| P-nitrophenol（PNP）is a typical nitroaromatic compound,which is widely used in the manufacturing process of pesticides,dyes and herbicides.At present,it has been detected in the groundwater of the United States,Canada and other countries.PNP has been listed as a priority pollutant by the US Environmental Protection Agency due to its high toxicity,potential carcinogenicity,and non-biodegradability.It has also been listed on the blacklist of 68 priority control pollutants in China.The Fenton system based on Fe（Ⅱ）/H2O2 is considered to be an effective method for in situ remediation of PNP-contaminated groundwater.However,practical experience has shown that H2O2 is easily decomposed in the underground environment,resulting in low utilization efficiency.In addition,the use of high concentrations of H2O2 during construction can release heat violently,which has certain safety risks.Therefore,developing an efficient and environmentally friendly in situ groundwater remediation technology without using H2O2 is a key issue to be solved urgently.In recent years,scholars have found that in the presence of appropriate ligands,Fe（Ⅱ）in the environmental medium can activate oxygen to produce reactive oxygen species（ROS）and degrade organic pollutants.This system has the advantages of reactants from natural environments,mild reaction conditions,and avoiding the excessive addition of H2O2.However,research on the Fe（Ⅱ）/O2/ligand system is mainly focused on water treatment.For more complex underground environments,the pathways and conversion efficiency of dissolved and solid-phase Fe（Ⅱ）activated O2 to produce ROS under ligand enhancement are still unclear.The regulatory mechanism of ligands on ROS production and pollutant degradation in the system needs to be further analyzed.In addition,the dissolved oxygen content in polluted groundwater is usually low,and achieving sufficient O2supply in the underground environment is also a prerequisite for the system to function efficiently.In response to the issues mentioned above,this study selected inorganic ligand tripolyphosphate（TPP）as a complex to construct the Fe（Ⅱ）/O2/TPP advanced oxidation system.Compared with organic ligands,TPP has the advantages of green,non-toxic,and does not compete with pollutants for ROS.The study clarified the pathway and mechanism of Fe（Ⅱ）(including dissolved Fe2+and Fe（Ⅱ）minerals)activating O2 to generate ROS in the environment under the action of TPP,deeply analysed the effect of TPP on the degradation effect and pathway of PNP,and revealed the regulatory mechanism of TPP on ROS generation and PNP degradation,which realized the directional generation of ROS and the efficient removal of many refractory pollutants in the system.The Fe（Ⅱ）/O2/TPP system was constructed in situ for the remediation of PNP-contaminated groundwater by aeration combined with the injection of Fe（Ⅱ）-TPP remediation agents.The study clarified the migration and distribution patterns of Fe（Ⅱ）and TPP complexes in the medium,elucidated the degradation pathways and mechanisms of PNP,and revealed the regulatory mechanism of TPP on pollution remediation in the temporal and spatial dimensions.In addition,the study quantified the influence of control parameters such as remediation agent ratio and injection method on remediation effectiveness and evaluated the remediation feasibility and efficacy of this technology.The main research conclusions are as follows:（1）The regulation mechanisms of ROS generation and PNP degradation in the homogeneous Fe2+/O2/TPP system.The study compared the enhancement effects of three ligands,TPP,ethylenediaminetetraacetic acid（EDTA）,and oxalic acid（OA）,on the Fe2+/O2 system.The results showed that TPP was more effective in promoting the degradation of PNP in the system compared to the other ligands.The reaction could occur within the pH range of 3-9,and higher concentrations of TPP did not inhibit the degradation of PNP.Mechanistic studies showed that the ROS in the Fe2+/O2/TPP system were primarily O2·-and·OH,which were generated through the one-electron activation process of O2.Through the detailed analysis of the PNP degradation pathway and the contribution of ROS in the system,it was found that the degradation of PNP in this system followed the“reduction-oxidation”process,with the reduction of O2·-playing the main role.By comparing the removal of refractory organic pollutant tetrabromobisphenol A（TBBPA）by the Fe2+/O2/TPP system with the traditional Fenton system,it can be found that the removal of TBBPA in the Fe2+/O2/TPP system was increased by 30%under the condition of generating the same·OH,which means that the system has a significant advantage for the refractory organic pollutant.The study systematically analyzed the influence of TPP on ROS generation and pollutant degradation in the system,combined with DFT theoretical calculations,clarified the different coordination modes of Fe（Ⅱ）and TPP in the system and their mechanisms for ROS generation,and revealed,for the first time,the regulation mechanism of TPP on ROS transformation and action in the system,including the following two pathways:（i）TPP concentration regulation.Under neutral conditions,when CFe2+≥CTPP,the O2·-was rapidly converted to·OH,and the PNP was degraded mainly by oxidation;when CFe2+<CTPP,the O2·-in the system accumulated,and the PNP was mainly degraded by reduction.