Catalytic Removal of Aqueous Contaminants on N-Doped Graphitic Biochars: Inherent Roles of Adsorption and Nonradical Mechanisms

Revisit the actual roles of catalytic sites in a Fenton-like system

The perception of catalytic site contributions in peroxymonosulfate (PMS)/biochar systems is biased due to the neglect of active site interactions. Here, random forest regression (RF), supporting vector regression (SVR), XGBoost (XGB), and gradient boosting decision tree (GBDT) are selected to construct models using active sites (A model) instead of elements (E model) as input features to revisit the relationship between C, N, and O-containing sites on biochar surface and Fenton-like activity. Consequently, the A models achieve twice the accuracy of the E models. For individual sites, a low CC or high CN concentration promotes the degradation of electron-donating organics in PMS systems, while system activity initially increases and then declines with rising CO concentration. Considering site interactions, CC&CO, CC&CN, and CO&CN show excellent synergy for PMS activation. Specifically, the Gibbs free energy (ΔG) of PMS at CC&CO (0.44 eV), CN&CO (0.78 eV), and CC&CN (0.82 eV) is significantly lower than that of single CC (2.26 eV), CN (1.44 eV), and CO&CO (1.14 eV) during the transition state formation process. The reduced ΔG facilitates OO bond cleavage, enhancing the generation of active species. This study employs A_SVR model and density functional theory (DFT) calculations to clarify the structure-activity relationships of biochar by considering the synergistic effects of active site. The results contribute to the precise design of Fenton-like catalysts for targeted pollutant degradation, improving water purification efficiency.

Mechanistic insight into enhancement of undissolved rice husk biochar on Tetracycline biodegradation by strain Serratia marcescens basing on electron transfer response

Undissolved biochar (UBC) plays a key role in persistently affecting bacterial characteristics after loss of dissolved biochar. However, its potential role as electron shuttle mediating tetracycline (TC) removal by bacteria is less understood. Result demonstrated UBC (700°C) coupled strain MSM2304 resulted in 72.19 % of TC biodegradation (37.76 % in free cells). UBC improved nutrients usage of TOC and TN to enhance cells proliferation, and facilitated biofilms formation and secretion of redox-active-related extracellular polymeric substances (EPS) including protein (40 % higher) and humus (30 % higher). Moreover, UBC optimized cells oxidative stress indicators including reactive oxygen species (40 % lower), total antioxidant capacity (30 % higher), superoxide dismutase (35 % higher), and catalase (30 % higher) during TC exposure. Importantly, UBC not only accelerated electron transfer from intracellular into extracellular by stimulating cytochrome C reductase activity and cytochrome C development, also decreased extracellular electron transfer resistance between MSM2304 and TC from 231.7 to 109.5 Ω, proved by cyclic voltammetry and electrochemical impedance spectra of EPS, and helped quinone moieties formation on UBC through CO and CC or CO production determined by FTIR and XPS. These findings indicate UBC could be as electron shuttle and contribute to provide a better understanding of interactions between biochar and microorganism.

Enhanced peroxymonosulfate photoactivation by nitrogen-doped multiwalled carbon nanotube-loaded CoFe2O4 for orange G degradation: Performance, mineralization and reaction mechanism

In this study, CF@N-MWCNT nanocomposites were successfully synthesized by anchoring cobalt ferrite (CoFe2O4, CF) on nitrogen-doped multiwalled carbon nanotube (N-MWCNT) and used to promote ultraviolet (UV) light-induced peroxymonosulfate (PMS) activation for the removal of the azo dye orange G (OG).The incorporation of N-MWCNT enhances the catalytic performance and photocatalytic activity of CF. A prominent synergistic effect on the removal of OG by UV irradiation, CF@N-MWCNT, and PMS was evident through comparisons of various reaction systems. Almost complete degradation (99.5%) of OG was achieved within 30 min at [OG]0 = 50 mg/L, [PMS]0 = 1 mM, CF@N-MWCNT = 0.1 g/L under pH = 7. Scavenging experiments indicated that non-radical pathways played a significant role in OG degradation apart from the contribution of hydroxyl radical (HO•) and sulfate radical (). The catalyst demonstrated superior catalytic performance across a broad pH range (3-11) while exhibiting strong resistance to interference from coexisting substances in aqueous environments. Furthermore, the UV/CF@N-MWCNT/PMS system can effectively remove numerous recalcitrant pollutants from dyeing wastewater. Overall, this study highlights the promising catalytic performance of the CF@N-MWCNT nanocomposite for UV-activated PMS, offering a potentially effective method for treating dyeing wastewater.

Activation of Persulfates Using Alkali-Modified Activated Coke to Promote Phenol Removal

Coke (AC) was modified and activated with sodium hydroxide (NaOH) and potassium hydroxide (KOH) to produce AC-Na and AC-K, respectively, and applied as a persulfate (PS) activator to promote phenol (Ph) removal in water. Under the given experimental conditions, compared to AC/PS (Ph removal effect was 77.09%), the Ph removal effects were 94.46% and 88.73% for AC-K/PS and AC-Na/PS, respectively. AC-K proved to be a more effective activator than AC-Na and was used for all the subsequent experiments. When PS/phenol molar ratio was 6.26:1:00, the initial system pH was 7 and the system temperature was 25 °C; the AC-K/PS system could effectively remove Ph (98.75%) from the simulated wastewater. After that, the stability of AC-K was verified. Electron paramagnetic resonance (EPR) and quenching analysis confirmed the hydroxyl free radical (•OH) to be predominant within this system. EPR combined with X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared (FTIR) spectroscopy, and Raman spectroscopy indicated that the sulfate radical (SO4•-) and •OH were generated due to the defects in AC-K, thereby enhancing the PS activation potency of AC-K. Additionally, the radical quenching experiments showed that the superoxide (O2-) radical is a key intermediate product promoting SO4•- and •OH, which aided Ph removal. Both radical (SO4•- and •OH) and non-radical (1O2) pathways were found to co-exist during the removal process. The Ph removal rate of the AC-K/PS system could still reach 29.50%, even after four repeated cycles. These results demonstrate that the unique AC-K/PS system has a potential removal effect on organic pollutants in water.

Impact of External Conditions on the Desorption and Degradation Capacity of Biochar for Rhodamine B

Biochar has attracted considerable interest owing to its high adsorption capacity; however, the mechanisms through which environmental factors influence the release of adsorbed pollutants remain unclear. This study investigates the adsorption and desorption dynamics of Rhodamine B (RhB) on biochars B2 and B6, which were pyrolyzed at temperatures of 200 °C and 600 °C, respectively, under varying conditions. The results indicated that there was no significant difference in removal efficiency at low RhB concentrations; however, at a concentration of 600 mg/L, biochar B2 had a higher removal efficiency than B6, likely attributable to more adsorption sites. Increased temperatures were found to enhance desorption from both B2 and B6, with B6 exhibiting a faster desorption rate. This phenomenon may be due to the stronger hydrogen bonding between B2 and RhB, which could inhibit desorption. In addition, elevated pH values facilitated desorption, presumably through electrostatic repulsion. Under alkaline conditions, B2 released a greater amount of dissolved organic matter (DOM), leading to increased RhB desorption relative to B6. This study offers a valuable reference for evaluating the environmental risk associated with the application of biochar in real-world settings.

Fe1-xS@BC prepared from natural pyrite and biomass as peroxydisulfate activator for sulfadiazine degradation

The efficient degradation of SAs is a significant challenge for the treatment of wastewater. To address this, the Fe1-xS@BC was prepared by calcining a mixture of pyrite and biomass, and used to activate peroxydisulfate (PDS) to degrade sulfadiazine (SDZ). The effect of carbon sources (wheat straw, rice husk, and corn cob) on catalytic activity of Fe1-xS@BC were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), total Fe dissolution and free radical quantification. The results indicate that Fe1-xS@WBC with carbon defects and oxygenated functional groups facilitate the dissolution of Fe and the generation of ·OH and ·SO4-. Additionally, the electron-rich the thiophene S facilitate the regeneration of Fe(II). In the Fe1-xS@WBC/PDS system, 90.3% of SDZ degradation could be achieved under optimal conditions: Fe1-xS@WBC = 0.5 g L-1, SDZ = 10 mg L-1, PDS = 2.0 mM, initial solution pH = 7.0. In addition, Fe1-xS@WBC/PDS system exhibits strong resistance to interference from Cl-, and NO3-, while elevated concentrations of HCO3-, HPO42-, and HA hinder SDZ degradation. The Fe1-xS@WBC/PDS system shows excellent selectivity and recoverability. Quenching experiments and electron spin resonance (ESR) reveal the involvement of ·OH, ·SO4-, and 1O2 in the degradation of SDZ within Fe1-xS@WBC/PDS system. Furthermore, four possible degradation pathways for SDZ were proposed based on density functional theory (DFT) and liquid chromatography-mass spectrometry (LC-MS) analysis, while assessing the toxicity of degradation intermediates. This study not only introduces a novel catalytic system for the efficient degradation of antibiotic-contaminated wastewater, but also provides a theoretical foundation for the development and application of iron sulfide-biomass composite catalysts.

