Oxidative ageing induces change in the functionality of biochar and hydrochar: Mechanistic insights from sorption of atrazine

A systematic review of biochar aging and the potential eco-environmental risk in heavy metal contaminated soil

Biochar is widely accepted as a green and effective amendment for remediating heavy metals (HMs) contaminated soil, but its long-term efficiency and safety changes with biochar aging in fields. Currently, some reviews have qualitatively summarized biochar aging methods and mechanisms, aging-induced changes in biochar properties, and often ignored the potential eco-environmental risk during biochar aging process. Therefore, this review systematically summarizes the study methods of biochar aging, quantitatively compares the effects of different biochar aging process on its properties, and discusses the potential eco-environmental risk due to biochar aging in HMs contaminated soil. At present, various artificial aging methods (physical aging, chemical aging and biological aging) rather than natural field aging have been applied to study the changes of biochar's properties. Generally, biochar aging increases specific surface area (SSA), pore volume (PV), surface oxygen-containing functional group (OFGs) and O content, while decreases pH, ash, H, C and N content. Chemical aging method has a greater effect on the properties of biochar than other aging methods. In addition, biochar aging may lead to HMs remobilization and produce new types of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs) and colloidal/nano biochar particles, which consequently bring secondary eco-environmental risk. Finally, future research directions are suggested to establish a more accurate assessment method and model on biochar aging behavior and evaluate the environmental safety of aged biochar, in order to promote its wider application for remediating HMs contaminated soil.

A review: Hydrochar as potential adsorbents for wastewater treatment and CO2 adsorption

To valorize the biomass and organic waste, hydrothermal carbonization (HTC) stands out as a highly efficient and promising pathway given its intrinsic advantages over other thermochemical processes. Hydrochar, as the main product obtained from HTC, is widely applied as a fuel source and soil conditioner. Aside from these applications, hydrochar can be either directly used or modified as bio-adsorbents for environmental remediation. This potential arises from its tunable surface chemistry and its suitability to act as a precursor for activated or engineered carbon. In view of the importance of this topic, this review offers a thorough examination of the research progress for using hydrochar and its modified forms to remove organic dyes (cationic and anionic dyes), heavy metals, herbicides/pesticides, pharmaceuticals, and CO2. The review also sheds light on the fundamental chemistry involved in HTC of biomass and the major analytical techniques applied for understanding surface chemistry of hydrochar and modified hydrochar. The knowledge gaps and potential hurdles are identified to highlight the challenges and prospects of this research field with a summary of the key findings from this review. Overall, this article provides valuable insights and directives and pinpoints the areas meriting further investigation in the application potential of hydrochar in wastewater management and CO2 capture.

Effects of chemical aging on carbonaceous materials: Stability of water-dispersible colloids and their influence on the aggregation of natural-soil colloid

Although hydrochar and biochar have been used as soil conditioners, there is not a clear understanding of how their properties changes due to aging impacts their colloidal particles behavior on the soil system. From this premise, we produced hydrochar and biochar from the same feedstock (cashew bagasse) and aged with different chemical methods: (i) using hydrogen peroxide, (ii) a mixture of nitric and sulfuric acids, and (iii) hot water. It was analyzed the effects of aging on the stability of the carbonaceous materials (CMs) colloids in aqueous medium with different ionic strength (single systems), as well as the stability of the natural-soil colloid when interacting with biochar and hydrochar colloids (binary systems). A chemical composition (C, H, N, and O content) change in CMs due to the chemically induced aging was observed along with minor structural modifications. Chemical aging could increase the amount of oxygen functional groups for both biochar and hydrochar, though in a different level depending on the methodology applied. In this sense, hydrochar was more susceptive to chemical oxidation than biochar. The effectiveness of chemical aging treatments for biochar increased in the order of water < acid < hydrogen peroxide, whereas for hydrochar the order was water < hydrogen peroxide < acid. While the increase in surface oxidation improved the biochar colloidal stability in water medium at different ionic strengths (single systems), the stability and critical coagulation concentration (CCC) slightly changed for hydrochar. Natural-soil clay (NSC) interactions with oxidized carbonaceous material colloids (binary systems) enhanced NSC stability, which is less likely to aggregate. Therefore, the aging of carbonaceous materials modifies the interaction and dynamics of soil small particles, requiring far more attention to the environmental risks due to their application over time.

