Abstract：Heavy metal contamination has become one of the most serious environmental problems. Among these elements, cadmium (Cd) is regarded as one of the major priority pollutants in drinking water due to its high biotoxicity, non-biodegradability and persistence in the whole environment. It has hazardous effects on ecological environment and human health. The major sources of cadmium release into the environment are waste water from electroplating factories, mining activities, chemical manufacturing processes and refining processes. It is of great social significance and economic benefits to study how to remove cadmium in water/wastewater friendly and efficiently. The commonly traditional methods used for the removal of cadmium from waste water include chemical precipitation, cementation, membrane separation, ion exchange, solvent extraction, adsorption process and the like processes. Among these methods, adsorption process is generally preferred and widely used because of its high efficiency, low cost, simplicity and availability. Activated carbons are well-known adsorbents extensively used for effluent treatment in many industrial processes. Activated carbon fiber (ACF) that has the advantages of uniform micropore structure, well-developed functional groups and good adsorption properties, is a new type of activated carbon, and has been gradually applied in water/wastewater purification systems. In this study, a comparative adsorption analysis of three activated carbon fibers (net, none-woven fabric and felt) was carried out and characterized with inductively coupled plasma optical emission spectrometry (ICP-OES) as the analysis method. Several characterization techniques (specific surface area analysis, X-ray diffraction, Fourier transform infrared spectroscopy and elemental analysis) were also employed to identify the structures of three activated carbon fibers. The structures of three fibers were similar with well-developed micropore structure according to X-ray diffraction and specific surface area analysis. The activated carbon none-woven fabric fiber has the strongest polarity and the highest oxygen content in the three ACFs. And it has a lot of oxygen-containing functional groups on the surface such as hydroxyls, carboxyls and aldehydes. Besides, the adsorption of cadmium on activated carbon none-woven fabric fiber obtained the most satisfied result with 97% removal efficiency. So that none-woven fabric was chosen as the target adsorbent in the subsequent experiments. The adsorption property of cadmium ions on none-woven fabric and its influential factors such as initial solution pH value and contact time between cadmium ions and the adsorbent were examined and optimized in the next procedure. The removal efficiency and adsorption capacity vary with pH value because pH affects not only the surface charge characteristics of activated carbon fiber but also the chemical forms of cadmium in the aqueous solution. The adsorbent charge turned into negative with an increase of pH value, which is favorable for removal of cadmium due to the emerging electrostatic attractions between cadmium ions and the oxygen-containing functional groups. The removal efficiency of cadmium increased with the increase of the initial pH of the solution. Cadmium removal efficiency by none-woven fabric increased significantly in the pH range of 1～6, and slightly from pH 6 to 9. Cadmium removal efficiency from 98.04% to 99.81% was obtained from pH 6～9. At lower pH, there was electrostatic repulsion between adsorbent and cadmium ions and competitive adsorption between H+ and Cd2+. The maximum uptake value was achieved at pH>6, which might be attributed to the presence of lone pair of electrons on oxygen atoms that are beneficial to coordinate with cadmium ions to give the corresponding complex compounds. At pH>9, the removal of cadmium was the synergistic effect of adsorption on adsorbent, complex formation and precipitation formation. Based on pH influence study, the pH value of solution was adjusted to 6～7 in subsequent adsorption experiments. In the initial adsorption stage, cadmium adsorption ratio increased rapidly with the increased contact time, and 72% of cadmium was removed in the first 10 min. Then the adsorption rate lowered down with the adsorption sites of none-woven fabric filled with cadmium ions. And finally, adsorption efficiency was up to a constant after 300 min, and the adsorption capacity reached a dynamic equilibrium. Further, any increase of contact time did not show any considerable changes in percent removal of cadmium. After the adsorption conditions were optimized, the isothermal adsorption experiment and kinetic