Immobilized microbial technology has been widely used in wastewater treatment, but it has been used less frequently for soil remediation, particularly in sites that are co-contaminated with organic compounds and heavy metals. In addition, there is limited knowledge on the efficiency of remediation and microbial preferences to colonize the immobilized carriers. In this study, biochar immobilized with Sphingobium abikonense was introduced to remediate soils that were co-contaminated with phenanthrene (PHE) and copper (Cu), and the mechanisms of microbial assemblage were investigated. The immobilized microbial biochar maintained a degradation rate of more than 96% in both the first (0–6 d) and second (6–12 d) contamination periods. The addition of biochar increased the proportion of Cu bound to organic matter, and Fe–Mn oxide bound Cu in the soil. In addition, both Cu and PHE could be adsorbed into biochar pellets in the presence or absence of immobilized S. abikonense. The presence of biochar significantly increased the abundance of bacteria, such as Luteibacter, Bordetella and Dyella, that could degrade organic matter and tolerate heavy metals. Notably, the biochar could specifically select host microbes from the soil for colonization, while the presence of S. abikonense affected this preference. The autonomous selection facilitates the degradation of PHE and/or the immobilization of Cu in the soil. These results provide a green approach to efficiently and sustainably remediate soil co-contaminated with PHE and Cu and highlight the importance of microbial preference colonized in immobilized carriers.Graphical Abstract

In the past few decades, numerous studies have been conducted to promote the use of biochar as a soil amendment and most recently, for compacted geo-engineered soils. In general, the definite trends of biochar effects on water retention and fertility of soils have been confirmed. However, the biochar effects on hydraulic conductivity, particularly unsaturated hydraulic conductivity of soil-biochar mix remain unclear, making it difficult to understand water seepage in both agricultural and geo-engineered infrastructures in semi-arid regions. This study examines the unsaturated hydraulic conductivity function derived based on the measurements of soil water characteristic curves of soil with biochar contents of 0%, 5% and 10%. A new parameter “biochar conductivity factor (BCF)” is proposed to evaluate the inconsistency in reported biochar effects on soil hydraulic conductivity and to interpret it from various mechanisms (inter- and intra- pore space filling, cracking, aggregation, bio-film formation and piping/internal erosion). The impact of biochar content on unsaturated hydraulic conductivity appears to reduce as the soil becomes drier with minimal effect in residual zone. Qualitative comparison of near-saturated hydraulic conductivity with test results in the literature showed that the BCF is generally higher for smaller ratio of sand to fine content (clay and silt). Moreover, the particle size of biochar may have significant influence on soil permeability. Future scope of research has been highlighted with respect to biochar production for its applications in agriculture and geo-environmental engineering. Long term effects such as root decay and growth, aggregation and nutrient supply need to be considered.Graphical Abstract

Root-knot nematodes (RKNs) are obligate endoparasites that feed on their host plants to complete its life cycle, representing a major threat to agriculture and economy worldwide. The development of new management strategies becomes essential as effective chemical nematicides are progressively being restricted. Hence, we analysed grape pomace-derived biochars, pyrolysed at 350 °C (BC350) and 700 °C (BC700), focusing on their potential for RKN control. The thermal treatment of grape pomace caused an increase in the concentration of carbon and plant macro- and micronutrients, which were largely present in a water-soluble form. Synchrotron radiation-based Fourier transform infrared microspectroscopy data showed a general loss of carboxylic functional groups during pyrolysis, partially contributing to the alkalinisation of both biochars, mostly in BC700. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis revealed a highly porous structure filled with different crystals composed of elements such as K, Ca, Mg, P, Si or Al, which could be a suitable environment for the growth of microorganisms. Biochar-derived aqueous extracts showed phytotoxicity to tomato seedlings at high concentrations, and disappeared upon dilution, but no toxic effect was observed on the nematode’s infective stage. However, the infective and reproductive traits of a Meloidogyne javanica population in tomato were significantly reduced (i.e. egg masses and eggs per plant) in washed-biochar-treated soil in pots (0.75%; BC350W). Therefore, the large amount of grape waste generated after wine production can be transformed into a valuable product such as biochar, effective for RKNs control, thus reducing the waste management problem and contributing to a circular economy.Graphical abstract

