| The moderate fluoride is greatly beneficial to cells enzyme reproductive system and to the strength of teeth and bones for human body. However, excess fluoride is harmful to human health, and may cause the fluorosis. In fact, the drinking water with high fluoride is the main cause of endemic fluorosis. In China, the fluoride pollution is quite serious, and the drinking water with high fluoride distributes throughout 29 provinces, autonomous regions and municipalities. The fluorosis may bring on the damage of dental and skeletal tissue of the serious patients, furthermore labor ability might lose. Therefore, it is very important to remove the excess fluoride in the drinking water for protecting human health. At present, the methods of defluoridation are as follows: coagulation-precipitation, adsorption, electrochemical exchange, membrane separation, etc, among which the adsorption method is extensive.The adsorption method is focused on the synthesis of the material with high defluoridation capacity. In this paper, the shell sand, with its chemical components and special microstructure, was selected to prepare a kind of fine defluoridation filters. In the process, the raw shell sand was reacted with citric acid or sucrose solution, following modification with the phosphoric acid solution and ammonia solution. After the former procedures, two kinds of new defluoridation materials, CPAS (Citric acid, Phosphoric acid, Ammonia, Shell) and SPAS (Sucrose, Phosphoric acid, Ammonia, Shell), were prepared. With elaborate experiments, the characteristics and performance of the prepared materials were investigated. The optimum reacting conditions in process of CPAS and SPAS were determined, and important factor includes aqueous pH, adsorption time, dosage, aqueous fluoride concentration and coexisting ions. Meanwhile filter-column experiments were researched and the regeneration of SPAS material was investigated. Finally, reaction mechanism was discussed. Some main conclusions are:(1) Using shell sand as raw material, two new and efficient defluoridation filters were prepared, and the citric acid, sucrose, phosphoric acid and ammonia solution were the successful reactants.(2) The optimum CPAS preparation procedures were mainly as follows: first, soaked in 5% citric acid with a solid/liquid ratio 1:5 (ms:mL) for 24 hours; then, washed for eight times and dried, and calcined at 600℃for 1 h; soaked in 0.6 mol·L-1 phosphoric acid with ms:mL=1:30, for 3 h, then adjusted with ammonia to aqueous pH=8, lately aging time at room-temperature for 3 h.(3) The optimum SPAS preparation procedures were mainly as follows:first, soaked in 5% sucrose solution with a solid/liquid ratio 1:10 (ms:mL) for 24 hours; then, filtered and dried, and calcined at 500℃for 2 h. soaked in 0.6 mol·L-1 phosphoric acid with mx:mL=1:30. for 0.25 h. then adjusted with ammonia to aqueous pH=8. lately aging time at room-temperature for 3 h.(4) It was found that the dosage and the aqueous pH influence on the defluoridation performance of CPAS and SPAS materials, and the coexisting ions scarcely ever compete with the fluoride ions. The fluoride removal percentages of CPAS material and SPAS material all could reach more than 95% under the optimum condition,1.33 g·L-1 dosage, aqueous pH 3～10 and 12 h reacting time.(5) The filter-column experiments of CPAS and SPAS materials indicated their removing fluoride capacities were 7.53 mg·g-1 and 7.67 mg·g-1. respectively. After some regeneration cycles, the defluoridation performance of SPAS material declines.(6) The mechanisms of defluoridation of the prepared materials. CPAS and SPAS, were ion-exchange or crystal lattices-exchange of the OH- anion and aqueous F-, as while as the subsurface adsorption of CPAS and SPAS. |
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