadhu et al. 2021). Under conditions where the pH is lower than the pH_PZC, the surfaces of adsorbent particles are positively charged, thereby attracting negatively charged fluoride ions and facilitating their adsorption (Sujana et al. 2009; Loganathan et al. 2013; Vinati et al. 2015; Sadhu et al. 2021). Conversely, at pH values exceeding the pH_PZC, a decrease in fluoride adsorption occurs due to electrostatic repulsion between fluoride ions and the negatively charged surfaces of adsorbent particles (Sujana et al. 2009; Loganathan et al. 2013; Vinati et al. 2015; Sadhu et al. 2021). Additionally, as pH increases, competition between fluoride and hydroxide ions intensifies for active sites, impeding fluoride ion adsorption (Vinati et al. 2015). Typically, at pH values beyond 7–8, the decline in fluoride adsorption removal results not only from the negatively charged surfaces of the adsorbent (Sujana et al. 2009) but also from intensified competition between fluoride and hydroxide ions (Loganathan et al. 2013. In this study, the pH_ZPC of unmodified and acid-modified red clay soils was measured as 6.5 and 2.7, respectively. The maximum fluoride adsorption was observed at pH 2 for both unmodified and acid-modified red clay soils. Subsequently, as the pH increased, a gradual decline in fluoride adsorption was observed (Figure 4). The observed patterns of pH effects on fluoride adsorption onto both unmodified and acid-modified red clay soils in this study differed from those reported by Sujana et al. (2009) for certain geomaterials. Sujana et al. (2009) reported that the highest fluoride adsorption removal occurred in the pH range of 3–5 for laterites containing iron and nickel when the pH range from 2.5 to 10 was investigated in the study of pH effects on fluoride adsorption.

Specific fluoride adsorption, on the other hand, involves surface protonation (Loganathan et al. 2013) and ligand exchange reactions (Sujana et al. 2009). Surface protonation at a pH lower than the pH_PZC enhanced the availability of hydrogen atoms on the surfaces of unmodified and acid-modified red clay soil particles, increased the number of hydrogen bonds between the hydrogen atoms and fluoride, and thereby improved fluoride adsorption (Loganathan et al. 2013). However, as the pH increased, the number of protonated sites decreased, eventually diminishing and leading to reduced efficiency in the adsorption removal of fluoride (Sadhu et al. 2021). Additionally, lower pH facilitates interactions through the ligand exchange mechanism between fluoride and the surfaces of the adsorbent (Loganathan et al. 2013). As mentioned above, unmodified and acid-modified red clay soils contained SiO₂, Fe₂O₃, and Al₂O₃; thus, the fluoride adsorption process might include specific adsorption through ligand exchange reactions with these oxide species (Sujana et al. 2009). This specific mechanism would be explored through the analysis of FTIR in Section 3.8.

The investigation revealed a decline in fluoride adsorption removal with an increase in the initial concentration (C₀) for both unmodified and acid-modified red clay soil. Specifically, fluoride adsorption removal decreased from 16.54 to 7.65% for unmodified red clay soil and from 62.33 to 31.38% for acid-modified red clay soil. This observation might be a result of enhanced competition among fluoride ions for active sites on the reaction surface due to the increased initial fluoride concentration (Thakre et al. 2010; Goswami & Purkait 2011; Vinati et al. 2015).

The outcomes derived from fitting data into three isotherm models presented distinct parameter values. In the fitted Langmuir model, q_max and K_L values were determined as 0.550 mg/g and 0.028 L/mg for unmodified red clay soil, and 1.935 mg/g and 0.069 L/mg for acid-modified red clay soil. Meanwhile, K_F and n values were 0.042 (mg/g)(L/mg)^1/n and 1.955 for unmodified red clay soil, and 0.289 (mg/g)(L/mg)^1/n and 2.360 for acid-modified red clay soil in the fitted Freundlich model. Additionally, A_T and b_T were determined as 0.215 L/g and 18,349 J/mol for unmodified red clay soil and 0.597 L/g and 5,591 J/mol for acid-modified red clay soil in the fitted Temkin model.

