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Graphical abstract Highlights • A facile approach to prepare N-doped carbon materials was developed. • N-doped biochar was synthesized by simply ball milling pristine biochar with NH 3 ·H 2 O. • NH 2 and C N were loaded on biochar through the dehydration of COOH and OH. • N-doped biochar showed enhanced sorption for CO 2 and reactive red in comparison to N-free biochar. Abstract Doping of nitrogen (N) on carbon materials often requires complicated synthetic steps or specific machinery. In this study, N-doped biochar was successfully synthesized by simply ball milling pristine biochar with ammonium hydroxide. The content and species of N in the resulting N-doped biochars were carefully characterized, the formation mechanisms of N-related groups were illustrated, and their applications in CO 2 uptake and reactive red removal were evaluated. Most of N introduced to biochar was loaded on its surface in forms of NH 2 and C N, resulting from the dehydration of COOH and OH. Biochars produced at 450 °C were doped with more N on the surface (XPS, 2.41%–2.65%) than those produced at 600 °C (XPS, 1.18%–1.82%) because the content of O-containing functional groups in biochar decreases with increasing pyrolysis temperature. The basic properties of N-related groups enhanced the sorption performance of biochar to both acidic CO 2 and anionic reactive red. For example, the CO 2 uptake of the N-doped, 450 °C hickory biochar was 31.6%–55.2% higher than the corresponding pristine and ball-milled biochars. The maximum sorption capacity of the N-doped, 600 °C bagasse biochar to reactive red was about 3.66–16.2 times of the corresponding biochars without N doping. This paper provides an alternative and facile approach to prepare N-doped biochar that can be extended to other carbon materials to meet the specific needs in different applications, especially adsorption. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Engineered biochar showed strong batch adsorption ability to P in real secondary treated wastewater. • Engineered biochar in fixed-bed columns also effectively removed P from the wastewater. • Commonly used fixed-bed dynamic adsorption models describe the P breakthrough curves well. • Exhaust P-laden biochar promoted mung bean seed germination and seedling growth. Abstract Phosphorus (P), a vital growth-limiting nutrient, is often lost in wastewater discharge, which may not only deteriorate water quality but also accelerate P depletion. In this study, laboratory experiments were conducted to investigate the reclamation and reuse of P from secondary treated wastewater (STWW) by engineered biochar loaded with aluminum oxyhydroxides (AlOOH). Biochar loaded with colloidal and nanosized AlOOH crystalline flakes was successfully produced through direct pyrolysis of hickory wood chips pretreated with aluminum salt. STWW samples were collected from a local wastewater treatment plant to evaluate the P sorption ability of the engineered biochar. The real wastewater was used in batch sorption experiments and the results showed that the engineered biochar effectively removed P from the STWW with relatively fast kinetics (<8 h) and good capacity (8,346 mg-P/kg), mainly through the electrostatic attraction mechanism. When the engineered biochar was packed in fixed-bed columns, it also effective removed P from the real STWW. The P removal efficiencies of the fixed-bed columns were affected by biochar dosage and flow rate. Various fixed-bed filtration models were applied to simulate the column experimental data and the model results matched the experimental breakthrough curves of P well. After the use, the engineered biochar was laden with P and thus significantly stimulated seed (Vigna radiata R. Wilczek) germination and early seedling growth. Findings from this study demonstrate that engineered biochar has promising applications in wastewater treatment because it can not only reduce P level in discharge to benefit the environment but also reclaim it as an agricultural fertilizer to improve P biogeochemical cycling and sustainability. [ABSTRACT FROM AUTHOR]

The synergistic adsorption and photocatalysis is of importance to pollutant degradation, and thus the design of both adsorption and photocatalytic sites on one site of the catalyst is worthy of investigation. In the work, a bamboo biochar with rich porous structure was synthesized to support Ag 3 PO 4 photocatalyst (AP/PBB) through a ball mill-pyrolysis-precipitation method. Owing to the large surface area of PBB, most of AP nanoparticles were well-dispersed and Ag0 species were in-situ generated in the preparation, which was beneficial for absorbing more visible light. Furthermore, a stronger interaction, i.e., built-in electric field between AP and PBB was observed, and thus the photo-generated electrons transfer quickly from AP to PBB via the Ag0 bridge. Furthermore, the transferred electrons reacted with the surface adsorbed oxygen species on PBB to produce ·OH radicals, and then oxidized the methylene blue (MB) molecules on PBB. Therefore, both adsorption and photocatalytic sites on one site, i.e., "two-in-one" of the AP/PBB photocatalyst exhibited higher MB degradation efficiency (95.6%), compared with AP/common biochar. This work affords a novel strategy for biochar-based photocatalysts for effective contaminant removal. [Display omitted] • The porous BC combined with Ag 3 PO 4 presented the best removal efficiency of MB. • The porous BC is helpful for Ag 3 PO 4 dispersion and built-in electronic field generation. • Adsorbed oxygen species on porous BC increase·OH radicals, and then oxidize MB. • Both adsorbed and photocatalytic sites on porous BC promote MB removal. [ABSTRACT FROM AUTHOR]

