Dual roles of biochar redox property in mediating 2,4-dichlorophenol degradation in the presence of Fe3+ and persulfate

The role of superoxide anion to Cr(VI) reduction by pine biochar

It has been reported that Cr(VI) can be reduced by biochar because of its redox activity. Considering the anionic form of Cr(VI), we hypothesize that the reduction in aqueous phase is significant. However, the contribution of different reactive oxygen species in the biochar-Cr(VI) reaction system has not been distinguished. Herein, we quantitatively identified Cr(VI) adsorption and reduction in biochar systems. The reduction content of Cr(VI) was 1.5 times higher in untreated conditions than in anaerobic conditions. The disappearance of·O2- under anaerobic conditions illustrated that·O2- may be involved in the reduction of Cr(VI). Quenching of·O2- resulted in a decrease of Cr(VI) reduction by 34%, while 1O2 was negligible, probably due to the stronger electron-donating capacity of·O2-. The degradation of nitrotetrazolium blue chloride (quenching agent of·O2-) confirmed that the reduction process of·O2- mainly occurred in the liquid-phase. Boehm titration and quantification of·O2- further elucidated the significant correlation (P < 0.05) between phenolic groups and the formation of·O2-, which implied that phenolic groups acted as the primary electron donors in generating·O2-. This study highlights the importance of the liquid-phase reduction process in removing Cr(VI), which provides theoretical support for biochar conversion of Cr(VI).

Biochar-Derived Persistent Free Radicals: A Plethora of Environmental Applications in a Light and Shadows Scenario

Biochar (BC) is a carbonaceous material obtained by pyrolysis at 200-1000 °C in the limited presence of O2 from different vegetable and animal biomass feedstocks. BC has demonstrated great potential, mainly in environmental applications, due to its high sorption ability and persistent free radicals (PFRs) content. These characteristics enable BC to carry out the direct and PFRs-mediated removal/degradation of environmental organic and inorganic contaminants. The types of PFRs that are possibly present in BC depend mainly on the pyrolysis temperature and the kind of pristine biomass. Since they can also cause ecological and human damage, a systematic evaluation of the environmental behavior, risks, or management techniques of BC-derived PFRs is urgent. PFRs generally consist of a mixture of carbon- and oxygen-centered radicals and of oxygenated carbon-centered radicals, depending on the pyrolytic conditions. Here, to promote the more productive and beneficial use of BC and the related PFRs and to stimulate further studies to make them environmentally safer and less hazardous to humans, we have first reviewed the most common methods used to produce BC, its main environmental applications, and the primary mechanisms by which BC remove xenobiotics, as well as the reported mechanisms for PFR formation in BC. Secondly, we have discussed the environmental migration and transformation of PFRs; we have reported the main PFR-mediated application of BC to degrade inorganic and organic pollutants, the potential correlated environmental risks, and the possible strategies to limit them.

N,S-codoped biochar outperformed N-doped biochar on co-activation of H2O2 with trace dissolved Fe(Ⅲ) for enhanced oxidation of organic pollutants

Co-activation of H2O2 with biochar and iron sources together provides an attractive strategy for efficient removal of refractory pollutants, because it can solve the problems of slow Fe(Ⅱ) regeneration in Fenton/Fenton-like processes and of low •OH yield in biochar-activated process. In this study, a wood-derived biochar (WB) was modified by heteroatom doping for the objective of enhancing its reactivity toward co-activation of H2O2. The performance of the co-activated system using doped biochars and trace dissolved Fe(Ⅲ) on oxidation of organic pollutants was evaluated for the first time. The characterizations using X-ray photoelectron spectroscopy (XPS), Raman spectra and electrochemical analyses indicate that heteroatom doping introduced more defects in biochar and improved its electron transfer capacity. The oxidation experiments show that heteroatom doping improved the performance of biochar in the co-activated process, in which the N,S-codoped biochar (NSB) outperformed the N-doped biochar (NB) on oxidation of pollutants. The reaction rate constant (kobs) for oxidation of sulfadiazine in NSB + Fe + H2O2 is 2.25 times that in NB + Fe + H2O2, and is 72.9 times that in the Fenton-like process without biochar, respectively. The mechanism investigations indicate that heteroatom doping enhanced biochar's reactivity on catalyzing the decomposition of H2O2 and on reduction of Fe(Ⅲ) due to the improved electron transfer/donation capacity. In comparison with N-doping, N,S-codoping provided additional electron donor (thiophenic C-S-C) for faster regeneration of Fe(Ⅱ) with less amount of doping reagent used. Furthermore, co-activation with NSB maintained to be efficient at a milder acidic pH than Fenton/Fenton-like processes, and can be used for oxidation of different pollutants and in real water. Therefore, this research provides a novel, sustainable and cost-efficient method for oxidation of refractory pollutants.