Vinati et al. (2015) conducted a comprehensive review on the adsorption of fluoride on clay and clay minerals demonstrating the Langmuir model's exceptional fit regarding fluoride adsorption across various minerals, notably including magnesium-incorporated bentonite (Thakre et al. 2010), Fe³⁺-modified bentonite (Gitari et al. 2015), palygorskitic clay (Hamdi & Srasra 2007), mechanochemically activated kaolinite (Meenakshi et al. 2008), montmorillonite (Ramdani et al. 2010), and pyrophyllite (Goswami & Purkait 2011). Notably, the maximum adsorption capacity of fluoride by acid-modified red clay soil in this study exceeded that of laterite (0.1328 mg/g) (Sarkar et al. 2006), chemically modified bentonite (0.56 mg/g) (Kamble et al. 2009), and calcium chloride-modified natural zeolite (1.766 mg/g) (Zhang et al. 2011). These findings highlighted the potential efficacy of utilizing acid-modified red clay soil as an adsorbent for fluoride removal.

The results obtained from fitting the data into the Weber and Morris intraparticle diffusion model indicated a distinct two-phase pattern in fluoride adsorption for both unmodified and acid-modified red clay soil (Figure 6(c) and 6(d)). Notably, acid modification significantly accelerated the adsorption reaction rate in Phase I. This acceleration is evident from the significantly higher slope of the first line (Phase I) for acid-modified red clay soil (0.068) (Figure 6(d)) compared to unmodified red clay soil (0.0054) (Figure 6(c)). Additionally, the near-zero intercept observed in the Phase I lines strongly indicated that the fluoride adsorption process during this phase was primarily governed by intraparticle diffusion (Hovsepyan & Bonzongo 2009; Wu et al. 2009; Largitte & Pasquier 2016; Wang & Guo 2022).

Moreover, acid modification notably enhanced the reaction rate in Phase II, as evidenced by the significantly higher slope for acid-modified red clay soil (0.012) (Figure 6(d)) compared to that for unmodified red clay soil (0.0006) (Figure 6(c)). In Phase II, the non-zero intercepts observed in both second lines suggested that the rate of the fluoride adsorption reaction was limited by both external mass transfer and intraparticle diffusion (Hovsepyan & Bonzongo 2009; Wu et al. 2009; Largitte & Pasquier 2016; Wang & Guo 2022). Furthermore, acid modification reduced the breakpoint time from 6.694 to 2.318 min^1/2 (Figure 6(c) and 6(d)), suggesting a shortened transition time from intraparticle diffusion-limited adsorption to both external mass transfer and intraparticle diffusion-controlled adsorption in fluoride adsorption (Hovsepyan & Bonzongo 2009; Wu et al. 2009; Largitte & Pasquier 2016; Wang & Guo 2022). Additionally, the higher slopes of the first lines compared to the second lines indicated that the adsorption reaction rate in Phase I was faster than that in Phase II.

Furthermore, the results indicated a decrease in fluoride adsorption removal with an increase in temperature for both unmodified and acid-modified red clay soil (Figure 7). This implies an exothermic nature of the adsorption process of fluoride by unmodified and modified red clay soil. This observation is consistent with previous research (Loganathan et al. 2013), including studies on fluoride adsorption by alum sludge (Sujana et al. 1998), chelating resin (Meenakshi & Viswanathan 2007), calcined Zn/Al layered double hydroxide (LDH) (Das et al. 2003), LDH/chitin (Sairam Sundaram et al. 2008), modified activated carbon (Daifullah et al. 2007), watermelon rind biochar (Sadhu et al. 2021), and geomaterials (Sujana et al. 2009).

