Fluoride uptake and translocation in food crops grown in fluoride-rich soils

Organic manures reduce the bioavailability of fluoride in soil via different mechanisms

Fluoride (F) pollution in soils, which can be caused by long-term application of phosphate fertilizers, has serious socio-economic and ecological impacts. Therefore, it is important to find economically viable and environmentally friendly remediation technologies that mitigating the potential hazards of F-contaminated soil while maintaining crop yields. In this study, a soil incubation experiment was carried out to investigate the effects of two different organic manures at various amounts on soil F content and soil properties. We demonstrated that organic manure application at a rate of 5% was optimal to control the content of available fluoride (AE-F) in soil. Organic manure decreased the AE-F content and increased the residual or organically bound content. Furthermore, organic manure application also increased the contents of carbohydrates, proteins, and amides, which can anchor AE-F to the dissolved organic matter (DOM) in soil through physicochemical reactions, thereby reducing the migration of F. Moreover, after organic manure application, complex humus compounds were formed through microbial action, thus reducing the bioavailability of soil F. In summary, the application of organic manures can reduce the accumulation of soil AE-F, thereby reducing the absorption of F by crops in F-contaminated soil and alleviating the potential risk of F contamination.

Fluoride in groundwater sources in Ghana: A multifaceted and country-wide review

A large portion of Ghana's population, particularly in rural areas, lack reliable access to safely managed water. Many of these communities rely on groundwater as their primary drinking water source. Distinguished by its thorough examination of F- occurrences in Ghana, this study complements previous studies by meticulously analyzing groundwater-soil and -plant dynamics, global implications, and region-specific insights, notably in the high-risk Bongo area. The study showed that Fluoride contamination in Ghana is evident in various regions, with primary data showcasing concentrations ranging from 0.05 mg/L-1 to 13.29 mg/L-1. The Bongo District in the north exhibits elevated fluoride levels, surpassing WHO safety limits of 1.5 mg/L-1 [62]. Additional studies in Sekyere South and Nalerigu disclose concentrations from 0.3 mg/L-1 to 4.0 mg/L-1 and 0.35 mg/L-1 to 3.95 mg/L-1, respectively. Contamination probabilities range from 50 % to 90 % in the north and northeast. While southern areas lack extensive data, the identified hotspots necessitate further investigation. Geological factors significantly influence fluoride levels, emphasizing the urgent need for comprehensive monitoring, mitigation, and public awareness. The identified contamination poses risks to public health, urging immediate action for sustainable solutions and ensuring safe drinking water in affected regions. The health implications of fluoride toxicity on the residents of regions prone to fluoride exposure are noteworthy. As a result, an inevitable surge in instances of dental and skeletal fluorosis can be anticipated. Notwithstanding the challenges, research indicates optimistic prospects for mitigating fluoride pollution in drinking water. Techniques like the utilization of "Bone Charcoal" and the "Contact Precipitation" approach offer promise for remediation. These methods can be implemented at a household level and some are economically viable, making them advisable for adoption in fluoride-prone areas of Ghana.

Advances and future perspectives of water defluoridation by adsorption technology: A review

Fluoride contamination in water sources poses a significant challenge to human health and the environment. In recent years, adsorption technology has emerged as a promising approach for water defluoridation due to its efficiency and cost-effectiveness. This review article comprehensively explores the advances in water defluoridation through adsorption processes. Various adsorbents, including natural and synthetic materials, have been investigated for their efficacy in removing fluoride ions from water. The mechanisms underlying adsorption interactions are elucidated, shedding light on the factors influencing defluoridation efficiency. Moreover, the review outlines the current state of technology, highlighting successful case studies and field applications. Future perspectives in the field of water defluoridation by adsorption are discussed, emphasizing the need for sustainable and scalable solutions. The integration of novel materials, process optimization, and the development of hybrid technologies are proposed as pathways to address existing challenges and enhance the overall efficacy of water defluoridation. This comprehensive assessment of the advances and future directions in adsorption-based water defluoridation provides valuable insights for researchers, policymakers, and practitioners working towards ensuring safe and accessible drinking water for all.

