Mechanisms Of Hydroxyl Radical Production And Pollutant Oxidation By Organic Acid-complexed Fe(Ⅱ) Activated Oxygen

Hydroxyl radical（·OH,with a standard reduction potential of 2.8 V）is the strongest oxidant in natural environments.It can oxidize most organics and redox-active inorganics without selectivity at near diffusion-controlled rate.·OH plays an important role in soil remediation and water treatment.In recent decades,it has been demonstrated that·OH can be produced under dark conditions.For example,microorganisms can produce reactive oxygen species（ROSs）,such as superoxide radicals and hydrogen peroxide,under dark conditions.Reduced substances such as natural organic matter（NOM）and Fe（II）in subsurface environments can activate O₂ to produce·OH.Different forms of Fe（II）can activate O₂ to produce·OH under dark conditions.·OH oxidation of pollutants has become a research hotspot.Existing studies are mainly focused on the production of·OH upon oxygenation of real sediments and different forms of Fe（II）.Among these forms of Fe（II）,Fe（II）-containing clay minerals and Fe（II）sulfide（pyrite or Mackinawite）,and dissolved/adsorbed/inorganic ligand-complexed Fe（II）has been studied in detail.Whereas,organic acid-complexed Fe（II）and magnetite（Fe₃O₄）,which can also activate O₂ to produce·OH under dark conditions and represent great environmental significance,have been much less studied.Therefore,this thesis investigated the mechanisms of hydroxyl radical production and pollutant oxidation by acid-complexed Fe（II）activated oxygen.The main conclusions are summarized as follows.（1）Mechanisms of hydroxyl radical production and pollutant oxidation byorganic acid-complexed Fe（II）in dissolved formThe promotion of·OH generation by organic acids is an important factor in Fe（II）-catalyzed O₂ and H₂O₂ oxidation processes.However,the quantitative effect of the molar ratio of organic acids to Fe（II）on·OH generation is still unclear.Citric acid and inorganic dissolved Fe（II）(Fe（II）_dis)were used as representative organic acid and Fe（II）species to quantify this relationship,respectively.The results showed that·OH production was highly dependent on the molar ratio of citric acid/Fe（II）_dis.Under p H6.0-7.5 conditions,when the citric acid/Fe（II）_dis ratios were low（0.25-0.5）,moderate（0.5-1）,and high（1-2）,the·OH accumulation in the presence of 0.25 m M Fe（II）_disduring oxidation was 2.0-8.5,3.4-28.5,and 8.1-42.3μM,respectively.When the citric acid/Fe（II）_dis ratio was low（<0.5）,the oxidation of inorganic Fe（II）played a major role in·OH generation.As the citric acid/Fe（II）_dis ratio increased to a higher level（1-2）,the citrate-complexed Fe（II）became the electron source for·OH generation.The change in Fe（II）form increased with increasing citric acid/Fe（II）_disratio,leading to an increase in·OH production.An appropriate citric acid/Fe（II）_disratio（0.5-1）may be optimal for Fe（II）-catalyzed O₂ and H₂O₂ oxidation processes.Changes in the citric acid/Fe（II）_dis ratio not only affected·OH production but also promoted the oxidation degradation of pollutants in Fe（II）oxidation processes.Under p H 7 conditions,in a system with a citric acid/Fe（II）ratio of 1 and 0.25 m M Fe（II）,the degradation efficiency of 1 mg/L phenol within 40 min was 53.6%.（2）Effects of minerals on hydroxyl radical production and pollutant oxidation by organic acid-complexed Fe（II）Organic acid-complexed Fe（II）can activate O₂ to produce·OH.However,in natural underground environments,organic acid-complexed Fe（II）generally coexists with minerals,and the effect and mechanism of minerals on the activation of O₂ and H₂O₂ by organic acid-complexed Fe（II）to produce·OH is unclear.Here we investigated the effects of silica,kaolin,alumina,montmorillonite,and hematite on the activation of oxygen and the production of hydroxyl radicals and the oxidation of pollutants by citrate-Fe（II）and EDTA-Fe（II）complexes.The results showed that in the absence of minerals,citrate-Fe（II）and EDTA-Fe（II）existed in the aqueous phase and could rapidly oxidize to produce·OH and oxidize the pollutant phenol.Silica