From Waste to Resource: Valorization of Lignocellulosic Agri-Food Residues through Engineered Hydrochar and Biochar for Environmental and Clean Energy Applications-A Comprehensive Review

Agri-food residues or by-products have increased their contribution to the global tally of unsustainably generated waste. These residues, characterized by their inherent physicochemical properties and rich in lignocellulosic composition, are progressively being recognized as valuable products that align with the principles of zero waste and circular economy advocated for by different government entities. Consequently, they are utilized as raw materials in other industrial sectors, such as the notable case of environmental remediation. This review highlights the substantial potential of thermochemical valorized agri-food residues, transformed into biochar and hydrochar, as versatile adsorbents in wastewater treatment and as promising alternatives in various environmental and energy-related applications. These materials, with their enhanced properties achieved through tailored engineering techniques, offer competent solutions with cost-effective and satisfactory results in applications in various environmental contexts such as removing pollutants from wastewater or green energy generation. This sustainable approach not only addresses environmental concerns but also paves the way for a more eco-friendly and resource-efficient future, making it an exciting prospect for diverse applications.

Aging rice straw reduces the bioavailability of mercury and methylmercury in paddy soil

Straw amendment is a prevalent agricultural practice worldwide, which can reduce air pollution and improve soil fertility. However, the impact of aging straw amendment on the bioavailability of mercury (Hg) and methylmercury (MeHg) in paddy soil remains unclear. To investigate this, incubation experiments were conducted using the diffusive gradient in thin-film technique. Results showed that amendments of dry-wet aging (DRS), photochemical aging (LRS), and freeze-thaw aging rice straw (FRS) reduced the bioavailable MeHg in paddy soil by 2.2-27.6%, 13.5-69.8%, and 23.5-86.1%, respectively, compared to fresh rice straw (RS) amendment. This result could be due to changes in soil properties such as soil pH and overlying water Fe and Mn as well as microbial abundance (including Clostridiaceae, Firmicutes, and Actinobacteriota). Simultaneously, The LRS and FRS amendments reduced bioavailable Hg in paddy soil by 20.0-40.8% and 17.1-48.6%, respectively, while DRS increased the bioavailable Hg by 15.8-120.0%. This could be attributed to changes in soil oxidation-reduction potential and overlying water SO42- content. Additionally, the results of sand culture experiments showed that the concentrations of Hg uptake by rice seedlings were 97.1-118.2%, 28.1-35.6%, and 198.0-217.1% higher in dissolved organic matter (DOM) derived from DRS, LRS, and FRS than RS, indicating that aging straw leached DOM may promote the Hg bioavailable when straw amendment. This result could be due to lower molecular weight and higher CO functional group content. These results provide new insight into how aging straw amendment affects the bioavailability of Hg and MeHg in paddy soil under different climates.

Low temperature pyrolytic biochar is a preferred choice for sulfonamide-Cu(II) contaminated soil remediation in tropical climate region

Biochar is getting increasing consideration for eco-friendly soil amendment and environmental remediation. Once added to the soil, biochar would undergo the natural ageing process, affecting its physicochemical properties and, as a result, the adsorption and immobilization of pollutants in the water and soil. To evaluate the high/low temperature pyrolyzed biochar performance on complex contaminants and the effect of climate ageing, the batch experiments were conducted on the adsorption of the pollutants of antibiotics sulfapyridine (SPY) and a typical coexisting heavy metal Cu²⁺ as one or binary system on low/high pyrolytic temperature biochars before and after the simulated tropical climate and frigid climate region ageing treatment. The results showed that high-temperature ageing could enhance the SPY adsorption in biochar-amended soil. The SPY sorption mechanism was fully elucidated, and the result confirmed that H-bonding was the dominant role in biochar-amended soil, and π-π electron-donor-acceptor (EDA) interaction and micro-pore filling was another factor for SPY adsorption. This study could lead to the conclusion that low-temperature pyrolytic biochar is a better option for sulfonamide-Cu(II) contaminated soil remediation in tropical regions.

Hydrogen Peroxide/Phosphoric Acid Modification of Hydrochars for Sulfamethoxazole and Carbamazepine Adsorption: The Role of Oxygen-Containing Functional Groups

Emerging pollutants, such as sulfonamide antibiotics and pharmaceuticals, have been widely detected in water and soils, posing serious environmental and human health concerns. Thus, it is urgent and necessary to develop a technology for removing them. In this work, a hydrothermal carbonization method was used to prepare the hydrochars (HCs) by pine sawdust with different temperatures. To improve the physicochemical properties of HCs, phosphoric acid (H₃PO₄) and hydrogen peroxide (H₂O₂) were used to modify these HCs, and they were referred to as PHCs and HHCs, respectively. The adsorption of sulfamethoxazole (SMX) and carbamazepine (CBZ) by pristine and modified HCs was investigated systematically. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results indicated that the H₂O₂/H₃PO₄ modification led to the formation of a disordered carbon structure and abundant pores. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy results suggested that carboxyl (-COOH) and hydroxyl (-OH) functional groups of HCs increased after modification, which is the main reason for the higher sorption of SMX and CBZ on H₃PO₄/H₂O₂-modified HCs when compared with pristine HCs. In addition, the positive correlation between -COOH/C=O and logK_d of these two chemicals also suggested that oxygen-containing functional groups played a crucial role in the sorption of SMX and CBZ. The strong hydrophobic interaction and π-π interaction between CBZ and pristine/modified HCs resulted in its higher adsorption when compared with SMX. The results of this study provide a novel perspective on the investigation of adsorption mechanisms and environmental behaviors for organic contaminants by pristine and modified HCs.