Hydrogen sulfide (H2S) removal has been a significant concern in various industries. In this study, food waste digestate-derived biochar (DFW-BC), a by-product of food waste treatment with abundant minerals, was assessed for removing H2S from different simulated biogas containing oxygen (O2) and carbon dioxide (CO2) and under different moisture (H2O) contents (0% and 20%) of biochar. The influencing mechanisms of the gas conditions combined with the moisture contents were also investigated. The results showed an H2S removal of 1.75 mg g−1 for dry biochar under pure H2S, 4.29 mg g−1 for dry biochar under H2S + O2, 5.29 mg g−1 for humid biochar under H2S, and 12.50 mg g−1 for humid biochar under H2S + O2. For dry DFW-BC, the high Fe content was responsible for the O2 enhancement. In contrast, O2 + H2O activated the catalytic H2S oxidation of the less reactive minerals (mainly Ca). The inhibition of CO2 on H2S adsorption was not obvious for dry DFW-BC; the specific pore structure may have provided a buffer against the physisorption competition of CO2. However, when H2O was present on DFW-BC, the changes in critical biochar properties and sulfur speciation as opposed to that without H2O implied an evident occurrence of CO2 chemisorption. This CO2 chemisorption partially hindered O2 + H2O enhancement, decreasing the H2S removal capacity from 12.50 to 8.88 mg g−1. The negative effect was ascribed to mineral carbonation of CO2, neutralizing the alkaline surface and immobilizing metal oxides, which thus reduced the acceleration in H2S dissociation and activation in catalytic H2S oxidation by O2 + H2O.Graphical Abstract

Co-contamination of groundwater with trichloroethene (TCE) and arsenic (As) is a widespread problem in industrial sites. The simultaneous biological removal of As and TCE has not yet been developed. This study incorporated biochar into anaerobic dechlorination system to achieve a greatly accelerated dissipation and co-removal of TCE and As. Biochar eliminated microbial lag (6 days) and achieved a 100% TCE removal within 12 days even at a relatively high initial concentration (TCE: 30 mg L−1; As(V): 4 mg L−1), while without biochar, only 75% TCE was removed until day 18. Biochar adsorbed TCE and the intermediate products allowing them to be degraded on its surface gradually, maintaining a high metabolic activity of microbes. Biochar facilitated the preferential colonization of its surfaces by dechlorinating microorganisms (Clostridium and Dehalococcoides) and suppressed hydrogen-competing microorganisms (Desulfovibrio) in water. Biochar itself cannot adsorb As, however, separation of biochar carrying the As-laden microorganisms achieved 50–70% As-removal from groundwater. The biochar-amended incubations were found to be enriched with microbes possessing more crucial As-transforming genes (K00537-arsC and K07755-AS3MT), and upregulated amino acid metabolism, thus enhancing the self-detoxification ability of microorganisms to transform As(V) to As(III) or volatile organic As. This study proposes a strategy of regulating microbes’ metabolic activity by biochar to achieve simultaneous removal of coexisting contaminations, which is an important step prior to examining the feasibility of biochar application for enhanced bioremediation.Graphical Abstract

Biochar with a highly accessible specific surface area can display a higher performance when it is used as the cathode of lithium-ion capacitors. Facing the complex composition and diversity of biomass precursors, there is a lack of a universally applicable method to construct hierarchical porous biochar controllably. In this work, a multi-stage activation strategy combining the feature of different activation methods is proposed for this target. To confirm the porous characteristic in prepared samples, N2 adsorption–desorption and transmission electron microscope were used. As the optimal sample, BC-P3K4S had the highest specific surface area of 3583.3 m2 g−1. Evaluated as the electrode for a lithium-ion capacitor, BC-P3K4S displayed a capacity of 139.1 mAh g−1 at 0.1 A g−1. After coupling it with pre-lithiated hard carbon, the full device exhibited a high energy density of 129.3 W h kg−1 at 153 W kg−1. The work outlined herein offers some insights into the preparation of hierarchical porous biochar from complex biomass by multistep activation method.Graphical Abstract

