Relationships between Chemical Characteristics and Phytotoxicity of Biochar from Poultry Litter Pyrolysis

Review on biochar as a sustainable green resource for the rehabilitation of petroleum hydrocarbon-contaminated soil

Petroleum pollution is one of the primary threats to the environment and public health. Therefore, it is essential to create new strategies and enhance current ones. The process of biological reclamation, which utilizes a biological agent to eliminate harmful substances from polluted soil, has drawn much interest. Biochars are inexpensive, environmentally beneficial carbon compounds extensively employed to remove petroleum hydrocarbons from the environment. Biochar has demonstrated an excellent capability to remediate soil pollutants because of its abundant supply of the required raw materials, sustainability, affordability, high efficacy, substantial specific surface area, and desired physical-chemical surface characteristics. This paper reviews biochar's methods, effectiveness, and possible toxic effects on the natural environment, amended biochar, and their integration with other remediating materials towards sustainable remediation of petroleum-polluted soil environments. Efforts are being undertaken to enhance the effectiveness of biochar in the hydrocarbon-based rehabilitation approach by altering its characteristics. Additionally, the adsorption, biodegradability, chemical breakdown, and regenerative facets of biochar amendment and combined usage culminated in augmenting the remedial effectiveness. Lastly, several shortcomings of the prevailing methods and prospective directions were provided to overcome the constraints in tailored biochar studies for long-term performance stability and ecological sustainability towards restoring petroleum hydrocarbon adultered soil environments.

Phytotoxicity and hormesis in common mobile organic compounds in leachates of wood-derived biochars

Although addition of pyrolyzed organic materials (biochars) to soil generally results in increased growth and physiological performance of plants, neutral and negative responses have also commonly been detected. Toxicity of organic compounds generated during pyrolysis, sorbed by biochars, and then released into the soil solution, has been implicated as a possible mechanism for such negative effects. Conversely, water-soluble biochar constituents have also been suggested to have "hormetic" effects (positive effects on plants at low concentrations); however, no specific compounds responsible have been identified. We investigated the relative phytotoxicity-and possible hormetic effects-of 14 organic compounds common in aqueous extracts of freshly produced lignocellulosic biochars, using seed germination bioassays. Of the compounds examined, volatile fatty acids (VFAs: acetic, propionic, butyric, valeric, caproic, and 2-ethylbutyric acids) and phenol, showed acute phytotoxicity, with germination-based ED50 values of 1-30 mmol L-1, and 2-ethylbutyric acid showed ED50 values of 0.1-1.0 mmol L-1. Other compounds (benzene, benzoic acid, butanone, methyl salicylate, toluene, and 2,4-di-tert-butylphenol) showed toxic effects only at high concentrations close to solubility limits. Although phytotoxic at high concentrations, valeric and caproic acid also showed detectable hormetic effects on seedlings, increasing radicle extension by 5-15% at concentrations of ~ 0.01-0.1 mmol L-1. These data support the hypothesis that VFAs are the main agents responsible for phytotoxic effects of lignocellulosic biochar leachates, but that certain VFAs also have hormetic effects at low concentrations and may contribute to positive effects of biochar leachates on early plant development in some cases.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s42773-024-00339-w.

Fuel Characteristics and Phytotoxicity Assay of Biochar Derived from Rose Pruning Waste

The aim of this study was the characterization and evaluation of applicability as a soil amendment of biochar derived from rose pruning waste at different pyrolysis temperatures (200-500 °C) and process durations (20-60 min). The biochar properties were compared to the raw material. The biochars produced at 300 °C for 40 and 60 min demonstrated the best fuel properties. These variants showed high energy gain rates (77.6 ± 1.5% and 74.8 ± 1.5%, respectively), energy densification ratios (1.35 ± 0.00 and 1.37 ± 0.00, respectively), high heating values (24,720 ± 267 J × g-1 and 25,113 ± 731 J × g-1, respectively), and relative low ash contents (5.9 ± 0.5% and 7.1 ± 0.3%, respectively). Regarding fertilizer properties, such as pH value, ash content, heavy metal content, and pollutant elution, the biochars showed better qualities than the raw material. All tested biochar did not exceed the permissible values for heavy metals, including Cr, Cd, Ni, and Pb. The most optimal properties for soil amendments were noted for biochar variants of 400 °C for 40 min, 450 °C for 20 min, and 500 °C for 20 min. Generally, biochars produced at temperatures ≥400 °C did not inhibit root elongation, except for the material produced at 450 °C for 60 min (4.08 ± 23.34%). Biochars obtained at ≥300 °C showed a positive impact on seed germination (86.67 ± 18.48-100 ± 24.14%).

A critical review on the development of lanthanum-engineered biochar for environmental applications

Biochar and lanthanum (La) have been widely used in environment. However, there is a lack of knowledge and perspective on the development of La-engineered biochar (LEB) for environmental applications. This review shows that LEBs with a variety of La species via pre-/post-doping routes are developed for environmental applications. Specifically, precipitation, gelation, and calcination are the common sub-processes involved in the pre-/post-doping of La on the resultant LEB. The dominant La species for LEBs is La(OH)₃, which is formed through precipitation of La ions with various bases. Various La carbonates, e.g., LaOHCO₃, La₂(CO₃)₃, La₂CO₅, and NaLa(CO₃)₂, are also involved in the preparation of LEBs. The LEBs are high-efficient in the adsorption of phosphate, arsenic, antimonate and fluoride ions, attributed to the strong affinity of La to oxyanions and Lewis hard base. Lanthanum is also favorable for co-doping with transition metal species to further enhance the performances in adsorption or catalysis. This review also analyzes the prospects and future challenges for the preparation and application of LEBs in environment. Finally, this review is beneficial to inspire new breakthroughs on the preparation and environmental application of LEBs.

