Research On The Mechanism Of Cr(Ⅲ)-Fe(Ⅲ) Particles Oxidation By Birnessite Under The Effect Of Dissolved Organic Matter

Since the implementation of the Soil Pollution Prevention and Control Action Plan in China,Chromium（Cr）has been one of the heavy metal pollutants targeted for control and remediation.Cr exhibits stable valence states,namely,Cr（Ⅲ）and Cr（Ⅵ）,of which Cr（Ⅵ）is more toxic and soluble.In recent years,iron-based reductants have been widely used in the remediation of sites and water bodies contaminated with Cr（Ⅵ）,as they can effectively reduce the highly toxic Cr（Ⅵ）to the less toxic Cr（Ⅲ）and form Cr（Ⅲ）-Fe（Ⅲ）hydroxides(Cr_xFe_1-x（OH）₃)in colloidal or precipitated state.Furthemore,Cr_xFe_1-x（OH）₃ tends to accumulate in environments containing Cr and Fe such as drinking water distribution systems and groundwater due to the ferrophilic nature of Cr.Compared to Cr（Ⅵ）,Cr（Ⅲ）exhibits lower biotoxicity and higher stability,making it a suitable chromium pollution remediation end product.However,during long-term remediation,Cr_xFe_1-x（OH）₃ was found to be oxidized by birnessite,causing re-forming of Cr（Ⅵ）and secondary contamination.Dissolved organic matter（DOM）,which is widely present in the earth surface and groundwater,can not only enhance the diffusion of Cr_xFe_1-x（OH）₃ through complexation but also act as an electron shuttle to regulate the oxidation process of Cr（Ⅲ）,and is an essential component in determining Cr morphology.However,few studies have been reported to analyze the reaction mechanism of birnessite and Cr（Ⅲ）-Fe（Ⅲ）particles in the presence of DOM at the kinetic and molecular levels,which limits our understanding of the Cr transformation mechanism in natural environments.In this study,different ratios of Cr_xFe_1-x（OH）₃ were prepared as typical Cr（Ⅲ）-Fe（Ⅲ）particles,and DOM was prepared using Suwannee River Natural organic matter.Oxidation kinetics,combined with X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）and Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR-MS）,the oxidation mechanism of Cr（Ⅲ）-Fe（Ⅲ）particles by birnessite under the effect of DOM was deeply analyzed,and the conversion mechanism of Cr,DOM and MnO₂ was clarified from the molecular level.The main results and conclusions were as follows:（1）The stability and migration ability of Cr（Ⅲ）-Fe（Ⅲ）particles are observed to be their highest levels near a pH value of 5.The maximum zeta potential values for Cr1Fe1,Cr1Fe4,and Cr1Fe9 are 46.18±3.02 m V,34.58±0.38 m V and 31.73±0.63 m V,respectively.Meanwhile,the stability of Cr（Ⅲ）-Fe（Ⅲ）particles became less stable with increasing Fe/Cr molar ratio,and more Fe（Ⅲ）was dissolved from Cr（Ⅲ）-Fe（Ⅲ）particles with higher Fe/Cr molar ratio,leading to enhanced hydrolysis of Fe（Ⅲ）.Increasing the pH in the pH=2 to pH=5,the Cr（Ⅲ）-Fe（Ⅲ）particles gradually formed and the particle size tended to increase.Increasing the pH in the pH=5 to pH=9,the Cr（Ⅲ）-Fe（Ⅲ）particles gradually destabilize and coalesce,eventually forming a precipitate.（2）The oxidation of Cr（Ⅲ）-Fe（Ⅲ）particles by MnO₂ is observed to be enhanced with the reduction of pH and the elevation of Fe/Cr molar ratio.The oxidation systems were analyzed,and it was observed that the most effective oxidation was achieved at pH=5,with Cr（Ⅲ）oxidation rates of 3.39%,4.14%and 4.58%for Cr1Fe1,Cr1Fe4 and Cr1Fe9,respectively.MnO₂ was identified as the primary oxidant for the direct oxidation of Cr（Ⅲ）-Fe（Ⅲ）particles,while reactive oxygen species（ROS）such as O₂^·–,H₂O₂ and^·OH were found to be involved in the indirect oxidation of Cr（Ⅲ）-Fe（Ⅲ）particles by MnO₂.（3）It is noted that the presence of DOM can promote the oxidation of Cr（Ⅲ）-Fe（Ⅲ）particles by MnO₂,and the oxidation effect is observed to be enhanced with the elevation of DOM concentration and the reduction of pH.After adding 0,1,5 and 10 mg C/L DOM at pH=5,the oxidation of Cr1Fe1 by MnO₂ was 3.38%,3.21%,3.40%and 4.05%,respectively.The O₂^·–concentration of the MnO₂-DOM experimental systems conditions was about twice that of the MnO₂ experimental systems under all pH.DOM enhanced the incomplete reduction of O₂ in oxygen vacancies and produced more O₂^·–.The^·OH of the oxidation systems with the addition of DOM under acidic conditions were mainly produced by the Fenton and Fenton-like reactions,which were different from that of the oxidation systems without DOM by the Haber-Weiss reaction.During the oxidation of Cr（Ⅲ）-Fe（Ⅲ）particles by MnO₂ under the effect of DOM,it was observed that the hydroquinone（phenol）-quinone component of DOM underwent a redox reaction,with the highest reactivity being exhibited under acidic conditions.