Biochar from bio-waste pyrolysis presents excellent CO2 sequestration capacity. This study innovated the design of cement-bonded particleboards utilizing a substantial amount of 50–70 wt.% pre-soaked biochar to render the products carbon-negative. We investigated the roles of biochar in magnesium oxysulfate cement (MOSC) system and demonstrated good mechanical and functional properties of biochar cement particleboards. In the presence of biochar, the amounts of hydration products were enriched in the cement systems as illustrated by the thermogravimetric analyses (TGA) and X-ray diffraction (XRD). We further incorporated supplementary cementitious materials (SCMs) and generated 5 Mg(OH)2⋅MgSO4·7H2O (5–1–7) phase in the MOSC system. As a result, our designs of biochar particleboards satisfied the standard requirements for flexural strength (> 5.5 MPa) and thickness swelling (< 2%). Moreover, our biochar particleboards presented a low thermal conductivity as the biochar pores disrupted thermal bridging within particleboards. We illustrated that the high dosage ratio of biochar could significantly offset the CO2 emissions of the particleboards (i.e., carbon-negative) via life cycle assessment. Noticeable economic profits could also be accomplished for the biochar particleboards. For instance, the 50BC-MOSC bonded particleboard (with 50 wt.% pre-soaked biochar as aggregate, 50 wt.% MOSC as binder) with promising mechanical properties could store 137 kg CO2 tonne−1 and yield an overall economic profit of 92 to 116 USD m−3 depending on the carbon prices in different countries. In summary, our new designs of carbon-negative biochar particleboards could curtail carbon emissions in the construction materials and promote the realization of carbon neutrality and circular economy.Graphical Abstract

Hydrothermal carbonization (HTC) technology has increasingly been considered for biomass conversion applications because of its economic and environmental advantages. As an HTC conversion product, hydrochar has been widely used in the agricultural and environmental fields for decades. A CiteSpace-based system analysis was used for conducting a bibliometric study to understand the state of hydrochar environmental application research from 2011 to 2021. Researchers had a basic understanding of hydrochar between 2011 and 2016 when they discovered hydrochar could apply to agricultural and environmental improvement projects. Keyword clustering results of the literature published in 2017–2021 showed that soil quality and plant growth were the major research topics, followed by carbon capture and greenhouse gas emissions, organic pollutant removal, and heavy metal adsorption and its bioavailability. This review also pointed out the challenge and perspective for hydrochar research and application, namely: (1) the environmental effects of hydrochar on soils need to be clarified in terms of the scope and conditions; (2) the influence of soil microorganisms needs to be investigated to illustrate the impact of hydrochar on greenhouse gas emissions; (3) combined heavy metal and organic contaminant sorption experiments for hydrochar need to be conducted for large-scale applications; (4) more research needs to be conducted to reveal the economic benefits of hydrochar and the coupling of hydrochar with anaerobic digestion technology. This review suggested that it would be valuable to create a database that contains detailed information on how hydrochar got from different sources, and different preparation conditions can be applied in the environmental field.Graphical Abstract