Evaluation and characterization of biochar on the biogeochemical behavior of polycyclic aromatic hydrocarbons in mangrove wetlands

As the inter-tidal regions between land and ocean, mangrove ecosystems have high polycyclic aromatic hydrocarbons (PAHs) content, and the over accumulation of PAHs in mangrove wetland poses a serious ecological risk to the health of plant and living creatures. Comparison to the agricultural sources -biochar, biochar produced from wetland plant has lower O/C (molar ratio), larger N contents, higher stability and more benefits. However, whether the rhizosphere action occurs in biochar- amended sediment and how to influence the biogeochemical behavior of PAH have rarely been reported. In this context, a leaching procedure and pot experiment (60-d) were performed on migration and transformation of PAH at the sediment, and toxicity and their bioavailability in plant affected by the presence of Kandelia obovate-derived biochar in Southeast China. Root exudates amendments significantly increased the cumulative leaching-loss of pyrene by 36-51 % with or without biochar amendment via continuous diffusion and partition process, and biochar amendments decreased the bioavailability of pyrene (16.8-25.8 %) probably due to a faster pyrene sorption on inter-phase transport against desorption. The regression analysis indicated a significant relationship (p < 0.05) between leachate pH and pyrene concentrations. Notably, the bioaccumulation of pyrene on K. obovate parts had significant negative correlation (p < 0.05) to biochar. The activities of four key antioxidizes (phenylalanine ammonia-lyase, dismutases, peroxidases and catalases) were significantly decreased with the application of biochar. Moreover, biochar plays a positive role in cytochrome C release and phosphatidylserine secretion, and a combined biochar-rhizosphere approach can improve the stress tolerance and resistance of K. obovate with an enhanced synergetic effect, which could be a feasible remediation strategy for alleviating the mangrove sediment contaminated by PAH.

Vermitoxicity of aged biochar and exploring potential damage factors

Although biochar is a promising soil amendment, its characteristics change owing to its aging in soil. Studies have shown that some aged biochar is hazardous to plants and soil microbiota. Earthworms are well-known soil ecosystem engineers; nevertheless, the toxic effects of aged biochar on them (vermitoxicity) are yet unknown, and it is necessary to explore the potential risk factors. Here, a series of soil culture experiments were conducted to systematically examine the vermitoxicity of aged biochar at various levels utilizing the earthworm Eisenia fetida and corncob biochar.. Acute toxicity bioassays were also used to evaluate several potential harm factors utilizing modified aged biochar/leaching solutions. The findings showed that both fresh and aged biochar might have adverse effects on earthworms, and that aged biochar was more toxic than fresh biochar with LC₅₀s reduced to 6.89%. Specifically, aged biochar caused earthworm death, growth inhibition with a maximum of 36.6%, and avoidance with 100% avoidance at the application rates of 2% at the individual-behavioral level. At the cellular and physiological-biochemical levels, aged biochar damaged coelomocyte lysosomal membrane stability, disrupted antioxidant enzyme activities, and improved the malondialdehyde (MDA) content in earthworms. Heat-treated and pH-modified aged biochar exhibited less acute toxicity on earthworms than aged biochar, whereas aqueous and acetone extracts showed weak vermitoxicity. As a result, earthworms may be harmed by volatile organic compounds (VOCs), an improper pH, and aqueous and acetone extracts. Additionally, the range of neural red retention times (NRRTs) was reviewed as ∼20-70 min mostly. This study, as far as we know, is the first to evaluate the vermitoxicity of aged biochar and its potential damage factors. The results may enhance our understanding of ecological toxicity of biochar, particularly over the long term, and lead to the development of application standards for biochar amendments to the soil.

Effects of biochar-based materials on the bioavailability of soil organic pollutants and their biological impacts

Motivated by the unique structure and superior properties, biochar-based materials, including pristine biochar and composites of biochar with other functional materials, are considered as new generation materials for diverse multi-functional applications, which may be intentionally or unintentionally released to soil. The influencing mechanism of biochar-based material on soil organisms is a key aspect for quantifying and predicting its benefits and trade-offs. This work focuses on the effects of biochar-based materials on soil organisms within the past ten years. 206 sources are reviewed and available knowledge on biochar-based materials' impacts on soil organisms is summarized from a diverse perspective, including the pollutant bioavailability changes in soil, and potential effects of biochar-based materials on soil organisms. Herein, effects of biochar-based materials on the bioavailability of soil organic pollutants are detailed, from the perspective of plant, microorganism, and soil fauna. Potential biological effects of pristine biochar (PBC), metal/metal compounds-biochar composites (MBC), clay minerals-biochar composites (CMBC), and carbonaceous materials-biochar composites (CBC) on soil organisms are highlighted for the first time. And possible mechanisms are presented based on the different characters of biochar-based materials as well as various environmental interactions. Finally, the bottleneck and challenges of risk assessment of biochar-based materials as well as future prospects are proposed. This work not only promotes the development of risk assessment system of biochar-based materials, but broadens the strategy for the design and optimization of environmental-friendly biochar materials.

