Assessing the effect of pyrolysis temperature on the molecular properties and copper sorption capacity of a halophyte biochar

Impact of Pyrolysis Temperature on the Physical and Chemical Properties of Non-Modified Biochar Produced from Banana Leaves: A Case Study on Ammonium Ion Adsorption

Given the current importance of using biochar for water treatment, it is important to study the physical-chemical properties to predict the behavior of the biochar adsorbent in contact with adsorbates. In the present research, the physical and chemical characteristics of three types of biochar derived from banana leaves were investigated, which is a poorly studied raw material and is considered an agricultural waste in some Latin American, Asian, and African countries. The characterization of non-modified biochar samples pyrolyzed at 300, 400, and 500 °C was carried out through pH, scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and specific surface area measurements. The adsorption properties of banana leaf-derived biochar were evaluated by ammonium ion adsorption experiments. The results demonstrated that the pyrolysis temperature has a large impact on the yield, structure, elemental composition, and surface chemistry of the biochar. Biochar prepared at 300 °C is the most efficient for NH4+ adsorption, achieving a capacity of 7.0 mg of adsorbed NH4+ on each gram of biochar used, while biochar samples prepared at 400 and 500 °C show lower values of 6.1 and 5.6 mg/g, respectively. The Harkins-Jura isotherm model fits the experimental data best for all biochar samples, demonstrating that multilayer adsorption occurs on our biochar.

The immobilization of cadmium by rape straw derived biochar in alkaline conditions: Sorption isotherm, molecular binding mechanism, and in-situ remediation of Cd-contaminated soil

The issue of cadmium (Cd) contamination in alkaline soils is escalating, necessitating the prompt implementation of effective passivation strategies. Biochar has gained significant attention for its potential in immobilizing heavy metals; however, the suitability of biochar as a remediation material and its micro-scale interaction mechanisms with Cd under alkaline conditions remain unclear. Rape straw (RS) were pyrolyzed at 400 °C (RB400) and 700 °C (RB700) to produce biochar. Adsorption and soil incubation experiments were carried out to assess the feasibility of using rape straw derived biochar pyrolyze at different temperatures and understanding their remediation mechanisms in alkaline environments. The sorption capacity for Cd immobilization was evaluated using sorption isotherms, revealing that RB700 exhibited enhanced Cd sorption performance with a maximum sorption capacity of 119.33 mg g-1 calculated from the Langmuir isotherm equation at pH 8. Cd L3-edge X-ray absorption near-edge structure (XANES) spectroscopy analysis confirmed that the dominant sorption species of Cd were organic Cd in RB400, with CdCO3 precipitation increased to 73.9% in RB700. Solid-state 13C nuclear magnetic resonance (13C-NMR) spectroscopy demonstrated that aromatic and carboxyl C functional groups are involved in the organic sorption of Cd through complexation and Cd2+-π interactions in alkaline solutions. The precipitation of CdCO3 in RB700 may resulted in a more effective passivation effect compared to RB400, leading to a significant 15.54% reduction in the DTPA-Cd content in Cd-contaminated soil. These findings highlight the effective Cd passivation Cd in alkaline environments by rape straw derived biochar, providing new molecular insights into the Cd retention mechanism of biochar. Furthermore, it presents novel ideas for improving remediation approaches for alkaline Cd-contaminated soils.

Remediation effect of walnut shell biochar on Cu and Pb co-contaminated soils in different utilization types

Biochar, with its dual roles of soil remediation and carbon sequestration, is gradually demonstrating great potential for sustainability in agricultural and ecological aspects. In this study, a porous biochar derived from walnut shell wastes was prepared via a facile pyrolysis coupling with in-situ alkali etching method. An incubation study was conducted to investigate its performance in stabilizing copper (Cu) and lead (Pb) co-contaminated soils under different utilization types. The biochar effectively decreased the bioavailable Cu (8.5-91.68%) and Pb (5.03-88.54%), while increasing the pH, CEC, and SOM contents in both soils. Additionally, the results of sequential extraction confirmed that biochar promoted the transformation of the labile fraction of Cu and Pb to stable fractions. The mechanisms of Cu and Pb stabilization were found to be greatly dependent on the soil types. For tea plantation yellow soil, the main approach for stabilization was the complexation of heavy metals with abundant organic functional groups and deprotonation structure. Surface electrostatic adsorption and cation exchange contributed to the immobilization of Cu and Pb in vegetable-cultivated purple soil. This research provides valuable information for the stabilization of Cu and Pb co-contaminated soils for different utilization types using environmentally-friendly biochar.

Camel Dung-Derived Biochar for the Removal of Copper(II) and Chromium(III) Ions from Aqueous Solutions: Adsorption and Kinetics Studies

This study explores an innovative approach to tackle the critical issue of heavy metal ion contamination in aqueous solutions through the utilization of camel dung-derived biochar. In the context of global environmental concerns and the adverse impacts of heavy metal pollution on ecosystems and human health, the investigation focuses on copper(II) and chromium(III) ions, which are among the most pervasive pollutants originating from industrial activities. The research revealed that camel dung-derived biochar exhibits exceptional potential for the removal of copper(II) and chromium(III) ions, with removal efficiencies of more than 90% and adsorption capacities of 23.20 and 23.36 mg/g, respectively. The adsorption processes followed second-order kinetics, and the data fitted both the Langmuir and Freundlich adsorption models. The underlying mechanisms governing this adsorption phenomenon seem to be grounded in complexation reactions, cation exchange, and cation-π interactions, underscoring the multifaceted nature of the interactions between the biochar and heavy metal ions. This research not only advances our understanding of sustainable materials for water purification but also harnesses the underutilized potential of camel dung as a valuable resource for environmental remediation, offering a promising avenue for addressing global water pollution challenges.

Pb(ΙΙ), Cd(ΙΙ), and Mn(ΙΙ) adsorption onto pruning-derived biochar: physicochemical characterization, modeling and application in real landfill leachate

The aim of this study was to systemically evaluate how different pyrolysis temperatures (400, 550, and 700 °C) and particle sizes (1-2 mm and 63-75 µm) were influenced biochar evolution, made from urban pruning waste, during pyrolysis process and to establish their relationships with biochar potential for removal of lead (Pb), cadmium (Cd), and manganese (Mn) from real municipal solid waste landfill leachate. The effects of pH (2-7), contact time (30-300 min) and adsorbent dosage (0.1-5 g L-1) on heavy metals removal were also examined. The results showed that physicochemical properties of biochar were greatly influenced by pyrolysis temperature. Particle size, however, showed little influence on biochar characteristics (p > 0.05). The yield, volatile matter, hydrogen and oxygen contents, and surface functional groups decreased consistently with increasing pyrolysis temperature. An increase in the pH, electrical conductivity, ash, fixed carbon, and specific surface area values was also found. In biochar samples formed at high temperatures (i.e., 550 and 700 °C), Fourier transform infrared spectroscopy-FTIR studies confirmed the increase in aromaticity. Field emission scanning electron microscopy-FESEM images showed differences in the microporous structure and lower size pores at higher temperatures. Biochar pyrolyzed at 700 °C with a particle size of 63-75 µm (i.e., Lv700-63) showed the highest removal efficiency performance. Pb and Cd ions were completely removed (100%) by 0.2 g L-1 Lv700-63 at 7.0 pH and contact times of 120 and 90 min, respectively. The maximum percentage removal of Mn was 86.20% at optimum conditions of 0.2 g L-1 Lv700-63 dosage, 7.0 pH, and 180 min contact time. The findings suggests that the surface complexation, π-electron coordination, and cation exchange were the dominant mechanisms for the Pb, Cd, and Mn removal onto Lv700-63.

