Enhanced adsorption of aromatic VOCs on hydrophobic porous biochar produced via microwave rapid pyrolysis.

To customize biochar suitable for efficient adsorption of benzene derivatives, this study presents programmed microwave pyrolysis to produce hydrophobic porous biochar with low-dose ferric chloride. Designated control of the ramping rates in the carbonization stage and the temperatures in the activation stage were conducive to enlarging the specific surface area. Iron species, including amorphous iron minerals, could create small-scale hotspots during microwave pyrolysis to promote microporous structure development. Compared with conventional pyrolysis, programmed microwave pyrolysis could increase the specific surface area from 288.6 m2 g-1 to 455.9 m2 g-1 with a short heating time (15 min vs. 2 h) under 650 °C. Engineered biochar exhibited higher adsorption capacity for benzene and toluene (136.6 and 94.6 mg g-1), and lower adsorption capacity for water vapour (6.2 mg g-1). These findings provide an innovative design of engineered biochar for the adsorption of volatile organic compounds in the environment.

4D structural changes and pore network model of biomass during pyrolysis.

Biochar is an engineered carbon-rich substance used for soil improvement, environmental management, and other diverse applications. To date, the understanding of how biomass affects biochar microstructure has been limited due to the complexity of analysis involved in tracing the changes in the physical structure of biomass as it undergoes thermochemical conversion. In this study, we used synchrotron x-ray micro-tomography to visualize changes in the internal structure of biochar from diverse feedstock (miscanthus straw pellets, wheat straw pellets, oilseed rape straw pellets, and rice husk) during pyrolysis by collecting a sequence of 3D scans at 50 °C intervals during progressive heating from 50 °C to 800 °C. The results show a strong dependence of biochar porosity on feedstock as well as pyrolysis temperature, with observed porosity in the range of 7.41-60.56%. Our results show that the porosity, total surface area, pore volume, and equivalent diameter of the largest pore increases with increasing pyrolysis temperature up to about 550 °C. The most dramatic development of pore structure occurred in the temperature range of 350-450 °C. This understanding is pivotal for optimizing biochar's properties for specific applications in soil improvement, environmental management, and beyond. By elucidating the nuanced variations in biochar's physical characteristics across different production temperatures and feedstocks, this research advances the practical application of biochar, offering significant benefits in agricultural, environmental, and engineering contexts.

Nature-Based Solution to Eliminate Cyanotoxins in Water Using Biologically Enhanced Biochar.

Climate change and high eutrophication levels of freshwater sources are increasing the occurrence and intensity of toxic cyanobacterial blooms in drinking water supplies. Conventional water treatment struggles to eliminate cyanobacteria/cyanotoxins, and expensive tertiary treatments are needed. To address this, we have designed a sustainable, nature-based solution using biochar derived from waste coconut shells. This biochar provides a low-cost porous support for immobilizing microbial communities, forming biologically enhanced biochar (BEB). Highly toxic microcystin-LR (MC-LR) was used to influence microbial colonization of the biochar by the natural lake-water microbiome. Over 11 months, BEBs were exposed to microcystins, cyanobacterial extracts, and live cyanobacterial cells, always resulting in rapid elimination of toxins and even a 1.6-1.9 log reduction in cyanobacterial cell numbers. After 48 h of incubation with our BEBs, the MC-LR concentrations dropped below the detection limit of 0.1 ng/mL. The accelerated degradation of cyanotoxins was attributed to enhanced species diversity and microcystin-degrading microbes colonizing the biochar. To ensure scalability, we evaluated BEBs produced through batch-scale and continuous-scale pyrolysis, while also guaranteeing safety by maintaining toxic impurities in biochar within acceptable limits and monitoring degradation byproducts. This study serves as a proof-of-concept for a sustainable, scalable, and safe nature-based solution for combating toxic algal blooms.

Assessing the suitability of municipal sewage sludge and coconut bran as breeding medium for Oryza sativa L. seedlings and developing a standardized substrate.