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.

The Influence of Biochar Augmentation and Digestion Conditions on the Anaerobic Digestion of Water Hyacinth

The augmentation of biochar (BC) during anaerobic digestion (AD) has been identified as a potential strategy for improving the AD of complex feedstocks. This study evaluates the influence of oak wood biochar 450 °C and fermentation conditions during the AD of the invasive aquatic plant, water hyacinth (WH). Factorial 22 design of experiments (DOE) allowed the evaluation of the effect of the crucial processing conditions, inoculum-to-substrate ratio (ISR) and biochar load. Further optimisation was performed to identify the best processing conditions for the AD of WH, at an ideal ISR of 1. The contour plots suggested that methane yield is favoured at biochar loads of ≤0.5%, whereas the production rate is favoured by increasing biochar loads. However, biochar addition offered no further improvement or significant effect on the digestion of WH. The subsequent AD of WH samples collected from different locations in India and Uganda exhibited variable biochemical methane potential (BMP) yields. BC addition had little effect on BMP performance, and in some cases, it even reduced the BMP. This study concludes that the amendment potential of biochar is influenced by digestion conditions and the substrate, particularly when working with complex substrates.

A comprehensive assessment on bioavailability, leaching characteristics and potential risk of polycyclic aromatic hydrocarbons in biochars produced by a continuous pyrolysis system

Biochar application as a soil amendment has attracted worldwide attention. Nevertheless, polycyclic aromatic hydrocarbons (PAHs) formed during biochar production might enter into ecosystems and threaten human health after application to soil. Continuous pyrolysis systems tend to cause an accumulation of PAHs in biochar owing to short residence time and rapid cooling. This study conducted a comprehensive assessment regarding potential risk of PAHs in biochars produced by a continuous pyrolysis system based on bioavailability, leaching behavior, toxic equivalent quantity, health risk and phytotoxicity of PAHs. Results showed that the concentrations of total PAHs in biochars were in the range of 93.40-172.40 mg/kg, exceeding the European Biochar Certificate standard. 3-rings PAHs were the predominant groups. The percentages of total freely dissolved and leachable PAHs were lower than 1%. RH contained the least bioavailable and leachable PAHs concentration and phytotoxicity compared with CS and PS, which might attribute to the characteristic of three biochars. CS and PS were acidic and exhibited high levels of DOC and VFAs, while RH was strongly alkaline and presented greater aromaticity and higher surface area, which might have resulted in high adsorptive capacity and decreased bioavailability of PAHs. When the biochar application rate was higher than 0.6 t/ha, the incremental lifetime cancer risk value for human exposure to biochar-borne PAHs through the biochar-amended soil was over 10^-6, suggesting carcinogenic risks. Germination index values of biochars ranged from 25.66 to 88.95%. Phytotoxicity mainly was caused by bioavailable PAHs and dissolved organic compounds. Overall, these findings highlighted that although the percentage of bioavailable PAHs was low, the potential health risk and phytotoxicity of PAHs in biochars produced by a continuous pyrolysis system was of a great concern. High biochar application rates should be avoided without processing both for soil safety and human health.

Correlating the chemical properties and bioavailability of dissolved organic matter released from hydrochar of walnut shells

Large amounts of lignocellulosic biomass are discarded, whereas the carbon source of sewage is deficient. This situation greatly impairs the efficiency of wastewater treatment. To address this concern, we evaluate the feasibility of using hydrochar as a potential carbon source by systematically investigating the effects of hydrothermal carbonization (HTC) conditions on the composition, content, and chemical structure of dissolved organic matter (DOM) released from hydrochar. Results show that the most important factor that affects the properties of hydrochar and DOM is temperature, followed by heating rate. Under optimal HTC conditions, the growth of Bacillus subtilis increased by 18.32% in hydrochar aqueous solution in comparison with the 6.64% growth of the untreated biomass group. Excitation emission matrix-parallel factor analysis and UV-vis analyses confirm that the DOM released by hydrochar produced at a low temperature mainly contains protein substances, which promote the growth of microorganisms. The DOM released by hydrochar at a high temperature mainly contains humic substances with an aromatic structure; such substances are toxic to microorganisms. This study demonstrates that hydrochar obtained under optimized conditions can be a potential carbon source of wastewater treatment plants.

Effects of biochar and thermally treated biochar on Eisenia fetida survival, growth, lysosomal membrane stability and oxidative stress

Despite its known positive impacts when added to soil, the negative effects of biochar on earthworms are not fully understood. Here, we investigated the toxicity of nine biochars of three feedstock origins, animal (cow dung), plant (corncob) and microorganism (sewage sludge), produced at three pyrolysis temperatures (350 °C, 550 °C and 750 °C) on earthworms. Vermitoxicity was first assessed using acute toxicity test, neutral red retention time (NRRT) assay and oxidative stress response assay. Furthermore, we evaluated whether the thermal treatment of biochars could reduce their vermitoxicity using an acute toxicity assay. We found that, according to LC₅₀ and earthworm weight loss, cow dung biochar was more toxic than corncob or sewage sludge biochar; thus, production feedstock is apparently important to biochar vermitoxicity. Furthermore, NRRTs indicated cow dung biochar disrupted lysosomal membrane stability in earthworm coelomocytes, providing further evidence for the toxicity of this biochar to earthworms. Disturbed antioxidant enzyme activities and elevated malondialdehyde content showed that earthworm suffer oxidative stress, also implying a potential vermitoxicity. However, thermal treatment of cow dung biochar substantially improved its LC₅₀ and decreased earthworm weight loss, implying that the PAHs in this biochar might be damage factors and that heating could reduce the potential toxicity of biochar. Besides, NRRT assay was first used to evaluate the effects of biochar on earthworms and clear dose-effect relationships indicated that NRRT assay might be a useful tool for assessing the potential negative effects of biochar. Overall, given the different effects of various biochars, including toxicity, reported here, our findings will help improve understanding of biochar vermitoxicity mechanisms, serve to improve biochar ecological risk assessments and provide a reference for the proper application of biochar amendments.