Application of biochar on soil bioelectrochemical remediation: behind roles, progress, and potential

Bioelectrochemical systems (BESs) that combine electrochemistry with biological methods have gained attention in the remediation of polluted environments, including wastewater, sludge, sediments, and soils. The most attractive advantage of BESs is that the solid electrode is used as an inexhaustible electron acceptor or donor, and biocurrent directly converted from organics can afford the reaction energy of contaminant breakdown, crossing the internal energy barrier of endothermic degradation, which achieves a continuous biodegradation process without the simultaneous use of exogenetic chemicals and bioelectricity recovery. However, soil BESs are hindered by expensive electrode materials, difficult pollutant and electron transfer, low microbial competitive activity, and biocompatibility in contamination remediation. Fortunately, introducing biochar into soil BESs could reveal a high potential in addressing these BES inadequacies. The characteristics of biochar, e.g., conductivity, transferability, high specific surface area, high porosity, large functional groups, and biocompatibility, can improve the performance of soil BESs. In fact, biochar not only carries electrons but also transfers nutrients, pollutants, and even bacteria by facilitating transmission in the bioelectric field of BESs. Consequently, the abilities of biochar make for better functionality of BESs. This review collates information on the roles, application, and progress of biochar in soil BESs, and future prospects are given. It is beneficial for environmental researchers and engineers to extend BES application in environmental remediation and to assist the progress of carbon sequestration and emission reduction based on the inertia of biochar and the blocking of electron flow to form methane.

Qualitative and quantitative analysis for Cd2+ removal mechanisms by biochar composites from co-pyrolysis of corn straw and fly ash

Removal of heavy metals (e.g., Cd) from contaminated water using waste-converted adsorbents is promising, but the efficiency still needs to be improved. Here, we prepared a functional biochar composite as novel Cd adsorbents by co-pyrolysis of two typical solid wastes, i.e., agricultural corn straw and industrial fly ash. The adsorption behavior and mechanism were investigated using batch and column adsorption experiments and modern characterization techniques. Results showed that alkali-modified fly ash (AMFA) was loaded onto the surface of the corn straw biochar as some fine particle forms, with quartz (SiO₂) and silicate being the main mineral phases on the surface. The maximum sorption capacity fitted by Langmuir model for functionalized biochar composite (FBC700) was up to 137.1 mg g^-1, which was 7.7 times higher than that of the original corn straw biochar (BC700). Spectroscopic analysis revealed that adsorption mechanisms of Cd onto the FBC700 included mainly precipitation and ion exchange, with complexation and Cd-π interaction also contributing. The AMFA could effectively improve the mineral precipitation with Cd. The adsorption columns filled with FBC700 exhibited a longer breakthrough time than that filled with BC700. The adsorption capacity calculated by Thomas model for FBC700 was also approximately 6.0 times higher than that for BC700, showing that FBC700 was more suited to practical applications. This study provided a novel perspective for recycling solid wastes and treating Cd-contaminated water.

Systematic optimization of biochars derived from corn wastes, pineapple leaf, and sugarcane bagasse for Cu(II) adsorption through response surface methodology

Many industrial wastewaters contain an appreciable amount of toxic copper (Cu(II)) that needs to be properly treated before discharging into receiving water body. Adsorption can effectively remove Cu(II) with optimized parameters. This study investigates the critical pyrolysis parameters of biochar derived from agricultural waste. Optimized biochar showed maximum Cu(II) adsorption capacity of 60.7, 36.8, and 35.5 mg g^-1 by PLB, SBB, and CWB at pyrolysis temperatures of 555 ℃, 559 ℃, 507 ℃, respectively, compared with commercial activated carbon (CAC, 40.8 mg g^-1). Surface characterization confirmed surface complexation, electrostatic interaction, and cation exchange capacity as Cu(II) removal mechanisms. The presence of humic acid reduced the Cu(II) removal of both CAC and optimized biochars. Optimized PLB displayed high reusability (87% Cu(II) removal efficiency) after five consecutive cycles using pressure cooker regeneration. With excellent Cu(II) adsorption capacity and reusability, the investigated biochars show high applicability potential to Cu(II)-laden wastewater treatment.

Catalytic modification of corn straw facilitates the remediation of Cd contaminated water and soil

Turning postharvest residue into high-value-added products is crucial for agricultural waste management and environmental remediation. In this proof-of-concept study, nanosized Pt/TiO₂ was used as a model catalyst to modify corn straw (CS) materials through a simple low-temperature oxidation process. This method was demonstrated to be self-sustainable, waste-free, and with high yields. At an optimal temperature of 220 °C, O₂ treatment with 1 wt% Pt/TiO₂ greatly changed ultra-micropore and mesopore structures, dissolved organic carbon, aromatic contents and surface oxygen (O)-containing functional groups in CS products. This treatment resulted in an approximately 5-fold increase of cadmium (Cd) adsorption from aqueous solution and immobilization rate of 43.1% at 7d for bioavailable Cd in soil. Spectroscopic and linear regression analysis demonstrated that both acidic and basic functional groups in CS contributed to Cd adsorption, suggesting chemical adsorption. According to the d-band theory, the unexpected role of catalysts in CS modification could be associated with dissociative adsorption of molecular O₂ on the Pt surface. These results provide insights for the development of economic and sustainable technologies to reutilize agricultural waste biomass for water and soil remediation.

Bamboo for producing charcoal and biochar for versatile applications

Bamboo, the fastest-growing plant, has several unique characteristics that make it appropriate for diverse applications. It is low-cost, high-tensile, lightweight, flexible, durable, and capable of proliferating even in ineffectual areas (e.g., incline). This review discusses the unique properties of bamboo for making charcoal and biochar for diverse applications. To produce bamboo charcoal and biochar, this study reports on the pyrolysis process for the thermal degradation of organic materials in an oxygen-depleted atmosphere under a specific temperature. This is an alternative method for turning waste biomass into products with additional value, such as biochar. Due to various advantages, bamboo charcoal is preferred over regular charcoal as it has four times the absorption rate and ten times more surface area reported. According to the reports, the charcoal yield ranges from 24.60 to 74.27%. Bamboo chopsticks were the most useful source for producing charcoal, with a high yield of 74.27% at 300 °C in nitrogen, but the thorny bamboo species have a tremendous amount of minimal charcoal, i.e., 24.60%. The reported biochar from bamboo yield ranges from 32 to 80%. The most extensive biochar production is produced by the bamboo D. giganteus, which yields 80% biochar at 300 °C. Dry bamboo stalks at 400 °C produced 32% biochar. One of the sections highlights biochar as a sustainable solution for plastic trash management produced during the COVID-19 pandemic. Another section is dedicated to the knowledge enhancement about the broad application spectrum of the charcoal and biochar. The last section highlights the conclusions, future perspectives, and recommendations on the charcoal and biochar derived from bamboo.

Graphical Abstract

Biochar as Sustainable Alternative and Green Adsorbent for the Remediation of Noxious Pollutants: A Comprehensive Review

The current water crisis necessitates the development of new materials for wastewater treatment. A variety of nanomaterials are continuously being investigated for their potential as adsorbents for environmental remediation. Researchers intend to develop a low-cost, simple, and sustainable material that can cater to removal of pollutants. Biochar derived from biowaste is a potential candidate for the existing problem of water pollution. The review focuses on the various aspects of biochar, such as its sources, preparation methods, mechanism, applications for wastewater treatment, and its regeneration. Compared with other adsorbents, biochar is considered as an environmentally friendly, sustainable, and cost-effective substitute for waste management, climate protection, soil improvement, wastewater treatment, etc. The special properties of biochar such as porosity, surface area, surface charge, and functional groups can be easily modified by various chemical methods, resulting in improved adsorption properties. Therefore, in view of the increasing environmental pollution and the problems encountered by researchers in treating pollutants, biochar is of great importance. This review also highlights the challenges and prospective areas that can be explored and studied in more detail in the future.

