| Trace amount of fluoride is indispensability to human, but an exeess of intake of fluoride is harmful to human body. In China, the permissible limit of fluoride in drinking water is between 0.5 and 1.0 mg/L, and the fluoride removal treatment is necessary when more than the standard. Among some treatment technologies, adsorption because of its simplicity of operation, cost-effectiveness, selectivity, high efficiency and the availability of a wide range of adsorbents, is still one of the most extensively used methods for the fluoride removal from water. Zeolite is a kind of normal adsorber.The adsorption capacity of natural zeolite is low,so it must be activated in order to attain a higher adsorption capacity. On this condition, a study on modification of natural zeolite and its defluorination ability was carried out.Natural zeolite was modified with acid, alkali, salt solution, respectively. Defluorinati-on ability of modified zeolite was studied with adsorption experiments, and optimal modification condition was presented. A series of batch sorption experiments were conducted to study the influence of various factors such as pH, adsorbent dose, initial fluoride concentration,temperature, equilibrium contact time and co-existing anions on the sorption of fluoride. According to the SEM, BET, XRF, FT-IR and XRD analysis, the adsorption mechanism of the modified zeolites were preliminary studied.The results showed that the effect of fluoride removal of natural zeolite modified by ferric chloride has improved significantly.After immersion for 24h with 10% ferric chloride solution in 1:1 ratio can to treatment the water with fluoride concentration of 2mg/L standards when the amount of 5g/150ml and reactiong time is 5h. The adsorption of fluoride was followed pseudo-second-order kinetics.The adsorption rate increased with the temperature increased first and then decreased.In the raw water fluoride concentration is less than 20mg/g, the equilibrium adsorption data fitted well to Langmuir adsorption isotherm and showing maximum fluoride adsorption capacity of 0.091 mg/g,compared to the unmodified zeolite is greatly improved and equilibrium constant of 0.28 L/mg. At low concentrations of fluoride,the higher the temperature,the higher the saturated adsorption capacity.In the raw water fluoride concentration is more than 20mg/g, the equilibrium adsorption data fitted well to Freundlich adsorption isotherm. When the dosage of 8g,the adsorption capacity is the largest.In the range of drinking water,pH has little influence to fluride removal effect.H2PO4-ã€Cl-ã€NO3-ã€SO42- have little impact on effect of fluoride removal of a variety of modifiednatural zeolite, but HCO3-ã€CO32-ã€HPO42- have greater impact.After modified, the zeolite pore structure have been adjusted and the surface area increase.The content of Fe element increased and Ca2+ content reduced; The modified zeolite crystal structure is not damaged.The mechanism of fluorine removing is mainly based on physical and chemical adsorption and ion exchange.Under different conditions,different adsorption of the dominant role.For the regeneration of the saturated modified zeolite,this paper researched the regeneration conditions by using NaOH solution as the desorption solution and FeCl3 solution as the activated solution.The adsorption capacity decreased is not obvious after five regeneration cycles. |
