Robust reactive oxygen species production in interfacial reaction between organic acids and biochar: The combined effect of electron acceptance and electron conduction

Interfacial electron transport and reactive oxygen species (ROS) generation in the redox action between biochar (BC) and low-molecular-weight organic acids (LMWOAs) have been overlooked during the utilization of BC in soil amelioration/remediation. Herein, BC and N-doped BC (NBC) with various physicochemical properties were prepared at pyrolysis temperatures of 350, 550 and 750 °C (namely BC/NBC350, 550 and 750) and their interactions with LMWOAs were systematically investigated. Results of ROS quenching and electrochemical cell experiments revealed that BC/NBC could strongly interact with LMWOAs, especially ascorbic acid (AA), and acted as an electron shuttle to mediate one electron transfer from AA to oxygen to generate O2•-, followed by the robust generation of H2O2 and •OH. Interestingly, the generation rates of •OH and H2O2 in the NBC-AA system were significantly higher than that in the BC-AA system, of which the NBC550-AA system exhibited the best performance, enabling its superior ability in bisphenol A degradation. Different from conventional wisdom, the electron shuttle capacity depended on a combined aromatization degree and electron-accepting capacity of BC/NBC. These findings complement the interfacial electron transfer mechanism during the natural BC-LMWOAs interaction and provide new inspiration for the development of green and efficient organic pollutant removal technologies.

Graphitic N-doped biochar for superefficient uranium recycling from nuclear wastewater

N-doped biochar (AL-N/BC) prepared by pyrolyzing lignin in various temperatures manifested superefficient performance for uranium (U) recycling from nuclear wastewater. The optimist AL-N/BC-700 showed higher adsorption capacity of 25,000 mg/g and faster kinetics of 4100 g·min^-1·mg^-1 than the most of reported adsorbents, and excellent adsorption-desorption capability (adsorption rate > 90 % and desorption rate > 70 % after 12 cycles). Moreover, the high applicability of AL-N/BC-700 was verified by its superefficient U(VI) adsorption performance in a broad working pH range, various water matrices, and high irradiation stability. Furthermore, the adsorption mechanism discloses the significant role of graphitic N, rather than pyridinic N or pyrrolic N, for U(VI) adsorption. Overall, this work not only presents an applicable approach to alleviate the increasingly serious energy crisis via recycling U(VI) from nuclear wastewater, but also enriches the method of synthesizing N-doped materials for U(VI) adsorption.

Super facile one-step synthesis of sugarcane bagasse derived N-doped porous biochar for adsorption of ciprofloxacin

A new N-doped biochar derived from sugarcane bagasse (NSB) was prepared by one-pot pyrolysis with sugarcane bagasse as feedstock, melamine as nitrogen source and NaHCO₃ as pore-forming agent, and then NSB was used to adsorb ciprofloxacin (CIP) in water. The optimal preparation conditions of NSB were determined based on the evaluation index of adsorbability of NSB for CIP. SEM, EDS, XRD, FTIR, XPS and BET characterizations were used to analyze the physicochemical properties of the synthetic NSB. It was found that the prepared NSB had excellent pore structure, high specific surface area and more nitrogenous functional groups. Meanwhile, it was demonstrated that the synergistic interaction between melamine and NaHCO₃ increased the pores of NSB and the largest surface area of NSB was 1712.19 m²/g. The CIP adsorption capacity of 212 mg/g was obtained under optimal parameters as follows: NSB amount 0.125 g/L, initial pH 6.58, adsorption temperature 30 °C, CIP initial concentration 30 mg/L and adsorption time 1 h. The isotherm and kinetics studies elucidated that the adsorption of CIP conformed both D-R model and Pseudo-second-order kinetic model. The high CIP adsorption capacity of NSB for CIP was due to the combined filling pore, π-π conjugation and hydrogen bonding. All results demonstrated that adsorption of CIP by the low-cost N-doped biochar of NSB is a reliable technology for the disposal of CIP wastewater.

