Preparation, Modification And Adsorption To Cd²⁺ Of Activated Carbons

In recent year, with the rapidly industrial development, waste water containing heavy metals is indiscriminately discharged, leading to serious environment pollution. Therefore, it becomes a significant key which a practical high efficiency method to remove heavy metals is desired highly. In the present, many methods such as ion exchange method, membrane separation technology and adsorption method were widely reported to remove heavy metals. In which, the adsorption are more and more used in removing heavy metal ions from wastewater due to the renewable, cost-effective and high efficiency. Activated carbon?AC? has been proven to be an effective adsorbent in the treatment of wastewaters due to its high surface areas and rich surface functional groups.In this article, based on abundant coal resources and agricultural forestry wastes from Xinjiang, coal-based activated carbons and cotton-based activated carbons were prepared by chemical activation method and treated by HNO₃, H₂O₂ and plasma. The surface physical and chemical properties of the activated carbons were determined by infrared spectroscopy, boehm titration, nitrogen adsorption desorption curves. These activated carbons were used to adsorb Cd²⁺. Effects of the initial concentration, adsorption time and p H value on the adsorption of Cd²⁺ on the modified activated carbon were investigated, and their adsorption mechanism was discussed. The main contents are as follows:?1?Using Kuche coal as raw material and KOH as active agent, coal based activated carbons were prepared by the chemical activation under the condition of the lower alkali/carbon ratio?1:2?. The adsorption capacity of Cd²⁺ for coal based activated carbons was only 6.8 mg/g. However, after the modification by nitric acid, hydrogen peroxide and plasma, the adsorption capacity of Cd²⁺ significantly increased. It was found that the modifid effect in 90%?V / V? of nitric acid is the best. Its maximum adsorption amount reaches 88mg/g, which is 13 times of unmodified activated carbons. Based on the adsorption properties of modified activated carbons, the optimum adsorption conditions were Cd²⁺ concentration of 100mg/L, the equilibrium time of 1h and the pH value of 6. Moreover, it was found that the adsorption process of Cd²⁺ on modified activated carbon was in accordance with the quasi two order kinetics and Langumir model.?2?Shawan cotton stalk as raw material, phosphoric acid as active agent, the cotton stalk based activated carbons were prepared by microwave method, which is fast and efficient. The preparation process of cotton stalk based activated carbons was optimized by tuning the activation temperature, phosphoric acid/raw ratio and calcination time. the adsorption capacity of the cotton stalk based activated carbons reach 28mg/g under the optimum condition of phosphoric acid/raw ratio of 1:1 and calcination for 10 min at 250?.?3?Cotton stalk based activated carbons were modified by nitric acid, hydrogen peroxide and plasma, respectively. Their adsorption properties of Cd²⁺ were improved in different degree. The modification methods by hydrogen peroxide and plasma were studied in detail. Based on the results of the modification by hydrogen peroxide, it is concluded that the optimal conditions for the Cd²⁺ adsorption were Cd²⁺ concentration of 80 mg / L, equilibrium adsorption time of 1h and solution p H of 6. The adsorption capacity is up to 58mg/g, which is 2 time of unmodified activated carbons. Studies of thermodynamics and kinetics indicate that the quasi two stage kinetics and Langumir model can better describe the adsorption process of activated carbons modified by hydrogen peroxide. In addition, after the modification by plasma treatment time for 3min activated carbons can achieve the best adsorption effect. The maximum adsorption amount of activated carbons by the plasma treatment is up to 50mg/g under the optimal conditions of 80mg/L Cd²⁺ concentration, the equilibrium adsorption time of 2h and p H of 6. Moreover, their adsorption processes match well with quasi second order kinetics and Freundlich model. The above results indicate that the two methods have their own advantages. Compared with the traditional chemical modification, the method of plasma is simple, high efficiency and non-pollution, which is an economic and environmental friendly new technology.