New Evidence for High Sorption Capacity of Hydrochar for Hydrophobic Organic Pollutants

The impact of biochar's physicochemical properties on sorption of perfluorooctanoic acid (PFOA)

To better characterize properties governing the sorption of per- and polyfluoroalkyl substances (PFAS) to biochar, twenty-three diverse biochars were characterized and evaluated as sorbents for perfluorooctanoic acid (PFOA). Biochars were produced at various temperatures, using two different technologies, and made from sewage sludge, food waste reject, wood wastes, and one reference substrate (wood pellets). The biochars were characterized in terms of surface area, pore volume and pore size distributions, elemental composition, leachable elements, ash content, pH, zeta potential, condensed aromatic carbon (ConAC) content (determined by benzenepolycarboxylic acid (BPCA) markers), and their -OH functional group content (infrared spectroscopy). PFOA sorption isotherms were determined using Polanyi-Dubinin-Manes (PDM) and Freundlich models. The sludge-based biochars [Freundlich coefficients (log KF) between 2.56 ± 0.11 and 6.72 ± 0.22 (μg/kg)/(μg/L)nF; fitted free energy of adsorption (E) and pore volume (Vo) from the PDM model between 13.27 and 17.26 kJ/mol, and 0.50 and 523.51 cm3/kg] outperformed wood biochars [log KF between 1.02 and 4.56 ± 0.22 (μg/kg)/(μg/L)nF; E between 9.87 and 17.44 kJ/mol; Vo between 0.21 and 7.16 cm3/kg] as PFOA sorbents. Multivariate statistical analysis revealed that the sorption capacity was mainly controlled by pore volume within the pore diameter region that could accommodate the molecular size of PFOA (3-6 nm). Hydrophobic interactions between PFOA and aromatic carbon rich regions controlled sorption affinity, especially in the wood biochars.

Reuse of straw in the form of hydrochar: Balancing the carbon budget and rice production under different irrigation management

Hydrochar is proposed as a climate-friendly organic fertilizer, but its potential impact on greenhouse gas (GHG) emissions in paddy cultivation is not fully understood. This two-year study compared the impact of exogenous organic carbon (EOC) application (rice straw and hydrochar) on GHG emissions, the net ecosystem carbon budget (NECB), net global warming potential (net GWP), and GHG emission intensity (GHGI) in a rice pot experiment using either flooding irrigation (FI) or controlled irrigation (CI). Compared with FI, CI increased ecosystem respiration by 23 - 44 % and N2O emissions by 85 - 137 % but decreased CH4 emissions by 30 - 58 % (p < 0.05). Since CH4 contributed more to net GWP than N2O, CI reduced net GWP by 16 - 220 %. EOC amendment increased crop yield by 5 - 9 % (p < 0.05). Compared with CK, hydrochar application increased initial GHG emission, net GWP and GHGI in the first year, while in the second year, there was no significant difference in net GWP and GHGI between CI-hydrochar and CK. Compared with straw addition, hydrochar amendment reduced net GWP and GHGI by 20 - 66 % and 21 - 66 %; and exhibited a lower net CO2 emission when considering the energy input during the hydrochar production. These findings suggest that integrated CI-hydrochar practices would be a sustainable and eco-friendly way for organic waste management in rice production as it holds potential to enhance the NECB and SOC sequestration of rice production, while also offsetting the extra carbon emissions from organic inputs.

Sludge-derived hydrochar modulates complete nonradical electron transfer in peroxydisulfate activation via pyrrolic-N and carbon defect: Implication for degrading electron-rich ionizable anilines compounds

Nonradical electron transfer process (ETP) is a promising pathway for pollutant degradation in peroxydisulfate-based advanced oxidation processes (PDS-AOPs). However, there is a critical bottleneck to trigger ETP by sludge-derived hydrochar due to its negatively charged surface, inferior porosity and electrical conductivity. Herein, pyrrolic-N doped and carbon defected sludge-derived hydrochar (SDHC-N) was constructed for PDS activation to degrade anilines ionizable organic compounds (IOC) through complete nonradical ETP oxidation. Degradation of anilines IOC was not only affected by the electron-donating capacity but also proton concentration in solution because of the ionizable amino group (-NH2). Diverse effects including proton favor, insusceptible and inhibition were observed. Impressively, addition of HCO3 with strong proton binding capacity boosted aniline degradation nearly 10 times. Moreover, characterizations and theoretical calculations demonstrated that pyrrolic-N increased electron density and created positively charged surface, profoundly promoting generation of SDHC-N-S2O82-* complexes. More delocalized electrons around carbon defect could enhance electron mobility. This work guides a rational design of sludge-derived hydrochar to mediate nonradical ETP oxidation, and provides insights into the impacts of proton on anilines IOC degradation.

Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials

The contemporary production of carbon materials heavily relies on fossil fuels, contributing significantly to the greenhouse effect. Biomass is a carbon-neutral resource whose organic carbon is formed from atmospheric CO2. Employing biomass as a precursor for synthetic carbon materials can fix atmospheric CO2 into solid materials, achieving negative carbon emissions. Hydrothermal carbonization (HTC) presents an attractive method for converting biomass into carbon materials, by which biomass can be transformed into materials with favorable properties in a distinct hydrothermal environment, and these carbon materials have made extensive progress in many fields. However, the HTC of biomass is a complex and interdisciplinary problem, involving simultaneously the physical properties of the underlying biomass and sub/supercritical water, the chemical mechanisms of hydrothermal synthesis, diverse applications of resulting carbon materials, and the sustainability of the entire technological routes. This review starts with the analysis of biomass composition and distinctive characteristics of the hydrothermal environment. Then, the factors influencing the HTC of biomass, the reaction mechanism, and the properties of resulting carbon materials are discussed in depth, especially the different formation mechanisms of primary and secondary hydrochars. Furthermore, the application and sustainability of biomass-derived carbon materials are summarized, and some insights into future directions are provided.

Activation of iron oxide minerals in an aquifer by humic acid to promote adsorption of organic molecules

In aquifers, the sequestration and transformation of organic carbon are closely associated with soil iron oxides and can facilitate the release of iron ions from iron oxide minerals. There is a strong interaction between dissolved organic matter (DOM) and iron oxide minerals in aquifers, but the extent to which iron is activated by DOM exposure to active iron minerals in natural aquifers, the microscopic distribution of minerals on the surface, and the mechanisms involved in DOM molecular transformation are currently unclear. This study investigated the nonbiological reduction transformation and coupled adsorption of iron oxide minerals in aquifers containing DOM from both macro- and micro perspectives. The results of macroscopic dynamics experiments indicate that DOM can mediate soluble iron release during the reduction of iron oxide minerals, that pH strongly affects DOM removal, and that DOM is more efficiently degraded at low rather than high pH values, suggesting that a low pH is conducive to DOM adsorption and oxidation. Spherical aberration-corrected scanning transmission electron microscopy (SACTS) indicates that the reacted mineral surfaces are covered with large amounts of carbon and that dynamic agglomeration of iron, carbon, and oxygen occurs. At the nanoscale, three forms of DOM are found in the mineral surface agglomerates (on the surfaces, inside the surface agglomerates, and in the polymer pores). The microscopic organic carbon and iron mineral reaction patterns can form through oxidation reactions and selective adsorption effects. Fourier transform ion cyclotron resonance mass spectra indicate that both synergistic and antagonistic reactions occur between DOM and the minerals, that the release of iron is accompanied by DOM decomposition and humification, that large oxygen- and carbon-containing molecules are broken down into smaller oxygen- and carbon-containing compounds and that more molecules are produced through oxidation under acidic rather than alkaline conditions. These molecules provide adsorption sites for sediment, meaning that more iron can be released. Microscopic evidence for the release of iron was acquired. These results improve the understanding of the geochemical processes affecting iron in groundwater, the nonbiological transformation mechanisms that occur at the interfaces between natural iron minerals and organic matter, groundwater pollution control, and the environmental behavior of pollutants.

Performance analysis of hydrochar derived from catalytic hydrothermal carbonization in the multicomponent emerging contaminant systems: Selectivity and modeling studies

In this work, as an alternative to pyrochar, catalytic hydrothermal carbonization has been employed to synthesize hydrochar to eliminate emerging contaminants in multicomponent systems. The hydrochar has been synthesized using a single step catalytic hydrothermal carbonization at low temperature (200 °C) without any secondary activation with high specific surface area and very good adsorption efficiency for the removal of emerging contaminants. The synthesized hydrochar (HC200) was characterized using various analytical techniques and found to have porous structure with 114.84 m2.g-1 of specific surface area and also contained various oxygen-containing functionalities. The maximum adsorption efficiencies of 92.4 %, 85.4 %, and 82 % were obtained for ibuprofen, sulfamethoxazole, and bisphenol A, respectively. Humic acid, a naturally occurring organic compound had a negligible effect on the adsorption of the selected contaminants. The hydrochar's selectivity towards the emerging contaminants in binary and ternary multicomponent systems was in the order of ibuprofen > sulfamethoxazole > bisphenol A.

