Iron Manganese Minerals-Mediated The Transformation Mechanism Of Polycyclic Aromatic Hydrocarbons And Their Derivatives Under Water Unsaturated Condition

Polycyclic aromatic hydrocarbons（PAHs）and their derivatives are common toxic organic compounds in soils.PAHs are carcinogenic,teratogenic,and mutagenic,and can endanger human health through respiration,diet,and skin contact.Soils are the important repository of PAHs and their derivatives,and are the site of their transformation and degradation.The chemical transformation of PAHs and their derivatives has received increasing attention due to its fast and efficient.Iron manganese minerals with strong oxidizing ability are important components of soils,and can effectively mediate the transformation of various toxic organic compounds.However,studies on the transformation of PAHs and its derivatives mediated by iron manganese minerals in soil are still scarce.PAHs and their derivatives are hydrophobic,and the interfacial interaction between PAHs and its derivatives with iron manganese minerals is weak in the suspension system.However,the topsoil is often in the unsaturated water state.Can iron manganese minerals in soils mediate the transformation of PAHs and their derivatives in the water unsaturated condition?What are its mechanisms and influencing factors?Research is urgently needed.Based on the transformation efficiencies of PAHs and their derivatives mediated by iron manganese minerals in soils,this study focused on the influence of the structural characteristics of iron manganese minerals,molecular properties,water contents,and metal ions on the transformation of PAHs and their derivatives mediated by iron manganese minerals.The research identified the transformation products and revealed the transformation mechanism.The main results are as follows:1.Confirmed the transformation of PAHs and their derivatives mediated by goethite.The transformation of anthracene and its amino（-NH₂）,methyl（-CH₃）,chlorinated（-Cl）and nitro（-NO₂）derivatives on goethite with 4.8%–14.5%water content was investigated.The transformation kinetics results showed that the transformation rate constants（k）of anthracene and its derivatives decreased from 0.069–3.681 d^-1 to 0.014–0.346 d^-1 with the water content increasing from 4.8%to 14.5%.The k values of anthracene was 0.075–1.536d^-1,and the k values of its derivatives were 0.169–3.681 d^-1 and 0.014–0.858 d^-1 after introducing electron-donating groups（CH₃ and-NH₂）and electron-absorbing groups（-Cl and-NO₂）,respectively.Mossbauer spectrogram results showed that Fe（Ⅲ）on goethite surface was reduced to Fe（Ⅱ）by electron transfer with anthracene and its derivatives,and the latter was oxidized to anthraquinone and its derivatives with high solubility and bioavailability.The theoretical calculation results indicated that water molecules form water films on the surface of goethite.The water films change from single layer to multi-layer with the decrease of water contents to prevent the binding of anthracene and its derivatives with Fe（Ⅲ）,and thus inhibited the transformation of anthracene and its derivatives.Orbital theory results showed that the introduction of electron-donating/electron-absorbing groups led to an increase/decrease in the highest occupied molecular orbital（HOMO）energy of anthracene,which affected the electron-donating ability of the derivatives,thus promoting/inhibiting the transformation of the derivatives.These results confirmed that goethite can effectively mediate the transformation of PAHs and its derivatives in soil even at lower water content.2.Elucidated the promoted transformation of PAHs and their derivatives on cryptomelane-type manganese minerals by metal ions.The effect of metal ions（Fe（Ⅲ）,Cu（Ⅱ）,Zn（Ⅱ））on the transformation of PAHs（fluorene,anthracene,pyrene）and their halogenated derivatives on cryptomelane-type manganese mineral（K-OMS-2）was investigated.The transformation kinetics results showed that the transformation rate constants（k）of fluorene and its halogenated derivatives,anthracene and its halogenated derivatives,pyrene and its halogenated derivatives were 1.176–3.744 d^-1,0.086–1.454 d^-1,0.064–0.294 d^-1,respectively.And the results were Fe（Ⅲ）>Cu（Ⅱ）>Zn（Ⅱ）>K（I）.The results of electron paramagnetic resonance spectroscopy（EPR）and theoretical calculation of state density showed that PAHs and their halogenated derivatives were converted into ketones and quinones through electron transfer and superoxide radical catalytic reaction.The results of oxidation potential measurement and quantum chemical calculations showed fluorene and its halogenated derivatives were weak acidic and had high proton activity.They can be abstracted H combined with-OH groups of manganese mineral,and can be quickly transformed.The transformation efficiencies of anthracene,pyrene and their halogenated derivatives were closely correlated with their oxidation potential.Metal ions can obviously promote the transformation of anthracene,pyrene and their halogenated derivatives with low oxidation potential,but have little effect on the transformation of fluorene and its halogenated derivatives with high oxidation potential.The results of energy spectrum and spectra showed that Fe（Ⅲ）,Cu（Ⅱ）and Zn（Ⅱ）are incorporated into K-OMS-2through ion exchange,and the K（I）content decreases from 10.56%to 3.86%–4.50%.The results of H₂ temperature-programmed reduction showed that the three metal ions promoted the oxidation capacity（electron transfer capacity）and the generation of active free radicals.These results elucidated the rule and mechanism of metal ions promoting the transformation of PAHs and its derivatives mediated by K-OMS-2.3.Elucidated the transformation and mechanism of PAHs and its derivatives on various manganese minerals.Five manganese minerals with different structures were synthesized by hydrothermal method to study the effect of manganese mineral structures on the transformation of benzo（a）pyrene.The results of transformation kinetics showed that the k values of benzo（a）pyrene are 0.49,0.080,0.0071,0.0055 and 0.0022 h^-1,respectively.After Fe（Ⅲ）saturated,the k values were increased to 22,2.7,0.25,0.0072 and 0.0098 h^-1,respectively.The results of EPR and theoretical calculation showed that benzo（a）pyrene was synthesized into benzo（a）pyrene-1,6-dione and benzo（a）pyrene-1,12-dione through electron transfer and superoxide radical catalytic reactions.The correlation analysis showed that the k value of benzo（a）pyrene was positively correlated with its oxidation capacity（electron transfer capacity）and superoxide radical concentration of manganese minerals with different structures.The results of energy spectrum analysis confirmed that superoxide radical concentration was consistent with the content of surface reactive oxygen species,which was mainly determined by the oxygen vacancies content of manganese minerals.In addition,the enhancement efficiency of benzo（a）pyrene was positively correlated with the permanent charge of manganese minerals after Fe（Ⅲ）saturation of manganese minerals with different structures,and the latter is mainly determined by the vacancies content of manganese minerals.Based on the above results,it was found that the vacancies and oxidation capacity of manganese minerals are the main reasons for the difference in the transformation efficiency of PAHs mediated by manganese minerals with different structures.In summary,this study found that the iron-manganese minerals effectively mediated the transformation of PAHs and their derivatives under low water content,and the coexistence of metal ions can promote the transformation of PAHs and their derivatives on manganese minerals.Manganese mineral vacancies and oxidation capacity are the main reasons for the difference in the transformation efficiency of PAHs mediated by manganese minerals with different structures.These results revealed that the transformation efficiencies and mechanism of iron manganese minerals to PAHs and their derivatives,and deepened that the understanding of the chemical transformation of PAHs and their derivatives in the soil environment.