Biochar application and conservation tillage are significant for long-term organic carbon (OC) sequestration in soil and enhancing crop yields, however, their effects on native soil organic carbon (native SOC) without biochar carbon sequestration in situ remain largely unknown. Here, an 11-year field experiment was carried out to examine different biochar application rates (0, 30, 60, and 90 Mg ha−1) on native SOC pools (native labile SOC pool I and II, and native recalcitrant SOC) and microbial activities in calcareous soil across an entire winter wheat–maize rotation. The proportions of C3 and C4-derived native SOC mineralization were quantified using soil basal respiration (SBR) combined with 13C natural isotope abundance measurements. The results showed that 39–51% of the biochar remained in the top 30 cm after 11 years. Biochar application rates significantly increased native SOC and native recalcitrant SOC contents but decreased the proportion of native labile SOC [native labile SOC pool I and II, dissolved organic carbon (DOC), and microbial biomass carbon (MBC)]. Biochar application tended to increase the indicators of microbial activities associated with SOC degradation, such as SBR, fluorescein diacetate hydrolysis activity, and metabolic quotient (qCO2). Meanwhile, higher biochar application rates (B60 and B90) significantly increased the C4-derived CO2 proportion of the SBR and enhanced C4-derived native SOC mineralization. The effect of the biochar application rate on the content and proportion of native SOC fractions occurred in the 0–15 cm layer, however, there were no significant differences at 15–30 cm. Soil depth also significantly increased native labile SOC pool I and II contents and decreased qCO2. In conclusion, the biochar application rate significantly increased native SOC accumulation in calcareous soil by enhancing the proportion of native recalcitrant SOC, and biochar application and soil depth collectively influenced the seasonal turnover of native SOC fractions, which has important implications for long-term agricultural soil organic carbon sequestration.Graphical Abstract

To improve the phosphorus (P) recovery efficiency from livestock wastewater, a novel MgO doped mildewed corn biochar with thermal pre-puffing treatment (Mg-PBC) and without pre-puffing (Mg-BC) was synthesized and tested. The thermal-puffing pretreatment improved the effectiveness of metal soaking and MgO dispersion. P recovery time with Mg-PBC (7 h) was significantly shorter than that with Mg-BC (12 h). Moreover, Mg-PBC showed significantly higher P recovery capacity (241 mg g−1) than Mg-BC (96.6 mg g−1). P recovery capacity of the Mg-PBC fitted to the Thomas model was 90.7 mg g−1, which was 4 times higher than that of Mg-BC (22.9 mg g−1) under column test conditions. The mechanisms involved in P recovery included precipitation, surface complexation, and electrostatic interaction. After adsorption, both Mg-BC and Mg-PBC showed relatively low regeneration abilities. The P loaded Mg-BC (Mg-BC-P) and Mg-PBC (Mg-PBC-P), the later particularly, obviously increased the available P content and promoted plant growth. The release of P increased with time in the Mg-PBC-P treated soil, while it decreased with time in the P fertilizer treated soil. A cost–benefit analysis revealed that thermal-puffing pretreatment greatly increased the profit of MgO doped biochar from −0.66 to 5.90 US$ kg−1. These findings highlight that biomass pre-puffing is a feasible treatment to produce MgO modified biochar and to recover P from livestock wastewater, and that the Mg-PBC-P can be used as a slow-release P fertilizer.Graphical Abstract

Long-term consumption of tea with high fluoride (F) content has a potential threat to human health. The application of different amounts of biochar to reduce F accumulation in tea leaves has been little studied. In this study, a pot experiment was conducted to investigate the effect of biochar amounts (0, 0.5%, 2.5%, 5.0%, 8.0%, and 10.0%, w/w) on tea F content during the tea plant growth. Changes in tea quality, soil F fraction, and soil properties caused by biochar and the relationship with tea F accumulation were also considered. The results showed that the application of biochar amendment significantly reduced water-soluble F contents in tea leaves compared to CK (without biochar), especially in the 8.0% treatment (72.55%). Overall, biochar contributed to improving tea polyphenols and caffeine, but had no significant impact on free amino acids and water leachate. Compared with CK, 5.0–10.0% biochar significantly increased soil water-soluble F content due to the substitution of F− with OH− under high pH. Additionally, biochar applied to tea garden soil was effective in decreasing the soil exchangeable aluminum (Ex-Al) content (46.37–91.90%) and increasing the soil exchangeable calcium (Ca2+) content (12.02–129.74%) compared to CK, and correlation analysis showed that this may help reduce F enrichment of tea leaves. In general, the application of 5.0–8.0% biochar can be suggested as an optimal application dose to decrease tea F contents while simultaneously improving tea quality.Graphical Abstract

