Degradation of organic pollutants by ferrate/biochar: Enhanced formation of strong intermediate oxidative iron species

Enhanced Oxidation of Organic Contaminants by Iron(II)-Activated Periodate: The Significance of High-Valent Iron-Oxo Species

Potassium periodate (PI, KIO₄) was readily activated by Fe(II) under acidic conditions, resulting in the enhanced abatement of organic contaminants in 2 min, with the decay ratios of the selected pollutants even outnumbered those in the Fe(II)/peroxymonosulfate and Fe(II)/peroxydisulfate processes under identical conditions. Both ¹⁸O isotope labeling techniques using methyl phenyl sulfoxide (PMSO) as the substrate and X-ray absorption near-edge structure spectroscopy provided conclusive evidences for the generation of high-valent iron-oxo species (Fe(IV)) in the Fe(II)/PI process. Density functional theory calculations determined that the reaction of Fe(II) with PI followed the formation of a hydrogen bonding complex between Fe(H₂O)₆²⁺ and IO₄(H₂O)^-, ligand exchange, and oxygen atom transfer, consequently generating Fe(IV) species. More interestingly, the unexpected detection of ¹⁸O-labeled hydroxylated PMSO not only favored the simultaneous generation of ^·OH but also demonstrated that ^·OH was indirectly produced through the self-decay of Fe(IV) to form H₂O₂ and the subsequent Fenton reaction. In addition, IO₄^- was not transformed into the undesired iodine species (i.e., HOI, I₂, and I₃^-) but was converted to nontoxic iodate (IO₃^-). This study proposed an efficient and environmental friendly process for the rapid removal of emerging contaminants and enriched the understandings on the evolution mechanism of ^·OH in Fe(IV)-mediated processes.

New insight into quinones triggered ferrate in-situ synthesized polynuclear Fe-hydroxyl complex for enhancing interfacial adsorption in highly efficient removal of natural organic matter

In this study, the effects of quinone on the formation of in-situ synthesized polynuclear Fe-hydroxide (PnFe-H) from ferrate activation and enhanced degradation of organics were investigated by in-situ UV linear differential absorbance spectra for the first time. Results indicated benzoquinone (BQ) efficiently activated ferrate for the flocculation of humic acid (HA) that the flocculation reactions rate constants in Fe(VI)-0.1 mM BQ was 3.3 times as much as the blank. Interestingly, quenching studies suggested PnFe-H derived from the high-valence iron species which were the active components by BQ activation, was proved the vital factor for removing of HA. According to the analysis of interaction energy, BQ promoted FeOH²⁺ converted to Fe(OH)₂⁺ and Fe₂(OH)₂⁴⁺ which weakened the polar property and increased hydrophobicity of compounds, further benefited for adsorption with lower Lifshitz-van del Waals (LW) and Lewis acid-base (AB) interfacial energy between PnFe-H-contaminant compounds. However, excessive BQ reduced freshly particulate Fe(III) to Fe(II), weakened the PnFe-H flocculation performance which retarded the transformation of iron species. In addition, the effects of HA concentration were also studied due to the existent of functional quinone-like moieties. The contribution of PnFe-H flocculation removal on the total removal (Re_flocculation/Re_total) improved from 2.6% to 17.09% with Fe(VI)/HA from 0.1 to 1.12. Fe(VI) sufficient oxidized electron-rich moieties and decreased the aromaticity due to π bond was broken, further cooperated with PnFe-H captured small fragment particles by sweep flocculation that Fe(VI) self-accelerating decay produced more Fe(III). The research elucidated a new insight into of ferrate activation by quinone which could expand our knowledge of activation pathway, further regulate the relationship between oxidation and flocculation for enhancing organic and colloidal particle removal in practical application.

Single-Atom Fe Catalyst Outperforms Its Homogeneous Counterpart for Activating Peroxymonosulfate to Achieve Effective Degradation of Organic Contaminants

Recently, reactive iron species (RFeS) have shown great potential for the selective degradation of emerging organic contaminants (EOCs). However, the rapid generation of RFeS for the selective and efficient degradation of EOCs over a wide pH range is still challenging. Herein, we constructed FeN₄ structures on a carbon nanotube (CNT) to obtain single-atom catalysts (Fe_SA-N-CNT) to generate RFeS in the presence of peroxymonosulfate (PMS). The obtained Fe_SA-N-CNT/PMS system exhibited outstanding and selective reactivity for oxidizing EOCs over a wide pH range (3.0-9.0). Several lines of evidences suggested that RFeS existing as an FeN₄═O intermediate was the predominant oxidant, while SO₄^·- and HO^· were the secondary oxidants. Density functional theory calculation results revealed that a CNT played a key role in optimizing the distribution of bonding and antibonding states in the Fe 3d orbital, resulting in the outstanding ability of Fe_SA-N-CNT for PMS chemical adsorption and activation. Moreover, CNT could significantly enhance the reactivity of the FeN₄═O intermediate by increasing the overlap of electrons of the Fe 3d orbital, O 2p orbital, and bisphenol A near the Fermi level. The results of this study can advance the understanding of RFeS generation in a heterogeneous system over a wide pH range and the application of RFeS in real practice.

Heterogeneous catalytic ozonation of atrazine with Mn-loaded and Fe-loaded biochar

After reaction with permanganate or ferrate, the resulted Mn-loaded and Fe-loaded biochar (MnOx/biochar and FeOx/biochar) exhibited excellent catalytic ozonation activity. O₃ (2.5 mg/L) eliminated 48% of atrazine (ATZ, 5 μM) within 30 min at pH 7.0, while under identical conditions, ozonation efficiency of ATZ increased to 83% and 100% in MnOx/biochar and FeOx/biochar (20 mg/L) heterogeneous catalytic systems, respectively. Radical scavenger experiment and electron paramagnetic resonance (EPR) analysis confirmed that hydroxyl radical (•OH) was the dominant oxidant. Total Lewis acid sites on MnOx/biochar and FeOx/biochar were 3.5 and 4.1 times of that on the raw biochar, which induced enhanced adsorption of O₃ and its subsequent decomposition into •OH. Electron transfer via redox pairs on MnOx/biochar and FeOx/biochar was observed by cyclic voltammetry scans, which also functioned in the improved catalytic capacity. Degradation pathways of ATZ in MnOx/biochar and FeOx/biochar ozonation systems were proposed, with 34.6% and 44.8% of dechlorination effect accomplished within 30 min of reaction, which was improved by 4.1 and 5.3 times compared to pure ozonation. After 12-hour treatment, acute toxicity of ATZ oxidation products was reduced from 38.3% of pure ozonation system to 14.5% and 6.3% of activated ozonation systems with MnOx/biochar and FeOx/biochar, respectively. Mn-loaded biochar and Fe-loaded biochar have great potential for heterogeneous catalytic ozonation of polluted water.