Occurrence,Vulnerability And Remediation Of High Fluoride Groundwater

Widespread concerns about aquifer contamination and their health risks due to endemic fluorosis have been raised worldwide.China and Kenya suffer from groundwater fluoride contamination severely.It is of great theoretical and practical significance to investigate the occurrence and hydrochemical characteristics of fluoride in groundwater,evaluate the risk of groundwater fluoride contamination,and carry out the remediation of fluorine contamination in groundwater for the scientific prevention and control of high-fluoride groundwater.In this study,hydrochemistry,graphical,multivariate statistical methods and modified Analytical Hierarchy Process（AHP）-DRASTICH model were integrated to delineate the occurrence and vulnerability of high fluoride groundwater in shallow aquifers.Yuncheng Basin,northern China and Kenya rift valley are selected as study areas.In order to groundwater protection from fluoride contamination,the biochars and Fe-Mn modified biochars were also prepared by corncob,to demonstrate the potential impact of Iron–Manganese oxides deposition（Fe-Mn）on the defluoridation ability.The main conclusions are drawn as followings:Spatially,fluoride concentration is low（<1.5 mg/L）in the south piedmont plain,medium（<4 mg/L）in the central basin,and high（up to 14.1 mg/L）in Kaolao lowland areas in shallow aquifers of Yuncheng Basin.In Kenya rift valley,over 40%of the shallow groundwater,with the highest F^-concentration of up to 23.5 mg/L,show elevated fluoride values over the WHO standards of 1.5 mg/L.High fluoride groundwater in Yuncheng Basin and Kenya rift valley,generally Na-rich and Ca-poor with relatively high p H and HCO₃^-concentrations,are controlled by different hydrochemical processes.Evidenced by hydrogeochemical and factor analysis,sediment（fluoride-bearing minerals and evaporites）weathering leaching and evapotranspiration are recognized as the first primary geochemical processes in fluoride contaminated shallow groundwater in the Yuncheng Basin.Human activities and alkaline condition also contribute to the groundwater chemistry.While in Kenya rift valley,hydrogeochemical and principal component analysis indicate that weathering and dissolution of minerals（hyper-alkaline volcanic rocks and fluorite）and cation exchange are the major factors controlling the occurrence of high fluoride shallow groundwater.Health risk assessment results show that chronic health risks by groundwater fluoride ingestion are identified to various individuals in Kenya rift valley.Children suffer from higher chronic health threats than adults.The vulnerability assessment built by AHP-DRASTICH model in Yuncheng basin indicates that over 40%of the areas,mainly distributed at the central part of the basin with shallow depth,flat topography,and vadose zone media with medium permeability,are under moderate and high vulnerability with more susceptible to fluoride contamination.The strong evaporation and long-term natural water-rock interaction together lead to the fluoride enrichment and salt accumulation of groundwater.The AHP-DRASTICH model also shows a stronger positive correlation between risk scores and F concentration,giving better vulnerability assessment to F contamination compared with DARSTIC and DRASTICH models.Plenty of corn production are found in China and Kenya,which are both agricultural countries.As a highly effective soil amendment,biochar has a broad application in the field of fluoride removal.Batch and kinetics adsorption of fluoride were performed by using the corncob biochars with different carbonization（300℃,400℃and 500℃）and Fe-Mn deposited biochars（BCM₁F₂,BCM₂F₁）.Batch sorption experiments showed that the obtained Fe-Mn modified biochar（BCM₁F₂）had strong sorption ability to fluoride with maximum sorption capacities,which were higher than those of the biochars.The characterization results based on scanning electron microscopy（SEM）,energy dispersive spectrometer（EDS）,Fourier transform infrared spectroscopy（FTIR）and X-ray diffractometer（XRD）indicated that BCM₁F₂had a rougher surface and more porous structure with Fe-Mn oxides coated on the surface of the modified biochar.The adsorption kinetics of fluoride on BCM₁F₂were well described by the pseudo-second-order model and the adsorption behavior coincided with heterogeneous adsorption materials fit the Freundlich model very well.It is suggested that Fe-Mn modified biochar BCM1F2 could be used as an inexpensive and highly efficient soil amendment for fluoride removal from aqueous solution and soil.Our study reveals that the deposition of Fe-Mn oxides in soil will not reduce the adsorption capacity of fluoride by biochar.