| Fluoride contamination in water has been recognized as one of the increasingly serious worldwide environmental problems and has received considerable attention for many years.People who drink fluoride contaminated water for a long time may cause a series of physical disorders,including dental and skeletal fluorosis,infertility,brain damage and thyroid disorder.A strict rule from World Health Organization(WHO)is adopted the maximum allowable fluoride concentration level for drinking water at 1.5mg/L.Various techniques have been developed to remove fluoride from aqueous solution,and the adsorption has developed very mature for defluoridation of water due to its low maintenance cost,flexible operation and simple design.In the study,the coal-fired power plant solid waste fly ash cenospheres were used as raw materials,and the paper mill solid waste lime mud was used as a modifier to prepare a high-efficiency,low-cost and wide-ranging fly ash cenospheres modified with paper mill lime mud(LCM)for fluorine removal.The main conclusions were as follows:(1)The surface morphology,microscopic and chemical composition of the LCM were analyzed by SEM,EDS,BET,XRD and FT-IR.The results of SEM and EDS showed that the modification of lime mud destroyed the original hollow sphere structure of fly ash cenospheres and produced a calcium-containing substance with a porous structure.The BET results show that the specific surface area of the modified material increases,and the average pore diameter changes from macropores to mesopores.XRD and FT-IR analysis indicated that the lime mud decomposed during the modification process and reacted with the fly ash cenospheres to form porous calcium-containing substances,while reducing the crystallinity of the fly ash cenospheres and increasing the mineral activity.(2)The dosage,initial pH and coexisting ions of the MCL in the process of fluorine removal were studied by single factor experiments.When the initial fluorine concentration is 10mg/L,the dosage of the material is 2g/L;when the pH is in the range of 3-7,the adsorption effect of the material is better;Nitrate and sulfate in the common coexisting ions in water have little effect on the removal of fluoride ions.Bicarbonate and phosphate have little effect on the fluorine removal performance of the material at lower concentrations.(3)The CCD experimental model in the Response surface method was used to optimize the 4 types of influencing factors in the adsorption process.The analysis results show that the model is highly dominant,and the correlation coefficient R~2reaches 0.99846,which can be used to optimize the defluoridation process of LCM.The results of model variance analysis showed that the order of influence on the fluorine removal process of the material was:pH>dosage>F~-concentration>adsorption temperature.Through the analysis of the expected function,when the F~-concentration was 11 mg/L,the pH was 5,the dosage was 1.9 g/L,and the adsorption temperature was 27°C,the defluoridation removal rate of the LCM material reached a theoretical maximum of 96.85%(experimental verification value was 95.65%)within the experimental range.(4)The process of adsorbing F~-in MCL material accords with Langmuir isotherm model and pseudo-second-order model,indicating that the adsorption process of the material is mainly based on the chemical adsorption.The maximum adsorption capacity was 7.37 mg/g.In addition,the determination of the zeta potential shows that the surface of the material has a positive charge in a wide pH range(2.3-10.3),which is favorable for the adsorption of F~-.The change of surface charge before and after adsorption indicates that the process of adsorbing fluoride ions forms a negatively charged inner-sphere complex on the surface of the material.(5)Using the generalized composite non-electrostatic model in the surface complex model,the adsorption process of the material was studied.It was found that two inner-spherical complexes,?SF and?SOHF~-,were formed in the adsorption process by means of the ligand exchange and surface complexation. |
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