Microwave Modification And Regeneration Of Activated Carbon In Wastewater Treatment

Chlorophenols are mainly produced in chemical industries, such as pesticide,petroleum refineries, plastics and wood preservation. Chlorophenols and related compoundsare carcinogenic, mutagenic and resistant to biodegradation, so their discharge leads to thecontamination of soils, surface and ground water and living organisms. Many efforts havebeen made for the treatment of chlorophenol-rich wastewater, the adsorption process givesthe best results as it can be used to remove different types of them. Takes the advantages ofgood opening structure and huge surface area, activated carbon (AC) has excellentadsorption capacity and becomes the most common adsorption materials. In order to getdifferent adsorption characteristic, AC will be modified by means of different physical andchemical processes to change its pore structure and surface chemistry. Microwave treatmentis one of the most popular method for AC modification, at the same time, as microwaveheating takes advantages of instantaneous heating, energy and time consuming, a lot ofresearches have been carried out regarding the application of microwave in AC regenerationboth at home and abroad.This dissertation focused on the modification of AC used for the treatment ofchlorophenois and its regeneration by using microwave technology. The adsorptionperformance of 2, 4-DCP onto microwave modified and regenerated activated carbon (AC)were investigated by comparing its adsorption ability with traditional AC modified orregenerated in other ways. Regularity for the change of the surface physicochemicalcharacters was studied with kinds of measurement thchnologies such as N₂ isothermadsorption, elemental analyzer, XPS, XRD, FTIR, SEM and Boehm titration. Theadsorptive ability of 2, 4-DCP on modified and regenerated ACs were characterized with 2,4-DCP isothermal adsorption. Combined with the above analysis of the surface chemistries,the effect mechanisms for the adsorption of 2, 4-DCP on modified and regenerated ACswere studied in details. At last, the activities of AC supported copper and iron catalysts for 2,4-dichlorophenol (2, 4-DCP) decomposition were studied in the presence of microwaveirradiation. Results show that: 1. After microwave modification, the adsorptive capacity of AC is improved, theamplitude is about 12.5%, and the adsorption isotherm model is changed from Langmuirequation to Freundlich equation after modification. Modified AC has little change in thesurface area, modest shrink on pore volume and a little change in pore size distribution,compared with the virgin AC. MW treatment of AC eliminate the oxygen-contained sufacefunctional groups, result in the decrease of surface oxygen content and the increase ofsurface basicity and hydrophobicity, as well, its surface C=O content and graphitizingincreases.2. The adsorption capacity of AC increases rapidly after the modification of nitrogendoping, the amplitude is about 22.70%, while oxidation treatment decreases its adsorptionability, especially when it was modified by nitric acid. With nitrogen doping treatment, thesurface area and pore volume of carbon increased, while with H₂O₂ treatment, the porestructure is somewhat improved, which suggests that the modest oxidizing corrosion openup the enclosed tiny hole. With high concentration HNO₃ treatment, the surface area andpore volume of carbon decreases, for the strong oxidation probably obstruct the entrance ofmicropore. After oxidation, surface oxygen content and acidity of AC increases, whilenitrogen doping treatment gives the contrary effects.3. Equilibrium and kinetic data of 2, 4-DCP adsorption onto nitrogen doping andmicrowave modified ACs gained from batch experiments were fitted to Freundlichequations and pseudo-second-order kinetic model respectively, and the results are verifiedto fit the adsorption data well. The thermodynamics constants of these two adsorptionprocess were estimated, which show that the adsorption of 2, 4-DCP is exothermic andspontaneous, although the chemisorption capability of ACs are enhanced after microwavemodification and nitrogen doping treatment, the adsorption studied here are assigned to aphysisorption mechanism.4. Adsorptive capacities of AC increases more or less after one adsorptionregenerationcycle, afterwards, the adsorptive capacity slowly decreases. This phenomenonnot only related to the textual change of regenerated AC but also affected by the variety ofits surface chemistry. The removal rate of 2, 4-DCP are enhanced with the increase ofmicrowave power and irradiation time. Most of them are removed by pyrolysis, only a little of them are removed through desorption.5. Microwave regeneration offers the advantage over conventional regeneration notonly for its time and energy consuming but also for its maintaining the porosity of AC. Theadsorptive capacities of microwave regenerated AC can maintain relatively high level afterseveral adsorption-regeneration cycles, whereas conventional heating can't give the soundresult because the porous structure blockage of regenerated AC, on the other hand, the massloss of AC is also higher during conventional heating regeneration.6. Elemental copper and iron are found highly dispersed on the surface of AC in theform of elment and oxide. AC-supported iron catalyst shows desirable activity in thedecomposition of 2, 4-DCP. Powder iron and its oxide are suggested to adsorb microwaveirradiation more intensively, so the temperature of catalyst bed rises more quickly, whilecopper brings the negative effect.