Degradation Of Typical TROCs In A Magnetic Field- Enhanced ZVI Catalytic Oxidation System And The Mechanistic Study

It has become a research hotspot recently to develop suitable technologies for the removal of TrOCs in the water environment.Zero valent iron?ZVI?can reduce kinds of organic/inorganic pollutants because of its low redox potential?-0.44 V?,while more attention is being paied to the ZVI-based catalytic oxidizing technologies.The later are able to selectively oxidize organics via the production of hydroxyl radicals from the catalytic decomposition of common oxidants,e.g.H₂O₂ and O₂.Nevertheless,the passivation of ZVI makes its low reactivity in the longterm and limited applicability to high pH.Thus,seeking for a green,cost-effective assisting method to counteract the limitation in the application of ZVI technology would be necessary to applying the ZVI catalytic oxidation technology to the treatment of TrOCs.Therefore,this dissertation established a kind of magnetic field assisting ZVI catalytic oxidation system,realized the effective degradation of TrOCs,and investigated the mechanism of the magnetic field effect on ZVI corrosion.The main results are as follows:?1?A comparative investigation between the ZVI/H₂O₂ and magntic field/ZVI/H₂O₂systems was conducted under different experimental conditions.Magnetic field could suppress the duration of initial lag degradation phase one order of magnitude in addition to significantly enhancing the overall 4-CP degradation,under the conditions of ZVI dosages in 0.025-0.5 g/L,H₂O₂ dosages in 0.5-3.5 mM,pH in 2-3 and temperature in 10-55°C.Monitoring of intermediates/products and detecting of the ROS species indicated that magnetic field would just accelerate the Fenton reactions to produce OH·more rapidly.Simultaneous evolutions of released dissolved iron species suggested that magnetic field not only improved mass-transfer of the initial heterogeneous reactions,but also modified the pristine ZVI surface.Characterizations of the specific prepared ZVI samples revealed a Fe_xO_y-layer based magnetic field-inducing evolution of the iron surface,i.e.initial rapid point dissolution of Fe_xO_y—pitting corrosion of the exposed Fe⁰ reactive sites—appearance of a particular rugged surface topography with numerous adjacent Fe⁰ pits and Fe_xO_y tubercles.?2?After the systematic investigation of the corrosion of iron electrode in the magnetic field,it was found the magnetic field would lead to the local liberation and ununiform distribution of corrosion products on the iron surface;the investigation of the static anodic polarization of iron surface indicated the effective counteraction to iron passivation by magnetic field;moreover,according to the static and dynamic polarization experiments with the electrodes modified by the nano Fe₃O₄ particles,the Lorentz force was found to enhance the mass transfer,while the gradient magnetic force would lead to surface evolution.Combining the actual distribution of the iron products and simulated distribution of magnetic field,it was found the former corresponded well with the distribution of magnetic gradient.It was concluded that the gradient magnetic force would migrate the ferrous iron,thus“directionally sweep”the corrosion products at the location of high magnetic gradient.In the result,the concentration polarization and surface passivation would be greatly alleviated,leading to the local corrosion of iron and the sustained exposure of active sites at the location of high magnetic gradient.?3?Investigation between the nano ZVI?nZVI?/O₂ and magntic field/nZVI/O₂ systems was conducted under different experimental conditions and it was found the magnetic field could effectively enhance the oxidation of diclofenac?DCF?.Through the quenching experiments and the analysis of degradation intermediates,the magnetic field was found not changing the radicals-oxidizing degradation mechanism of DCF,but increasing the production of OH·.According to the results of the quenching experiments for OH·,O₂^-·and H₂O₂,the OH·was confirmed to be generated from the single-electron activation of O₂and the magnetic field affected the transform of H₂O₂ to OH·.Combining the simultaneous liberation of Fe?II?and the evaluation of the utilization efficiency of Fe?II?,as well as the analysis for the rate-controlling parameters for O₂ activation and the characterization of the corrosion products,the mechanism for the magnetic field enhancement was proposed.In the external magnetic field,the high magnetic gradient around the nZVI particals would migrate the generated Fe?II?,thus promoting the liberation of Fe?II?.As a result,the single electron decomposion of H₂O₂,which genenrates OH·,would be enhanced,while the double electron decomposion of H₂O₂,which consumes Fe⁰ and generates H₂O,would be suppressed,ultimately leading to the increased production of OH·.?4?An applied research for the treatment of DCF in the continuous flow under neutral pH was conducted with the magnetic field assisting packed-iron-quatz sand reaction columns.It was found the magnetic field could significantly alleviate the attenuation of the iron reactivity.According to the quenching experiments and the analysis for the products of DMSO,the reactive oxidants was confirmed to be OH·instead of Fe?IV?.The operational parameters were investigated and the optimum condition was:pH 6,rate of 0.2 mL/min,6mM H₂O₂.In addition,either increasing the packed iron quantity or decreasing the iron particle size would benefit the degradation of DCF.Four common anions were investigated and the inhibiting effects on DCF degradation were HCO₃^->SO₄^2->Cl^->NO₃^-at the moderate concentration?0.3-4.2 mM?,while HCO₃^->NO₃^->SO₄^2->Cl^-at the high concentration?100 mM,and 20 mM only for HCO₃^-?.The longterm run was also conducted,and it could be separated into three stages,i.e.rapidly attenuating stage?0-800 PV?,gradually attenuating stage?800-3000 PV?and stable stage?>3000 PV?.Althouth the degradation of DCF was partly inhibited in the simulated ground water,the removal ratio was maintained upper than 50%within 140 PV.According to the morphology analysis of iron products,it could be concluded:in the rapidly attenuating stage,there are a lot of reactive sites and it would be consumed rapidly;after the significant consumption of Fe⁰,the the residual Fe⁰ would be coated by the thick passivating layer and catalyze H₂O₂through the electron transfer between the core and shell in the gradually attenuating stage.