Tobacco Stems Base Preparation Of Activated Carbon Adsorption Treatment Of Heavy Metal Wastewater

To deal with the resource present status of agricultural and forestry waste-tobacco stems, and serial problems in conventional preparation methods of activated carbon, and combined the features of microwave heating, a novel process of manufacturing activated carbon from tobacco stems by microwave heating was put forward. Meanwhile, researches on the adsorption of waste water containing heavy metals on activated carbons were investigated systematically. The reaction mechanism of preparing activated carbon by microwave heating was investigated through optimum process conditions being studied, comparison with one-step carbonization of conventional heating method, characterizations of structures, studying of pyrolysis process and temperature rising process in microwave field. At last, the activated carbons prepared by microwave heating have been used successfully for the removal heavy metals in wastewater. The thermodynamics, kinetics and the mechanism of adsorption were studied and analyzed systematically.The preparation process of one-step carbonization of tobacco stems by conventional heating method and microwave heating were put forward with a series of agents activation. The process flow of preparing activated carbons from tobacco stems activation with zinc chloride, phosphoric acid, potassium hydroxide by congventional and microwave heating was investigated systematically. The effects of impregnating and activation time, agents concentration, activation temperature or microwave power on the properties of activated carbons (iodine number, methylene blue adsorption and yield) were studied by using orthogonal test, and the optimal process conditions were obtained. The effects of gas flow rate, activation time, and temperature or microwave power on the properties of activated carbons activated with water steam and carbon dioxide were studied, and obtained the optimal process conditions. The activated carbon with high adsorption properties was manufactured with tobacco stems carbonaceous as raw materials and potassium hydroxide, potassium carbonate as activation agents, and the effects of ratio of agents to materials, microwave-heating time on properties of adsorption of activated carbons were studied. It was shown from the experiments that the activated carbons prepared by microwave heating reached the national standard, the process flow of the microwave heating method is shorter and more efficient than that of conventional method, and the process time of microwave heating was shorten 61.60-80% comparing with the conventional process.The pore structures of the samples were characterized with nitrogen adsorption measurement. The adsorption isotherms were analyzed and classified. Compared with the adsorption isotherm of activated carbons by the conventional heating, the "tail" phenomenon of the adsorption isotherm curves and the values of the slope of adsorption platform by microwave heating were smaller. Above results indicated that the activated carbon manufactured by microwave heating has well-developed microporous structures. The Astakhov quotients of the activated carbons were analyzed by Dubinin-Astakhov equation. It was found that the pore size distribution of the carbon activated by carbon dioxide was the narrowest among all activation agents used. For the carbons activated by zinc chloride, phosphoric acid and potassium hydroxide, the pore size distributions of the carbons manufactured by microwave heating were narrower than those of conventional heating. The microporous structures of the carbons were characterized by Horvath-Kawazoe equation. It was found that the ultramicropore ratio of the carbons by microwave heating was larger than that by conventional heating. The pore size distributions of the carbons were also characterized using DFT (Density Functional Theory), indicating that the ratio of microstructures of activated carbons prepared by microwave heating was larger than that of activated carbon prepared by conventional heating. Generally, it was shown from pore structure analyses that the microstructures of carbons prepared by microwave method were more well developed, and having fewer mesostructures and macrostructures than those of carbons prepared by conventional method.Microstructures of the samples were characterized with X-ray analysis, scanning electron microscopy and transmission electronic microscopy. It can be found from XRD analysis that activated carbon is a kind non-crystalline graphitic carbon material. The d₀₀₂, L_c and L_a of the activated carbon decreased comparing with graphite. The micro-crystal size of carbons activated by carbon dioxide was the smallest among all the activation agents, and the micro-crystal size of carbons activated by phosphoric acid with conventional heating was the biggest among all the activation agents. The results were consistent with the results of nitrogen adsorption analysis. According to SEM analysis, the basic structures of the carbon activated by chemical agents were destroyed seriously and the microstructures are complex, but the structures of the carbon of the tobacco stems activated by physical agents were partially maintained. The nano-scale pore structures were observed by TEM, which certificated the results of nitrogen adsorption and XRD, indicating that the carbons prepared have micropores, mesopores and macropores. It was found that the physical activation appeared in the chemical activation process in the microwave field, which was remarkably different from that of conventional heating method.The TG/DTG curves and temperature rising curves of tobacco stems and tobacco stems impregnated with chemical activation agents were measured by thermogravimetric analysis. The kenitics parameters of pyrolysis were calculated, showing that the activation energy of the impregnated tobacco stems was significantly reduced and the yield of the carbon was enhenced. The temperature rising curves of the samples impregnated with ZnCl₂ and H₃PO₄ in microwave field were determined, and analyzed the proces of the temperature rising to obtain rate equations of the temperature rising. The intrinsic regularity and reaction mechanism of activated carbon prepared by microwave heating was investigated combined with the optimum process parameters and research of the carbons structures.A systematic study was carried out for disposal of wastewater containing copper or lead ion by the activated carbons activated by zinc chloride and microwave heating. The effects of the carbon dosage, initial concentration, shaking time, pH value and solution temperature on the heavy metal removal rate were studied, and obtained the optimal process parameters. The thermodynamic parameters of adsorption were calculated by Van't Hoff equation, and analyzed the equilibrium data at different temperatures using Langmuir and Freundlich isotherms. The calculated Langmuir limit adsorption capacities of copper and lead ion were 10.6577, 8.0887 mg/g, respectively. The experimental results were analyzed by pseudo-second-order kinetics model of solid-liquid phase, obtaining the initial sorption rate and rate constants. It was found that the sorption kinetics fit the pseudo-second-order kinetics model well. It was shown from the adsorption mechanism analyses and investigations of the controlling step of adsorption that the adsorptions of heavy metals onto activated carbon were all film diffusion controlling processes. There was a big difference from those reported in the literatures overseas, showing that the activated carbon derived from tobacco stems with zinc chloride activation by microwave heating has perfect pore structure and was very suitable for the removal of heavy metals from aqueous solutions.Generally, a novel process of preparing activated carbon by microwave heating was put forward. This process had many advantages, such as reduction of environmental pollution, enhancement of additional values, widening of raw materials for activated carbon, giving novel path for utilization of agricultural and forestry wastes. The process could overcome the disadvantages of the conventional heating and develop the low cost and high quality activated carbon products which could reach the first-grade or excellent-grade of the national standard. It could greatly promote the grade of our national activated carbons. Furthermore, activated carbons prepared could be used in the disposal of wastewater containing heavy metal, which is of great significance to control environmental pollution, utilize wastes and develop recycling economy for nonferrous industry.