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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]

The interactive effects of straw-derived biochar and bio-based polymer-coated urea (BPCU) was examined with a pot experiment conducted in 2014 and 2015. Using a split-plot design, the main plot factor was the form of straw use and the sub-plot factor was the type of N fertilizer. The soil inorganic nitrogen (N), organic carbon and lint yield of biochar treatments were significantly higher than for straw treatments. Meanwhile, the BPCU treatments enhanced nitrogen use efficiency (NUE) and yield over urea treatments. Biochar combined with BPCU resulted in the highest lint yield, 14.3-108.2% increasing over the other treatments, with NUE 27.1-63.5% increased. We attributed this superior performance to the interactive effects between BPCU's controlled supply of N according to cotton's N requirements and biochar's functionalities in enhancing soil quality. Thus, the application of biochar and BPCU is a sustainable strategy to improve soil quality and increase cotton yield. [ABSTRACT FROM AUTHOR]

Biochar application as a soil amendment has been proposed as a strategy to improve soil fertility and increase crop yields. However, the effects of successive biochar applications on cotton yields and nutrient distribution in soil are not well documented. A three-year field study was conducted to investigate the effects of successive biochar applications at different rates on cotton yield and on the soil nutrient distribution in the 0–100 cm soil profile. Biochar was applied at 0, 5, 10, and 20 t ha-1 (expressed as Control, BC5, BC10, and BC20, respectively) for each cotton season, with identical doses of chemical fertilizers. Biochar enhanced the cotton lint yield by 8.0–15.8%, 9.3–13.9%, and 9.2–21.9% in 2013, 2014, and 2015, respectively, and high levels of biochar application achieved high cotton yields each year. Leaching of soil nitrate was reduced, while the pH values, soil organic carbon, total nitrogen (N), and available K content of the 0–20 cm soil layer were increased in 2014 and 2015. However, the changes in the soil available P content were less substantial. This study suggests that successive biochar amendments have the potential to enhance cotton productivity and soil fertility while reducing nitrate leaching. [ABSTRACT FROM AUTHOR]

Controlling water pollution by phosphorus (P) and satisfying high demand of P fertilizer in agriculture are two global challenges for sustainable development. This paper presents a novel application of iron modified biochar as an adsorbent to recover P from wastewater and reuse it as P fertilizer. Granular iron biochar (GIB) and ball milled powder iron biochar (PIB) were prepared from pinewood pretreated with iron salt. The biochars were characterized to determine their surface properties. Their effectiveness in P removal from wastewater was evaluated with packed column filters for GIB and continuous flow reactors for PIB. The spent biochar was tested to determine if it is safe for agricultural application as alternative P fertilizer. The results showed that GIB and PIB were highly porous, had high specific surface area (385 and 331 m2 g−1, respectively), and contained high levels of iron (mainly γ-Fe 2 O 3). Both GIB and PIB showed excellent performance for P removal from wastewater. The P adsorption capacity of GIB in the column filter was 16 times larger than that of sand. A fast P adsorption kinetic rate (0.144 min−1) was observed for PIB in the flow reactor. The spent biochars showed no negative effects on bean germination or even some positive effects on seedling growth, indicating they can be safely used as P fertilizer. This study provides the technical basis of a sustainable wastewater treatment strategy that can capture the full values of water, P, and biochar. [Display omitted] • Granular iron biochar (GIB) and ball milled power iron biochar (PIB) were prepared. • GIB and PIB are high in specific surface area and rich in iron oxides. • GIB and PIB remove P efficiently from wastewater in column filters and flow reactors. • Spent biochars showed no negative effects on bean germination and seedling growth. [ABSTRACT FROM AUTHOR]

Mg–Fe layered double hydroxide (MgFe-LDH) particles were incorporated into wheat-straw biochar by liquid-phase deposition. The obtained biochar/MgFe-LDH composite was used to remove nitrate from aqueous solutions. X-ray diffraction patterns demonstrated that LDH particles were successfully deposited onto the carbon surface within the biochar matrix. Adsorption kinetic and isotherm studies and the effects of co-existing anions and adsorbents dosages were investigated using laboratory batch sorption experiments. The biochar/MgFe-LDH composite showed a strong sorption ability to nitrate in aqueous solutions with the Langmuir maximum adsorption capacity of 24.8 mg/g. A high selectivity for nitrate of the biochar/MgFe-LDH composite was also observed irrespective of the co-existence of sulfate and phosphate. The biochar/MgFe-LDH composite can thus be potentially used as an alternative adsorbent for the treatment of nitrate-contaminated waters as well as a soil amendment for the reduction of soil nitrate leaching. [ABSTRACT FROM AUTHOR]

This study determined the interactive effects of biochar and lead toxicity on the soil microbial community in a phytoextraction experiment. Arranged with a completely randomized design in a greenhouse, banana liners were planted singly in a sandy soil spiked with Pb(NO 3) 2 at 0, 400 and 1200 mg kg−1 and amended with bamboo biochar (pyrolyzing at 600 °C) at 0, 1, 3%. Soil samples were taken from triplicated pots five months after planting and measured for (i) content of lead and organic carbon; (ii) lead speciation; and (iii) microbial community composition through 16S rRNA gene sequencing. DNA sequencing results showed that lead and biochar treatments had significant individual and interactive effects on soil microbial dissimilarities from taxonomic levels of phyla to genera. While some specific taxa were lead resistant, biochar addition apparently alleviated lead toxicity and increased their richness (e.g., Alkanibacter, Muciaginibacter , Burkholderiaceae , and Beggiatoaceae). Soil analysis data indicated that biochar not only helped retain more lead in the soil matrix but created a soil environment inducive for transformation of lead into highly insoluble pyromorphite. This study highlights the effectiveness of biochar for lead remediation and the sensitivity of soil microorganisms in sensing changes in soil environment and lead bioavailability. [Display omitted] • Lead had a greater effect on soil microbial communities than biochar • Biochar amendment enhanced Pb sorption and pyromorphite formation • Biochar had interactive effects in alleviating Pb toxicity and increasing richness of Pb resistant taxa [ABSTRACT FROM AUTHOR]

Ball milling was used to fabricate a nanocomposite of 20% hickory biochar (600 °C) and 80% expanded vermiculite (20%-BC/VE). This novel composite adsorbent had much higher removal of As(V) from aqueous solutions than ball-milled biochar and expanded vermiculite. Characterization of these adsorbents showed that the enhanced As(V) adsorption was ascribed to much larger surface area and pore volume (2–6 times), notable changes in crystallinity, activation of cations, and increased functional groups in the nanocomposite compared with the ball-milled products of their pristine counterparts. The As(V) adsorption process by the 20%-BC/VE fitted well with the pseudo-second-order kinetic model (R 2 = 0.990) and Langmuir isotherm model (R 2 = 0.989) with a maximum adsorption capacity of 20.1 mg g−1. The 20%-BC/VE best performed at pH about 6. The adsorption efficiency was not sensitive to the competition of N O 3 − , C l − , S O 4 2 − , as well as the coexistence of humic acid. However, the adsorption capacity for As(V) was significantly reduced by coexisting with P O 4 3 − . The 20%-BC/VE composite can potentially serve as a superior low-cost adsorbent for As(V) removal in real-world applications. Image 1 • Ball milling was used to fabricate a novel biochar/vermiculite nanocomposite. • Ball milling enhanced physiochemical properties and activated cations in the composite. • The composite feature Langmuir maximum As(V) sorption capability of 20.1 mg g−1. • Adsorption of As(V) by the composite was insensitive to humic acid and anions. [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]

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]

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]