This observation may be attributed to the increasing tendency of molecules to escape from the reaction interface due to a rise in temperature, resulting in decreased adsorption removal (Sujana et al. 1998, 2009; Sadhu et al. 2021). Higher temperatures might also increase the desorption of fluoride from the reaction interface, resulting in a net reduction in fluoride adsorption removal (Sujana & Anand 2011). Additionally, the rise in temperature might alter the particle structures and properties in both unmodified and acid-modified red clay soils, potentially making the surfaces less favorable for fluoride adsorption. This observation highlights the complex relationship between temperature and fluoride adsorption onto unmodified and acid-modified red clay soil, necessitating further investigation to explore the underlying mechanisms.

After fluoride adsorption in unmodified red clay soil, a notable spectral change was observed: the peak at 1,030/cm broadened and intensified, indicating the strengthening of silicon–oxygen–silicon bonds due to fluoride adsorption on red clay soil particles. Following hydrolysis, silicon–oxygen–silicon bonds transformed into −SiOH functional groups on the surfaces of red clay soil (Sujana et al. 2009; Loganathan et al. 2013; Vinati et al. 2015). Once these −SiOH functional groups were protonated, they formed (Sujana et al. 2009; Loganathan et al. 2013; Vinati et al. 2015). The positively charged more effectively electrostatically attracted negatively charged fluoride ions to the soil particle surfaces (Sujana et al. 2009; Loganathan et al. 2013; Vinati et al. 2015). These attracted fluoride ions ultimately engaged in a complex ligand exchange, displacing and forming SiF groups (Sujana et al. 2009; Loganathan et al. 2013; Vinati et al. 2015).

After fluoride adsorption in sulfuric acid-modified red clay soil, the peak at 3,086/cm was replaced by two small peaks at 3,621/cm and 3,420/cm, suggesting the regeneration of hydroxyl groups on the soil particle surfaces due to fluoride adsorption (Jozanikohan & Abarghooei 2022). The peak at 2,521/cm disappeared after fluoride adsorption, implying ion exchange between fluoride ions and carbonate ions (Zhang & Huang 2019). The peak at 1,671/cm shifted back to 1,637/cm and became smaller, indicating a decrease in water content and the restoration of hydrogen bonding on the soil particle surfaces (Nabbou et al. 2019). The peak at 1,080/cm might be associated with the hydrolysis of silicon–oxygen–silicon bonds and the protonation of the resulting -SiOH functional groups due to acid modification. This point should be further confirmed. This peak became broadened after fluoride adsorption, displaying a fluoride adsorption mechanism similar to that in unmodified red clay soil. The peaks at 887/cm, 593/cm, and 581/cm disappeared after fluoride adsorption in acid-modified red clay soil, indicating an ion exchange mechanism between fluoride ions and carbonate ions or between fluoride ions and sulfate ions (Kumari et al. 2019; Zhang & Huang 2019).

Additionally, five peaks occurred in both unmodified and acid-modified red clay soils at 797/cm and 778/cm representing Si–O stretching, Si–O–Al stretching, (Al, Mg)-OH, or Si–O-(Mg, Al) stretching, at 694/cm representing Si–O stretching or Si–O–Al stretching, at 529/cm representing Si–O bending or Si–O–Al stretching, and at 470/cm representing Si–O bending or Si–O-Fe stretching, respectively (Jozanikohan & Abarghooei 2022). These peaks did not significantly change after fluoride adsorption, implying that fluoride adsorption was not significantly impacted by these functional groups at these bands.

To explore the impact of common co-existing anions on fluoride adsorption by acid-modified red clay soil, competitive adsorption experiments were conducted in triplicate, following the methodology outlined by Kemer et al. (2009). The experiments were carried out under controlled parameters: dosage of 20 g/L, pH of 6, temperature of 25 °C, initial fluoride concentration (C₀) of 20 mg/L, adsorption duration of 3 h, and agitation speed of 200 rpm. Each experiment maintained a concentration of 100 mg/L for the sodium salts of ⁠. The results indicated that the fluoride adsorption removal rate was 86.15 ± 0.81% in the control group, and none of the tested anions exhibited a statistically significant influence on fluoride adsorption at a significance level of P < 0.05. This observation aligned with findings reported by Kemer et al. (2009) using waste mud as the adsorbent.

