| Fluoride contamination in groundwater is a worldwide problem and manyregions have fluoride concentration higher than prescribed by WHO, which is aserious threat to flora and fauna including humans. Adsorption method is an attractivealternative to other treatment because of its environmental respectability and ease ofoperation.In recent years, considerable attention has been focused on the study of fluorideremoval using natural, synthetic and biomass materials. They have shown a certaindegree of fluoride adsorption capacities but some of them can only be used in anarrow pH range (5.0-6.0) and some of them are too expensive to be considered forfull-scale water treatment. Furthermore, most of them are fine particles or powderswhich would be suspended in water, making separation difficult and blocking flumes.Therefore, an effective and low-cost adsorbent with coarse particles is desired as anefficient treatment technology for fluoride in large-scale water samples. This researchfocused on the fluoride removal from contaminated groundwater and discussed thefluoride adsorption characteristics by using natural clay materials and artificialsynthesis porous mineral materials. This study focused on solving the key technicalproblems for the fluoride removal methods, which will provide the theory andscientific basis for fluoride removal from groundwater.The first section investigated the fluoride adsorption ability using one kind ofnatural mud in a batch and column study. The influences of contact time, solution pH,adsorbent dosage, initial fluoride concentration and co-existing ions were investigatedby batch equilibration studies. Batch experiments indicate that the time to attainequilibrium was 2 h and adsorption was followed the pseudo-second-order kineticmodel. Maximum adsorption for fluoride removal was achieved at pH range of5.0-7.0. The adsorption of fluoride on Kanuma mud in batch systems can be describedby the Dubnine-Radushkevich (D-R) and Freundlich isotherm models. The adsorptionprocess was an endothermic process. According to the enthalpy change of H = 11.782 kJ/mol, it can be inferred that the adsorption force was hydrogen bondingforce and the coupling reaction force. Chloride ion and nitrate ion had no effect onfluoride adsorption by natural Kanuma mud; sulfate ion and bicarbonate ion had aslightly negative influence on fluoride adsorption process; while carbonate ion andphosphate ion had a significant inhibitory effect on fluoride adsorption. The fixed-bedcolumn breakthrough curves were analyzed at different flow rates, bed depth andinitial fluoride concentration. Thomas and BDST model can be used for predicting ofbreakthrough curves for fluoride removal by a fixed bed of Kanuma mud for differentflow rates and bed depths.The second section has successfully combined Kanuma mud, with starch, zeoliteand FeSO4·7H2O salts to calcine clay materials (particle size: 3-5 mm) andinvestigated the fluoride adsorption capability of these adsorbents. Both GC(FeSO4 7H2O) and GC (Fe2O3) adsorbents can be used for fluoride removal fromaqueous solution, while GC (FeSO4 7H2O) is more effective for fluoride removal thanGC (Fe2O3). Maximum adsorption of fluoride on GC (FeSO4 7H2O) and GC (Fe2O3)at pH 7.0 and 4.0 were 94.23% and 60.48%, respectively. The equilibrium data ofsamples fitted well to both Langmuir and Freundlich isotherms. The adsorptioncapacity of GC (FeSO4 7H2O) and GC (Fe2O3) was 2.16 mg/g and 1.70 mg/g. Both ofthese two granular adsorbents followed second-order kinetics and were governed byintra-particle diffusion model. Chloride ion and nitrate ion had no effect on fluorideadsorption by GC (FeSO4 7H2O); sulfate ion had a slightly negative influence onfluoride adsorption process; while carbonate ion and phosphate ion had a significantinhibitory effect on fluoride adsorption by GC (FeSO4 7H2O). The calculatedthermodynamic parameters showed that both of the adsorption processes werethermodynamically favorable, spontaneous and endothermic in nature. For theadsorbent of GC (FeSO4 7H2O), the fluoride adsorption process was mainly chemicalreaction.The third section has developed another kind of adsorbent, which was preparedby mixing Knar clay/King Kong clay, zeolite and starch with mass ratio 1:1:1. Porousgranular clay adsorbents that contain dispersed aluminum and iron oxides have been synthesized by impregnating with salt solutions followed by precipitation at process.This adsorbent was sphere in shape, 2-3 mm in particle size, highly porous andshowed particularly high specific surface area. Both of the materials can achieve themaximum adsorption capacity when the initial pH was 6.0, and the fluoride removalefficiency will decrease when the initial pH less than 4.0 or higher than 10.0. Theexperimental data revealed that both the Langmuir and Freundlich isotherm modelsfitted well with the fluoride sorption process. The monolayer adsorption capacity was1.79 mg/g and 3.38 mg/g, respectively. The adsorption process was well explainedwith pseudo-second order kinetic model. The 1/n value of Freundlich isotherm wasless than 1, which showed that the adsorption process was favorable or preferential forboth kinds of materials. Besides, the fluoride adsorption capacity of King Kong claymaterial was two times than the Knar clay material. The King Kong clay material hadgood adsorpotion ability under various ions coexistence conditions. The adsorptionefficiency can achieve 80% under the initial fluoride concentration of 10 mg/L,adsorbent dosage of 20 g/L conditions by the King Kong clay material. Results fromthis study demonstrated potential utility of Fe/Al coated porous granular clay materialthat could be developed into a viable technology for fluoride removal from aqueoussolution and groundwater.In short, Kanuma mud, iron-impregnated porous synthstic materials and Fe/Almodified porous clay materials have good fluoride adsorption performance. Theseresults may provide the theory basis and the technical guidance for groundwaterfluoride removal in the practical application.
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