Supporting nanoscale zero-valent iron onto shrimp shell-derived N-doped biochar to boost its reactivity and electron utilization for selenite sequestration

Nanoscale zero-valent iron (nZVI) has been widely used in the reductive removal of contaminants from water, yet it still fights against the inherent passive cover and the raise of medium pH. In this study, nZVI was supported onto a nitrogen-doped biochar (NBC) that was prepared by pyrolyzing shrimp shell for efficiently sequestrating aqueous selenite (Se(IV)). The resultant composite (NBC-nZVI) revealed a higher reactivity and electron utilization efficiency (EUE) than the bare nZVI in Se(IV) sequestration because of the positive charge, the buffering effect and the good conductivity of NBC. The kinetic rate and EUE of NBC-nZVI were increased by 143.4% and 15.3% compared to the bare nZVI, respectively, at initial pH of 3.0. The high removal capacity of 605.4 mg g^-1 for NBC-nZVI was obtained at Se(IV) concentration of 1000 mg L^-1, initial pH of 3.0, NBC-nZVI dosage of 1.0 g L^-1 and contact time of 12 h. Moreover, NBC-nZVI exhibited a strong tolerance to solution pHs and coexisting compounds (e.g., humic acid) and could reduce the Se(IV) concentration from 5.0 mg L^-1 to below the limit of drinking water (50 μg L^-1) in real-world samples. This work exemplified a utilization of shrimp shell-derived NBC to simultaneously enhance the reactivity and EUE of nZVI for reductively removing contaminants.

Nitrogen-doped biochar (N-doped BC) and iron/nitrogen co-doped biochar (Fe/N co-doped BC) for removal of refractory organic pollutants

The presence of refractory organic pollutants (ROPs) in the ecosystem is a serious concern because of their impact on environmental constituents as well as their known or suspected ecotoxicity and adverse health effects. According to previous studies, carbonaceous materials, such as biochar (BC), have been widely used to remove pollutants from ecosystems owing to their desirable features, such as relative stability, tunable porosity, and abundant functionalities. Nitrogen (N)-doping and iron/nitrogen (Fe/N) co-doping can tailor BC properties and provide supplementary functional groups as well as extensive active sites on the N-doped and Fe/N co-doped BC surface, which is advantageous for interaction with and removal of ROPs. This review investigates the impact of N-doped and Fe/N co-doped BC on the removal of ROPs through adsorption, activation oxidation, and catalytic reduction due to the synergistic Fe, N, and BC features that modify the physicochemical properties, surface functional groups, and persistent free radicals of BC to aid in the degradation of ROPs. Owing to the attractive properties of N-doped and Fe/N co-doped BCs for the removal of ROPs, this review focuses and evaluates previous experimental investigations on the manufacturing (including precursors and influencing parameters during manufacturing) and characterizations of N-doped and Fe/N co-doped BCs. Additionally, the effective applications and mechanisms of N-doped and Fe/N co-doped BCs in adsorption, activation oxidation, and reductive remediation of ROPs are investigated herein. Moreover, the application of N-doped and Fe/N co-doped BC for progressive environmental remediation based on their effectiveness against co-pollutants, regeneration, stability, affordability, and future research prospects are discussed.

Nitrogen availability in biochar-based fertilizers depending on activation treatment and nitrogen source

Different activation and N-doping treatments were used to produce biochar-based fertilizers (BBFs) with increased N concentration and slow N release. Pristine biochars were produced by pyrolysis of olive tree pruning feedstock at low and high temperatures (400 and 800 °C). These biochars were activated either by ultrasonication, or oxidation with hydrogen peroxide (H₂O₂) or nitric acid (HNO₃) to increase their N retention potential. Subsequently biochars were enriched with N with either urea or ammonium sulfate. The activation of low-temperature biochars with HNO₃ was the most effective treatment leading to new surface carboxylic groups that facilitated the later enrichment with N. When treated with urea, BBFs reached 7.0 N%, whereas the H₂O₂ activation only allowed an increase up to 2.0 N%. The use of urea as the external N source was the most efficient for incorporating N. Urea treated biochars had a water-soluble fraction that represented up to 14.5 % of the total N. The hydrolyzable N fraction, composed by amides and simple N heterocycles originated by the N-doping treatments, and nitro groups generated from HNO₃ activation, represented up to 60 % of the total N. This study relates the N chemical forms in the new BBFs to potential N availability in soil. The presence of water-soluble, hydrolyzable and non-hydrolyzable N implied that these BBFs may supply N that would be progressively available for plants, acting as slow-release fertilizers.

