Design Of Advanced Oxidation Systems With Molecular Oxygen Activation By Fe(Ⅱ)-TPP Complex And Their Performance On Organic Pollutants Degradation

With the development of the industry, a large of organic pollutants get into the water, causing serious enviromental pollution. The development of green oxidation technology is vital to eliminate the organic pollutants without secondary pollution. The typical feature of green oxidation technology is the utilization of green oxidants and environmental friendly catalysts to generate·OH, which could oxidize the pollutants, even mineralized the pollutants to CO2, H2O and salt. Molecular oxygen, accounting for about 21% of air, is the most green and economic oxidant. However, molecular oxygen could not oxidize organic pollutants at room temperature and pressure because the reaction between mocular oxygen and organic pollutants is spin-forbidden. It is therefore of great significance to develop efficient, low cost and eco-friendly methods to activate dioxygen for the aerobic degradation of toxic organic pollutants with using environmental friendly and inexpensive reagents.The utilization of low valent iron-containing materials for pollutant remediation has attracted extensive attention in view of its abundance in earth and environmental compatibility. Recent studies showed that Fe2+ and nanoscale zero valent iron (nZVI) could react with molecular oxygen to produce reactive oxygen species (ROS) to oxidize organics. Unfortunately, low yield of ROS in the Fe2+ /Air and nZVI/Air system restricts its application. It is known that the addition of organic ligands could enhance oxidant yield during the reaction of Fe2+ or nZVI with oxygen, but the utilization of organic additives would not only increase the expense, but also bring on the undesirable consequences of reactive oxygen species consumption and molecular oxygen activation efficiency decline caused by the simultaneous decomposition of organic additives along with target pollutants. Therefore, inorganic ligands might be more attractive to enhance ROS production and promote the degradation of target pollutants in the Fe2+/Air or nZVI/Air system in view of their high stability.This dissertation mainly focused on the development of advanced oxidation systems based on molecular oxygen activation induced by Fe(Ⅱ)-tetrapolyphosphate complex, and thus investigated their perforance on typical organic pollutants (dyes, atrazine, chlorophenol). The route of molecular oxygen activation, the enhancement mechanism of molecular oxygen activation efficiency and the pathway of pollutants degradation were studied systematically. The detailed results were shown as the following.1. We demonstrated that a readily available and nontoxic inorganic ligand tetrapolyphosphate (TPP) could form a complex with low-cost ferrous ions to activate molecular oxygen to produce reactive oxygen species for highly efficient aerobic degradation of toxic organic pollutants at room temperature and pressure. CV study revealed TPP could significantly reduce the redox potential of Fe(Ⅲ)/Fe(Ⅱ), beneficial for the activation of molecular oxygen. The formation of·O2-, H2O2 and·OH confirmed that molecular oxygen was activated via a one-electron transfer process. The results of scavenging experiments and degradation intermediates detection suggested that the predominant oxidant for PCPNa degradation was·OH, which could even mineralized PCPNa. Fe(Ⅱ)-TPP was oxidized to Fe(Ⅲ)-TPP accompaning with the degradation of pollutants. The utilization of dissimilatory iron-reducing bacteria could effectively reduce Fe(Ⅲ)-TPP to Fe(Ⅱ)-TPP under anaerobic conditions and therefore regenerate the deactivated Fe(Ⅱ)/TPP/Air system.2. In this study, we comparatively investigated the degradation kinetics and mechanism of 4-chlorophenol (4-CP),2,4-dichlorophenol (2,4-DCP) and 2,4,6-trichlorophenol (2,4,6-TCP) in the Fe/TPP/Air system. It was found that the pseudo-first-order degradation rate constants gave the following sequence:2,4-DCP> 2,4,6-TCP> 4-CP, while the decline order of dechlorination was 2,4,6-TCP> 2,4-DCP> 4-CP. The discrepancy between the orders of degradation and dechlorination was attributed to the synergistic effect of·O2- and ·OH,as·O2- and ·OH produced in the Fe/TPP/Air system could react with chlorophenols via dechlorination and hydroxylation, respectively. GC-MS results showed that the dechlorinaiton intermediates disappeared when·O2- was trapped, confirming the indispensible dechlorination effect of ·O2-. It was found that the contribution of superoxide radical to the degradation, dechlorination and mineralization increased with the increasing of chlorine number.3. The effects of an inorganic ligand tetrapolyphosphate on the molecular oxygen activation and the subsequent aerobic atrazine degradation by Fe@Fe2O3 core-shell nanowires were investigated systematically at a circumneutral to alkaline pH range (pH 6.0-9.0). We interestingly found that the addition of tetrapolyphosphate could enhance the aerobic atrazine degradation rate by 955 times, which was even 10 times that of traditional organic ligand ethylenediamine tetraacetate. This tetrapolyphosphate induced dramatic aerobic atrazine degradation enhancement could be attributed to two factors. One was that the presence of tetrapolyphosphate strongly suppressed hydrogen evolution from the reduction of proton by Fe@Fe2O3 core-shell nanowires through proton confinement, leaving over more electrons for the reduction of Fe(Ⅲ) to Fe(Ⅱ) and the subsequent molecular oxygen activation. The other was that the complexation of tetrapolyphosphate with ferrous ions not only guaranteed enough soluble Fe(Ⅱ) for Fenton reaction, but also provided another route to produce more-OH in the solution via the single-electron molecular oxygen reduction pathway. We employed gas chromatography mass spectrometry and liquid chromatography-mass spectrometry to identify the degradation intermediates of atrazine and proposed a possible aerobic atrazine degradation pathway.4. A novel E-Fenton system was developed with iron wire, activated carbon fiber, and sodium tetrapolyphosphate (Na6TPP) as the anode, the cathode, and the electrolyte, respectively. This Na6TPP-E-Fenton system could efficiently degrade atrazine in a wide pH range of 4.0-10.2. The utilization of Na6TPP instead of Na2SO4 as the electrolyte enhanced the atrazine degradation rate by 130 times at an initial pH of 8.0. This dramatic enhancement was attributed to the formation of ferrous-tetrapolyphosphate (Fe(II)-TPP) complex from the electrochemical and chemical corrosion of iron electrode in the presence of Na6TPP. The Fe(Ⅱ)-TPP complex could provide additional molecular oxygen activation pathway to produce more H2O2 and ·OH via a series single-electron transfer process, producing the Fe(Ⅲ)-TPP complex. The cycle of Fe(Ⅱ)/Fe(Ⅲ) was easily realized through the electrochemical reduction process on the cathode. More interestingly, we found that the presence of Na6TPP could prevent the iron electrode from excessive corrosion via phosphorization in the later stage of Na6TPP-E-Fenton process, avoiding the generation of iron sludge. Gas chromatograph-mass spectrometry, liquid chromatography-mass spectrometry, and ion chromatography were used to investigate the degradation intermediates to propose a possible atrazine oxidation pathway in the Na6TPP-E-Fenton system.