Insights into naphthalene degradation in aqueous solution and soil slurry medium: Performance and mechanisms

Ascorbic acid enhanced the circulation between Fe(II) and Fe(III) in peroxymonosulfate system for fluoranthene degradation

In this research, ascorbic acid (AA) was used to enhance Fe(II)/Fe(III)-activated permonosulfate (PMS) systems for the degradation of fluoranthene (FLT). AA enhanced the production of ROS in both PMS/Fe(II) and PMS/Fe(III) systems through chelation and reduction and thus improved the degradation performance of FLT. The optimal molar ratio in PMS/Fe(II)/AA/FLT and PMS/Fe(III)/AA/FLT processes were 2/2/4/1 and 5/10/5/1, respectively. In addition, the experimental results on the effect of FLT degradation under different groundwater matrixes indicated that PMS/Fe(III)/AA system was more adaptable to different water quality conditions than the PMS/Fe(II)/AA system. SO4·- was the major reactive oxygen species (ROS) responsible for FLT removal through the probe and scavenging tests in both systems. Furthermore, the degradation intermediates of FLT were analyzed using gas chromatograph-mass spectrometry (GC-MS), and the probable degradation pathways of FLT degradation were proposed. In addition, the removal of FLT was also tested in actual groundwater and the results showed that by increasing the dose and pre-adjusting the solution pH, 88.8 and 100% of the FLT was removed for PMS/Fe(II)/AA and PMS/Fe(III)/AA systems. The above experimental results demonstrated that PMS/Fe(II)/AA and PMS/Fe(III)/AA processes have a great perspective in practice for the rehabilitation of FLT-polluted groundwater.

Persulfate-based remediation of organic-contaminated soil: Insight into the impacts of natural iron ions and humic acids with complexation/redox functionality

The use of persulfate (PDS) for in-situ chemical oxidation of organic contaminants in soils has garnered significant interest. However, the presence of naturally occurring iron-containing substances and humic acid (HA) in environmental compartments can potentially influence the effectiveness of soil remediation. Thus, this study aimed to investigate the role of key functional groups (adjacent phenolic hydroxyl (Ar-OH) and carboxyl groups (-COOH)) in HA that interact with iron. Modified HAs were used to confirm the significance of these moieties in iron interaction. Additionally, the mechanism by which specific functional groups affect Fe complexation and redox was explored through contaminant degradation experiments, pH-dependent investigations, HA by-products analysis, and theoretical calculations using six specific hydroxybenzoic acids as HA model compounds. The results showed a strong positive correlation between accessible Ar-OH and -COOH groups and Fe3+/Fe2+ redox. This was attributed to HA undergoing a conversion process to a semiquinone-containing radical form, followed by a quinone-containing intermediate, while Fe3+ acted as an electron shuttle between HA and PDS, with Fe3+ leaching facilitated by generated H+ ions. Although the stability of HA-Fe3+ complexes with -COOH as the primary binding sites was slightly higher at neutral/alkaline conditions compared to acidic conditions, the buffering properties of the soil and acidification of the PDS solution played a greater role in determining the Ar-OH groups as the primary binding site in most cases. Therefore, the availability of Ar-OH groups on HA created a trade-off between accelerated Fe3+/Fe2+ redox and quenching reactions. Appropriate HA and iron contents were found to favor PDS activation, while excessive HA could lead to intense competition for reactive oxygen species (ROS), inhibiting pollutant degradation in soil. The findings provide valuable insights into the interaction of HA and Fe-containing substances in persulfate oxidation, offering useful information for the development of in-situ remediation strategies for organic-contaminated soil.

Waste self-heating bag derived iron-based composite with abundant oxygen vacancies for highly efficient Fenton-like degradation of micropollutants

