Hydroxylamine promoted hydroxyl radical production and organic contaminants degradation in oxygenation of pyrite

Iron coupled with hydroxylamine turns on the "switch" for free radical degradation of organic pollutants under high pH conditions

Reducing iron by hydroxylamine (HA) can promote the generation of reactive oxygen species (ROS) in the Fenton reaction and play a crucial role in the degradation of organic pollutants. However, the performance of this system at wider environmental thresholds is still not sufficiently understood, especially in the highly alkaline environments resulting from human activities. Here, we assessed the impact of solution pH on organic pollutant degradation by goethite with the addition of HA and H2O2. The solid phase variation and ROS generation were analyzed using Mössbauer spectroscopy, X-ray absorption near edge structure spectroscopy, and electron paramagnetic resonance analysis. This study found that under alkaline conditions, the system can continuously scavenge organic pollutants through oxygen-mediated generation of free radicals. At lower pH levels, organic pollutant decomposition, exemplified by the breakdown of bisphenol A (BPA), is primarily driven by the Fenton reaction facilitated by iron. As pH increases, hydroxyl radical (•OH) production decreases, accompanied by decreased BPA removal efficiency. However, the removal efficiency of BPA increased significantly at pH > 9. At pH 12, the removal of BPA exceeded that of the acidic condition after one hour, which is consistent with observations in soil system studies. Unlike the Fenton reaction, which is not sensitive to oxygen content, the removal of BPA under alkaline conditions occurs only under aerobic conditions. H2O2 is hardly involved in the reaction, and the depletion of HA becomes a critical factor in the decomposition of BPA. Importantly, in contrast to acidic conditions, where the dramatic decomposition of BPA occurs mainly in the first 10 min, the decomposition of BPA under alkaline conditions continued to occur over the 2 h of observation until complete removal. For natural systems, the remediation of pollutants depends more on the active time of ROS than on their reactivity. Therefore, this idea can reference pollution remediation strategies in anthropogenically disturbed environments.

Monoethanolamine enhanced iohexol degradation in the Co(II)/sulfite system: Nonnegligible role of complexation in accelerating cobalt redox cycling

Generation of sulfate radicals (SO4•-) from sulfite activation has emerged as a promising method for abatement of organic pollutants in the water and wastewater treatment. Co(II) has garnered attention due to its high catalytic activity in the sulfite activation, which is compromised by the slow Co(II)/Co(III) redox cycling. Regarding the regulation of Co(II) electronic structure via the complexation effect, monoethanolamine (MEA), a common chelator, is introduced into the Co(II)/sulfite system. MEA addition results in a significant improvement in iohexol abatement efficiency, increasing from 40% to 92%. The superior iohexol abatement relies on the involvement of SO4•-, hydroxyl radicals (HO•) and Co(IV). Hydrogen radical (•H) is unexpectedly detected, acting as a strong reducing agent, contributing to the reduction of Co(III). This enhancement of sulfite activation by MEA is due to the formation of the Co(II)-MEA complex, in which the complexation ratio of Co(II) and MEA is critical. Electrochemical characterization and theoretical calculations demonstrate that the complexation can facilitate the Co(II)/Co(III) redox cycling with the concomitant enhancement of sulfite activation. This work provides a new insight into the Co(II)/sulfite system in the presence of organic ligands.

Hydroxyl radical production by abiotic oxidation of pyrite under estuarine conditions: The effects of aging, seawater anions and illumination

Pyrite is widely distributed in estuarine sediments as an inexpensive natural Fenton-like reagent, however, the mechanism on the hydroxyl radical (HO·) production by pyrite under estuarine environmental conditions is still poorly understood. The batch experiments were performed to investigate the effects of estuarine conditions including aging (in air, in water), seawater anions (Cl-, Br- and HCO3-) and light on the HO· production by pyrite oxidation. The one-electron transfer dominated the process from O2 to HO· induced by oxidation of pyrite. The Fe (oxyhydr)oxide coatings on the surface of pyrite aged in air and water consumed hydrogen peroxide while mediating the electron transfer, and the combined effect of the two resulted in a suppression of HO· production in the early stage of aging and a promotion of HO· production in the later stage of aging. Corrosion of the surface oxide layers by aggressive anions was the main reason for the inhibition of HO· production by Cl- and Br-, and the generation of Cl· and Br· may also play a role in the scavenging of HO·. HCO3- increased the average rate of HO· production through surface-CO2 complexes formed by adsorption on the surface of pyrite. The significant enhancement of HO· production under light was attributed to the formation of photoelectrons induced by photochemical reactions on pyrite and its surface oxide layers. These findings provide new insights into the environmental chemical behavior of pyrite in the estuary and enrich the understanding of natural remediation of estuarine environments.