The utilization of organic solid waste (OSW) for preparing standardized seedling substrates is a main challenge due to its temporal and spatial variability. This study aims to form models based on data from the literature and validate them through experiments to explore a standardized seedling substrate. The typical OSW in Hainan Province, including municipal sewage sludge (MSS), coconut bran (CB), seaweed mud (SM), and municipal sewage sludge biochar (MSSB), was used as raw material. A series of six mixing ratios was tested, namely: T1 (0% MSS: 90% CB), T2 (10% MSS: 80% CB), T3 (30% MSS: 60% CB), T4 (50% MSS: 40% CB), T5 (70% MSS: 20% CB), and T6 (90% MSS: 0% CB). SM and MSSB were added as amendment materials at 5% (w/w) for each treatment. The physicochemical properties of substrates, agronomic traits of rice seedlings and microbial diversity were analyzed. The results showed that the four kinds of OSW played an active role in providing rich sources of nutrients. The dry weight of the above-ground part was 2.98 times greater in T3 than that of the commercial substrate. Furthermore, the microbial analysis showed a higher abundance of Actinobacteria in T3, representing the stability of the composted products. Finally, the successful fitting of the results with the linear regression models could establish relationship equations between the physicochemical properties of the substrate and the growth characteristics of seedlings. The relevant parameters suitable for the growth of rice seedlings were as follows: pH (6.46-7.01), EC (less than 2.12 mS cm-1), DD (0.13-0.16 g cm-3), and TPS (65.68-82.73%). This study proposed relevant parameters and models for standardization of seedling substrate, which would contribute to ensuring the quality of seedlings and OSW resource utilization.

The Effect of Biochar Addition on Thermal Stability and Decomposition Mechanism of Poly(butylene succinate) Bionanocomposites.

In the present study, poly(butylene succinate) (PBSu) and its bionanocomposites containing 1, 2.5, and 5 wt.% biochar (MSP700) were prepared via in situ melt polycondensation in order to investigate the thermal stability and decomposition mechanism of the materials. X-ray photoelectron spectroscopy (XPS) measurements were carried out to analyze the surface area of a biochar sample and PBSu/biochar nanocomposites. From XPS, it was found that only physical interactions were taking place between PBSu matrix and biochar nanoadditive. Thermal stability, decomposition kinetics, and the decomposition mechanism of the pristine PBSu and PBSu/biochar nanocomposites were thoroughly studied by thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). TGA thermograms depicted that all materials had high thermal stability, since their decomposition started at around 300 °C. However, results indicated a slight reduction in the thermal stability of the PBSu biochar nanocomposites because of the potential catalytic impact of biochar. Py-GC/MS analysis was employed to examine, in more detail, the thermal degradation mechanism of PBSu nanocomposites filled with biochar. From the decomposition products identified by Py-GC/MS after pyrolysis at 450 °C, it was found that the decomposition pathway of the PBSu/biochar nanocomposites took place mainly via ß-hydrogen bond scission, which is similar to that which took place for neat PBSu. However, at higher biochar content (5 wt.%), some localized differences in the intensity of the peaks of some specific thermal degradation products could be recognized, indicating that α-hydrogen bond scission was also taking place. A study of the thermal stability and decomposition pathway of PBSu/biochar bionanocomposites is crucial to examine if the new materials fulfill the requirements for further investigation for mulch films in agriculture or in electronics as possible applications.

Comparative characterisation and phytotoxicity assessment of biochar and hydrochar derived from municipal wastewater microalgae biomass.

Microalgae, originating from a tertiary treatment of municipal wastewater, is considered a sustainable feedstock for producing biochar and hydrochar, offering great potential for agricultural use due to nutrient content and carbon storage ability. However, there are risks related to contamination and these need to be carefully assessed to ensure safe use of material from wastewater microalgae. Therefore, this study compared the properties and phototoxicity of biochar and hydrochar produced via pyrolysis and hydrothermal carbonisation (HTC) of microalgae under different temperatures and residence times. While biochar promoted germination and seedling growth by up to 11.0% and 70.0%, respectively, raw hydrochar showed strong phytotoxicity, due to the high content of volatile matter. Two post-treatments, dichloromethane (DCM) washing and further pyrolysis, proved to be effective methods for mitigating phytotoxicity of hydrochar. Additionally, biochar had 35.8-38.6% fixed carbon, resulting in higher carbon sequestration potential compared to hydrochar.

Revealing roles of CO2 and N2 in pressurized hydrothermal carbonization process for enhancing energy recovery and carbon sequestration.