In situ preparation of highly graphitized N-doped biochar geopolymer composites for efficient catalytic degradation of tetracycline in water by H2O2

Advanced oxidation processes (AOPs) hold great prospects for the treatment of antibiotic wastewater. N-doped biochar (NB) has received increasing attention as a catalyst for AOPs because of its green nature, abundant biomass resources, and low cost. However, NB catalysts are complicated to prepare and difficult to recover, limiting their practical application. In this study, an N-doped biochar geopolymer composite (NBGC) was synthesized via in situ doping, simultaneous carbonization, and activation (ISCA) of lignin and urea in the porous geopolymer flake, without additional activators. The ISCA process used a low-cost geopolymer flake that not only served as a carrier to immobilize NB and facilitate the recovery, but also applied its inherent strong alkalinity to activate NB. The composite catalyst obtained at 600 °C (NBGC-600) exhibited excellent activity in activating H₂O₂ to degrade tetracycline (∼100%, 50 mg/L). The EPR results indicated that NBGC-600 had a strong ability to activate and decompose H₂O₂ to •OH, which could be attributed to its rich persistent radicals, graphitized N and CO groups, as well as the high degree of graphitization of biochar. The degradation pathway and intermediates of tetracycline in the NBGC-600-H₂O₂ system were also discussed according to the HPLC-MS results. Moreover, NBGC-600 had excellent reusability and showed great potential for continuous treatment of tetracycline in water. This work paves a new way for the synthesis of cost-effective N-doped biochar composite catalysts for AOPs.

A Review on N-Doped Biochar for Oxidative Degradation of Organic Contaminants in Wastewater by Persulfate Activation

The Persulfate-based advanced oxidation process is the most efficient and commonly used technology to remove organic contaminants in wastewater. Due to the large surface area, unique electronic properties, abundant N functional groups, cost-effectiveness, and environmental friendliness, N-doped biochars (NBCs) are widely used as catalysts for persulfate activation. This review focuses on the NBC for oxidative degradation of organics-contaminated wastewater. Firstly, the preparation and modification methods of NBCs were reviewed. Then the catalytic performance of NBCs and modified NBCs on the oxidation degradation of organic contaminants were discussed with an emphasis on the degradation mechanism. We further summarized the detection technologies of activation mechanisms and the structures of NBCs affecting the PS activation, followed by the specific role of the N configuration of the NBC on its catalytic capacity. Finally, several challenges in the treatment of organics-contaminated wastewater by a persulfate-based advanced oxidation process were put forward and the recommendations for future research were proposed for further understanding of the advanced oxidation process activated by the NBC.

N-doped biochar-catalyzed dechlorination of carbon tetrachloride in sulfide-containing aqueous solutions: Performances, mechanisms and pathways

Nitrogen-doped biochar (N-BC) has been widely concerned in the field of environmental protection. This study verified the alfalfa-based N-BC pyrolyzed at different temperatures is able to catalyze carbon tetrachloride (CT) dechlorination in sulfide-containing aqueous solutions under normal environmental pH range (6.3 ∼ 8.3) effectively, with Cl^-, trichloromethane (CF), CS₂ and HCO₃^- as predominated products. Higher pyrolysis temperature resulted in larger specific surface area, more pores and better catalytic capacity. The N-BC had a good tolerance to water matrix in catalyzing CT dechlorination by sulfide, while the higher pH value or higher dosage of sulfide or N-BC was beneficial to catalytic CT dechlorination. The generation of CS₂ was the major CT dechlorination pathway, attributing to the SN₂ nucleophilic substitution by newborn C-SS- structure generating from the interaction between pyridine-N and sulfide. Besides, generation of CF via hydrogenolysis process was the minor CT dechlorination pathway, owing to the enhanced electron transfer by pyrrole-N, graphitic-N and quinones on surface of N-BC. It was the first time that N-BC was found to be effective in catalyzing the hydrogenolysis process of CT dechlorination. This study emphasized the importance of N-BC in restoring chlorinated hydrocarbons polluted aquatic environment containing sulfide, such as sediments.