and kaolin had almost no effect on the production of hydroxyl radicals during the oxidation of citrate-Fe（II）and EDTA-Fe（II）,while alumina and montmorillonite reduced the efficiency of·OH production and the degradation of the phenol.Hematite reduced the efficiency of hydroxyl radical production during the oxidation of citrate-Fe（II）,but had no significant effect on the efficiency of hydroxyl radical production during the oxidation of EDTA-Fe（II）.When the minerals did not adsorb the organic acid complexed Fe（II）,the minerals had no significant effect on·OH production and almost no effect on the oxidation and degradation of pollutants.When significant adsorption occurred,the Fe（II）concentration in the aqueous phase decreased,while the Fe（II）concentration in the solid phase increased.The efficiency of hydroxyl radical production by solid phase Fe（II）activated oxygen was much lower than those of xitrate-Fe（II）and EDTA-Fe（II）in the aqueous phase.Therefore,the adsorption effect reduced the efficiency of hydroxyl radical production during Fe（II）oxidation and also reduced the efficiency of the oxidation and degradation of pollutants.（3）Enhancement of organic acids on hydroxyl radical production and pollutant oxidation by magnetite activated O₂Existing research has shown that Fe（II）-containing clay minerals,Fe（II）sulfides and siderite can activate O₂ to produce·OH.Magnetite formed by microbial reduction is widely present in subsurface environments,but it is unclear whether Fe（II）in microbially formed magnetite can activate O₂ to produce·OH.Moreover,the mechanism of organic acid coexistence with magnetite Fe（II）in activating O₂ to produce·OH and degrade pollutants is not clear.This study investigated the process of Fe（II）in microbially formed magnetite activating O₂ to produce·OH and explored the influencing mechanism of organic acids,citric acid and EDTA,in activating Fe（II）in magnetite to produce·OH and degrade pollutants.Results showed that·OH production increased with increasing magnetite concentration.The accumulated·OH produced within 5 h of oxidation for magnetite concentrations of 0.7,1.4,2.8 and 4.9 g/L were6.3,13.7,27.9,and 52.1μM,respectively.In the presence of 2 m M citric acid,the hydroxyl radical produced by 1.4 g/L magnetite within 5 h of oxidation was 80.2μM,which was 5.9 times that produced in the absence of organic acid.In the presence of 2m M EDTA,the hydroxyl radical produced by 1.4 g/L magnetite within 5 h of oxidation was 276.1μM,which was 20.2 times that produced in the absence of organic acid.Citric acid and EDTA both effectively promoted the oxidation of Fe（II）in magnetite to produce·OH,and the efficiency of·OH production increased with increasing concentrations of citric acid and EDTA.The mechanism of organic acid promoted activation of magnetite Fe（II）to produce·OH is that the dissolved Fe（II）in the process of magnetite oxidation in the presence of organic acid can be quickly complexed by citric acid and EDTA to form dissolved Citrate-Fe（II）and EDTA-Fe（II）.Citrate-Fe（II）and EDTA-Fe（II）can then activate O₂ to produce·OH,and the complexed Fe（III）produced by oxidation can be reduced by solid-phase Fe（II）in magnetite to form dissolved complexed Fe（II）,thus generating more hydroxyl radicals in a cycle.EDTA was more effective than citric acid in promoting the activation of magnetite Fe（II）due to the faster reduction rate of EDTA-Fe（III）in magnetite than that of citrate-Fe（III）.As a representative pollutant,under neutral p H 7.0 conditions and with an initial concentration of 2 mg/L phenol,1.4 g/L magnetite oxidation could degrade 10%of phenol.The addition of 2 m M EDTA could degrade 80%of phenol,while the addition of 2 m M citric acid could degrade 40%of phenol.The novelty of this thesis includes:（1）a quantitative clarification of·OH production during oxygenation of citriate-complexed Fe（II）through kinetic modeling;and（2）the elucidation of influence of organic acid on·OH production and phenol degradation during magnetite oxygenation through regulating Fe species distribution and valance interconversion.