Carbon dioxide as a carrier gas and mixed feedstock pyrolysis decreased toxicity of sewage sludge biochar

Contrasting environmental impacts of nano-biochar and conventional biochar on various organisms

The environmental hazards of nanobiochar (n-BC) require attention due to limited knowledge. This study is the first to explore the effects of biochar size reduction across various organisms, including bacteria (Allivibrio fischeri), plants (Lemna minor, Lepidium sativum), and invertebrates (Daphnia magna, Folsomia candida). Bulk biochar (b-BC) and n-BC were applied in both liquid and solid-phase tests to assess their ecotoxicity. The resulting leachates were tested at concentrations of 2, 10, and 100 mg/L on organisms such as Daphnia magna and Lemna minor. In the solid-phase tests, b-BC and n-BC were added to the OECD soil at concentrations of 1 % and 5 % to evaluate toxicity in Folsomia candida and at concentrations of 1 % to evaluate toxicity in Lepidium sativum. We found n-BC to be significantly more toxic (by 18 % to 2886 %) to A. fischeri than b-BC, with toxicity increasing over time. Low doses (1 %) of both b-BC and n-BC did not cause mortality or inhibit reproduction in F. candida, though b-BC enhanced reproduction (by 30 % to 56 %) compared to n-BC. At a 5 % dose, both b-BC and n-BC inhibited reproduction F. candida, with n-BC being 0.5 to 1.8 times more toxic. Neither b-BC nor n-BC immobilized D. magna, but both inhibited reproduction (by 28 % to 35 %). The nanoscale dimensions of n-BC facilitated bioaccumulation in D. magna, leading to adhesion on the organism's body. The n-BC had a greater impact on plants, both b-BC and n-BC were non-toxic to L. minor, but all n-BC inhibited root growth in L. sativum. These findings highlight the importance of considering biochar size, feedstock, and pyrolysis conditions when evaluating environmental risks, ensuring safe use in sustainable agriculture.

Increased oxygen content in biochar lowered bioavailability of polycyclic aromatic hydrocarbons-related toxicity to various organisms

Agricultural or environmental application of biochar (BC) is connected with the introduction of biochar-derived components among which polycyclic aromatic hydrocarbons (PAHs) and heavy metals are the most toxic. Their presence and bioavailability are crucial considering biochar toxicity. The effect of feedstock and pyrolysis temperature on the physicochemical properties of produced biochar and contaminant content was established and combined with toxicity to a broad range of living organisms. The obtained data revealed that predicting the bioavailability of PAHs using the total content is misleading. The toxicity was influenced by factors in the following way: the bioavailable PAHs > ash > total PAHs content in BC stressing the role of BC physicochemical characteristics. Among tested BC properties, surface functionalization, e.g. presence of oxygen-containing functional groups was crucial in revealing the toxicity. The data clearly indicate that additional research is required to determine BC's impact on various organisms and performing one ecotoxicity test is not sufficient.

Biochar from Co-Pyrolyzed Municipal Sewage Sludge (MSS): Part 2: Biochar Characterization and Application in the Remediation of Heavy Metal-Contaminated Soils

The disposal of municipal sewage sludge (MSS) from wastewater treatment plants poses a major environmental challenge due to the presence of inorganic and organic pollutants. Co-pyrolysis, in which MSS is thermally decomposed in combination with biomass feedstocks, has proven to be a promising method to immobilize inorganic pollutants, reduce the content of organic pollutants, reduce the toxicity of biochar and improve biochar's physical and chemical properties. This part of the review systematically examines the effects of various co-substrates on the physical and chemical properties of MSS biochar. This review also addresses the effects of the pyrolysis conditions (temperature and mixing ratio) on the content and stability of the emerging pollutants in biochar. Finally, this review summarizes the results of recent studies to provide an overview of the current status of the application of MSS biochar from pyrolysis and co-pyrolysis for the remediation of HM-contaminated soils. This includes consideration of the soil and heavy metal types, experimental conditions, and the efficiency of HM immobilization. This review provides a comprehensive analysis of the potential of MSS biochar for environmental sustainability and offers insights into future research directions for optimizing biochar applications in soil remediation.

Biochar from Co-Pyrolyzed Municipal Sewage Sludge (MSS): Part 1: Evaluating Types of Co-Substrates and Co-Pyrolysis Conditions

With the increasing production of municipal sewage sludge (MSS) worldwide, the development of efficient and sustainable strategies for its management is crucial. Pyrolysis of MSS offers several benefits, including volume reduction, pathogen elimination, and energy recovery through the production of biochar, syngas, and bio-oil. However, the process can be limited by the composition of the MSS, which can affect the quality of the biochar. Co-pyrolysis has emerged as a promising solution for the sustainable management of MSS, reducing the toxicity of biochar and improving its physical and chemical properties to expand its potential applications. This review discusses the status of MSS as a feedstock for biochar production. It describes the types and properties of various co-substrates grouped according to European biochar certification requirements, including those from forestry and wood processing, agriculture, food processing residues, recycling, anaerobic digestion, and other sources. In addition, the review addresses the optimization of co-pyrolysis conditions, including the type of furnace, mixing ratio of MSS and co-substrate, co-pyrolysis temperature, residence time, heating rate, type of inert gas, and flow rate. This overview shows the potential of different biomass types for the upgrading of MSS biochar and provides a basis for research into new co-substrates. This approach not only mitigates the environmental impact of MSS but also contributes to the wider goal of achieving a circular economy in MSS management.