Innovative strategy for the treatment of oily wastewater by in-situ synthesis of nitrogen-doped biochar supported FeS for activation of peroxymonosulfate

Disposing of oily wastewater poses a significant challenge in treating oilfield-produced wastewater treatment. This study developed a FeSNC-9/PMS system for the effective degradation of total petroleum hydrocarbons (TPHs) in oily wastewater, while increasing the value of excess sludge, thus achieving the dual purpose of waste treatment. This work involved the in-situ preparation of a porous nitrogen-doped biochar-supported iron sulphide catalyst material using surplus sludge from SBR. Compared to undoped FeS (NC-9), FeSNC-9 exhibited excellent pore structure and abundant functional groups. Fe-Nx served as an effective connecting site between FeS species and the graphite network of biochar. The FeSNC-9/PMS system significantly degraded 74.21% of TPHs within 300 min. The FeSNC-9/PMS system demonstrated remarkable TPHs degradation efficiency across a wide temperature range and under both weak acidity and near-neutral conditions The dominant reactive oxygen species were identified as SO4•- and •OH, with O2•- and 1O2 also confirmed as active species. Gas chromatography semi-quantitative analysis showed that the long-chain alkanes of C20-C30 in total petroleum hydrocarbons were significantly degraded into short-chain alkanes or completely mineralized. This work provides new insights for the low-cost and high-efficiency treatment of TPHs in oilfield-produced water, and delves into the activation mechanism of PMS and the degradation pathways of TPHs.

Electron transfer tuning for persulfate activation via the radical and non-radical pathways with biochar mediator

Electron mediator-based in-situ chemical oxidation (ISCO) offers a novel strategy for groundwater remediation due to diverse reaction pathways. However, distinguishing and further tuning the reaction pathway remains challenging. Herein, biochar as an electron mediator targeted active peroxysulphate (PDS) via the radical or non-radical pathway. Exemplified by the triazin pesticides removal, the complex radical (•OH and SO4•-) and non-radical active species (electron transfer oxidation) were generated and identified in different biochar/PDS systems. The electron transfer process between biochar and PDS was significantly distinguished via an innovatively in-situ visualization of radical pathway, and the electron transfer oxidation non-radical pathway is directly unveiled via a galvanic cell experiment combined with LC-MS analyses. The electron transfer mechanism was revealed via establishing the quantitative structure-activity relationships between biochar and ln kobs. The redox capacity of biochar was assessed as a key for tuning the atrazine degradation by non-radical pathway, and the surface carbon-centered persistent free radicals (PFRs) were identified as key electron donors for triggering the radical pathway. This study gives new insights into the electron transfer mechanism during tuning radical and non-radical activation pathways and the enhanced utilization of oxidants in ISCO technology.

Constructing a Graphene-like Layered Carbocatalyst by the Dual Templating Effect for an Efficient Fenton-like Reaction

Two-dimensional (2D) carbon materials are receiving increasing attention due to their partly groundbreaking performance in catalysis and electrochemistry based on distinct physiochemical and textural properties. We focus on the challenge to directly achieve a well-developed layered morphology with a high doping level of heteroatoms as the active sites, a standard conflict of interests of high-temperature synthesis. Here, we report a dual-templating strategy to yield graphene-like layered carbon (GLC) by direct carbonization of a texturally prealigned zeolitic imidazolate framework-8 (ZIF-8). The recrystallization of ZIF-8 in an aqueous NaCl solution discloses a 2D packing mode that was retained after freeze-drying with recrystallized NaCl as an exotemplate and a space-confining nanoreactor. Further promoted by the chemical interaction of NaCl in promoting and stabilizing the carbonization process, the final product came with a well-separated layered morphology and high amounts of heteroatoms (16.6 wt % N and 7.5 wt % O). The structurally and catalytically special GLC functioned well in activating peroxymonosulfate-based Fenton-like reactions. It was shown that the reaction proceeded via nonfree-radical-mediated pathways, as reflected in significantly enhanced electron-transfer processes and ultrafast kinetics for pollutant removal. The proposed strategy is expected to afford a broader applicability for the bottom-up design of 2D carbon materials.

Three-dimensionally structured MoS2@biochar breaks through the bottleneck in antibiotic wastewater treatment: Greater efficiency and self-motivated oxidation pathway

Two-dimensional (2D) MoS2 has been widely used to remove antibiotics. However, low selectivity for antibiotic pollutants, dependence on applied energy and oxidant, and secondary contamination are still the bottlenecks of this system for treating antibiotic wastewater. In this study, we proposed a three-dimensional (3D) material (3MoS2/BMBC@MF) based on MoS2 and biochar with melamine sponge as the backbone. Compared with the 2D material (MoS2/BMBC), 3MoS2/BMBC@MF performed significantly better in enrofloxacin (ENR) removal, with an increase in the removal degree from 60.8 % to 88.1 %, and acted mainly through the degradation pathway rather than relying solely on the adsorption effect. It was shown that the direct oxidation process (DOP) behind the 3D materials is the key to the self-activated oxidation pathway. The three-dimensional structure enhances the generation and transfer pathways of persistent free radicals (PFRs) and electrons, realizing a multi-dimensional activation mechanism through its unique three-dimensional network, which greatly improves the redox capacity of the material. Upon exposure to pollutants, 3MoS2/BMBC@MF generates carbon-centered radicals of PFRs, which degrade ENR through mediated electron transfer. Coupled with the three-dimensional structure that contributes to the homogeneous dispersion of the active substances, dense steric active centers are formed in the grid skeleton by redox cycling of Mo ions to degrade antibiotics via DOP. Meanwhile, 3MoS2/BMBC@MF possesses good recyclability and maintains high efficiency in recycling. The structural design of this material not only enhances the removal efficiency and reduces the environmental impact, but also provides new potentials and solutions for practical water treatment of antibiotic contaminants.

Activation of peroxymonosulfate by sustainable biomass-based carbon nanotubes for controlling the spread of plant viruses in water environments

With the increasing demand for water in hydroponic systems and agricultural irrigation, viral diseases have seriously affected the yield and quality of crops. By removing plant viruses in water environments, virus transmission can be prevented and agricultural production and ecosystems can be protected. But so far, there have been few reports on the removal of plant viruses in water environments. Herein, in this study, easily recyclable biomass-based carbon nanotubes catalysts were synthesized with varying metal activities to activate peroxymonosulfate (PMS). Among them, the magnetic 0.125Fe@NCNTs-1/PMS system showed the best overall removal performance against pepper mild mottle virus, with a 5.9 log10 removal within 1 min. Notably, the key reactive species in the 0.125Fe@NCNTs-1/PMS system is 1O2, which can maintain good removal effect in real water matrices (river water and tap water). Through RNA fragment analyses and label free analysis, it was found that this system could effectively cleave virus particles, destroy viral proteins and expose their genome. The capsid protein of pepper mild mottle virus was effectively decomposed where serine may be the main attacking sites by 1O2. Long viral RNA fragments (3349 and 1642 nt) were cut into smaller fragments (∼160 nt) and caused their degradation. In summary, this study contributes to controlling the spread of plant viruses in real water environment, which will potentially help protect agricultural production and food safety, and improve the health and sustainability of ecosystems.

Effect of persulfate dosage on organic degradation using N-doped biochar: Reaction pathway and environmental implications

Persulfate-based advanced oxidation processes (PS-AOPs) catalyzed by carbon-based catalysts are promising for removing organic pollutants via radical/non-radical pathways. However, the activation efficiency of peroxymonosulfate (PMS) or peroxydisulfate (PDS) usage and the reaction mechanism remain insufficiently understood. In this study, the effects of PMS/PDS dosage on the degradation of bisphenol A (BPA, 10 mg/L) were evaluated using N-doped biochar (N-BC, 0.2 g/L) assisted PS-AOPs. The reaction pathways were comprehensively investigated through a combination of characterization techniques and molecular simulations. With low PS dosages (0.05 and 0.1 mM), the degradation rate constants (k obs $$ {k}_{obs} $$ ) were higher in N-BC/PDS (0.04 and 0.07 min-1) compared to N-BC/PMS (0.02 and 0.04 min-1), likely due to higher PDS utilization, which enhanced the contribution of the non-radical pathway. Interestingly, with higher PS dosages (0.5 and 1.5 mM), thek obs $$ {k}_{obs} $$ values were 0.16 min-1 and 0.18 min-1 in N-BC/PMS, respectively, significantly exceeding those determined in N-BC/PDS (0.11 and 0.11 min-1). This result stemmed from the greater adsorption capacity of N-BC for PMS compared to PDS, leading to increased formation of 1O2. The contribution of non-radical pathways for both PMS and PDS increased with higher PS dosage. The results highlighted that BPA degradation improved significantly with the increase in PMS dosage; meanwhile, BPA degradation was insensitive to PDS dosage. The optimal PMS dosage for BPA degradation was found to be 1.5 mM and 0.1 mM for PDS. This study offered valuable insights for optimizing PS-AOPs in environmental remediation, helping to guide the selection of appropriate oxidants and dosages for maximizing pollutant removal. PRACTITIONER POINTS: Effect of PMS/PDS dosage on BPA degradation by N-doped biochar was revealed. Contribution of dominated non-radical pathway increased as PMS/PDS dosage increased. The greater PDS utilization and non-radical pathway resulted in the higherk obs $$ {k}_{obs} $$ at low dosage. N-BC adsorbed more PMS than PDS, leading to an increase ink obs $$ {k}_{obs} $$ at high dosage.

