Preparation Of Activated Carbon With Grapefruit Peel And Study On Adsorption Mechanism Of Application

Activated carbons, known as very effective adsorbents, are used widely due to their highly developed porosity, large surface area, variable characteristics of surface chemistry, and high degree of surface reactivity. There are many studies on the development of preparation of activated carbon and application for the removal of specific pollutants from aqueous phase, mainly heavy metals, dyes and etc, at home and abroad. However, the costs of activated carbon preparation from coal and other non-renewable sources are too expensive. In search of new and alternative source as a precursor for the preparation of activated carbon, many agricultural wastes have been studied. The raw materials obtained from agricultural wastes have attracted people’s attention because of their wide resources, renewability and no second pollution. Grapefruit is one of the principal fruits in the south of China with high production. After processing, grapefruit peels are generally discarded as a waste in large scale. Till now, nothing on the preparation of activated carbon from grapefruit peel waste by zinc chloride and treatment of the wastewater has been reported. This dissertation is concerned with the synthesis of activated carbons derived from grapefruit peel by chemical activation with zinc chloride and the removal of different pollution from aqueous solution. Samples were characterized by using FTIR, SEM and XRD techniques, etc. The principles of kinetics, isotherms and thermodynamics about the sorption of ammonia nitrogen, phosphate, Methylene Blue (MB), Congo Red (CR) and hexavalent chromium on the prepared samples were studied. The influence of several operating parameters, such as pH, contact time and initial concentrations of adsorbate on the adsorption capacity, were also investigated. The main conclusions of this study are as follows:(1) The low-cost activated carbon was prepared from grapefruit peel, an agricultural waste material, by chemical activation with zinc chloride. The optimal conditions for the production of activated carbon by orthogonal test were:20%zinc chloride solution concentration,3.0:1mass ratio of liquid to solid,500℃activation temperature and80min. activation time, resulting in35.36%of carbon yield and851mg/g of iodine adsorption value. At this optimal condition, the BET surface area of GAC was found as1045m2/g. The pore volume of GAC is estimated to be7.471cm3/g. The mean pore size of GAC is estimated to be1.430nm. The FT-IR spectroscopy result indicates that the carbons produced are rich in surface functional groups, such as hydroxy, amide, carboxyl and pyrone groups. The pHpzc value of GAS was5.67.(2) Adsorption of Cr(VI) onto GAC was investigated in a batch system. The results show that it takes12hours to reach the equilibrium. Cr(VI) removal is pH dependent and found to be maximum at pH1.0. Temkin and Langmuir isotherm model fitted the data well. The maximum monolayer adsorption capacity of Cr(VI) onto GAC was calculated as145.47mg/g. The mean free sorption energy was calculated as9.93,17.72and20.82kJ/mol respectively. It is very likely that Cr(VI) adsorption onto GAC is chemical in nature. The adsorption kinetic was well fitted to the pseudo-second-order model. Intraparticle diffusion model and Boyd model suggested that average values of the film diffusion coefficient (D1) and the pore diffusion coefficient (D2) were3.72×10-13and5.99×10-12cm2/s. Adsorption was both by film diffusion and intraparticle diffusion and the external mass transfer was the rate-determining. Thermodynamic parameters for the adsorption system were determinated,△G=-0.32kJ/mol～-20.06kJ/mol,△H=39.85kJ/mol～342.93kJ/mol,△S=134.91kJ/(mol·K)-829.51kJ/(mol·K). The negative value of△G showed spontaneous nature of adsorption. The positive values of both△H and△S suggest the adsorption process is an endothermic reaction increasing in randomness at the solid-liquid interface. Possible adsorption mechanism involved three processes:(Ⅰ) adsorption of Cr(VI) ions onto charged groups such as hydroxy, carboxyl and amines groups;(Ⅱ) reduction of Cr(Ⅵ) to Cr(Ⅲ) in the acidic medium;(Ⅲ) reduction releasing Cr(Ⅲ) to the aqueous phase or complexation.