Composting of recovered rock wool from hydroponics for the production of soil amendment

Due to its fibrous structure and high water holding capacity, rock mineral wool (RMW) has boosted the development of hydroponics. Consequently, the amount of waste RMW has also increased tremendously, which has stimulated the research and development of RMW reuse options. In this study, composting and degradability of RMW from hydroponics (gRMW) were tested in combination with different ratios of biowaste compost, including physical and chemical properties of the starting and final materials, and potential ecological hazards of the final product. gRMW had high water holding capacity and low organic matter content, which was easily degradable. Limits of toxic elements according to EU regulation were not exceeded. Degraded gRMW mixtures with compost did not exhibit toxicity to plants or aquatic bacteria and showed intermediate or limited habitat function for earthworms, which preferred the sole gRMW not mixed with compost. Overall, degraded gRMW exhibited parameters of safe soil amendment.

Formation mechanisms and degradation methods of polycyclic aromatic hydrocarbons in biochar: A review

Biochar has been widely used in soil amendment and environmental remediation. Polycyclic aromatic hydrocarbons (PAHs) could be produced in preparation of biochar, which may pose potential risks to the environment and human health. At present, most studies focus on the ecotoxicity potential of biochar, while there are few systematic reviews on the formation mechanisms and mitigation strategies of PAHs in biochar. Therefore, a systematical understanding of the distribution, formation mechanisms, risk assessment, and degradation approaches of PAHs in biochar is highly needed. In this paper, the distribution and content of the total and bioavailable PAHs in biochar are reviewed. Then the formation mechanisms, influencing factors, and potential risk assessment of PAHs in biochar are systematically explored. After that, the effective strategies to alleviate PAHs in biochar are summarized. Finally, suggestions and perspectives for future studies are proposed. This review provides a guide for reducing the formation of biochar-associated PAHs and their toxicity, which is beneficial for the development and large-scale safe use of environmentally friendly biochar.

Biotoxicity attenuation and the underlying physicochemical mechanism of biochar aged under simulated natural environmental conditions

Biochar (BC), with the benefits of enhancing soil fertility, absorbing heavy metals, carbon sequestration, and mitigating the greenhouse effect, has been extensively used for soil remediation. However, the long-term changes in the biotoxicity of BC under complex environmental conditions, which are the key factors influencing the sustainable application of BC in soil, are still unclear. Herein, the biotoxicity of BC aged with various processes, including dry‒wet cycle (DW) aging, freeze‒thaw cycle (FT) aging, ultraviolet irradiation (UV) aging, and low molecular weight organic acid (OA) aging, was systematically investigated by Escherichia coli (E. coli) culture experiments. The toxicity attenuation rate (%·week-1) was proposed to more concisely and clearly compare the influence of different aging methods on BC toxicity. The results indicated that after 5 weeks of aging, the toxicity attenuation rate during the four aging modes followed the order OA aging > FT aging > UV aging > DW aging. BC was nontoxic after 1 week of OA aging, 4 weeks of FT aging, 7 weeks of UV aging, and 14 weeks of DW aging. Spectroscopic characterizations revealed that humic acids in the dissolved organic matter of BC were the main reason for the biotoxicity. In addition, the attenuation of environmentally persistent free radicals on BC during aging was also an important factor for reducing environmental toxicity. This work provides insight into the detoxification mechanism of the BC aging process under ordinary environmental conditions and guidance for the safe application of BC in soil.

Effect of chemical aging on phosphate adsorption and ecotoxicological properties of magnesium-modified biochar

Using magnesium-biochar composites (Mg-BC) in adsorption allows for the efficient and economically relevant removal of phosphate (PO43-) from water and wastewater. Applying Mg-BC for pollutant removal requires evaluating the adsorption capacity of composites and their ecotoxicological properties. Investigating the composite aging during the application of these composites into the soil is also essential. In the present study, nonaged and aged (at 60 or 90 °C) Mg-BC composites were investigated in the context of pyrolysis temperature (500 or 700 °C). All analyzed biochars were examined by Fourier transform infrared spectroscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and surface area. The content of polycyclic aromatic hydrocarbons (PAHs) (bioavailable Cfree and organic solvent-extractable Ctot), heavy metals (HMs), and environmentally persistent free radicals (EPFRs) were determined. Ecotoxicity was evaluated using tests with Folsomia candida and Allivibrio fischeri. The dependence of adsorption on pyrolysis temperature and composite aging time was observed. Changes in physicochemical properties occurring as a result of aging reduced the adsorption of PO43- on Mg-BC composites. It was found that nonaged Mg-BC700 was more effective (9.55 mg g -1) in the adsorption of PO43- than Mg-BC500 (5.75 mg g-1). The adsorption capacities of aged composites were from 21 to 61% lower than those of the nonaged composites. Due to aging, the content of Cfree PAHs increased by 3-5 times depending on the pyrolysis temperature. However, aging reduced the Ctot PAHs in all composites from 24 to 35% depending on the pyrolysis temperature. Ecotoxicological evaluation of Mg-BC composites showed increased toxicity after aging to both organisms. The use of aged BC potentially increases the contaminant content and toxicity of Mg-BC composites.