Pruned litter decomposition primes fluorine bioavailability in soils planted with different tea varieties

Tea (Camellia sinensis L.) plant is fluoride (F) hyperaccumulator. The decomposition of pruned litter in tea plantations releases a large amount of F back into the soil. However, the effect of pruned litter return on soil F bioavailability has remained unclear. We investigated the decomposition dynamics of pruned litter from four tea varieties (Chuannong Huangyazao, Chuancha No. 3, Chuanmu No. 217 and C. sinensis 'Fuding Dabaicha') and its effect on soil F bioavailability. The decomposition of pruned litter occurred in two distinct periods, with an early period of rapid decomposition during the first 120 days, releasing 26-33 % of F, followed by a late period of slow decomposition during 120-360 days, releasing 2-9 % of F. The decomposition of pruned litter enhanced soil F bioavailability by increasing the concentrations of soil water-soluble F (WS-F), exchangeable F (EX-F), and organic matter-bound F (OR-F). The increase in WS-F, EX-F, and OR-F concentrations was higher than the amount of F released from pruned litter, suggesting that the increases in soil F availability did not solely originate from the release of F from pruned litter. The findings reveal the pathway of pruned litter decomposition priming soil F bioavailability through both the direct release of F and transformation from other fractions. Furthermore, the traits (C, N, lignin, and cellulose) of pruned litter from different tea varieties were the dominant factors controlling F release and soil F bioavailability. Compared with other tea varieties, the pruned litter of Chuanmu No. 217 with low lignin and cellulose content promoted higher mass loss and F release, resulting in the highest soil F bioavailability. These findings provide new insights into the mechanisms underlying the accumulation of bioavailable F in soil. These insights offer valuable support for devising effective management strategies for the incorporation of pruned litter into soil.

Fluoride distribution in selected foodstuffs from Nakuru County, Kenya, and the risk factors for its human overexposure

Critical data on the impacts of fluoride (F) in food systems along the Eastern Africa Rift Valley System (EARS) is needed for public health risk assessment and for the development of strategies for ameliorating its deleterious effects among the affected communities. Long-term F overexposure causes dental and skeletal fluorosis, and leads to neurotoxicity, which impacts several important body functions. Investigating F exposure pathways is of essence to inform and safeguard public health of the affected communities. The current study assessed the F levels in potatoes (Solanum tuberosum L.), beans (Phaseolus vulgaris L.) and garden peas (Possum sativa) from Nakuru County, Kenya, by potentiometric analysis using F ion-selective electrodes. It then evaluated the risk factors for excessive human exposure to F through contaminated foodstuffs. The mean F levels in the potatoes (8.50 ± 4.70 mg/kg), beans (8.02 ± 4.12 mg/kg) and peas (4.99 ± 1.25 mg/kg) exceeded recommended dietary allowances (RDA) level of 4 mg/kg endorsed by US Institute of Medicine for the different categories of people. The F distribution trends in beans and potatoes reflected the environmental patterns of F contamination of the study area but the spatial extent Fin the peas indicated existence of partial resistance of the pea plants to environmental F uptake. The results indicated that both the beans and the potatoes were more liable to accumulating greater amounts of F from the environment than garden peas and that all the three foodstuffs contained high F levels that posed greater risk of F overexposure and its deleterious impacts among the young children, male populations, and in people of greater body weight and high physical activity levels.

