| Coal gangue is a common solid waste obsolete discharged by coal mining and washing process, bulk deposition of which exerts harm to ecological environment. Instead, it can be used as raw materials for the synthesis of zeolite molecular sieves in virtue of Al2O3 and SiO2 as its main chemical composition. In this paper, zeolite NaX with complete surface morphology was prepared using coal gangue from Baishui coal mine in Shaanxi Province. The adsorption of three kinds of heavy metal ions (Cd2+, Cr3+ and Cu2+) to zeolite NaX prepared by coal gangue in single system, dual system and ternary system were discussed. What’s more, adsorbabilities of gangue based zeolites and modified zeolites to Cr (Ⅵ), whose toxicity is much higher than its tervalence state, were also under discussion. Methods for modification of gangue based zeolites NaX by three reagents, as well as conditions and mechanisms of modified zeolites’adsorption were investigated. And the topographys of zeolite before and after modification were characterized by scanning electron microscope. In the end, fitting process was conducted on each adsorption process and the isothermal adsorption model, as well as the adsorption kinetics model. Based on the above research, conclusions were drawn as followed:(1) The collected coal gangue were synthesized into zeolite molecular sieves with complete surface topography by in a sequence of steps including breaking, grinding, acid leaching, washing, drying, alkali fusion, aging, crystallization and so on. There into, in step of alkali fusion, the ratio of coal gangue and NaOH was 1:1.2. The alkali fusion temperature was 650 ℃, while aging time 6 h, crystallization temperature 110 ℃, crystallization time 12 h.(2) In single factor experiments focusing on adsorption of Cd2+, Cu2+ and Cr3+ to coal gangue base zeolite NaX, the optimum pH was 4, while the optimum zeolite content was 0.15 g. With the increase of initial concentration, the unit adsorption quantity of three kinds of heavy metal ions increased to varying degrees, while removal rate reversed. The adsorption of Cr3+ to zeolite reached adsorption equilibrium after 60min whether in single system or polycomponent system. Nevertheless, the adsorption of Cd2+ and Cu2+ to zeolite could reach adsorption equilibrium after 5-10 min in single system and system without Cr3+ participation. When Cr3+ participated, the adsorption equilibrium time also reached after 60 min.(3) Results of surface features showed that the crystal type of gangue based zeolite modified by aluminum chloride and hexadecyltrimethylammonium (HDTMA) did not change much, and the modified zeolite volume slightly expanded than before, and modification agents are attached to the surface. The crystal of gangue based zeolite modified by ferrous sulfate was destroyed.(4) Before modification, absorption efficacy of coal gangue base zeolite NaX to Cr (VI) was the best when pH was 8, however, the adsorption rate was not very high.(5) The optimum synthetic conditions of zeolites modified by different modifying agents were listed as followed:ferrous sulfate (concentration 0.3 mol/L,2 h), aluminium chloride (concentration 20%,4 h), and HDTMA (concentration 0.5%,2 h).(6) According to discussion of different factors affecting the adsorption processes, conclusions can be drawn:among three modifying agents, the optimum adsorption initial pH of zeolites modified by ferrous sulfate was 4, while the values of zeolites modified by aluminium chloride and zeolites modified by HDTMA were both 2. The removal ratios of three kinds of modified zeolites decreased as initial concentrations increased. Meanwhile, the unit adsorption quantities increased. Under the same initial concentration and volume, it can achieve harmless only when the dosages are more than 0.25 g in practical applications. The three kinds of modified zeolites were verified by experiments to be able to reach equilibrium in 120 min.(7) With the method of model fitting, it was found that adsorption processes were in accord with Langmuir isotherm adsorption model and pseudo secondary dynamic adsorption model in all single systems. The isothermal adsorption of three kinds of heavy metals to coal gangue base NaX zeolites were more aligned with extended Langmuir model.(8) Through the comparison of the adsorption effect of Cr (Ⅵ) by different adsorbents at their own optimal pH value and other the same adsorption conditions, the adsorption capacities of gangue based zeolite NaX on Cr (Ⅵ) have been improved in different degree after being modified by different methods, of which the adsorption effect of Cr (Ⅵ) onto gangue based zeolite NaX modified by organics is the best. |
