| Silica sand is the main raw material for the preparation of silicate glass. In addition, silica sand can be as filter material used in water treatment. The efficiency of silica sand toward metal ions in wastewater is low. Coating silica sand with porous silica can increase the surface area of the silica sand greatly, thus adsorption ability of silica sand toward metal ions in waste water can be improved. In the same time, concerning the composition, the silica coating layer will not affect on the recycling of silica sand in the glass industry.Based on the above considerations, four types of silica sands were carefully examined, including the composition, loss-on-ignition(LOI) test, acid and alkali dissolution, and glass melting. Results show that the silica sand A is highly pure with the smallest LOI(0.07%) and acid dissolubility(0.24%), and easier to be melted. Under the optimal pH, the porous silica prepared from the dissolved sand A is big has the biggest surface area(1293 m2/g). Therefore, silica sand A was choosen for further coating experiment.Partially dissolved silica from sand A was used as the silica source, cetyltrimethylammonium bromide(CTAB) as the surfactant, SiO2-surfactant composite coating layer was formed on the surface of the undissolved sand particles, leading to porous silica coated silica sand after drying and calcining. In the experiments, the pH, reaction time and temperature, the amounts of CTAB water, silica sand were changed. The optimal coating conditions are, pH of 11.5, 1.5g CTAB, 100 ml H2O, 25 oC, 1h, 9g silica sand. The surface area of the resultant sand is 61.67m2/g, which is 743 times than the 0.08m2/g of the raw sand. In order to increase the adsorption rate of hexavalent chromium of coated silica, the coated sand with 3-ammonia propyl triethoxy silane(KH550), the modified silica sand has a surface area of 10.75m2/g.The coated and modified silica sands were used to adsorb hexavalent chromium ion in solution and waste water. The results show that the best conditions for the adsorption of hexavalent chromium ion in the solution by the coated silica sand is pH of 2, 100mg/L Cr6+, 25 oC, 90 min, 1g coated silica sand. The removal effiency is only 5.77%,the adsorption amount is 0.288mg/g. The best adsorption conditions of the modified silica sand is pH of 3, 100mg/L Cr6+, 25 oC, 90 min, 1g modified silica sand. The adsorption rate is 24.84%,the adsorption amount is 1.242mg/g.The best conditions for the adsorption of hexavalent chromium ions in the waste water by the modified silica sand is pH of 3, 174.162mg/L Cr6+, 25 oC, 90 min, 8 cycles of adsorption. The hexavalent chromium ion concentration is 0.444 mg/L after 8 times of adsorption, meeting requirement of the standard of wastewater emission. The adsorptions of hexavalent chromium ions on both the coated and modified silica sands follow the second-order kinetic modes. The adsorption processes of the coated and modified sands fit well the Langmuir and Freundlich models, respectively.The coated and modified silica sands which have adsorbed hexavalent chromium ions were used to prepare the green glass successfully. The colour of the glass made from the coated silica was light than that from the modified silica, indicating the modified silica introduced more hexavalent chromium ions to the glass. The spectrum of the glass made from the modified silica is similar to that of a beer bottle, demonstrated that the recycling of adsorbed hexavalent chromium ions in waste water by the modified silica sand is feasible. |
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