Removal Of The Excessive Fluoride Ions In Brick Tea Liquor By Modified Chitosan Beads

Tea leaves are abundant in fluoride by selectively absorbing fluoride ions from soil,water and air. Long-term consumption of tea with high fluoride concentration leads tochronic fluorosis. The current defluoridation methods have many shortcomings, forexample, fluoride removers for absorbing fluoride ions from tea liquor are mainlychemical agents or natural minerals, whose adsorption ability is low and separationproccess is difficult. Therefore, it is significant to develop efficient, available, safe,low-cost, renewable and green defluoridation agents.In this paper, chitosan-based beads modified by bentonite, La(III) and mixed rareearths are prepared as new fluoride removers. The adsorption capacities of modifiedchitosan beads as well as the defluoridation from brick tea liquor are studied. The mainresearch and results are shown as below:(1) Several novel adsorbents, i.e., chitosan beads (CB), bentonite/chitosan beads(BCB), La-loaded chitosan beads (La-CB), rare earths-loaded chitosan beads (Re-CB),La-loaded bentonite/chitosan beads (La-BCB) and rare earths-loaded bentonite/chitosanbeads (Re-BCB) are prepared as novel fluoride removers using inverse suspensionpolymerization. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction(XRD) and Differential scanning calorimeter (DSC) analysis are used to characterizethe modified chitosan beads and they show that bentonite can mixed with chitosan andrare earth ions can react with–NH2in chitosan, and the beads are spherical with highthermal stability. The adsorption capacities of modified chitosan beads are: CB＜BCB＜Re-CB＜Re-BCB＜La-CB＜La-BCB. The dosage of bentonite and rare earths canaffect the adsorption capacities of the beads. When La(III) dosage is2.0g, La-BCB hasan adsorption ability of2.07mg/g, and Re-BCB has an adsorption ability of1.51mg/gat rare earths dosage of3.0g. La-BCB and Re-BCB are potential fluoride removers with excellent defluoridation capacity.(2) Defluoridation experiments of La-BCB and Re-BCB for aqueous solution showthat La-BCB has an adsorption capacity of2.87mg/g and Re-BCB is2.20mg/g whensolution pH5,30℃,8h. Acidic condition is beneficial for defluoridation. Theco-existing anions especially the carbonate and bicarbonate ions can influence thefluoride adsorption. The exhausted La-BCB and Re-BCB could be regenerated andreused with respectively17%and23%loss for the tenth time, and their accumulatedadsorption capacities are19.2mg/g and13.3mg/g, respectively. Defluoridationprocesses of La-BCB and Re-BCB fit well with both Langmuir and Freundlich isothermmodels and their adsorption processes are available and favorable. The calculatedmaximum defluoridation capacities of La-BCB and Re-BCB are8.547mg/g and7.943mg/g, respectively. Pseudo-second order kinetic model and the particle and intraparticlediffusion models could describe the adsorption data of La-BCB and Re-BCB, whichindicate that chemical factors and intraparticle diffusion mainly controll the adsorptionrate. The mechanism of fluoride sorption onto La-BCB is governed by both physicaladsorption and ion exchange. FTIR and Energy dispersive X-ray Spectroscopy (EDX)analysis show the La-BCB successfully adsorbed fluoride ions from aqueous solution.La-BCB possesses high fluoride adsorption capability, good reuse efficiency, fastadsorption rate, wonderful properties, and therefore it is worth to be developed forpractical application.(3) The fluoride content of different types of tea is tested using fluoride ionselective electrode, and the ascending order is: Puer ripe tea (high-quality), Laoshangreen tea (class one), Tie Guanyin, Puer fresh tea, Laoshan green tea (normal), Laoshandark tea, Fuzhuan brick tea, Zang brick tea. High-quality tea has less fluoride contentthan normal tea. Brick tea has a fluoride content of794±25mg/kg, higher than nationalstandard of300mg/kg. La-BCB is employed to remove fluoride ions from brick tealiquor. The factors such as initial fluoride concentration, La-BCB dosage, pH have greatinfluence on the defluoridation rate and adsorption capacity of La-BCB. When theinitial fluoride concentration is4.49mg/L,30℃, La-BCB dosage is0.1mg/50mL, thedefluoridation rate of brick tea liquor is56.7%. The defluoridation process is fast in the first2hours and obtains stable after8hours. Column experiments show that La-BCB of4.465g could treat1.05L brick tea liquor when tea flow rate is0.92mL/min. Low flowrate is beneficial for defluoridation and column experiment is better than batch fordefluoridation of brick tea liquor.(4) The effect of La-BCB treatment on the chemical quality, nutrition, and safetyof brick tea liquor is studied. After defluoridation, the water extract, total water solublepolysaccharide, free amino acids, total polyphenols, caffeine and co-existing anionsdecrease to some extent. But no similar trend is observed for chemical elements in bricktea liquor. The content of La, Cl and Al show an increase after La-BCB treatment,however it has no influence for the safety of tea. FTIR and XRD analysis indicate thatLa-BCB can adsorb the fluoride ions from brick tea liquor. Therefore, La-BCB haspractical value as fluoride remover for brick tea liquor.