In this study, CO2- and N2-pressurized hydrothermal carbonization processes were investigated to understand the catalytic effects of CO2 on hydrochar production and its quality (e.g., surface properties, energy recovery, and combustion behaviour). Both CO2- and N2-pressurized HTC processes could enhance the energy recovery (from 61.5% to 63.0-67.8%) in hydrochar by enhancing the dehydration reactions. Nonetheless, the two systems exhibited contrasting trends in volatile release, oxygen removal, and combustion performance as a function of increasing pressure. High N2 pressure enhanced deoxygenation reaction, facilitating the release of volatiles and increasing the hydrochar aromaticity and combustion activation energy (172.7 kJ/mol for HC/5N). Without the contribution of CO2, excessively high pressure may cause an adverse impact on the fuel performance owing to higher oxidation resistance. This study presents an important and feasible strategy to utilise CO2-rich flue gas in the HTC process to produce high-quality hydrochar for renewable energy and carbon recovery.

Removal of NO at low concentrations from polluted air in semi-closed environments by activated biochars from renewables feedstocks.

Efficient measures are urgently required in large cities for nitric oxide (NO) elimination from air in urban semi-closed environments (parking lots and tunnels), characterized by low NO concentrations (<10 ppmv) and temperatures. One of the most promising abatement alternatives is the NO oxidation to NO2, which can be further easily captured in an alkali solution or over a porous solid. However, most of the research devoted to this topic is focused on the elimination of NO from fuel exhaust gases, with high NO concentrations (400-2000 ppmv). In this work, sustainable and low-cost activated biochars of different origin and having very different ash contents were employed in NO removal at very low concentrations. Thus, low ash content forestry (oak woodchips, OAK) and high ash content from agriculture (oilseed rape straw, OSR) biochars were subjected to physical activation with CO2 at 900 °C (OAK550-A900CO2 and OSR700-A900CO2, respectively). The NO removal performance tests of such activated carbons were carried out at different experimental conditions: i.e., temperature, relative humidity (0-50 vol% RH), NO-containing gas (N2 or air), amount of activated carbon, and NO concentration, to assess how the activated biochar properties influence their NO removal capacity. The sample OSR700-A900CO2 contained a higher population of oxygen surface functionalities, which might play an important role in the NO removal efficiency in dry conditions since they could assist NO oxidation on carbon active sites when used above room temperature (50-75 °C). However, at room temperature (25 °C), the presence of narrow micropore size distribution at 6 Å became a more relevant property, since it facilitates an intimate contact between NO and O2. Accordingly, the activated biochar from OAK was much more efficient, achieving complete removal of NO from air flow (dry or with 50 vol% RH) at 25 °C during 400 min of testing, making it an ideal candidate as biofilter for purifying air streams of semi-closed spaces contaminated with low concentrations of NO.

Rhizosphere effect on the relationship between dissolved organic matter and functional genes in contaminated soil.

Arsenic contamination in a mining area is a potential threat to the local population. In the context of one-health, biological pollution in contaminated soil should be known and understandable. This study was conducted to clarify the effects of amendments on arsenic species and potential threat factors (e.g., arsenic-related genes (AMGs), antibiotic resistance genes (ARGs) and heavy-metal resistance genes (MRGs)). Ten groups (control (CK), T1, T2, T3, T4, T5, T6, T7, T8, and T9) were set up by adding different ratio of organic fertilizer, biochar, hydroxyapatite and plant ash. The maize crop was grown in each treatment. Compared with CK, the bioavailability of arsenic was reduced by 16.2%-71.8% in the rhizosphere soil treatments, and 22.4%-69.2% in the bulk soil treatments, except for T8. The component 2 (C2), component 3 (C3) and component 5 (C5) of dissolved organic matter (DOM) increased by 22.6%-72.6%, 16.8%-38.1%, 18.4%-37.1%, respectively, relative to CK in rhizosphere soil. A total of 17 AMGs, 713 AGRs and 492 MRGs were detected in remediated soil. The humidification of DOM might directly correlate with MRGs in both soils, while it was influenced directly on ARGs in bulk soil. This may be caused by the rhizosphere effect, which affects the interaction between microbial functional genes and DOM. These findings provide a theoretical basis for regulating soil ecosystem function from the perspective of arsenic contaminated soil.

Application of heavy metal immobilization in soil by biochar using machine learning.