Artificial aging induced changes in biochar,s properties and Cd2+ adsorption behaviors

Fresh biochar has been widely applied to the remediation of heavy metals in soil by its property of adsorption, but the changes in its physicochemical properties and in situ adsorption performance over time cannot be ignored. In this study, the sorption of Cd²⁺ by corn straw biochars (CB) and municipal sludge biochars (SB) produced at 350 °C and 650 °C before and after H₂O₂ oxidation, and dry-wet and freeze-thaw aging were investigated using batch sorption experiments. The changes of physicochemical properties of biochar before and after aging were analyzed by various characterization methods. Based on these results, the impact of aging on the Cd²⁺ adsorption behavior could be clarified, which showed that CB650 was able to display the highest adsorption capacity in fresh biochars. Aging treatments reduced the ash content and pH value of CB, and significantly diminished the adsorption performance of Cd²⁺. These changes indicated that precipitation was a critical factor in the adsorption of Cd²⁺ on CB. The adsorption capacity of SB was enhanced after H₂O₂ oxidation, but weakened after dry-wet and freeze-thaw aging. This was closely related to the increase or decrease in the content of oxygen-containing functional groups, which in turn enhanced or inhibited its ability to compound with heavy metals. These results are of great significance for evaluating its long-term application prospects in the natural environment.

Mechanistic insight into adsorptive removal of ionic NOR and nonionic DEP organic contaminates by clay-biochar composites

The synthesis of clay-biochar composite has been recognized as an effective way to enhance the removal of pollutants. The interaction between clay mineral and biomass during thermal pyrolysis and the sorption capacity for ionic/nonionic organic containments have not been elaborated. In this study, two types of biochar were obtained from pyrolytic carbonization of the cellulosic-rich corn straw (C) and lignin-rich pine wood (P) at 500 or 700 °C. Typical clay minerals kaolinite and montmorillonite were selected to prepare clay-biochar composite. The results showed that the addition of clay mineral could strengthen dehydration reaction of corn straw biomass and reinforce its carbon structure. Montmorillonite-biochar composite owned more CC functional groups and porous structure than kaolinite-biochar composite. The addition of clay minerals could promote electrostatic attraction of ionic formed norfloxacin (NOR) on clay-pine wood biochar. However, the sorption capacity of nonionic diethyl phthalate (DEP) adsorption on clay-corn straw biochar decreased, owing to that clay increased the compactness of the biochar carbon structure, thus inhabited hydrophobic partition of nonionic organic compounds on disordered carbon fraction. The results from this study provide insights into the suitable contaminated site remediation by clay-biochar composite.

Norfloxacin adsorption and subsequent degradation on ball-milling tailored N-doped biochar

N-doping is an effective way to modify biochar for enhancing the adsorption capacity. The synthesis of N-doped biochar by the ball-milling method has been attractive due to its facile and eco-friendly approach with low energy consumption. However, the commonly used N-precursor NH₃·H₂O is environmentally harmful. It is needed to prepare safe and non-toxic N-doped biochar for large-scale production. Here, a urea N-doped biochar (U-MBC) was prepared by the ball-milling method and used for norfloxacin (NOR) removal. The results showed that U-MBC exhibited almost 4-fold higher adsorption capacity for NOR than pristine biochar in a wide pH range (3-9). The adsorption enhancement was owing to the enhancement of H-bonds, π-π electron donor-acceptor, and pore-filling interactions due to the N-doping and ball-milling method. Additionally, 89% of adsorbed NOR can be further removed after 6 h milling. The regenerated U-MBC still had a good adsorption capacity (46.27 mg g^-1) and performed well in three cycles. The knowledge gained from this study could encourage researchers to use urea or similar safe N-precursors with the ball-milling method for the large-scale production of N-doped biochar to remove antibiotic organic pollutants in the environment.