Effect of zinc-biochar composite aging on its physicochemical and ecotoxicological properties

The stability of Zn-biochar composites is determined by environmental factors, including the aging processes. This paper focused on the ecotoxicological evaluation of Zn-biochar (Zn-BC) composites subjected to chemical aging. Pristine biochars and composites produced at 500 or 700 °C were incubated at 60 and 90 °C for six months. All biochars were characterized in terms of their physicochemical (elemental composition, Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and porous structure), ecotoxicological properties (tested with Folsomia candida and Aliivibrio fischeri) and contaminant content (polycyclic aromatic hydrocarbons (PAH), heavy metals (HM) and environmentally persistent free radicals (EPFR)). An increase in the number of surface oxygen functional groups and increased hydrophilicity and polarity of all Zn-BC composites were observed due to oxidation during aging. It was also found that Zn-BC aging at 90 °C resulted in a 28-30% decrease in solvent-extractable PAHs (Ʃ16 Ctot PAHs) compared to nonaged composites. The aging process at both temperatures also caused a 104 fold reduction in EPFRs in Zn-BC composites produced at 500 °C. The changes in the physicochemical properties of Zn-BC composites after chemical aging at 90 °C (such as pH and HM content) caused an increase in the toxicity of the composites to Folsomia candida (reproduction inhibition from 19 to 24%) and Aliivibrio fischeri (luminescence inhibition from 96 to 99%). The aging of composites for a long time may increase the adverse environmental impact of BC-Zn composites due to changes in physicochemical properties (itself and its interactions with pollutants) and the release of Zn from the composite.

Preparation and application of biochar from co-pyrolysis of different feedstocks for immobilization of heavy metals in contaminated soil

Co-pyrolysis is a potentially effective method for both biomass waste management and multi-functional biochar-based product design. It involves the thermochemical decomposition of biomass waste under anoxic conditions, which can reduce the cost of disposal and produce biochar with beneficial properties. Herein, this study aimed to investigate the properties and environmental applications of biochar from single- and mixed- feedstocks of wheat straw, rice husk, pig manure, and oyster shell at 450 ℃, respectively. A pot experiment with Chinese cabbage was carried out to compare the effects of biochars with limestone on soil Cd and Pb immobilization at two harvest periods. The results indicated that co-pyrolysis of various biomasses exhibited synthetic effects on promoting the calorific value of syngas and enhancing the quality of produced biochar. The pot experiment revealed a significant promotion on soil pH, soil organic matter, cation exchange capacity, and soluble Ca, which consequently reduced Cd and Pb availability. In contrast with limestone treatment, soil amendment with single biomass-derived and co-pyrolysis-derived (COPB) biochars had a significant positive impact on soil fertility and microbial biomass. Application of COPB at a 0.5% dosage consistently and most effectively enhanced the shoot biomass, increased leaf Vitamin C content but reduced leaf content of nitrate and heavy metals in both harvests. Using COPB for soil remediation would be financially visible due to the enhancement of crop yield. Therefore, this study proposes a strategy for targeted enhancement of the functions of biochar derived from co-pyrolysis of selected biomass waste for soil remediation and agricultural production.

Chemical properties of soil determine the persistence and bioavailability of polycyclic aromatic hydrocarbons in sewage sludge- or sewage sludge/biomass-derived biochar-amended soils

In this study the persistence (organic solvent extractable) and bioavailability (freely dissolved) of polycyclic aromatic hydrocarbons (PAHs) in soils with various properties amended with sewage sludge (BCSSL)- or sewage sludge/biomass (BCSSLW)-derived biochars was examined. Biochars produced at 600 °C were applied to soils (acidic, neutral, or alkaline) at a dose of 2% and subsequently incubated for 180 days. Here, the use of biochars regarding the soil's type was examined for the first time. Depending on the soil pH and the feedstock, the content of sum of 16 organic solvent extractable PAHs was found to decrease from 7.5 to 37% (soil + BCSSL) and from 24 to 40% (soil + BCSSLW). The decrease in the content of sum of 16 freely dissolved PAHs ranged from 18 to 36% (soil + BCSSL) and from 17 to 54% (soil + BCSSLW). In acidic BCSSL-amended soil and the alkaline BCSSLW-amended soil no statistically significant differences in the content of sum of 16 freely dissolved PAHs were noted between the beginning and end of the study. BCSSLW was characterized by a greater reduction content of organic solvent extractable PAHs in the acidic and alkaline soils, while in the neutral one - BCSSL. In turn, a larger reduction in freely dissolved PAH content in the acidic and neutral soils could be seen in the presence of BCSSLW, whereas in the alkaline soil in the presence of BCSSL. The persistence and bioavailability of PAHs in the biochar-amended soils were closely related to the chemical properties of these soils. This was confirmed by numerous statistically significant (P ≤ 0.05) relationships between organic solvent extractable PAHs and pH, cation exchange capacity, available magnesium, potassium and phosphorus, and dissolved organic carbon as well as between freely dissolved PAH and pH, dissolved organic carbon, available potassium and phosphorus content, and electrical conductivity.