[Display omitted] • H 2 O 2 -modified ball-milled biochar were synthesized to adsorb methylene blue (MB). • H 2 O 2 modification increased the oxygen-containing functional groups of biochar. • H 2 O 2 modification improved the adsorption capacity of biochar to MB. In this work, hickory chip biochars developed at distinctive pyrolysis temperatures were ball milled (BMHC) and then post-modified with a 10% hydrogen peroxide (H 2 O 2) solution to obtain a set of novel sorbents (BMHC-H 2 O 2). The specific surface area (SSA) was dramatically increased after ball-milling while the hydroxyl and carboxyl groups on the surface of the biochars were further increased through H 2 O 2 modification. Additionally, thermal stability of the biochar treated with ball-milling was not greatly reduced by H 2 O 2 modification and hydrodynamic radius was decreased. Ball milling enhanced the adsorption efficiency to methylene blue (MB) by the biochar, and this ability was further increased by H 2 O 2 modification, because of the increasing in oxygen-containing functional groups (OCFG) to interact with MB. The rate of MB adsorption to BMHC-H 2 O 2 was faster than that of BMHC, reaching equilibrium after about 6 h. Among adsorbents tested, the 450 °C BMHC-H 2 O 2 had the greatest MB adsorption capacity (310 mg g−1). [ABSTRACT FROM AUTHOR]

A facile ball-milling method was developed to synthesize MgO/biochar nanocomposites as a dual-functional adsorbent. The physicochemical properties of the synthesized nanocomposites indicated that the composites achieved nano-scaled morphologies and mesoporous structure with MgO nanoparticles, which is approximate 20 nm and dispersed uniformly on the surface of the biochar matrix. Batch sorption experiments yielded 62.9% removal of phosphate, an anion, and 87.5% removal of methylene blue, a cationic organic dye, at low adsorbent dosages of 1.0 g L−1 and 0.2 g L−1, respectively. This work indicates that ball milling, as a facile and promising method for synthesis of carbon-metal oxide nanocomposites, lends the advantage of operational flexibility and chemical adjustability for targeted remediation of diverse environmental pollutants. Image 1 • Solvent-free synthesis of MgO/biochar nanocomposites was achieved through ball milling. • MgO nanoparticles (20 nm) dispersed uniformly on the surface of the biochar matrix. • MgO/biochar nanocomposites showed dual functions to effectively adsorb cationic dye and anionic phosphate. • Ball milling method has the advantage of operational flexibility and chemical adjustability. [ABSTRACT FROM AUTHOR]

Graphic abstract Highlights • Governing adsorption mechanisms of heavy metals onto carbon adsorbents are summarized. • Various methods for introduction of oxygen, nitrogen, and sulfur functional groups onto carbon surfaces are categorized. • Effects of various functional groups on heavy metal adsorption onto different carbon materials are discussed. • Perspectives of future work on functionalized carbon adsorbents for heavy metal removal is also presented. Abstract Carbon-based adsorbents such as graphene and its derivatives, carbon nanotubes, activated carbon, and biochar are often used to remove heavy metals from aqueous solutions. One of the important aspects of effective carbon adsorbents for heavy metals is their tunable surface functional groups. To promote the applications of functionalized carbon adsorbents in heavy metal removal, a systematic documentation of their syntheses and interactions with metals in aqueous solution is crucial. This work provides a comprehensive review of recent research on various carbon adsorbents in terms of their surface functional groups and the associated removal behaviors and performances to heavy metals in aqueous solutions. The governing removal mechanisms of carbon adsorbents to aqueous heavy metals are first outlined with a special focus on the roles of surface functional groups. It then summarizes and categorizes various synthesis methods that are commonly used to introduce heteroatoms, primarily oxygen, nitrogen, and sulfur, onto carbon surfaces for enhanced surface functionalities and sorptive properties to heavy metals in aqueous solutions. After that, the effects of various functional groups on adsorption behaviors of heavy metals onto the functionalized carbon adsorbents are elucidated. A perspective of future work on functional carbon adsorbents for heavy metal removal as well as other potential applications is also presented at the end. [ABSTRACT FROM AUTHOR]