Bubbles in microfluidics: an all-purpose tool for micromanipulation

In recent decades, the integration of microfluidic devices and multiple actuation technologies at the microscale has greatly contributed to the progress of related fields. In particular, microbubbles are playing an increasingly important role in microfluidics because of their unique characteristics that lead to specific responses to different energy sources and gas-liquid interactions. Many effective and functional bubble-based micromanipulation strategies have been developed and improved, enabling various non-invasive, selective, and precise operations at the microscale. This review begins with a brief introduction of the morphological characteristics and formation of microbubbles. The theoretical foundations and working mechanisms of typical micromanipulations based on acoustic, thermodynamic, and chemical microbubbles in fluids are described. We critically review the extensive applications and the frontline advances of bubbles in microfluidics, including microflow patterns, position and orientation control, biomedical applications, and development of bubble-based microrobots. We lastly present an outlook to provide directions for the design and application of microbubble-based micromanipulation tools and attract the attention of relevant researchers to the enormous potential of microbubbles in microfluidics.

Molecular characterization and environmental impacts of water-soluble organic compounds of bio-oil from the thermochemical treatment of domestic sewage sludge

Water-soluble organic compounds derived from bio-oil (WOCB) are regarded as potential risk sources of sludge thermochemical treatment. This study showed that 10.35 mg of water-soluble organic carbon and 1.32 mg of water-soluble organic nitrogen were released per gram of sludge when the final temperature of thermochemical treatment was 600 °C. WOCB was mainly formed at 300-500 °C. Furthermore, FT-ICR MS results indicated that high temperatures promoted deamination reactions, and low molecular weight (LMW) compounds with low oxygen number polymerized into aromatic compounds with increasing temperature. Noteworthily, WOCB released at 20-600 °C showed strong phytotoxicity to wheat. LMW compounds with lignin/carboxylic rich alicyclic molecules (CRAM)-like structures derived from low temperatures (200-400 °C) induced this inhibitory effect, but lipids containing nitrogen and sulfur from high temperatures (400-600 °C) can act as nutrients to promote wheat growth. This study provides theoretical support for the risk control and benefits assessments of sludge thermochemical treatment.

THE DARK SIDE OF BLACK GOLD: Ecotoxicological aspects of biochar and biochar-amended soils

Biochar, a product of biomass pyrolysis, is characterized by significant surface area, porosity, high water holding capacity, and environmental persistence. It is perceived as a material that can counteract climate change due to its high carbon stability and is also considered suitable for soil amendment (fertility improvement, soil remediation). However, biochar can have a toxic effect on organisms as harmful substances may be present in it. This paper reviews the literature regarding the current knowledge of harmful substances in biochar and their potential negative impact on organisms from different trophic levels. The effects of biochar on the content and toxicity of harmful substances in biochar-amended soils are also reviewed. Application of biochar into soil does not usually have a toxic effect and very often stimulate plants, bacteria activity and invertebrates. The effect however is strictly determined by type of biochar (especially the feedstock used and pyrolysis temperature) as well as contaminants content. The pH, electrical conductivity, polycyclic aromatic hydrocarbons as well as heavy metals are the main factor usually responsible for biochar toxicity.

Biochar from pyrolyzed Tibetan Yak dung as a novel additive in ensiling sweet sorghum: An alternate to the hazardous use of Yak dung as a fuel in the home

Yak dung is used as fuel in Tibetan homes; however, this use is hazardous to health. An alternative use of the dung that would be profitable and offset the loss as a fuel would be very beneficial. Sweet sorghum silage with yak dung biochar as an additive was compared with a control silage with no additives and three silages with different commercial additives, namely Lactobacillus buchneri, Lactobacillus plantarum and Acremonium cellulase. Biochar-treated silage had a significantly greater concentration of water-soluble carbohydrates than the other silages (76 vs 12.4-45.8 g/kg DM) and a greater crude protein content (75.5 vs 61.4 g/kg DM), lactic acid concentration (40.7 vs 27.7 g/kg DM) and gross energy yield (17.8 vs 17.4 MJ/kg) than the control silage. Biochar-treated and control silages did not differ in in vitro digestibility and in total gas (507 vs 511 L/kg DM) and methane production (57.9 vs 57.1 L/kg DM). Biochar inhibited degradation of protein and water-soluble carbohydrates and enhanced lactic acid production, which improved storability of feed. It was concluded that yak dung biochar is an efficient, cost-effective ensiling additive. The profit could offset the loss of dung as fuel and improve the health of Tibetan people.