Comediating Adsorption and Electron Transfer via Dual-Active Site Catalyst Construction for Improving the Treatment of Extraction Wastewater

Solvent extraction is widely applied, while extraction wastewater treatment remains a huge challenge because of the stability of extractants. Heterogeneous Fenton-like catalysis is a promising method, but the short half-life of hydroxyl radicals (⋅OH) generated by hydrogen peroxide (H2O2) activation results in unsatisfactory ⋅OH utilization and organics removal. Herein, an efficient strategy for treating extraction wastewater based on comediating adsorption and electron transfer by fluorine and nitrogen co-doped carbon (FNC) catalyst with dual-active site was developed. Specially, N sites adsorb organics and F sites activate H2O2, shortening the migration distance of ⋅OH. Theoretical calculation and di(2-ethylhexyl) phosphoric acid (D2EHPA) extraction wastewater degradation experiment showed that F site with electron acquisition can transfer electrons provided by electron-rich D2EHPA enriched at N sites to H2O2, facilitating the continuous generation of ⋅OH through lowering the energy barrier for H2O2 activation. As a result, 96.49 % D2EHPA in simulated wastewater and 90.26 % total organic carbon in real extraction wastewater were removed. Moreover, FNC catalyst exhibited excellent reusability and ionic adaptability, and can be extended to the removal of various extractants. The proposed dual-active site catalyst provides an effective strategy for Fenton-like reaction to treat refractory extraction wastewater, promoting sustainable development of solvent extraction industry.

Upcycling Waste Biomass to Biochar: Feedstocks, Catalytic Mechanisms, and Applications in Advanced Oxidation for Wastewater Decontamination

Advanced oxidation technology plays an important role in wastewater treatment due to active substances with high redox potential. Biochar is a versatile and functional biomass material. It can be used for resource management of various waste biomasses. In addition, carbonaceous materials are commonly used to enhance the synergistic mechanisms of advanced oxidation processes, because of their good electrical conductivity and metal-free leaching. Biochar produced from waste biomass through pyrolysis has catalytic potential, is cost-effective, and is environmentally friendly. It is commonly used to activate hydrogen peroxide, persulfate, ozone, photocatalysis, and other systems for degrading organic pollutants in water. This review provides a summary of the feedstocks, pyrolysis conditions, and modification methods used in biochar production. It also described the effects of these factors on the yield, structure, and active sites of the biochar. The review summarized the mechanisms of various catalytic systems and their applications in wastewater decontamination, as well as their potential for practical application. Eventually, the limitations of this current technique and the outlook for future research were noted.

Cu-doped waste-tire carbon as catalyst for UV/H2O2 oxidation of ofloxacin

Ofloxacin (OFX), commonly employed in the treatment of infectious diseases, is frequently detected in aquatic environments and poses potential ecological risks. UV/H2O2 oxidation has been recognized as an efficient approach for removing antibiotics. In this study, Cu-doped waste-tire carbon was prepared and used as a UV/H2O2 catalyst for the degradation of OFX. The results showed that the OFX removal was 89.3% within 90 min under the optimal reaction conditions. It was found that ZnO, used in the tire manufacturing to promote rubber vulcanization for enhanced stability and durability, played an important role in UV/H₂O₂ oxidation for OFX degradation. Toxicity experiments conducted with a microbial degradation respirometer demonstrated that the treated water exhibited low toxicity. This study introduces a sustainable catalyst derived from waste tires, and facilitating the development of metal-carbon catalysts for the effective removal of antibiotics from wastewater.

Novel catalytic behavior of defective nanozymes with catalase-mimicking characteristics for the degradation of tetracycline

Although nanozymes have shown significant potential in wastewater treatment, enhancing their degradation performance remains challenging. Herein, a novel catalytic behavior was revealed for defective nanozymes with catalase-mimicking characteristics that efficiently degraded tetracycline (TC) in wastewater. Hydroxyl groups adsorbed on defect sites facilitated the in-situ formation of vacancies during catalysis, thereby replenishing active sites. Additionally, electron transfer considerably enhanced the catalytic reaction. Consequently, numerous reactive oxygen species (ROS) were generated through these processes and subsequent radical reactions. The defective nanozymes, with their unique catalytic behavior, proved effective for the catalytic degradation of TC. Experimental results demonstrate that •OH, •O2-, 1O2 and e- were the primary contributors to the degradation process. In real wastewater samples, the normalized degradation rate constant for defective nanozymes reached 26.0 min-1 g-1 L, exceeding those of other catalysts. This study reveals the new catalytic behavior of defective nanozymes and provides an effective advanced oxidation process for the degradation of organic pollutants.

Key role of sulfur in sulfidated zerovalent iron during persulfate activation for the dynamic equilibrium of oxidative radicals including SO4•- and •OH

Surface sulfidation has been widely investigated to effectively enhance the utilization and selectivity of iron electrons for enhanced pollutant reduction. However, there is relatively less knowledge on whether sulfidation facilitates the catalytic oxidation process and the mechanism of enhancement. Therefore, in this study, the role of surface sulfidation in modulating the oxidant decomposition pathway and reactive oxygen species generation was investigated with the sulfidated zerovalent iron (S-ZVI) activated persulfate (PS) system. The results revealed that sulfur on the surface of S-ZVI not only facilitates PS activation to generate more SO4•-, but also acts as an essential in the dynamic equilibrium between SO4•- and •OH. Specifically, the S-ZVI surface sulfide first forms sulfur monomers during catalysis, which promotes electron transfer to accelerate Fe3+ to Fe2+ cycling, prompting the generation of more SO4•- also generates SO32-. Then, SO32- is further reacted with •OH to generate the [O--O-SO3-] intermediate of SO4•-, which leads to a dynamic equilibrium of SO4•- and •OH, mitigating the further conversion of SO4•- to •OH. These findings unveiled the dynamic variation of sulfur on the surface of S-ZVI during PS activation, elevating new insights for the sulfate radical-based efficient degradation.

Revealing the essence of anion ligands in regulating amorphous MnOx to activate peracetic acid for micropollutant removal

How the anion ligands of manganese precursors affect the catalytic activity of amorphous manganese oxides (MnOx) in Fenton-like process is poorly understood. Here, five amorphous MnOx synthesized by Mn(II) precursors with different ligands were characterized and adopted to activate peracetic acid (PAA) for bisphenol A (BPA) degradation. Although > 90 % BPA removal was achieved in the five MnOx/PAA processes via both adsorption and oxidation, the oxidation kobs greatly differentiates by the ligands types with the order of MnOx-N > MnOx-S > MnOx-Cl > MnOx-AA > MnOx-OA. Ligands types would affect the specific surface area of MnOx and their ability to adsorb BPA, however which is not the decisive factor in determining the contaminant oxidation efficiency. Multiple experimental results indicate that the generation of oxygen vacancies induced by the ligands alters the Mn(III)/Mn(IV) ratio, ultimately contributing to the different efficiency of BPA oxidation driven by the direct electron transfer mechanism. Moreover, amorphous MnOx holds the promise of practical applications in catalytic PAA of various micropollutants with good stability. This study advances the fundamental understanding of ligand-regulated amorphous MnOx-catalyzed PAA process.

Remediation of chromium contaminated water and soil by nitrogen and iron doped biochars

Chromium (Cr) is a toxic and redox-sensitive contaminant that has accumulated in water and soil systems, becoming a serious issue worldwide. Producing novel remedial materials with enhanced removal efficiency from plentiful available sources is a pleasing aspect for Cr removal. This review explores valuable insights into the production of nitrogen doped biochar (N/BC), iron doped biochar (Fe/BC), and iron‑nitrogen doped biochar (Fe-N/BC) and their application for Cr (trivalent (Cr(III)) and hexavalent (Cr(VI)) removal. Specifically, this review focuses on conferring knowledge about producing environmentally friendly N and Fe doped BCs with enhanced surface functionalities, physicochemical properties, and holding capacities for removing Cr(VI) through adsorption and reduction. Affecting factors for Cr(VI) removal by N/BC, Fe/BC, and Fe-N/BC through reviewing the literature on the reaction system pH, mass transfer driving forces, effect of coexisting ions, BC production conditions, and redox potential are overviewed. Notably, isotherm and kinetic models and removal mechanisms of Cr(VI) by N/BC, Fe/BC, and Fe-N/BC with the assistance of characterization analyses, experimental results, and computational modeling methods are explored. Finally, the regeneration, cost evaluation, and environmental implications, as well as the real-world applications and environmental risks of N/BC, Fe/BC, and Fe-N/BC are discussed. This review shows that N and Fe doped BCs are remedial materials that can potentially remediate Cr(VI) contaminated water and soil.