(3) Adsorption of MB onto GAC was investigated in a batch system. The results show that it takes2hours to reach the equilibrium. The MB removal was not affected over pH range of2.0～10.0. Temkin and Langmuir isotherm model fitted the data well. The maximum monolayer adsorption capacity of MB onto GAC was calculated as234.26mg/g. The adsorption kinetic was well fitted to the pseudo-second-order model. The mean free sorption energy was calculated as18.53kJ/mol. The adsorption of MB onto GAC was mainly attributed to the chemical adsorption. Intraparticle diffusion model and Boyd model suggested that average values of the film diffusion coefficient (D1) and the pore diffusion coefficient (D2) were2.85×10-12and2.30×10-10cm2/s. Adsorption process was affected by both film diffusion and intraparticle diffusion and film-diffusion dominate the adsorption rate. Thermodynamic parameters for the adsorption system were determinated,△G=-19.44 kJ/mol～-24.77kJ/mol,△H=64.63kJ/mol～67.50kJ/mol,△S=276.65kJ/(mol·K)-277.98kJ/(mol·K). The adsorption process of MB was endothermic and spontaneous. Possible adsorption mechanism was proposed as follows:electrostatic interactions, hydrogen bonding formation and electron donor-acceptor interactions.(4) Adsorption of CR onto GAC was investigated in a batch system. The results show that it takes2hours to reach the equilibrium. The CR removal was not affected over pH range of3.0-10.0. Freundlich model fitted the data well. The mean free sorption energy was calculated as12.73kJ/mol. The adsorption of CR onto GAC was mainly attributed to the chemical ion exchange adsorption. The adsorption kinetic was well fitted to the pseudo-second-order model and Elovich model. Intraparticle diffusion model and Boyd model suggested that average values of the film diffusion coefficient (D1) and the pore diffusion coefficient (D2) were3.72×10-13and5.99×10-12cm2/s. Adsorption process was both handled by film diffusion and intraparticle diffusion and film-diffusion dominate the adsorption rate. Thermodynamic parameters for the adsorption system were determinated,△G=-3.72kJ/mol～-15.17kJ/mol,△H=45.35kJ/mol～49.54kJ/mol,△S=154.00kJ/(mol·K)～166.05kJ/(mol·K). The adsorption process of CR was endothermic and spontaneous. Possible adsorption mechanism was proposed as follows:electrostatic interactions, hydrogen bonding formation.(5) Adsorption of ammonia nitrogen onto GAC was investigated in a batch system. The results show that it takes5hours to reach the equilibrium. Temkin and Freundlich model fitted the data well. The adsorption kinetic was well fitted to the pseudo-second-order model and Elovich model. The mean free sorption energy was calculated as9.19kJ/mol. The adsorption of ammonia nitrogen onto GAC was mainly attributed to the Chemical ion-exchange adsorption. Intraparticle diffusion model and Boyd model suggested that average values of the film diffusion coefficient (D1) and the pore diffusion coefficient (D2) were6.65×10-13and1.15×10-11cm2/s. Adsorption process was both handled by film diffusion and intraparticle diffusion and film-diffusion was the rate-limiting step. Thermodynamic parameters for the adsorption system were determinated,△G=-4.26kJ/mol～-4.73kJ/mol,△H=1.48kJ/mol～2.58kJ/mol,△S=19.74kJ/(mol-K)-23.29kJ/(mol·K). The adsorption process of ammonia nitrogen was endothermic and spontaneous. Possible adsorption mechanism was proposed as follows:electrostatic interactions and Van der Wals forces.(6) Adsorption of phosphate onto GAC was investigated in a batch system. The results show that it takes3hours to reach the equilibrium. Freundlich and Dubinin-Radushkevich model fitted the data well. The adsorption kinetic was well fitted to the pseudo-second-order model and Elovich model. The mean free sorption energy was calculated as8.43kJ/mol. The adsorption of phosphate onto GAC was mainly attributed to the Chemical ion-exchange adsorption. Intraparticle diffusion model and Boyd model suggested that average values of the film diffusion coefficient (D1) and the pore diffusion coefficient (D2) were5.16×10-13and5.03×10-12cm2/s. Adsorption process was dominated by film diffusion. Thermodynamic parameters for the adsorption system were determinated,△G=-2.29kJ/mol～-4.03kJ/mol,△H=2.68kJ/mol～23.15kJ/mol,△S=18.38kJ/(mol·K)～86.48kJ/kJ/(mol—K). The adsorption process of phosphate was endothermic and spontaneous. Possible adsorption mechanism was proposed as follows:electrostatic interactions and Van der Wals forces.