| Fluoride contamination in water has been recongnized as one of the serious problems around the world. It is reported that drinking water fluorosis have occurred in almost50countries over five continents. In China, the spread area of water-drinking endemic fluorosis covers about2.2million km2, more than70millions of people suffered from high-fluoride drinking water. In this study, a novel alumina-iron oxide composite nano-magnetic adsorbent with high adsorption capacity (50mg/g), wide available pH range (pH4.0-10.0) and cost-effective separation for water treatment was prepared for fluoride removal, the adsorption behavior of fluoride on the adsorbent was examined, and the mechanism of fluoride removal by the adsorbent was investigated. The results were obtained as follows:(1) The adsorbent for fluoride removal was prepared by coating Fe3O4nanoparticles with aluminum sulfate. The optimum preparation condition was:Fe3O4/Al molar ratio1:2, preparation pH at5.0. Analyses showed that the adsorbent consistent an outer layer of aluminum oxide embedded with crystalline Fe3O4magnetic nanoparticles and intercalated with low levels of sulfate ions. The other properties of the adsorbent were analyed as follows: average particle siza20nm, BET surface area63.37m2/g, pHpzc11.30and saturation magnetization15.63emu/g.(2) The optimum fluoride adsorption performance occurred when adsorbent dosage1.0g/L, reaction pH range4.0-10.0. The effect of the co-existing anions on fluoride adsorption was investigated. The results showed that fluoride removal in the presence of anion increased in the order: PO43>Br-≈SO42->Cr≈NO3-, which closely correlated with the Z/r (charge/radius) values of the anions.(3) The adsorption kinetic results showed that:fluoride adsorption was clearly biphasic:including an initial rapid adsorption stage (first1h) and then a much slower stage. The rapid fluoride uptake may due to the availability of active sites on adsorbent surfaces at the beginning, whereas the observed plateau corresponding to a slow rate of adsorption may due to the saturation of active site. The kinetic data of fluoride adsorption were fitted well by the pseudo-second-order kinetic model with the regression coefficients R2above0.9980, indicating the chemical reaction was the rate-controlling step.(5) It is indicated that the fluoride adsorption isotherms can be fitted by Langmuir model well with regression coefficients R2>0.9500. The fluoride adsorption capacity was founded to be45-50mg/g. The calculation of thermodynamic parameters showed the adsorption of fluoride was a spontaneous(△G°<0), endothermic(△H°>0) and entropy increasing(△S°>0) process.(6) The fluoride removal by the adsorbent was through three mechanisms:(i) electrostatic abstraction,(ii) ligand exchange mechanism with formation of inner-sphere surface complexes at the Al3+center and (iii) ion-exchange with sulfates intercalated in adsorbents with possible formation of outer-sphere complex. |
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