Engineered digestate-derived biochar mediated peroxymonosulfate activation for oxytetracycline removal in sustainable wastewater remediation

Nowadays, biochar is extensively used in wastewater remediation with the aim of achieving water security and circularity with minimal impacts on ecosystems and the environment. In this study, digestate biochar was prepared and modified using different methods and then employed as a peroxymonosulfate (PMS) activator to oxidize oxytetracycline, a model antibiotic pollutant in wastewater. The optimal biochar catalyst was characterized, spin trapping tests were carried out to confirm the dominant catalytic mechanism, and in silico toxicity prediction was conducted based on structure-activity relationships. Assessment of the catalytic performance of the pristine and engineered biochar showed that nitrogen doping increased oxytetracycline degradation efficiency by 1.92-fold (i.e., 100% oxytetracycline degradation with the engineered biochar compared to 52% with pristine biochar), while pyrrolic nitrogen was identified as a major PMS activation site. It was discovered that several parameters, such as catalyst dose, pH, PMS concentration, and competing ions, affected oxytetracycline degradation efficiencies. Additionally, the toxicity of the degradation intermediate was studied. Scavenger trapping tests showed that 1O2 and SO4•- were the most prevalent species during oxytetracycline degradation in the system, with four possible degradation pathways proposed, including secondary alcohol oxidation, hydroxylation, dehydration, and deamidation. Overall, it is anticipated that this study would contribute to our understanding of metal-free biochar activation of PMS as an attractive treatment scheme for antibiotic-polluted water.

Highly stable carbon-coated nZVI composite Fe0@RF-C for efficient degradation of emerging contaminants

Nanoscale zerovalent iron (nZVI) has garnered significant attention as an efficient advanced oxidation activator, but its practical application is hindered by aggregation and oxidation. Coating nZVI with carbon can effectively addresses these issues. A simple and scalable production method for carbon-coated nZVI composite is highly desirable. The anti-oxidation and catalytic performance of carbon-coated nZVI composite merit in-depth research. In this study, a highly stable carbon-coated core-shell nZVI composite (Fe0@RF-C) was successfully prepared using a simple method combining phenolic resin embedding and carbothermal reduction. Fe0@RF-C was employed as a heterogeneous persulfate (PS) activator for degrading 2,4-dihydroxybenzophenone (BP-1), an emerging contaminant. Compared to commercial nZVI, Fe0@RF-C exhibited superior PS activation performance and oxidation resistance. Nearly 95% of BP-1 was removed within 10 min in the Fe0@RF-C/PS system. The carbon layer promotes the enrichment of BP-1 and accelerates its degradation through singlet oxygen oxidation and direct electron transfer processes. This study provides a straightforward approach for designing highly stable carbon-coated nZVI composite and elucidates the enhanced catalytic performance mechanism by carbon layers.

Biochar Meets Single-Atom: A Catalyst for Efficient Utilization in Environmental Protection Applications and Energy Conversion

Single-atom catalysts (SACs), combining the advantages of multiphase and homogeneous catalysis, have been increasingly investigated in various catalytic applications. Carbon-based SACs have attracted much attention due to their large specific surface area, high porosity, particular electronic structure, and excellent stability. As a cheap and readily available carbon material, biochar has begun to be used as an alternative to carbon nanotubes, graphene, and other such expensive carbon matrices to prepare SACs. However, a review of biochar-based SACs for environmental pollutant removal and energy conversion and storage is lacking. This review focuses on strategies for synthesizing biochar-based SACs, such as pre-treatment of organisms with metal salts, insertion of metal elements into biochar, or pyrolysis of metal-rich biomass, which are more simplistic ways of synthesizing SACs. Meanwhile, this paper attempts to 1) demonstrate their applications in environmental remediation based on advanced oxidation technology and energy conversion and storage based on electrocatalysis; 2) reveal the catalytic oxidation mechanism in different catalytic systems; 3) discuss the stability of biochar-based SACs; and 4) present the future developments and challenges regarding biochar-based SACs.

Quantification of the Contribution of Heterogeneous Surface Processes to Pollutant Abatement during Heterogeneous Catalytic Ozonation

Heterogeneous surface processes such as adsorption and oxidation with surface-adsorbed reactive oxygen species (ROSad, e.g., adsorbed oxygen atom (*Oad) and hydroxyl radicals (•OHad)) have been suggested to play an important role in pollutant abatement during heterogeneous catalytic ozonation (HCO). However, to date, there is no reliable method to quantitatively evaluate the contribution of heterogeneous surface processes to pollutant abatement (fS) during HCO. In this study, we developed a method by combining probe compound-based experiments with kinetic modeling to distinguish heterogeneous surface processes from homogeneous bulk reactions with aqueous O3 and ROS (•OH and superoxide radicals (O2•-) in the abatement of various pollutants (e.g., atrazine, ibuprofen, tetrachloroethylene, and perfluorooctanoic acid) during HCO with reduced graphene oxide. The results show that the pollutants that have a low affinity for the rGO surface (e.g., ibuprofen and tetrachloroethylene) were essentially abated by homogeneous bulk reactions, while the contribution of heterogeneous surface processes was negligible (fS < 5%). In contrast, heterogeneous surface processes played an important or even dominant role in the abatement of pollutants that have a high surface affinity (e.g., fS = 32-82% for atrazine and perfluorooctanoic acid). This study is a critical first step in quantitatively evaluating the role of heterogeneous surface processes for pollutant abatement during HCO, which is crucial to understanding the mechanism of HCO and designing catalysts for effective pollutant abatement.

Synergistic enhancement of microbes-to-pollutants and inter-microbes electron transfer by Fe, N modified ordered mesoporous biochar in anaerobic digestion

Extracellular electron transfer was essential for degrading recalcitrant pollutants by anaerobic digestion (AD). Therefore, existing studies improved AD efficiency by enhancing the electron transfer from microbes-to-pollutants or inter-microbes. This study synthesized a novel Fe, N co-doped biochar (Fe, N-BC), which could enhance both the microbes-to-pollutants and inter-microbes electron transfer in AD. Detailed characterization data indicated that Fe, N-BC has an ordered mesoporous structure, high specific surface area (463.46 m2/g), and abundant redox functional groups (Fe2+/Fe3+, pyrrolic-N), which translate into excellent biocompatibility and electrochemical properties of Fe, N-BC. By adding Fe, N-BC, the stability and efficiency of the medium-temperature AD system in the treatment of methyl orange (MO) wastewater were improved: obtained a high degradation efficiency of MO (96.8 %) and enhanced the methane (CH4) production by 65 % compared to the control group. Meanwhile, Fe, N-BC reduced the accumulation of volatile fatty acids in the AD system, and the activity of anaerobic granular sludge electron transport system and coenzyme F420 was enhanced. In addition, Fe, N-BC showed positive enrichment of azo dyes decolorization bacteria (Georgenia) and direct interspecies electron transfer (DIET) synergistic partners (Syntrophobacter, Methanosarcina). Overall, the rapid degradation of MO and enhanced CH4 production in AD systems by Fe, N-BC is associated with enhancing two electronic pathways, i.e., microbes to MO and DIET between syntrophic bacteria and methanogenic archaea. This study introduced an enhanced "two-pathways of electron transfer" theory, realized by Fe, N-BC. These findings provided new insights into the interactions within AD systems and offer strategies for enhancing their performance with recalcitrant pollutants.

Analyzing the active sites of carbocatalyst for peroxydisulfate activation: Specific surface area or electrochemical surface area?

Persulfates activation by various nanomaterials has been intensively reported for advanced oxidation processes (AOPs), and substantial progress has been made in understanding the mechanism. However, most of the published articles only present the unnormalized catalytic properties, which generated confusion to compare different catalysts and identify the active sites. Herein, we presented electrochemical surface area (ECSA) as a practical normalized method and confirmed the primary active sites in N-doped graphene. By controlling the aggregation state of graphene sheets to adjust the activity of doped graphite-N species, the active sites for peroxydisulfate (PDS) activation were accurately estimated. In further experiments, specific surface area (SSA, by N2-physisorption and methylene blue adsorption) and ECSA were adopted to conclude the normalized oxidation rate constant and graphitic-N was confirmed as the primary site in nitrogen-doped graphene for the carbocatalyst/PDS system. The normalized results revealed that SSA derived from inert gas on materials could not reflect the true active sites at solid-liquid interface, while ECSA considering the operated solid-liquid situation can be used for accurate estimation of the active sites. Therefore, this study suggests that ECSA integrates the properties of both kinetics and thermodynamics, which can be adopted as a useful methodology for analyzing nano-sized environmental catalysts performance.

