As a class of famous carbon materials, biochars (BCs) and their derivative materials with excellent physicochemical properties and diversified functionalities present great potential in wastewater treatment fields. This review focuses on the latest development in reported biochar-based materials as superior adsorbents or catalysts for removing harmful organic contaminants from wastewater. The construction and properties of biochar-based materials are briefly introduced at the beginning. As one of the major factors affecting the properties of BCs, the wide diversity of feedstocks, such as agricultural and forest residues, industrial by-products as well as municipal wastes, endows BCs different chemical compositions and structures. Woody and herbaceous BCs usually have higher carbon contents, larger surface areas and strong aromaticity, which is in favor of the organic contaminant removal. Driven by the desire of more cost-effective materials, several types of biochar-based hybrid materials, such as magnetic BC composites (MBC), nanometal/nanometallic oxides/hydroxide BC composites and layered nanomaterial-coated BCs, as well as physically/chemically activated BCs, have also been developed. With the help of foreign materials, these types of hybrid BCs have excellent capacities to remove a wide range of organic contaminants, including organic dyestuff, phenols and chemical intermediates, as well as pharmaceutically active compounds, from aquatic solutions. Depending on the different types of biochar-based materials, organic contaminants can be removed by different mechanisms, such as physical adsorption, electrostatic interaction, π–π interaction and Fenton process, as well as photocatalytic degradation. In summary, the low cost, tunable surface chemistry and excellent physical–chemical properties of BCs allow it to be a potential material in organic contaminant removal. The combination of BCs with foreign materials endows BCs more functionalities and broader development opportunities. Considering the urgent demand of practical wastewater treatment, we hope more researches will focus on the applications and commercialization of biochar-based materials.

After entering the twenty-first century, biochar has become a focal point of multidisciplinary research because of its special characteristics, broad application, and promising development prospects. Basic and applied research on the application of biochar in the areas of agriculture, environment, and energy have increased dramatically in the face of food security, environmental pollution, and energy shortage. Although there are some disputes about biochar research, many studies have demonstrated the importance of biochar research from the perspective of scientific advancement and practical application. This paper briefly recalls the history of biochar application; introduces research progress on the basic characteristics of biochar and its associated production technologies; summarizes the research status and existing problems of biochar application in the areas of agriculture, environment, and energy; and analyzes the potential problems and development trends of biochar research in the future.

Biochar produced from straw has been shown to improve soil physicochemical properties. This review introduces the fundamental concepts, the broad applications, and underlying theory of straw biochar returning. Current developments in biochar industry and the production practices prevalent among enterprises in China are critiques. This review analyzes current knowledge gaps, challenges, and opportunities in the industrial application of straw biochar returning. Biochar standards, the quantitative and qualitative analysis methods for biochar, and high-value-added products that are based on biochar are critically examined with goal of providing recommendations for future studies. We propose production and modification of biochar that is application oriented to enhance its fitness for purpose as well as long-term and large-space–scale field study to better understand its impact on soil properties and ecotoxicology. Finally, we make prospects for the future development of SBR, including constructing a standard system about straw biochar returning and promoting self-discipline of biochar industry and the establishment of a biochar-based agricultural production model.

Field studies were conducted over 2 years to determine the response of soil microbial biomass pool to biochar and N fertilizer combinations in a rain-fed rice cropping system. Biochar was applied at four doses: 0 t ha⁻¹, 3 t ha⁻¹, 6 t ha⁻¹ and 12 t ha⁻¹ in combination with N fertilizer at four rates: 0 kg ha⁻¹, 30 kg ha⁻¹, 60 kg ha⁻¹ and 90 kg ha⁻¹ to a Typic Paleustalf Alfisol. Soil samples from two depths (0–10 and 10–20 cm) were collected to determine microbial biomass C (MBC), N (MBN), P (MBP), MBC/N ratio, MBC/P ratio, soil CO₂ flux, microbial qCO2, cultivable bacterial and fungal abundance. Biochar and N fertilizer combination effects on MBC, MBN and MBP pools were dependent on biochar doses, N fertilizer rates and soil depth. MBC/N and MBC/P ratios were decreased after 2 years. Soil CO₂ flux was maximum at post-seeding stage of rice plant, while decreasing trends occurred at active tillering and harvest stage. Increasing doses of biochar irrespective of its combination with N fertilizer rates decreased CO₂ flux and microbial qCO2. Combinations of biochar and N fertilizer increased fungal/bacterial ratio and induced a shift to a more fungal-dominated population after 2 years. Our results suggest that combination of biochar doses (3–12 t ha⁻¹) with N fertilizer rates had stimulatory effects on microbial biomass pools and activity with positive implications for organic carbon accumulation, nitrogen (N) and phosphorus (P) retention in tropical soils.