In this study, iron-rich waste self-heating bag was reutilized as the raw material to prepare oxygen vacancies (OV) functionalized iron-based composite (iron oxide (Fe₃O₄)-carbon-vermiculite, viz. OV-ICV), which exhibited excellent performance in the Fenton-like degradation of micropollutants via peroxydisulfate (PDS) activation. Above 95% of 1.0 mg/L carbaryl (CB) was efficiently eliminated in the presence of 0.1 g/L of OV-ICV and 0.5 mmol/L of PDS over a wide pH range of 3-10 within 30 min. Besides, OV-ICV also showed acceptable adaptability, stability, and renewability. Imbedding OV into Fe₃O₄ structure significantly generated more active iron sites and localized electrons, promoted the charge transfer ability, and assisted the redox cycle of ≡Fe(III)/≡Fe(II) for PDS activation. Mechanism investigation demonstrated that superoxide radicals (O₂^•-) derived from the activation of molecular oxygen mediated the generation of H₂O₂, and both of them further enhanced the formation of more sulfate radicals (SO₄^•-) and hydroxyl radicals (^•OH), which led to the efficient degradation and mineralization of CB. Furthermore, the degradation pathways of CB were proposed based on the intermediates identification. This work lays a foundation for the rational reutilization of iron-containing wastes modified with defect engineering in heterogeneous Fenton-like catalysis for the remediation of micropollutants wastewater.

Surfactant enhanced thermally activated persulfate remediating PAHs-contaminated soil: Insight into compatibility, degradation processes and mechanisms

Although advanced oxidation processes (AOPs) based on persulfate (PS) is an attractive approach for repairing polycyclic aromatic hydrocarbons (PAHs) contaminated soils, limited oxidizability of PAHs and efficient in-situ activation of PS hinder its practical applications. In this study, we comprehensively examined the contributions of five representative surfactants on the oxidative remediation of PAHs-contaminated soil in terms of degradation kinetics of the pollutants, and further proposed an innovative coupling strategy of surfactant-enhanced thermally activated PS remediating PAHs-contaminated soil. The results showed that the degradation process of PAHs in soil was significantly facilitated only via adding sodium dodecyl benzenesulfonate (SDBS) and fitted the pseudo-first-order kinetic pattern. The removal of phenanthrene (PHE) reached 98.56% at 50 mM PS, 50 °C, 5 g L^-1 SDBS and 48 h reaction time, accompanying an increase of 25% in reaction rate constant from 0.0572 h^-1 (without SDBS) to 0.0715 h^-1. More importantly, SDBS-enhanced thermally activated PS degrading PAHs with higher benzene rings were more effective as the reaction rate constants of pyrene (PYR) and benzo(a)anthracene (BaA) were significantly increased by 49.40% and 56.86%. Additionally, only appropriate dosages (5-10 g L^-1) of SDBS facilitated the oxidative degradation of PHE, as well as the aging time of contaminant-soil contact slowed down the enhancement of oxidative degradation of PHE by SDBS. Scavenger experiments demonstrated that SO₄^·- and ¹O₂ were the dominant reactive oxygen species. Finally, a possible oxidative degradation pathway of PHE was proposed, and the toxicity of derived intermediates got alleviation by the assessment using the Toxicity Estimation Software Tool. This investigation was promising for in situ scale-up remediation of PAHs-contaminated soil.

Metal ion endogenous cycles of CoFe2O4-x induced boosted photocatalytic/PMS degradation toward polycyclic aromatic hydrocarbons

The rational design of photocatalytic nanomaterials with unique structures is critical for remediating environmental problems and thus reducing ecological risks. In this work, we employed H₂ temperature-programmed reduction to modify MFe₂O₄ (M = Co, Cu, and Zn) photocatalysts for obtaining additional oxygen vacancies. After activation of PMS, naphthalene and phenanthrene degradation rates in the soil phase were increased by 3.24-fold and 1.39-fold, respectively, and 1.38-fold for naphthalene in the aqueous phase by H-CoFe₂O_4-x. The extraordinary photocatalytic activity is attributed to the oxygen vacancies on the H-CoFe₂O_4-x surface, which promote electron transfer and thus enhance the redox cycle from Co(III)/Fe(III) to Co(II)/Fe(II). In addition, oxygen vacancies are used as electron traps to hinder the recombination of photogenerated carriers and accelerate the generation of hydroxyl and superoxide radicals. Quenching tests showed that the addition of p-benzoquinone resulted in the greatest decrease in the degradation rate of naphthalene (inhibition of about 85.5%), demonstrating that O₂˙^- radicals are the main active species in the photocatalytic degradation of naphthalene. H-CoFe₂O_4-x showed improved degradation performance in synergy with PMS (82.0%, k_app = 0.00714 min^-1) while maintaining excellent stability and reusability. Hence, this work provides a promising approach for the design of efficient photocatalysts to degrade persistent organic pollutants in soil and aqueous environments.