Tempospatially Confined Catalytic Membranes for Advanced Water Remediation

The application of homogeneous catalysts in water remediation is limited by their excessive chemical and energy input, weak regenerability, and potential leaching. Heterogeneous catalytic membranes (CMs) offer a new approach to facilitate efficient, selective, and continuous pollutant degradation. Thus, integrating membranes and continuous filtration with heterogeneous advanced oxidation processes (AOPs) can promote thermodynamic and kinetic mass transfers in spatially confined intrapores and facilitate diffusion-reaction processes. Despite the remarkable advantages of heterogeneous CMs, their engineering application is practically restricted due to the fuzzy design criteria for specific applications. Herein, the recent advances in CMs for advanced water remediation are critically reviewed and the design flow for tempospatially confined CMs is proposed. Further, state-of-the-art CM materials and their catalytic mechanisms are reviewed, after which the tempospatial confinement mechanisms comprising the nanoconfinement effect, interface effect, and kinetic mass transfer are emphasized, thus clarifying their roles in the construction and performance optimization of CMs. Additionally, the fabrication methods for CMs based on their catalysts and pore sizes are summarized and an overview of their application and performance evaluations is presented. Finally, future directions for CMs in materials research and water treatment, are presented.

Sewage sludge pyrolysis 'kills two birds with one stone': Biochar synergies with persulfate for pollutants removal and energy recovery

The disposal and resource utilization of sewage sludge (SS) have always been significant challenges for environmental protection. This study employed straightforward pyrolysis to prepare iron-containing sludge biochar (SBC) used as a catalyst and to recover bio-oil used as fuel energy. The results indicated that SBC-700 could effectively activate persulfate (PS) to remove 97.2% of 2,4-dichlorophenol (2,4-DCP) within 60 min. Benefiting from the appropriate iron content, oxygen-containing functional groups and defective structures provide abundant active sites. Meanwhile, SBC-700 exhibits good stability and reusability in cyclic tests and can be easily recovered by magnetic separation. The role of non-radicals is emphasized in the SBC-700/PS system, and in particular, single linear oxygen (1O2) is proposed to be the dominant reactive oxygen. The bio-oil, a byproduct of pyrolysis, exhibits a higher heating value (HHV) of about 30 MJ/kg, with H/C and O/C ratios comparable to those of biodiesel. The energy recovery rate of the SS pyrolysis system was calculated at 80.5% with a lower input cost. In conclusion, this investigation offers a low-energy consumption and sustainable strategy for the resource utilization of SS while simultaneously degrading contaminants.

Comparative study of organic removal by pre-adsorption oxidation and synchronous adsorption oxidation processes: Performance and mechanism

Both adsorption and oxidation occur and contribute to organics removal in carbonaceous materials based advanced oxidation processes, while the correction of adsorption and oxidation, and the role of adsorption in the veritable removal of organic are not clear. Herein, we investigated the performance of carbamazepine (CBZ) removal by peroxymonosulfate (PMS) activated by magnetic Fe-doped biochar through two models of pre-adsorption oxidation and synchronous adsorption oxidation processes. The adsorption process was better fitted by pseudo-second-order kinetic model and the adsorption mechanism was obtained by comprehensive analysis of equilibrium adsorption capacities, surface functional groups, specific surface area, pore volume, and ID/IG value. It is noted that pre-adsorption highly inhibited the further oxidation of CBZ in 0.5Fe@LSBC700/PMS system due to the occupied catalytic active sites. Total CBZ removal in pre-adsorption oxidation (45 %) was inferior to synchronous adsorption oxidation (∼100 %), as well as the veritable CBZ oxidation removal of 27 % for pre-adsorption oxidation vs ∼100 % in synchronous adsorption oxidation at 30 min. Oxidation degradation of CBZ based on radical oxidation was identified by quenching experiments and electron paramagnetic resonance measurements. This work is conducive to identifying the role of adsorption during the removal of organics in the adsorption-oxidation process, as well as veritable adsorption and oxidation removal of organics.

A comprehensive review of the "black gold catalysts" in wastewater treatment: Properties, applications and bibliometric analysis

A significant amount of effort has been devoted to the utilization of biochar-based catalysts in the treatment of wastewater. By virtue of its abundant functional groups and high specific surface area, biochar holds significant promise as a catalyst. This article presents a comprehensive systematic review and bibliometric analysis covering the period from 2009 to 2024, focusing on the restoration of wastewater through biochar catalysis. The production, activation, and functionalization techniques employed for biochar are thoroughly examined. In addition, the application of advanced technologies such as advanced oxidation processes (AOPs), catalytic reduction reactions, and biochemically driven processes based on biochar are discussed, with a focus on elucidating the underlying mechanisms and how surface functionalities influence the catalytic performance of biochar. Furthermore, the potential drawbacks of utilizing biochar are also brought to light. To emphasize the progress being made in this research field and provide valuable insights for future researchers, a scientometric analysis was conducted using CiteSpace and VOSviewer software on 595 articles. Hopefully, this review will enhance understanding of the catalytic performance and mechanisms pertaining to biochar-based catalysts in pollutant treatment while providing a perspective and guidelines for future research and development efforts in this area.

Ranitidine degradation in layered double hydroxide activated peroxymonosulfate system: impact of transition metal composition and reaction mechanisms

Ranitidine, a competitive inhibitor of histamine H2 receptors, has been identified as an emerging micropollutant in water and wastewater, raising concerns about its potential impact on the environment and human health. This study aims to address this issue by developing an effective removal strategy using two types of layered double hydroxide (LDH) catalysts (i.e., CoFeLDH and CoCuLDH). Characterization results show that CoFeLDH catalyst has superior catalytic properties due to its stronger chemical bond compared to CoCuLDH. The degradation experiment shows that 100% degradation of ranitidine could be achieved within 20 min using 25 mg/L of CoFeLDH and 20 mg/L of peroxymonosulfate (PMS). On the other hand, CoCuLDH was less effective, achieving only 70% degradation after 60 min at a similar dosage. The degradation rate constant of CoFeLDH was 10 times higher than the rate constant of CoCuLDH at different pH range. Positive zeta potential of CoFeLDH made it superior over CoCuLDH regarding catalytic oxidation of PMS. The catalytic degradation mechanism shows that sulfate radicals played a more dominant role than hydroxyl radicals in the case of LDH catalysts. Also, CoFeLDH demonstrated a stronger radical pathway than CoCuLDH. XPS analysis of CoFeLDH revealed the cation percentages at different phases and proved the claim of being reusable even after 8 cycles. Overall, the findings suggest that CoFeLDH/PMS system proves to be a suitable choice for attaining high degradation efficiency and good stability in the remediation of ranitidine in wastewater.

Sustainable efficient utilization of magnetic porous biochar for adsorption of orange G and tetracycline: Inherent roles of adsorption and mechanisms

Iron-doped biochar has been widely used as an adsorbent to remove contaminants due to the high adsorption performance, but it still suffers from complicated preparation methods, unstable iron loading, unsatisfactory specific surface area, and uneven distribution of active sites. Here, a novel magnetic porous biochar (FeCS800) with nanostructure on surface was synthesized by one-pot pyrolysis method of corn straw with K2FeO4, and used in orange G (OG) and tetracycline (TC) adsorption. FeCS800 exhibited outstanding adsorption capacities for OG and TC after K2FeO4 activation and the adsorption data were fitted satisfactorily to Langmuir isotherm and Pseudo-second-order kinetic model. The maximum adsorption capacities of FeCS800 for OG and TC were around 303.03 mg/g and 322.58 mg/g, respectively, at 25 °C and pH 7.0, which were 16.27 and 24.61 times higher than that before modification. Thermodynamic studies showed that the adsorption of OG/TC by FeCS800 were thermodynamically favorable and highly spontaneous. And the adsorption capacity of OG and TC by FeCS800 remained 77% and 81% after 5 cycles, respectively, indicating that FeCS800 had good stability. The outstanding adsorption properties and remarkable reusability of FeCS800 show its great potential to be an economic and environmental adsorbent in contaminants removal.

High-performance electrode materials of heteroatom-doped lignin-based carbon materials for supercapacitor applications

Supercapacitors are the preferred option for supporting renewable energy sources owing to many benefits, including fast charging, long life, high energy and power density, and saving energy. While electrode materials with environmentally friendly preparation, high performance, and low cost are important research directions of supercapacitors. At present, the growing global population and the increasingly pressing issue of environmental pollution have drawn the focus of numerous researchers worldwide to the development and utilization of renewable biomass resources. Lignin, a renewable aromatic polymer, has reserves second only to cellulose in nature. Ten million tonnes of industrial lignin are produced in pulp and paper mills annually, most of which are disposed of as waste or burned for fuel, seriously depleting natural resources and polluting the environment. One practical strategy to accomplish sustainable development is to employ lignin resources to create high-value materials. Based on the high carbon content and rich functional groups of lignin, the lignin-based carbon materials generated after carbonization treatment display specific electrochemical properties as electrode materials. Nevertheless, low electrochemical activity of untreated lignin precludes it from achieving its full potential for application in energy storage. Heteroatom doping is a common modification method that aims to improve the electrochemical performance of the electrode materials by optimizing the structure of the lignin, improving its pore structure and increasing the number of active sites on its surface. This paper aims to establish theoretical foundations for design, preparation, and optimizing the performance of heteroatom-doped lignin-based carbon materials, as well as for developing high-value-added lignin materials. The most reported the mechanism of supercapacitors, the doping process involving various types of heteroatoms, and the analysis of how heteroatoms affect the performance of lignin-based carbon materials are also detailed in this review.