Drought conditions and nutrients loss have serious impacts on soil quality as well as crop yields in agroecosystems. New techniques are needed to carry out effective soil water and nutrient conservation and fertilizer application tools. Here, calcium alginate (CA) beads impregnated with ball-milled biochar (BMB) were investigated as a new type of water/nutrients retention agent. Both CA and Ca-alginate/ball milled biochar composite (CA-BMB) beads showed high kinetic swelling ratios in KNO 3 solution and low kinetic swelling ratios in water, indicating that CA-BMB beads have the potential to retain mineral nitrogen and nutrients by ion exchange. Pseudo-second-order kinetic model well-described the swelling kinetics of both beads in KNO 3 solution. Over a range of temperatures, the characteristics of dehydration suggested that impregnation with BMB improved the water holding capacity and postponed the dehydration time of Ca-alginate. The cumulative swelling and release characteristics of water, K + , and NO 3 − indicated that CA-BMB beads have great potential as a soil amendment to improve its nutrient retention and water holding capacity. [ABSTRACT FROM AUTHOR]

Sorption is one of the most efficient and low cost strategies for volatile organic compound (VOC) removal, but VOC sorption by biochar has seen limited research. In this work, gas phase sorption experiments were conducted to determine the sorption potential and mechanisms of VOCs onto biochar. A total of 15 biochars produced from 5 common feedstocks at 300, 450, and 600 °C were evaluated as sorbents. Three common VOCs (acetone, cyclohexane, and toluene) were chosen as sorbates. The results showed that all the tested biochars had VOC sorption capacity in the range of 5.58–91.2 mg g −1 . The sorption capacities were mainly influenced by both the surface area of biochar and its noncarbonized organic matter content. The vapor sorption process was exothermic, and the removal of VOCs by the biochars decreased with increasing feedstock temperature. Both the physical adsorption and partition mechanisms played important roles in controlling the VOC removal by the biochars. Biochar maintained its VOC removal ability after five consecutive sorption-desorption cycles, which indicated good reusability. Findings of this work suggest that biochar is a promising alternative sorbent for gaseous VOC removal. [ABSTRACT FROM AUTHOR]

In this study, biochar derived from bamboo pretreated with aluminum salt was synthesized for the removal of two sulfonamide antibiotics, sulfamethoxazole (SMX) and sulfapyridine (SPY), from wastewater. Batch sorption experiments showed that Al-modified bamboo biochar (Al-BB-600) removed both sulfonamides effectively with the maximum sorption capacity of 1200–2200 mg/kg. The sorption mechanism was mainly controlled by hydrophobic, π-π, and electrostatic interactions. Fixed bed column experiments with Al-modified biochar packed in different dosages (250, 500 and 1000 mg) and flow rates (1, 2 and 4 mL/min) showed the dosage of 1000 mg and flow rate of 1 mL/min performed the best for the removal of both SMX and SPY from wastewater. Among the breakthrough (BT) models used to evaluate the fixed bed filtration performance of Al-BB-600, the Yan model best described the BT behavior of the two sulfonamides, suggesting that the adsorption process involved multiple rate-liming factors such as mass transfer at the solid surface and diffusion Additionally, the Bed Depth Service Time (BDST) model results indicated that Al-BB-600 can be efficiently used in fixed bed column for the removal of both SMX and SPY in scaled-up continuous wastewater flow operations. Therefore, Al-modified biochar can be considered a reliable sorbent in real-world application for the removal of SMX and SPY from wastewater. [Display omitted] • Fixed bed column packed with Al-modified biochar effectively removed SMX and SPY from wastewater. • SMX and SPY adsorption on Al-modified biochar was controlled by multiple mechanisms. • Yan model best described the breakthrough behaviors of SMX and SPY. • BDST model showed a strong linear relationship between service time and the amount of Al-modified biochar. [ABSTRACT FROM AUTHOR]

