Current progress on fluoride occurrence in the soil environment: Sources, transformation, regulations and remediation

Root-specific expression of CsNPF2.3 is involved in modulating fluoride accumulation in tea plant (Camellia sinensis)

Fluoride (F) is a nonessential but potentially harmful element for plants, especially when present in excess. The tea plant is known for its ability to hyperaccumulate F from the soil and eventually accumulates in the leaves; however, how the tea plant transports F to the leaves remains unclear. Here, we found that Se can significantly decrease the transport efficiency of F from root to leaf. Therefore, RNA-Sequencing was performed on tea roots cotreated with selenite and fluoride, and then we isolated a plasma membrane-localized F transporter CsNPF2.3 from tea plant roots and examined its role in transport of F in tea plants. The results showed that CsNPF2.3 exhibited F transport activity when heterologously expressed in yeast. Expression pattern analysis revealed that CsNPF2.3 is expressed in epidermal cells, cortex cells, and xylem parenchyma cells in roots. Overexpression of CsNPF2.3 in tea roots significantly increased F content in the root, stem, and leaf, and enhanced the transport efficiency of F from root to leaf. Furthermore, in nine tea cultivars, CsNPF2.3 expression in the root was significantly positively correlated with F content in the leaf and root, and the transport efficiency of F from root to leaf. Altogether, these findings suggest that CsNPF2.3 was involved in uptake and transport of F in tea plants.

Integrating multi-omics and biomarkers to reveal the stress mechanisms of high fluoride on earthworms

Excessive fluorine accumulation poses a significant threat to soil ecology and even human health, yet its impact on soil fauna, especially earthworms, remains poorly understood. This study employed multi-omics and biomarkers to investigate high fluorine-induced biochemical changes that cause tissue damages in Eisenia fetida. The results demonstrated that earthworms exhibited obvious damage with fluorine addition exceeding 200 mg kg-1, with stress levels escalating as fluorine contents increased. Further analysis of the underlying mechanisms revealed that fluorine could upregulate genes encoding mitochondrial respiratory chain complexes I-III and downregulate those for IV-V, leading to reactive oxygen species (ROS) accumulation despite antioxidant system activation. The resulting ROS interfered with deoxyribonucleoside triphosphate synthesis, prompting homologous recombination as the main DNA repair mechanism. Additionally, fluorine-induced ROS also attacked and disrupted protein and lipid related metabolisms ultimately causing oxidative damages. These cumulative oxidative damages from high fluorine contents subsequently triggered autophagy or apoptosis, resulting in tissue ulceration and epithelial exfoliation. Therefore, high fluorine could threaten earthworms by inducing ROS accumulation and subsequent biomolecule damages.

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

The adsorption of fluoride by soils influences its mobility and bioavailability. Therefore, the fluoride adsorption process in soils has garnered widespread attention. Yet research on assessing environmental risk based on the characteristics of fluoride adsorption in soil is still limited. Here, a suite of batch experiments were conducted using three soil types with distinct properties. The results demonstrate that soil organic matter (SOM) and pH are critical factors determining fluoride adsorption in soils. Paddy soil (PS) with its higher SOM content has a higher adsorption capacity compared with loessal soil (LS) and brown soil (BS). Under acidic conditions, BS and LS whose Ca2+ content is higher exhibited a higher adsorption capacity. The fluoride adsorption process in soils may involve electrostatic adsorption, complexation, and precipitation. The desorption results showed stronger fluoride binding to PS and LS than BS, while the fluoride adsorbed onto BS was almost completely desorbed. This research demonstrates that a deeper understanding of regional differences in soil properties is crucial for better studying the migration and accumulation characteristics of fluoride and its bioavailability in various soils. This study provides a theoretical basis for evaluating the bioavailability, exposure risk, and groundwater pollution risk of fluoride in different soils.