Soil exposure is the major fluoride exposure pathways for residents from the high-fluoride karst region in Southwest China

In the karst areas of southwest China, soil fluoride levels are higher than in China (478 mg kg^-1) and world (200 mg kg^-1). High levels of F in the environment might pose a health risk to humans. The comprehensive exposure risk must be studied in this area. Herein, samples of crops and soil were collected from Bijie City, a typical karst area in southwest China, to investigate the pollution level and evaluate the comprehensive F exposure risk. The single-factor index (P_Fw) and the geological accumulation index (I_geo) were used. The hazard index (HI) was applied to assess exposure risk from multiple exposure routes. The results revealed that there is considerable F contamination in soil and crops in the study area. Average soil total fluorine (Ft) was 1139.13 mg kg^-1, and soil water soluble F (Fw) was 3.792 mg kg^-1. In corn, rice, wheat, and potatoes, F contents were 1.167-9.585, 1.222-6.698, 1.587-9.976, and 1.797-9.143 mg kg^-1, respectively. The mean values of HI were 4.45 and 2.42 for children and adults, respectively, > 1, showing potential health risk exists. Youngsters are at a greater exposure risk than adults. From the results of contribution ratios of different exposure routes for health risk, the major exposure risk was determined to be from soil exposure. Based on this, we suggest that risk managers mainly strive to control the soil fluoride level and implement the risk education and communication.

Potential fluoride exposure from selected food crops grown in high fluoride soils in the Makueni County, south-eastern Kenya

Makueni County, located in south-eastern Kenya, faces challenges such as limited potable water and restricted food supplies as the result of semi-aridity. High fluoride (F) concentrations have been reported in drinking water with resultant dental fluorosis affecting the local population. To determine the potential F exposure through the consumption of food crops grown in the area, F concentration was assessed in the main five locally grown and consumed crops. Additionally, the water-soluble F fraction was determined from 30 soil samples with mineralogical determination of 20 samples. Mean F concentration in the food crops was in the order; 700, 288, 71.2, 36.6, and 29 mg/kg in kale, cowpeas leaves, green grams, cowpeas (legume portion), and maize, respectively. The F concentration in farm soils ranged from 0 to 3.47 mg/kg (mean of 0.87 mg/kg) and showed a significant strong positive correlation (p = 0.03, r = 0.89) with F values in the crops. Apatite, muscovite, and biotite were identified as the F-rich minerals present. While considering two hypothetical F absorption fractions (75 and 100%), the estimated average daily dose (EADD) of F from consuming the crops ranged between 0.004 and 65.17 mg/kg/day where the highest values were from the vegetables. Most of these values were higher than the F reference dose (RfD) of 0.06 mg/kg. The estimated EADD values of several hypothetical meals prepared from the analyzed crops revealed that steamed kale and maize porridge pose the highest health risk of F associated diseases to the local population, whereas boiled cowpeas pose no health risk. Children, due to their higher daily energy requirement and low body weight, were the most vulnerable group at risk of high daily F intake relative to the RfD. These results suggest that consumption of the analyzed food crops in Makueni County may significantly contribute to F related diseases in the local population. This creates a food security issue for the area because of the potential health risks associated with these crops which are highly relied upon in the semi-arid area with a limited selection of food crops available and viable to grow.

Spatial distribution and risk assessment of fluorine and cadmium in rice, corn, and wheat grains in most karst regions of Guizhou province, China

