Study On The Mechanism Of Fluoride Ion Removal From Wastewater By Thermal Dehydration Modified Phosphogypsum

China is a high fluoride area,and most of the country has excessive fluoride concentration in drinking water.And the fluoride concentration in the water exceeds the standard will cause harm to human health.Therefore,it is important to reduce the concentration of fluoride in water for drinking water safety.At present,the main methods to remove fluoride pollution in water are:chemical precipitation method,electric flocculation method,ion exchange method,membrane separation method,adsorption method,etc.Phosphogypsum is a solid by-product of phosphorus chemical wet production of phosphoric acid,the main chemical composition of calcium sulfate dihydrate（Ca SO₄-2H₂O）,China’s phosphogypsum storage is huge,and the annual production is much larger than its resource utilization,which leads to more and more phosphogypsum is stored in piles.Therefore,enhancing the resource utilization rate of phosphogypsum is the main task of solid waste resourceization at present.In this paper,by summarizing the domestic and international methods for the removal of fluoride ions from water,we conducted a study on the removal of fluoride ions by thermal modification of solid waste phosphogypsum as raw material.In this study,the most suitable phosphogypsum pyrolysis mechanism function was calculated by thermogravimetric experiments,and the activation energy and finger front factor were solved.The best phosphogypsum modification method was selected,and its performance on the removal of fluoride ions from water was investigated.The effects of phosphogypsum modification conditions（calcination temperature,calcination time）and operating conditions（reaction temperature,dosage,p H and ion concentration）on the fluoride removal performance were investigated,and finally characterized with the help of X-ray crystal diffraction（XRD）,scanning electron microscopy（SEM-EDS）,X-ray photoelectron spectroscopy（XPS）and Fourier infrared spectroscopy（FTIR）and X-ray fluorescence spectrometry（XRF）To investigate the mechanism of fluoride ion removal from water by heat-treated phosphogypsum.The results show that there are two most suitable mechanism functions under nitrogen atmosphere,g（α）=[-ln（1-α）]^1/3 and g（α）=（1-α）^-1/2,respectively.where the activation energy Es obtained from the mechanism function g（α）=[-ln（1-α）]^1/3 are52.61,57.16,54.80 and 54.07 k J·mol^-1 of different heating rates,referring to the prefactor lg（As）of 7.31,7.40,7.02 and 7.22 min^-1;the activation energy Es obtained from the mechanism function g（α）=（1-α）^-1/2 are 63.72,56.98,62.26 and 59.71k J·mol^-1 of different heating rates,respectively,lg（As）are 6.30,5.80,6.63 and 5.73min^-1.The most suitable mechanism function for air atmosphere is g（α）=α^1/3 and Es are 62.08,66.45,76.82 and 71.18 k J·mol^-1 and lg（As）of 5.96,6.07,6.20,and 6.30min^-1,respectively.The effects of modification methods and conditions,input quantity,reaction temperature,initial concentration of fluoride ions and p H on the efficiency of phosphogypsum in removing fluoride ions from water were compared.The results showed that phosphogypsum had the highest fluoride removal efficiency after low-temperature thermal modification,and the best fluoride removal conditions were:calcination temperature 140^oC,calcination time 5h,fluoride ion concentration200mg/L,reaction temperature 20^oC,input amount 40g/L,p H=2,and the fluoride removal efficiency was as high as 94.23%.However,considering that the fluoride removal efficiency of the sample phosphogypsum was better when the input amount was 10g/L and the utilization rate was higher,the input amount of 10g/L was chosen for the rest of the single-factor experiments;The p H of the actual water is mostly neutral,so the initial p H of the fluorine solution is chosen as 7 and the initial concentration of fluorine ion is 100mg/L.Compared with the pre-modification period,the efficiency of phosphogypsum fluoride removal after modification increased by18.14%,which is a significant effect.SEM-EDS and BET analysis showed that the morphological characteristics of phosphogypsum after heat treatment and defluoridation were not significantly different,both were irregularly flaky and multilayered.Ca F₂ was found in the phosphogypsum samples after defluoridation by FTIR,XRD and XPS,indicating that Ca²⁺dissolved from Ca SO₄-0.5H₂O in phosphogypsum reacted with F^-in water to form a more insoluble Ca F₂ precipitate after thermal dehydration and reduced the concentration of F^-in solution.The simple low-temperature thermal dehydration（140°C）treatment not only saves energy for heat treatment of phosphogypsum,but also effectively removes fluoride ions from the water,achieving the purpose of"treating waste with waste".