The Removal of Cr(VI) from Aqueous Solutions with Corn Stalk Biochar

The discharge of wastewater containing hexavalent chromium (Cr(VI)) into the environment is very harmful to living things. Therefore, before effluent that contains Cr(VI) can be discharged into the environment, this toxin should be removed from the contaminated water. In this study, corn stalk biochar was investigated to evaluate the Cr(VI) removal efficiency from an aqueous solution. The effects of pH (2-10), biochar concentration (0.5 to 10 g/L), Cr(VI) concentration (10-500 mg/L), and contact time (10-1440 min) were studied. The actual experimental value of the Cr(VI) removal efficiency was 28.67%, largely consistent with the predicted model value of 29.31%, under the optimal conditions of a Cr(VI) concentration of 60 g/L, pH 4, contact time of 270 min, and a biochar concentration of 4.5 g/L. A significant interaction between the Cr(VI) concentration and pH was observed, along with significance in the interaction between Cr(VI) concentration and biochar concentration, which had a greater impact on the removal of Cr(VI). Biosorption onto corn stalk biochar is an affordable and economical adsorption process to treat wastewater contaminated with Cr(VI). The aim of this study is to provide data to serve as a basis for future studies on the use of raw agricultural waste to remove Cr(VI).

A Single Soil Washing with Humic Substance Can Achieve the Risk-Based Remedial Target for Nickel Contaminated Soil

Risk-based soil remediation and management have become a global environmental issue. Here, a nickel (Ni)-contaminated site was selected to conduct the risk-based remediation strategy. The Health and Environment Risk Assessment Software was used to calculate the human health risk and the remedial target value (RTV) of Ni. Soil highly contaminated with Ni (424.30 mg kg^-1) could cause an unacceptable carcinogenic risk (1.41 × 10^-6), which needs further remediation. Hence, a soluble humic substance (HS) was used as the washing agent to remove Ni. After a single wash at pH 4 and 8, the Ni concentrations in soil were reduced to 278.05 and 288.27 mg kg^-1, both below the RTV (300 mg kg^-1). Furthermore, sequential extraction analysis revealed that the residual Ni was maintained stably in the soil after HS washing. These findings suggested that HS is a promising washing agent for Ni-contaminated soil remediation under the guidance of risk control.

Combined remediation effects of biochar, zeolite and humus on Cd-contaminated weakly alkaline soils in wheat farmland

Threats posed by Cd-contaminated arable soils to food security have attracted increasing attention. The combination of organic and inorganic amendments has been extensively applied to immobilize Cd in paddy soils. However, the regulatory mechanism of Cd fractionation under these combined amendments and the effect on wheat Cd accumulation remain unclear in upland soils. In this work, different combinations of organic and inorganic amendments were prepared with biochar, zeolite and humus, and the Cd-immobilization mechanism was also investigated in field experiments. The results demonstrated that the mixture of biochar, zeolite and humus had excellent Cd immobilization performance in highly Cd-contaminated (4.26 ± 1.25 mg kg^-1) weakly alkaline soils, resulting in 76.5-84.8% decreases in soil available Cd. The contribution of single components to Cd immobilization in the combined amendment follows the order of humus > biochar > zeolite. The combined amendment converted the acid soluble Cd to the Cd bound to the reducible fraction with higher stability, thereby decreasing Cd bioavailability. The maximum Cd decrease rate in wheat roots, straw and grains could reach 68.2%, 45.0% and 59.3%, respectively, and the Cd content in grains (0.098 mg kg^-1) was lower than the food security standards of China (0.1 mg kg^-1). Wheat planting for two successive years in a large-scale field further verified the superior Cd immobilization performance and stability of the combined amendment in moderately to slightly Cd-contaminated soil. The present study provides references for the remediation of Cd-contaminated weakly alkaline upland soils and certain guidance for safe food production.

A Novel Low-Cost Bio-Sorbent Prepared from Crisp Persimmon Peel by Low-Temperature Pyrolysis for Adsorption of Organic Dyes

In order to properly reuse food waste and remove various contaminants from wastewater, the development of green, sustainable and clean technologies has demonstrated potential in the efficient inhibition of secondary pollution to the environment. In this study, an economical and green method was used to prepare biochar from crisp persimmon peel (CPP) using flash-vacuum pyrolysis at different temperatures (200–700 °C; referred to as CPP200–CPP700). CPP200 has high polarity, low aromaticity and high oxygen-containing functional groups that exhibit superior MB adsorption capabilities. CPP200 that was prepared at a relatively low temperature of 200 °C exhibited a high adsorption capacity of 59.72 mg/g toward methylene blue (MB), which was relatively higher than that for alizarin yellow R (4.05 mg/g) and neutral red (39.08 mg/g), indicating that CPP200 possesses a higher adsorption selectivity for cationic dyes. Kinetics investigation revealed that the kinetic data of CPP200 for the adsorption of MB was better fitted by a linear pseudo-second-order model. Isothermal studies indicated that the linear Langmuir model was more suitable for describing the adsorption process. The adsorption thermodynamics illustrated that the adsorption of MB onto CPP200 was spontaneous and endothermic. EDS and IR analyses of CPP200 for both pre- and post-adsorption of MB showed that electrostatic interactions between oxygen-containing groups on biochar and target MB dominated the adsorption procedure, in addition to hydrogen bonding interactions. Reusability tests confirmed the excellent regeneration characteristics of CPP200, indicating that CPP200 may be used as a green, sustainable, highly efficient and recyclable adsorbent for the selective removal of cationic organic dyes.

Binding characteristics of copper onto biochar-derived DOM using general, heterospectral and moving-window two-dimensional correlation analyses

Dissolved organic matter released from biochar (biochar-derived DOM, BDOM) could dominate the environmental behavior and fate of trace metals by forming BDOM-metal complexes. Here general, heterospectral as well as moving-window (MW) two-dimensional correlation spectroscopy (2DCOS) analyses of synchronous fluorescence and Fourier transform infrared spectra were employed to explore the heterogeneous binding characteristics between sludge BDOM and Cu(II). The results revealed that Cu-BDOM binding first occurred in the fulvic-like (368-380 nm), then humic-like (428 nm) fluorescent fractions, followed by infrared groups of phenolic hydroxyl groups, carboxylate, COH of polysaccharide groups, CC of aromatic carbon, CH of aliphatics and COC of aliphatic ethers. The binding affinity of the hydrophilic groups was stronger than that of hydrophobic groups in BDOM towards Cu(II). Fluorescence components in BDOM played a decisive role in the binding of Cu(II) with trace concentration (1 μM), while infrared functional groups made a substantial contribution in the complexation of Cu(II) with higher concentration (10-100 μM). The concentration of final configuration transformation point (11.7 μmol/mg in this study) by MW2DCOS analysis was suggested as an actual binding threshold that was helpful for evaluating their environmental behaviors.

Biochar-based fertilizers and their applications in plant growth promotion and protection

Soil is an integral part of the ecosystem because it serves as a habitat for various microorganisms and lays the foundation for supporting plant growth and development. Therefore, factors such as increased anthropogenic activities hand by hand with other natural processes that harm the ecosystem may eventually lead to a decline in soil quality and fertility, hindering the growth of plants and soil microbial communities. Given the current global scenario of increasing human intervention, it is essential to find effective measures and reliable technologies to restore soil quality. Biochar is an emerging soil ameliorant employed for soil health restoration and is primarily generated through the anoxygenic pyrolysis of biomass. The biochar application in soil remediation may be beneficial due to biochar's unique physicochemical properties, including high carbon and metal fixation abilities. In addition, biochar possesses abilities to reduce the plant's environmental stress injuries. This review briefly overviewed the ingredients and mechanism of biochar productions. We then emphatically reviewed the advances in biochar applications in soil bioremediation, soil microflora growth stimulation, and the alleviation of various biotic and abiotic stresses in plants.