Biochar is a carbon-containing material prepared through thermal treatment of biomass in limited supply of oxygen, and used for an array of applications including waste management, climate change mitigation, soil fertility improvement, bio-energy production, and contaminant remediation. The data related to biochar, its production, and the wide applicability were collected using Web of Science Core Collection Database (on 25/10/2022), while bibliometric network analysis was performed using VOSviewer software to analyse year-wise, author-wise, country-wise, and journal-wise publication trends, construct keyword co-occurrence maps, and identify research areas receiving greater focus. Further, the applications of biochar were reviewed and mechanistic insights were provided. Some of the findings include: > 50% of documents (> 13,000) getting published in the past 3 years, > 90% of documents (> 21,000) being research articles, ~ 50% of publications (> 10,000) being related to environmental sciences, pyrolysis being the most widely used (~ 40% articles) production technique (followed by carbonization, gasification, combustion, and torrefaction), China being the most active country in terms of publications (> 11,000), and biochar being mostly used for removing contaminants (followed by soil improvement, waste management, energy production, and climate change mitigation). Various strengths, weaknesses, opportunities, and threats (SWOT analysis) of biochar production and wide-ranging applicability were identified. Lastly, gaps were identified including the need for performing elaborate life cycle assessments, exploring machine learning and artificial intelligence for upgrading conversion technology and producing application-specific biochar, and investigating mechanistic aspects of soil-biochar interactions and nano-scale transformation of biochar. The study covers a broad spectrum of biochar applicability to identify areas receiving lesser attention, which could guide the future researchers for augmenting biochar research.Graphical Abstract

This work demonstrated that Enteromorpha biochar with introduced iron (SFB900-3) could activate peroxymonosulfate (PMS) efficiently for NTP remediation. It removed 83.9%–95.1% of NTP in 60 min under a wide pH range from 3.15 to 8.95. Density functional theory (DFT) calculations revealed the synergistic relationship between internal Fe single atoms and introduced Fe compounds—Fe3C. The adsorption capacity of SFB900-3 for persulfate improved from −0.953 eV to −4.214 eV, and the Bader charge analysis showed that Fe atoms as active sites (0.658 e) enhanced the adsorption capacity more than carbon (0.050 e). Moreover, the energy barrier for PMS dissociation reduced from 0.072 eV to −5.372 eV due to the longer length of O–O bond under the synergistic effect of Fe single atom and Fe3C which increased from 1.467 Å to 3.890 Å. The quenching experiment confirmed that 1O2 was the main active substance in NTP degradation and its contribution rate was 88.2%, which was further verified by EPR detection. The effect factor experiments proved that the SFB900-3/PMS system had stable and efficient activity for NTP removal, which remained at 73.6% removal rate after three rounds of tests. This work provided novel guidance for constructing efficient and stable biochar-based materials for organic pollutant remediation.Graphical Abstract

Biochar addition has been widely used in the field to mitigate soil nitrous oxide (N2O) emissions, and can be considered as a potential method to reduce N2O emissions during vermicomposting. However, excessive biochar addition may inhibit earthworms’ activity. Thus, it is crucial to clarify the optimum addition volumes of biochar during vermicomposting. This study evaluated the impact of addition of various amounts of biochar (0, 5, 10, 15, 20 and 25% of total amount of feedstock) on earthworms’ (Eisenia fetida) activity, N2O emission and compost quality during vermicomposting. Compared with the treatment without biochar added, 5% of biochar application significantly increased earthworm total biomass (from 177.5 to 202.2 g pot−1), and cumulative burrowing activity (from 47.0% to 52.2% pixel per terrarium). The increased earthworms activity stimulated the vermicomposting process and led to the best quality of compost, which showed the highest total nutrient content (5.38%) and a significantly higher germination percentage of seeds (88%). Although N2O emissions were slightly increased by 5% biochar addition, a non-significant difference was found between the treatment with 5% biochar and the treatment without biochar added. On the contrary, 20% and 25% biochar addition not only lowered N2O emissions, but also significantly decreased the quality of compost. The results suggest that 5% biochar application is an appropriate amount to improve the quality of compost without significant N2O emissions.Graphical Abstract

