Performance And Mechanism Of The Oxidation Of Organic Pollutants By Bimetallic Fenton Reactions

As a common oxidant, H2O2is low-cost, safe and with no secondary pollution to the environment. However, the oxidation capability of H2O2is not such strong that it has to be activated in pratical applications. Hence, the researches have been focused on looking for an effective method to activate H2O2. The most simple method is to activate H2O2by ferrous ion, known as Fenton reaction system. Fenton reaction system has the advantages of being simple in both reagents and operation, but it also has the inherent defects, such as narrow pH range with the most effective performance at pH3.0, precipitation of ferric ion and formation of large amount of iron sludge. Accordingly, many studies have been focused on how to improve the performance of Fenton system, such as introducing light, electric or ultrasonic energy to the system, adding ligands into the system to avoid the precipitation of ferric ion, loading iron on carriers and using iron oxide as catalyst. In fact, a successful method for the improvement should be relatively simple, but can raise the production efficiency of HO and correspondingly improve the oxidation of organic compounds.In the thesis, the capabilities of Cu(II), Mn(II), Ce(III), Co(II) and Ni(II) to decompose H2O2and their influence on the decomposition of H2O2by Fe(II) and Fe(III) were studied first. The results show that these metal ions have no capability to decompose H2O2, but Cu(II), Mn(II) and Ce(III) impact on the decomposition of H2O2by both Fe(II) and Fe(III). Accordingly, these three metal ions influence the oxidation of organic compounds in the Fe(II)/H2O2and Fe(III)/H2O2system, and the degree of influence closely depends on the reaction conditions. Based on the above, three new bimetalic Fenton systems with better performance were established, Fe/Mn, Fe/Cu and Fe/Ce.In order to further investigate the oxidation performance and mechanism of Fe/Mn bimetallic Fenton system, benzoic acid (BA) wa s employed as the target compound. The effect of Mn(II) on Fenton system is mainly expressed during the reaction process of Fe(III) and H2O2. In Fe(III)/Mn(II)/H2O2system, oxidation of BA exhibits a slow phase and a following rapid phase, which is attributed to the catalytic effect of quinone-like substances formed during the reaction. The oxidation rate of BA increases with the initial concentration of H2O2and Mn(II), respectively. When the initial concentration of Mn(II) increases from0to300μM, the initial pseudo-order rate constant (initial kobs) increases three times, while the half-life of BA (t1/2) decreases by half. The experimental results of inhibition of hydroxyl radical and quantification of pHBA show that HO is still the main oxidative species in Fe(III)/Mn(II)/H2O2system. During the oxidation, the presence of Mn(II) promotes the formation of HO2/O2-by following reactions: Mn(II) reacts with HO to produce Mn(III) which is transformed to MnO2+by H2O2, and MnO2+rapidly disproportionates to Mn(II) and HO2/O2-. HO2/O2-can reduce Fe(III) directly or reduce benzoquinone to form reductive products to reduce Fe(III) indirectly, both of which leads to the improvement of the oxidation of BA.2,4,6-trichlorophenol (2,4,6-TCP) was selected as the target compound to investigate the oxidation performance and the mechanism of Fe/Cu bimetallic Fenton system which shows an obvious advantage at pH3.0. The removal efficiency of2,4,6-TCP by Fe(II)/Cu(II)/H2O2system is28.1%higher than Fe(II)/H2O2system after10min of the reaction under the initial concentration of5mM H2O2. The oxidation efficiency of Fe(II)/Cu(II)/H2O2system increases with the initial concentration of Cu(II). When2,4,6-TCP is completely oxidized, the dechlorination efficiency of Fe(II)/H2O2and Fe(II)/Cu(II)/H2O2system are83%and95%, respectively, which indicates that the chlorinated byproducts in later system are less. The results from the experiment of inhibition of hydroxyl radical and quantification of the formation of pHBA show that Fe(II)/Cu(II)/H2O2system still obeys the mechanism of HO. Cu(II) is beneficial to the system because it can react with HO2/O2-to form Cu(I) which shows both the good catalytic and reductive activity. At the initial fast phase of the oxidation of2,4,6-TCP by Fe(II)/Cu(II)/H2O2system, the promotive effect is due to the catalytical decomposition of H2O2by Cu(I) to produce HO and hence to accelerate the formation rate of HO. However, at the slow phase, the contribution of reduction of Fe(III) by Cu(I) is more significant that the instantaneous concentration of Fe(II) is always above5μM. Oxidation of atrazine by Cu(II)/hydroquinone/H2O2system and Cu(II)/reductants system demonstrates that Cu(I) can catalytically decompose H2O2to produce HO.Phenol was employed as the target compound to investigate the oxidation performance and mechanism of Fe/Ce bimetallic Fenton system. Similar with Mn(II), the enhancement of Ce(III) is mainly reflected in the reaction process of Fe(III) and H2O2. The oxidation efficiency of Fe(III)/Ce(III)/H2O2system increases with the initial concentration of H2O2at pH4.0, while it keeps almost constant with the initial concentration of Ce(III) varying in the range of20~200μM. Compared to the Fe(III)/H2O2system, Fe(III)/Ce(III)/H2O2system has a significant advantage in the oxidation of compounds which show the autocatalysis in Fenton system such as phenol, BA, para-hydroxylbenzoic acid (pHBA) and nitrobenzene (NB), while it has little advantage in the oxidation of atrazine. Ce(III) in the Fenton system, like the effect of Mn(II), also promotes the production of HO2/O2-by the partial capture of HO. Under the experimental conditions stated, the capture of HO by20μM and200μM Ce(III) are less than1%and8%, respectively, while the production rates of HO2/O2-correspondingly increase to7.5%and80%, respectively. During the oxidation of phenol in Fe(III)/Ce(III)/H2O2system, the maxium concentration of BQ significantly decreases and the formation rate of Fe(II) is accelerated, which indicates that the HO2/O2-has impact on the cycle of quinone-like substrates and the reduction of Fe(III). The mechanism of the enhancement caused by Ce(III) is similar with Mn(II). Based on the research of the three bimetallic Fenton system, multi-metal Fenton system were established, Fe(II)/Ce(III)/Cu(II)/H2O2and Fe(II)/Mn(II)/Cu(II)/H2O2system. Both of them have the advantages of the fast oxidation rate and wide effective range of pH. Fe(II)/Mn(II)/Cu(II)/H2O2system is still effective to oxidize phenol at pH4.4, while Fe(II)/Ce(III)/Cu(II)/H2O2system is effective even at pH5.0.