Removal of heavy metals from wastewaters with biochar pyrolyzed from MgAl-layered double hydroxide-coated rice husk: Mechanism and application

This study reports a MgAl-LDH rice husk biochar composite (MgAl-LDH@RHB) with a regular hydrotalcite structure synthesized by a simple hydrothermal method, which was then used to remove Cd(II) and Cu(II) from water. The influencing factors on the adsorption performance were determined through batch adsorption experiments, and the adsorption characteristics and cycling capacity were evaluated with eight models and adsorption-desorption experiments. The results showed that the adsorption of Cd(II) and Cu(II) by MgAl-LDH@RHB conformed to the Langmuir-Freundlich model and PSO kinetics model, indicating single-layer chemical adsorption. In addition, the experimental maximum adsorption capacities for Cd(II) and Cu(II) were 125.34 and 104.34 mg g^-1, respectively. The adsorption of Cd(II) and Cu(II) by MgAl-LDH@RHB was dominated by surface precipitation and ion exchange. The findings reveal the mechanism for the heavy metal removal by MgAl-LDH@RHB and provide a theoretical reference for agricultural waste disposal and water pollution control.

Biomass derived functional carbon materials for supercapacitor applications

Biochar produced from the thermochemical conversion of biomass, provides a green and sustainable platform for the preparation of various functional carbon materials (porous carbon, heteroatom doped biochar, carbon nanotubes, graphene, carbon quantum dots, etc.) towards advanced application. Their preparation involves the physical as well as chemical activation of biochar or directly from the biomass. The inherent versatile physicochemical properties of these versatile materials have been explored for the construction of the electrochemical energy storage devices like supercapacitors. In the present review, the various methodologies for the preparation of various biomass-derived carbon materials are summarized. Further utilization of these materials in supercapacitor electrodes and the properties associated with their charge storage ability, along with associated challenges and perspectives are also discussed.

Novel insights into the adsorption of organic contaminants by biochar: A review

With rising concerns in the practical application of biochar for the remediation of environment influenced by various organic contaminants, a critical review to facilitate insights the crucial role that biochar has played in wastewater and polluted soil decontamination is urgently needed. This research therefore aimed to describe different intriguing dimensions of biochar interactions with organic contaminants, which including: (i) an introduction of biochar preparation and the related physicochemical properties, (ii) an overview of mechanisms and factors controlling the adsorption of organic contaminants onto biochar, and (iii) a summary of the challenges and an outlook of the further research needs in this issue. In the light of the survey consequences, the appearance of biochar indicates the potential in substituting the existing costly adsorbents, and it has been proved that biochar is one promising adsorbent for organic pollutants adsorption removal from water and soil. However, some research gaps, such as dynamic adsorption, potential environmental risks, interactions between biochar and soil microbes, novel modification techniques, need to be further investigated to facilitate its practical application. This research will be conductive to better understanding the adsorption removal of organic contaminants by biochar.

Probing changes in humus chemical characteristics in response to biochar addition and varying bulking agents during composting: A holistic multi-evidence-based approach

Despite the various benefits of humus, the changes in its chemical characteristics during composting in response to biochar addition and varying bulking agents remain to be further explored. In this study, three treatments were conducted, in which swine manure, bulking agent, and biochar were mixed at ratios of 4:1:0, 8:1:0, and 8:1:1. Fourier transform infrared spectroscopy (FTIR), carbon nuclear magnetic resonance spectroscopy (¹³C-NMR), three-dimensional excitation-emission matrix fluorescence spectroscopy (3D-EEM), and near-edge X-ray absorption fine structure (NEXAFS) were employed to characterize the chemical and structural properties of humus from multiple perspectives. The 3D-EEM spectra in this study showed a larger increase in humic acids (HAs) content (56%) and HAs to fulvic acids ratio (128%) during composting, indicating stronger humification in biochar-amended treatment. FTIR, ¹³C-NMR, and NEXAFS all confirmed the essential properties of HA as the core agronomic functional substance with rich aromatic and carboxyl groups, and that its aromaticity increased gradually during composting. In addition, ¹³C-NMR demonstrated that biochar addition and a relatively higher bulking agent ratio aided an increase in the carboxyl C proportion in HA after composting. In particular, NEXAFS revealed that biochar addition promoted the diversification of C, N, and O species in HA, with the emergence of quinone C and O-alkyl C as the main representatives. This work suggests that biochar addition and a relatively high bulking agent ratio could enhance humification and improve the agronomic function of humus.

Preparation and characterization of biochar derived from co-pyrolysis of Enteromorpha prolifera and corn straw and its potential as a soil amendment

Single biomass feedstock approach may not meet the requirements for developing biochar with desired characteristics for use as soil amendment. In this study, biochars were prepared by co-pyrolysis of nutrients-rich Enteromorpha prolifera and lignocellulose-rich corn straw (CPECs) at different mass ratios (3:7, 1:1, and 7:3). CPECs presented higher water-soluble N/P contents than corn straw biochar, and exhibited larger surface area, low Na content, and slower nutrient release rate than Enteromorpha prolifera biochar. The modification in physicochemical and properties of CPECs enhanced its potential application as a soil amendment. A pot experiment showed that CPECs derived from co-pyrolysis of appropriate ratios of Enteromorpha prolifera and corn straw (1:1, 7:3) significantly increased the biomass of cherry tomato plant by 64.05%, 40.03% and 81.88%, 55.25%, when compared with corn straw biochar and Enteromorpha prolifera biochar, respectively. The positive effects of CPECs were primarily attributed to improved soil properties (e.g., water holding capacity, soil organic matter, pH, soil nutrients content) and increased total N/P uptake by plants. The results of this work provided potentials of developing "designer" biochars to meet the multiple soil requirements by co-pyrolysis.

Overlooked contributions of biochar-derived dissolved organic matter on the adsorption of Pb (Ⅱ): Impacts of fractionation and interfacial force

Comprehensive understanding of how the release of biochar-derived dissolved organic matter (BDOM) affects the immobilization of heavy metals when biochar (BC) is applied for long-term soil remediation is extremely important. In this study, BCs prepared under different pyrolysis temperatures were fractionated into residual BC (RBC), nano-sized BC (NBC), and BDOM, in order to clarify the contribution of BDOM for lead (Pb(II)) adsorption on BC and to explore the interfacial mechanisms. Results demonstrated that the adsorption capacity (Q_e) of Pb(II) on BC improved from 166.1 to 423.9 mg g^-1 with the increase in the pyrolysis temperature from 350 to 800 °C. The sum of Q_e of Pb(II) on NBC and RBC was lower than that on BC, due to the complexation between BDOM and Pb(II) rather than pH variance and cation exchange. Ultraviolet-visible and fluorescence spectroscopy revealed that fulvic-like substances as well as small molecules with low aromaticity in BDOM underwent favorable association with Pb(II) and got re-adsorbed on RBC. With the increase in the Pb(II) concentration, the contribution of van der Waals interaction for adsorption of BDOM350-Pb complexes was improved, whereas adsorption mechanism in BDOM800-Pb complexes was more dependent on ligand exchange. This study provides mechanistic insights into the impact of BDOM on Pb(II) immobilization, which can provide valuable information for the long-term remediation of Pb-contaminated soils using BC.

Preparation of Si-Mn/biochar composite and discussions about characterizations, advances in application and adsorption mechanisms

A novel Si-Mn binary modified biochar composite material (SMBC) was prepared after being sintered 450 °C for 2 h. The crystal structure, surface functional groups, surface morphology and element composition, specific surface area and pore structure were characterized by XRD, FTIR, XPS, SEM + EDS and BET etc. The results showed that the surface of SMBC was rough and loose, and the specific surface area increased to 35.4284 m²/g. Si and Mn were uniformly attached to the surface of biochar in the form of SiO₂, MnO_x, MnSiO₃. Batch adsorption experiments showed that SMBC had a higher removal efficiency (139.06 mg/g, above 98%) for Cu(II) when the dosage was 2 g/L and pH = 6. The cycle experiments showed that SMBC had good reusability, and its regeneration efficiency still reached 80.24%. The leaching amount of Mn (0.65 mg/L) was greatly reduced and avoid second-pollution resulted from ion exchange, which was attributed to the existence of Si-O-Mn bonds, and they could help Mn adhere to the surface of biochar more stable. The adsorption process was dominated by single-layer chemical adsorption and mainly occurred in the membrane diffusion stage. Cu(II) mainly formed -COOCu, -OCu, Cu(OH)₂, Cu(OH)₂CO₃, Si-O-Cu, Mn-O-Cu by the mechanisms such as precipitation (4.74%), ion exchange (13.81%), complexation and physical adsorption (total 81.45% of the two mechanisms). Among them, complexation was dominant in the adsorption process.