Removal of per- and polyfluoroalkyl substances by activated hydrochar derived from food waste: Sorption performance and desorption hysteresis

Carbonaceous materials, derived from waste biomass, have proven to be a viable and appealing alternative for removing emerging micro-pollutants, such as per- and polyfluoroalkyl substances (PFAS). To assess the feasibility and efficacy of using material derived from food waste to alleviate PFAS pollution, this study prepared activated hydrochar (AHC) for sorbing ten PFAS, including five perfluoroalkyl carboxylic acids (PFCA; C4-C8), three perfluoroalkyl sulfonic acids (PFSA; C4, C6, C8), and two emerging PFAS, namely hexafluoropropylene oxide dimer acid (commercial name GenX, an alternative to perfluorooctanoic acid (PFOA)) and 6:2 fluorotelomer sulfonic acid (6:2 FTS). The results demonstrated that AHC possessed a relatively high specific surface area (207 m2/g) and hydrophobic surface properties. At environmentally relevant concentrations (40 μg/L), the sorption partition coefficients (log Kd) of PFAS on AHC ranged from 2.33 to 6.49 L/kg. Notably, GenX exhibited a lower log Kd value (2.33 L/kg) than PFOA (3.88 L/kg). The AHC showed favorable sorption performance for all tested PFAS, with log Kd values surpassing other reported sorbents (e.g., 0.83 for GenX on pyrochar, and 2.83 for PFOA on commercial biochar). Additionally, desorption hysteresis was observed for all PFAS, except for PFOA, and was particularly pronounced in PFBA, GenX, and 6:2 FTS at high initial concentrations, with Hysteresis Index (HI) values varying from 0.31 to 1.45, 0.68 to 1.88, and 0.51 to 1.85, respectively. Given its robust sorption capacity and desorption hysteresis toward PFAS, AHC is expected to be a favorable candidate for remediating PFAS-contaminated water. This study underscores, for the first time, the potential of food waste-derived hydrochar as an efficient sorbent for alleviating PFAS contamination, and further study is needed to investigate the sorption and desorption behaviors of PFAS on AHC at various environmental conditions.

Chemodiversity of soil organic matters determines biodegradation of polychlorinated biphenyls by a graphene oxide-assisted bacterial agent

A promising strategy for degrading persistent organic pollutants (POPs) in soil is amendment with nanomaterial-assisted functional bacteria. However, the influence of soil organic matter chemodiversity on the performance of nanomaterial-assisted bacterial agents remains unclear. Herein, different types of soil (Mollisol soil, MS; Ultisol soil, US; and Inceptisol soil, IS) were inoculated with a graphene oxide (GO)-assisted bacterial agent (Bradyrhizobium diazoefficiens USDA 110, B. diazoefficiens USDA 110) to investigate the association between soil organic matter chemodiversity and stimulation of polychlorinated biphenyl (PCB) degradation. Results indicated that the high-aromatic solid organic matter (SOM) inhibited PCB bioavailability, and lignin-dominant dissolved organic matter (DOM) with high biotransformation potential was a favored substrate for all PCB degraders, which led to no stimulation of PCB degradation in MS. Differently, high-aliphatic SOM in US and IS promoted PCB bioavailability. The high/low biotransformation potential of multiple DOM components (e.g., lignin, condensed hydrocarbon, unsaturated hydrocarbon, etc.) in US/IS further resulted to the enhanced PCB degradation by B. diazoefficiens USDA 110 (up to 30.34%) /all PCB degraders (up to 17.65%), respectively. Overall, the category and biotransformation potential of DOM components and the aromaticity of SOM collaboratively determine the stimulation of GO-assisted bacterial agent on PCB degradation.

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.