Ecotoxicological characteristics and properties of zinc-modified biochar produced by different methods

Despite the dynamic progress of BC engineering, there is a lack of knowledge on the toxicity and environmental impact of modified BC. The aim of this study was the ecotoxicological evaluation of BC modified with zinc (Zn) using different methods: impregnation of feedstock with Zn before pyrolysis (PR), impregnation with Zn after pyrolysis (PS) and impregnation with Zn after pyrolysis with an additional calcination step (PST). The ecotoxicological assessment was based on tests with invertebrates (Folsomia candida, Daphnia magna) and bacteria (Aliivibrio fischeri). The post-treated and calcined composites had a higher content of total (C_tot) PAHs (144-276 μg kg^-1) than pre-treated BC-Zn (68-157 μg kg^-1). All BC-Zn treatments stimulated the reproduction of F. candida at the lowest BC dose (0.5%) by 4-24%. Increasing the biochar dose to 1% and 3% retained the stimulating effect of the pre-modified biochars (from 19 to 41%). Pre-modified BC-Zn reduced the luminescence of A. fischeri from 40% to 80%. Post-treated BCs reduced bacterial luminescence by 99%, but the calcination step limited the toxic effects to the level observed for the control. Post-treated BCs had a toxic effect on D. magna, with EC50 values ranging from 433 to 783 mg L^-1. The ecotoxicity of composites depends on modification methods, BC dose and pyrolysis temperature. The application of limiting conditions for HM leaching (i.e., pre-modification, calcination) increased the safety of using Zn-biochar composites.

Sewage sludge derived biochar for environmental improvement: Advances, challenges, and solutions

With the rapid growth yield of global sewage sludge, rational and effective treatment and disposal methods are becoming increasingly needed. Biochar preparation is an attractive option for sewage sludge treatment, the excellent physical and chemical properties of sludge derived biochar make it an attractive option for environmental improvement. Here, the current application state of sludge derived biochar was comprehensively reviewed, and the advances in the mechanism and capacity of sludge biochar in water contaminant removal, soil remediation, and carbon emission reduction were described, with particular attention to the key challenges involved, e.g., possible environmental risks and low efficiency. Several new strategies for overcoming sludge biochar application barriers to realize highly efficient environmental improvement were highlighted, including biochar modification, co-pyrolysis, feedstock selection and pretreatment. The insights offered in this review will facilitate further development of sewage sludge derived biochar, towards addressing the obstacles in its application in environmental improvement and global environmental crisis.

Biochar application for greenhouse gas mitigation, contaminants immobilization and soil fertility enhancement: A state-of-the-art review

Rising global temperature, pollution load, and energy crises are serious problems, recently facing the world. Scientists around the world are ambitious to find eco-friendly and cost-effective routes for resolving these problems. Biochar has emerged as an agent for environmental remediation and has proven to be the effective sorbent to inorganic and organic pollutants in water and soil. Endowed with unique attributes such as porous structure, larger specific surface area (SSA), abundant surface functional groups, better cation exchange capacity (CEC), strong adsorption capacity, high environmental stability, embedded minerals, and micronutrients, biochar is presented as a promising material for environmental management, reduction in greenhouse gases (GHGs) emissions, soil management, and soil fertility enhancement. Therefore, the current review covers the influence of key factors (pyrolysis temperature, retention time, gas flow rate, and reactor design) on the production yield and property of biochar. Furthermore, this review emphasizes the diverse application of biochar such as waste management, construction material, adsorptive removal of petroleum and oil from aqueous media, immobilization of contaminants, carbon sequestration, and their role in climate change mitigation, soil conditioner, along with opportunities and challenges. Finally, this review discusses the evaluation of biochar standardization by different international agencies and their economic perspective.

Effect of carrier gas change during sewage sludge or sewage sludge and willow pyrolysis on ecotoxicity of biochar-amended soil

Different pyrolysis conditions determine the properties of the biochar. The properties of biochar may affect directly or indirectly their influence on living organisms. The aim of this study was to determine the toxicity of biochar obtained under different conditions (temperature: 500 or 700 °C, carrier gas: N₂ or CO₂, feedstock: sewage sludge or sewage sludge/biomass mixture) after adding to the soil in long-term pot experiment (180 days). Biochars were added to the podzolic loamy sand at a 2% (w/w) dose. Samples were collected at the beginning of the experiment and after 30, 90 and 180 days. The bacteria Aliivibrio fischeri (luminescence inhibition - Microtox), the plant Lepidium sativum (root growth and germination inhibition test - Phytotoxkit F), and the invertebrate Folsomia candida (mortality and reproduction inhibition test - Collembolan test) were used as the test organisms. In the long-term perspective for most tests, changing the carrier gas from N₂ to CO₂ resulted in reduced toxicity of the biochar. A particularly beneficial effect of changing the gas to CO₂ was observed for the solid-phase test with L. sativum. The CO₂ during pyrolysis had the least beneficial effect on toxicity towards A. fischeri.

Building a risk matrix for the safety assessment of wood derived biochars

Biochar is recognized as an efficient amendment and soil improver. However, environmental and quality assessments are needed to ensure the sustainability of its use in agriculture. This work considers the biochar's chemical-physical characterization and its potential phyto- and geno-toxicity, assessed with germination and Ames tests, obtaining valuable information for a safe field application. Three biochar types, obtained from gasification at different temperatures of green biomasses from the Tuscan-Emilian Apennines (in Italy), were compared through a broad chemical, physical and biological evaluation. The results obtained showed the relevance of temperature in determining the chemical and morphological properties of biochar, which was shown with several analytical techniques such as the elemental composition, water holding capacity, ash content, but also with FTIR and X-ray spectroscopies. These techniques showed the presence of different relevant surface aliphatic and aromatic groups. The procedures for evaluating the potential toxicity using seeds germination and Ames genotoxicity assay highlights that biochar does not cause detrimental effects when it enters in contact with soil, micro- and macro-organisms, and plants. The genotoxicity test provided a new highlight in evaluating biochar environmental safety.