Towards a Soil Remediation Strategy Using Biochar: Effects on Soil Chemical Properties and Bioavailability of Potentially Toxic Elements

Soil contamination with potentially toxic elements (PTEs) is considered one of the most severe environmental threats, while among remediation strategies, research on the application of soil amendments has received important consideration. This review highlights the effects of biochar application on soil properties and the bioavailability of potentially toxic elements describing research areas of intense current and emerging activity. Using a visual scientometric analysis, our study shows that between 2019 and 2020, research sub-fields like earthworm activities and responses, greenhouse gass emissions, and low molecular weight organic acids have gained most of the attention when biochar was investigated for soil remediation purposes. Moreover, biomasses like rice straw, sewage sludge, and sawdust were found to be the most commonly used feedstocks for biochar production. The effect of biochar on soil chemistry and different mechanisms responsible for PTEs' immobilization with biochar, are also briefly reported. Special attention is also given to specific PTEs most commonly found at contaminated soils, including Cu, Zn, Ni, Cr, Pb, Cd, and As, and therefore are more extensively revised in this paper. This review also addresses some of the issues in developing innovative methodologies for engineered biochars, introduced alongside some suggestions which intend to form a more focused soil remediation strategy.

Characterization of pruned tea branch biochar and the mechanisms underlying its adsorption for cadmium in aqueous solution

In the present study, discarded pruned tea branch was used to prepare a new biochar, and the physicochemical properties and adsorption characteristics were investigated by characterization and batch experiments. With increasing pyrolysis temperature from 400 to 800 °C, the yield, specific surface area, and acidic functional groups had significant differences. The optimum adsorption conditions were determined as pH = 6 and dosage of 2 g L^-1. The pseudo-second-order kinetic and Langmuir isothermal model could fit well to the adsorption data, which showed that the adsorption process was dominated by monolayer chemical adsorption. The highest adsorption property (74.04 mg g^-1) was obtained by the pyrolysis of tea branch biochar (TBB) at 700 °C owing to the adsorption mechanisms, including surface complexation, precipitation, metal ion exchange, and Cd²⁺-π interaction. After five cycles of desorption, biochar still showed superior adsorption (80%). Hence, the TBB acted as a regenerable adsorbent for treating Cd²⁺-containing wastewater.

Biochar Modified by Nano-manganese Dioxide as Adsorbent and Oxidant for Oxytetracycline

Biochar has limited capacity to adsorb oxytetracycline (OTC). Here we have used bamboo willow biochar (BC) as a carrier to produce nMnO₂-loaded biochars (MBC) by a co-precipitation method. Their chemical compositions, morphological features, specific surface area, and surface functional groups were observed or determined. Batch experiments were conducted to assess the effects of reaction time, initial OTC concentrations, pH, salt concentrations, and natural organic matter (NOM) on OTC removal. Kinetics and isotherms indicated that OTC was mainly adsorbed via chemical interactions, and mono- and multi-layer adsorption occurred on the surface. MBC removed 19-25 times more OTC than BC, and the removal was highest at near-neutral pH, not influenced by NaCl (2, 10 mM), slighted reduced by NOM (0-20 mg L^-1), and enhanced by NaHCO₃ (2, 10 mM). Besides being an adsorbent, MBC acted as an oxidant and degraded 58.5% of OTC at 24 h.

Biochar derived from chicken manure as a green adsorbent for naphthalene removal

In this study, biochar was generated from chicken manure by using a tube furnace under different temperatures (300, 500, and 700 °C), and the treatments were noted as J300, J500, and J700, respectively. In comparison, another type of biochar was prepared under 500 °C with a muffle furnace, and the treatment was noted as JM500. Biochar in treatment group J500 was subsequently modified with HNO₃ and NaOH, and the treatments were noted as J500-HNO₃ and J500-NaOH, respectively. The sorption efficiencies of naphthalene by the above six types of biochar were evaluated. Characteristic results showed that the surface pores of the biochar were improved with the increase of temperature, and biochar under the treatments J300, J500, J700, and JM500 experienced a high speed of adsorption within 1 h after the naphthalene adsorption started. The adsorption capacity of naphthalene increased with the increase of the initial concentration of naphthalene. Treatment J700 exhibited the largest adsorption capacity since its biochar surface pore structure was more fully developed with a crystal structure formed, and its specific surface area was increased by about 20 times compared to the original chicken manure. After biochar modification using HNO₃ and NaOH, the infrared spectrum changed, and the adsorption active sites were increased. The biochar modification by HNO₃ had a high naphthalene adsorption efficiency compared to NaOH. The order of adsorption capacity was as follows: J500 ≈ JM500 < J300 < J500-NaOH < J500-HNO₃ < J700.

Study of soil microorganisms modified wheat straw and biochar for reducing cadmium leaching potential and bioavailability

The application of crops straw and biochar in trace metals remediation from the contaminated environment attracted more and more attention during the past decade. Although there has been some review work on the mechanism of trace metals stabilization by crops straw, the effects and mechanisms of interaction among soil indigenous-microbes and crops-straw for trace metal adsorption and stabilization is still unclear. In this study, the dynamic effects along with potential mechanisms of wheat-straw (WS), wheat-straw biochar (WBC) and biologically modified wheat-straw (BMWS) were conducted to investigate the adsorption, leaching behaviour, chemical fractions and bioavailability of cadmium (Cd). The results showed that the biosorption capacity (q_e) was most elevated in the BMWS treatment (14.42 mg g^-1) as compared to WBC (6.28 mg g^-1) and WS (4.20 mg g^-1). The application of BMWS, WBC and WS at the rate of 3% significantly reduced Cd concentration in leachate to 53, 45 and 21% respectively, as compared to control. The addition of BMWS reduced the exchangeable Cd fraction resulted an increase in organic matter and carbonate bound Cd fraction in the soil. The DTPA extractable Cd was significantly decreased by 31.2 and 28.6% with the application of BMWS and WBC at 3% w/w respectively as compared to control. The research results may provide a novel perceptive for the development of functional materials and strategies for eco-friendly and sustainable trace metal remediation in contaminated soil and water by combination of straw and soil-indigenous microorganisms.

Adsorption mechanism and effectiveness of phenol and tannic acid removal by biochar produced from oil palm frond using steam pyrolysis

In order to meet the growing demand for adsorbents to treat wastewater effectively, there has been increased interest in using sustainable biomass feedstocks. In this present study, the dermal tissue of oil palm frond was pyrolyzed with superheated steam at 500 °C to produce nanoporous biochar as bioadsorbent. The effect of operating conditions was investigated to understand the adsorption mechanism and to enhance the adsorption of phenol and tannic acid. The biochar had a microporous structure with a Brunauer-Emmett-Teller surface area of 422 m²/g containing low polar groups. The adsorption capacity of 62.89 mg/g for phenol and 67.41 mg/g for tannic acid were obtained using 3 g/L biochar dosage after 8 h of treatment at solution pH of 6.5 and temperature of 45 °C. The Freundlich model had the best fit to the isotherm data of phenol (R² of 0.9863), while the Langmuir model best elucidated the isotherm data of tannic acid (R² of 0.9632). These indicated that the biochar-phenol interface was associated with a heterogeneous multilayer sorption mechanism, while the biochar-tannic acid interface had a nonspecific monolayer sorption mechanism. The residual concentration of 26.3 mg/L phenol and 23.1 mg/L tannic acid was achieved when treated from 260 mg/L three times consecutively with 1 g/L biochar dosage, compared to a reduction to 72.3 mg/L phenol and 69.9 mg/L tannic acid using 3 g/L biochar dosage in a single treatment. The biochar exhibited effective adsorption of phenol and tannic acid, making it possible to treat effluents that contain varieties of phenolic compounds.

