Photocatalytic degradation mechanism of phenanthrene over visible light driven plasmonic Ag/Ag3PO4/g-C3N4 heterojunction nanocomposite

Phenanthrene sorption studies on coffee waste- and diatomaceous earth-based adsorbents, and adsorbent regeneration with cold atmospheric plasma

Phenanthrene (PHE) is a polycyclic aromatic hydrocarbon categorized as a high priority organic pollutant being toxic for the ecosystem and human health, and its sorption on natural organic or inorganic substances seems a well-promising method for its removal from water streams. The goals of the present work are (i) to assess the capacity of low-cost adsorbents fabricated by treating coffee wastes and diatomaceous earth to remove PHE from water; (ii) to elucidate the role of the pore structure on PHE sorption dynamics; and (iii) to assess the potential to regenerate adsorbents loaded with PHE, by using the novel technology of cold atmospheric plasma (CAP). Diatomaceous earth (DE) and DE pre-treated with sodium hydroxide (NaOH) or phosphoric acid (H3PO4) were chosen as inorganic adsorbents. Coffee waste (CW) and activated carbons (AC) produced from its pyrolysis at 800 °C (CWAC), either untreated (CWAC-800) or pre-treated with NaOH (CWAC-NaOH-800) and H3PO4 (CWAC-H3PO4-800), were chosen as organic adsorbents. The adsorbents were characterized with nitrogen adsorption-desorption isotherms, attenuated total reflectance-Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and mercury intrusion porosimetry. Based on the PHE sorption capacity and pore structure/surface characteristics, the CWAC-NaOH-800 was chosen as the most efficient adsorbent for further equilibrium and kinetic sorption studies. The multi-compartment model was used to describe the PHE sorption dynamics in CWAC-NaOH-800 by accounting for the pore/surface diffusion and instantaneous sorption. The CWAC-NaOH-800 exhibited remarkable values for (i) the specific surface area (SBET = 676.5 m2/g) and meso- and micro-pore volume determined by nitrogen sorption (VLN2 = 0.415 cm3/g); (ii) the macro- and meso-pore volume determined by mercury intrusion porosimetry (VMIP = 3.134 cm3/g); and (iii) the maximum PHE sorption capacity (qmax = 142 mg/g). The percentage of adsorbent recovery after its regeneration with CAP was found to be ~ 35%. From the simulation of sorption dynamics, it was found that at early times, the sorption kinetics is governed by the film diffusion towards the external surface of grains, but at late times, most of the adsorbed mass is transferred primarily to meso-/macro-pores via diffusion, and secondarily to micro-porosity via surface diffusion. Based on the adsorbent characteristics, effect of pH on sorption efficiency, and numerical analysis of sorption dynamics, it was concluded that probably the dominant adsorption mechanism is the π-π interactions between hydrophobic PHE aromatic rings and CWAC-NaOH-800 graphene layers. The high PHE removal efficiency of CWAC-NaOH-800, the successful interpretation of sorption dynamics with the multi-compartment model, and the potential to regenerate PHE-loaded adsorbents with the green and economic technology of CAP motivate a strategy for testing CWACs towards the adsorption of other PAHs, application of adsorbents to real wastewaters, and scaling-up to pilot units.

Recent updates on remediation approaches of environmentally occurring pollutants using visible light-active nano-photocatalysts

Photocatalysis emerges as a potential remedy for the issue of an unreliable light source. Recognized as the most dependable and potent energy source sustaining life on Earth, sunlight offers a promising solution. Sunlight is abundant and free, operational costs associated with running photocatalytic system using nanoparticles are often lower compared to system relying on artificial light source. The escalating problem of water pollution, particularly in highly industrialized nations, necessitates effective wastewater treatment methods. These methods aim to combat elevated pollution levels, encompassing pharmaceuticals, dyes, flame retardants, and pesticide components. Advanced oxidation processes within photocatalytic wastewater treatment exhibit substantial promise for removing complex organic pollutants. Doped nanomaterials, with their enhanced properties, enable efficient utilization of light. Coupled nanomaterials present significant potential in addressing both water and energy challenges by proficiently eliminating persistent pollutants from environment. Photocatalysis when exposed to sunlight can absorb photons and generate e- h + pairs. This discussion briefly outlines the wastewater treatment facilitated by interconnected nanomaterials, emphasizing their role in water-energy nexus. In exploring the capabilities of components within a functional photocatalyst, a comprehensive analysis of both simple photocatalysts and integrated photocatalytic systems is undertaken. Review aims to provide detailed explanation of the impact of light source on photon generation and significance of solar light on reaction kinetics, considering various parameters such as catalyst dosage, pH, temperature, and types of oxidants. By shedding light on these aspects, this review seeks to enhance our understanding of intricate processes involved in photocatalysis and its potential applications in addressing contemporary environmental challenges.