Norfloxacin adsorption and subsequent degradation on ball-milling tailored N-doped biochar

N-doping is an effective way to modify biochar for enhancing the adsorption capacity. The synthesis of N-doped biochar by the ball-milling method has been attractive due to its facile and eco-friendly approach with low energy consumption. However, the commonly used N-precursor NH₃·H₂O is environmentally harmful. It is needed to prepare safe and non-toxic N-doped biochar for large-scale production. Here, a urea N-doped biochar (U-MBC) was prepared by the ball-milling method and used for norfloxacin (NOR) removal. The results showed that U-MBC exhibited almost 4-fold higher adsorption capacity for NOR than pristine biochar in a wide pH range (3-9). The adsorption enhancement was owing to the enhancement of H-bonds, π-π electron donor-acceptor, and pore-filling interactions due to the N-doping and ball-milling method. Additionally, 89% of adsorbed NOR can be further removed after 6 h milling. The regenerated U-MBC still had a good adsorption capacity (46.27 mg g^-1) and performed well in three cycles. The knowledge gained from this study could encourage researchers to use urea or similar safe N-precursors with the ball-milling method for the large-scale production of N-doped biochar to remove antibiotic organic pollutants in the environment.

Investigations on the pyrolysis of microalgal-bacterial granular sludge: Products, kinetics, and potential mechanisms

This study investigated the pyrolysis of microalgal-bacterial granular sludge for producing bio-oil and biochar. Results showed that the bio-oil productivity of pyrolyzed MBGS reached 39.5-45.4 wt%, while 23.8-41.2% for the nitrogen-containing bio-oil at the temperature of 673-1073 K. Meanwhile the biochar with a nitrogen content of 3.7-7.0 wt% could also be produced. Moreover, the Van-Krevelen diagram revealed that produced bio-oil had a H/C ratio higher than that from agroforestry biomass, but its O/C ratio was found to be similar to those of coal and biochar. It further appeared from a mass conservation analysis that the highest bio-oil production yield was achieved at a pyrolysis temperature of 773 K, while the pyrolytic kinetics of MBGS in the temperature range studied was governed by the 3-D diffusion mechanism with the activation energy of 224.96 kJ·mol^-1.

From food waste and its digestate to nitrogen self-doped char and methane-rich syngas: Evolution of pyrolysis products during autogenic pressure carbonization

Thermal conversion of solid digestate following anaerobic digestion (AD) can be a solution for producing value-added products and closing the material loop. The products of the novel autogenic pressure carbonization (APC) of food waste and two digestates were characterized to evaluate the temperature effect, product distribution and physicochemical composition. As the temperature increased from 300 to 700 ℃, char yields slightly decreased from 60.0% to 53.3% for the solid digestate from high-solid AD (SDH) while those of the solid digestate from low-solid AD (SDL) varied from 78.7% to 73.2%. X-ray photoelectron spectroscopy (XPS) results indicated that pyridinic N accounts for 40.0% of total N in char of SDL at 700 ℃ that can be upgraded to functional N-doped carbon materials. Maximum yield (39.8%) of syngas was obtained at 700 ℃ with SDH as a feedstock. After APC at 700 ℃, syngas, mainly consisted of CH₄ and CO₂, had heating values ranging from 22.4 to 24.6MJm^－3, which can be jointly utilized with biogas from AD as fuel. Overall, results from this study demonstrate that APC could be used as a potential thermal conversion process for producing value-added products (N-doped biochar) and biofuel (syngas).

Nitrogen-doped lignin-derived biochar with enriched loading of CeO2 nanoparticles for highly efficient and rapid phosphate capture

Development of lignin-derived carbon adsorbents with ultrahigh phosphate adsorption activity and rapid adsorption kinetics is of great importance, yet limited success has been achieved. Herein, we develop a CeO₂ functionalized N-doped lignin-derived biochar (Ce@NLC) via a cooperative modification strategy for effective and fast phosphate capture. The novel modification strategy not only contributes greatly to the loading of well-dispersed CeO₂ nanoparticles with a smaller size, but also significantly increases the relative concentration of Ce(III) species on Ce@NLC. Consequently, an enhanced capture capacity for phosphate (196.85 mg g^-1) as well as extremely rapid adsorption kinetics were achieved in a wide operating pH range (2-10). Interestingly, Ce@NLC exhibited a strong phosphate adsorption activity at even low-concentration phosphorus-containing water. The removal efficiency and final P concentration reached 99.87% and 2.59 μg P L^-1 within 1 min at the phosphate concentration of 2 mg P L^-1. Experiments and characterization indicated that Ce(III) species plays a predominant role for the phosphate capture, and ligand exchange, together with electrostatic attraction, are the main adsorption mechanism. This work develops not only an efficient carbon-based adsorbent for phosphate capture, but also promotes the high-value application of industrial lignin.