Harnessing the power of natural minerals: A comprehensive review of their application as heterogeneous catalysts in advanced oxidation processes for organic pollutant degradation

The release of untreated wastewater into water bodies has become a significant environmental concern, resulting in the accumulation of refractory organic pollutants that pose risks to human health and ecosystems. Wastewater treatment methods, including biological, physical, and chemical techniques, have limitations in achieving complete removal of the refractory pollutants. Chemical methods, particularly advanced oxidation processes (AOPs), have gained special attention for their strong oxidation capacity and minimal secondary pollution. Among the various catalysts used in AOPs, natural minerals offer distinct advantages, such as low cost, abundant resources, and environmental friendliness. Currently, the utilization of natural minerals as catalysts in AOPs lacks thorough investigation and review. This work addresses the need for a comprehensive review of natural minerals as catalysts in AOPs. The structural characteristics and catalytic performance of different natural minerals are discussed, emphasizing their specific roles in AOPs. Furthermore, the review analyzes the influence of process factors, including catalyst dosage, oxidant addition, pH value, and temperature, on the catalytic performance of natural minerals. Strategies for enhancing the catalytic efficiency of AOPs mediated by natural minerals are explored, mainly including physical fields, reductant addition, and cocatalyst utilization. The review also examines the practical application prospects and main challenges associated with the use of natural minerals as heterogeneous catalysts in AOPs. This work contributes to the development of sustainable and efficient approaches for organic pollutant degradation in wastewater.

Novel strategy for copper precipitation from cupric complexes wastewater: Catalytic oxidation or reduction self-decomplexation?

Cupric (Cu(II)) complexes in industrial wastewater are responsible for the failure of conventional alkaline precipitation, but the properties of cuprous (Cu(I)) complexes at alkaline circumstance have not been focused. This report proposed a novel strategy for the remediation of Cu(II)-complexed wastewater by coupling alkaline precipitation with green benign reductant, namely, hydroxylamine hydrochloride (HA). This remediation process (HA-OH) exhibits superior Cu removal efficiency that cannot be achieved with the same dosage of oxidants (3 mM). The possibility of Cu(I) activated O₂ catalysis and self-decomplexation precipitation were investigated, and the results identified that ¹O₂ was generated from Cu(II)/Cu(I) cycle, but it was insufficient to annihilate organic ligands. Cu(I) self-decomplexation was the dominate mechanism of Cu removal. For real industrial wastewater, HA-OH process can realize the efficient Cu₂O precipitation and Cu recovery. This novel strategy utilized intrinsic pollutant in wastewater without introducing other metals, complicated materials, and expensive equipment, broadening the insight for the remediation of Cu(II)-complexed wastewater.

Generation of hydroxyl radicals via activation of Cr(VI) by UVA-LED for rapid decontamination: The important role of Cr(V)

Hexavalent chromium (Cr(VI)) was activated by ultraviolet-A light-emitting diode (UVA-LED), resulting in efficient removal of various pollutants, including dye, pharmaceuticals, and pesticides, with pseudo-first-order rate constants of 0.0610-0.159 min^-1. Comparatively, UVA-LED or Cr(VI) alone barely degraded selected pollutants. Both HO• and Cr(V) were produced in the UVA-LED/Cr(VI) system based on scavenging and probing experiments, UV-visible and electron spin resonance spectra analysis. HO• was demonstrated to be the dominant reactive species via stepwise regeneration of Cr(V) to Cr(VI). The quantum yield of HO• was determined to be 7.79 × 10^-4 mol Es^-1 at a Cr(VI) dosage of 0.5 mM and pH of 6.0. Additionally, the degradation efficiency of sulfamethoxazole (SMX) as a model compound decreased linearly as UVA-LED wavelengths increased from 365 to 405 nm, while SMX was barely degraded at visible light irradiation wavelength ranges (449-505 nm). SMX degradation efficiency increased from 71.0 % to 97.5 % as Cr(VI) dosage increased from 0.05 to 0.7 mM. pH displayed a negative impact on SMX degradation with its removal efficiency decreasing from 99.4 % to 13.3 % as pH increased from 3.0 to 9.0. This study first reported that HO• was generated via activation of Cr(VI) by UVA-LED, which is instructive for the removal of pollutants co-existed in chromium-containing wastewater.

Carbon Quantum Dots Bridged TiO2/CdIn2S4 toward Photocatalytic Upgrading of Polycyclic Aromatic Hydrocarbons to Benzaldehyde

Conversion of hazardous compounds to value-added chemicals using clean energy possesses massive industrial interest. This applies especially to the hazardous compounds that are frequently released in daily life. In this work, a S-scheme photocatalyst is optimized by rational loading of carbon quantum dots (CQDs) during the synthetic process. As a bridge, the presence of CQDs between TiO₂ and CdIn₂S₄ improves the electron extraction from TiO₂ and supports the charge transport in S-scheme. Thanks to this, the TiO₂/CQDs/CdIn₂S₄ presents outstanding photoactivity in converting the polycyclic aromatic hydrocarbons (PAHs) released by cigarette to value-added benzaldehyde. The optimized photocatalyst performs 87.79% conversion rate and 72.76% selectivity in 1 h reaction under a simulated solar source, as confirmed by FT-IR and GC-MS. A combination of experiments and theoretical calculations are conducted to demonstrate the role of CQDs in TiO₂/CQDs/CdIn₂S₄ toward photocatalysis.