Ecotoxicity of sewage sludge- or sewage sludge/willow-derived biochar-amended soil

Co-pyrolysis of sewage sludge (SL) with plant biomass gains attention as a way to minimize SL-derived biochar drawbacks, such as high amount of toxic substances, low specific surface area and carbon content. The toxicity of soil amended with SL- (BCSL) or SL/biomass (BCSLW)-derived biochar was evaluated in long-term pot experiment (180 days). The results were compared to SL-amended soil. Biochars produced at 500, 600, or 700 °C were added to the soil (podzolic loamy sand) at a 2% (w/w) dose. Samples were collected at four different time points (at the beginning, after 30, 90 and 180 days) to assess the potential toxicity of SL-, BCSL- or BCSLW-amended soil. The bacteria Aliivibrio fischeri (luminescence inhibition - Microtox), the plant Lepidium sativum (root growth and germination inhibition test - Phytotoxkit F), and the invertebrate Folsomia candida (mortality and reproduction inhibition test - Collembolan test) were used as the test organisms. Depending on the organism tested and the sample collection time point variable results were observed. In general, SL-amended soil was more toxic than soil with biochars. The leachates from BCSLW-amended soil were more toxic to A. fischeri than leachate from BCSL-amended soil. A different tendency was observed in the case of phytotoxicity. Leachate from BCSL-amended soil was more toxic to L. sativum compared to BCSLW-amended soil. The effect of biochars on F. candida was very diversified, which did not allow a clear trend to be observed. The toxic effect of SL-, BCSL- or BCSW-amended soil to particular organisms was observed in different time, point's periods, which may suggest the different factors affecting this toxicity.

Suppression of polycyclic aromatic hydrocarbon formation during pyrolytic production of lignin-based biochar via nitrogen and boron co-doping

Polycyclic aromatic hydrocarbons are toxic byproducts of biochar production. The effects of pyrolysis atmosphere (i.e., N₂ and CO₂) and temperature (i.e., 300-900 °C) and element doping (i.e., N, B, O, and S) on the production of sixteen high priority polycyclic aromatic hydrocarbons in lignin-based biochar was investigated. N₂ atmosphere at 300 °C produced the highest total polycyclic aromatic hydrocarbon content (1698 ± 50 ng/g). Polycyclic aromatic hydrocarbon formation decreased with increase in temperature (31 ± 15 ng/g at 900 °C). CO₂ atmosphere significantly decreased yield of polycyclic aromatic hydrocarbons. The effects of heteroatom doping on polycyclic aromatic hydrocarbon formation were investigated for the first time in the pyrolysis synthesis of lignin-based biochar. N-, B-, O, N-B-, and N-S-doping of biochar reduced polycyclic aromatic hydrocarbon formation by 90, 85, 87, 97, and 89%, respectively. Results bring new insights into the role of heteroatom-doping and pyrolysis conditions in controlling polycyclic aromatic hydrocarbon formation in biochars.

Insights into CO2 adsorption on KOH-activated biochars derived from the mixed sewage sludge and pine sawdust

The environment issues associated with global warming and climate change caused by continuous increase in greenhouse gas emissions have attracted worldwide concerns. As renewable resources with good adsorption property, biochar is an efficient and environmental friendly adsorbsent for CO₂ capture. In this study, the CO₂ adsorption behavior of biochars derived from feedstock mixtures of 70% pine sawdust and 30% sewage sludge by KOH modification was investigated. The textual properties and functional groups of the pristine biochars have been significantly enhanced after KOH activation. With highly developed microporosity, the specific surface area (SSA) of the KOH-modified biochars increased by 3.9-14.5 times. Furthermore, higher CO₂ adsorption capacities of 136.7-182.0 mg/g were observed for the modified biochars, compared to pristine ones (35.5-42.9 mg/g). The development of micropores by KOH activation significantly increased the CO₂ adsorption capacity. Meanwhile, the presence of hetero atoms (O and K) also positively influenced CO₂ adsorption capacity of biochar. Noticeably, both physical and chemical adsorption played a crucial role in CO₂ capture, which was verified by different characterization methods including high resolution scanning electron microscope, X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The Findings of this study demonstrate the -significance of chemical sorption by identifying the transformation of CO₂ by biochar composites and in situ characterization of weakly adsorbed and newly formed mineral species during the CO₂ sorption process. Moreover, BC700K showed 97% recyclability during 10 consecutive adsorption-desorption cycles at 25 °C, 1 bar. The results obtained in the present study may inspire new research interest and provide a comprehensive insight into the research subject to biochars derived from feedstock mixtures for CO₂ capture.