Magnetic loofah sponge biochar facilitates microbial interspecies cooperation in surface and subsurface sediments for enhanced PAH biodegradation

Magnetic biochar had been used for the bioremediation of polycyclic aromatic hydrocarbon (PAH)-contaminated sediments. However, the long-term remediation pattern of vertical stratification driven by the application of magnetic biochar and the assembly of microbes had received little attention. In this study, magnetic loofah sponge biochar (MagLsBC), magnetic iron oxide (MagOx) and magnetic coconut shell activated carbon (MagCoAC) were applied for the 900-day remediation of contaminated sediments. Significant (p < 0.05) PAH biodegradation was observed in both the surface and subsurface sediments with MagLsBC addition. However, enhanced PAH biodegradation was observed only in the surface sediments with MagOx and MagCoAC treatments. Magnetotactic bacteria (Magnetococcus) was dominant genera in surface sediments and indigenous PAH degradation bacteria were more abundant in subsurface sediments of MagLsBC relative to other bacterial communities. The network interaction between microbes in surface and subsurface sediments with MagLsBC treatments was a less complex and tighter than those with MagCoAC, MagOx or Control treatments. Long-distance electron transfer rates could be enhanced through cooperation between magnetotactic bacteria and indigenous degradation bacteria, thus accelerating PAH degradation in sediment with MagLsBC treatment, especially in the underlying sediment.

The promoted degradation of biochar-adsorbed 2,4-dichlorophenol in the presence of Fe(III)

Organic pollutant degradation by biochar could be promoted by Fe because of the Fenton-like reaction. However, studies have also confirmed that reactive oxygen species (ROS) play only a limited role in organic pollutant degradation by biochar. Herein, we quantitatively identified 2,4-dichlorophenol (2,4-DCP) adsorption and degradation in Fe-biochar systems and obtained degradation (k₁) and adsorption rate constants (k₂) by two-compartment first-order kinetics modeling. The k₁ was approximately 7-10 times lower than the corresponding k₂ and the positive correlation between k₁ and k₂ illustrated that adsorption and degradation were kinetically associated. ROS quenching only slightly inhibited 2,4-DCP degradation. Chemicals with similar structures to ROS quenchers (without quenching ability) also inhibited 2,4-DCP degradation, probably because of the competition of the active degradation sites on biochars. Electrochemical analysis and pH-impact experiments further elucidated that 2,4-DCP underwent oxidation-dominated degradation in the adsorbed phase via direct electron transfer. Fe(III) obviously increased 2,4-DCP adsorption through cation bridging and enhanced electron density by Fe-O conjugations on the biochar surface, which facilitated subsequent degradation. This study emphasized the importance of degradation on the biochar solid phase and that a breakthrough of the mass transfer bottleneck of adsorption will greatly promote degradation.