Curcumin mitigates sodium fluoride toxicity in Caenorhabditis elegans

Fluoride, a naturally occurring element found in water, soil, food, and atmospheric precipitation, can lead to fluorosis and various health issues when consumed excessively. However, the mechanism of fluorosis is still under investigation. This study utilizes Caenorhabditis elegans as a model organism to investigate the effects of fluoride exposure on biological systems and to explore the mechanisms by which curcumin mitigates fluoride-induced toxicity. Three groups were established: a blank control, a sodium fluoride (NaF) exposure group (concentration 5 mmol/L), and a curcumin intervention group (concentration 25 μmol/L). Physiological parameters, lipofuscin levels, intracellular reactive oxygen species (ROS) levels, mitochondrial membrane potential, and mitochondrial copy numbers were measured to assess the effects of fluoride toxicity and curcumin protection. RNA-seq and qRT-PCR were utilized to investigate the molecular mechanisms underlying fluoride-induced damage and curcumin's mitigating effects. Results indicated that fluoride-exposed nematodes displayed physiological abnormalities, increased ROS production, higher lipofuscin levels, altered mitochondrial membrane potential and mitochondrial copy number, and activated MAPK signaling pathway genes. Curcumin exhibited protective effects on these parameters, suggesting its potential in preventing fluoride-induced harm by modulating oxidative stress and preserving mitochondrial function. This research enhances our understanding of the mechanisms of fluoride toxicity and highlights the potential benefits of curcumin.

Enhanced fluoride extraction from contaminated soil combining chelator and surfactant: Insights into adsorptive controlment of soil surface charge

Biodegradable chelators and surfactants are promising alternatives to conventional washing agents for remediating soil contaminated with toxic elements, owing to their excellent extractability and environmental compatibility. Most previous studies have primarily aimed at maximizing removal efficiency. However, understanding their underlying extraction mechanism is essential to expand the application potential of chelator- or surfactant-assisted washing systems. This study evaluated the effectiveness of chelators and surfactants in remediating fluoride (F)-contaminated soil and explored their associated extraction mechanisms. Our findings highlight a biodegradable chelator, HIDS (3-hydroxy-2,2'-imino disuccinic acid) as uniquely effective in F extraction with minimal F-bearing minerals dissolution (Ca, Fe, and Al). Chelator recovery rates and zeta potential measurements in post-washed solutions suggests that HIDS adsorbs onto soil surfaces, displacing the originally adsorbed F and enhancing the negative surface charge to inhibit F re-adsorption. Additionally, applying an anionic surfactant to enhance F extraction from soil showed promising results. Notably, a binary blend of HIDS and in-lab designed anionic surfactant, SDT (sodium N-dodecanoyl-taurinate), achieved the highest F removal rate (132 mg kg-1) under optimized washing conditions (HIDS: 10 mmol L-1, SDT: 10 mmol L-1, solution pH: 3, and washing time: 1 h), enhancing F extraction by 22% compared to HIDS-only washing (108 mg kg-1; washing time: 3 h). The FT-IR and zeta potential measurements suggested that SDT adsorbed onto the soil surface. The action of the HIDS-SDT blend towards F extraction involves the complexation and acid dissolution of F-bearing soil minerals, followed by F replacement through chelator and surfactant adsorption. This process mitigated F back-adsorption and enhanced F extraction by generating a negatively charged soil surface.

A novel fluorescent probe for efficient detection of fluoride ions in living animal and plant tissues

This work presents the design and synthesis of a new fluorescent probe IF-Br-F for the specific detection of fluoride ions. The IF-Br-F probe has excellent fluorescence properties, and the mechanism of the probe response to fluoride ions was successfully verified via HRMS and DFT calculations. IF-Br-F has high sensitivity and low detection limit (5.82 × 10-7 mol L-1) and successfully detects fluoride ions in actual water samples. The probe can be applied to fluorescence imaging of zebrafish, cells, and Arabidopsis roots and exhibits low cytotoxicity and good biocompatibility.

Remediation of fluoride-contaminated wastes: Chelator-assisted washing and subsequent immobilization using CaO and H3PO4

