Removal And Control Of Phthalic Acid Esters From Domestic Wastewater

As the ever-accelerated of urbanization and industrialization, sewage collected by municipal drainage system not only contains numerous normal pollutants, but also includes various environmental trace pollutants. However, the conventional wastewater treatment processes are not designed to remove recalcitrant emerging contaminants, such as Endocrine Disrupting Chemicals (EDCs). This increases the load and difficulty of sewage treatment plants as well as threats the safety and health of environment and human.This research selected six kinds of phthalic acid esters (PAEs), which were considered to be EDCs as targets to study their degradation using advanced oxidation process (AOPs). Nano-α-Fe2O3 catalyst was synthesized using hydrothermal method which was combined with UV and Fenton reaction to study the degradation efficiency and influence factors of DMP、 DEP、DBP、BBP、DEHP、DnOP, and to investigate the optimal degradation conditions. Their application for treatment of real urban wastewater effluent was also studied. After analyzing the mechanism of degradation process and the intermediate products, the degradation pathway of DEP was investigated. The main research content and results include:(1) Manipulation methods of solid phase extraction (SPE) aiming to pretreat the real domestic wastewater and liquid chromatography-tandem mass spectrometry (LC-MS-MS) which investigated the concentration of PAEs were established in the present work. Gas chromatography-mass spectrometry (GC-MS) was used to investigate the intermediate products of DEP.(2) Based on our previous research, pure phase of nano-α-Fe2O3 crystal was prepared with uniform size (about 80nm) and sphere or sphere-like shape by the hydrothermal synthesis method. α-Fe2O3-AC composites were prepared by sintering method. Catalyst loading on activated carbon could avoid many drawbacks which existed in powder catalyst system. For example, the mutual shading phenomenon of powder catalyst which could result in the decrease of light utilization efficiency, could be avoided by using a-Fe2O3-AC composites. It would also be more easy to reuse catalyst and reduce the loss of catalyst powder.(3) The removal efficiency can be effectively enhanced by application of nano-a-Fe2O3. The influence factors of degradation of phenol simulation wastewater by heterogeneous photo-Fenton catalysis using homemade photo-catalytic reactor were investigated. When the amount of H2O2、Fe2+ or nano-α-Fe2O3 increased as appropriate, degradation efficiencies of phenol was improved. While, excessive dose led to decrease or stagnation of removal efficiency of phenol. Lower dose of phenol simulation wastewater did not always get higher degradation efficiency. Furthermore, as the increase of pH value, removal efficiency reduced. The result showed that with the optimum conditions:pH value was 3.0, initial concentration of H2O2 and Fe2+ were 20mM and 0.5mM, respectively, the dosage of α-Fe2O3 or α-Fe2O3-AC were 10mg·L-1 or 10g·L-1, respectively, the removal efficiency of phenol could reach to 98%.(4) The photocatalytic degradation efficiencies and degradation mechanisms of six selected PAEs are different. The results of degradation of PAEs simulation wastewater by UV irradiation alone, H2O2 oxidation alone, and Fenton reaction alone were unsatisfactory. The removal efficiency increased dramatically when combining UV and Fenton reaction together, and it was also improved significantly by adding nano-α-Fe2O3 as catalyst which surpassed 94%, while it was not helped a lot by adding AC. However, the removal efficiency reduced a bit in UV/Fenton/α-Fe2O3-AC system compared with UV/Fenton/α-Fe2O3 system. The degradation of PAEs in real urban wastewater was not good as that in simulation wastewater because that it was influenced by various ions or other pollutants existing in real urban wastewater. We also discovered the degradation mechanism of different PAEs. Different PAEs compounds which were relatively simple and with short side chain were relatively easy to be degraded, on the contrary, complex PAE owning long side chain were difficult to be degraded. The degradation mechanism on the surface of α-Fe2O3 was photocatalysis which rely on the photogenerated electron hole interacting with hydroxyl groups to generate OH. Besides, photogenerated electron reacted with dissolved oxygen (O2) to produce reactive oxygen species (O2-) which had similar strong oxidizability as OH. While, in UV/Fenton/α-Fe2O3 system, there were two kinds of degradation mechanism:photocatalysis and heterogeneous photo-Fenton catalysis. The mechanism of coupling nano-α-Fe2O3 photo-Fenton catalysis and adsorption of AC was established in UV/Fenton/α-Fe2O3-AC system to improve the degradation efficiency of PAEs.(5) Under the optimum conditions of UV/Fenton/α-Fe2O3 system, the degradation factor, intermediates and degradation pathway of DEP were investigated. The results showed that OH was the degradation factor of DEP and its intermediate products included ethyl benzoate and dimethyl phthalate. Degradation of DEP mainly began on its side chains and was through two pathways:the carbon atoms on the end of two side chains were attached by OH, which led to the rupture of C-C bonds; the other one was that OH attached the a position between benzene ring and ester groups, which also ruptured the C-C bond. These processes degraded DEP into smaller molecule substances.The application of nano-α-Fe2O3 catalyst and the α-Fe2O3-AC composites in photocatalytic oxidation degradation of PAEs contributes to the improvement of control technologies for PAEs in urban wastewater treatment plants, which possess essential practical application value.