The pollution status of crops planted in Guizhou province of Southwestern China with high background values of Fluorine (F) and Cadmium (Cd) has attracted people’s concern. The present study aimed to investigate the spatial distributions of F and Cd in rice, corn and wheat grains, and further evaluate their health risks to residents in Guizhou province. The contents of F and Cd were measured by fluoride ion-selective electrode and inductively coupled plasma mass spectrometry (ICP-MS) methods, respectively. Additionally, the inverse distance weighted (IDW) technique was conducted to analyze spatial distribution, and the health risk was estimated by target hazard quotient (THQ) and hazardous index (HI). The results indicate that Cd contents in samples varied from 0.000 to 0.463 for rice, 0.000 to 0.307 for corn, and 0.012 to 0.537 (mg/kg) for wheat, while F contents ranged from 0.825 to 5.193 (rice), 0.946 to 8.485 (corn), and 0.271 to 9.143 (wheat) mg/kg. The Cd exceeding ratios were 11.600% for rice, 13.500% for corn, and 45.100% for wheat grains, respectively. In terms of spatial distribution, high levels of F and Cd in rice were found in the northern and central in Guizhou, while Cd in corn was distributed in the eastern and F in corn were distributed in the west area of Guizhou. Moreover, the high levels of F and Cd in wheat were distributed in the western and eastern areas. The mean carcinogenic risks (R) of Cd in rice, corn, and wheat in children were 4.150 × 10^–4, 1.670 × 10^–4 and 3.470 × 10^–4, respectively, and that in adults were 3.430 × 10^–4, 0.471 × 10^–4, and 2.190 × 10^–4, respectively. The HI for adults in rice, corn and wheat grains were 0.756, 0.154, and 0.514, respectively, and that for children were 0.913, 0.549, and 0.814, respectively. Collectively, the potential risks produced by F and Cd to the local residents should not be ignored.

Remediation of Soils Contaminated by Fluoride Using a Fermentation Product of Seaweed (Eucheuma cottonii)

This study investigated the efficacy of fermented seaweed (Eucheuma cottonii) on the remediation of fluoride-contaminated soil. The soil was amended with either 1.25, 3.0, or 5.0% (w/w) fermented seaweed (FSW), parallel with the controls (0%). The amendment improved the physicochemical properties of the soil particularly pH regulated from strong alkaline (9.3) to neutral (7.0) which is essential for germination, crop growth, and yield. The amount of water soluble-fluoride (Ws-F) dropped from 81.7 ± 3.1 mg/kg to 42.7 ± 2.4, 33.7 ± 1.2, 19.6 ± 0.9, and 12 ± 1.3 mg/kg following 0, 1.25, 3, and 5% amendment dosage, respectively. Most of the Ws-F was converted into exchangeable fluoride (Ex-F) and to fluoride-bound to iron and manganese (Fe/Mn-F). Furthermore, the amendment also enhanced microbial mass and diversity in the soil. The FSW contains organic acids which participate in ionic bonding with the multivalent cations in the soil. The formed compound participates in ion exchange with clay or with anionic adsorption to positively charged clay sites at the edges. This interaction is further essential for enhancing the fluoride holding capacity of the soil. The use of seaweed reduced the bioavailability of fluoride in the agricultural soils and had positive effects on promoting soil fertility. However, further studies to observe its effects on crop performance is of significance.

Co-exposure of potentially toxic elements in wheat grains reveals a probabilistic health risk in Southwestern Guizhou, China

Bijie is located at a typical karst landform of Southwestern Guizhou, which presented high geological background values of potentially toxic elements (PTEs). Recently, whether PTE of wheat in Bijie is harmful to human health has aroused people's concern. To this end, the objectives of this study are to determine the concentrations of PTE [chromium (Cr), nickel (Ni), arsenic (As), lead (Pb), cadmium (Cd), and fluorine (F)] in wheat grains, identify contaminant sources, and evaluate the probabilistic risks to human beings. A total of 149 wheat grain samples collected from Bijie in Guizhou were determined using the inductively coupled plasma mass spectrometer (ICP-MS) and fluoride-ion electrode methods. The mean concentrations of Cr, Ni, As, Cd, Pb, and F were 3.250, 0.684, 0.055, 0.149, 0.039, and 4.539 mg/kg, respectively. All investigated PTEs met the standard limits established by the Food and Agriculture Organization except for Cr. For the source identification, Cr and Pb should be originated from industry activities, while Ni, As, and Cd might come from mixed sources, and F was possibly put down to the high geological background value. The non-carcinogenic and carcinogenic health risks were evaluated by the probabilistic approach (Monte Carlo simulation). The mean hazard quotient (HQ) values in the three populations were lower than the safety limit (1.0) with the exception of As (children: 1.03E+00). However, the mean hazard index (HI) values were all higher than 1.0 and followed the order: children (2.57E+00) > adult females (1.29E+00) > adult males (1.12E+00). In addition, the mean carcinogenic risk (CR) values for Cr, As, Pb, and Cd in three populations were all higher than 1E-06, which cannot be negligible. The mean threshold CR (TCR) values were decreased in the order of children (1.32E-02) > adult females (6.61E-03) > adult males (5.81E-03), respectively, all at unacceptable risk levels. Moreover, sensitivity analysis identified concentration factor (C _W ) as the most crucial parameter that affects human health. These findings highlight that co-exposure of PTE in wheat grains revealed a probabilistic human health risk. Corresponding measures should be undertaken for controlling pollution sources and reducing the risks for the local populace.