Biochar is the carbon-rich product obtained from the thermochemical conversion of biomass under oxygen-limited conditions. Biochar has attained extensive attention due to its agronomical and environmental benefits in agro-ecosystems. This work adopts the scientometric analysis method to assess the development trends of biochar research based on the literature data retrieved from the Web of Science over the period of 1998–2018. By analysing the basic characteristics of 6934 publications, we found that the number of publications grew rapidly since 2010. Based on a keyword analysis, it is concluded that scholars have had a fundamental recognition of biochar and preliminarily found that biochar application had agronomic and environmental benefits during the period of 1998–2010. The clustering results of keywords in documents published during 2011–2015 showed that the main research hotspots were “biochar production”, “biochar and global climate change”, “soil quality and plant growth”, “organic pollutants removal”, and “heavy metals immobilization”. While in 2016–2018, beside these five main research hotspots, “biochar and composting” topic had also received greater attention, indicating that biochar utilization in organic solid waste composting is the current research hotspot. Moreover, updated reactors (e.g., microwave reactor, fixed-bed reactor, screw-feeding reactor, bubbling fluidized bed reactor, etc.) or technologies (e.g., solar pyrolysis, Thermo-Catalytic Reforming process, liquefaction technology, etc.) applied for efficient energy production and modified biochar for environmental remediation have been extensively studied recently. The findings may help the new researchers to seize the research frontier in the biochar field.

Application of biochar to soils changes soil physicochemical properties and stimulates the activities of soil microorganisms that influence soil quality and plant performance. Studying the response of soil microbial communities to biochar amendments is important for better understanding interactions of biochar with soil, as well as plants. However, the effect of biochar on soil microorganisms has received less attention than its influences on soil physicochemical properties. In this review, the following key questions are discussed: (i) how does biochar affect soil microbial activities, in particular soil carbon (C) mineralization, nutrient cycling, and enzyme activities? (ii) how do microorganisms respond to biochar amendment in contaminated soils? and (iii) what is the role of biochar as a growth promoter for soil microorganisms? Many studies have demonstrated that biochar-soil application enhances the soil microbial biomass with substantial changes in microbial community composition. Biochar amendment changes microbial habitats, directly or indirectly affects microbial metabolic activities, and modifies the soil microbial community in terms of their diversity and abundance. However, chemical properties of biochar, (especially pH and nutrient content), and physical properties such as pore size, pore volume, and specific surface area play significant roles in determining the efficacy of biochar on microbial performance as biochar provides suitable habitats for microorganisms. The mode of action of biochar leading to stimulation of microbial activities is complex and is influenced by the nature of biochar as well as soil conditions.

