Mechanism Of Peroxodisulfate Activated By Soil Iron Minerals Coupling With Tea Polyphenols For Organophosphorus Flame Retardants Degradation

The widespread use of organophosphorus flame retardants（OPFRs）has resulted in their massive release into soil,which has caused serious environmental pollution.It is urgent to explore efficient removal methods.The activation of peroxodisulfate（PDS）to generate reactive oxygen species（ROS）for the degradation of organic contaminates is a current research hotspot.However,the commonly used exogenous activation methods have many limitations with low economic benefits.Because soil is rich in active constituents（such as soil iron minerals）,it is promising to use it to activate PDS for the remediation of soil contaminated by organic contaminates.However,the current research on the activation of PDS by soil constituents is still very limited,and further research is urgently needed.The specific research contents and results of this study are as follows:（1）In this study,a typical OPFR,triphenyl phosphate（TPHP）,was used as the research object to explore the influencing factors and mechanism of soil constituent-activated PDS on TPHP degradation.Electron paramagnetic resonance（EPR）analysis,ROS chemical quenching experiments,and model ROS verification experiments,degradation product identification were carried out.As showed,soil constituents can promote the decomposition of PDS to degrade TPHP in soil.Iron minerals and soil organic matter（SOM）jointly mediated the formation and propagation of ROS,e.g.,sulfate radicals(SO₄^·-),hydroxyl radicals（·OH）,singlet oxygen（¹O₂）,superoxide radicals(O₂^·-),and soil organic matter free radicals（SOM·）,in the reaction system.Among them,the solid phase iron-containing minerals（Fe-minerals）in soil and their redox cycle（Fe（II）?Fe（III））process are the keys to promoting the activation of PDS to generate ROS.Besides promoting the formation and propagation of ROS,SOM also rapidly depletes ROS,which has a significant negative impact on TPHP degradation.ROS can degrade TPHP to less toxic para-hydroxy triphenyl phosphate（p-OH-TPHP）,diphenyl phosphate（DPHP）,and phosphate(PO₄^3-).SO₄^·-and·OH could completely mineralize TPHP.¹O₂ could degrade both of TPHP and p-OH-TPHP to DPHP,but could not further mineralize them to PO₄^3-.O₂^·-couldn’t degrade TPHP and intermediates directly.With the increase of soil-water ratio,the inhibitory effect of SOM on degradation was significantly enhanced.Cl^-significantly inhibited the degradation of TPHP,while HCO₃^-and H₂PO₄^-had little effects on the degradation of TPHP.（2）The complexing-reducing reagents（CRs）were proposed for the first time to enhance the potential of soil iron minerals to activate PDS.The degradation efficiency,reaction mechanism,and influencing factors were explored in CRs coupling with soil minerals-activated PDS process.It’s found that tea polyphenols（TP）can effectively complex and reduce solid insoluble Fe（III）atoms and dissolved Fe（III）ions,transforming them into solid insoluble Fe（II）atoms,TP-Fe complexes,and Fe（II）ions,which may further promote the activation efficiency of PDS in soil.When the dosage of TP was 5-40m M,the content of dissolved Fe（Ⅱ）in soil increased from 0.05 mg L^-1 to 26.73 mg L^-1,and the content of insoluble Fe（Ⅱ）increased from 729.97 mg kg^-1 to 3803.68 mg kg^-1,and the degradation efficiency of TPHP increased from 10.8%to 58.6%,respectively.ROS chemical quenching experiments,EPR analysis,and kinetic solvent isotope effect（KSIE）experiments showed that PDS is mainly activated in heterogeneous ways.SO₄^·–(especially surface-bound SO₄^·–(≡SO₄^·–)),·OH（especially surface-bound·OH（≡OH·））,and ¹O₂ play major roles in the degradation of TPHP,with the contribution of about 39.0%,44.0%,and17.0%,respectively.Soil total iron content is not the key factor to determine TP complex reduction efficiency and TPHP degradation.The