Study On The Mechanism Of Plant Phenolic Acids Complexing Iron Ions To Strengthen Fenton Oxidation Of Phenanthrene

Polycyclic aromatic hydrocarbons（PAHs）are a prevalent class of persistent organic pollutants found in the Earth’s environment.PAHs are highly toxic,carcinogenic,and capable of bioaccumulation.Due to their environmental persistence and ability to migrate over long distances,the US Environmental Protection Agency and the European Union have designated16 PAHs as priority pollutants.The primary source of PAH emissions in my country is from the combustion of fossil fuels and their derivatives.As a result,some regions of the country are exposed to increased ecological risks.The traditional Iron-based Fenton technology is a commonly used method for degrading organic pollutants due to its high efficiency,eco-friendliness,simple operation,and ability to achieve high rates of organic pollutant mineralization.However,this technology is limited by strict pH conditions（between 2.8-3.5）,as well as factors such as Fe³⁺/Fe²⁺circulation blockage and inefficient H₂O₂ decomposition.To address these limitations,improved Fenton technologies have been developed,including the use of complexing agents to enhance the degradation of organic pollutants by stabilizing H₂O₂ and Fe³⁺/Fe²⁺cycles through complexation of iron ions.Natural plant phenolic acid is an organic complexing agent with a catechol group in its chemical structure that can complex and reduce Fe³⁺,overcoming the traditional Fenton reaction’s conundrum of limited cycles.Moreover,plant phenolic acids are widely found in nature,have low costs,and are environmentally friendly,making them promising candidates for use in the Fenton process.This study selected four plant phenolic acids（catechol,protocatechuic acid,gallic acid,and tannic acid）to construct Fe³⁺-plant systems and investigated their degradation characteristics,reaction kinetics,and degradation mechanisms on the simulated pollutant phenanthrene.The study demonstrated that the plant phenolic acids’ability to stabilize H₂O₂ explained their multi-effect strengthening mechanism on the degradation of phenanthrene.This paper draws the following main findings:（1）By optimizing reaction conditions at an initial pH of 6.0,the Fe³⁺/Catechol/H₂O₂,Fe³⁺/PCA/H₂O₂,Fe³⁺/GA/H₂O₂,and Fe³⁺/TA/H₂O₂ Fenton-like systems were constructed using four plant phenolic acids:catechol（Catechol）,protocatechuic acid（PCA）,gallic acid（GA）,and tannic acid（TA）.The optimum molar ratios for the Fe³⁺/Catechol/H₂O₂,Fe³⁺/PCA/H₂O₂,Fe³⁺/GA/H₂O₂,and Fe³⁺/TA/H₂O₂ systems were found to be 1/0.5/10,1/0.5/10,1/0.1/10,and1/0.1/10,respectively.Under these optimal conditions,the four systems showed enhancement effects on the degradation of phenanthrene in water,increasing the degradation rate by 61.79%,51.95%,62.63%,and 62.06%,respectively,compared to the system without plant phenolic acid（35.39%）.The degradation kinetics of the four Fenton-like systems exhibit conformity to the pseudo-first-order kinetic equation,with the reaction kinetic constants（Kobs）arranged as follows:Fe³⁺/PCA/H₂O₂<Fe³⁺/Catechol/H₂O₂<Fe³⁺/TA/H₂O₂<Fe³⁺/GA/H₂O₂.（2）The mechanism by which plant phenolic acids enhance the degradation of phenanthrene can be categorized into three types:（1）Complexation with Fe³⁺:Plant phenolic acids can form different types of complexes with Fe³⁺under different pH conditions.The formation of these complexes prevents the precipitation of iron and activates H₂O₂.（2）Promotion of Fe³⁺/Fe²⁺cycle:Plant phenolic acids have the ability to reduce Fe³⁺,thus promoting the cycle of Fe³⁺/Fe²⁺in the reaction system and overcoming the limitation of Fe³⁺/Fe²⁺cycle observed in the traditional Fenton reaction.（3）Stabilization of H₂O₂:Plant phenolic acids can form dicomplexes and tricomplexes with Fe³⁺under neutral and alkaline conditions,occupying the active sites of Fe3+and preventing the binding of Fe³⁺with H₂O₂.This,in turn,slows down the ineffectiveness of H₂O₂ breakdown.（3）The electron paramagnetic resonance spectroscopy（EPR）results demonstrate that under the initial pH=6.0,Fe³⁺/Catechol/H₂O₂,Fe³⁺/PCA/H₂O₂,Fe³⁺/GA/H₂O₂,and Fe³⁺/TA/H₂O₂ systems produce active species such as hydroxyl radicals（^·OH）and superoxide radicals(O₂^·-).The contribution rate experiments of free radicals indicate that^·OH and O₂^·-are the primary active species in the four Fe3+-plant phenolic acid Fenton systems,while Fe（IV）may also be present.