experiment of cadmium were carried out subsequently. The results showed that the saturated adsorption capacity of cadmium on none-woven fabric fiber were 3.04 mg·g-1 and 0.035 mg·m-2 when the equilibrium concentration of Cd2+ was 20.0 mg·L-1, pH was 6.0, and the adsorption time was 300 min at 25 ℃. As the concentration of cadmium in the solution continued to increase, the adsorption amount tended to be a dynamic balance. The isothermal adsorption data was simulated by Langmuir model and Freundlich model. The main assumption of Langmuir isotherm is the monolayer formation of the solute on the surface of adsorbent without interaction between the solute molecules. The Freundlich isotherm is most commonly used to explain adsorption on a surface having heterogeneous energy distribution. In Langmuir model, the linear correlation coefficient was 0.997, and Langmuir factor was 1.796 L·mg-1. In Freundlich model, the linear correlation coefficient was 0.895, and Freundlich factor was 0.918 L·mg-1, and n was 2.12. The linear correlation coefficient of Langmuir model was much higher than that of Freundlich model. The adsorption capacity calculated according to the Langmuir model was 3.07 mg·g-1, which was just approximated to the experimental data of 3.04 mg·g-1. It indicated that the adsorption system conformed to the Langmuir equation better, and it dominated that adsorption on none-woven fabric was based on monolayer adsorption. The value of Langmuir separating factor was estimated for the entire concentrations range and it was calculated between 0 and 1, confirming favorable cadmium adsorption condition. In order to investigate the adsorption processes of cadmium on none-woven fabric, the kinetic data were fitted by four dynamic models, which were pseudo-first-order kinetic equation, pseudo-second-order kinetic equation, intra-particle diffusion equation and Elovich equation. In the first 5 min of the kinetic process, the adsorption capacity of cadmium accorded with intra-particle diffusion equation fairly since the linear correlation coefficient was calculated 0.985. It indicated that the diffusing rate in the interior surface layer of the particle was the control?step in the first 5 min. However, in the 5～300 min, the adsorption kinetic data could not accord with that equation at all. The whole adsorption process of cadmium on activated carbon none-woven fabric fiber accorded with the pseudo-second-order kinetic equation approximately with linear correlation coefficient 0.999 and reaction rate constant 0.367 g·mg-1·min-1. And the adsorption capacity obtained by calculation of pseudo-second-order kinetic equation was just approximated to that obtained by experiment. The whole adsorption process accorded with the Elovich equation and the pseudo-first-order kinetic equation relatively lower. In the Elovich equation, the linear correlation coefficient was 0.981, and Elovich factors were 0.271 mg·g-1 and 0.083 mg·g-1(lg min)-1. In the pseudo-first-order kinetic equation, the linear correlation coefficient was 0.927, and reaction rate constant was calculated to be 0.008 8 min-1. And the adsorption capacity obtained by calculation of pseudo-first-order kinetic equation differed from the experimental data. So that adsorption of cadmium on ACF was based on chemical reactions, such as electrostatic interaction and hydrogen bond. In removal of heavy metals by none-woven fabric, taking a 5.0 mg·L-1 cadmium contained synthetic wastewater sample for example, the cadmium content was less than 0.10 mg·L-1 after adsorption by none-woven fabric fiber, and it met Integrated Waste Water Discharge Standard (GB 8978—1996). Besides cadmium, heavy metals such as copper, lead and chromium could be adsorbed by none-woven fabric with removal efficiency higher than 95%. None-woven fabric had poor selection of cadmium adsorption in water with variety of heavy metals. When it met electroplating waste water and mining waste water, various heavy metals could be removed with more adsorbent added in adsorption treatment. The results showed that it was suitable, simple and effective in treating water containing cadmium by activated carbon fiber due to its good effect and convenient operation. And this study provided technical assistance and theoretical support in real waste water treatment.
张辰凌，韩 梅，贾 娜，刘冰冰，刘 佳，王振兴，刘玲霞. ICP-OES研究活性炭无纺布对镉的吸附[J]. 光谱学与光谱分析, 2019, 39(03): 931-936.
ZHANG Chen-ling, HAN Mei, JIA Na, LIU Bing-bing, LIU Jia, WANG Zhen-xing, LIU Ling-xia. A Study on Adsorption of Cd on Activated Carbon Fiber by Using ICP-OES. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2019, 39(03): 931-936.
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