Hydrochar more effectively mitigated nitrous oxide emissions than pyrochar from a coastal soil of the Yellow River Delta, China

Application of char amendments (e.g., pyrochar or biochar, hydrochar) in degraded soils is proposed as a promising solution for mitigating climate change via carbon sequestration and greenhouse gases (GHGs) emission reduction. However, the hydrochar-mediated microbial modulation mechanisms underlying N₂O emissions from coastal salt-affected soils, one of essential blue C ecosystems, were poorly understood. Therefore, a wheat straw derived hydrochar (SHC) produced at 220 °C was prepared to investigate its effects on N₂O emissions from a coastal salt-affected soil in the Yellow River Delta and to distinguish the microbial regulation mechanisms in comparison with corresponding pyrochar pyrolyzed at 500 °C (SPC) using a 28-day soil microcosm experiment. Compared with SPC, the acidic SHC (pH 4.15) enriched in oxygenated functional groups, labile C and N constituents. SHC application more efficiently depressed cumulative soil N₂O emissions (48.4-61.1 % vs 5.57-45.2 %) than those of SPC. SHC-induced inhibition of ammonia-oxidizing gene (amoA)-mediated nitrification and promotion of full reduction of N₂O to N₂ by nitrous oxide reductase gene (nosZ) were the underlying microbial mechanisms. Structural equation models further revealed that SHC-modulated bacterial N-transformation responses, i.e., inhibited nitrification and promoted heterotrophic denitrification, mainly contributed to reduced N₂O emissions, whereas modification of soil properties (e.g., decreased pH, increased total C content) by SPC dominantly accounted for decreased N₂O emissions. These results address new insights into microbial regulation of N₂O emission reduction from the coastal salt-affected soils amended with hydrochar, and provide the promising strategies to enhance C sequestration and mitigate GHG emissions in the blue C ecosystems.

Calcium-pyro-hydrochar derived from the spent mushroom substrate as a functional sorbent of Pb2+ and Cd2+ from aqueous solutions

A calcium-pyro-hydrochar (Ca-PHC) can be distinguished as a novel sorbent of Pb²⁺ and Cd²⁺ from an aqueous solution. It was obtained using hydrothermal treatment of the spent mushroom substrate (SMS), followed by a CaCl₂·5H₂O activation and pyrolysis. The characterisation of chars before and after modifications was done by scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET) and Fourier transform infrared (FTIR). Batch experiments were performed to examine Ca-PHC's sorption properties and binding mechanisms to selected metal ions. The maximum sorption capacities of Ca-PHC for Pb²⁺ and Cd²⁺ were 297 mg g^-1, and 131 mg g^-1, respectively. The obtained results demonstrated that the sorption of Pb²⁺ and Cd²⁺ by Ca-PHC follows a pseudo-second kinetic model and Freundlich isotherm. The binding of the selected metals onto Ca-PHC was enabled by the ion-exchange mechanism, surface complexation, mineral precipitation and cation-π interaction. Thermodynamic parameters indicate that metal ions binding by Ca-PHC are spontaneous and endothermic. Due to the high adsorption capacities, the obtained Ca-PHC has good potential for application in industrial wastewater treatment. In addition, the demonstrated use of SMS highlights another possibility of applying this specific biomass relevant to sustainable and economical waste management in the growing mushroom industry.

Influence of pyrolysis atmosphere and temperature co-regulation on the sorption of tetracycline onto biochar: structure-performance relationship variation

Presently, as the prevalent pyrolysis atmospheres, N₂ is widely used, while air-limitation and CO₂ are rarely considered, to produce biochar to adsorb tetracycline. This study thus used N₂, CO₂, and air-limitation to produce various biochars at 300 ∼ 750 °C, and explored their structure-performance relationship for tetracycline sorption. The maximum sorption capacities of biochars produced in CO₂ and air-limitation were 55.36 mg/g and 71.11 mg/g (at 750 °C), respectively, being 2.34 and 3.01 times that of biochars produced in N₂ (23.60 mg/g at 750 °C). Interestingly, except for high pore volume and specific surface area supported pore filling and sites providing effect, ash (containing metal cations, P-O, and S=O) induced complexing effect was the primary mechanism for tetracycline sorption, rather than hydrophobic effect, π-π interaction, and hydrogen bond caused by C composition. This study provides important information about adjusting the pyrolysis atmosphere to improve the sorption performance of biochar toward tetracycline.

