Research On The Redox Process Involved Reaction Mechanism Of Modified Zerovalent Iron For The Removal Of Organic Pollutants In Water

The safety of water resources is closely related to human life.A large number of persistent organic pollutants frequently detected in water bodies have aroused widespread concern.The treatment of organic pollutants is imminent.Zerovalent iron(ZVI)has become the most widely used restoration material in the field of environmental restoration due to its excellent reducibility.How to slow down the surface passivation and the oxidation of zerovalent iron,improve the electron transfer efficiency of zerovalent iron,and explore the mechanism of degradation of organic pollutants by zerovalent iron through material modification and other means have been the research hotspot and difficulty in this field in recent years.And it is also the key scientific issues affecting the engineering application of zerovalent iron technology.In this paper,two strategies,hydroxylamine hydrochloride assisted and surface sulfide,have been selected to improve the remediation efficiency of zerovalent iron.The mechanism of high efficiency degradation of p-nitrophenol(PNP)by zerovalent iron has been systematically studied.The root cause of PNP directly reduced degradation by zerovalent iron has been confirmed.The oxidation process of PNP dominated by hydroxyl radicals in oxygen-free environment has been found.The ecological toxicity of PNP product has also been illustrated during the process of oxidation and reduction.By further introducing calcium peroxide and persulfate,the effects of the degradation and transformation of organic pollutants enhanced by slow-release oxygen agent were discussed during the catalytic oxidation of zero valent iron.This paper mainly includes the following three aspects,four systems of research content:1.The degradation mechanism of p-nitrophenol by zerovalent iron/hydroxylamine hydrochloride(ZVI/HA)system was studied,and the effect of the addition of hydroxylamine hydrochloride on the degradation efficiency of p-nitrophenol by zerovalent iron was also discussed.By investigating the effects of the dosage of ZVI and HA,pH value of the solution and dissolved oxygen on the degradation efficiency of PNP,it was confirmed that the introduction of hydroxylamine hydrochloride could significantly improve the reduction efficiency of zerovalent iron,and the removal rate of PNP was increased nearly 90 times.Selected potassium bromate,tertiary butyl alcohol and 1,10-phenanthroline as electron capture agent,active hydrogen indicator and zerovalent iron surface Fe(?)passivation agent,it was clear that zerovalent iron directly donated electronic to reduce PNP was the root cause of the high degradation efficient,and the surface generated by Fe(?)of zerovalent iron and active hydrogen's contribution to the PNP degradation could be neglected.The characterization results of morphology and chemical composition of the catalysts by XRD,TEM and XPS showed that hydroxylamine hydrochloride could also promote the Fe(?)/Fe(?)circulation,slow passivation and broaden the reaction pH range in the system.It can be concluded that that hydroxylamine hydrochloride was an effective strategy to improve the reduction efficiency of zerovalent iron.2.In this chapter,the strategy of surface sulfide was used to modify zerovalent iron(S-ZVI)to explore the degradation mechanism of p-nitrophenol in groundwater by sulfidated zerovalent iron.Compared with the degradation effect of different iron sulfide ratios on the degradation of PNP,it was found that there was also certain oxidative degradation in the process except the reduction degradation process in anaerobic environment.Based on the analysis of degradation products and free radical probe experiments,the oxidation process of PNP was proved to exist in the S-ZVI system dominated hydroxyl radicals,in which the contribution of PNP reduction removal was90%,and the remaining 10%of PNP is oxidized to benzoquinone by hydroxyl radicals.XPS characterization and O18 isotope tracer experiments revealed that the iron sulfide on the surface of the sulfidated zerovalent iron can react with water molecules to generate hydrogen peroxide in anaerobic environment.The surface bonded divalent iron species and hydrogen peroxide follow the Fenton-like reaction mechanism to generate hydroxyl radicals.It is worth noting that the oxidation of PNP generates relatively high toxicity of p-benzoquinone,which is of great significance to evaluate of the ecological toxicity of nano-zero-valent iron products in the degradation process of pollutants.3.The eco-toxic effect of degradation products of organic pollutants is a nonnegligible environmental problem.Strengthening the oxidation process involving zerovalent iron and promoting the efficient degradation and harmless transformation of organic pollutants are the key to solve the above problems.Therefore,calcium peroxide and persulfate were further introduced to explore the catalytic oxidation process of zerovalent iron enhanced by slow-release oxygen agent and the effect on the degradation and transformation of organic pollutants in this paper.It is divided into two systems to study the above problems.(1)Calcium peroxide was selected as the representative of slow-release oxygen agent to explore the influence of pH value of the solution,the amount of calcium peroxide and zerovalent iron sulfide on the degradation efficiency of ibuprofen by sulfidated zerovalent iron.The results showed that the introduction of calcium peroxide could significantly improve the degradation efficiency of ibuprofen,and the removal rate of ibuprofen was increased by more than 18 times compared with that of sulfidated zerovalent iron alone.Based on the quantitative analysis of dissolved oxygen,hydrogen peroxide and free radicals,it is confirmed that calcium peroxide can slowly release dissolved oxygen or hydrogen peroxide,which greatly improves the utilization efficiency of sulfidated zerovalent iron for in-situ hydrogen peroxide generation.And the contribution of oxygen reduction process for the degradation of ibuprofen by sulfidated zerovalent iron was about32.8%.XPS analysis revealed that the continuous single-electron transfer process on the surface was the key to improve the oxygen reduction efficiency of sulfidated zerovalent iron.Besides,the degradation products of ibuprofen were also characterized by TOC,GC/MS.The results showed that the introduction of slow-release oxygen agent improved the mineralization efficiency of ibuprofen by sulfidated zerovalent iron.To sum up,it can be concluded that adding calcium peroxide as a slow-release oxygen agent is an effective method to enhance the degradation efficiency and mineralization efficiency of pollutants.(2)Persulfate,a commonly groundwater remediation oxidant,was used to explore the influence on the degradation of bisphenol A by the enhanced sulfidated zerovalent iron system.The experimental results showed that the introduction of persulfate significantly improved the degradation efficiency of bisphenol A,and the removal rate of bisphenol A was 64 times higher than that of sulfidated zerovalent iron alone.Based on the design of free radical probe experiment and persulfate decomposition intermediate analysis found that S-ZVI/PS system can effective active persulfate to form the middle species of[O^--O-SO₃^-]and suppresses sulfate radicals translate into hydroxyl free radical side effects.The higher oxidation reduction potential(3.1 V)of sulfate radical is the root cause of bisphenol A efficient degradation.The degradation products of bisphenol A were further analyzed by TOC and GC/MS.The results showed that the presence of persulfate improved the mineralization efficiency of ibuprofen by sulfidated zerovalent iron.In summary,it can be concluded that persulfate may be a more effective means to improve the degradation efficiency and mineralization efficiency of pollutants than calcium peroxide.