Following the methodology provided by Kemer et al. (2009) for assessing the reusability of acid-modified red clay soil, fluoride adsorption experiments were conducted under the same controlled parameters as those described in the control groups in Section 3.9. After each adsorption cycle, the fluoride-loaded adsorbent was filtered, air-dried, and reintroduced into a fresh 20 mg/L fluoride solution for subsequent adsorption cycles. This reuse experiment was repeated five times. The fluoride adsorption removal rates were recorded as 42.82, 29.46, 27.78, 25.67, and 22.72% for the first through fifth cycles, respectively. Notably, there was a significant decrease in fluoride adsorption removal after the first cycle; however, from the second to fifth cycles, the removal rate gradually decreased. This observation indicates that acid-modified red clay soil can be reused for at least five cycles, consistent with the findings reported by Kemer et al. (2009).

As for desorption experiments, initial investigations involved the adsorption of fluoride onto acid-modified red clay soil, conducted under controlled parameters: a dosage of 20 g/L, an initial fluoride concentration (C₀) of 5 mg/L, pH 6, a temperature of 25 °C, a duration of 3 h, and a shaking speed of 200 rpm. Following adsorption, the fluoride-loaded acid-modified red clay soil was subjected to filtration and air-drying. Subsequently, desorption experiments were performed using ultrapure water under the following conditions: the dosage of fluoride-loaded red clay soil = 20 g/L, pH = 6, initial concentration of fluoride = 0 mg/L, temperature = 25 °C, desorption time = 3 h, and a shaking speed of 200 rpm. The findings showed a desorption rate of 1.6%, indicating a predominantly irreversible adsorption process. This observation suggests the formation of strong chemical bonds between fluoride ions and acid-modified red clay soil, highlighting its potential for sustained efficacy in fluoride removal in water or wastewater treatment.

This study highlights the enhancement of red clay soil's ability to adsorb fluoride in aqueous solutions through sulfuric acid modification. Sulfuric acid-modified red clay soil exhibited significantly higher fluoride removal compared to unmodified, sodium hydroxide-modified, and thermally modified red clay soils. The pH significantly influenced fluoride adsorption in both unmodified and acid-modified red clay soil, with acidic conditions proving preferable for efficient fluoride removal. The Langmuir model better described the adsorption data, and acid modification significantly improved the maximum monolayer adsorption capacity of fluoride by 252% (from 0.550 to 1.935 mg/g). Weber–Morris model analysis revealed that acid modification significantly improved adsorption reaction rates and shortened the duration of Phase I. Intraparticle diffusion limited the adsorption reaction rate in Phase I, while both external mass transfer and intraparticle diffusion governed the adsorption process in Phase II. FTIR analysis demonstrated that acid modification strengthened the complex ligand exchange of fluoride with in groups and generated carbonates and sulfates, providing opportunities for ion exchange for fluoride removal. Therefore, this study signifies advancements in the field of fluoride adsorption through the innovative and sustainable utilization of clay soil. The novel findings in this study have the potential to offer economical, viable, and environmentally friendly solutions for addressing fluoride contamination in the world. Further research, including the exploration of enhanced acid modification methods and the investigation into fluoride transport under diverse hydraulic conditions in acid-modified red clay soils, signifies significant strides in advancing a sustainable global water environment.

This work is supported by Fundamental Research Program of Shanxi Province, China (Grant No. 202103021224082). We are deeply appreciative of the anonymous reviewers for their diligent review and constructive comments, which have substantially improved the quality of this paper.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.