higher the abundance of highly active iron components（such as exchangeable,absorbed,organic,and amorphous iron）in the soil,the higher the yield of Fe（II）species and the higher the degradation efficiency of TPHP in soil.The increase of SOM content and pH value both inhibited the degradation of TPHP by ROS.Inorganic anions(including Cl^-,H₂PO₄^-,HCO₃^-,NO₃^-,and SO₄^2-)and manganese minerals had limited effects on TPHP degradation.The complexing and reducing of soil iron minerals by tea polyphenols can realize the in-situ reduction of soil solid phase iron,the activation of PDS and TPHP degradation in soil column.（3）In order to reveal the reaction mechanism of soil iron minerals reacting with TP to form"iron-polyphenol complex（Fe-TP）"particles,its environmental behavior and activation mechanism for PDS,we synthesized Fe-TP particles and further evaluated its migration in porous media and activation of PDS to degrade TPHP.The dark purple Fe²⁺/Fe³⁺-coexisted amorphous iron-polyphenol complex（Fe-TP）formed after TP complexed with Fe ions.Under weak acid conditions,the particle size of Fe-TP complexes is about 3.0μm based on the scanning electron microscope（SEM）image,and the hydrodynamic particle size（D_h）is 2-4μm.The reduction potential of Fe-TP complex solution（-0.134 V）is lower than those for Fe and TP solutions,and the reduction potential of Fe-TP complex solution decreases gradually as pH arising.Under strong acid conditions,the D_h of Fe-TP particles is small（<1μm）,and the migration capacity is large(the penetration efficiency in the plateau phase:99%;the maximum interception concentration:60 mg kg^-1).Under weak acid conditions,Fe-TP particles have a larger D_h and a weaker penetration ability(the penetration efficiency in the plateau phase:68%;the maximum interception concentration:163 mg kg^-1).Under the weak acid condition,the maximum migration distance(L_max)and deposition coefficient（k_d）of Fe-TP particles in the quartz sand column are 2.15 m and 2.80 s^-1,respectively.One-dimensional convection-dispersion equation（ADE）and two-point kinetic retention model can be used to simulate the penetration and retention behavior of Fe-TP particles（R²>90%）.The two-site interception concentration and interception coefficients,S_max1/C₀,S_max2/C₀,k_sw1,and k_sw2,are 1.88 cm³kg^-1,8.82 cm³ kg^-1,1.23 min^-1,and 1.0×10^-12 min^-1,respectively.Na Cl and Ca Cl₂ had little effect on the zeta（ζ）potential,particle size and migration ability of Fe-TP complexes,while Na H₂PO₄ had a greater effect on them via interacting with Fe ions.Fe-TP particles can promote the activation of PDS to generate SO₄^·-and·OH for TPHP degradation.Both Fe³⁺concentration and pH could affect the degradation efficiency of TPHP.Higher concentrations of SOM inhibited TPHP degradation,while lower concentrations of SOM promoted TPHP degradation.The oxidation products of TP had insignificant effect on the degradation efficiency of TPHP.The presence of Cl^-,HCO₃^-,and H₂PO₄^-significantly inhibited the degradation of TPHP.The entrapment of Fe-TP complexes would accelerate ROS consumption and lead to lower degradation efficiency of TPHP.This study first clarified the mechanism of soil constituents-activated PDS to form a variety of ROS for TPHP degradation in soil.We proposed an innovative strategy that introducing TP to complex and reduce soil iron minerals in soil,thereby enhancing the activation of PDS to generate ROS for TPHP degradation in soil.In addition,from the perspective of colloid science,we elaborated the formation,the migration and transformation mechanism of Fe-TP colloids under different environment-related conditions and their impacts on PDS activation and TPHP degradation.In conclusion,this study provides a theoretical reference and application guidance for the application of soil iron minerals to activate PDS technology to remediate OPFRs-contaminated soil.