Migration And Transformation Of Fluoride In Groundwater-Soil-Crop In Hotan Area,Xinjiang

Fluoride is ubiquitous in various media,such as water,soil,air,plants and animals,with high biological activity and non-biodegradability.Different forms of fluorine in soil can be transformed into each other under certain conditions,and cause toxicity to human body through biological enrichment and food chain.It has been shown that the groundwater in the Hotan area of Xinjiang has a high fluorine content and is rich in fluoride ores due to primary geological reasons,providing a rich source of fluoride for the local soil and groundwater.Therefore,it is of great significance to study the migration and transformation of fluoride in groundwater-soil-crops in Hotan.In this paper,the oasis area in Hotan Prefecture of Xinjiang is taken as the research area,and the groundwater,soil and crops in this area are taken as the research objects.Through field investigation,sampling and laboratory test analysis,the distribution characteristics,influencing factors and migration and transformation rules of fluoride in groundwater,soil and crops in this area are studied,so as to provide a theoretical basis for the prevention and control of fluoride pollution in this area,and also provides a reference for the study of soil fluoride migration in similar areas.The conclusions are as follows:1.The distribution characteristics of fluoride in groundwater are affected by the recharge and discharge conditions of groundwater,and high-fluorine groundwater is scattered in a small range in the relatively low-lying areas of the central plains.Vertically,the fluoride content decreased with the increase of well depth.Dry climatic conditions,strong evaporation and concentration,alternating cation adsorption,and mineral dissolution or precipitation caused by water-rock interaction during runoff jointly lead to the enrichment of fluoride in groundwater.2.The order of fluoride content in different forms of soil in the tillage layer was residual state>>organic bound state>water soluble state>Fe/Mn bound state>exchangeable state.Except water-soluble fluoride,the content of speciation fluoride in profile soil was the maximum in surface soil.Correlation analysis showed that residual fluoride is significantly correlated with Al₂O₃,K,Zn and Ca O;Fe/Mn bound fluoride was significantly positively correlated with SOC and N;exchangeable fluoride was significantly correlated with B,K and Ca O;organic bound fluoride and water-soluble fluoride had no significant correlation with soil properties and other elements.The stepwise regression analysis results showed that the main factor affecting the total fluoride and residual fluoride content was Al₂O₃,and the main factor affecting the exchangeable fluoride content was B.The results of redundancy analysis showed that Al₂O₃and B had the strongest effect on the speciation of fluorine in soil.3.The content of fluorine in wheat grain ranged from ND（not detected）to 1.0 mg/kg,with an average of 0.65 mg/kg,which met the national safety standards.The correlation between wheat seed fluoride content and different forms of soil fluoride content was weak.4.The adsorption capacity of soil to water-soluble fluoride ions increased with the increase of initial content;under different initial contents,positive adsorption was observed;the quasi-second-order kinetic model can more comprehensively describe the adsorption process of fluorine ions in soil.The isothermal adsorption of water-soluble fluoride ions by soil is nonlinear;Freundlich isothermal adsorption model had the best fitting effect;the adsorption of fluorine ions by soil is the adsorption of extremely rare solution by the multi-molecule layer on the uneven surface.The adsorption process of water-soluble fluoride ions in different types of soil is spontaneous entropy reduction exothermic reaction,and high temperature will inhibit the adsorption reaction to a certain extent.The increase of p H will decrease the adsorption capacity of soil for fluoride.5.Fluorine was enriched in the profile and migrated.The activity coefficients of the cultivated layer at the sampling points ranged from 0.005 to 0.015;the activity coefficients at the profile points ranged from0.005 to 0.024.The effective mass-based apparent bioconcentration factor has better discrimination ability than the total mass-based apparent bioconcentration factor.The apparent fluorine bioconcentration factors（total and effective basis）in wheat grains decreased with the increase of soil total and effective fluorine contents.There was a significantly positive correlation between Fe and Mn in the topsoil and the enrichment of F in wheat grains.Multivariate linear regression model can predict the fluorine content in wheat grains to a certain extent.