Biochar application is a promising strategy for the immobilization of heavy metal (HM)-contaminated soil, while it is always time-consuming and labor-intensive to clarify key influenced factors of soil HM immobilization by biochar. In this study, four machine learning algorithms, namely random forest (RF), support vector machine (SVR), Gradient boosting decision trees (GBDT), and Linear regression (LR) are employed to predict the HMimmobilization ratio. The RF was the best-performance ML model (Training R2 = 0.90, Testing R2 = 0.85, RMSE = 4.4, MAE = 2.18). The experiment verification based on the optimal RF model showed that the experiment verification was successful, as the results were comparable to the RF modeling results with a prediction error<20%. Shapley additive explanation and partial least squares path model method were used to identify the critical factors and direct and indirect effects of these features on the immobilization ratio. Furthermore, independent models of four HM (Cd, Cu, Pb, and Zn) also achieved better model prediction performance. Feature importance and interactions relationship of influenced factors for individual HM immobilization ratio was clarified. This work can provide a new insight for HM immobilization in soils.

Non-Isothermal Crystallization Kinetics of PBSu/Biochar Composites Studied by Isoconversional and Model Fitting Methods.

Non-isothermal crystallization of Poly(butylene succinate) (PBSu)/biochar composites was studied at various constant cooling rates using differential scanning calorimetry. The analysis of the kinetics data revealed that the overall crystallization rate and activation energy of the PBSu polymer were significantly influenced by the addition of biochar. Specifically, the PBSu/5% biochar composite with a higher filler content was more effective as a nucleation agent in the polymer matrix, as indicated by the nucleation activity (ψ) value of 0.45. The activation energy of the PBSu/5% biochar composite was found to be higher than that of the other compositions, while the nucleation activity of the PBSu/biochar composites decreased as the biochar content increased. The Avrami equation, which is commonly used to describe the kinetics of crystallization, was found to be limited in accurately predicting the non-isothermal crystallization behavior of PBSu and PBSu/biochar composites. Although the Nakamura/Hoffman-Lauritzen model performed well overall, it may not have accurately predicted the crystallization rate at the end of the process due to the possibility of secondary crystallization. Finally, the combination of the Sesták-Berggren model with the Hoffman-Lauritzen theory was found to accurately predict the crystallization behavior of the PBSu/biochar composites, indicating a complex crystallization mechanism involving both nucleation and growth. The Kg parameter of neat PBSu was found to be 0.7099 K2, while the melting temperature and glass transition temperature of neat PBSu were found to be 114.91 °C and 35 °C, respectively, very close to the measured values. The Avrami nucleation dimension n was found to 2.65 for PBSu/5% biochar composite indicating that the crystallization process is complex in the composites.

Application of life cycle assessment and machine learning for the production and environmental sustainability assessment of hydrothermal bio-oil.

The hydrothermal bio-oil (HBO) production from biomass conversion can achieve sustainable and low-carbon development. It is always time-consuming and labor-intensive to quantitative relationship between influential variables and bio-oil yield and environmental sustainability impact in the hydrothermal conditions. Machine learning was used to predict bio-oil yield. Life cycle assessment (LCA) is further conducted to assess its environmental sustainability effect. The results demonstrated that gradient boosting decision tree regression (GBDT) have the most optimal prediction performance for the HBO yield (Training R2 = 0.97, Testing R2 = 0.92, RMSE = 0.05, MAE = 0.03). Lipid content is the most significant influential factor for HBO yield. LCA result further suggested that 1 kg of bio-oil production can cause 0.02 kg ep of SO2, 2.05 kg ep of CO2, and 0.01 kg ep of NOx emission, and environmental sustainability assessment of HBO is exhibited. This study provides meaningful insights to ML model prediction performance improvement and carbon footprint of HBO.

Removal of NO at low concentration from air in urban built environments by activated miscanthus biochar.

This work presents an innovative and sustainable approach to remove NO emissions from urban ambient air in confined areas (underground parking areas or tunnels) using low-cost activated carbons obtained from Miscanthus biochar (MSP700) by physical activation (with CO2 or steam) at temperatures ranging from 800 to 900 °C. The NO removal capacity of the activated biochars was evaluated under different conditions (temperature, humidity and oxygen concentration) and compared against a commercial activated carbon. This last material showed a clear dependence on oxygen concentration and temperature, exhibiting a maximum capacity of 72.6% in air at 20 °C, whilst, its capacity notably decreased at higher temperatures, revealing that physical NO adsorption is the limiting step for the commercial sample that presents limited oxygen surface functionalities. In contrast, MSP700-activated biochars reached nearly complete NO removal (99.9%) at all tested temperatures in air ambient. Those MSP700-derived carbons only required low oxygen concentration (4 vol%) in the gas stream to achieve the full NO removal at 20 °C. Moreover, they also showed an excellent performance in the presence of H2O, reaching NO removal higher than 96%. This remarkable activity results from the abundance of basic oxygenated surface groups, which act as active sites for NO/O2 adsorption, along with the presence of a homogeneous microporosity of 6 Å, which enables intimate contact between NO and O2. These features promote the oxidation of NO to NO2, which is further retained over the carbon surface. Therefore, the activated biochars studied here could be considered promising materials for the efficient removal of NO at low concentrations from air at moderate temperatures, thus closely approaching real-life conditions in confined spaces.

