| Activated carbons are widely used in chemical products separation, water supply purification and wastewater treatment. However, the high costs of adsorption using activated carbons as adsorbent restrict their extensive application. The main factors affecting adsorption operating costs include adsorption rate and capacity as well as regeneration efficiency and costs. Therefore, improving adsorption rate and capacity, enhancing regeneration efficiency and reducing regeneration costs are the effective ways to reduce the adsorption operating costs, which is the focus in this research area. An effective and economic electrochemical polarization method is used to enhance adsorption rate and capacity, which can be combined with in situ electrochemical regeneration.The adsorption rate and capacity of phenol, aniline, m-cresol and acid orange 7 (AO7) on activated carbon fibers (ACFs) at potential polarization are investigated in this study. The electrosorption rate equation is developed and verified on the basis of Lagergren adsorption rate law. The mechanism of electrochemically enhanced adsorption is discussed, and the feasibility of the combination of electrosorption and in situ electrochemical regeneration cycles is explored.The results indicate that electrochemical polarization can effectively accelerate the adsorption of phenol, aniline, m-cresol and AO7 on ACFs. The adsorption kinetics at different polarization potentials follow the Lagergren adsorption rate law. Compared with the OC condition, the adsorption rate of phenol in basic solution at electrochemical polarization of 700 mV increases by 71%. With electrosorption at bias potential of 600 mV, 33%, 77% and 120% enhancement of adsorption rate constant are achievable for aniline, m-cresol and AO7, respectively. The enhancements of adsorption rate at bias potential supplied may be caused by the affinity and adsorption sites on the surface of ACFs, enhanced by the polarization under electric field. In order to quantitatively describe the contribution of polarization potential to adsorption rate, electrosorption rate equation is developed on the basis of Lagergrenadsorption rate law. The prediction results are in good agreement with experimental data for the electrosorption of organic substances (phenol, aniline and m-cresol).The adsorption capacity of phenol, aniline, m-cresol and AO7 on ACFs can also be effectively enhanced by electrochemical polarization. The electrosorption isotherms are in agreement with two classical models of Langmuir and Freundlich, but the former shows more satisfactory results. Compared with the adsorption capacity of phenol in basic solution at OC, the one at electrochemical polarization of 700 mV increased by tenfold. With electrosorption at bias potential of 600 mV, 2.04, 0.61 and 1.15 folds enhancement of adsorption capacity are achievable for aniline, m-cresol and AO7, respectively. The enhancement of adsorption capacity are not due to direct electrochemical degradation or secondary reactions caused by active oxidizing free radicals such as hydroxyl free radicals produced by electrochemical water splitting, but might mainly be due to the increase of adsorption interaction forces, change of adsorption junction state and the improvement of surface properties of ACFs under electric field.Different groups substitution can affect the enhancement of electrosorption. The adsorption of benzene with electron-donating group is obviously enhanced by anodic polarization. Anodic and cathodic polarization have less effect on the adsorption of electron-accepting aromatic compounds, but they both accelerate the adsorption rate of benzene bearing donor-conjugate bridge-acceptor.The results for the combination of electrosorption and in situ electrochemical regeneration suggest that the integrative electrosorption/regeneration cycle process can be realized. The regeneration efficiency can reach more than 90% when the regeneration voltage is -5 V. It hardly changes after 3 consecutive electrosorption/regeneration cycles, and is still more than 70%. This electrosorption/regeneration cycles process may be operated in the same reactor, which doses not need to take down, install and transport ACFs to the special factory. The maximum current during the regeneration course is only 0.08 A, indicating that the energy consumption is very low.In summary, electrochemical polarization can effectively enhance the adsorption rate and capacity of organic pollutants on ACFs, which may be due to the enhancement of adsorptive affinity and adsorption sites on the surface of ACFs under electric field. The results, which are predicted by electrosorption rate equation, are in good agreement with experimental data. The integrative electrosorption/regeneration cycle process with low costs can be realized, whichmay be promising processes for the purifications of industrial effluents and supplying water.
|
PDF Full Text Request