Enhanced visible light photocatalytic activity of octopod Ag3PO4 microcrystals with high index crystal faces

Novel octopod shaped Ag3PO4 microcrystals were successfully fabricated by a simple ion exchange method under the conditions of a hot water bath using [Ag(NH3)2]+ solution and Na2HPO4 solution as the precursors. Meanwhile, sphere and cube shaped Ag3PO4 microcrystals were also prepared followed by changing reaction materials as well as temperature. The surface morphology, microstructure and photocatalytic performance were investigated on the three different shaped crystals respectively. Compared to sphere and cube counterparts, the obtained octopod shaped Ag3PO4 crystals possess 8 symmetric feet with sharp tips and exhibit higher photocatalytic activity and better cycle stability. After further exploring its formation process, UV-vis diffusion reflectance properties as well as photocurrent transient response, it was found that the Ag3PO4 octopod had exposed high index crystal faces, and possessed a narrow band gap as well as high photoexcited transient charge separation efficiency. The results show that the improved photocatalytic activity of octopod shaped Ag3PO4 is mainly due to the synergistic action of the strong light absorption capacity and high carrier separation efficiency. These results highlight the tremendous practical application of octopod Ag3PO4 microcrystals in visible light photocatalysis.

Ag3PO4-anchored La2Ti2O7 nanorod as a Z-Scheme heterostructure composite with boosted photogenerated carrier separation and enhanced photocatalytic performance under natural sunlight

Developing wide spectra-responsive photocatalysts has attracted considerable attention in the photocatalytic technology to achieve excellent catalytic activity. Ag₃PO₄, with strong response to light spectra shorter than 530 nm, shows extremely outstanding photocatalytic oxidation ability. Unfortunately, the photocorrosion of Ag₃PO₄ is still the biggest obstacle to its application. Herein, the La₂Ti₂O₇ nanorod was used to anchor Ag₃PO₄ nanoparticles in this study, and a novel Z-Scheme La₂Ti₂O₇/Ag₃PO₄ heterostructure composite was constructed. Remarkably, the composite showed strong responsive to most of the spectra in natural sunlight. The Ag⁰ formed in-situ acted as the recombination center of photogenerated carriers, which promoted their efficient separation and contributed to the improved photocatalytic performance of the heterostructure. When the mass ratio of Ag₃PO₄ in the La₂Ti₂O₇/Ag₃PO₄ catalyst was 50%, the degradation rate constant of Rhodamine B (RhB), methyl orange (MO), chloroquine phosphate (CQ), tetracycline (TC), and phenol under natural sunlight irradiation were 0.5923, 0.4463, 0.1399, 0.0493, and 0.0096 min^-1, respectively. Furthermore, the photocorrosion of the composite was greatly inhibited, 76.49% of CQ and 83.96% of RhB were still degraded after four cycles. Besides, the holes and O₂^•- played a significant role in RhB degradation, and it included multiple mechanisms of deethylation, deamination, decarboxylation, and cleavage of ring-structures. Moreover, the treated solution can also show safety to the water receiving environment. Overall, the synthesized Z-Scheme La₂Ti₂O₇/Ag₃PO₄ composite exhibited immense potential for removing various organic pollutants through photocatalytic technology under natural sunlight irradiation.

Adsorption of Phenanthrene on Multi-Walled Carbon Nanotubes in the Presence of Nonionic Surfactants

The bioavailability and mobility of phenanthrene (Phe) adsorbed by multi-walled carbon nanotubes (MWCNTs) may be substantially influenced by nonionic surfactants used both in the synthesis and dispersion of MWCNTs. The adsorption mechanisms of Phe adsorbed onto MWCNTs under the different nonionic surfactants Tween 80 (TW-80) and Triton X-100 (TX-100) in the aqueous phase were investigated in terms of changes in the MWCNTs' compositions and structures. The results showed that TW-80 and TX-100 were easily adsorbed onto MWCNTs. Phe adsorption data onto MWCNTs were better suited to the Langmuir equation than the Freundlich equation. Both TW-80 and TX-100 reduced the adsorption capacity of Phe onto MWCNTs. When TW-80 and TX-100 were added in the adsorption system, the saturated adsorption mass of Phe decreased from 35.97 mg/g to 27.10 and 29.79 mg/g, respectively, which can be attributed to the following three reasons. Firstly, the hydrophobic interactions between MWCNTs and Phe became weakened in the presence of nonionic surfactants. Secondly, the nonionic surfactants covered the adsorption sites of MWCNTs, which caused Phe adsorption to be reduced. Finally, nonionic surfactants can also promote the desorption of Phe from MWCNTs.

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.