Biochars have the potential to reclaim mine-impacted soils; however, their variable physico-chemical properties incite speculation about their successful remediation performance. This investigation examined the capability of biochars produced from three different feedstocks along with a compost blend to improve switchgrass growth conditions in a mine-impacted soil by examining influences on soil pH, grass metal contents, and soil-extractable metal concentrations. Cadmium (Cd)- and zinc (Zn)-contaminated mine soil was collected from a site near Webb City, Missouri, USA—a location within the Tri-State Mining District. In a full factorial design, soil was treated with a 0%, 2.5%, and 5% (w/w) compost mixture (wood chips + beef cattle manure), and 0%, 2.5% and 5% of each biochar pyrolyzed from beef cattle manure, poultry litter, and lodgepole pine feedstocks. Switchgrass (Panicum virgatum, ‘Cave-In-Rock’ variety) was grown in a greenhouse for 50 days and the mass of shoots (above-ground biomass) and roots was assessed, while soil pH, deionized H₂O- and 0.01 M CaCl₂-extractable Cd and Zn concentrations were measured. Poultry litter biochar and compost had the greatest ability to raise soil pH (from 4.40 to 6.61), beef cattle manure biochar and compost moderately raised pH (from 4.4 to 5.92), and lodgepole pine biochar and compost weakly raised pH (from 4.40 to 5.05). Soils treated with beef cattle manure biochar, poultry litter biochar significantly reduced deionized H₂O- and 0.01 M CaCl₂-extractable Cd and Zn concentrations, while lodgepole pine biochar-treated soils showed mixed results. Switchgrass shoot and root masses were greatest in soil treated with compost in combination with either beef cattle manure biochar or poultry litter biochar. Soils treated with 5% beef cattle manure biochar + 5% compost had greater reductions in total Cd and Zn concentrations measured in switchgrass shoots and roots compared to the other two treatments. The three biochars and compost mixtures applied to heavy metal, mine-impacted soil had considerable performance dissimilarities for improving switchgrass productivity. Switchgrass growth was noticeably improved after treatment with the compost in combination with biochar from beef cattle manure or poultry litter. This may be explained by the increased soil pH that promoted Zn and Cd precipitation and organic functional groups that reduced soil-available heavy metal concentrations. Our results imply that creating designer biochars is an important management component in developing successful mine-site phytostabilization programs.

Soil β-glucosidase (BG), the rate-limiting enzyme in the final step of cellulose hydrolysis, plays a key role in microbial metabolism, carbon (C) cycling and sequestration in terrestrial ecosystems. Biochar application is known to affect soil BG activity; however, most of the biochar studies have focused on the potential activity of BG, and it is not clear how biochar influences the kinetic and thermodynamic behavior of BG in the soil. The objective of this study was to investigate the effect of maize residue biochar on soil BG kinetic and thermodynamic parameters. Soil BG kinetic (V_max and K_m) and thermodynamic (E_a, ΔH_a and Q₁₀) parameters were determined within soils (clayey and sandy loam soils) amended with either maize residue (as positive control) or its biochar (600 °C) at 0.5 and 1.0% ratios (w/w), and the mixtures were incubated for 90 days. BG showed an increase in potential enzymatic activity (81%), enzyme concentration (higher V_max value) (25%) and substrate affinity (lower K_m value) (32%) in the biochar-amended sandy loam soil only at high addition rates compared with the control, and an increase by about 86% of the catalytic efficiency (V_max/K_m). In the clayey soil, biochar addition decreased potential BG activity (by 10–29%), increased the V_max value (by 20–25%) and had no impact on enzyme–substrate binding affinity, but still increased the catalytic efficiency by 47–72%. Adsorption of soil BG by biochar particles did not affect the catalytic efficiency in the soil. Generally, application of maize residue biochar to the soil decreased the E_a, ΔH_a and Q₁₀ values of BG compared with the negative controls at both biochar rates in the light-textured soil and only at low biochar rate in heavy-textured soil. The direction and magnitude of BG responses (activity, kinetics, and thermodynamics) to biochar were more related to the soil characteristics. Biochar would increase soil BG thermal stability and decrease its sensitivity to increasing temperature and global warming.

Converting waste biomass into value-added biochar has been considered as a green and sustainable strategy for resource management and pollution control. In this study, graphitic carbon nitride (g-C₃N₄) modified biochars (BCs) were produced through one-pot pyrolysis of urea and hickory chips in differential ratios at 520 °C. The resulting BC/g-C₃N₄ composites were evaluated in laboratory for their physicochemical, adsorptive, and photocatalytic properties. The characterization tests showed the successful synthesis of the BC/g-C₃N₄ composites that introduced g-C₃N₄ structure, N-containing surface functional groups, reduced surface area, and better thermal stability to the biochar. After modification, the BC/g-C₃N₄ composites showed better adsorption ability to reactive red 120 (RR120) than the pristine BC, due to the strong electrostatic attrition between N-containing functional groups of g-C₃N₄ on biochar surface and anionic RR120. The BC/g-C₃N₄ composites also inherited g-C₃N₄’s photocatalytic activity, which is visible light responsive to generate free radicals for RR120 degradation. In addition, the composites with higher urea modification ratios were more effective in the degradation of RR120. Overall, this study demonstrates the feasibility and promising potential of combining biochar and photocatalyst for the removal of aqueous dye. Because of the synergistic adsorption and photodegradation ability, BC/g-C₃N₄ composites present a novel and cost-effective solution for the removal of aqueous dye and other photodegradable contaminants under natural conditions.