Role of rice cultivation on fluorine distribution behavior in soda saline-alkali land

Fluorine stability in the soil is crucial to protecting the groundwater and the food chain from pollution by fluorine, which was abundantly present (762.78-1330.66 mg/kg) in soda saline-alkali soil areas of western Jilin Province. This study investigated the fluorine distribution and pollution level in soil. Two representative areas in Zhenlai County, namely Chagan and Hatu villages, were selected to gather soda saline-alkali soil profiles (0-20, 20-40, and 40-60 cm) from different cultivation years (i.e. 3, 5, 8, 10, 13, 15, 18, and 20). The results revealed that fluorine content in soil decreased with increasing cultivation years. In the soil profiles, fluorine and water-soluble fluorine concentrations followed the order of 40-60 > 20-40 > 0-20 cm, while the contents of exchangeable and organic matter-bound fluorine decreased with the increasing profile depth. Rice planting increased the content of macroaggregates with diameters >2 and 2-0.25 mm, which significantly improved the aggregate structure of the soil. The fluorine content was the greatest in microaggregate with particle sizes <0.053 mm in the aggregates' sizes. Rice planting in saline-alkali soil reduced the contents of water-soluble and exchangeable fluorine, while enhancing the content of organic matter-bound fluorine, thereby reducing the bioavailability of fluorine in the soil and the risk of fluorine migration through the groundwater. The degree of soil pollution decreased with increasing plantation years, and the soil became slightly polluted after 15 years of rice planting. The rice plants grew normally after years of improving fluorine-containing saline-alkali land, and the fluorine content was within the safe limit.

Fluoride toxicity in cropping systems: Mitigation, adaptation strategies and related mechanisms. A review

Environmental fluoride (F^-) contamination, mainly due to natural geogenic processes, and in spot cases also of anthropogenic origin, is a widespread global issue, which has been recognized to affect all living organisms. From the contaminated soil and water, F^- is absorbed by plants which can manifest symptoms of abiotic stress including oxidative stress and interference with essential physiological and biochemical processes involved in seed germination and plant growth and development. Depending on the diet of the population living in the high F-polluted areas, F-contaminated crops can be key contributors to excessive F^- intake along food chains which can lead to human and animal health issues. Various strategies are being explored with the objective of reducing both F^- bioaccumulation and its damage on plants (e.g. by means of immobilization or phytoextraction processes) or aimed at limiting the F^- anthropogenic input in the soil (e.g. through the use of alternative phosphate fertilizers) but the literature is still fragmented. After a brief overview on the effects of F^- on the production and safety of food crops, its sources, mobility and bioavailability in agricultural soils, this paper reviews the available F^- mitigation and adaptation options and the involved mechanisms with the aim of providing stakeholders with knowledge to make informed decisions when selecting methods for coping with F^- impacts in agricultural systems. Research gaps and possible areas for future studies have also been suggested.