Synthesis and Study of Fully Biodegradable Composites Based on Poly(butylene succinate) and Biochar.

Biodegradable polymers offer a promising alternative to the global plastic problems and especially in the last decade, to the microplastics problems. For the first time, samples of poly(butylene succinate) (PBSu) biocomposites containing 1, 2.5, and 5 wt% biochar (BC) were prepared by in situ polymerization via the two-stage melt polycondensation procedure. BC was used as a filler for the PBSu to improve its mechanical properties, thermal transitions, and biodegradability. The structure of the synthesized polymers was examined by 1H and 13C nuclear magnetic resonance (NMR) and X-Ray diffraction (XRD) along with an estimation of the molecular weights, while differential scanning calorimetry (DSC) and light flash analysis (LFA) were also employed to record the thermal transitions and evaluate the thermal conductivity, respectively. It was found that the amount of BC does not affect the molecular weight of PBSu biocomposites. The fine dispersion of BC, as well as the increase in BC content in the polymeric matrix, significantly improves the tensile and impact strengths. The DSC analysis results showed that BC facilitates the crystallization of PBSu biocomposites. Due to the latter, a mild and systematic increase in thermal diffusivity and conductivity was recorded indicating that BC is a conductive material. The molecular mobility of PBSu, local and segmental, does not change significantly in the biocomposites, whereas the BC seems to cause an increase in the overall dielectric permittivity. Finally, it was found that the enzymatic hydrolysis degradation rate of biocomposites increased with the increasing BC content.

Abundance, spatial distribution, and characteristics of microplastics in agricultural soils and their relationship with contributing factors.

Agro-ecosystem contamination with microplastics (MPs) is of great concern. However, limited research has been conducted on the agricultural soil of tropical regions. This paper investigated MPs in the agro-ecosystem of Hainan Island, China, as well as their relationships with plastic mulching, farming practices, and social and environmental factors. The concentration of MPs in the study area ranged from 2800 to 82500 particles/kg with a mean concentration of 15461.52 particles/kg. MPs with sizes between 20 and 200 µm had the highest abundance of 57.57%, fragment (58.16%) was the most predominant shape, while black (77.76%) was the most abundant MP colour. Polyethylene (PE) (71.04%) and polypropylene (PP) (19.83%) were the main types of polymers. The mean abundance of MPs was significantly positively correlated (p < 0.01) with all sizes, temperature, and shapes except fibre, while weakly positively correlated with the population (p = 0.21), GDP (p = 0.33), and annual precipitation (p = 0.66). In conclusion, plastic mulching contributed to significant contamination of soil MPs in the study area, while environmental and social factors promoted soil MPs fragmentation. The current study results indicate serious contamination with MPs, which poses a concern regarding ecological and environmental safety.

Hydrothermal recycling of carbon absorbents loaded with emerging wastewater contaminants.

Adsorption using carbon materials is one of the most efficient techniques for removal of emerging contaminants such as pharmaceuticals from wastewater. However, high costs are a major hurdle for their large-scale application in areas currently under economic constraints. While most research focuses on decreasing the adsorbent price by increasing its capacity, treatment costs for exhausted adsorbents and their respective end-of-life scenarios are often neglected. Here, we assessed a novel technique for recycling of exhausted activated biochars based on hydrothermal treatment at temperatures of 160-320 °C. While a treatment temperature of 280 °C was sufficient to fully degrade all 10 evaluated pharmaceuticals in solution, when adsorbed on activated biochars certain compounds were shielded and could not be fully decomposed even at the highest treatment temperature tested. However, the use of engineered biochar doped with Fe-species successfully increased the treatment efficiency, resulting in full degradation of all 10 parent compounds at 320 °C. The proposed recycling technique showed a high carbon retention in biochar with only minor losses, making the treatment a viable candidate for environmentally sound recycling of biochars. Recycled biochars displayed potentially beneficial structural changes ranging from an increased mesoporosity to additional oxygen bearing functional groups, providing synergies for subsequent applications as part of a sequential biochar system.