Resource utilization of medical waste incineration fly ash to activate peroxydisulfate for tetracycline degradation: Synergy between adsorption and PDS activation

Medical waste incineration fly ash (MWI FA) is classified as a hazardous solid waste. Therefore, the development of recycling technologies to convert MWI FA into useful products is necessary and challenging. In this study, we developed a sustainable approach for preparing a catalyst through the pyrolysis of water-washed MWI FA (WW FA-x, where x corresponds to the pyrolysis temperature). Subsequently, it was applied as a potent peroxydisulfate (PDS) activator to remove tetracycline (TC) from water. The results showed that the WW FA-800 exhibited remarkable adsorption performance as well as highly efficient catalytic activation of PDS, with a 115 mg/g maximum TC adsorption capacity and 93.5% (reaction kinetic rate = 315 μmol/g/h) TC removal within 60 min. A synergistic effect was achieved by adsorption and PDS activation. TC degradation was primarily driven by non-radical (1O2 and electron transfer) processes. WW FA-800 possesses multiple active sites, including defects, π-π*, O-CO groups, Fe0, and Cu(I). Three possible pathways for TC decomposition have been proposed, with the majority of intermediates exhibiting less toxicity than TC. Furthermore, the WW FA/PDS system exhibited an excellent anti-interference ability, and universality in the degradation of various organic contaminants. Notably, energy consumption was minimal, approximately 2.80 kWh/(g·TC), and the leachability of heavy metals in the WW FA-800 was within acceptable limits. This study provides a MWI FA recycling route for the development of highly active catalysts.

Novel enhanced defluorination of perfluorooctanoic acids by biochar-assisted ultrasound coupling ferrate: Performance and mechanism

An ultrasound (US)/biochar (BC)/ferrate (Fe (VI)) system was firstly proposed to enhance perfluorooctanoic acid (PFOA) defluorination. It achieved 93 % defluorination optimally, higher than the sum of 77 % (28 % and 49 % for US/BC and US/Fe (VI) respectively), implying synergistic effect. Besides, the mechanism study confirmed that, this system can not only increase the specific surface area of BC and the generation of reactive oxidant species (ROS), enriching the active sites and forming new oxygen-containing functional groups, but also promote the formation of intermediate iron species. The PFOA degradation in the US/BC/Fe (VI) was probably an adsorption-degradation process, both ROS and electron transfer promoted the defluorination. Additionally, its sustainability was also demonstrated with 14 % reduced defluorination percentage after five cycles of BC. Overall, the synergistic effect of the US/BC/Fe (VI) and its enhancing mechanism for PFOA defluorination were clarified firstly, which contributes to the development of biochar for assisting polyfluoroalkyl substances degradation.

Insights into the efficient mineralization of antibiotic trimethoprim in aqueous media by Fe2+ catalytically enhanced vacuum-UV irradiation: Kinetics, mechanisms, and toxicity evaluation

The widespread existence of antibiotics in the environment has attracted growing concerns regarding the potential adverse effects on aquatic organisms, ecosystems, and human health even at low concentrations. Extensive efforts have been devoted to developing new methods for effective elimination of antibiotics from wastewater. Herein, a novel process of Fe2+ catalytically enhanced vacuum ultraviolet (VUV) irradiation was proposed as a promising approach for the removal of antibiotic trimethoprim (TMP) in water. Compared with UVC photolysis, VUV photolysis, and UVC/Fe2+, VUV/Fe2+ could increase the pseudo-first-order reaction rate constant of TMP removal by 6.6-38.4 times and the mineralization rate by 36.5%-59.9%. The excellent performance might originate from the synergistic effect of VUV and Fe2+, i.e., VUV irradiation could effectively split water and largely accelerate the Fe3+/Fe2+ cycle to generate more reactive oxygen species (ROS). EPR results indicated that •OH and O2•- were identified as the main ROS in the UVC/Fe2+ and VUV/Fe2+ processes, while •OH, O2•-, and 1O2 were involved in the VUV process. The operating parameters, such as Fe2+ dosage and initial TMP contents, were evaluated and optimized. Up to 8 aromatic intermediates derived from hydroxylation, demethylation, carbonylation, and methylene group cleavage were identified by UPLC-QTOF-MS/MS technique, the possible pathways of TMP degradation were proposed. Finally, the acute and chronic toxicity of intermediates formed during TMP degradation in the VUV/Fe2+ process were also evaluated.

Knowledge graph and development hotspots of biochar as an emerging aquatic antibiotic remediator: A scientometric exploration based on VOSviewer and CiteSpace

As an emerging material in the field of environmental remediation, biochar produced by carbonisation of organic solid waste has been widely used in the remediation of antibiotic wastewater due to its environmental friendliness and excellent adsorption properties. This study analyses the current literature in the field in a comprehensive and scientific manner using CiteSpace and VOSviewer technologies. Between 2011 and 2023, a total of 1162 papers were published in this domain, spanning three distinct stages: applied methods, mechanism investigation, and enhanced improvement. The results of keyword clustering indicate that the remediation of antibiotics complexed with multiple pollutants by biochar is the main research topic, followed by the remediation of antibiotics by biochar in combination with other technologies. Furthermore, drawing from current research hotspots in antibiotic remediation using biochar, this study identified the pivotal mechanisms involved: (1) The primary mechanisms by which raw biochar remediates antibiotics include π-π electron donor-acceptor interactions, hydrophobic interactions, electrostatic interactions, hydrogen-bonding, and pore filling. (2) Steam activation, acid/base, metal salt/metal oxide, and clay mineral modification can improve the physical/chemical properties of biochar, enhancing its adsorptive removal of antibiotics. (3) Biochar activated persulfate and degraded antibiotics via free radical pathways (SO4-•, •OH and O2-•) as well as non-free radical pathways (1O2 and electron transfer). In addition, the challenge and prospect of biochar engineering applications for antibiotic remediation lies in improving the main mechanism of antibiotic remediation by biochar. The prospective utilization of biochar in enhancing the remediation of antibiotic-related pollutants holds tremendous value for the future.

Molten salt synthesis of porous graphene-like carbons as peroxydisulfate catalyst for the efficient removal of rhodamine B dye

Carbon materials have been receiving considerable attention as effective green catalysts for peroxydisulfate (PDS) activation to degrade organic pollutants. Herein, the porous graphene-like carbons (PGCs) were synthesized by pyrolyzing a nitrogen-rich biomass (peanut shell, PS) in the eutectic mixture of FeCl3 and ZnCl2. The results suggested that involvement of molten salts attributed the biochar the amazing properties such as high specific surface area (SBET = 2529.4 m2 g-1), abundant structural defects, high nitrogen content (6.5%), and oxygen-containing functional groups on its surface. Especially when pyrolyzed at activation temperature of 800 °C, mass ratio of 1:3:15 (PS:ZnCl2:FeCl3), and activation time of 2 h, the optimized PGCs-op exhibited outstanding performance in the catalytic degradation of rhodamine B (RhB). Almost all of RhB (99.02%) was removed in 40 min and basically not influenced by initial pH in the range of 3.00 to 9.98. Although the RhB degradation was influenced by anions (Cl-, HCO3-, HPO42-), the inhibition would be significantly alleviated within 120 min unless these substances were high in concentration. Furthermore, the quenching tests revealed that the reactive species were involved in RhB degradation in the sequence of 1O2 > O2∙-  > SO4∙-  > ∙OH, among which singlet oxygen played a crucial role. Combined with characterization analysis, a possible mechanism of RhB degradation in PGCs-op/PDS system was proposed. Overall, this study provided a promising metal-free catalyst for the removal of organic pollutants while achieving reutilization of the waste biomass.

