N-doped citrate-sludge-derived carbon (NCSC) effectively promotes peroxymonosulfate activation for perfluorooctanoic acid (PFOA) removal with surface-mediated electron transfer mechanism

In traditional wastewater treatment methods, the removal of emerging contaminants including perfluorooctanoic acid (PFOA) can be challenging. To address this, biochar is commonly utilized as an activator for peroxymonosulphate (PMS) to effectively eliminate organic pollutants. Sewage sludge has shown potential as a biochar precursor, but its complex composition and variable iron content, as well as the low specific surface area of the product limit the practical use of iron-dominated sludge-derived catalysts. To overcome this limitation, N-doped citrate-sludge-derived carbon (NCSC) was synthesized, possessing a low iron content (0.29 at%) and a large specific surface area (315.31 m2 g-1). As a comparison, Fe-/N-doped citrate-sludge-derived carbon (Fe-NCSC) was prepared by introducing exogenous iron, resulting in a higher iron content (2.12 at%) but a significantly reduced specific surface area (73.87 m2 g-1). In performance evaluation, the NCSC/PMS system achieved impressive removal efficiency, effectively eliminating 99.8% of PFOA (at an initial concentration of 2 mg L-1) within 60 min, while Fe-NCSC/PMS only achieved 84.6%. The slightly lower reaction rate per specific surface area of NCSC/PMS proved that large specific surface area was NCSC's key advantage. The lower sensitivity of NCSC to pH and water substrates than FeNCSC suggested different activation mechanisms. Further analysis of reactive sites and species showed that the main oxidation mechanism of NCSC/PMS was forming the surface-bound PMS-NCSC complexes at the N sites, followed by PFOA donating electrons to the complexes to be oxidized, which was different from the Fe/N-dominated singlet oxygen mechanism of Fe-NSC/PMS. Furthermore, the reusability of the NCSC was demonstrated, with the removal rate decreasing to only 90.1% after four cycles and recovering to 94.8% after heated regeneration. In conclusion, this study provides a viable method for the elimination of emerging contaminants such as PFOA in water remediation.

Enhanced degradation of herbicides in groundwater using sulfur-containing reductants and spinel zinc ferrite activated persulfate

A challenge to successfully implementing an injection-based remedial treatment in aquifers is to ensure that the oxidative reaction is efficient and lasts long enough to contact the contaminated plume. Our objective was to determine the efficacy of zinc ferrite nanocomposites (ZnFe2O4) and sulfur-containing reductants (SCR) (i.e., dithionite; DTN and bisulfite; BS) to co-activate persulfate (S2O82-; PS) and treat herbicide-contaminated water. We also evaluated the ecotoxicity of the treated water. While both SCRs delivered excellent PS activation in a 1:0.4 ratio (PS:SCR), the reaction was relatively short-lived. By including ZnFe2O4 in the PS/BS or PS/DTN activations, herbicide degradation rates dramatically increased by factors of 2.5 to 11.3. This was due to the SO4- and OH reactive radical species that formed. Radical scavenging experiments and ZnFe2O4 XPS spectra results revealed that SO4- was the dominant reactive species that originated from S(IV)/PS activation in solution and from the Fe(II)/PS activation that occurred on the ZnFe2O4 surface. Based on liquid chromatography mass spectrometry (LC-MS), atrazine and alachlor degradation pathways are proposed that involve both dehydration and hydroxylation. In 1-D column experiments, five different treatment scenarios were run using 14C-labeled and unlabeled atrazine, and 3H2O to quantify changes in breakthrough curves. Our results confirmed that ZnFe2O4 successfully prolonged the PS oxidative treatment despite the SCR being completely dissociated. Toxicity testing showed treated 14C-atrazine was more biodegradable than the parent compound in soil microcosms. Post-treatment water (25 %, v/v) also had less impact on both Zea Mays L. and Vigna radiata L. seedling growth, but more impact on root anatomies, while ≤4 % of the treated water started to exert cytotoxicity (<80 % viability) on ELT3 cell lines. Overall, the findings confirm that ZnFe2O4/SCR/PS reaction is efficient and relatively longer lasting in treating herbicide-contaminated groundwater.

The potential of green biochar generated from biogas residue as a heterogeneous persulfate activator and its non-radical degradation pathways: Adsorption and degradation of tetracycline

Advanced oxidation aided by sulfate radicals (SO₄^-) is an effective option for the treatment of refractory pollutants from aqueous solutions. In this work, a metal-free biochar catalyst was prepared using pyrolyzed biogas residue as the raw material. The biogas residue carbon (BRC) obtained at 800 °C showed excellent catalytic activity and adsorption capacity for the removal of tetracycline (TC) with 97.9% of removal efficiency. Such performance is accounted for by the rich pores and accelerated electron transformability conferred by its defect structure with the crucial role of pyrolysis temperature in regulating catalyst properties. The BRC-800/peroxymonosulfate (PMS) system worked predominantly through non-radical pathways with high stability/recyclability without being interfered by organic/inorganic compounds in an actual water environment. The exceelent removal performance is also supported by the kinetic reaction rate of the BRC-800/PMS system as estimated to be 0.03017 min^-1. This work provides a simple and effective path for modifying biogas residue waste for versatile applications.