Spatio-temporal variation of fluoride in groundwater and agricultural soil and crops of Unnao district, UP: Monitoring and assessment

Fluoride (F^-) contamination in groundwater of Unnao district, Uttar Pradesh was reported for the first time in 1994, however comprehensive monitoring of F^- in different environmental matrices remains to be undertaken. The presented study reports spatio-temporal monitoring of F^- content in groundwater, crops and soil from F^- affected district Unnao, in pre-monsoon (PRM), monsoon (MO) and post-monsoon (PMO), to establish F^- groundwater-soil-plant continuum. More than 80% of groundwater samples were contaminated with F^-> 1.0 mg L^-1 with highest level (mg L^-1), at Patiyara (3.6 ± 0.64), during PRM > Pathakpur (2.73 ± 0.57) during PMO > Sarukheda (2.40 ± 0.43) during PRM. High Cr in groundwater was observed in Jajmau (7.08 ± 1.42). The level of F^- (mg Kg^-1) in agricultural soils followed 3.4 ± 0.71 at Patiyara (MO) > 2.9 ± 0.14 at Badlikheda (PRM) 1.89 ± 0.28 at Jagatkhera (PRM). Among the different edible parts of crops in selected sites, highest F^- content (mg Kg^-1), F^- level in grains of Oryza sativa ranged between 0.23 ± 0.02 to 2.01 ± 0.24. Whereas in the edible fruit of Trichosanthes diocia contained 1.47 ± 0.32 and Momordica charantia 1.47 ± 0.02. Leaf of spinach (1.03 ± 0.22) and seed of Brassica juncea (0.73 ± 0.08). Overall, comparing across all the three seasons, level of F^- was highest in all the plants during MO, as compared to PRM and PMO. The regression analysis of physiochemical properties of groundwater show negative relationship between Na⁺ and F^- whereas soil alkalinity exhibited strong influence in soil F^-. The high F^- content in soil and groundwater at Patiyara and Shekhpur also coincided with presence of several brick kilns, possibly contributing to the high F^-.

Deterministic and probabilistic human health risk assessment for fluorides in drinking groundwater from Lukavac, Bosnia and Herzegovina

Although fluorides are essential for dental health, there are growing health concerns regarding the risk-benefit ratio of fluoride exposure. The objectives of the study were to obtain data on the amount and human health risks of fluoride in drinking groundwater, as well as to compare and evaluate the differences in the outputs obtained by two different approaches in health risk assessment (deterministic vs probabilistic). From a densely populated industrial area of north-eastern Bosnia and Herzegovina, a total of 54 groundwater samples were collected. Fluoride concentrations varied from 1.69 to 3.52 mg/L. The WHO's threshold value for fluoride in drinking water was exceeded in all the samples analysed, indicating an increased daily intake of fluoride from groundwater. Deterministic and semi-probabilistic techniques were used for exposure assessment and health risk quantification. Generally, the deterministic approach resulted in acceptable health risks in most adult exposure scenarios. However, the Monte Carlo simulation revealed that 20.6, 20.8, and 99.8% of adult males, females, and children, respectively, were above the tolerable upper intake level, indicating that both adults and children face a significant health risk. Nevertheless, small children are more vulnerable to environmental hazards than youth and adults. Hence, a more in-depth risk-benefit analysis is required to reduce/or optimize fluoride content in drinking water to prevent tooth decay and fluorosis at all ages. Considering that optimal daily intake of fluorides is a crucial factor for preserving human health, decision-makers should take steps to emphasize the importance of continuous monitoring of fluoride concentrations in drinking water.

