| Treatment of wastewater containing phenolic substances is of most concern in China’s water pollution control. Catechol is the main intermediate in degradation of phenolic pollutants, presenting higher toxicity than phenol. Fenton reaction is an effective method to treat most organics, but it has some intrinsic drawbacks which limit the widespread application in practice, such as highly acidic condition, large amount of H2O2 addition, and inhibition from chloride ion, etc. Heterogeneous Fenton with a representative of nano-scaled particles catalyzing hydrogen peroxide shows an technical advantage of overcoming some drawbacks of classical homogeneous Fenton, especially heterogeneous Fenton with iron-containing nano-particles as catalyst gaining extensive attention and research. This paper first studied Fenton treatment of tannery wastewater, a representative of real phenolic wastewater, with an aim to find the problems of Fenton practical application. Then, with an objective to address the above Fenton drawbacks, this paper studied the performance of classical homogeneous Fenton and heterogeneous Fenton catalyzed by nanoscaled iron oxides in catechol oxidation, explored the mechanisms about inhibition on Fenton reaction from acetic acid, and excellent performance of nanoscaled iron oxides catalyzing UV-Fenton. COD was used to show the efficiency of catechol oxidation, in order to quickly remove the interference of residual H2O2 on COD analysis, a new method was developed, which is the most simple and effective method to the best of our knowledge.A new method based on Na2SO3 reduction and O2 oxidation to eliminate the H2O2 interference on COD analysis was developed. Under the optimal operation conditions obtained from the experiment, it only took 30 min to remove the H2O2 interference on the COD analysis in a simple manner.Response surface methodology was proved to be a powerful tool to optimize the reaction conditions of Fenton in treatment of tannery wastewater, the obtained optimal conditions were: initial p H of 4.0, 14.00 mmol?L-1 H2O2, H2O2/Fe2+ molar ratio of 10.6:1, reaction time of 3 h, with the highest COD removal efficiency of 55.87%. Significant p H decrease was found in Fenton treatment of tannery wastewater, taking the reaction under initial p H of 7.0 as an example, the reaction p H quickly dropped to be 3.9. The relatively lower treatment efficiency was probably caused by inhibition on Fenton oxidation from high concentration chloride ion present in tannery wastewater.The main reasons of efficient catechol oxidation by Fenton under neutral condition were the complexation between Fe3+ and catechol and the drop of solution p H due to generation of acidic intermediates. The level of chloride ion affecting Fenton oxidation of catechol was dependent on Cl- concentration and initial p H value: adjusting initial p H to 6.0 could only weaken the adverse effect of low concentration Cl-, however, the inhibition effect of high concentration Cl- could be eliminated by UV-Fenton.Formic acid, acetic acid, and propionic acid were hard to oxidize by Fenton reaction, acetic acid showing the strongest resistance to Fenton. The difficulty of Fenton to oxidize these simple organic acids was caused by the absence of Fe2+ in the reaction solution and the subsequent end of HO? generation. The level of acetic acid affecting Fenton oxidation of catechol was determined by initial p H: the concentration of acetic acid causing inhibition was lower under acidic p H, and higher under neutral p H. The possible mechanism of acetic acid inhibiting Fenton reaction was as follows: in the presence of Fe2+ and acetic acid, HO? would oxidize Fe2+ with priority, causing the stop of Fe2+ regeneration and the following end of Fenton reaction; however, in the presence of Fe2+, acetic acid and catechol, HO? would attack catechol preferentially, in this case, regeneration of Fe2+ continued and Fenton reaction proceeded. When the coexisting acetic acid had a much higher concentration, Fenton reaction would still be inhibited.Nanoscaled iron oxides catalyzing UV-Fenton could overcome the drawbacks of classical homogeneous Fenton, showing a bright future as a technique for organic pollutant removal. This oxidation system could effectively oxidize catechol in the initial p H scale of 2.0~8.0, and required at least 20% less H2O2 than the theoretical amount. There were similar technical features among nano-Fe3O4 catalyzing UV-Fenton and nano-Fe2O3 catalyzing UV-Fenton, i.e., a wide range of initial p H and a low requirement of H2O2. Acidic intermediates including formic acid, acetic acid, oxalic acid, and maleic acid could decrease the reaction p H, which was the key reason of maintaining the high oxidation efficiency under neutral p H. Dioxygen gas could enhance the oxidation performance, oxygen displacing the role of partial H2O2 maybe the cause of H2O2 utilization efficiency higher than 1.0. The H2O2 utilization efficiency increased with the increase of initial p H, the optimal value occurred under the conditions as: 5.9 mmol?L-1 H2O2, 0.25 g?L-1 Fe3O4 and 0.125 g?L-1 Fe2O3, and UV intensity of 4 k W?m-3. The biggest value could reach as high as 1.8. The first-order kinetics could well describe catechol oxidation with regard to COD removal by nanoscaled iron oxides catalyzing UV-Fenton, the rate constant increased linearly with the increase of H2O2 dose and reaction temperature. Under the same conditions, the oxidation rate of nano-Fe3O4 catalyzing UV-Fenton was 1.5~2.0 times of the value in nano-Fe2O3 catalyzing UV-Fenton for catechol oxidation. Repeated use of the same batch of nanoparticles for six cycles in catechol oxidation, the COD removal efficiency would almost keep at the same high level, however, the H2O2 utilization efficiency would drop slightly. Nano-Fe3O4 and nano-Fe2O3 exhibited a good stability, and the dissolved iron amount was little.Under the respective optimal conditions, the activation energy of nano-Fe2O3 catalyzing UV-Fenton treating catechol was 45.1(±6.3) k J?mol-1, much higher than the Ea of 18.0(±1.18) k J?mol-1 in nano-Fe3O4 catalyzing UV-Fenton; hence, the former oxidizing system was more susceptible to be affected by temperature change, whereas the latter one could more strongly endure the adverse effect of low temperature. According to the size of activation energy, nano-Fe3O4 catalyzing UV-Fenton was a diffusion controlling process while nano-Fe2O3 catalyzing UV-Fenton was a surface controlling process. HO?, O21 and O2?- participated in the oxidation process of both nano-Fe3O4 catalyzing UV-Fenton and nano-Fe2O3 catalyzing UV-Fenton, the function mode of radicals was similar. HO? was the direct product, while O21 and O2?- were the secondary oxidants coming from HO? involved reactions. Most of pollutant removal was caused by oxidation of O21 and O2?-, rather than HO? oxidation. The contribution of O21 was bigger than the contribution of O2?-. |
PDF Full Text Request