Biochar reduces the toxicity of silver to barley (Hordeum vulgare) and springtails (Folsomia candida) in a natural soil

The use of biochar in soil remediation is a promising method to deal with metal contamination. In the present study, the influence of biochar amendment on the toxicity of silver (as AgNO₃) to terrestrial organisms was assessed. For this, toxicity tests were conducted with terrestrial plant barley (Hordeum vulgare) and invertebrate springtails (Folsomia candida) in the standard natural Lufa soil amended or not with a wood-derived biochar at 5% (w/w). Biochar addition increased root length and mass in barley, compared to unamended soil. However, the effects of Ag on barley growth were masked by a great variation among replicates in biochar-amended soil. Photosynthetic pigment contents (total chlorophyll and carotenoids) were lower in plants exposed to Ag in Lufa soil, but not in biochar-amended soil. Moreover, Ag drastically decreased dehydrogenase activity in Lufa soil. For springtails, the addition of biochar clearly decreased the toxicity of Ag. The LC50 was 320 mg Ag/kg in Lufa soil, while no mortality was observed up to 500 mg Ag/kg in biochar-amended soil. The EC50 for effects on reproduction was significantly higher in biochar-amended soil compared to unamended Lufa soil (315 and 215 mg Ag/kg, respectively). The wood-derived biochar used in this study has shown a potential for remediation of contaminated soils, as a decrease in Ag toxicity was observed in most endpoints analysed in barley and springtails.

Characteristics of Biochars Derived from the Pyrolysis and Co-Pyrolysis of Rubberwood Sawdust and Sewage Sludge for Further Applications

This study investigated the characteristics of biochars derived from the pyrolysis of rubberwood sawdust (RWS) and sewage sludge (SS) and their co-pyrolysis at mixing ratios of 50:50 and 75:25. Biochars were produced at 550 °C through slow pyrolysis in a moving bed reactor and then characterized. Results showed that the rubberwood sawdust biochar (RWSB) had high carbon content (86.70 wt%) and low oxygen content (7.89 wt%). By contrast, the sewage sludge biochar (SSB) had high ash content (65.61 wt%) and low carbon content (24.27 wt%). The blending of RWS with SS at the mentioned ratios helped enhance the gross and element contents of the biochar samples. The elemental analysis of the biochars was also reported in the form of atomic ratios (H/C and O/C). The functional groups of biochars were observed by Fourier-transform infrared spectroscopy (FTIR). X-ray fluorescence spectroscopy (XRF) revealed that the biochar from SS contained a high content of inorganic elements, such as Si, Ca, Fe, K, Mg, P, and Zn. The pH of the biochars ranged from 8.41 to 10.02. Brunauer, Emmett, and Teller (BET) and scanning electron microscopy (SEM) showed that RWSB had a lower surface area and larger pore diameter than the other biochars. The water holding capacity (WHC) and water releasing ability (WRA) of the biochars were in the range of 1.01–3.08 mL/g and 1.19–52.42 wt%, respectively. These results will be the guideline for further application and study of biochar from RWS, SS, and blended samples.

The Negative Effects of High Rates of Biochar on Violas Can Be Counteracted with Fertilizer

Increasing costs and environmental issues regarding excessive use of peat moss is impacting the horticultural industry. Biochar is a valuable substrate additive that has the potential to reduce the use of peatmoss in greenhouse production. However, its varying effects on ornamentals requires that individual species and cultivars of crops must be evaluated to determine the threshold for benefits. Viola cornuta is a high value ornamental crop; however, information on how different rates of biochar rates affect productivity and physiology of Viola cultivars in container production is not known. To determine if biochar rates could increase the productivity of Viola, we mixed a peat-based substrate with 10, 25, and 50% (w:w) hardwood biochar in two studies on four cultivars. Without fertilizers, 10 and 25% biochar improved plant biomass, growth, root length, and flowering, but 50% biochar was found to have negative effects on plant growth and flowering. Cultivars varied in their response to biochar rates. When fertilizer was applied in the second experiment, biochar rates did not impact growth parameters or flowering. These results suggest that up to 25% biochar can be used in Viola production without detrimental impacts. However, 50% biochar can be used with the addition of fertilizer without negatively affecting plant growth. Biochar can have a short-term impact on the growth characteristics of Viola plants in container production, but fertilization and growing period of Viola may influence these effects. These results indicate that biochar could be the suitable replacement for peat moss, with up to 50% biochar rate in Viola production reducing the environmental and economic burden for peat moss.

Effect of temperature on PTEs deportment and ecological risks of the biochars obtained from sewage sludge

Sewage sludge-derived biochars (SSBCs) were obtained at temperatures of 300, 500, and 700 °C to investigate the potentially toxic elements (PTEs) behaviors and assess the environmental acceptability for the possible application in the environment. Results indicated that PTEs exhibited diversely in the distribution of chemical speciation, while all elements tended to be immobilized in biochar matrix and the total amount elevated during the pyrolysis. The risk assessment of biochars implied a low degree of environmental risk for the utilization of SSBCs prepared at high temperatures. In addition, higher pyrolysis temperature alleviated the inhibition on the early seedling growth of Triticum aestivum L., with root elongation more sensitive to the biochar addition. PTEs, especially Cr, contributed much to the phytotoxicity of biochars as revealed by the principle component analysis (PCA) and leaner correlation analysis. Findings from this work illustrated that SSBCs prepared at higher temperatures might be more conductive to a wide range of applications with acceptable environmental risk.

