Preparation Of Granular Adsorbent Based On Fly Ash With Fiber As Pore Former And Its Adsorption Performance For Dye Removal

Reactive dyes are characterized by high solubility in water and low rates of fixation. Large amounts of effluent from textile industry pose great threat to the environment and human beings, which in turn draws widespread attention. Adsorption has received considerable attention for its advantages of removing varieties of dyes, producing high quality treated water and simplifying the operation procedures. Generally, activated carbon is widely used due to its high surface area and abundant pores, but its narrow pores limit the use in large-scale pollutants removal, like dyes. Fly ash is the solid waste and by-products from thermal power plant, and its main component is SiO2 and Al2O3. Moreover, it has a large number of pores and a high specific surface area and thus can be used as adsorbent. Paper sludge, which is generated during the production cycle of the residue, can not be further utilized. According to the data, China’s paper industry is growing with the annual rate more than 10% and the waste sludge caused severe pollution to the surroundings. Paper sludge contains a large number of fine fibers, which produces a large amount of gas during pyrolysis and can be used as the pore former.In this paper, a granular adsorbent based on fly ash with bentonite as binder and paper sludge as pore former was prepared and characterized by a certain mechanical strength, low-cost and high adsorption performance. The test mainly includes preparation of GAFF, the adsorption studies GAFF onto methylene blue and crystal violet, and GAFF performance with granular activated carbon as comparison and its characterization.The preparation process of GAFF includes:(1) dispersing paper sludge into fibers(2) fiber suspension was centrifuged; (3) mixing raw materials in water; (4) drying materials to its original weight; (5) crushing materials into powder and mixing them; (6) materials shaped by machine; (7) dehydration; (8) calcination. First, we dispersed paper sludge into fibers with ultrasonic treatment and next used an inverted fluorescence microscope to observe the fiber made in different conditions. Ultimately we determined the best condition for dispersing fibers was:lurry concentration is 0.2g /100mL, ultrasonic power is 400W. ultrasonic time is 15min. Second, the drying stage after material forming is also crucial. In order to prevent the material from drying stage cracks and thus affecting the absorption performance of GAFF, five different drying system were chosen and compared with methylene blue and crystal violet dye adsorption capacity and material loss rate as index, and ultimately we determined the best drying system:drying at room temperature for 24h, followed placed 70℃,105℃ in the oven for 12h. Third, we used the single-factor experimental to study the effect of sintering temperature, ash:bentonite proportion, fiber adding amount and heating rate on the performance of GAFF, and to determine the appropriate level of value for each factor in orthogonal experiment. Finally, we carried out the orthogonal experiment and determined the optimal preparation conditions for GAFF with crystal violet adsorption capacity and loss rate as an indicator:sintering temperature of 630℃, ash:bentonite ratio of 2:1, paper sludge adding amount of 3% and heating rate of 0.2℃/min.Basic dyes, methylene blue and crystal violet, are similar in molecular weight but different in molecular size. In addition, they are widely used and caused severe pollution to the surroundings. The adsorption of GAFF onto methylene blue and crystal violet dye were carried out under different initial pH, dye concentration and temperature. The experimental results showed that the adsorption amount of the dyes increased as the pH value increased until 5, the adsorption amount changed little. Moreover, when the reaction temperature and initial dye concentration increased, the adsorption amount also increased. And when the pH value is 7 and the reaction temperature is 50℃, the maximum amount of GAFF onto methylene blue and crystal violet were 216.57mg/g,682.59mg/g. Langmuir, Freundlich and Tempkin adsorption isotherms were used to fit adsorption data. The results showed that the R2 value of Langmuir isotherm was higher than the others, indicating that Langmuir isotherm is more suitable for fitting the adsorption of GAFF onto methylene blue and crystal violet. Adsorption kinetics were verified by pseudo-first-order, pseudo second-order and intra-particle models. The results indicated that the dye uptake process followed the pseudo second-order model. Absorption thermodynamics results showed that the adsorption process of GAFF onto methylene blue and crystal violet can occur spontaneously and physical adsorption played a dominant role.GAFF and activated carbon GAC1, GAC2 were conducted SEM/EDX characterization, surface area and pore size distribution test and adsorption of methylene blue and crystal violet research. SEM/EDX results showed the presence of a large number of irregular pores on GAFF, while the surface of GAC1 and GAC2 were all small pores. The specific surface area of GAFF, GAC1 and GAC2 is 19.54m2/g,1166.95m2/g and 755.22m2/g respectively. The average pore diameter of GAFF, GAC1 and GAC2 is 90.10nm.2.27nm and 0.47nm respectively. The adsorption capacity of GAFF, GAC1 and GAC2 onto methylene blue was 200.20mg/g, 362.78mg/g and 256.0 1mg/g respectively. The adsorption amount of GAFF, GAC1 and GAC2 onto crystal violet was 474.27mg/g,312.99mg/g and 230.61mg/g respectively. These results indicated that the surface area is not the only factor to determine the adsorption amount, pore size distribution and structure of adsorbate are also important factors and can not be ignored. Furthermore, XRD test results showed that the phase composition of un-calcined GAFF, calcined GAFF and adsorbed GAFF has not changed.FTIR results showed that the adsorption of GAFF onto methylene blue and crystal violet was physical adsorption.