Agent-assisted electrokinetic treatment of sewage sludge: Heavy metal removal effectiveness and nutrient content characteristics.

Sewage sludge (SS) is rich in nutrient elements such as phosphorus (P), nitrogen (N), and potassium (K), and therefore a candidate material for use in agriculture. But high content of heavy metals (HMs) can be a major obstacle to its further utilization. Therefore, an appropriate HM removal technology is required before its land application. In this study, an innovative biodegradable agent (citric acid, FeCl3, ammonium hydroxide, tetrasodium iminodisuccinate (IDS), and tea saponin) assisted electrokinetic treatment (EK) was performed to investigate the HM removal efficiency (RHMs) and nutrient transportation. Citric acid, IDS, and FeCl3-assisted EK showed a preferable average RHMs (Rave) reduction of 52.74-59.23%, with low energy consumption. After treatment, the content of Hg (0.51 mg kg-1), Ni (13.23 mg kg-1), and Pb (26.45 mg kg-1) elements met the criteria of national risk control standard, in all cases. Following the treatment, most HMs in SS had a reduced potential to be absorbed by plants or be leached into water systems. Risk assessment indicated that the Geoaccumulation index (Igeo) value of HMs has decreased by 0.28-2.40, and the risk of Pb (Igeo=-0.74) reduced to unpolluted potential. Meanwhile, no excessive nutrient loss in SS occurred as a result of the treatment, on the contrary, there was a slight increase in P content (18.17 mg g-1). These results indicate that agent-assisted EK treatment could be an environmentally-friendly method for RHMs and nutrient element recovery from SS, opening new opportunities for sustainable SS recycling and its inclusion into circular economy concepts.

Algae, biochar and bacteria for acid mine drainage (AMD) remediation: A review.

Acid mine drainage (AMD) is a global issue and causes harmful environmental impacts. AMD has high acidity and contains a high concentration of heavy metals and metalloids, making it toxic to plants, animals, and humans. Traditional treatments for AMD have been widely used for a long time. Nevertheless, some limitations, such as low efficacy and secondary contamination, have led them to be replaced by other methods such as bio-based AMD treatments. This study reviewed three bio-based treatment methods using algae, biochar, and bacteria that can be used separately and potentially in combination for effective and sustainable AMD treatment to identify the removal mechanisms and essential parameters affecting AMD treatment. All bio-based methods, when applied as a single process and in combination (e.g. algae-biochar and algae-bacteria), were identified as effective treatments for AMD. Also, all these bio-based methods were found to be affected by some parameters (e.g. pH, temperature, biomass concentration and initial metal concentration) when removing heavy metals from AMD. However, we did not identify any research focusing on the combination of algae-biochar-bacteria as a consortium for AMD treatment. Therefore, due to the excellent performance in AMD treatment of algae, biochar and bacteria and the potential synergism among them, this review provides new insight and discusses the feasibility of a combination of algae-biochar-bacteria for AMD treatment.

Composition of PAHs in Biochar and Implications for Biochar Production.

The content of polycyclic aromatic hydrocarbons (PAHs) in biochar has been studied extensively; however, the links between biomass feedstock, production process parameters, and the speciation of PAHs in biochar are understudied. Such an understanding is crucial, as the health effects of individual PAHs vary greatly. Naphthalene (NAP) is the least toxic of the 16 US EPA PAHs but comprises the highest proportion of PAHs in biochar. Therefore, we investigate which parameters favor high levels of non-NAP PAHs (∑16 US EPA PAHs without NAP) in a set of 73 biochars. On average, the content of non-NAP PAHs was 9 ± 29 mg kg-1 (median 0.9 mg kg-1). Importantly, during the production of the biochars with the highest non-NAP PAH contents, the conditions in the post-pyrolysis area, where pyrolysis vapors and biochar are separated, favored condensation and deposition of PAHs on biochar. Under these conditions, NAP condensed to a lower degree because of its high vapor pressure. In biochars not contaminated through this process, the average non-NAP content was only 2 ± 3 mg kg-1 (median 0.5 mg kg-1). Uneven heat distribution and vapor trapping during pyrolysis and cool zones in the post-pyrolysis area need to be avoided. This demonstrates that the most important factor yielding high contents of toxic PAHs in biochar was neither a specific pyrolysis parameter nor the feedstock but the pyrolysis unit design, which can be modified to produce clean and safe biochar.