Post-processing of biochars to enhance plant growth responses: a review and meta-analysis

A number of processes for post-production treatment of "raw" biochars, including leaching, aeration, grinding or sieving to reduce particle size, and chemical or steam activation, have been suggested as means to enhance biochar effectiveness in agriculture, forestry, and environmental restoration. Here, I review studies on post-production processing methods and their effects on biochar physio-chemical properties and present a meta-analysis of plant growth and yield responses to post-processed vs. "raw" biochars. Data from 23 studies provide a total of 112 comparisons of responses to processed vs. unprocessed biochars, and 103 comparisons allowing assessment of effects relative to biochar particle size; additional 8 published studies involving 32 comparisons provide data on effects of biochar leachates. Overall, post-processed biochars resulted in significantly increased average plant growth responses 14% above those observed with unprocessed biochar. This overall effect was driven by plant growth responses to reduced biochar particle size, and heating/aeration treatments. The assessment of biochar effects by particle size indicates a peak at a particle size of 0.5-1.0 mm. Biochar leachate treatments showed very high heterogeneity among studies and no average growth benefit. I conclude that physiochemical post-processing of biochar offers substantial additional agronomic benefits compared to the use of unprocessed biochar. Further research on post-production treatments effects will be important for biochar utilization to maximize benefits to carbon sequestration and system productivity in agriculture, forestry, and environmental restoration.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s42773-021-00115-0.

Insights into the molecular transformation in the dissolved organic compounds of agro-waste-hydrochars by microbial-aging using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Hydrochars-based dissolved organic matters (DOM) are easily available to organisms and thus have important influence on the biota once applying hydrochars as environment amendment. Thus, positive modifications on molecular composition of DOM is indispensable before hydrochars application. In this study, the impacts of microbial-aging by anaerobic fermentation on DOM of agro-waste-hydrochars was systematically assessed. Results revealed that microbial-aging caused lower DOM release but higher DOM molecular diversity. Moreover, microbial-aging resulted in the production of more biodegradable compounds, including lipids and proteins, and reduced the aromaticity of DOM. The highly oxygenated molecules (O/C > 0.6) were shifted into lower-order ones in the hydrochars-based DOM, suggesting the transformation of hydrophilic compounds into hydrophobic ones. Additionally, microbial-aging promoted the degradation of phenols by 99.0-98.9%, phenolic acids 37.8-73.5%, and polycyclic aromatic hydrocarbons by 83.4-90.4% in hydrochar-based DOM. Overall, this study demonstrates that microbial-aging changes the molecular characteristics of hydrochars-based DOM in a positive manner.

An emerging environmental concern: Biochar-induced dust emissions and their potentially toxic properties

Amending soils with biochar is increasingly proposed as a solution to many pressing agricultural and environmental challenges. Biochar, created by thermochemical conversion of biomass in an oxygen-limited environment, has several purported benefits, including remediation of contaminated soils, increased crop yields, reduced fertilizer demands, increased plant available water, and mitigation of climate change. Due to these potential benefits, biochar-related research has flourished in the past decade, though there remains a critically understudied area of research regarding biochar's potential impact on human health. Because biochar characteristically has low bulk density and high porosity, the material is susceptible to atmospheric release via natural or mechanical soil disturbance. The specific risks of biochar inhalation have not been elucidated; however, recent publications have demonstrated that biochar can increase soil dust emissions of particles <10 μm (PM₁₀) or possess elevated levels of toxic chemicals. These data should not be interpreted to suggest that all biochars are problematic, but rather to highlight an important and overlooked field of study, and to stress the need to critically assess parameters for biochar production and management strategies that safeguard human health. Here the literature on biochar-related dust emissions and potentially toxic properties (PTPs) is reviewed in order to summarize what is known, highlight areas for future study, and aggregate solutions to minimize potential harm.

Strain-Specific Effects of Biochar and Its Water-Soluble Compounds on Bacterial Growth

Previous studies have revealed that biochar could induce the disturbance of a microbial community above the family level. So far, very little is known about how individual bacteria are affected by biochar at genus or species levels. In this study, effects of biochar and its water-soluble compounds on the growth of individual soil bacteria were examined. Biochar derived from different feedstock showed disproportionate impacts on bacterial growth. Corncob biochar could significantly stimulate the growth of most tested strains, whereas the growth of four strains, including Bacillus pumilus ACCC04306 (Agricultural Culture Collection of China, ACCC), B. licheniformis, B. cereus, and Kitasatospora viridis, were inhibited by addition of rice husk biochar. All the biochars greatly supported the growth of B. mucilaginosus but inhibited that of K. viridis. More importantly, different strains exhibited discrepant growth response towards the same biochar sample, even when strains belong to the same species, suggesting that the effect of biochar on bacteria growth is strain-specific. Corncob biochar showed the strongest adsorption on B. thuringiensis but the greatest growth promotion was observed in B. mucilaginosus, indicating that the porous structure of biochar is not the sole factor that influences cell growth. Due to the possible stimulation or inhibition of water-soluble compounds existing in biochar, the growth variation of tested strains decreased or increased correspondingly when the washed biochar was applied, indicating that water-soluble compounds in fresh biochar play an important role in cell growth and such effect is also strain-dependent. Biochar application could also enhance potassium-/phosphate-solubilizing activities through promoting bacterial growth. All these results suggested that biochar might influence bacterial growth under different mechanisms. Our findings should be valuable for an in-depth understanding of the potential mechanism of soil bacteria changes following biochar incorporation and for biochar application in agriculture.