Agriculture under changing climate scenario is facing major challenges of water scarcity and resource imbalances. Crop water productivity (WP) may act as an indicator of crop responses to water limitation. Organic amendments such as biochar and manure application to soil are suggested for improving soil quality and reducing water requirements from agricultural sector. However, studies exploring the impact of biochar as sole or in combination with organic and/or chemical fertilizers on WP in dry tropical agro-ecosystems are limited. In this study, we observed the effect of rice-husk ash (RHA, biochar) along with farm-yard manure (FYM) and chemical fertilizers (CF) under varying water conditions on soil hydro-physical properties, yield and WP of wheat crop. Water-filled pore space (WFPS), grain and straw yield, irrigation and total water productivity varied significantly (at P < 0.001) at treatment level. Grain and straw yield were found higher under sole and combined CF applied treatments. Sole and combined RHA and FYM amendment improved water holding capacity (WHC) and WFPS, whereas a decrease in crop yield was observed as compared to the control. Irrigation and total water productivity were found higher under combined RHA + FYM and sole CF treatments with reduced water supply (except sole CF) as compared to control and sole RHA treatments with full water irrigation. Crop water productivity was found positively correlated with grain and straw yields, however, significant correlations were not observed with WHC and WFPS. Results indicate that increasing soil hydro-physical properties in silty-loam soil may hinder crop yield and WP under sole biochar applied soils. Overall, the implications of the study would help in devising agro-management practices based on combined application of RHA and FYM with reduced chemical fertilizer and water inputs to mitigate the impacts of climate change without compromising crop yield in the highly vulnerable dry tropical agro-ecosystem of India. Moreover, long-term studies are needed in these ecosystems to identify the appropriate agricultural package for mitigating the forthcoming water scarcity conditions.

In paddy fields, the opposing transformation of arsenic (As) and cadmium (Cd) poses many challenges for their simultaneous remediation. In our previous study, we reported that combined biochar and zero-valent iron (ZVI) amendment had great potential for the simultaneous alleviation of As and Cd bioavailability in contaminated acid paddy soil. In this study, an As- and Cd-contaminated alkaline paddy soil was further studied, and the same ZVI–biochar mixtures amendments were applied to evaluate the impact of the mixtures on As and Cd transformation and translocation in the soil–rice system by performing pot experiments with rice. In line with our previous study, the ZVI–biochar composites significantly reduced As and Cd accumulation in different rice tissues, leading to a 42% and 47% decrease in rice grain As and Cd levels, respectively, compared with the control values. The ZVI–biochar mixtures exhibited synergistic effects of biochar and ZVI by enhancing the transformation of bioavailable As and Cd fractions into less bioavailable fractions, and by increasing iron plaque formation to reduce As and Cd bioavailability. Although the bioaccumulation and translocation factors of As and Cd in alkaline paddy soil were generally lower than those in acid paddy soil, particularly in the presence of the ZVI–biochar mixtures, the grain As and Cd levels did not achieve the desired food safety standard levels, probably related to the high soil As content and the small changes in soil pH. Nevertheless, for treating lightly and moderately contaminated paddy soils, ZVI–biochar mixtures can still be a good choice in the future.