Fe-N co-doped biochar derived from biomass waste triggers peracetic acid activation for efficient water decontamination

In this study, the porous carbon material (FeN-BC) with ultra-high catalytic activity was obtained from waste biomass through Fe-N co-doping. The prominent degradation rate (> 96.8%) of naproxen (NAP) was achieved over a wide pH range (pH 3.0-9.0) in FeN-BC/PAA system. Unlike previously reported iron-based peracetic acid (PAA) systems with •OH or RO• as the dominated reactive species, the degradation of contaminants was attributed to singlet oxygen (1O2) produced by organic radicals (RO•) decomposition, which was proved to be thermodynamically feasible and favorable by theoretical calculations. Combining the theoretical calculations, characteristic and experimental analysis, the synergistic effects of Fe and N were proposed and summarized as follows: i) promoted the formation of extensive defects and Fe0 species that facilitated electron transfer between FeN-BC and PAA and continuous Fe(II) generation; ii) modified the specific surface area (SSA) and the isoelectric point of FeN-BC in favor of PAA adsorption on the catalyst surface. This study provides a strategy for waste biomass reuse to construct a heterogeneous catalyst/PAA system for efficient water purification and reveals the synergistic effects of typical metal-heteroatom for PAA activation.

A state-of-art review on the redox activity of persistent free radicals in biochar

Biochar-bound persistent free radicals (biochar-PFRs) attract much attention because they can directly or indirectly mediate the transformation of contaminants in large-scale wastewater treatment processes. Despite this, a comprehensive top-down understanding of the redox activity of biochar-PFRs, particularly consumption and regeneration mechanisms, as well as challenges in redox activity assessment, is still lacking. To tackle this challenge, this review outlines the identification and determination methods of biochar-PFRs, which serve as a prerequisite for assessing the redox activity of biochar-PFRs. Recent developments concerning biochar-PFRs are discussed, with a main emphasis on the reaction mechanisms (both non-free radical and free radical pathways) and their effectiveness in removing contaminants. Importantly, the review delves into the mechanism of biochar-PFRs regeneration, triggered by metal cations, reactive oxygen species, and ultraviolet radiations. Furthermore, this review thoroughly explores the dilemma in appraising the redox activity of biochar-PFRs. Components with unpaired electrons (particular defects and metal ions) interfere with biochar-PFRs signals in electron paramagnetic resonance spectra. Scavengers and extractants of biochar-PFRs also inevitably modify the active ingredients of biochar. Based on these analyses, a practical strategy is proposed to precisely determine the redox activity of biochar-PFRs. Finally, the review concludes by presenting current gaps in knowledge and offering suggestions for future research. This comprehensive examination aims to provide new and significant insights into the redox activity of biochar-PFRs.

Phosphate-mediated degradation of organic pollutants in water with peroxymonosulfate revisited: Radical or non-radical oxidation?

Whilst it is generally recognized that phosphate enables to promote the removal of some organic pollutants with peroxymonosulfate (PMS) oxidation, however, there is an ongoing debate as to whether free radicals are involved. By integrating different methodologies, here we provide new insights into the reaction mechanism of the binary mixture of phosphates (i.e., NaH2PO4, Na2HPO3, and NaH2PO2) with peroxymonosulfate (PMS) or hydrogen peroxide (H2O2). Enhanced degradation of organic pollutants and observation of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) adducts (i.e. DMPOOH and 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX)) with electron paramagnetic resonance (EPR) in most phosphates/PMS system seemly support a radical-dominant mechanism. However, fluorescence probe experiments confirm that no significant amount of hydroxyl radicals (•OH) are produced in such reaction systems. PMS in the phosphate solutions (without any organics) remains relatively stable, but is only consumed while organic substrates are present, which is distinct from a typical radical-dominant Co2+/PMS system where PMS is continuously decomposed. Through density functional theory (DFT) calculation, the energy barriers of the phosphates/PMS reaction processes are greatly decreased when non-radical mechanism dominates. Complementary evidence suggests that the reactive intermediates of PMS-phosphate complex, rather than the free radicals, are capable of oxidizing electron-rich substrates such as DMPO and organic pollutants. Taking the case of phosphate/PMS system as an example, this study demonstrates the necessity of acquisition of lines of evidence for resolving paradoxes in identifying EPR adducts.

Unveiling activation mechanism of persulfate by homologous hemp-derived biochar catalysts for enhanced tetracycline wastewater remediation

Advancements in biochar activating persulfate advanced oxidation processes (PS-AOP), have gained significant attention. However, the understanding of biochar-based catalysts in activating PS remains limited. Herein, biochar (BC) and N-doped biochar (NBC) were synthesized from hemp for activating PS to treat tetracycline (TC) wastewater and analyzed their mechanisms separately. Surprisingly, N-doped in biochar leads to a change in the activation mechanism of PS. The BC-PS system operates mainly through a radical pathway, advantageous for treating soil organic pollution (68%) with pH adaptability (less than 10% variation). Nevertheless, the NBC-PS system primarily employs an electron transfer non-radical pathway, demonstrating stability (only 7% performance degradation over four cycles) and enhanced resistance to anionic interference (less than 10% variation) in organic wastewater treatment. This study provides a technical reference and theoretical foundation for enhancing biochar activation of PS in the removal of organic pollutants from aquatic and terrestrial environments.

Nanostructured, Biodoping-Activated Fungi-Modified Wood for Enhanced Cd2+ Removal: Performances and Insight on Adsorption Mechanisms

Nanostructured activated carbon (AC) adsorbents derived from woody biomass have garnered attention for their potential usage to remove toxic substances from the environment due to their high specific surface area, superior micro/mesoporosity, and tunable surface chemistry profile. However, chemical dopants widely used to enhance the chemical reactivity with heavy metals would pollute the environment and conflict with the vision of a cleaner and sustainable environment. Herein, we report a facile, green, and sustainable approach using fungi modification combined with alkali activation to produce AC for heavy metal removal. The decayed wood-derived AC (DAC) exhibited a high specific surface area of 2098 m2/g, and the content of O and N functional groups was 18 and 2.24%, respectively. It showed remarkable adsorption capacity toward Cd2+ of 148.7 mg/g, which was much higher than most reported Cd2+ adsorbents. Such excellent adsorption capacity was primarily based on enhanced physical adsorption (pore filling, π-π) and chemical adsorption (functional group complexation, ion exchange, and precipitation). Additionally, the DAC showed rapid kinetics and remarkable applicability in both dynamic environments and actual water samples. These results suggest that decayed wood has excellent potential for efficient use in the removal of Cd2+ from wastewater. Furthermore, these results indicate that decayed wood can be cleanly produced into high efficiency heavy metal adsorbents to realize value-added utilization of decayed wood.

S vacancies-introduced chalcopyrite switch radical to non-radical pathways via peroxymonosulfate activation: Vital roles of S vacancies

Regulation of peroxymonosulfate (PMS) activation from radical to non-radical pathways is an emerging focus of advanced oxidation processes (AOPs) due to its superiority of anti-interference to complex wastewater. However, the detailed correlation mechanism between the defect structure of the catalyst and the regulation of radicals/non-radicals remains unclear. Herein, natural chalcopyrite (CuFeS2) with different levels of S vacancies created by a simple NaBH4 reduction process was employed to explore the above-mentioned underlying mechanism for constructing high efficiency and low cost of catalyst towards AOPs. With the assistance of simulated solar light, S-deficient chalcopyrite (Sv-NCP) exhibited prominent performance for PMS activation. More interestingly, the different degrees of S vacancies regulated the active species from radicals to non-radical 1O2, thus showing excellent purification of complex wastewater as well as actual pharmaceutical wastewater. Mechanistic analysis reveals that PMS tends to loss electrons on S vacancies sites and is dissociated into 1O2 rather than ·OH/SO4·- due to electron deficiency. Meanwhile, the improved adsorption performance makes the degradation sites of pollutants change from solution to surface. Most importantly, Sv-NCP presented excellent detoxication for antibiotic wastewater due to the high selectivity of 1O2. This work provides novel insights into the regulation of active species in Fenton-like reactions via defect engineering for high efficiency of pollutant degradation.

Excess sludge-based biochar loaded with manganese enhances catalytic ozonation efficiency for landfill leachate treatment

This study developed an efficient and stable landfill leachate treatment process, which was based on the combination of biochar catalytic ozonation and activated sludge technology for intensive treatment of landfill leachate, aiming to achieve the standard discharge of leachate. The focus is to investigate the effect of manganese loading on the physicochemical properties of biochar and the mechanism of its catalytic ozonation. It was found that more surface functional groups (CO, Mn-O, etc.) and defects (ID/IG = 1.27) were exposed via the change of original carbon structure by loading Mn, which is conducive to the generation of lattice oxygen. Meanwhile, generating different valence states of Mn metal can improve the redox properties and electron migration rate, and encourage the production of reactive oxygen species (ROS) during the reaction process and enhance the catalytic efficiency. The synergistic action of microorganisms, especially denitrifying bacteria, was found to play a key role in the degradation of nitrogenous pollutants during the activated sludge process. The concentration of NH+4-N was reduced from the initial 1087.03 ± 9.56 mg/L to 9.05 ± 1.91 mg/L, while COD was reduced from 2290 ± 14.14 mg/L to 86.5 ± 2.12 mg/L, with corresponding removal rates of 99.17% and 99.20%, respectively. This method offers high efficiency and stability, achieving discharge standards for leachate (GB16889-2008). The synergy between Mn-loaded biochar and microorganisms in the activated sludge is key to effective treatment. This study offers a new approach to solving the challenge of waste leachate treatment.