In this paper, bismuth oxybromide (BiOBr)/biochar composites were synthesized by a facile ball milling method for synergistic adsorption and photodegradation of Reactive red 120 (RR120). The characterizations show that ball milling changed the degree of crystallization, increased the surface area, and promoted the charge transfer ability of biochar. The 70% BiOBr/BC composite showed the best removal efficiency for RR120 removal with or without light illumination, which proves its enhanced removal ability by adsorption and photodegradation. The biochar is served as a support of BiOBr for preventing its aggregation and a transporter of charges for promoting the separation of photo-induced carriers in composites. BiOBr can release the adsorption sites on the surface of composites by degradation, which facilitated the RR120 removal and regenerated the photocatalyst for reusing. The strong interactions between BiOBr and biochar in composites resulted from ball milling were beneficial for the charge transfer and synergistic removal of adsorption and degradation. Findings of this work indicate that ball milling method is an effective method to prepare highly efficient biochar-based composites for RR120 removal through synergistic adsorption and photodegradation. [Display omitted] • BiOBr/biochar composites were firstly synthesized by ball milling method. • Biochar increased the surface area and promoted charge transfer in composites. • There were strong interactions between BiOBr and biochar after ball milling. • Synergy between adsorption and photodegradation in BiOBr/biochar is elucidated. [ABSTRACT FROM AUTHOR]

There are challenges in developing multifunctional materials that can not only effectively adsorb but also completely eliminate organic contaminants in water. In this work, novel ZnO/biochar nanocomposites were synthesized using a facile ball-milling method. A series of characterization results showed that the ZnO nanoparticles dispersed uniformly on carbon surface within the biochar matrix. Ball milling increased the mesopores and macropores of the nanocomposites by breaking biochar and squeezing ZnO. The addition of appropriate amount of ZnO into biochar enhanced both the adsorption capacity and photocatalytic ability of the nanocomposites for methylene blue (MB) removal. When the initial concentration of MB was 160 mg/g, the nanocomposites exhibited high MB removal efficiency (up to 95.19%) under visible light through the combination of adsorption and photocatalysis. This work provides a feasible synthesis of metal oxide/biochar nanocomposites with excellent adsorption and photocatalysis properties for the treatment of organic dye wastewater. [Display omitted] • ZnO/biochar nanocomposites were fabricated through simple ball milling. • Varied sized ZnO nanoparticles dispersed uniformly in the biochar matrix. • Ball milling increased the mesopores and macropores of nanocomposites. • Adsorption and photocatalytic activity were enhanced with proper ZnO addition. • BZ-X exhibited efficient MB removal by combining adsorption and photocatalysis. [ABSTRACT FROM AUTHOR]

As the first of its kind, a novel biochar/iron oxide composite (BM-Fe-HC) was successfully prepared by simply ball milling iron-laden biochar (Fe-HC). The performance and mechanisms of Cr(VI) removal by BM-Fe-HC were investigated. Ball milling effectively reduced particle size, increased specific surface area, more importantly, enhanced the distribution and increased the exposure of iron oxides on biochar surface. As a result, Cr(VI) removal by BM-Fe-HC showed fast kinetics and large adsorption capacity with the Langmuir maximum capacity of 48.1 mg/g, higher than that of other biochar/iron composites reported in the literature. Acidic pH promoted Cr(VI) removal while competition ions (Cl-, SO 4 2- and PO 4 3-) inhibited Cr(VI) removal by BM-Fe-HC. Comparison of pre- and post-adsorption samples revealed that iron oxides of the BM-Fe-HC played the dominant role in the adsorption and reduction of Cr(VI) during the removal. After adsorption, part of adsorbed Cr(VI) was reduced by Fe(II) and then stabilized by Fe(III) in the form of amorphous Cr x Fe 1−x (OH) 3 on the composite surface. All the results demonstrate that novel ball-milled biochar/iron oxide composites can be used as an effective adsorbent to remove Cr(VI) from water. [Display omitted] • Ball milling iron-laden biochars produced novel biochar/iron oxide composites • Ball milling increased the exposure of iron oxides on biochar surface • Ball-milled biochar/iron oxide composites showed enhanced removal of Cr(VI) • Iron oxides of the composites are the main sites that can adsorb and reduce Cr(VI) [ABSTRACT FROM AUTHOR]