Advanced oxidation processes for the removal of mono and polycyclic aromatic hydrocarbons - A review

Aromatic hydrocarbons (AHs) are toxic environmental contaminants presented in most of the environmental matrices. Advanced oxidation processes (AOPs) for the removal of AHs in the account of complete mineralization from various environmental matrices have been reviewed in this paper. An in-depth discussion on various AOPs for mono (BTEX) and polyaromatic hydrocarbons (PAHs) and their derivatives is presented. Most of the AOPs were effective in the removal of AHs from the aquatic environment. A comparative study on the degradation of various AHs revealed that the oxidation of the AHs is strongly dependent on the number of aromatic rings and the functional groups attached to the ring. The formation of halogenated and nitrated derivatives of AHs in the real contaminated water containing chloride, nitrite, and nitrate ions seems to be a challenge in using the AOPs in real systems. The phenolic compounds, quinone, alcohols, and aliphatic acids are the important byproducts formed during the oxidation of AHs, initiated by the attack of reactive oxygen species (ROS) on their electron-rich center. In conclusion, AOPs are the adaptable method for the removal of AHs from different environmental matrices. The persulfate-based AOPs were applied in the soil phase removal as an in situ chemical oxidation of AHs. Moreover, the combination of AOPs will be a conclusive solution to avoid or minimize unexpected or other toxic intermediate products and to obtain rapid oxidation of AHs.

Insights into enhanced removal of fluoranthene by sulfidated nanoscale zero-valent iron: In aqueous solution and soil slurry

In this study, 90.9% fluoranthene (FLT) was degraded in sodium percarbonate (2Na₂CO₃·3H₂O₂, SPC) oxidation system by Fe(II) combined with sulfidated nano zero valent iron (S-nZVI) activation within 60 min in aqueous solution. Scavenging experiments and electron paramagnetic resonance detection suggested that HO•, O₂^-•, and ¹O₂ contributed to the removal of FLT in SPC/Fe(II)/S-nZVI system. Based on the FLT degradation intermediates that were analyzed by GC-MS in SPC/Fe(II)/S-nZVI process, three potential FLT degradation pathways were speculated. The removal efficiency of FLT was inhibited with the presence of humic acid (HA) unless the concentration of HA was controlled at 1.0 mg L^-1, and the presence of 1.0 mg L^-1 HA favored the generation of HO•. The excellent removal performance of FLT (88.6%) could be achieved in actual groundwater by increasing the chemical dosages and adjusting the initial solution pH to acid environment. In soil slurry tests, the optimal reaction time and soil/water ratio were obtained as 24 h and 2/10, respectively, and the desired FLT degradation performances were obtained at pH 3 and 5 with the soil/water ratio of 2/10. This work effectively demonstrates the application potential of SPC/Fe(II)/S-nZVI system for the remediation of PAHs contamination in actual industrial sites.

Remediation of sulfathiazole contaminated soil by peroxymonosulfate: Performance, mechanism and phytotoxicity

Peroxymonosulfate (PMS) was successfully adopted to remove organic pollutants in water, but it was rarely applied to soil remediation. Sulfathiazole (STZ) is a widely used sulfonamide antibiotic, while its residues have negative impacts on soil. To the best of our knowledge, this is the first attempt to apply PMS for the treatment of STZ-contaminated soil. The results showed that 4 mM PMS can degrade 96.54% of STZ in the soil within 60 min. Quenching and probe experiments revealed that singlet oxygen rather than hydroxyl radical and sulfate radical was the predominant reactive oxygen species responsible for STZ removal. The presence of Cl^-, SO₄^2-, NO₃^-, Fe³⁺, and HA enhanced the degradation efficiency of STZ, while HCO₃^- and Mn²⁺ presented an obstructive effect on STZ elimination at high concentrations. Different chemical extraction procedures were used to determine the bioavailability of the heavy metals. PMS oxidation process caused an unnoticeable influence of the concentrations of heavy metals except for the increase of Mn concentration and the decrease of Ba concentration. Moreover, the germination rate and stem length of wheat and radish both increased, indicating PMS oxidation reduced the toxicity of STZ, and the increase of Mn concentration did not cause a negative impact on their growth. Besides, the results of XRD and FTIR tests showed oxidation processes have negligible impacts on soil structure and composition. Based on intermediates identified, STZ degradation pathways in the PMS system were proposed. According to the results of this study, using PMS alone to repair STZ-contaminated soil is a relatively feasible, safe, and environmentally friendly technology.