Phytotoxicity of Corncob Biochar before and after Heat Treatment and Washing

Biochar from crop residues such as corncobs can be used for soil amendment, but its negative effects have also been reported. This study aims to evaluate the phytotoxic effects of different biochar treatments and application rates on cress (Lepidium sativum). Corncob biochar was produced via slow pyrolysis without using purging gas. Biochar treatments included fresh biochar (FB), dried biochar (DB), washed biochar (WB), and biochar water extract (WE). Biochar application rates of 10, 20, and 30 t/ha were investigated. Significant phytotoxic effects of biochar were observed on germination rates, shoot length, fresh weight, and dry matter content, while severe toxic effects were identified in FB and WE treatments. Germination rate after 48 h (GR48) decreased with the increase of biochar application rates in all treatments. The observed order of performance of the biochar treatments for germination, shoot length, and shoot fresh weight for every biochar application rate was WB>DB>WE>FB, while it was the reverse order for the shoot dry matter content. WB treatment showed the best performance in reducing the phytotoxicity of biochar. The mitigation of the phytotoxicity in fresh corncob biochar by washing and heat treatment was found to be a simple and effective method.

Getting to the root of the matter: Water-soluble and volatile components in thermally-treated biosolids and biochar differentially regulate maize (Zea mays) seedling growth

The use of thermally treated biomass, including biochar, as soil amendments can improve soil fertility by providing nutrients, stable C and improving soil water-holding capacity. However, if the degree of carbonization is low, these soil amendments can lower crop productivity as a result of high salinity or organic compounds. The overall effect of these soil amendments is mediated by complex relationships between production conditions, soil properties and environmental conditions. This study aimed to 1) characterize the physiochemical properties and organic compounds released by three soil amendments (softwood biochar or pyrogenic carbonaceous biosolids), 2) determine the effects of these amendments on maize (Zea mays) seedling productivity, and 3) relate properties of these amendments to effects on maize seedling productivity under controlled environment conditions. Physicochemical properties and mobile organic compounds (water-soluble and volatile organic compounds were determined. The amendments were tested in maize germination and greenhouse experiments. Chemical fingerprinting of volatile and water-soluble compounds revealed over 100 mobile organic species. Increasing treatment temperature from 270 to 320°C reduces phytotoxicity of pyrogenic carbonaceous biosolids soil amendments. Water-soluble components of pyrogenic carbonaceous biosolids produced at 270°C (inorganic N, Na and/or organic compounds) were associated with reduced maize seedling productivity. Volatile components of pyrogenic carbonaceous biosolids produced at 320°C were associated with improved maize seedling productivity; nitrogen uptake was increased in spite of smaller root systems as a result of increased mineralization of soil or amendment N and/or uptake of organic N compounds. These results suggest that pyrogenic carbonaceous biosolids have potential benefits to provide plant nutrients when the amount of organic and inorganic species are limited during early growth stages, under greenhouse conditions. Future studies should examine these effects under field conditions to confirm whether controlled environment results translate into effects on yield.

Effect of management of organic wastes on inactivation of Brassica nigra and Fusarium oxysporum f.sp. lactucae using soil biosolarization

BACKGROUND

Soil biosolarization is a promising alternative to conventional fumigation. Volatile fatty acids (VFAs) produced in the soil through fermentation of amended organic matter can affect pest inactivation during biosolarization. The objective was to determine how soil amended with organic wastes that were partially stabilized through either composting or anaerobic digestion affected the inactivation of Brassica nigra (BN; a weed) and Fusarium oxysporum f. sp. lactucae (FOL; a phytopathogenic fungus).

RESULTS

The mortality of BN seeds in the biosolarized soil was 12% higher than in the solarized soil, although this difference was not significant. However, a significant correlation between BN mortality and VFA accumulation was observed. The number of FOL colony-forming units (CFU) in solarized samples at 5 cm was 34 CFU g^-1 of soil, whereas in the biosolarized samples levels were below the limit of quantification. At 15 cm, these levels were 100 CFU g^-1 for solarized samples and < 50 CFU g^-1 of soil for the biosolarized samples. Amendment addition positively affected the organic matter and potassium content after the solarization process.

CONCLUSION

The organic waste stabilization method can impact downstream biosolarization performance and final pest inactivation levels. This study suggests that organic waste management practices can be leveraged to improve pest control and soil quality. © 2018 Society of Chemical Industry.

Production Temperature Effects on the Structure of Hydrochar-Derived Dissolved Organic Matter and Associated Toxicity

Hydrochar is a carbonaceous material derived from hydrothermal liquefaction, and it carries good potential as a new material for environmental applications. However, little is known about the dissolved organic matter (DOM) associated with hydrochar and the consequences of its release. The relationship between the production temperature and the characteristics of DOM released from hydrochar as well as the associated biotoxicity was investigated using a suite of advanced molecular and spectroscopic tools. With the increase in production temperature, the resulted hydrochar-based DOM contained a higher content of phenols and organic acids but less sugars and furans. Meanwhile, the molecular structure of DOM shifted to lower molecular weight with higher organic contents containing <6 O atoms per compound, aromatics, and N-containing substances. While low-temperature hydrochar-derived DOM showed minimal biotoxicity, increase in production temperature to 330 °C led to a great rise in toxicity. This might be attributed to the increased contents of phenols, organic acids, and organics containing <6 O atoms and 1 N atom per compound. These results suggest that hydrochar-derived DOM have more negative impacts on the environment than the organics associated with biochar production. Such understanding highlights the importance of controlling the hydrochar production process.

