Changes in the catalytic activity of oxygen-doped graphitic carbon nitride for the repeated degradation of oxytetracycline

Using waste to treat waste: facile synthesis of hollow carbon nanospheres from lignin for water decontamination

Lignin, the most abundant natural material, is considered as a low-value commercial biomass waste from paper mills and wineries. In an effort to turn biomass waste into a highly valuable material, herein, a new-type of hollow carbon nanospheres (HCNs) is designed and synthesized by pyrolysis of biomass dealkali lignin, as an efficient nanocatalyst for the elimination of antibiotics in complex water matrices. Detailed characterization shows that HCNs possess a hollow nanosphere structure, with abundant graphitic C/N and surface N and O-containing functional groups favorable for peroxydisulfate (PDS) activation. Among them, HCN-500 provides the maximum degradation rate (95.0%) and mineralization efficiency (74.4%) surpassing those of most metal-based advanced oxidation processes (AOPs) in the elimination of oxytetracycline (OTC). Density functional theory (DFT) calculations and high-resolution mass spectroscopy (HR-MS) were employed to reveal the possible degradation pathway of OTC elimination. In addition, the HCN-500/PDS system is also successfully applied to real antibiotics removal in complex water matrices (e.g. river water and tap water), with excellent catalytic performances.

Oxygen doped graphite carbon nitride as efficient metal-free catalyst for peroxymonosulfate activation: Performance, mechanism and theoretical calculation

In this study, oxygen-doped graphitic carbon nitride (g-C3N4), named O-g-C3N4, was successfully fabricated and characterized, and its performance in activating peroxymonosulfate (PMS, HSO5-) for the removal of phenol, 2,4-dichlorophenol (2,4-DCP), bisphenol A (BPA), rhodamine B (RhB), reactive brilliant blue (RBB) and acid orange 7 (AO7) was evaluated. The catalytic performance of O-g-C3N4 for AO7 removal increased by 14 times compared to g-C3N4. In the presence of 0.2 g L-1 O-g-C3N4, 3.5 mM PMS at natural pH 5.8, 96.4% of AO7 could be removed in 60 min, reduced toxicity of the treated AO7 solution was obtained, and the mineralization efficiency was 47.2% within 120 min. Density functional theory (DFT) calculations showed that the charge distribution changed after oxygen doping, and PMS was more readily adsorbed by O-g-C3N4 with the adsorption energy (Eads) of -0.855 kcal/mol than that of the pristine g-C3N4 (Eads: -0.305 kcal/mol). Mechanism investigation implied that AO7 was primarily removed by the sulfate radicals (SO4•-) and hydroxyl radicals (•OH) on the surface of O-g-C3N4, but the role of singlet oxygen (1O2) to AO7 elimination was negligible. The results of cyclic experiments and catalyst characterization after reaction confirmed the favorable catalytic activity and structural stability of O-g-C3N4 particles. Furthermore, the O-g-C3N4/PMS system was very resistant to most of the environmental impacts, and AO7 removal was still acceptable in natural water environment. This study may provide an efficient metal-free carbonaceous activator with low dosage for PMS activation to remove recalcitrant organic pollutants (ROPs).

Mechanism insight into the facet-dependent photoaging of polystyrene microplastics on hematite in freshwater

Hematite, as an extensive natural mineral with multiple crystal facets, profoundly affects the migration and transformation of pollutants in the natural environment. However, little is known about the photochemical behavior of microplastics on different facets of hematite in the aquatic environment. In this work, the photoaging of polystyrene microplastics (PS-MPs) on different crystal planes ({001}, {100}, and {012} facets) and related mechanisms were studied. Two-dimensional correlation spectroscopy analysis illustrated that the reaction pathways of PS-MPs photoaging on hematite tended to preferential chemical oxidization. The stronger performance of PS-MPs photoaging, expressed by particle size reduction and surface oxidation, was observed on the {012} crystal facet. Under irradiation, {012} facet-dominated hematite with a narrower bandgap (1.93 eV) reinforced the photogenerated charge carrier separation, and the lower activation energy barrier (1.41 eV calculated from density functional theory) led to effective •OH formation from water oxidation. These findings elucidate the underlying photoaging mechanism of MPs on hematite with different mineralogical phases.

Degradation of phenol by ball-milled activated carbon (ACBM) activated dual oxidant (persulfate/calcium peroxide) system: Effect of preadsorption and sequential injection

This study explored pre-adsorption and sequential injection of dual oxidant (DuOx) of persulfate (PS) and calcium peroxide (CP) for phenol degradation in an aqueous solution. Ball-milled activated carbon (AC_BM) was used as the catalyst in the following systems: pre-adsorption and sequential injection of PS and CP (AC_BM + PS + CP), pre-adsorption and simultaneous injection of PS and CP (AC_BM + PS/CP), simultaneous injection of AC_BM, PS, and CP (AC_BM/PS/CP), simultaneous injection of AC_BM and PS (AC_BM/PS), and simultaneous injection of AC_BM and CP (AC_BM/CP). The AC_BM had a larger specific surface area, more graphitic structures, and more defects. Moreover, it showed better phenol removal when introduced simultaneously with PS and CP. The phenol removal was most the efficient in AC_BM + PS + CP (98.8%) with a near-neutral final pH, followed by AC_BM + PS/CP, AC_BM/PS, AC_BM/PS/CP, and AC_BM/CP. This indicates that pre-adsorption and separate injection of PS and CP were the key strategy for improved performance and maintained favorable pH for the activation of PS and CP. The dual oxidant system (PS/CP) is superior to single oxidant systems (PS or CP). Scavenger experiments and the electron spin resonance spectra (ESR) demonstrated that non-radical species (¹O₂) were dominantly involved in AC_BM + PS + CP, but radical species (HO^•, SO₄^•-) also contributed. HCO₃^- and HPO₄^2- inhibited phenol degradation in AC_BM + PS + CP, whereas Cl^- and HA had negligible effects. The AC_BM + PS + CP showed high total organic carbon removal and AC_BM was recyclable with a slight decrease in activity. This work is important as it provides a detailed insight into the strategy of pre-adsorption and sequential injection of dual oxidants for a practical and cost-effective method of groundwater remediation.