Characteristics of greenhouse gas emissions from farmland soils based on a structural equation model: Regulation mechanism of biochar

Greenhouse gas (GHG) emissions from soil carbon and nitrogen cycles during freeze-thaw cycles (FTCs) provide positive feedback to climate warming. Biochar is a new type of soil conditioner that shows potential in soil GHG emissions reduction. To explore the mechanisms of the effects of biochar on soil GHG emissions in seasonally frozen soil areas, this study focused on farmland soil in the Songnen Plain. Variations in soil environmental factors, available carbon and nitrogen and microbial biomass were analyzed using an indoor simulation of soil FTCs. A structural equation model (SEM) was established to reveal the key driving factors and potential mechanism of biochar on soil GHG emissions under FTCs. The results showed that biochar increased carbon dioxide (CO₂) emissions by 3.40% and methane (CH₄) absorption by 2.52% and decreased nitrous oxide (N₂O) emissions by 35.90%. SEM showed that soil temperature (ST) was the main environmental factor determining CO₂ emissions and that soil moisture (SM) was the main environmental factor determining CH₄ and N₂O emissions. Soil available carbon and nitrogen and microbial biomass are important for soil GHG emissions as the reaction substrates and main participants in the biochemical transformation of soil carbon and nitrogen, respectively. This study showed that the application of biochar in farmland is a feasible choice to address climate change in the long term via soil carbon sequestration and GHG emissions reduction. The research results provide a theoretical basis and scientific guidance for soil GHG emissions reduction during FTCs in middle to high latitudes.

Effect of ageing on biochar properties and pollutant management

Biochar is a carbon-rich pyrogenic material for multifunctional environmental applications such as carbon sequestration, soil amendment, and pollutant management, etc. Owing to long-term existing in the soil, biochar would inevitably suffer from natural geochemical weathering. Such ageing process could pose nonnegligible impacts on the physicochemical property and functionality of biochar. For an object-oriented design of biochar under different application ageing conditions, the latest research progress on ageing methods, biochar properties, and pollutant sorption performance needs to be fully understood. Specifically, the effect of soil components on biochar ageing is critically reviewed, which is of importance but not fully explored so far. The decrease of ash in aged biochar can inhibit the adsorption of heavy metals. The loss of aromatic components and the formation of three-dimensional water clusters during the ageing process have a negative impact on high-temperature biochar (>500° C) for organic pollutants adsorption. For long-term soil remediation, these results remind us to carefully use high-ash biochar for heavy metals and high-temperature biochar for organic pollutants. The interaction between soil minerals and biochar can form organometallic complexes and change functional groups to enhance the oxidation resistance of biochar. In the present review, the current research on biochar ageing are critical reviewed, and the further researches are prospected including developing advanced artificial ageing methods, exploring the impact of soil components on biochar ageing, and clarifying the long-term environmental behavior of modified biochar.

Mechanism of aging of biochars obtained at different temperatures from sewage sludges with different composition and character

The aim of this study was to determine the effect of abiotic aging of sewage sludge (SSL)-derived biochars on their physicochemical properties and in consequence on their stability. Biochars produced at 500 or 700 °C from SSLs with a different composition and properties were incubated at different temperatures (-20, 4, 20, 60, and 90 °C) for 6 and 12 months. Pristine and aged biochars were characterized in terms of their composition and properties using a range of complementary methods. The results showed that SSL-derived biochars will not be as stable as previously thought in the long term. The stability of the SSL-derived biochars was closely related to the content and character of C. The biochars that had more C in their composition and, apart from aromatic C, also aliphatic matter/carbon substances deposited in surface pores (i.e. those produced from SSL with a lower initial ash content and a lower degree of aromaticity) were less stable than the biochars with a lower C content and a typically aromatic character of C (i.e. those derived from SSL with a higher initial ash content and a higher degree of aromaticity). Their oxidation led to partial mineralization of aliphatic chains or organic surface film and manifested itself in a greater changes in their properties. The low-temperature biochars (BC-500) with lower aromaticity were found to be more susceptible to oxidation than the high-temperature ones (BC-700) with higher aromaticity. The more aromatic structure of C limited access of O₂ molecules to biochar interior, due to which the processes occurring during aging were concentrated in their surface layer and their properties were less change. It can therefore be concluded that pyrolysis of SSL with higher aromaticity and a lower organic content and higher pyrolysis temperatures will lead to obtaining more stable SSL-derived biochars.

Effective adsorptive removal of atrazine herbicide in river waters by a novel hydrochar derived from Prunus serrulata bark

In this work, a novel and effective hydrochar was prepared by hydrothermal treatment of Prunus serrulata bark to remove the pesticide atrazine in river waters. The hydrothermal treatment has generated hydrochar with a rough surface and small cavities, favoring the atrazine adsorption. The adsorption equilibrium time was not influenced by different atrazine concentrations used, being reached after 240 min. The Elovich adsorption kinetic model presented the best adjustment to the kinetic data. The Langmuir model presented the greatest compliance to the isotherm data and indicated a higher affinity between atrazine and hydrochar, reaching a maximum adsorption capacity of 63.35 mg g^-1. Thermodynamic parameters showed that the adsorption process was highly spontaneous, endothermic, and favorable, with a predominance of physical attraction forces. In treating three real river samples containing atrazine, the adsorbent showed high removal efficiency, being above 70 %. The hydrochar from Prunus serrulata bark waste proved highly viable to remove atrazine from river waters due to its high efficiency and low precursor material cost.