Changes of total and freely dissolved polycyclic aromatic hydrocarbons and toxicity of biochars treated with various aging processes

The aim of this study was to determine the effect of biochar aging on the content of polycyclic aromatic hydrocarbons (PAHs) (the total content - C_tot, and the freely dissolved - C_free) in biochar and its ecotoxicity. Two biochars (BCS and BCM) with varying properties were aged for 420 days at different temperatures (-20 °C, 4 °C, 20 °C, 70 °C), at a variable temperature (-20/20 °C), in the presence of nutrients, and in the presence of inoculum and nutrients. After the aging process, C_tot and C_free PAHs were determined in samples obtained and an ecotoxicological analysis was performed, which involved tests with bacteria (Vibrio fischeri), invertebrates (Folsomia candida) and plants (Lepidium sativum). Aging significantly affected all the parameters tested. The range of changes in the studied parameters depended on the type of biochar and ageing conditions. In the case of most of the aging methods, PAH content (C_tot, C_free) and toxicity were found to decrease. Aging in the presence of microorganisms and nutrients and in the presence of nutrients alone caused the greatest reduction in C_tot PAH content (a reduction from 30 to 100% relative to non-aged biochar), C_free PAH content (a reduction from 12 to 100%), root growth inhibition (a reduction from 73 to 90%), and luminescence inhibition (a reduction from 24 to 100%). In the case of C_free PAHs and toxicity to F. candida, some aging methods caused their increase. The study also found a significant relationship between the changes in C_tot PAH content during aging and inhibition of root growth (BCS, BCM) and inhibition of V. fischerii luminescence (BCM). In no case was a significant correlation (P ≥ 0.05) between C_free PAHs and the investigated toxicity parameters found.

Sustainability in art conservation: a novel bio-based organogel for the cleaning of water sensitive works of art

Organo- and hydrogels have been proposed in the restoration field to treat different types of surfaces. The possibility to retain solvents and to have a controlled and superficial action allowed to use these materials for the removal of very thin layers applied on ancient historical objects, when the under paint layers are particularly delicate and water sensitive. In the last years, an increased attention has been devoted to the proposal of more healthy products to guarantee the safeguard of the operators. Few attention has been devoted to the development of green methods which foresee the use of renewable and biodegradable materials. The aim of this paper is to test a green organo-gel for the cleaning of water sensitive surfaces like varnished egg tempera paintings. The gel has been tested experimented on mock ups varnished with natural and synthetic materials and has been validated on a small portion of a Cimabue painting for the removal of two varnishes applied on two different test areas of the painting.

Evaluation of biochars from different stock materials as carriers of bacterial strain for remediation of heavy metal-contaminated soil

Two kinds of biochars, one derived from corn straw and one from pig manure, were studied as carriers of a mutant genotype from Bacillus subtilis (B38) for heavy metal contaminated soil remediation. After amendment with biochar, the heavy metal bioavailability decreased. Moreover, the heavy metal immobilization ability of the biochar was enhanced by combining it with B38. The simultaneous application of B38 and pig manure-derived biochar exhibited a superior effect on the promotion of plant growth and the immobilization of heavy metals in soil. The plant biomass increased by 37.9% and heavy metal concentrations in the edible part of lettuce decreased by 69.9–96.1%. The polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) profiles revealed that pig manure-derived biochar could enhance the proliferation of both exotic B38 and native microbes. These results suggest that B38 carried by pig manure-derived biochar may be a promising candidate for the remediation of soils contaminated by multiple heavy metals.

Source and Biological Response of Biochar Organic Compounds Released into Water; Relationships with Bio-Oil Composition and Carbonization Degree

Water-soluble organic compounds (WSOCs) were extracted from corn stalk biochar produced at increasing pyrolysis temperatures (350-650 °C) and from the corresponding vapors, collected as bio-oil. WSOCs were characterized by gas chromatography (semivolatile fraction), negative electron spray ionization high resolution mass spectrometry (hydrophilic fraction) and fluorescence spectroscopy. The pattern of semivolatile WSOCs in bio-oil was dominated by aromatic products from lignocellulose, while in biochar was featured by saturated carboxylic acids from hemi/cellulose and lipids with concentrations decreasing with decreasing H/C ratios. Hydrophilic species in poorly carbonized biochar resembled those in bio-oil, but the increasing charring intensity caused a marked reduction in the molecular complexity and degree of aromaticity. Differences in the fluorescence spectra were attributed to the predominance of fulvic acid-like structures in biochar and lignin-like moieties in bio-oil. The divergence between pyrolysis vapors and biochar in the distribution of WSOCs with increasing carbonization was explained by the hydrophobic carbonaceous matrix acting like a filter favoring the release into water of carboxylic and fulvic acid-like components. The formation of these structures was confirmed in biochar produced by pilot plant pyrolysis units. Biochar affected differently shoot and root length of cress seedlings in germination tests highlighting its complex role on plant growth.

Assessment of Two Solid Anaerobic Digestate Soil Amendments for Effects on Soil Quality and Biosolarization Efficacy

Anaerobic digestion is an organic waste bioconversion process that produces biofuel and digestates. Digestates have potential to be applied as soil amendment to improve properties for crop production including phytonutrient content and pest load. Our objective was to assess the impact of solid anaerobic digestates on weed seed inactivation and soil quality upon soil biosolarization (a pest control technique that combines solar heating and amendment-induced microbial activity). Two solid digestates from thermophilic (TD) and mesophilic (MD) digesters were tested. The solarized TD-amended samples presented significantly higher mortality of Brassica nigra (71%, P = 0.032) than its equivalent incubated at room temperature. However, biosolarization with digestate amendment led to decreased weed seed mortality in certain treatments. The plant-available water, total C, and extractable P and K were significantly increased (P < 0.05) in the incubated amended soils. The results confirm the potential of digestates as beneficial soil amendments. Further studies are needed to elucidate the impacts of digestate stability on biosolarization efficacy and soil properties.