Valorization of rubberwood sawdust and sewage sludge by pyrolysis and co-pyrolysis using agitated bed reactor for producing biofuel or value-added products

This study investigated experimentally pyrolysis of rubberwood sawdust (RWS), sewage sludge (SS), and their blends (25:75, 50:50, and 75:25 by weight) in an agitated bed pyrolysis reactor. The yields and characteristics of liquid product and biochar were determined for pyrolysis at 450, 500, and 550 °C and were affected both by temperature and feedstock type. The liquid and biochar yields were in the ranges 27.30-52.42 and 21.43-49.66 (wt%). Pyrolysis of RWS at 550 °C provided the highest liquid yield, while SS gave a high biochar yield. Co-pyrolysis of SS with RWS improved yield and quality of liquid and biochar products. The liquid product had 57.54-70.70 wt% of water and a low hydrocarbon content. The higher heating value (HHV) of water-free liquid product was 14.73-22.45 MJ/kg. The major compounds of liquid product included acetic acid, 2-propanone, 1-hydroxy, and phenols according to GC-MS. The biochar from RWS had a high carbon content (83.37 wt%) and a high HHV (33.57 MJ/kg), while SS biochar was mainly ash (67.62 wt%) with low carbon content. The SS biochar also had high contents of Si, Ca, Fe, K, and Mg as determined by XRF. Co-pyrolysis of SS with RWS improved the biochar by increasing its carbon content and reducing ash and inorganic elements. The surface of RWS biochar was more porous, while SS biochar had the larger specific surface according to SEM and BET. Based on these results, co-pyrolysis of 75:25 feedstock mix is recommended for further studies on applications of liquid product and biochar.

Sustainable remediation of hazardous environmental pollutants using biochar-based nanohybrid materials

Biochar is a well-known carbon material with diversified functionalities and excellent physicochemical characteristics with high wastewater treatment potential. This review aims to summarize recent advancements in the development of biochar and biochar-based nanohybrid materials as a potential tool for the removal of harmful organic compounds such as synthetic dyes/effluents. The formation of biochar using pyrolysis of renewable feedstocks and their applications in various industries are explained hereafter. The characteristics and construction of biochar-based hybrid materials are explained in detail. Diversity of feedstocks, including municipal wastes, industrial byproducts, agricultural, and forestry residues, endows different biochar types with a wide structural variety. The production of cost-effective biochar drives the interest in manipulating biochars and induces desire functionality using nanoscale reinforcements. Various types of biochars, such as magnetic biochar, layered nanomaterial coated biochar, nanometallic oxide composites, chemically and physically functionalized biochar, have been produced. With the aid of nanomaterial, hybrid biochar exhibits a high potential to remove toxic contaminants. Depending upon biochar type, dyes/effluents can be removed via different mechanisms, including the Fenton process, photocatalytic degradation, π-π interaction, electrostatic interaction, and physical adsorption. In conclusion, desired physicochemical features, and tunable surface properties of biochar present high potential material in removing organic dyes and other effluents. The blended biochar with different materials/nanomaterials endows broader development and multi-functional opportunities for treating dyes/effluents.

Preparation, characterization and agri applications of biochar produced by pyrolysis of sewage sludge at different temperatures

Sewage sludge (SS) is an abundantly available feedstock, which is generally considered as potential threat to human health and environment. Its utilization in any process would be of great help for environmental sustainability. Accordingly, this work aimed to prepare and characterize the sewage sludge biochar (SSB) at temperatures, i.e. (500, 450, 400, and 350 °C), and further analyze the available nutrients and contaminants as well as agri application potential. The results indicated that the total nitrogen (TN), electrical conductivity (EC), and total organic carbon (TOC) content in SSBs decreased with increasing pyrolysis temperature. The overall concentration of polycyclic aromatic hydrocarbons (PAHs) in SSBs was substantially lower (1.8-9.7-fold depending on pyrolysis temperature) than in SS. Pyrolysis of SS enriched the heavy metals content in SSBs and the relative enrichment factor (RE) factor varied between 1.1 and 2.1 depending on the pyrolysis temperature. Furthermore, compared to SS, the leaching rate of heavy metals was significantly decreased in SSBs (1.1-100-fold depending on the pyrolysis temperature) and the pyrolysis temperature of 400-450 °C prevented the Ni, Pb, Cr, and Zn leaching in SSB. The total PAH and heavy metals content in biochars were below the control standard for land application. Finally, testing of the growth-promoting effect of biochar extracts on fenugreek plants revealed that SSB prepared at 350 °C significantly stimulated the root and shoot length of 5-days old seedlings. This study provides important data for potential environmental risks of SSB applications.

The inhibiting effects of biochar-derived organic materials on rice production

The effects of PBC and HBC on rice production, NUE and corresponding mechanisms were examined. Six treatments, P05, P30, H05, H30 (P: PBC; H: HBC; 05 and 30 represented the application rate of 0.5 and 3.0% w/w), CKU (urea application without char) and CK (no application of char and urea), were set up. Results showed that P05, P30 and H05 increased grain yield by 1.8-7.3% (P > 0.05), whereas H30 reduced grain yield by 60.4% (P < 0.05), compared to CKU. Meanwhile, HI under P05, P30 and H05 increased by 3.4-3.6%, while H30 decreased by 9.1% (P < 0.05). NUE and NAE showed similar trends with rice yield. By investigation, the excessive introduction of BDOM plays a crucial role in the reduction of rice production and NUE under higher HBC application. GC-MS/MS analysis showed that the soluble BDOM of HBC and PBC was quite different, and compounds such as 2,6-dimethoxyphenol might stress rice growth. ESI-FT-ICR-MS analysis showed that the BDOM of HBC contained a certain quantity of aromatic compounds, which may also stress rice growth. Overall, HBC pretreatment should be conducted, and the application rate should be strictly controlled before its agricultural application.

