Study Of Enhanced Reductive/Oxidative Zero-Valent Iron Technologies In The Degradation Of Chlorophenols

Zero-valent iron(ZVI) has been successfully used in the remediation of various groundwater and soil contaminants including a large variety of organic(e.g.,chlorinated organic solvent,PCBs) and inorganic(e.g.,heavy metal ion,arsenic,nitrate) compound. However,with respect to chlorophenols(CPs),treated by sole ZVI technology was reported inefficient.Therefore,it is essential to develop new ZVI technologies for the treatment of CPs.In this dissertation,CPs was chosen as the main model pollutant.Enhanced ZVI reductive/oxidative systems were established to evaluate the degradation of CPs.Firstly, enhanced ZVI reductive catalyst—nanoscale Pd/Fe was synthesized and dechlorination of 3 CPs in this reductive system was investigated.Secondly,a ZVI/H₂O₂ Fenton like oxidative system was established.The degradation kinetics,pathways and effect factors were further studied.Thirdly,degradation of CPs in a novel ultrasound(US) enhanced Fenton like system(Fe/EDTA/air) was investigated.The reaction mechanism and enhanced role of US were studied.Lastly,treatment of simulated textile wastewater and desoprtion waste from heavy metal contaminated soil in the Fenton like(Fe/EDTA/air) was investigated.The main results of this dissertation are as follows:1.Results of reductively dechlorination of CPs by nanoscale Pd/Fe:â‘ Nanoscale Pd/Fe bimetal catalyst with mean particle size of 80 nm and BET specific area of 26±3 m²/g was synthesized.â‘¡The reductive dechlorination of three CPs by nano scale Pd/Fe was investigated respectively.The sequence of dechlorinated rate of CPs is TCP＜DCP＜MCP.In the degradation of CPs,direct transformation from model parent CPs compound to phenol are all identified as the main dechlorinated degradation pathway.â‘¢The effect of some experiment parameters such as catalyst loading,Pd ratio, reaction temperature and initial pH etc on the reductive dechlorination of 246TCP were investigated.â‘£Evolution of hydrogen gas in the system was observed.Catalytic reductive dechlorination mechanism of CPs by nanoscale Pd/Fe was proposed.Within the system,the role of ZVI is to produce hydrogen through metal corrosion reaction.Hydrogen gas can be associated on the Pd surface and transformed to atomic hydrogen with strong reductive ability.The degradation of CPs can be ascribed to catalytic hydrogendechlorination on Pd surface.2.Results of oxidative degradation of 4CP in a heterogeneous ZVI/H₂O₂ Fenton like system:â‘ Initial pH was proven to have significant effect on the degradation of 4CP and lower pH brought faster degradation rate.â‘¡When initial pH was 4,two-stage first order degradation kinetic of 4CP could be applied very well in all experiments.The two-stage kinetic is composed of an initial slow degradation stage(first-stage) and a followed rapid degradation stage(second-stage),where kinetic constant(k_obs) of the latter stage was one magnitude larger than that of the former stage.â‘¢A scheme of 4CP degradation pathways in ZVI/H₂O₂ system was proposed and two pathways--4CC pathway(attacking by hydroxyl radical(·OH)) and orthoparachlorophenolperoxyl radicals(ClPP·) pathway(participating by O₂) were expected to happen in the system.Both pathways would finally lead to the production of low molecule aliphatic organic acid.3.Results of Degradation of CPs in a novel ultrasound assisted Fenton like system:â‘ The sequence of the degradation rate of 24DCP in different system is US/Fe/EDTA＞Fe/EDTA＞US/Fe＞US.It showed that ultrasound presented synergistic effect on the Fenton like system.Pseudo-first-order kinetics could be well applied in degradation of DCP.â‘¡Both 4CP and EDTA were found to follow pseudo-first-order degradation kinetic in the novel US/Fenton like system at neutral pH.A non-radical degradation pathway was proposed for the neutral US/Fenton like system.The competitive degradation relationship between 4CP and EDTA is very weak.EDTA and ZVI play important roles in the system and variation of their initial concentration showed significant effect on the degradation rate of both 4CP and EDTA.Ultrasound presented significant positive effect on the degradation of 4CP and EDTA.By ultrasonic cavitation effect,kinetic barrier of O-O band was overcome,which eventually improve H₂O₂ production rate.Sufficient DO is indispensable to the neutral US/Fenton like system and degradation of 4CP and EDTA was almost inhibited at low DO concentrations.Reaction temperature also had positive effect on the degradation rate of 4CP and EDTA.Activation energy(Ea) of 4CP and EDTA was calculated as 38.6 and 24.8 kJ mol^-1 respectively.A non-radical degradation pathway was proposed for the neutral US/Fenton like system.Ferryl-EDTA complex([Fe^â…£O]EDTA) instead of OH·was identified as the dominant oxidant in the system.â‘¢By the use of PLS method and 19 quanchemical descriptors computed by PM3 Hamilitonian,an optimized QSPR model was developed for degradation rate constants of 5 CPs in a US irradiated Fenton-like system.The model can be used to explain the degradation mechanism and enhancement role of ultrasound.CPs with greater, E_lumo-E_homo,(E_lumo-E_homo)²,Mw,¹X^V and CCR values tend to be degraded slower, whereas CPs with higher values of E_homo,μ,TE,EE and HOF values tend to be degraded faster.The degradation of CPs in the US/Fenton-like system can be ascribed to homogeneous Fenton oxidation in the bulk solution.Ultrasound irradiation is expected to enhance Fenton reaction instead of directly decompose CPs in the system.4.Results of degradation of textile wastewater and desorption wastewater from heavy metal contaminated soil in the Fenton like system:â‘ The degradation of simulated textile wastewater was investigated.It indicated that decolorization of the wastewater and degradation of EDTA could be completely achieved at 15min and 3h respectively.Effect of some experiment parameters was also investigated. Two types of external energy--ultrasound(US) and ultraviolet(UV) was introduced into the Fenton like system respectively and the effect of the external energies on the degradation of the wastewater was assessed.Ultrasound presented significant synergistic improvement on both the decolorization rate of the wastewater and degradation rate of EDTA.In contrast, ultraviolet light presented slight inhibition on degradation of the wastewater in the Fenton like system.â‘¡Desorption wastewater from heavy metal contaminated soil in the Fenton like system could be treated by Fenton like system.The result showed that heavy metal ions could be completely removed from the water after 24h's treatment and the removal rate of EDTA achieved 94.8%as well.