Stabilising fluoride in contaminated soils with monocalcium phosphate and municipal solid waste compost: microbial, biochemical and plant growth impact

This study evaluated the influence of a municipal solid waste compost (MSWC) and monocalcium phosphate (MCP), alone or combined, on the mobility, toxicity, bioavailability and health risk of fluoride (1000 mg F^-·kg^-1) in an artificially polluted soil (pH 7.85). The addition of MCP (0.2% w/w) and MSWC (1% w/w) (alone and combined) to the contaminated soil reduced water-soluble (e.g. by more than 50% in MCP and MCP + MSWC-treated soils) and exchangeable F^- fractions and increased the residual one. The addition of MSWC and MSWC + MCP to the contaminated soil significantly increased microbial biomass C (SMB-C; 1.3-3.6-fold) while all treatments increased the abundance of culturable heterotrophic bacteria (up to twofold in MSWC + MCP). Overall, dehydrogenase, β-glucosidase, urease and phosphomonoesterase activities were enhanced in treated soils and positively correlated with SMB-C, but not with labile F^-. All treatments increased carrot yield (up to 3.4-fold in MSWC + MCP), while bean growth was significantly enhanced only by MCP and MCP + MSWC (~ twofold). The opposite trend applied for F^- uptake which was especially reduced in the edible part of carrot after soil amendment. A limited influence of MCP and MSWC on hazard quotient (HQ), due to bean and carrot consumption, was also recorded (i.e. HQ generally > 1). Results suggest that MCP and MSWC can be used in the recovery of soil chemical, microbial and biochemical status of F-rich agricultural soils. They also indicate that the bean and carrot cultivars employed in this study are likely unsuitable in such soils due to high F^- uptake in edible parts.

Hydrogeochemical evidence for fluoride behavior in groundwater and the associated risk to human health for a large irrigation plain in the Yellow River Basin

A hydrochemical analysis of groundwater (GW) was conducted to investigate the factors controlling GW fluoride (F) in a large irrigation plain in the Yellow River Basin, Guanzhong Plain, China. Area-dependent variations in F were observed in the study region. The F concentrations of 93% of samples on the south bank of the Weihe River and the western part of the Qishui River were <1 mg L^-1, whereas those of 73% of GW samples for the eastern part of the Qishui River exceeded the national limit. A forward model based on mass budget equations identified carbonate weathering as the dominant factor regulating hydrochemistry in low-F GW, whereas the factors in the high-F zone were evaporate dissolution and evaporation. The high-F GW displayed a distinctive major ion chemistry, which could be attributed to a high pH, low Ca²⁺, and high HCO₃^- and Na⁺ concentrations. An analysis of the correlation between F/Cl and F concentrations and fluid-mineral equilibria indicated distinct forces driving the behavior of F in the subparts of the high-F GW zone, including irrigation-induced F dilution, F enrichment through Na-Ca exchange, and adsorption of F on clay minerals. The order of vulnerable segments of the population in terms of risk posed by F in GW was: infants > children > adults. These results can enhance the understanding of F behaviors in GW and provide insights into the effect of irrigation practices on GW F concentration.

Human health risk assessment in aluminium smelting site: Soil fluoride bioaccessibility and relevant mechanism in simulated gastrointestinal tract

Incidental oral ingestion is considered to be an important exposure route for humans to soil contaminants, such as fluoride (F). For 25 soil samples containing 4000 mg F/kg from aluminium smelting site in southwestern China, this study investigated F bioaccessibility in the human gastrointestinal tract in vitro. Fluoride bioaccessibility (2.4-48.8%) in the gastric phase was primarily caused by the dissolution of F-Ca and F-Al compounds (assigned to residual phase), identified by X-ray photoelectron spectroscopy and sequential extraction. Following modification to the small intestinal phase, the variation in F bioaccessibility (2.5-38.8%) should be the result of concurrent processes, including the formation of F complexes and competitive adsorption, and inversely the precipitation of fluorite and surface adsorption of formed F-Al complexes. The colon incubation with human gut microbiota yielded a 1.3-fold increase in F bioaccessibility (3.9-45.7%), probably due to the dissolution of F bound to Fe (hydr)oxides. Bioaccessibility adjustment can reduce hazard quotient of fluoride, and non-carcinogenic risk for children should be noted that soil F intake contributed 21.7% on average, up to 76.6% of oral reference dose. This will result in better understanding of human health risk assessment associated with F exposures.