Study On The Performance And Mechanism Of Fluoride Removal From Groundwater By Adsorption Of Different Manganese Dioxide Crystal

Fluoride contamination in groundwater is recognized as one of the important issues of concern worldwide.In the northwest of China,groundwater fluoride exceeds the standard most seriously in rural areas,among which the fluoride content of rural drinking water fluoride samples in Shaanxi Province is mainly distributed around 2.0 mg/L,and the highest exceeded fluoride content is 9.8 mg/L.Among the main methods to remove fluoride ions from water,metal-based adsorption has been widely studied for its advantages such as better adsorption effect and simplicity of process.Manganese dioxide（MnO₂）is widely present in the earth’s crust,has good spatial structure and irregularity.And MnO₂has a strong adsorption effect on a variety of organic matter and metal ions.Four common crystalline forms of MnO₂areα,β,γ,δ.Its structural morphology is related to its physical and chemical properties,and the removal ability of MnO₂for fluorine ions is different for different crystalline forms.To screen out the crystalline types with better fluoride removal performance in MnO₂,so as to provide a technical reference for the practical application of MnO₂adsorption for fluoride ion removal.In this project,four crystalline types（α,β,γ,δ）of MnO₂materials were synthesized by hydrothermal method for the application of fluoride（fluoride ion concentration of 2.0 mg/L）treatment in groundwater.In view of the limited performance of different crystalline MnO₂for the removal of high fluoride groundwater,the subject modified the lanthanum of different crystalline MnO₂by impregnation roasting method and applied to the removal of high fluoride water（fluoride ion concentration of 10.0 mg/L）.The results obtained in this subject are as follows:（1）Characterization and analysis of different crystalline types of MnO_2.The char-acterization results of scanning electron microscopy and X-ray energy spectrometer in-dicate the successful preparation of these four crystalline types of MnO₂with high purity.The order of specific surface area and pore volume size of different crystalline types of MnO₂isα-MnO₂>γ-MnO₂>δ-MnO₂>β-MnO₂.（2）The adsorption performance and mechanism of different crystalline types of MnO₂.The results of static adsorption experiments showed that the removal rates of fluorine ions by different crystalline types of MnO₂were in the following order:α-MnO₂>γ-MnO₂>δ-MnO₂>β-MnO₂.After five times of adsorption and desorption experiments,the regeneration efficiency ofα-MnO₂could still reach more than 80%.The mechanisms of fluoride removal by adsorption of different crystalline MnO₂include ion exchange and surface complexation,as shown by Fourier infrared spectroscopy and X-ray photoelectron spectroscopy analysis.The reason for the variability of the adsorption and fluoride removal performance of different crystalline MnO₂is the difference in the number of surface hydroxyl groups.（3）Preparation and characterization of lanthanum-modified MnO₂of different crystalline forms.The lanthanum-modified MnO₂of different crystalline forms was prepared under the optimum modification conditions（lanthanum solution impregnation concentration of 0.3mol/L,calcination temperature of 300℃,and calcination time of 3 h）.The characterization of the lanthanum-modified MnO₂of different crystalline forms showed that the crystalline shape,specific surface area and pore size of the modified MnO₂did not change significantly.La-γ-MnO₂has the most lanthanum content.（4）The properties and mechanism of lanthanum-modified MnO₂of different crystalline forms were studied.The results of static adsorption experiments showed that the removal rates of fluoride ions by lanthanum-modified MnO₂of different crystalline forms were in the following order:La-γ-MnO₂>La-α-MnO₂>La-β-MnO₂>La-δ-MnO₂.The regeneration effic-iency of La-γ-MnO₂could still reach more than 75%after five adsorption and desorption experiments.The analysis by Fourier infrared spectroscopy and X-ray photoelectron spectr-oscopy shows that the mechanisms of lanthanum-modified MnO₂of different crystalline forms adsorption and defluorination include ion exchange and surface complexation.The surface hydroxyl content of the modified material is increased,and the complexation reaction between lanthanum and fluoride ions will occur to improve the adsorption and fluoride removal performance of the material.