The unsatisfactory nutrient slow-release and water-retention performance of traditional biochar-based compound fertilizers (BCF) severely limit their practical application. Herein, a new type of slow-release fertilizer with high water retention was fabricated via the incorporation of hydrotalcite and starch into BCF, named as HS-BCF. The water-retention and nutrient releasing performance of the prepared HS-BCF and related nutrient slow-release mechanism were investigated. The results showed that the incorporation of hydrotalcite and starch into BCF could increase the soil water-retention ratio by 5–10% points. The accumulated N, P, and K leaching amounts of HS-BCF in soil within 30 days were 49.4%, 13.3%, and 87.4% of BCF at most, respectively. Kinetic analysis indicated that the release of nutrients from HS-BCF was attributed to the coupling of the diffusion-controlled and relaxation-controlled mechanism. Moreover, hydrotalcite could bind with P in HS-BCF, contributing to the enhanced durability of P in HS-BCF. Finally, pot experiments showed that the N–P–K utilization efficiencies of HS-BCF were all higher than those of BCF due to a better synchronization between the nutrient release of HS-BCF and the uptake of tomato plants. Overall, the study may provide a promising strategy for simultaneously improving the water-retention and slow-release performance of traditional biochar-based fertilizers.Graphical Abstract

Biochar and biochar-based materials have been studied extensively in multidisciplinary areas because of their outstanding physicochemical properties. In this review article, biochar and biochar-based materials in the removal of environmental pollutants, hydrogen generation and carbon dioxide capture were summarized and compared. The interaction mechanisms were discussed from the experimental results and characterization analysis. The high porous structures, active surface sites, (co)doping of single metals/nonmetals, and incorporation of metal oxides or other materials improved the high activity of biochar-based materials in their applications. However, there are still some challenges such as: (1) the fact that H2 generation with high selectivity or the produced syngas to meet the real application requirement in industrial is the main challenge in H2 production; (2) the fact that the selective capture of CO2 with high stability, high adsorption capacity and recyclability at low-cost should be considered and focused on; (3) the sorption-(photo)degradation of the organic chemicals; and (4) the fact that the sorption-reduction-extraction/solidification of metals/radionuclides are efficient methods for the elimination of environmental pollutants. In the end, the perspectives, challenges and possible techniques for biochar-based materials’ real application in future were described.Graphical Abstract

Microbially induced calcite precipitation (MICP) technique utilizes ureolytic bacteria to decompose urea and generate carbonate ions for metal combination. MICP can remediate heavy metal (e.g., Cd) contaminated soils while maintaining or even improving soil functions, but its efficiency in agricultural soil practical application still needs to be enhanced. Here, we constructed a biochar-bacteria (2B) partnership in which biochar provides high nutrition and diverse sorption sites. Using the 2B system, Cd immobilization effectiveness and the underlying mechanism were examined along with the soil properties and soil functions. Results showed that compared to the single biochar and ureolytic bacteria systems, soil Cd mobility was reduced by 23.6% and 45.8% through co-precipitating with CaCO3 as otavite (CdCO3) in the 2B system, whereas soil fertility, bacterial diversity, and richness increased by 11.7–90.2%, 5.4–16.1%, and 6.8–54.7%, respectively. Moreover, the abundances of Proteobacteria and Firmicutes were enhanced in the 2B system. Notably, Sporosarcina and Bacillus (Firmicutes genus) that carry the ureC gene were boosted in the system, further implicating the microbiological mechanism in reducing Cd migration and its bioavailability in soil. Overall, the constructed 2B system was efficient in soil Cd immobilization by strengthening the ureolytic bacteria growth and their nutrient supply in the bacteria-rich soil ecosystem.