Insights into the mechanism of persulfate activation with biochar composite loaded with Fe for 2,4-dinitrotoluene degradation

Iron in biochar composite loaded with Fe (Fex@biochar) is crucial for persulfate activation. However, the iron dosages-driven mechanism linked to the speciation, electrochemical property, and persulfate activation with Fex@biochar remains ambiguous. We synthesized and characterized a series of Fex@biochar and evaluated its catalytic performance in 2,4-dinitrotoluene removal experiments. With increasing FeCl₃ dosage, iron speciation in Fex@biochar changed from γ-Fe₂O₃ to Fe₃O₄, and the variation in functional groups was as follows: Fe-O, aliphatic C-O-H, O-H, aliphatic C-H, aromatic CC or CO, and C-N. The electron accepting capacity of Fex@biochar increased as the FeCl₃ dosage increased from 10 to 100 mM but decreased at 300 and 500 mM FeCl₃. 2,4-dinitrotoluene removal first increased and subsequently decreased, reaching 100% in the persulfate/Fe100@biochar system. The Fe100@biochar also showed good stability and reusability for PS activation, verified by five test cycles. The mechanism analysis indicated that the iron dosage altered the Fe (Ⅲ) content and electron accepting capacity of Fex@biochar during pyrolysis, further controlling persulfate activation and 2,4-dinitrotoluene removal. These results support the preparation of eco-friendly Fex@biochar catalysts.

Enhanced visible light assisted peroxymonosulfate process by biochar in-situ enriched with γ-Fe2O3 for p-chlorophenol degradation: performance, mechanism and DFT calculation

In this study, a novel γ-Fe₂O₃/biochar (BF_γ) composite by a plant in-situ enrichment and one-step pyrolysis strategy was prepared, which was applied as a photocatalyst to activate peroxymonosulfate (PMS) for the degradation of p-chlorophenol (4-CP) under visible light irradiation (BF_γ/PMS/Vis) system. The characterization results exhibited that γ-Fe₂O₃ with localized carbon doping was evenly embedded in biochar during the pyrolysis. BF_γ exhibited better photoresponse properties than biochar (BC) and γ-Fe₂O₃. The removal efficiency of this system for 4-CP reached 96.41% under optimal conditions. This system showed high removal efficiency with a wide pH range (3.0-13.0) and under conditions of different organic pollutants. It also showed strong resistance to interference with co-existing inorganic ions and humic acid (HA). Electron paramagnetic resonance (EPR) and radical scavenging experiments revealed that the reactive oxygen species (ROS) in this system included SO₄^-·, ·OH, ·O₂^- and ¹O₂. The density functional theoretical (DFT) calculations further revealed the promotion of localized carbon doping in γ-Fe₂O₃ on electron transfer and photoresponse, including C-O bond (d=1.29 Å), C-Fe bond (d=1.80 Å) and band gap value (E_gap < 0.72 eV). This study provides new insights into constructing environmentally-friendly catalysts and the possibility of the solid waste recycling for other wetland plants.

Waste to catalyst: Role of agricultural waste in water and wastewater treatment

Presently, owing to the rapid development of industrialization and urbanization activities, a huge quantity of wastewater is generated that contain toxic chemical and heavy metals, imposing higher environmental jeopardies and affecting the life of living well-being and the economy of the counties, if not treated appropriately. Subsequently, the advancement in sustainable cost-effective wastewater treatment technology has attracted more attention from policymakers, legislators, and scientific communities. Therefore, the current review intends to highlight the recent development and applications of biochars and/or green nanoparticles (NPs) produced from agricultural waste via green routes in removing the refractory pollutants from water and wastewater. This review also highlights the contemporary application and mechanism of biochar-supported advanced oxidation processes (AOPs) for the removal of organic pollutants in water and wastewater. Although, the fabrication and application of agriculture waste-derived biochar and NPs are considered a greener approach, nevertheless, before scaling up production and application, its toxicological and life-cycle challenges must be taken into account. Furthermore, future efforts should be carried out towards process engineering to enhance the performance of green catalysts to improve the economy of the process.