Anchoring ZnO on spinel cobalt ferrite for highly synergic sono-photo-catalytic, surfactant-assisted PAH degradation from soil washing solutions

In this study, the photocatalytic activity of ZnO was effectively improved via its combination with spinel cobalt ferrite (SCF) nanoparticles. The catalytic performance of ZnO@SCF (ZSCF) was investigated in coupling with UV irradiation and ultrasound (US), as a heterogeneous sono-photocatalytic process, for the decontamination of phenanthrene (PHE) from contaminated soil. Soil washing tests were conducted in a batch environment, after extraction assisted by using Tween 80. Several characterization techniques such as XRD, FESEM-EDS, BET, TEM, UV-vis DRS, PL and VSM were utilized to determine the features of the as-prepared catalysts. ZSCF showed an excellent catalytic activity toward degradation of PHE in the presence of US and UV with a significant synergic effect. It was found that more than 93% of PHE (35 mg/L) and 87.5% of TOC could be eliminated by the integrated ZSCF/US/UV system under optimum operational conditions (pH: 8.0, ZSCF: 1.5 g/L, UV power: 6.0 W and US power: 70 W) within 90 min of reaction. After five times of use, ZSCF illustrated good reusability in the decontamination of PHE (87%) and TOC (79%). Quenching tests revealed the contribution of h⁺, HO^• and e^- species during PHE degradation over ZSCF/UV/US and an S-scheme photocatalytic mechanisms was proposed for the possible charge transfer routes under the ZSCF system. This study provides the important role of SCF in enhancing the ZnO photocatalytic activity due to its high performance, easy recovery and excellent durability, which it make an efficient and promising catalyst in environmental clean-up applications.

Construction of Ag nanocluster-modified Ag3PO4 containing silver vacancies via in-situ reduction: With enhancing the photocatalytic degradation activity of sulfamethoxazole

Photocatalytic removal of sulfonamide antibiotics is an effective strategy to solve environmental pollution. Ag3PO4 is a promising anode material for photocatalytic material with photocatalytic degradation ability under ultraviolet light or natural light. Unfortunately, due to its instability, Ag+ could be reduced to Ag0 which loaded onto the surface of Ag3PO4 during the photocatalytic process, causing self-photocorrosion and resulting in the reduction of photocatalytic activity and stability. Herein, Ag3PO4 nanoparticles loaded with Ag nanoclusters containing Ag vacancies (Ag/Ag3PO4-VAg) were constructed by an in-situ reduction strategy to achieve effectively photocatalytic degradation behavior. The Ag nanoclusters loaded on the surface of Ag3PO4 can not only effectively inhibit the self-photocorrosion but also affords a localized surface plasmon resonance (LSPR) effect in the photocatalytic process, thus leading to the efficient generation and rapid transfer of photogenerated carriers behavior. In addition, the Ag vacancies in Ag3PO4 are crucial to increasing the adsorption energy of H2O for further enhancing the capture and accumulation of electrons. In detail, according to Zeta potential analysis, the strong adsorption sites of sulfamethoxazole (SMX) molecules are generated at the interface of Ag and Ag3PO4, which promote the activation of SMX molecules. A 100 ml of 20 mg/L SMX could be completely degraded within 15 min with an apparent rate constant (Kapp) of 0.306 min-1, which far exceeds the activity of most of the photocatalysts. This work may provide an attractive strategy to address the activity, stability of Ag3PO4 and and realizing the green remediation of SMX 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.

Synergistic degradation of fluorene in soil by dielectric barrier discharge plasma combined with P25/NH2-MIL-125(Ti)

Plasma techniques to degrade pollutants are generally more efficient than conventional methods, but exist some problems such as high energy consumption, incomplete degradation of pollutants, and secondary pollution caused by highly toxic intermediates. In this study, the dielectric barrier discharge plasma (DBDP) combined with the Ti-based metal organic frameworks (MOFs) catalysts (P25/NH₂-MIL-125(Ti)) was used to degrade fluorene in the soil. The synergistic treatment technique used in soil remediation can realize a green and promising treatment efficiency with relatively low energy consumption. Compared with DBDP system alone, the synergetic treatment system of DBDP and P25/NH₂-MIL-125(Ti) considerably increased the degradation efficiency of fluorene in the soil to above 90% at 10 min, even with a relatively low discharge voltage (5 kV). The synergistic treatment system achieved 88.8% of fluorene mineralization at 60 min. Optical emission spectroscopy and electron paramagnetic resonance spectroscopy both showed that •OH and •O₂^- played an important role in the synergetic treatment system. Nine main intermediates were identified using gas chromatography-mass spectrometry and Fourier transform infrared analysis. The main degradation of fluorine in soil was caused by the electronic transition of the catalytic material excited by DBDP, and finally mineralized into CO₂ and H₂O. The fluorene and its toxic intermediates were effectively removed. This study provides an insight for achieving high efficiency and environmentally friendly application perspective in soil remediation.

Iron-doped g-C3N4 catalysts fabricated by forming Fe–N moieties with outstanding photo-Fenton activity toward tetracycline degradation

Porous Fe-doped g-C3N4 photo-Fenton catalysts are successfully prepared by a convenient one-step pyrolysis strategy and applied in the degradation of tetracycline hydrochloride.