Study On The Preparation Of Bamboo-Based Activated Carbon And Enhancement Of Wet Oxidation For The Treatment Of Phenol-Containing Wastewater Using Microwave Method

The discharge of organic wastewater is an important reason for the pollution of water bodies, and its treatment is now given much attention. Phenolic compounds are a kind of typical organic pollutants, the phenol-containing wastewater is characterised of highly toxic, high concentration and large discharge amount, resulting in an urgent need for the economical and effective treatment technique. Of those processes developed for this purpose, activated carbon adsorption has the advantages of high efficiency and simple operation, however, this method still has the problems of high cost of production and difficult regeneration of the exhausted activated carbon. This study is carried out to resolve these problems. A kind of bamboo-based activated carbon with high specific surface area has been prepared by microwave radiation process, making possible the clean and low-cost production of activated carbon; with 4-chlorophenol as the target pollutant, an activated carbon adsorption-microwave enhanced wet oxidation process has been established; the adsorption behavior and mechanism of phenolic compounds onto activated carbons have also been investigated.In this study, a microwave radiation process for the preparation of activated carbon has been developed, using bamboo, a kind of renewable resource, as the raw material, and phosphoric acid as the activation agent. The effects of various factors including microwave radiation power, microwave radiation time, phosphoric acid amount and impregnation time on the preparation were investigated. The optimal process conditions were determined as: microwave power 350W, microwave radiation time 20min, phosphoric acid to bamboo ratio 1:1, impregnation time 24h. Under the optimal conditions microporous activated carbon with a specific surface area of 1335m²/g and a carbon yield of 45% could be achieved. The surface properties of the activated carbon were characterized by various methods, and the results showed that there existed a large amount of acidic goups on the surface, resulting in a strong acidic nature. During the activation process, most of the chemical bonds in the bamboo material were destroyed. The contents of oxygen and nitrogen decreased and the content of carbon slightly increased, while a small amount of phosphorus was introduced. The prepared bamboo-based activated carbon was thermally treated under microwave radiation, and it was found that the micropores were lost with the extension of time, but the mesopores were further developed to some extent. After microwave thermal treatment the amount of acidic groups on carbon surface decreased, leading to an enhanced alkalinity in nature. The adsorption performance of the prepared bamboo-based activated carbon was then evaluated, and it was found that its adsorption ability for phenol was 105mg/L, comparable to a commercial activated carbon.In this study, an activated carbon adsorption–microwave enhanced wet oxidation process for the treatment of 4-chlorophenol wastewater has been established, using granular activated carbon and activated carbon fibers as the adsorbents. With this process, the efficient degradation of 4-chlorophenol and regeneration of activated carbon were achieved. The effects of vaious factors including the amount of activated carbon, adsorption time, pH, temperature and inorganic salt on the adsorption of 4-CP on activated carbon have been investigated. In the study of microwave enhanced wet oxidation regenearation process, the effects of various factors including system pressure, peroxide dosage, reaction time and and addition of catalysts on the regeneration efficiency have been investigated. The optimal process conditions were determined as: system pressure 0.5MPa, peroxide dosage 10mmol, reaction time 20min, 0.15mmol/L Cu²⁺ was used as catalyst for the regeneration of granular activated carbon, 0.10mmol/L Fe³⁺ and 0.05mmol/L Cu²⁺ were used as catalyst for the regeneration of activated carbon fibers, under the optimal conditions a regeneration efficiency of about 95% could be achieved. Analysis of the effluent from the microwave enhanced wet oxidation process showed that the concentration of the residual 4-CP was only 0.4mg/L. The main degradation products monitored were found to be formic acid and acetic acid, and the biodegradability was significantly improved. By comparison, the degradation efficiency of 4-chlorophenol using the combined process of activated carbon adsorption-microwave enhanced wet oxidation was notably higher than the homogeneous wet oxidation process, indicating that the reactions occurred at the liquid-solid interface played an important role for the regeneration of activated carbon, and the activated carbon also acted as catalyst in the wet oxidation. The effects of microwave enhanced wet oxidation process for the regeneration of activated carbon was obviously superior to conventional wet oxidation process, implying that the coupling between microwave and active carbon strengthened the ablity for the degration of organic matters. At high temperature, the interaction between 4-CP and the activated carbon was weakened and the desorption rate of 4-CP was accelerated, which facilitated its oxidative degradation. After repeated microwave wet oxidation regenerations, the adsorption performance of activated carbon declined to some extent, but remained in relatively high level. Further analysis revealed that the decline in the adsorption performance after wet oxidation regeneration derived mainly from the accumulation of degradation products. The activated carbon after repeated regenerations has been activated again under microwave radiation, and the results proved that after the treatment the activated carbon largely restored its adsorption ability.In this study the adsorption behavior and mechanism of phenolic compounds onto granular activated carbon and activated carbon fibers have been studied. The adsorption isotherm, kinetics and thermodynamics have been fitted and analyzed. The fitting of adsorption isotherm indicated that Redlich-Peterson model gave the best result. The pseudo-second-order kinetic model fitted the adsorption kinetics well, while the fitting of intraparticle diffusion model demonstrated that the diffusion process was influenced by the porous structure and the adsorbed organics. Thermodynamic analysis showed that theΔG°value was negative, whose absolute value increased with the increase of substitute number, indicating an enhancement in the adsorption interactons; theΔH°was also negative, whose absolute value decreased with the increase of substitute number, suggesting an enhancement in the physical adsorption; the value ofΔS°increased with the increase of substitute groups, suggesting that it became more disordered at the liquid-solid interface. The adsorption mechanisms have also been explored. TheÏ€-Ï€interactions between activated carbon and phenols were of great importance, other factors including the solvent effects, the hydrophobicity of adsorbates and the molecular size also made some contributions. The adsorption dynamics and isotherm analysis indicated that the steric effects on adsorption kinetics were primarily related with the largest molecular dimension of the adsorbate, and the steric effects on adsorption equilibrium were primarily related with the second molecular dimension of the adsorbate.