Modified Activated Carbons Remove Sulfadiazine From The Aqueous Phase

Because of their structural stability and persistence in theenvironment,sulfonamides can easily stay in the environment for a long time, and beharmful to human health. The biotoxicity of sulfonamides results in low removalrate of it by using biological method. The most common way amongphysico-chemical methods is the adsorption method. The activated carbon （AC） iscommonly used as the adsorbent because of its vast specific surface area. Becausethe activated carbon is of low-polarity and sulfonamides belongs to polar substance,the rate of removal might be low to remove the sulfonamides using the activatedcarbon as the adsorbent. The polarity of the activated carbon can be increased afteroxidation. In this study, the oxidant and oxidation technology of modifying theactivated carbon were chosen and optimized. The AC after modification wascharacterized. The study also investigated regeneration of the modified activatedcarbon （MAC） which came to saturation adsorption, and the adsorption capacity ofthe MAC towards four kinds of sulfonamides. The operation function of fixed-bedreactor with MAC inside was also studied.In this research, activated carbons were modified by concentratedH2SO4,concentrated HNO₃,30%H₂O₂and KMnO4solution to improve theirsulfonamides adsorption capacity from aqueous solution. Microwave-assistedoxidation, impregnated oxidation, reflux oxidation and ultrasound-assistedoxidation were applied to modify AC to search for the most efficient way toimprove their adsorption performance. The H2SO4oxidant and reflux oxidation wasregarded as the most suitable method by the standards of adsorption quantity ofsulfonamides and feasibility of large-scale industrial production.100℃,18mol/Lconcentrated sulfuric acid and refluxing of4hours was the best modificationmethod. The major principle of MAC oxidation is that surface oxygen-containinggroups of activated carbons are increased.In this research, MAC after H2SO4refluxing were used to adsorbsulfonamidesin the water phase and the adsorption behaviour of MAC was studieddeeply. An increase in the temperature resulted in a rise in sulfadiazine adsorptionrate, so the adsorption process was proved to be the endothermicprocess.Experimental data were in line with the pseudo second-order kinetics modeland the Freundlich isotherm equation.The calculation result of activation energy foradsorption was52.362KJ/mol, what indicated that chemisorption was predominantin the adsorption process.The research also studied mass transfer processes on the surface of sulfonamides and MAC. It was found that this process was controlled byintraparticle diffusion and film diffusion.After the adsorption experiment, sulfuricacid under microwave irradiation was used to regenerate MAC. The regenerationexperimental result suggested that the loss rate of MAC is not significant during theregeneration process. But the removal rate of these four kinds of sulfonamidesdecreased with the times of regeneration.In the final step, the fixed-bed reactor with MAC inside was used in thisresearch. The experimental data showed that the reactor with MAC had a72hbreakthrough time which was more than10times longer than the one with AC. Itmeant that the MAC had a better capacity and a higher rate of adsorbingsulfonamides. The sequence of penetration of these four kinds of sulfonamides inthe reactor wasSD, SM2, SMZ and SQ. At the penetration, the concentration ofsulfonamides could be decreased to the level less than the penetration level byreducing water inflow. But the penetration was formed again quickly. The fixed-bedreactor with regenerated MAC inside was also operated. The effect of regenerationwas good because the breakthrough time for four sulfonamides exceeded60h. Thesurface area and pore volume of MAC were reduced after regeneration.