Fluoride (F) contamination in industrial waste is a significant challenge for sustainable materials recycling. Existing techniques for mitigating F contamination focus on immobilization, converting F compounds to insoluble forms while leaving the total F content untreated. Chelator-assisted washing is considered a promising alternative remediation strategy that can indirectly release F by entrapping and dissolving F-bearing minerals. This study evaluates the effectiveness of chelator-assisted washing in removing F from three real F-contaminated waste samples (TCS-49, TCS-51, and TCS-52) by treating with four different chelators, ethylenediaminetetraacetic acid (EDTA), ethylenediamine N,N'-disuccinic acid (EDDS), diethylenetriaminepentaacetic acid (DTPA), and 3-hydroxy-2,2'-imino disuccinic acid (HIDS). The influence of key washing variables, including chelator type, solution pH, chelator concentration, washing time, and liquid-to-solid (L/S) ratio toward F extraction was assessed and optimized for attaining the maximum extraction. All chelators exhibited the highest F extraction from TCS-49 and TCS-51 at pH 11, whereas in TCS-52 it showed a discrete extraction pattern. Under optimized conditions (concentration, 10 mmol L-1; pH, 11; washing duration, 3 h; and L/S ratio, 10:1), EDTA outperformed the other chelators, enhancing F extraction by 2.1 and 1.2 times for TCS-49 and TCS-51, respectively, compared with those of control treatments. However, for TCS-52, the efficiency of EDTA was analogous to that of the control under the same washing conditions. The linear correlation between the extracted F and F-containing minerals suggests that the chelator-induced F removal from contaminated waste involves the entrapment and dissolution of F-bearing minerals, especially Ca and Fe. The subsequent post-washing immobilization of chelator-washed waste residues using CaO or H3PO4 significantly reduced the content of leachable F under the regulatory limit.

Mechanism for leaching of fluoride ions from carbon dross generated in high-temperature and low-lithium aluminum electrolytic systems

Carbon dross, a hazardous solid waste generated during aluminum electrolysis, contains large amounts of soluble fluoride ions for the main components of the electrolyte (such as Na3AlF6 and NaF). Response surface methodology (RSM) was used to investigate the mechanism for fluoride ion leaching from carbon dross via water leaching, acid leaching and alkali leaching, and the kinetic and thermodynamic principles of the leaching process were revealed. The RSM predicted the optimum conditions of water leaching, alkali leaching and acid leaching, and the conditions are as follows: temperature, 50 °C; shaking speed, 213 r·min-1; particle size, 0.075 mm; shaking speed, 194 r·min-1; liquid-solid ratio, 12.6 mg·L-1; sodium hydroxide concentration, 1.53 mol·L-1; liquid-solid ratio, 25.0 mg·L-1; sulfuric acid concentration, 2.00 mol·L-1; and temperature, 60 °C，and actual results which were almost consistent with the predicted results were gained. The fluoride ions in the alkaline and acid leaching solutions were mainly the dissociation products of fluorides such as Na3AlF6, Na5Al3F14 and CaF2, as indicated by thermodynamics calculations. In particular, the fluoride compounds dissolved in alkali solution were Na3AlF6, Na5Al3F14, AlF3, ZrF4, K3AlF6, while the acid solution could dissolve only Na3AlF6 and CaF2. The leaching kinetics experiments showed that the leaching rate fit the unreacted shrinking core model [1-2/3α-(1-α)2/3 =kt] and that the leaching process was controlled by internal diffusion. This study provides theoretical guidance for the removal of soluble fluoride ions from carbon dross and will also assist in the separation of electrolytes from carbon dross. ENVIRONMENTAL IMPLICATION: Carbon dross, a hazardous waste generated during the aluminum electrolysis production process, contains a large amount of soluble fluoride. Improper storage will lead the fluoride ions pollution in soil, surface water or groundwater under the direct contact between carbon dross and rainfall, snow or surface runoff. The influence of wind will cause carbon dross dust to pollute further areas. With the human body long-term contact with fluoride ion contaminated soil or water, human health will be seriously harmed.

Industrial fluoride emissions and their spatial characteristics in the Nansi Lake Basin, Eastern China

Excessive fluoride emissions threaten ecological stability and human health. Previous studies have noted that industrial sources could be significant. However, quantifying industrial fluoride emissions has not been yet reported. In this study, both bottom-up and top-down approaches were used to estimate the fluoride emissions in the Nansi Lake Basin. Global and local spatial autocorrelation were adopted to reveal the spatial agglomeration effects. The fluoride emissions calculated by the bottom-up approach were larger than those calculated by the top-down method. The highest fluoride input mainly occurred in Zoucheng and Mudan. The highest fluoride emissions mainly occurred in Zoucheng and Rencheng using the bottom-up approach. The highest fluoride emissions mainly occurred in Zoucheng and Yanzhou using the top-down approach. Mining and washing of bituminous coal and anthracite (BAW) was the most significant source of fluoride input and emissions. A significant spatial agglomeration effect of fluoride emissions was found. These findings could provide a method for accurate industrial fluoride emission estimation, complement the critical data on the fluoride emissions of main industrial sectors, and provide a scientific basis for tracing fluoride sources.