| Diatomite is widely used to adsorb heavy metal from waste water due to its unique physicochemical properties, such as large specific surface area and electronegativity. But the flawed structure limited its adsorption capacity. It is need to modify the nature diatomite. The adsorption capacity of modified diatomite which was treated with single method dose not shows a great improvement. Consequently, the composite modification of diatomite is of great significance.The optimum adsorption conditions and adsorption mechanism for Cd2+、Mn2+on natural raw diatomite were investigated to confirm the adsorption capacity. Modified diatomite was prepared by activation-sodium modification-pillaring (polyhydroxy aluminum, polyhydroxy iron, polyhydroxy aluminurn/iron)-calcination process. X-ray diffraction analysis, Fourier transform infrared spectroscopy, Scanning electron microscope were applied to study the microstructure and phase constitution of modified diatomite. Besides, the adsorption mechanism for Cd2-、Mn2+on modified diatomite were investigated in this paper. The main results obtained from this study are summarized as follows:(1) The adsorption characteristic of Cd2+、Mn2+on natural raw diatomite was analyzed by FAAS analysis. Batch sorption of Cd, Mn in aqueous solutions was investigated under different time, original adsorbate concentration, adsorbent amount, pH value and temperature. Results showed that the adsorption capacity increased with the increasing of original adsorbate concentration and pH value (<6). It needed about30min to reach equilibrium adsorption. Increasing original adsorbate concentration and pH, extending the adsorption time, and lowering the amount of adsorbent could improve the adsorption capacity. And the optimal condition for Cd2+was W06g·6g·L-1,C0200mg·L-1, pH5.0, T25℃; for Mn2+was W010g·L-1, Co100mg·L-1, pH5.0, T25℃. Equilibrium adsorption of Cd2+、Mn2+by natural raw diatomite followed the Langmuir model with adsorption of Cd2+5.44mg·g-1, Mn2+4.52mg·g-1. The adsorption process followed second-order kinetics.(2) The experiment took adsorption capacity as the standard to judge the modification effect of diatomite. Results showed that, the composited diatomite could obtained through treating the natural raw diatomite with hot10%H2SO4for2h and followed with hot6%NaCl for30min, and then using the pillaring agent which was made under OH/A1molar ratio2.2to pillared the diatomite at Al/diatomite10mmol/g at60℃for2d, and sequentially treating with calcinations at400℃for2h. After modification, the adsorption capacity of Cd2+、Mn2+increased from5.10mg·g-1、4.11mg·g-1to9.59mg·g-1、6.36mg·g-1.(3) The result of XRD, SEM, and FTIR showed that the impurities which exist on the raw diatomite surface and channel were removed when the diatomite was modified by composite process. The modified diatomite had higher surface roughness, specific surface area, and more micropores. It showed a great amount of Si-OH group, larger distance of interlayers and stronger electronegativity. Cation exchange capacity increased from27.78mmo·100g-1to38.56mmol·100g-1. All of these improved the adsorption capacity of modified diatomite.(4) The adsorption isotherms model and kinetics model of Cd2+, Mn2+on composite modification diatomite were studied. Results showed that Cd2+and Mn2+ions were easily adsorbed by modified diatomite. Equilibrium adsorption still followed the Langmuir model with adsorption of Cd2+9.60mg·g-1, Mn2+6.49mg·g-1which were76.47%and43.59%, respectively, higher than that of natural raw diatomite. The adsorption process also followed second-order kinetics. |
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