Hydrochar and pyrochar for sorption of pollutants in wastewater and exhaust gas: A critical review

Pollutants in wastewater and exhaust gas bring out serious concerns to public health and the environment. Biochar can be developed as a sustainable adsorbent originating from abundant bio-wastes, such as agricultural waste, forestry residue, food waste and human waste. Here we highlight the state-of-the-art research progress on pyrochar and hydrochar for the sorption of pollutants (heavy metal, organics, gas, etc) in wastewater and exhaust gases. The adsorption performance of pyrochar and hydrochar are compared and discussed in-depth, including preparation procedures (carbonization and activation), sorption possible mechanisms, and physiochemical properties. Challenges and perspective for designing efficient and environmental benign biochar-based adsorbents are finally addressed.

Efficient Removal of Cu(II), Zn(II), and Cd(II) from Aqueous Solutions by a Mineral-Rich Biochar Derived from a Spent Mushroom (Agaricus bisporus) Substrate

This study evaluated the novel application of a mineral-rich biochar derived from a spent Agaricus bisporus substrate (SAS). Biochars with various pyrolysis temperatures (350–750 °C) were used to remove Cu(II), Zn(II), and Cd(II) from aqueous solutions. The adsorption characteristics and removal mechanisms of the biochars were investigated. The adsorption kinetics and isotherm data were fitted well by pseudo-second-order and Freundlich models. The Langmuir maximum removal capacity (Q_max) values of Cu(II), Zn(II), and Cd(II) were ordered as SAS750 > SAS350 > SAS550, and the Q_max values of SAS750 were 68.1, 55.2, and 64.8 mg·g⁻¹, respectively. Overall, the removal mechanisms of biochar at a low production temperature (350 °C) to Cu(II), Zn(II), and Cd(II) were mainly via ion exchange (54.0, 56.0, and 43.0%), and at a moderate production temperature (550 °C), removal mechanisms were mainly via coordination with π electrons (38.3, 45.9, and 55.0%), while mineral precipitation (65.2, 44.4, and 76.3%, respectively) was the dominant mechanism at a high produced temperature (750 °C). The variation of the mutual effect of minerals and heavy metals was the predominant factor in the sorption mechanism of mineral precipitation and ion exchange. The results demonstrated that spent Agaricus bisporus substrate biochar is a potential candidate for the efficient removal of heavy metals, which provides a utilization route for spent mushroom substrates.

Remediation of cadmium-contaminated coastal saline-alkaline soil by Spartina alterniflora derived biochar

Following oil extraction in the wetland of the Yellow River Delta, heavy metal contamination of coastal saline-alkaline soil, especially with cadmium (Cd), has become a serious environmental problem in some regions. Biochar application has been proposed to remedy Cd-contaminated soil, but the remediation effect is related to preparation conditions of biochar (e.g., pyrolysis temperature and raw material) and soil properties. The invasive plant, Spartina alterniflora, produces a high amount of biomass, making it suitable for biochar production in coastal China. We investigated the effect of S. alterniflora-derived biochar (SDB) pyrolyzed at four temperatures (350, 450, 550, and 650 °C) crossed with three addition ratios (1, 5, and 10%) and control on Cd contamination of coastal saline-alkaline soil. Pyrolysis temperature affected pH, surface area, and functional groups of SDB. SDB markedly improved soil pH and soil organic matter, but the degree of improvement was affected by pyrolysis temperature and addition ratio. SDB significantly altered available Cd content in soil, but reduced it only at low pyrolysis temperatures (350 and 450 °C). Available Cd content had a positive correlation with soil pH (R² = 0.298, P < 0.01), but was not related to salinity and soil organic matter content. Thus, SDB pyrolyzed at 350 °C with 5% addition was optimal for passivating Cd in coastal saline-alkaline soil, since available Cd content in soil decreased mostly (by 26.9%). These findings act as a reference for the development of an application strategy for SDB to ameliorate Cd-contaminated coastal saline-alkaline soil.

A critical review on the biochar production techniques, characterization, stability and applications for circular bioeconomy

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Recent research on remediation of toxic pollutants by biochar has been summarized.

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The production techniques of the biochar have been narrated.

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Biochar properties, stability and its environmental issues have been analysed.

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Applications of biochar in soil fertility and removing pollutants have been reported.

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The major stumbling block in biochar production is cost of production.

There is an upsurge enthusiasm for utilizing biochar produced from waste-biomass in different fields, to address the most important ecological issues. This review is focused on an overview of remediating harmful contaminants utilizing biochar. Production of biochar utilizing various systems has been discussed. Biochar has received the consideration of numerous analysts in building up their proficiency to remediate contaminants. Process parameters are fundamentally answerable for deciding the yield of biomass. Biochar derived from biomass is an exceptionally rich wellspring of carbon produced from biomass utilizing thermal combustion. Activating biochar is another particular region for the growing utilization of biochar for expelling specific contaminations. Closed-loop systems to produce biochar creates more opportunities. Decentralized biochar production techniques serve as an effective way of providing employment opportunities, managing wastes, increasing resource proficiency in circular bioeconomy. This paper also covers knowledge gaps and perspectives in the field of remediation of toxic pollutants using biochar.

Preparation of biochar by mango peel and its adsorption characteristics of Cd(ii) in solution

Biochars were prepared by pyrolyzing mango peel waste at 300, 400, 500, 600 and 700 °C. Various characterizations were carried out to explore the effect of pyrolysis temperature on the biochars. The data indicated that the physical and chemical properties of biochar such as pH, element ratio, specific surface area and functional groups changed with the increase of pyrolysis temperature. The yield and contents of hydrogen, nitrogen and oxygen decreased, while contents of the ash and carbon, pH and specific surface area of the biochars increased. In addition, the molar ratios of H/C, O/C and (O + N)/C decreased. In this study, batch adsorption experiments for Cd(ii) adsorption were performed with initial Cd(ii) concentrations of 10-300 mg L-1, contact times of 0-2880 min, various pH (2-8) and biochar dose (1-20 g L-1). Langmuir isotherm and pseudo-second-order kinetics models were better fits than other models, suggesting the dominant adsorption of mango peel biochars is via monolayer adsorption. Biochar derived at 500 °C was found to have the highest adsorption capacity of 13.28 mg g-1 among all biochars and the adsorption efficiency was still 67.7% of the initial adsorption capacity after desorption for 4 times. Based on adsorption kinetics and isotherm analysis in combination with EDS, FTIR and XRD analysis, it was concluded that cation exchange, complexation with surface functional groups and precipitation with minerals were the dominant mechanisms responsible for Cd adsorption by mango peel biochar. The study suggested that mango peel can be recycled to biochars and can be used as a low-cost adsorbent for Cd(ii) removal from wastewater.

Efficient immobilization of toxic heavy metals in multi-contaminated agricultural soils by amino-functionalized hydrochar: Performance, plant responses and immobilization mechanisms

A novel amino-functionalized hydrochar material (referred to NH₂-HCs) was prepared and used as the soil amendment to immobilize multi-contaminated soils for the first time. The results showed that the application of NH₂-HCs significantly improved (P < 0.05) soil properties (i.e., pH value, cation exchange capacity and organic content). By introduction of NH₂-HCs, the contaminated soil showed the highest value of 96.2%, 52.2% and 15.5% reductions in Cu, Pb and Cd bioavailable concentrations and the leaching toxicity of Cu, Pb and Cd were remarkably reduced by 98.1%, 31.3% and 30.4%, respectively. Most of exchangeable Cu, Pb and Cd reduced were transformed into its less available forms of oxidizable and residual fractions. Potential ecological risk assessment indicated that the element Cd accounted for the most of total risks in NH₂-HCs amended soils. The mechanism study indicated that surface complexation, chemical chelating and cation-pi interaction of NH₂-HCs played a vital role in the immobilization of heavy metals. Pot experiments further verified that the application of NH₂-HCs significantly improved plant growth and reduced metal accumulations. The present study offered a novel approach to prepare amino-functionalized hydrochars with great potential as the green and alternative amendments for efficiently immobilizing heavy metals in multi-contaminated soil.

