| A huge quantity of dye wastewater is discharged by textile, paper, plastic and leather industries every year. As many dyes are complex organic contained azo and poly aromatic ring, which may cause people abnormal, cancerous and mutagenic, the removal of dyes from these wastewaters is a very important subject. Lots of dyes are resistant to oxidation and degradation, so adopting adsorbent to remove the dyes may be superior to other methods. The activated carbon is a very effective adsorbent, but its high price limits its extensive use. The diatomite is abundant, porous and has high specific surface area, which make it hopeful to be one effective and low-cost adsorbent. However, the adsorption capability of raw diatomite still needs to be improved and the studies on its application and theory on dyes adsorption are still expected to be carried out further.In this paper, basing on the structure characteristics of diatomite, the preparation technique of adsorbent of Mg(OH)2-modified diatomite and polyacrylamide-modified diatomite were developed with the method of activated and combined the second composition, which improved the adsorption capability for dyes. Hence, the system study from preparation process, materials characterization and dye-adsorption property to the dye-adsorption theory and application of diatomite-based adsorbent was carried out. On the study of adsorption theory, the traditional pseudo-first-order and pseudo-second-order kinetics equations were rearranged to be concentration form in the paper, and then they were applied to study the kinetics of dyes adsorption onto diatomite, which verified their practicability. Moreover, the adsorption mechanisms of dyes onto diatomite-based adsorbent were presented in this paper after the study of adsorption isotherms, kinetics and thermodynamics. The main achievements are as follows:(1) It was found that when the dye solution was adsorbed by the raw diatomite, the impurity on the raw diatomite made the dye solution after adsorbed increase the absorbance of visible light under the wavelength of 480 nm, which impacted the adsorption capability of diatomite. The impurity was removed when the diatomite was calcined at 450 ℃, with the specific surface area (SBET) decreasing from 67.17 to 52.56m2·g-1, the average pore diameter increasing from 8.847 to 10.29 run, the zero charge point (pHzpc) increasing from 3.2 to 4.5, and the removal of dye solution improving.(2) The preparation processes of adsorbents of Mg(OH)2-modified diatomite (MGMD) and polyacrylamide-modified diatomite (HPMD) were developed first time. It was found that the optimum content of Mg(OH)2 in MGMD was about 14% and that of polyacrylamide in HPMD was about 0.8%. The characterization study of MGMD showed that the SBET of MGMD was 57.84 m2·g-1 and the average pore diameter was 9.686 nm, and there were three times of charge reverse during the determination of zeta potential, which denoted the isoelectric point of diatomite, the precipitation start of Mg(OH)2 and the isoelectric point of Mg(OH)2. Moreover, the dye removal of anionic dyes onto MGMD was higher than that onto activated carbon while the removal of cationic dyes onto HPMD was close to that onto activated carbon.(3) The effects of adsorbent concentration, salt, adsorption time and pH value on the dyes adsorption of diatomite adsorbent were analyzed. It was found that the absorbance of visible light of Methylene blue (MB) solution decreased at high pH because of hydrolysis or forming dimer of MB. On the basis of this find, we proposed that there were two reasons for the increasing removal of MB onto diatomite or other mineral adsorbents with the increasing pH value: one was the increasing negative charge on the adsorbents surface with pH increasing and the other was the hydrolysis or dimer forming of MB which causing the removal increase. As MB is one of the most popular dyes for characterization of various adsorbents, this find may be also taken as reference to other adsorbent studies.(4) The concentration forms of pseudo-first-order and pseudo-second-order kinetics equations were rearranged in the paper. When they are applied to fit the kinetic data, they can describe the variation process of the dye concentration with time more visually, and under the condition of without equilibrium adsorption capacity (qe) determination, both of them can be used to predict the qe value directly after the fitting.(5) The adsorption isotherms model, kinetics model and thermodynamic parameters of dyes onto heat activated diatomite and MGMD were firstly studied and their dyes adsorption mechanisms were then proposed in the paper. The adsorption isotherms were studied using Langmuir, Freundlich and Redlich-Peterson models. It showed that theRedlich-Peterson model was the best one. The adsorption process of dyes can be better described by the pseudo-second-order model than the pseudo-first-order model. And the adsorption mechanisms of dyes were proposed as: adsorption by an electrostatic force, hydrogen bond, n-π interaction and Van der Waals force, of which the electrostatic force and hydrogen bond were the main effects.
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