Lindane degradation in wet-dry cycling soil as affected by aging and microbial toxicity of biochar

This study determined the degradation of lindane in soil amended with biochar to evaluate the effects of biochar aging and microbial toxicity. Two biochars were prepared at 400 and 600 °C (BC₄₀₀ and BC₆₀₀) and subjected to acid washing to remove nutrition (WBC₄₀₀ and WBC₆₀₀). After 89 days of incubation under the alternate "wet-dry" conditions, scanning electron microscopy showed that acid washing rendered biochars especially susceptible to aging with structural collapse and fragmentation, with less surface covering. Aging impeded the release of toxic substances in BC₄₀₀ and BC₆₀₀ with reduced toxicity to degrading microorganisms. Lindane degradation was somewhat stimulated by biochar nutrition but mainly inhibited by adsorption. Acid washing facilitated the release of toxic substances and additionally reduced lindane degradation. The variations in fatty acid saturation degree (SFA/UFA) in soils confirmed the microbial toxicity of 5% WBC₄₀₀ > 5% BC₄₀₀ > 5% BC₆₀₀ > 5% WBC₆₀₀. High-throughput DNA sequencing showed that biochar delayed the formation of dominant degrading microbial communities in soil. Lindane degradation was completed by joint Sphingomonas, Flaviolibacter, Parasegetibacter, Azoarcus, Bacillus and Anaerolinaea. These findings are helpful for better understanding the effect of biochar in soil on long-term degradation of persistent organic pollutants.

Polycyclic aromatic hydrocarbons (PAHs) persistence, bioavailability and toxicity in sewage sludge- or sewage sludge-derived biochar-amended soil

Soils can be contaminated with polycyclic aromatic hydrocarbons (PAHs) when either sewage sludge (SSL) or biochar (BC) are used. There are no comparative studies regarding the effects of soil amendment with SSL or BC on the persistence, bioavailability and toxicity of PAHs. This research compared the persistence of PAHs (based on the extractable content, C_tot) and their bioavailability (freely dissolved, C_free) as well as the toxicity (solid phase: Phytotoxkit F with Lepidium sativum and the Collembolan test with Folsomia candida; leachates: Phytotestkit F with L. sativum and Microtox® with Aliivibrio fischeri) of soil amended with SSL or with SSL-derived BCs. BCs were produced from three different sewage sludges at a temperature of 500 °C. SSLs or BCs were added to the soil at a rate of 1% (30 t/ha). Adding SSL to the soil increased more the PAH content in it than after BC application, which was associated with a higher content of PAHs in SSL. Losses of Σ16 C_tot and C_free PAHs were higher than those observed for biochar only in the case of one SSL. In the other cases, PAH losses were either higher for biochar or did not differ significantly between SSL and BC. On the other hand, the analysis of the individual groups of PAHs showed significant differences between SSL and BC, both for C_tot and C_free. Nonetheless, these differences were largely driven by the type of sewage sludge and biochar. Only in the case of root growth inhibition the toxicity higher was for the SSL-amended soils than for the BC-amended ones. In the other cases, varying results were observed which were determined by the type of sewage sludge/biochar, similarly to PAH losses.

Co-pyrolysis of sewage sludge and biomass in carbon dioxide as a carrier gas affects the total and leachable metals in biochars

The total and available (water extracted) content of selected metal were determined in co-pyrolyzed SSL with willow (8:2, 6:2 w/w) at 500, 600, and 700 °C using two different carrier gases (N₂ or CO₂). The study investigated the relationship of metal content and bioavailability to toxicity of biochars towards bacteria (Vibrio fischeri), plants (Lepidium sativum), and arthropods (Folsomia candida). For the biochar produced at a ratio of 6:4 (SSL:willow), the content of most metals significant decrease (P ≤ 0.05) from 5.9% to 28.9%. Co-pyrolysis of SSL with willow decreased water extractable metal concentrations (Ba, Cd, Cu, Fe, Mn, Ni, and Zn) from 8.0% to 100%. The CO₂ resulted from 6 to 200 % increase (P ≤ 0.05) of metal content relative to the biochar pyrolyzed in N₂. An increase in pyrolysis temperature caused a higher concentration of the metals in the biochars. The available content of most metals decreased from 9 to 100 %. The adverse effect of these biochars on living organisms was reduced due to a stronger binding of the metals (especially Cu and Cd) with the biochar matrix. The negative impact of Cd, Cr, and Cu on living organisms was also confirmed by principal component analysis (PCA).

A critical review of the production and advanced utilization of biochar via selective pyrolysis of lignocellulosic biomass

Biochar is a carbon-rich product obtained from the thermo-chemical conversion of biomass. Studying the evolution properties of biochar by in-situ modification or post-modification is of great significance for improving the utilisation value of lignocellulosic biomass. In this paper, the production methods of biochar are reviewed. The effects of the biomass feedstock characteristics, production processes, reaction conditions (temperature, heating rate, etc.) as well as in-situ activation, heteroatomic doping, and functional group modification on the physical and chemical properties of biochar are compared. Based on its unique physicochemical properties, recent research advances with respect to the use of biochar in pollutant adsorbents, catalysts, and energy storage are reviewed. The relationship between biochar structure and its application are also revealed. It is suggested that a more effective control of biochar structure and its corresponding properties should be further investigated to develop a variety of biochar for targeted applications.