Low-cost field production of biochars and their properties

Biochar has been intensively investigated for carbon sequestration, soil fertility enhancement, and immobilization of heavy metals and organic pollutants. Large-scale use of biochar in agricultural production and environmental remediation, however, has been constrained by its high cost. Here, we demonstrated the production of low-cost biochar ($20/ton) in the field from Robinia pseudoacacia biowaste via a combined aerobic and oxygen-limited carbonization process and a fire-water-coupled method. It involved aerobic combustion at the outer side of biomass, oxygen-limited pyrolysis in the inner core of biomass, and the termination of the carbonization by water spray. The properties of biochar thus produced were greatly affected by exposure time (the gap between a burning char fell to the ground and being extinguished by water spray). Biochar formed by zero exposure time showed a larger specific surface area (155.77 m²/g), a higher carbon content (67.45%), a lower ash content (15.38%), and a higher content of carboxyl and phenolic-hydroxyl groups (1.74 and 0.86 mol/kg, respectively) than biochars formed with longer exposure times (5-30 min). Fourier-transform infrared spectroscopic (FTIR) spectra indicated that oxygen-containing functional groups of biochar played a role in Cd and oxytetracycline sorption though a quantitative relationship could not be established as the relative contribution of carbon and ash moieties of biochar to the sorption was unknown. Outcomes from this research provide an option for inexpensive production of biochar to support its use as a soil amendment in developing countries.

Comparative study on Pb2+ removal from aqueous solutions using biochars derived from cow manure and its vermicompost

Waste emissions have increased the amount of water and soil contaminated with heavy metals such as Pb. To broaden the methods for the recycling and environmental usage of cow manure (CM) and its vermicompost (CV), CM, CV, and their derived biochars produced by the pyrolysis of CM or CV at 350 and 700 °C were used as adsorbents for Pb²⁺ removal in this batch adsorption experiment to reveal their different Pb²⁺ removal efficiencies and the underlying mechanisms. The batch experiment results revealed that all adsorbents rapidly removed Pb²⁺ within 30 min. A pH between 2.0 and 6.0 positively affected Pb²⁺ removal by CM and its biochar, whereas that by CV and its biochar was only positively affected by pH between 2.0 and 3.0. CV-derived biochar was more effective in the removal of Pb²⁺ than the other absorbents, with the maximum adsorption capacities (Qm) fitted from the Langmuir model reaching approximately 230.0 mg·g^-1 and the desorption rate (DR) being approximately 0.00-0.02%. Material physiochemical characterization, including X-ray diffraction analysis, showed that high pH, high ash content, rich mineral content, and high mineral contents might have been the main reasons for more effective removal of Pb²⁺ from aqueous solutions by CV-derived biochar. Fourier-transform infrared analysis indicated that surface functional groups such as -OH, CO, -COO-, and C-O; original and newly produced carbonate; and phosphate in CV also led to more effective Pb²⁺ removal efficiency from aqueous solution via surface functional group binding. Thus, pyrolyzing CVs may be used to produce biochar as a cost-effective adsorbent for heavy metal remediation in soil and water in the future.

Removal of methylene blue from aqueous solutions by biochar prepared from the pyrolysis of mixed municipal discarded material

This paper investigates the adsorption of organic compounds in aqueous solution to biochar adsorbent, using methylene blue as an indicator for adsorption. Biochar was produced by the pyrolysis of mixed municipal discarded material in an innovative heat pipe reactor, the pyrolysis temperature was held at 300°C for 12 h. Biochar produced under these conditions was found to have oxygen containing functional groups that are beneficial to the adsorption of methylene blue as well as graphitic structures suggesting potential sites for π-π interactions with methylene blue. Methylene Blue followed the pseudo second order kinetic model with higher R² values than both the pseudo first order kinetic and intraparticle diffusion models. The adsorption also closely fit the Langmuir isotherm rather than the Freundlich model, suggesting monolayer adsorption rather than multilayer adsorption. Maximum adsorption capacity was observed at 7.2 mg/g for initial concentration of 100 mg/l Methylene blue in aqueous solution. The amount of Methylene blue adsorbed increased with increasing initial concentration as expected. The adsorption mechanisms are likely π-π interactions between methylene blue and the graphitic structures in the biochar which are shown to be present in Raman spectroscopy, as well as electrostatic attraction and ionic bonding between negatively charged surface sites on the char and the positive charge on the dissolved methylene blue molecules. The results show that biochar obtained from mixed waste could be employed as a low-cost and effective tool in water treatment for the removal of basic dyes and potentially other organic impurities.

Biochar surface complexation and Ni(II), Cu(II), and Cd(II) adsorption in aqueous solutions depend on feedstock type

Biochar is a promising material for efficient removal of toxic metals from wastewater to meet standards for discharge into surface water. We characterized adsorption behaviour of willow (Salix alba) wood (WW) and cattle manure (CM) and their biochars, willow wood biochar (WWB) and cattle manure biochar (CMB), and elucidated the mechanisms for the removal of Ni(II), Cu(II) and Cd(II) from aqueous solutions. The kinetic adsorption suggests that the adsorption of Ni(II), Cu(II) and Cd(II) by feedstock and their biochars was controlled by mass transport, and chemisorption also played a role in the adsorption process. The Elovich model also well described the adsorption kinetics for WW and CM (R² > 0.92), indicating that heterogeneous diffusion was the mechanism. The Sips isotherm model fitted best (R² > 0.98) for Ni(II), Cu(II) and Cd(II) adsorption by the feedstocks and their biochars, indicating that both monolayer and multilayer adsorption played roles on the heterogeneous surfaces of the four adsorbents. The WWB had a higher while the CMB had a lower adsorption capacity than their respective feedstock due to the presence of abundant -COOH functional group on WWB surface to interact with Ni(II), Cu(II) and Cd(II) to form surface complexes. The higher specific surface area and lower pH of point of zero charge (PZC) of WWB were other contributing factors for its greater removal capacity. Therefore, we conclude that proper feedstocks need to be selected to produce biochars that are efficient for the removal of toxic metals from wastewater.

Biochar and bacteria inoculated biochar enhanced Cd and Cu immobilization and enzymatic activity in a polluted soil

The application of biochar in the remediation of heavy metal contaminated soil has received increasing global attention during the past decade. Although there has been some review work on the mechanism of heavy metals stabilization by biochar, the effects and mechanisms of interaction between biochar and functional microbes such as heavy metal tolerant, adsorption and transformation microbial strains remains unclear. In this paper, maize biochar and a heavy metal-tolerant strain Pseudomonas sp. NT-2 were selected to investigate the dynamic effects and potential mechanisms of biochar and bacteria loaded biochar on the stabilization of Cd and Cu mixed contaminated soil by a 75-day pot experiment. The results showed that, compared to the single biochar amendment, the application of biochar inoculated with NT-2 strain at the rate of 5% significantly increased the soil pH at the initial stage of incubation, and followed by a slight decline to a neutral-alkaline range during the reaction. The addition of NT-2 loaded biochar could also significantly increase the proportion of residual fraction of Cd and Cu, thus reduce the proportion of exchangeable and carbonate bound species in the soil, which lead to the decreasing of plant and human bioavailability of the metal in the soil indicated by DTPA and simulated human gastric solution extraction (UBM), respectively. Finally, the application of bacterial loaded biochar also markedly enhanced soil urease and catalase activities during the later stage of the incubation, and improved soil microbial community at the end of incubation, which indicates a recovery of soil function after the metal stabilization. The research results may provide some new insights into the development of functional materials and technologies for the green and sustainable remediation of heavy metal contaminated soil by the combination of biochar and functional microorganisms.