Effect of pyrolysis temperature on potential toxicity of biochar if applied to the environment

Biochars have increasingly been used as adsorbents for organic and inorganic contaminants in soils. However, during the carbonization process of pyrolysis, contaminants, including polycyclic aromatic hydrocarbons (PAHs) and polychlorinated dioxins and furans (PCDD/DF) can be generated. In this study, biochars made from sawdust, were prepared at various temperatures ranging from 250 to 700 °C. The Microtox^® and rat hepatoma cell line H4IIE-luc assays were used to characterize the general toxic and effects, mediated through the aryl hydrocarbon receptor (AhR), or dioxin-like potencies of organic extracts of biochars. The greatest total concentrations of PAHs (8.6 × 10² μg kg^-1) and PCDD/DF (6.1 × 10² pg g^-1) were found in biochar generated at 400 °C and 300 °C, respectively. Results of the H4IIE-luc assay, which gives total concentrations of 2,3,7,8-TCDD equivalents (TEQ_H4IIE-luc), indicated that total potencies of aryl hydrocarbon receptor (AhR) agonists were in decreasing order: 300 °C > 250 °C > 400 °C > 500 °C > 700 °C. The 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEQ_chem) calculated as the sum of products of 16 PAHs and 17 PCDD/DF congers multiplied by their respective relative potencies (RePs) was less than that of TEQ_H4IIE-luc determined by use of the bioanalytical method, with the H4IIE-luc assay, which measures the total dioxin-like potency of a mixtures. The ratio of TEQ_chem/TEQ_H4IIE-luc was in the range of 0.7%-3.8%. Thus, a rather small proportion of the AhR-mediated potencies extracted from biochars were identified by instrumental analyses. Results of the Microtox test showed similar tendencies as those of the H4IIE-luc test, and a linear correlation between EC50 of Microtox test and EC20 of H4IIE-luc test was found. The results demonstrated that biochars produced at higher pyrolysis temperatures (>400 °C) were less toxic and had lower potencies of AhR-mediated effects, which may be more suitable for soil application.

Conversion of poultry wastes into energy feedstocks

In this study, conversion of wastes from poultry farming and industry into biochar and bio-oil via thermochemical processes was investigated. Fuel characteristics and chemical structure of biochars and bio-oils have been investigated using standard fuel analysis and spectroscopic methods. Biochars were produced from poultry litter through both hydrothermal carbonization (sub-critical water, 175-250°C) and pyrolysis over a temperature range between 250 and 500°C. In comparison to hydrothermal carbonization, pyrolysis at lower temperatures produced biochar with greater energy yield due to the higher mass yield. Biochars obtained by both processes were comparable to coal. Hydrothermal liquefaction of poultry meal at different temperatures (200-325°C) was conducted and compared to optimize its process conditions. Higher temperatures favored the formation of bio-crude oil, with a maximum yield of 35wt.% at 300°C. The higher heating values of bio-oils showed that bio-oil could be a potential source of synthetic fuels. However, elemental analysis demonstrated the high nitrogen content of bio-oils. Therefore, bio-oils obtained from hydrothermal liquefaction of poultry meal should be upgraded for utilization as a transport and heating fuel.

Thermal treatment and leaching of biochar alleviates plant growth inhibition from mobile organic compounds

Recent meta-analyses of plant responses to biochar boast positive average effects of between 10 and 40%. Plant responses, however, vary greatly across systems, and null or negative biochar effects are increasingly reported. The mechanisms responsible for such responses remain unclear. In a glasshouse experiment we tested the effects of three forestry residue wood biochars, applied at five dosages (0, 5, 10, 20, and 50 t/ha) to a temperate forest drystic cambisol as direct surface applications and as complete soil mixes on the herbaceous pioneers Lolium multiflorum and Trifolium repens. Null and negative effects of biochar on growth were found in most cases. One potential cause for null and negative plant responses to biochar is plant exposure to mobile compounds produced during pyrolysis that leach or evolve following additions of biochars to soil. In a second glasshouse experiment we examined the effects of simple leaching and heating techniques to ameliorate potentially phytotoxic effects of volatile and leachable compounds released from biochar. We used Solid Phase Microextraction (SPME)-gas chromatography-mass spectrometry (GC-MS) to qualitatively describe organic compounds in both biochar (through headspace extraction), and in the water leachates (through direct injection). Convection heating and water leaching of biochar prior to application alleviated growth inhibition. Additionally, growth was inhibited when filtrate from water-leached biochar was applied following germination. SPME-GC-MS detected primarily short-chained carboxylic acids and phenolics in both the leachates and solid chars, with relatively high concentrations of several known phytotoxic compounds including acetic acid, butyric acid, 2,4-di-tert-butylphenol and benzoic acid. We speculate that variable plant responses to phytotoxic organic compounds leached from biochars may largely explain negative plant growth responses and also account for strongly species-specific patterns of plant responses to biochar amendments in short-term experiments.