Although research on biochar has received increasing attention for environmental and agricultural applications, the significance of nanobiochar for environmental pollutant remediation is poorly understood. In contrast to bulk biochar, nanobiochar has superior physicochemical properties such as high catalytic activity, unique nanostructure, large specific surface area and high mobility in the soil environment. These unique characteristics make nanobiochar an ideal candidate for pollution remediation. Thus far, the research on nanobiochar is still in its infancy and most of the previous studies have only been conducted for exploring its properties and environmental functions. The lack of in-depth summary of nanobiochar’s research direction makes it a challenge for scientists and researchers globally. Hence in this review, we established some key fabrication methods for nanobiochar with a focus on its performance for the removal of pollutants from the environment. We also provided up-to-date information on nanobiochar’s role in environmental remediation and insights into different mechanisms involved in the pollutant removal. Although, nanobiochar application is increasing, the associated drawbacks to the soil ecosystem have not received enough research attention. Therefore, further research is warranted to evaluate the potential environmental risks of nanobiochar before large scale application.Graphical Abstract

Torrefaction operation is an essential pathway for solid biofuel upgrading, and good hydrophobicity of torrefied biochar is conducive to its storage. Herein, a two-stage treatment of torrefaction followed by modification by hexadecyltrimethoxysilane was adopted to improve the moisture resistance performance of biochar. This two-stage treatment process led to a longer torrefied microalgal biochar preservation time (60–200% improved) and great superhydrophobicity and superlipophilicity. Therefore, the modified microalgal biochar could significantly adsorb leaking oil for environmental remediation and further improve the calorific value of the biochar. The obtained results indicated that the oil adsorption capacity of modified microalgal biochar was correlated to torrefaction temperature and oil species. Specifically, the oil adsorption capacity was enhanced up to 70–80% from the modification process when comparing to raw microalga. Increasing the torrefaction temperature enhanced the adsorption quantity of the modified microalgal biochar. By adsorbing the oil, the calorific value of oilchar, namely, biochar with adsorbed oil, could be higher than 40 MJ kg− 1. Furthermore, the pyrolysis and combustion characteristics suggested that biochar stability gradually rose as the torrefaction temperature increased. By comprehensively analyzing and comparing the fuel performance of the modified microalgal biochar with previous literature, the obtained modified microalgal biochar possessed better fuel properties and environmental sustainability.Graphical Abstract

The nanoscale biochar (N-BC) generated during the production and weathering of bulk biochar has caused significant concerns for its cotransport with contaminants spreading the contamination. In this study, the cotransport behaviors of N-BC with Cd2+ under variable solution chemistry were investigated for the first time, which can pose environmental contamination risks but have received little attention. The column experiment results showed that increasing ionic strength (IS) or decreasing pH retarded the transport of N-BC but promoted the transport of Cd2+ in their individual transport. In cotransport scenarios, Cd2+ facilitated the deposition of N-BC on the quartz sand with increasing IS or decreasing pH by providing additional sorption sites and led to the ripening of N-BC via cation bridging. N-BC retarded the transport of Cd2+ under all conditions. However, lower pH and higher IS could facilitate the release of Cd2+ from the immobile N-BC. The cotransport modeling results demonstrated that the Cd2+ adsorption on and desorption from the immobile N-BC controlled the retention and release of Cd2+ under variable pH and IS, while the influence of mobile N-BC on Cd2+ transport was minor. This study provided new insight for evaluating the potential contamination-spreading risks and suggested that rational use of biochar with great caution is necessary.Graphical Abstract