Clanis bilineata larvae skin-derived biochars for immobilization of lead: Sorption isotherm and molecular mechanism

Clanis bilineata larva skin (CBLS), a new residue from the food industry, was first used to produce biochars by pyrolysis at 300 °C (CBLS300) and 700 °C (CBLS700), respectively, for Pb immobilization. The sorption isotherms and immobilization mechanisms of Pb on two biochars were investigated. CBLS700 exhibited more high-efficiency in sorption of Pb than CBLS300 due to the predicted maximum sorption capacity of CBLS700 (77.52 mg/g) was larger than that of CBLS300 (49.02 mg/g). Synchrotron-based microfocused X-ray fluorescence analysis exhibited the co-distribution of Pb and P in the sorption product of CBLS700 rather than CBLS300. Microfocused X-ray absorption near-edge structure analysis highlighted the significance of organic ligand in the biochar for Pb immobilization due to both sorption products have organic complexed Pb. Moreover, 25% of total Pb was present as hydrocerussite on CBLS300 but partially transformed into stable hydroxylpyromorphite on CBLS700 (~21%), which was in accordance with the analysis of scanning electron microscopy coupled with energy disperse spectra. Additionally, addition of CBLS700 was more effective in reducing the leachable Pb in shooting range soil than that of CBLS300. These results strongly suggested the potential application of the new biochar (CBLS700) for the remediation of Pb-contaminated soils.

Novel pectin based composite hydrogel derived from grapefruit peel for enhanced Cu(II) removal

Novel biochar/pectin/alginate hydrogel beads (BPA) derived from grapefruit peel were synthesized and used for Cu(II) removal from aqueous solution. FTIR, SEM-EDS, XRD, TGA and XPS, etc. were applied for characterization analysis. The synergistic reinforcing effect of polymer matrix and biochar fillers improved the adsorptive, mechanical and thermostabilized performance of BPA. Factors like component contents of biochar and pectin, pH, contact time, Cu(II) concentration and coexisting inorganic salts or organic ligands were systematically investigated in batch mode. The adsorption isotherms were fitted well by the Freundlich model and the experimental maximum adsorption capacity of optimized BPA-9 beads (mass ratio of pectin to alginate = 10:1) with 0.25% biochar, was ∼80.6 mg/g at pH 6. Kinetic process was well described by the pseudo-second-order model and film diffusion primarily governed the overall adsorption rate, followed by intraparticle diffusion. Thermodynamics analysis suggested spontaneous feasibility and endothermic nature of adsorption behavior. Moreover, BPA also showed better environmental adaptability in the presence of NaCl, MgCl₂, CaCl₂, EDTA-2Na and CA as well as good adsorption potential for other heavy metal [e.g. Pb(III)]. Crucially, the BPA beads showed good regeneration ability after five cycles. All these results indicated the potential of BPA for removing heavy metal from water.

Limited Cu(II) binding to biochar DOM: Evidence from C K-edge NEXAFS and EEM-PARAFAC combined with two-dimensional correlation analysis

Multiple spectroscopic technologies and chemometric analyses were combined to explore the compositional characteristics and Cu binding performance of biochar-derived dissolved organic matter (DOM). The DOM samples were extracted from biochars produced from lignocellulose-rich rapeseed cake (RSC) by pyrolysis at 300, 500, and 700 °C (i.e., RSC300, RSC500, RSC700). Fourier transform infrared spectroscopy (FTIR) and carbon K-edge near-edge X-ray absorption fine structure spectroscopy (NEXAFS) analyses were combined to elucidate the molecular-level C species in the DOM. With the increasing pyrolysis temperature, DOM aromaticity increased, whereas the proportion of metal complexing sites (e.g., carboxyl and phenolic groups) decreased. Fluorescence excitation-emission matrix (EEM) spectroscopy with parallel factor analysis (PARAFAC) indicated that biochar DOM, irrespective of pyrolysis temperature, was mostly composed of three types of humic-like components (C1-C3), and a small amount of a protein-like component (C4). As charring temperature increased, DOM concentrations decreased substantially, but the humic-like C3 with abundant aromatic structures became predominant. Fluorescence quenching experiment and two-dimensional correlation spectroscopy (2D-COS) analysis suggested that the preferential Cu(II) binding fractions of the DOM were the humic-like substances. Moreover, the quenching curve fitting results for individual components indicated that despite the Cu(II) binding affinity was slightly enhanced as the pyrolysis temperature increased, the binding capacities of the four components decreased. In general, the DOM components from RSC biochar exhibited limited Cu(II) binding capacities (2.18-17.7 μmol L^-1). Results from this study improved understanding of the mechanisms by which biochar DOM interacts with Cu, and provided tools for fast screening of biochars to reduce their environmental risks.

Adsorption of Cr(VI) and Cu(II) from aqueous solutions by biochar derived from Chaenomeles sinensis seed

In order to utilize the discarded Chaenomeles sinensis seed (CSS) and develop low-cost biochar for heavy metal pollution control, this study pyrolyzed CSS to prepare biochar at three different temperatures (300, 450 and 600 °C). The physicochemical properties of CSS biochar such as elemental composition, surface area, surface morphology and surface functional groups were characterized. Its adsorption properties including kinetics, isotherms and thermodynamics were studied. The results showed that the adsorption equilibrium was reached at 5 h, which was relatively fast. CSS biochar prepared at 450 °C (CSS450) had the maximum adsorption capacity for Cr(VI) and Cu(II), which was 93.19 mg/g and 105.12 mg/g, respectively. The thermodynamic parameter ΔG⁰ < 0 and the isotherm parameter R_L between 0 and 1 all revealed the feasibility and spontaneity of the adsorption process. The removal of Cr(VI) exhibited high efficiency in a wide pH range (1-10), while the removal of Cu(II) was pH-dependent and optimal at pH = 6. The coexisting ions in the solution showed slight inhibition of the adsorption of Cr(VI) and Cu(II). Additionally, Cu(II) exhibited better affinity for CSS450 than Cr(VI) in dynamic adsorption. This is the first study to prepare biochar from CSS and confirms its potential application for heavy metal remediation.

Carboxylated graphene oxide-chitosan spheres immobilize Cu2+ in soil and reduce its bioaccumulation in wheat plants

Due to the strong interaction with pollutants and the huge adsorption capacity, graphene adsorbents are widely applied in water decontamination. However, graphene adsorbents are seldom used in soil remediation, because the adsorptive sites on graphene would be occupied by soil components. In this study, we prepared carboxylated graphene oxide-chitosan (GO-COOH/CS) spheres for the immobilization of Cu²⁺ from water and soil. The pores in GO-COOH/CS allowed the internal diffusion of Cu²⁺ solution, while they blocked the direct contact between the solid soil and the adsorptive sites on graphene sheets. Therefore, the high adsorption capacity of GO-COOH/CS spheres (78 mg/g) was largely retained for the soil Cu²⁺ fixation. The partition coefficient (PC) for Cu²⁺ adsorption onto GO-COOH/CS spheres was 4.2 mg/g/μM at C_e of 0.48 mg/L and q_e of 31 mg/g, while the PC value decreased to 0.096 mg/g/μM at C_e of 91.4 mg/L and q_e of 78 mg/g. At initial Cu²⁺ concentrations of 120 mg/L and lower, the fixation efficiencies were all higher than 99% and the corresponding free Cu²⁺ concentrations in leachates were lower than 1.0 mg/L. The Cu²⁺ fixation on GO-COOH/CS spheres largely reduced its bioaccumulation in wheat roots from 127.8 μg/g to 51.2 μg/g. The toxicity evaluations suggested that GO-COOH/CS spheres were of low toxicity to wheat seedlings and did not amplify the toxicity of Cu²⁺. The implications to the design of graphene adsorbents for soil remediation are discussed. Overall, our results collectively indicated that porous GO-COOH/CS spheres were high-performance adsorbents for the immobilization of Cu²⁺ to reduce Cu²⁺ bioaccumulation in plants.