Deep investigation on different effects of Cl- in transformation of reactive species in Fe(II)/NH2OH/PDS and Fe(II)/NH2OH/H2O2 systems

Deciphering the Formation of Fe(IV) in the Fe(II)/Peroxydisulfate Process: The Critical Role of Sulfate Radical

This study delves into the formation of ferryl ions (Fe(IV)) within the Fe(II)/peroxydisulfate (PDS) process, a pivotal reaction in advanced oxidation processes (AOPs) aimed at organic pollutant removal. Our findings challenge the conventional view that Fe(IV) predominantly forms through oxygen transfer from PDS to Fe(II), revealing that sulfate radicals (SO4•-) play a crucial role in Fe(IV) generation. By employing competitive kinetics, the second-order rate constant for Fe(III) oxidation by SO4•- was quantified as 4.58 × 108 M-1 s-1. Factors such as the probe compound concentration, chloride presence, and iron species influence Fe(IV) generation, all of which were systematically evaluated. Additionally, the study explores Fe(IV) formation in various Fe(II)-activated AOPs, demonstrating that precursors like peroxymonosulfate and H2O2 can also directly oxidize Fe(II) to Fe(IV). Through experimental data, kinetic modeling, and oxygen-18 labeling experiments, this research offers a comprehensive understanding of the Fe(II)/PDS system, facilitating the optimization of AOPs for pollutant degradation. Finally, introducing HSO3- was proposed to shift the Fe(II)/PDS process from Fe(IV)-dominated to SO4•--dominated mechanisms, thereby enhancing pollutant removal efficiencies.

High-valent metal-oxo species transformation and regulation by co-existing chloride: Reaction pathways and impacts on the generation of chlorinated by-products

High-valent metal-oxo species (HMOS) have been extensively recognized in advanced oxidation processes (AOPs) owing to their high selectivity and high chemical utilization efficiency. However, the interactions between HMOS and halide ions in sewage wastewater are complicated, leading to ongoing debates on the intrinsic reactive species and impacts on remediation. Herein, we prepared three typical HMOS, including Fe(IV), Mn(V)-nitrilotriacetic acid complex (Mn(V)NTA) and Co(IV) through peroxymonosulfate (PMS) activation and comparatively studied their interactions with Cl- to reveal different reactive chlorine species (RCS) and the effects of HMOS types on RCS generation pathways. Our results show that the presence of Cl- alters the cleavage behavior of the peroxide OO bond in PMS and prohibits the generation of Fe(IV), spontaneously promoting SO4•- production and its subsequent transformation to secondary radicals like Cl• and Cl2•-. The generation and oxidation capacity of Mn(V)NTA was scarcely influenced by Cl-, while Cl- would substantially consume Co(IV) and promote HOCl generation through an oxygen-transfer reaction, evidenced by density functional theory (DFT) and deuterium oxide solvent exchange experiment. The two-electron-transfer standard redox potentials of Fe(IV), Mn(V)NTA and Co(IV) were calculated as 2.43, 2.55 and 2.85 V, respectively. Due to the different reactive species and pathways in the presence of Cl-, the amounts of chlorinated by-products followed the order of Co(II)/PMS > Fe(II)/PMS > Mn(II)NTA/PMS. Thus, this work renovates the knowledge of halide chemistry in HMOS-based systems and sheds light on the impact on the treatment of salinity-containing wastewater.

Natural mineral-derived Fe/Mn-BC as efficient peroxydisulfate activator for 2,4-dichlorophenol removal from wastewater: Performance and sustainable catalytic mechanism

Iron and manganese oxides/biochar composite materials (Fe/Mn-BC) are promising catalysts in the field of advanced oxidation. High purity chemical reagents are popular precursors for preparing Fe/Mn-BC, while the potential of low-cost natural minerals as precursors has been neglected. In this study, high-efficiency Fe/Mn-BC was synthesized by one-step pyrolysis method using hematite, phosphoromanganese, and bagasse. The synthesized Fe/Mn-BC removed 83.7% 2, 4-dichlorophenol (2, 4-DCP) within 30 min, about 8.8 and 10.6 times better than biochar (BC) and Fe/Mn complex, respectively. The removal of 2, 4-DCP in the Fe/Mn-BC + peroxydisulfate (PDS) system was influenced by catalyst dosage, PDS concentration, initial pH, organic acids, and chromium. Sulfate radical (SO₄^•-) and hydroxyl radicals (•OH) generated by Fe/Mn-BC-activated PDS have similar contribution to the degradation of 2,4-DCP. A possible removal mechanism of 2, 4-DCP in the Fe/Mn-BC + PDS system was proposed based on Electron Spin Resonance spectroscopy, free radical quenching experiments, X-ray photoelectron spectroscopy, X-ray diffraction, and electrochemical measurement. Fe⁰ and Fe(II) in Fe/Mn-BC play significant role in catalytic degradation of 2, 4-DCP at the early stage of the reaction (within 0-5 min). Then, the interaction between Mn and BC or structural Mn and structural Fe gradually became dominant in the later stage. Similarly, the electron transfer promoted by biochar also played an important role in this catalysis. This discovery provided a new strategy for developing iron and manganese oxides/biochar composite materials to activate PDS for the elimination of refractory organic pollutants.

Iodide ions as invisible chemical scissors tailoring carbon nitride for highly efficient photocatalytic H2O2 evolution

The obtained TCN/NaI possesses Na+ and –OH co-modification, which shows the highest H2O2 generation rate. DFT calculations further reveal that the electron of β spin-orbital in TCN/NaI can transfer to the π* orbital of O2 more facile than BCN.