（ii）Solution pH regulation.When the molar ratio of Fe2+and TPP is 1:2 in the system,pH can regulate the degradation of PNP by affecting the degree of protonation of TPP.When the solution pH=3,PNP was mainly oxidatively degraded by·OH;when pH=5 or 7,PNP in the system was predominantly reductively degraded.（2）The degradation and regulation mechanism of PNP in heterogeneous Fe（Ⅱ）/O2/TPP system.FeCO3 and Fe3O4,which are commonly found in geological media,were selected to construct the heterogeneous Fe（Ⅱ）/O2/TPP system.The results showed that TPP effectively promotes the degradation of PNP in the Fe（Ⅱ）/O2 system under both acidic and neutral conditions.Mechanistic studies proved that the main ROS in the heterogeneous system are still O2·-and·OH.By quantifying different ROS contents,it was found that TPP enhances the effective electron utilization of heterogeneous Fe（Ⅱ）activating O2 to produce ROS by 2-3 times compared to homogeneous systems.For FeCO3,which has an unstable lattice structure,the generation of ROS in the heterogeneous system is dominated by homogeneous reactions of dissolved Fe2+.The slow dissolution of Fe2+in FeCO3 under the action of TPP makes O2·-in the system more inclined to be converted into·OH,·OH plays a major role in the degradation of PNP;For Fe3O4 with stable lattice structure,the generation of ROS in the system is dominated by the interfacial reaction.TPP in the solution can spontaneously bind to the surface of Fe3O4 to form Fe（Ⅱ）-TPP complex,which can effectively activate O2 to generate O2·-,and under its action,O2·-can be directionally converted to·OH.The complex structure between TPP and Fe3O4 surface was constructed through DFT calculation,and its mechanism of action on ROS production was clarified at the molecular level,revealing the interfacial regulation mechanism of TPP on the generation and transformation of ROS in the Fe3O4/O2 system.（3）The regulatory effect of TPP on in-situ remediation of PNP contaminated groundwater in Fe（Ⅱ）/O2 system.A new method for the remediation of contaminated groundwater by Fe（Ⅱ）/O2/TPP system was constructed in situ by aeration combined with remediation agent injection.The research results showed that the system could effectively remove PNP in the aquifer,and the oxidation/reduction degradation pathway of PNP could be regulated by changing the ratio of Fe2+and TPP in the remediation agent.When the molar ratio of Fe2+and TPP was 1:1,PNP was mainly oxidatively degraded.When the molar ratio of Fe2+and TPP was 1:2,the degradation of PNP in the system was the coexistence of oxidation and reduction.The study compares the removal effect of PNP in the aquifer under two conditions of one-time injection and continuous injection of remediation agents.The results showed that due to the density effect,remediation agents are prone to sinking in the aquifer.The continuous injection of the remediation agent can significantly improve the formation range of the reaction zone and the removal effect of PNP.The migration of iron ions and TPP in the remediation agent in the aquifer was synchronous,and the adsorption of TPP by the river sand was not significant（about6.25 mg/g）.The spatial distribution of the reduced product PAP and oxidized product HQ of PNP degradation in the aquifer has a clear sequence,indicating that the degradation of PNP is dominated by the process of"reduction-oxidation".The heterogeneous Fe（Ⅱ）/O2/TPP system constructed in situ by Fe3O4 in the medium was used to remediate PNP-contaminated aquifers.The results showed that after 168 hours of reaction,the average removal rate of PNP in the simulated tank was 65.9%,and the effective degradation area accounted for 83.4%of the total area of the simulated tank.TPP can promote an advanced oxidation reaction of Fe（Ⅱ）in the medium Fe3O4 with oxygen,which in turn degrades PNP in the aquifer.In summary,this study provides a new idea for in-situ remediation of PNP-contaminated groundwater based on Fe（Ⅱ）/O2/TPP system.The regulation method of TPP on ROS production and pollutant degradation in the system was established,resulting in a green,efficient,and sustainable degradation of organic pollutants in the aquifer. |
PDF Full Text Request