Study On The Performance Of Metal Salt Modified Lignite For Removal Of Fluoride Ion In Circulating Water Of The Coking Plant

Regarding defluorination,domestic and foreign researches mainly focus on defluorination of drinking water and industrial discharge sewage,but pay little attention to defluorination of circulating water.Fluoride is an essential element for animals and plants.Excessive fluoride intake by animals can cause a variety of complications.Excessive fluoride in the soil can inhibit plant growth.Excessive fluoride in the circulating water will corrode the primary cooler pipeline,causing personnel poisoning or environmental accident due to leakage of toxic and harmful substances.Lignite itself has certain pores and rich functional groups.It is a natural adsorbent.In order to achieve the purpose of efficiently removing fluoride ions,lignite needs to be modified.This paper mainly studies the optimization test of lignite modification conditions and fluoride ion adsorption conditions to analyze the adsorption mechanism,the modified lignite is also subjected to a defluorination test in actual fluorine-containing circulating water to explore the effect of defluorinating in actual fluorine-containing circulating water.The modification process of lignite is to crush,sieve,clean and dry large pieces of lignite successively to make pretreated lignite（LE）with a particle size of 0.074-0.125mm.The LE was oxygenized and carbonized in a tube furnace.Isolate the LE in a tube furnace for oxygenation and carbonize it.Impregnate the carbonized lignite（LE-C）with FeCl₃ solution,and then wash it with deionized water several times to remove the residue of the impregnating solution to obtain modified lignite（LE-C-Fe）.The specific surface area,morphology,surface elements,surface functional groups and zeta potential of LE,LE-C,LE-C-Fe and LE-C-Fe-F（modified lignite after adsorption of fluoride ion）were analyzed,and the following conclusions were obtained.The carbonization process increases the specific surface area of LE,the total pore volume increases,the proportion of oxygen on the surface decreases significantly,and the types and number of surface functional groups change greatly.The impregnation process makes the iron and chlorine elements evenly distributed on the surface of the lignite,and the reduction of the proportion of oxygen on the surface can be seen that the loading process makes the lignite experience the breaking of chemical bonds and the generation of new chemical bonds.A large amount of elemental fluorine appears on the surface of LE-C-Fe-F,and the proportion of elemental chlorine decreases,so ion exchange occurs between fluorine and chlorine.When the fluoride ion concentration in the fluorine-containing solution is 200mg/L,the dosage of LE-C-Fe is 8g/L.After adsorption equilibrium,the adsorption capacity of LE-C-Fe reaches 21.14mg/g,which is 8 times that of LE.The amount of adsorption is affected by the initial concentration of fluoride ions,the dosage of LE-C-Fe,the adsorption time,the temperature,the initial pH of the solution and the presence of weak acid ions in the solution.When the dosage is 8g/L,the initial concentration increases from 20mg/L to 400mg/L,the adsorption capacity of LE-C-Fe increases by 36.5mg/g.The dosage increases from 0.5g/L to12g/L,the adsorption capacity of LE-C-Fe decreases by 46.8mg/g.adsorption temperature increased from 303K to 323K,adsorption capacity of LE-C-Fe increased by 3.83mg/g.initial solution pH value increased from 4 to 11,adsorption capacity of LE-C-Fe decreased by14.25mg/g.The concentration of CO₃^2-ion increased from 25 to 800mg/L,the adsorption capacity of LE-C-Fe decreased by 17.25mg/g.According to the kinetic model fitting of the adsorption data,it can be seen that the LE-C-Fe adsorption of fluoride ion has a high degree of fitting with the pseudo-secondary kinetic model and internal particle diffusion model.The adsorption is mainly chemical adsorption.The adsorption rate is controlled by the three stages of liquid film diffusion,particle internal diffusion and adsorption.The adsorption by the isothermal adsorption model fits the Freundlich isotherm model and the Redlich-Peterson isotherm model.The adsorption is a multi-molecular layer adsorption on a heterogeneous surface,which is affected by the adsorption force of uneven size.In summary,the adsorption mechanism of LE-C-Fe for fluoride ion is electron donor-acceptor interaction,electrostatic attraction and ion exchange.According to the F^-adsorption test results of the actual fluorine-containing circulating water,LE-C-Fe has a strong adaptability to the solution environment.In the case of the same initial fluoride ion concentration,the amount of fluoride ion adsorption in the actual fluorine-containing circulating water is slightly reduced than that in the fluorine-containing solution.When treating the actual fluorine-containing circulating water with F^-concentration of21mg/L,the dosage of LE-C-Fe is 4g/L,the F^-concentration in the actual fluorine-containing circulating water can be reduced to less than 10mg/L.Reduce the risk of corrosion of circulation pipelines,and reach the standard value of industrial wastewater discharge in China.The paper has 35 figures,15 tables,and 95 references.