Catalytic activation of persulfate by nanoscale zero-valent iron-derived supported boron-doped porous carbon for bisphenol A degradation

In this study, boron-doped porous carbon materials (BCs) with high surface areas were synthesized employing coffee grounds as carbon source and sodium bicarbonate and boric acid as precursors; afterward, nanoscale zero-valent iron (nZVI) and BCs composites (denoted as nZVI@BCs) were further prepared through reduction of FeSO4 by NaBH4 along with stirring. The performance of the nZVI@BCs for activating persulfate (PS) was evaluated for the degradation of bisphenol A (BPA). In comparison with nZVI@Cs/PS, nZVI@BCs/PS could greatly promote the degradation and mineralization of BPA via both radical and non-radical pathways. On the one hand, electron spin resonance and radical quenching studies represented that •OH, SO4•-, and O2•- were mainly produced in the nZVI@BCs/PS system for BPA degradation. On the other hand, the open circuit voltages of nZVI@BCs and nZVI@Cs in different systems indicated that non-radical pathway still existed in our system. PS could grab the unstable unpaired electron on nZVI@BCs to form a carbon material surface-confined complex ([nZVI@BCs]*) with a high redox potential, then accelerate BPA removal efficiency via direct electron transfer. Furthermore, the performances and mechanisms for BPA degradation were examined by PS activation with nZVI@BC composites at various conditions including dosages of nZVI@BCs, BPA and PS, initially pH value, temperature, common anions, and humid acid. Therefore, this study provides a novel insight for development of high-performance carbon catalysts toward environmental remediation.

Efficient removal of Rhodamine B dye using biochar as an adsorbent: Study the performance, kinetics, thermodynamics, adsorption isotherms and its reusability

Removal of toxic dyes such as Rhodamine B is essential as it pollutes aqueous and soil streams as well. This comprehensive study explores the potential of Calophyllum inophyllum seed char as an efficient bio-adsorbent based on their characteristic properties and a comparative study between various carbon-based adsorbents on the adsorption capacity of Rhodamine B dye. In this study, the char was prepared from Calophyllum inophyllum seed using a slow pyrolysis process (298 K/min) at an optimum temperature of 823 K and used as an adsorbent for the removal of Rhodamine B from water. The resulting char was mesoporous and had 155.389 m2/g surface areas (BET) and 0.628 cc/g pore volume. The formation of pores was observed from the SEM analysis. The adsorption studies were tested and optimized through various parameters such as solution pH, adsorbent dosage, initial dye concentration, stirring speed, contact time, and solution temperature. Maximum 95.5 % removal of Rhodamine B was possible at the pH: 2, stirring speed: 100 rpm, time: 25 min, temperature 308 K, and dose: 1.2 g/L. The highest adsorption capacity at equilibrium was determined to be 169.5 (mg/g) through Langmuir adsorption isotherm studies and followed pseudo 2nd order kinetics. The thermodynamics study confirmed the adsorption processes were spontaneous (ΔG°=-0.735 kJ/mol) and endothermic (ΔH° = 4.1 kJ/mol) processes. The reusability study confirmed that the mesoporous char can be reused as an efficient adsorbent for up to 3 cycles for environmental remediation.

Definitive Review of Nanobiochar

Nanobiochar is an advanced nanosized biochar with enhanced properties and wide applicability for a variety of modern-day applications. Nanobiochar can be developed easily from bulk biochar through top-down approaches including ball-milling, centrifugation, sonication, and hydrothermal synthesis. Nanobiochar can also be modified or engineered to obtain "engineered nanobiochar" or biochar nanocomposites with enhanced properties and applications. Nanobiochar provides many fold enhancements in surface area (0.4-97-times), pore size (0.1-5.3-times), total pore volume (0.5-48.5-times), and surface functionalities over bulk biochars. These enhancements have given increased contaminant sorption in both aqueous and soil media. Further, nanobiochar has also shown catalytic properties and applications in sensors, additive/fillers, targeted drug delivery, enzyme immobilization, polymer production, etc. The advantages and disadvantages of nanobiochar over bulk biochar are summarized herein, in detail. The processes and mechanisms involved in nanobiochar synthesis and contaminants sorption over nanobiochar are summarized. Finally, future directions and recommendations are suggested.

Paint sludge derived activated carbon encapsulating with cobalt nanoparticles for non-radical activation of peroxymonosulfate

Industrial solid waste management and recycling are important to environmental sustainability. In this study, cobalt (Co) nanoparticles encapsulated in paint sludge-derived activated carbon (AC) were fabricated. The Co-AC possessed high conductivity, magnetic properties and abundant metal oxide impurities (TiAlSiOx), which was applied as multifunctional catalyst for peroxymonosulfate (PMS) activation. Compared to pure AC, the Co-AC exhibited significant enhanced performance for degradation of tetracycline hydrochloride (TCH) via PMS activation. Mechanism studies by in situ Raman spectroscopy, Fourier infrared spectroscopy, electrochemical analysis and electron paramagnetic resonance suggested that surface-bonded PMS (PMS*) and singlet oxygen (1O2) are the dominant reactive species for TCH oxidation. The non-radical species can efficiently oxidize electron-rich pollutants with high efficiency, which minimized the consumption of PMS and the catalyst. The removal percentages of TCH reached 97 % within 5 min and ∼ 99 % within 15 min in the Co-AC/PMS system. The Co active sites facilitated PMS adsorption to form the PMS* and the TiAlSiOx impurities provided abundant oxygen vacancy for generation of the 1O2. In addition, the Co-AC/PMS system achieved high efficiency and stability for oxidation of the target pollutants over a long-term continuous operation. This work not only offers a cost-effective approach for recycling industrial waste but also provides new insights into the application of waste-derived catalyst for environmental remediation.

Confined Co@NCNTs as highly efficient catalysts for activating peroxymonosulfate: free radical and non-radical co-catalytic mechanisms

A series of transition metal (Co, Ni, Fe) nanoparticles were confined in N-doped carbon nanotubes (NCNTs) prepared (Co@NCNTs, Ni@NCNTs, and Fe@NCNTs) by the polymerization method. The structure and composition of catalysts were well characterized. The catalytic activity of catalysts for activating peroxymonosulfate (PMS) was conducted via acid orange 7 (AO7) degradation. Among the catalysts, Co@NCNTs performed the best catalytic activity. Additionally, Co@NCNTs performed good catalytic activity in pH values of 2.39-10.98. Cl- and SO42- played a promoting roles in AO7 degradation. NO3- presented a weak effect on the catalytic performance of Co@NCNTs, while HCO3- and CO32- significantly suppressed the catalytic performance of Co@NCNTs. Both non-radical (1O2 and electron transfer) and free-radical (·OH and SO4·-) pathways were detected in the Co@NCNTs/PMS system. Notably, 1O2 was identified to be the main active specie in this study. The catalytic activity of Co@NCNTs gradually decreased after cycle reuse of Co@NCNTs. Finally, the toxicity of the AO7 degradation solution in the study was evaluated by Chlorella pyrenoidosa.

The potential application of biochar and salicylic acid to alleviate salt stress in soybean (Glycine max L.)

Salt stress has been one of the major contributor which affect soybean seed germination, its establishment, growth, and physiology stages. Utilization of strategies such as soil amendment and elicitors are of significant importance to reduce the disadvantageous effects of salt stress. In this regard, the objectives of the present study were to evaluate the effect of biochar and salicylic acid on morphological and physiological properties of soybean subjected to salinity. The first experiment was carried out based on completely randomized design with three replications including 11 soybean cultivars such as Williams, Saba, Kowsar, Tapor, Sari, Telar, Caspian, Nekador, Amir, Katol and Sahar and various levels of salinity such as 0, 2, 4, 6 dS/m of NaCl. The second experiment was performed as factorial design in a randomized complete block design with three replications consisting of treatments of biochar (0, 5 and 10 WP), salicylic acid (0, 0.5 and 1 mM), and NaCl (0, 2.5, 5, 7.5 dS/m). With respect to seed germination result, various concentrations of salt stress showed negative impact not only on all studied traits, but also varied among soybean cultivars indicating Amir cultivar as the best salt tolerant soybean genotype among others. In addition, our data exhibited that the interaction effect of biochar and salicylic acid on salt treated soybean plant were positively significant on some morphological traits such as leaf area, shoot dry/fresh weight, total dry/fresh weight and physiological attributes including chlorophyll a, flavonoid, proline contents, catalase and peroxidase activities. Moreover, the resultant data showed that the combination treatment of 5 and 10 WP of biochar and 1 mM of salicylic acid caused increase of the aforementioned parameters in order to improve their performance subjected to higher concentration of salinity. In final, it was concluded that the coupled application of biochar alongside salicylic acid was recommended as proficient strategy to mitigate the injurious influences of salt stress in soybean or other probable crops.