Degradation of ramipril by residues from the brewing industry: A new carbon-based photocatalyst compound

This study is a pioneer in the use of hydrochar as a support for photocatalytic oxide and its application and evaluation as a catalyst in degradation reactions of ramipril. Novel composites were easily prepared by the support TiO₂ or ZnO nanoparticles on the malt bagasse hydrochar. The preparation of the hydrochar requires low synthesis temperature (250 °C), generating the energy savings of the process. The production of the new composites was well supported by different analytical techniques XRD, FTIR, SSA, SEM, EDS, and reflectance diffuse. The effect of different proportions of TiO₂ or ZnO on the composites was investigated on the degradation efficiency of the pharmaceutical ramipril, without pH adjustment. Composites with a 5:1 hydrochar/TiO₂ or ZnO ratio (MH5T and MH5Z, respectively) showed degradations of 72 and 98% of ramipril at 120 min. This remarkable performance may be associated with the decrease in band gap energy and the electron-hole recombination rate. In addition, the composites were more efficient than metal oxides pristine, and this may be related to the fact that hydrochar have a high concentration of phenolic, hydroxyl, and carboxylic functional groups on their surface. Radical identification tests indicated that h⁺, O₂•^-, and •OH were the reactive species involved in the degradation. The proposed mechanism was studied via LC-MS/MS indicated that the ramipril molecule degrades into low m/z intermediates in the first 60 min of reaction using the MH5Z.

Biochars ages differently depending on the feedstock used for their production: Willow- versus sewage sludge-derived biochars

The aim of this study was to determine the effect of abiotic aging of biochars under controlled laboratory conditions on its physicochemical properties and in consequence on their stability. Biochars (BCs) produced at 500 and 700 °C from willow or sewage sludge were incubated at different temperatures (-20, 4, 20, 60, or 90 °C) for 6 and 12 months. Pristine (i.e. immediately after their production) and aged BCs were characterized using a range of complementary methods. As a result of simulated temperature aging, there was a change in all biochar properties studied, with the direction of these changes being determined by both the type of feedstock and biochar production temperature. At all temperatures, aging was the most intense during the first 6 months and led to oxidation of the biochars and removal of the most labile components from them. The intensity of these processes increased with increasing aging temperature. Incubation of the biochars for another 6 months did not have such a significant effect on the biochar properties as that observed during the first months of incubation, which is evidence that the biochars had reached stability. The sewage sludge-derived biochars with a higher mineral content than the willow-derived biochars were less stable. The low-temperature biochars (BC-500) with lower aromaticity were more prone to abiotic oxidation than the high-temperature biochars (BC-700) with higher aromaticity and structurally ordered C. Based on this study, it can be concluded that aging induced changes will be specific for each biochar, i.e. they will depend on both the type of feedstock and pyrolysis temperature. Nonetheless, all biochars will be oxidized to a smaller or greater extent, which will result in an increase in the number of surface oxygen functional groups, an increased degree of their hydrophilicity and polarity, and a decrease in pH.

High-temperature and freeze-thaw aged biochar impacts on sulfonamide sorption and mobility in soil

Biomass-derived biochar is a carbon-rich product for soil amendment and sulfapyridine (SPY) is a typical sulfonamide of antibiotics in the soil. Amendment with biochar for soil could control SPY sorption or mobility. However, the pristine biochar inevitably goes through the long-term ageing in the environment and the information on such ageing impact on SPY sorption is not fully recognized. The simulated ageing process methods were employed for high-temperature and freeze-thraw climate to treat the biochar for two months in the present study. The batch adsorption of SPY and leaching column experiments were conducted for comparison of the fresh/aged biochar-soil system. The results showed that biochar addition could increase soil pH and saturated moisture, aged biochars own more O-containing functional groups and exhibit higher hydrophilicity and polarity. The sorption mechanism of unamended soil with SPY primarily resulted from the weak hydrophobic distribution. All fresh and aged biochar amended soil increased SPY sorption due to improvement of H-bonding interaction between SPY and biochar surface functional groups, indicating such initiative adsorption was stronger than passive partitioning. It is of importance for us to reconsider that aged biochar-amended soil, especially two-month high-temperature aged biochar-amended soil showed the highest adsorption performance and the lowest desorption capacity towards SPY. Both SPY leaching column experiments and the acid rain leaching tests suggested that the application of biochar in tropical or high-temperature climate regions for organics polluted soil remediation is favorable, but we should be aware of the uncertainty of soil amendment with biochar in cold regions.