Structural analysis and heavy metal adsorption of N-doped biochar from hydrothermal carbonization of Camellia sinensis waste

N-doped biochar as adsorption material for heavy metal removal has attracted increasing concern in environmental application due to its unique features. Here, N-doped biochar was prepared by hydrothermal carbonization of Camellia sinensis branch waste using KOH/NH₄Cl at 120-280 °C for 2 h under 0.4-6.5 MPa, followed by structural analysis. The results showed that the highest N content determined by elemental analysis could reach up to 6.18% in biochar, and the major N species were involved in graphitic N, pyrrolic N, and pyridinic N. Interestingly, these N-doped biochar exhibited the effective adsorption ability of Cu²⁺, Pb²⁺, Zn²⁺, and Cr⁶⁺. The batch adsorption behavior had a better adjustment to the pseudo-second-order kinetic and the Langmuir adsorption isotherm models. In brief, the present results are attributed to develop low-cost adsorbent for removing heavy metal ions.

Urea/ZnCl2 in situ hydrothermal carbonization of Camellia sinensis waste to prepare N-doped biochar for heavy metal removal

Environmental benefits of biochar require a simple and effective method for preparation of functional N-doped biochar. In this study, urea/ZnCl₂ was developed to prepare N-doped biochar via in situ hydrothermal carbonization (HTC) of Camellia sinensis waste at 120-280 °C for 2 h under 1.0-9.8 MPa. Physicochemical and structural properties of the N-doped biochar were investigated by Raman spectra, elemental analysis, BET surface area, SEM, TEM, XRD, and XPS. The results showed that the N content in biochar could reach up to 7.79% at 280 °C. Surface chemistry suggested that pyridinic N, pyrollic N, and graphitic N were the major N species on the biochar. Moreover, the N-doped biochar was successfully employed to remove metal ions Cu²⁺, Pb²⁺, Zn²⁺, and Cr⁶⁺. Adsorption data fit closely to the pseudo-second-order kinetic equation and the Langmuir adsorption isotherm model for all metal ions.

Influence of NH3 concentration on biomass nitrogen-enriched pyrolysis

In this study, nitrogen was used to replace oxygen through biomass N-enriched pyrolysis in a fixed-bed reactor to obtain N-containing chemicals and N-doped biochar. Influence of NH₃ concentration on the formation mechanism of N-species and electrochemical performance of N-doped biochar was investigated in depth. Results showed that increasing NH₃ concentration promoted bio-oil and gas generation, and increased H₂, CH₄ and CO yield at the diminishing of CO₂. Simultaneously, bio-oil showed lower oxygen content with non-methoxy phenols and N-heterocyclics as the main components, and the maximums were 57.73% and 16.21% at 80 vol% NH₃ concentration, respectively. With regard to solid N-doped biochar, nitrogen content (4.85 wt%), N-containing groups and specific surface area (369.59 m²/g) increased greatly, and excellent electrochemical property (120 F/g) was shown with NH₃ concentration increasing. However, NH₃ conversion efficiency decreased gradually with NH₃ increasing, and 40 vol% may be the optimum NH₃ concentration for biomass N-enriched pyrolysis.

One-step synthesis of a novel N-doped microporous biochar derived from crop straws with high dye adsorption capacity

N-doping is one of the most promising strategies to improve the adsorption capacity and selectivity of carbon adsorbents. Herein, synthesis, characterization and dye adsorption of a novel N-doped microporous biochar derived from direct annealing of crop straws under NH3 is presented. The resultant products exhibit high microporosity (71.5%), atomic percentage of nitrogen (8.81%), and adsorption capacity to dyes, which is about 15-20 times higher than that of original biochar. Specifically, for the sample NBC800-3 pyrolyzed at 800°C in NH3 for 3 h, its adsorption for acid orange 7 (AO7, anionic) and methyl blue (MB, cationic) is up to 292 mg g(-1) and 436 mg g(-1), respectively, which is among the highest ever reported for carbonaceous adsorbents. The influences of N-doping and porous structure on dye adsorption of the synthesized carbons are also discussed, where electrostatic attraction, π-π electron donor-accepter interaction, and Lewis acid-base interaction mainly contribute to AO7 adsorption, and surface area (especially pore-filling) dominates MB adsorption. The N-doped biochar can be effectively regenerated and reused through direct combustion and desorption approaches.