Pristine and ball milled wheat stalk biochars pyrolysed at 300 °C, 450 °C, 600 °C were studied for tetracycline hydrochloride (TCH) adsorption from aqueous solution. Surface characteristics of ball milled biochar (BM-biochar) were significantly enhanced over their pristine counterparts. TCH adsorption occurred largely on external surface and by filling pores of biochars as evidenced by strong positive correlation between adsorption and external specific surface area (SSA), total pore volume, or mesoporous volume. A two-stage intra-particle diffusion model, limited by the TCH diffusion through the boundary liquid layer, well described TCH adsorption. Maximum TCH adsorption occurred at about pH = 6–8. While solution cations including Na+, K+ and Mg2+ subdued TCH adsorption as they competed for adsorption sites, Ca2+ promoted TCH adsorption due to formation of tetracycline-Ca2+ complexes. The best performing BM-biochar was the one pyrolysed at 600 °C with TCH adsorption amount of 84.54 mg/g. Therefore, this BM-biochar has the potential for TCH removal from aqueous solutions. And the research shed light on the management of organic contaminants in real wastewater by BM-biochar. Image 1 • Ball milling enhanced external specific surface area and mesoporous volume of biochar. • Ball milling increased sorption ability of biochars to TCH. • TCH adsorption by BM-biochars was controlled by surface adsorption and pore filling. • TCH adsorption on BM-biochars was affected by pH and cation in solution. [ABSTRACT FROM AUTHOR]

• Biochar/Fe oxyhydroxide composites showed effective CO 2 capture performance via chemical sorption mechanism. • The sorption mechanisms of CO 2 were verified by in situ DRIFT spectroscopy. • The formed (oxy)hydroxycarbonate during CO 2 chemical sorption could be regenerated at 50–125 °C. • Ball milling could speed up CO 2 mineralization rate on the biochar composites. Despite its importance, chemical process has been often overlooked in CO 2 sorption on carbon based oxyhydroxide composites. In this study, pristine and ball-milled biochar/Fe oxyhydroxide composites were fabricated for CO 2 sorption at 25 °C. The composites, particularly the ones with high Fe content, were effective sorbents for CO 2 with the capacities of up to 160 mg g−1. The primary mechanism of CO 2 sorption on biochar composites with low Fe content was physical adsorption. When the Fe content increased, biochar/Fe oxyhydroxide composites showed enhanced CO 2 sorption capacities, but the sorption kinetics became slower. This is because the governing CO 2 sorption mechanism was shifted from physical adsorption to chemical reaction between Fe oxyhydroxides and CO 2. The formed (oxy)hydroxycarbonate could be decomposed at a temperature between 50 and 125 °C. Furthermore, ball milling could speed up CO 2 mineralization rate on the composites, especially for those with high Fe content, to favor the relative significance of chemical sorption. Both physical and chemical CO 2 sorption mechanisms were verified by different characterization methods including in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Findings of this study not only demonstrate the importance of chemical sorption, but also provide new insights on CO 2 capture by low-cost and environmentally benign biochar/Fe oxyhydroxide composites. Besides, the low regeneration temperature of chemically-sorbed CO 2 gives biochar/Fe oxyhydroxide composite a competitive edge over other CO 2 sorbents, which often need a high regeneration temperature or are not regenerable. [ABSTRACT FROM AUTHOR]

A novel biochar composite was fabricated via the pyrolysis of distillers grains treated phosphogypsum for phosphate removal from water. Batch adsorption experiments were performed on the adsorption characteristics of phosphate. Effects of pyrolysis temperature, solution pH, the dosage of adsorbent, ambient temperature on phosphate adsorption were also investigated. The results demonstrated that the optimum initial solution pH for phosphate adsorption was 6.0, and high pyrolysis temperature was favorable for phosphate adsorption. The optimal dosage of biochar was 1.25 g L−1. A pseudo-second-order kinetic model can well explain the adsorption kinetics, indicative of the energetically heterogeneous solid surface of the composite. The maximum phosphate adsorption capacity of the phosphogypsum modified biochar obtained from Langmuir isotherm reached 102.4 mg g−1 which was almost five times that of distillers grains biochar alone (21.5 mg g−1). The mechanism is mainly attributed to electrostatic adsorption, surface precipitation and ligand exchange. The ideal adsorption performance indicated that biochar supported phosphogypsum can be used as high-quality adsorbent for phosphate removal in wastewater treatment. • A novel biochar composite was synthesized for phosphate adsorption. • The maximum phosphate adsorption capacity reached 102.4 mg g−1. • The adsorption mechanism is mainly attributed to electrostatic adsorption, surface precipitation and ligand exchange. • The composite can be used as high-quality adsorbent for phosphate removal. [ABSTRACT FROM AUTHOR]