Contrasting impacts of chemical and physical ageing on hydrochar properties and sorption of norfloxacin with coexisting Cu2

The conversion of agricultural biomass into hydrochar has enormous potential to improve soil quality. In particular, hydrochar particles introduced into the natural environment readily bind environmental pollutants. The interaction of hydrochar and pollutants will, however, be impacted by long term natural ageing in the earth surface. The adsorption performance and the associated mechanisms that could be affected by physical or chemical ageing are not yet fully understood. To elucidate the influence of different types ageing on the physicochemical properties and sorption capacity of hydrochar, we systematically characterized the elemental composition, specific surface area, total organic carbon, and functional groups of fresh and aged hydrochar. Norfloxacin (NOR), a typical antibiotic as a model in this study, was used for the sorption performance of different aged hydrochars in the presence or absence of Cu²⁺. The various artificial accelerated ageing methods have been employed such as H₂O₂ oxidation, HNO₃/H₂SO₄ acidification, high temperature, and freeze-thaw cycles. The results showed that ageing could increase hydrochar polarity and surface functional groups, which both increased NOR hydrophobic partition and H-bonding interaction on hydrochars. The chemical ageing largely increased the abundance of CO than physical ageing. H-bonding dominated NOR sorption on hydrochars after acidification, high temperature, and freeze-thaw cycles. The hydrophobic partition was the main sorption mechanism of NOR on oxidative aged hydrochars. The coexisting Cu²⁺ inhibited NOR sorption on most aged hydrochars (acidification, high temperature, and freeze-thaw cycles), whereas specially for oxidative ageing, Cu²⁺ increased hydrophobic sorption sites on hydrochars surface and enhanced the sorption capacity for NOR. The results from this study are likely to reveal the mechanisms of pollutant adsorption on hydrochars and their different susceptibilities under various ageing environment, suggesting us to comprehensively consider the reciprocal effects of natural ageing and coexisting pollutants on a long-term use of hydrochar in the field.

Sorption of anthracene (C14H10) and 9-anthroic acid (C15H10O2) onto biochar-amended soils as affected by field aging treatments

There is a lack of information on how aging affects the sorption of ionizable organic compounds on biochar-amended soils. To that end, this study investigates the sorption of two hydrophobic solutes (anthracene (ANT) and 9-anthroic acid (9-ANCA)) onto biochar-amended (5%, w/w) soils as a function of aging period (0, 6, and 12 months), electrolyte (5 mM CaCl₂ and 10 mM KCl), and aqueous pH. The isotherm plot of both solutes was fairly linear (r² > 0.998) and the linear sorption coefficient (K_d, L kg^-1) was obtained from this. In CaCl₂ solution at pH 5, the log K_d of ANT tended to decrease (from 3.90 to 3.72) with an increasing aging period, which was attributed to clogged pore surface, whereas the differences in 9-ANCA sorption (from 2.56 to 2.51) were not significant (α = 0.05). The increased ANT sorption at acidic pH (<4) could be attributed to π-π interaction. Aqueous Ca²⁺ ions played an important role in 9-ANCA sorption by forming a Ca-bridge between anionic solute and negatively charged adsorbent surface, thus accounting for up to 35% of its sorption at alkaline pH (>8). The spectroscopic data and isoelectric point measurement results indicated that the number of oxygen-containing functional groups and the content of elemental oxygen were both higher in aged samples, resulting in a more polar (negatively charged) surface. The formation of surface polar groups and the associated deformation altered the adsorbent nature of the tested biochar, thereby fortifying the hydrophilic retention propensity for ionizable organic solutes.

In Situ hydrochar regulates Cu fate and speciation: Insights into transformation mechanism

Cu is one of the dominant heavy metals toxic to human health and environmental ecosystems. Understanding its fate and chemical speciation is of great importance for hydrothermal liquefaction (HTL) of Cu-rich hazardous streams. Herein, we investigated its evolution during the HTL of wastewater algae through ICP-MS, XRD, XANES, and EXAFS. Cu-cysteine complexes (51.5%) and Cu₂S (40.4%) were the main components of Cu in algae, whereas the predominant form was CuS (70.9%) in 220 °C-hydrochar. Model compound experiments indicated that Cu-cysteine could be converted into CuS, while Cu₂S was stable during HTL. However, Cu₂S was partially converted into CuS in the hydrochar. Subsequently, the positive Gibbs free energy (36.8 KJ/mol) indicates that the oxidation from Cu⁺ to Cu²⁺ can't occur spontaneously. Furthermore, cyclic voltammograms demonstrated that hydrochar facilitated the oxidation of Cu₂S due to its higher capability of electron acceptance. All these results prove that hydrochar serves as a catalyst for the conversion of Cu₂S to CuS during HTL. This study firstly elucidated that Cu₂S was oxidized into CuS in the presence of hydrochar, and Cu-cysteine was converted into CuS under HTL. This study provides a critical insight into the transformation mechanism of Cu during the HTL of hazardous streams.