In this study, we report on the extraction, characterization, and potential applications of colloidal biochar derived from pyrolyzed wood—an untapped source of carbonaceous particles. A series of characterizations was performed on biochar colloids to unravel their colloidal properties and surface chemistry through which it was found that they have a net negative charge and are stable between pH 3 and 10. Moreover, our initial toxicity tests showed that biochar colloids themselves are not toxic and they can be used in remediation applications, which led us to investigate (1) their copper sorption, a model inorganic contaminant, in a scenario that biochar colloids are released into the environment and (2) their potential use in organic pollutants adsorption and degradation. Copper sorption studies showed that biochar colloids have a copper sorption capacity as high as 22 mg ${\text{g}}^{-1}$ in sub-ppm copper solutions. This increased the acute 48 h lethal concentration (${\text{LC}}_{50}$) of copper for Daphnia magna by 21 ppb, which is comparable to the previously reported effect by dissolved organic matter. Adsorption and degradation of methylene blue (MB), an often-used proxy for organic contaminants in water, were studied by coupling the biochar colloids to positively charged ${\text{TiO}}_2$ nanoparticles and using it as a photocatalyst. The hybrid MB photodegradation efficiency was $21\mathrm{\%}$ higher than that of ${\text{TiO}}_2$ nanoparticles alone. Enhancement of demethylation is proposed as the main degradation mechanism of MB, as confirmed by liquid chromatography–mass spectroscopy (LC/MS), and the positive impact of biochar colloids is ascribed to their abundant adsorption sites, which may facilitate MB adsorption and its photocatalytic degradation.

Dairy pastures can be a major source of soil nitrous oxide (N₂O) emissions due to the combination of intensive nitrogen (N) fertiliser use and high soil water content, from either rainfall and/or irrigation. Biochar application is a promising approach to lower soil greenhouse gas emissions, particularly under high soil moisture conditions where denitrification is the primary N-transformation pathway. In a replicated field trial, we evaluated the effects of two contrasting biochars derived from poultry litter and from hardwood on soil N₂O emissions, soil ammonium (NH₄⁺) and nitrate (NO₃⁻) status, pasture productivity and herbage nutrient content. A liming treatment to mimic the liming equivalence of the poultry litter biochar was used to separate any effects observed from changes in soil pH. To further separate the effects of biochars on soil N status, N₂O emissions and pasture N uptake, high and low N fertiliser doses (annual application of 672 kg N ha⁻¹, 336 kg N ha⁻¹) were superimposed across all of the treatments. The N fertiliser dose had no significant impact on pasture yield. Application of poultry litter biochar resulted in significant increases in pasture productivity under both high and low N inputs. This was achieved by alleviating soil P, and possibly K nutritional constraints that are typical in Australian Ferralsols. Under the high N fertiliser dose, emissions of N₂O from the treatments and control were not significantly different (p > 0.05) and ranged between 1.14 and 1.78 kg N₂O-N ha⁻¹ across the 11-month study. The low N dose resulted in significantly lower emissions of N₂O of between 0.80 and 0.84 kg N₂O-N ha⁻¹, but biochar had no significant effect on net emissions across the season. The lack of impact of biochar on N₂O emissions was attributed to the relatively dry conditions over the trial period resulting in nitrification being the most likely N-transformation pathway. During brief episodes of high soil moisture, peak emissions from the biochar plots were lower than from the control or lime treatment, but these differences did not impact on the emission budget over the 11-month sampling campaign.

The impacts of biochar addition with nitrogen fertilizer (Urea-N) on greenhouse gas (GHG) fluxes and grain yields are not comprehensively understood. Therefore, we designed a field experiment in an intensive rice–wheat cropping system located in the Taihu Lake region of China and measured CH₄ and N₂O emissions for 2 consecutive years to examine the impacts of biochar combined with N-fertilizer on rice production and GHG flux. Three field experimental treatments were designed: (1) no N-fertilizer application (N0); (2) 270 kg N ha⁻¹ application (N270); and (3) 270 kg N-fertilizer ha⁻¹ plus 25 t ha⁻¹ biochar application (N270 + C). We found that, compared with urea application alone, biochar applied with Urea-N fertilizer increased N use efficiency (NUE) and resulted in more stable growth of rice yield. In addition, biochar addition increased CH₄ emissions by 0.5–37.5% on average during the two consecutive rice-growing seasons, and decreased N₂O–N loss by ~ 16.7%. During the first growing season, biochar addition did not significantly affect the global warming potential (GWPt) or the greenhouse gas intensity (GHGI) of rice production (p > 0.05). By contrast, during the second rice-growing season, biochar application significantly increased GWPt and GHGI by 28.9% and 18.8%, respectively, mainly because of increased CH₄ emissions. Our results suggest that biochar amendment could improve grain yields and NUE, and increased soil GWPt, resulting in a higher potential environmental cost, but that biochar additions enhance exogenous carbon sequestration by the soil, which could offset the increases in GHG emissions.