Fe3O4 loaded on ball milling biochar enhanced bisphenol a removal by activating persulfate: Performance and activating mechanism

In this study, pristine biochar (BC), ball milling biochar (MBC), Fe₃O₄ modified BC (Fe₃O₄@BC), and Fe₃O₄ modified MBC (Fe₃O₄@MBC) were prepared to compare the Bisphenol A (BPA) removal efficiency by activating persulfate (PDS). All catalysts exhibited excellent degradation rather than adsorption in the PDS system, and Fe₃O₄@MBC800 had the best BPA removal efficiency, with 96.73% degradation and negligible 1.43% adsorption due to the synergistic effect between MBC800 and Fe₃O₄ particles. Radical quenching experiments and electron paramagnetic resonance analysis indicated radical pathways, namely, SO₄∙^- and ∙OH, O₂∙^-, and non-radical pathway (¹O₂) involving BPA degradation. The abundant oxygen-containing groups, increased graphitization and mesopores of MBC800, and Fe³⁺/Fe²⁺ conversion of Fe₃O₄ particles facilitated PDS activation to produce reactive oxygen species. In addition, the superior electrochemical performance accelerated the electron transfer between the catalyst and PDS, promoting BPA degradation in the Fe₃O₄@MBC800/PDS system. More importantly, Fe₃O₄@MBC800 is resistant to environmental interference, including pH, anions, cations, and humic acid, and has good catalytic reusability and stability, which fulfills the requirements of engineering applications. Therefore, Fe₃O₄ loaded on ball-milled biochar provides a convenient strategy for preparing environmentally friendly, economical, and efficient carbon-based catalysts to remove organic contaminants.

A comprehensive review on persulfate activation treatment of wastewater

With increasingly serious environmental pollution and the production of various wastewater, water pollutants have posed a serious threat to human health and the ecological environment. The advanced oxidation process (AOP), represented by the persulfate (PS) oxidation process, has attracted increasing attention because of its economic, practical, safety and stability characteristics, opening up new ideas in the fields of wastewater treatment and environmental protection. However, PS does not easily react with organic pollutants and usually needs to be activated to produce oxidizing active substances such as sulfate radicals (SO₄^-) and hydroxyl radicals (OH) to degrade them. This paper summarizes the research progress of PS activation methods in the field of wastewater treatment, such as physical activation (e.g., thermal, ultrasonic, hydrodynamic cavitation, electromagnetic radiation activation and discharge plasma), chemical activation (e.g., alkaline, electrochemistry and catalyst) and the combination of the different methods, putting forward the advantages, disadvantages and influencing factors of various activation methods, discussing the possible activation mechanisms, and pointing out future development directions.

Co-catalysis of trace dissolved Fe(iii) with biochar in hydrogen peroxide activation for enhanced oxidation of pollutants

Activation of hydrogen peroxide (H₂O₂) with biochar is a sustainable and low-cost approach for advanced oxidation of organic pollutants, but faces the challenge of a low yield of hydroxyl radical (˙OH). Herein, we hypothesize that the activation efficiency of H₂O₂ can be enhanced through co-catalysis of trace dissolved iron (Fe) with biochar. Two biochar samples derived from different feedstock, namely LB from liquor-making residue and WB from wood sawdust, were tested in the co-catalytic systems using trace Fe(iii) (0.3 mg L⁻¹). The cumulative ˙OH production in [Fe(iii) + LB]/H₂O₂ was measured to be 3.28 times that in LB/H₂O₂, while the cumulative ˙OH production in [Fe(iii) + WB]/H₂O₂ was 11.9 times that in WB/H₂O₂. No extra consumption of H₂O₂ was observed in LB/H₂O₂ or WB/H₂O₂ after addition of trace Fe(iii). Consequently, the reaction rate constants (k_obs) for oxidation of pollutants (2,4-dichlorophenoxyacetic acid and sulfamethazine) were enhanced by 3.13–9.16 times. Other iron species including dissolved Fe(ii) and iron minerals showed a similar effect on catalyzing 2,4-D oxidation by biochar/H₂O₂. The interactions involved in adsorption and reduction of Fe(iii) by biochar in which the defects acted as electron donors and oxygen-containing functional groups bridged the electron transfer. The fast regeneration of Fe(ii) in the co-catalytic system resulted in the sustainable ˙OH production, thus the efficient oxidation of pollutants comparable to other advanced oxidation processes was achieved by using dissolved iron at a concentration as low as the concentration that can be found in natural water.

The yield of ˙OH and oxidation of pollutants by biochar/H₂O₂ were enhanced dramatically by trace dissolved Fe(iii).