Physicochemical properties of aged hydrochar in a rice-wheat rotation system: A 16-month observation

The importance of hydrochar properties for soil application is well known, but the effects of natural aging on hydrochar properties remain ambiguous. The present study aimed to determine the shift patterns in the physicochemical properties of hydrochar through a 16-month soil column aging experiment conducted in a rice-wheat rotation system with hydrochars derived from a wheat straw at 220 °C and 260 °C. Obvious decreasing hydrophilic/polarity indices and increasing porosity, ash content, and stability occurred in aged hyrdrochar, which were due to the dissolved organic matter (DOM) leaching and the interaction with mineral content and fertilizer during the 16-month aging process. Besides, fewer C-OH, slightly more CO, and higher aromaticity (C-C/CC) in aged hydrochar were observed. Meanwhile, the relative abundance of the compounds containing only C, H, and O atoms in water extract of aged hydrochar decreased, while that of the compounds containing C, H, O, and N atoms increased during aging; these findings were attributed to the less labile DOM and microbial degradation and the retention of some plant-derived dissolved organic carbon, respectively. This study provided 16-month aging characterization data regarding alteration in hydrochar physicochemical properties, which was conducive to make a better understanding of the use of hydrochars as sustainable soil amendments from agroecosystems and environmental perspective.

Biochar Aging: Mechanisms, Physicochemical Changes, Assessment, And Implications for Field Applications

Biochar has triggered a black gold rush in environmental studies as a carbon-rich material with well-developed porous structure and tunable functionality. While much attention has been placed on its apparent ability to store carbon in the ground, immobilize soil pollutants, and improve soil fertility, its temporally evolving in situ performance in these roles must not be overlooked. After field application, various environmental factors, such as temperature variations, precipitation events and microbial activities, can lead to its fragmentation, dissolution, and oxidation, thus causing drastic changes to the physicochemical properties. Direct monitoring of biochar-amended soils can provide good evidence of its temporal evolution, but this requires long-term field trials. Various artificial aging methods, such as chemical oxidation, wet-dry cycling and mineral modification, have therefore been designed to mimic natural aging mechanisms. Here we evaluate the science of biochar aging, critically summarize aging-induced changes to biochar properties, and offer a state-of-the-art for artificial aging simulation approaches. In addition, the implications of biochar aging are also considered regarding its potential development and deployment as a soil amendment. We suggest that for improved simulation and prediction, artificial aging methods must shift from qualitative to quantitative approaches. Furthermore, artificial preaging may serve to synthesize engineered biochars for green and sustainable environmental applications.

Oxidative ageing of biochar and hydrochar alleviating competitive sorption of Cd(II) and Cu(II)

Biochar and hydrochar have been served as attractive adsorbents for remediation of polluted water and soil, but it is lack of the long-term ageing effects on competitive adsorption of co-existing heavy metals by these carbonized materials. By this, corn stalk was used as carbon precursor to prepare biochar (500 °C) and hydrochar (200 °C). The single-metal and binary-metal Cd(II)/Cu(II) sorption were conducted on biochar and hydrochar before and after ageing using artificial accelerated ageing of 5% H₂O₂ treatment. The elemental analysis, BET, SEM, FTIR, XRD and Zeta potential were used to characterize the physicochemical properties of carbonized material samples. The results showed that oxidative ageing could increase O content and O-containing functional groups but decrease C content, metal content and aromaticity degree. Ageing hardly affected the SSA and crystallographic structures of biochar and hydrochar. The reduction of metal content in Aged-BC caused a decline of sorption capacity, indicating that cation exchange would be the predominant factor involved in biochar sorption for Cd(II) and Cu(II). As for hydrochar with more O-containing functional groups than biochar, the dominated sorption mechanism would be surface complexation, due to higher sorption capacity of Aged-HC with richer O-containing functional groups. In binary-metal system, the competitive sorption of Cd(II) and Cu(II) on biochar was observed obviously but that on hydrochar was limited. Ageing could increase the sorption capacity of Cd(II) in binary-metal system, resulting in alleviating competitive adsorption. The total sorption amount of Cd(II) and Cu(II) by biochar was markedly greater than that of hydrochar before or after ageing, suggesting that biochar can be still more capable than hydrochar for handling Cd(II) and Cu(II) in single-metal or binary-metal. These findings suggest us to consider the long-term effect on immobilization of co-existing heavy metals and alleviating competitive adsorption of carbonized materials as alternative amendment for contaminated sites.

Removal of aqueous-phase lead ions by dithiocarbamate-modified hydrochar

Hydrochar produced from agricultural and forestry wastes and its application into the environment are very attractive. Herein, a high-efficiency dithiocarbamate-modified hydrochar (DTHC) was prepared successfully and then applied to eliminate Pb(II) from aqueous solutions. DTHC was characterized by various techniques. It was found that dithiocarbamate and amine groups were successfully grafted onto the surface of hydrochar. The surface area of DTHC was 7.94 m²·g^-1, which was four folds less than pristine hydrochar (31.60 m²·g^-1), but its adsorption capacity obviously increased. Adsorption experiments showed that the Pb(II) adsorption process onto DTHC well accorded with pseudo-2nd-order kinetics and Langmuir isotherms. The highest Pb(II) uptake by DTHC at 293 K determined from the Langmuir model was 151.51 mg·g^-1. Fourier transform infrared spectra and X-ray photoelectron spectroscopy verified that dithiocarbamate, carboxylate, amine and sulfonate groups all facilitated the Pb(II) adsorption. The adsorption mechanism was ascribed to the inner-sphere surface complexation of Pb(II) by these groups and to the ion exchange between Pb(II) and Na(I). Thus, DTHC is an effective adsorbent for Pb(II) removal from water.

Deep Learning Neural Network Approach for Predicting the Sorption of Ionizable and Polar Organic Pollutants to a Wide Range of Carbonaceous Materials

Most contaminants of emerging concern are polar and/or ionizable organic compounds, whose removal from engineered and environmental systems is difficult. Carbonaceous sorbents include activated carbon, biochar, fullerenes, and carbon nanotubes, with applications such as drinking water filtration, wastewater treatment, and contaminant remediation. Tools for predicting sorption of many emerging contaminants to these sorbents are lacking because existing models were developed for neutral compounds. A method to select the appropriate sorbent for a given contaminant based on the ability to predict sorption is required by researchers and practitioners alike. Here, we present a widely applicable deep learning neural network approach that excellently predicted the conventionally used Freundlich isotherm fitting parameters log K_F and n (R² > 0.98 for log K_F, and R² > 0.91 for n). The neural network models are based on parameters generally available for carbonaceous sorbents and/or parameters freely available from online databases. A freely accessible graphical user interface is provided.

The residue from the acidic concentrated lithium bromide treated crop residue as biochar to remove Cr (VI)

In this work, the hydrolysis residue produced from the acidic concentrated lithium bromide hydrolysis (ALBH) of wheat straw, corn stover and elephant grass were characterized as biochar. The ALBH biochar as the black power had high content of carbon (49.65-55 wt%), specific surface areas (4.53-7.79 m²/g), porous structures (micropores, mesopores and macropores) and abundant oxygen functional groups (hydroxy, carbonyl, ester and ketone groups). These properties made ALBH biochar as a potential adsorbent for environmental remediation, with relatively high removal efficiency for a variety of heavy metal ions, especially hexavalent chromium (Cr(VI)). Therefore, ALBH technology may be an efficient strategy for synthesis of bio-char along with fermentable sugars, which met the concern of sustainability and green chemistry.

A comparative study on biochar properties and Cd adsorption behavior under effects of ageing processes of leaching, acidification and oxidation

Biochar has potential to control the bioavailability and migration of potentially toxic heavy metals in soil by adsorption. Natural ageing in the environment may change the physicochemical properties and adsorption function of biochar over the long-term. The present study compared the effects of different simulated ageing treatments on Cd adsorption of high and low temperature biochar from straw of corn (Zea mays). Fresh and aged biochars were systematically characterized by elemental analysis, FTIR, XPS, Zeta, SEM-EDS, XRD and the composition of their mineral ash. The adsorption of Cd to fresh and aged biochars was then assessed under the influence of pH. Drawing the results together the effects of ageing on the extent and mode of Cd adsorption could be elucidated. The results showed that the adsorption capacity of fresh biochar produced at 650 °C was higher than of biochar made at 350 °C, and that mineral co-precipitation plays a dominant role in Cd sorption. Leaching removed organic and inorganic ash components from biochars, markedly diminishing the capacity of the high temperature biochar to adsorb Cd. The adsorption performance of the low temperature biochar was dependent on surface complexation. The adsorption capacity of low-temperature biochar was markedly enhanced by oxygen-containing functional groups formed through acidification and oxidation. The long-term benefits of biochar in the management of polluted soil require a rethink, considering the contrasting ageing behavior of different temperature biochar and their response to different ageing environments.