Novel Long-Chain Fatty Acid (LCFA)-Degrading Bacteria and Pathways in Anaerobic Digestion Promoted by Hydrochar as Revealed by Genome-Centric Metatranscriptomics Analysis

A large amount of long-chain fatty acids (LCFA) are generated after lipids hydrolysis in anaerobic digestion (AD), and LCFA are difficult to be biodegraded. This study showed that hydrochar (HC), which was produced during the hydrothermal liquefaction of organic wastes, significantly increased the methane production rate (by 56.9%) of oleate, a typical refractory model LCFA. Genomic-centric metatranscriptomics analysis revealed that three novel microbes (Bin138 Spirochaetota sp., Bin35 Smithellaceae sp., and Bin54 Desulfomonilia sp.) that were capable of degrading LCFA were enriched by HC, which played an important role in the degradation of oleate. LCFA was degraded to acetate through the well-known LCFA β-oxidation pathway and the combined β-oxidation and butyrate oxidation pathway. In addition, it was found that HC promoted the direct interspecies electron transfer (DIET) between Methanothrix sp. and Bin54 Desulfomonilia sp. The enriched new types of LCFA-degrading bacteria and the promotion of DIET contributed to the improved methane production rate of oleate by HC. IMPORTANCE Long-chain fatty acids (LCFA) are difficult to be degraded in anaerobic digestion (AD), and the known LCFA degrading bacteria are only limited to the families Syntrophomonadaceae and Syntrophaceae. Here, we found that hydrochar effectively promoted AD of LCFA, and the new LCFA-degrading bacteria and a new metabolic pathway were also revealed based on genomic-centric metatranscriptomic analysis. This study provided a new method for enhancing the AD of organic wastes with high content of LCFA and increased the understanding of the microbes and their metabolic pathways involved in AD of LCFA.

Sub- and Near-Critical Hydrothermal Carbonization of Animal Manures

To produce hydrochar with less volatile matter (VM) and more fixed carbon (FC) to increase its stability, this study compared the hydrothermal carbonization (HTC) of hen (HM) and swine (SM) manures at typical HTC sub-critical temperature of 210 °C and slightly super-critical temperature of 400 °C. Physico-chemical properties such as proximate analysis; ultimate analysis; Brunauer–Emmett–Teller (BET) surface area; higher heating value (HHV); chemical oxygen demand (COD); and inorganic nutrients of hydrochar, gaseous, and liquid products were determined. As expected, both VM and yield decreased with temperature. The heats of HTC reactions were estimated to be exothermic, ranging from −5.7 to −8.6 MJ/kg. The FC approximately doubled, while VM significantly decreased with a yield of 42.7%, suggesting the high potential of producing more stable hydrochar via near-critical HTC (NCHTC) treatment of SM. Additional work is needed before recommendations on carbonization temperatures can be made. Specifically, there is a need to experimentally investigate how the chars produced from each carbonization condition influence plant growth and soil emissions.

Recent Advances on Innovative Materials from Biowaste Recycling for the Removal of Environmental Estrogens from Water and Soil

New technologies have been developed around the world to tackle current emergencies such as biowaste recycling, renewable energy production and reduction of environmental pollution. The thermochemical and biological conversions of waste biomass for bioenergy production release solid coproducts and byproducts, namely biochar (BC), hydrochar (HC) and digestate (DG), which can have important environmental and agricultural applications. Due to their physicochemical properties, these carbon-rich materials can behave as biosorbents of contaminants and be used for both wastewater treatment and soil remediation, representing a valid alternative to more expensive products and sophisticated strategies. The alkylphenols bisphenol A, octylphenol and nonylphenol possess estrogenic activity comparable to that of the human steroid hormones estrone, 17β-estradiol (and synthetic analog 17α-ethinyl estradiol) and estriol. Their ubiquitous presence in ecosystems poses a serious threat to wildlife and humans. Conventional wastewater treatment plants often fail to remove environmental estrogens (EEs). This review aims to focus attention on the urgent need to limit the presence of EEs in the environment through a modern and sustainable approach based on the use of recycled biowaste. Materials such as BC, HC and DG, the last being examined here for the first time as a biosorbent, appear appropriate for the removal of EEs both for their negligible cost and continuously improving performance and because their production contributes to solving other emergencies, such as virtuous management of organic waste, carbon sequestration, bioenergy production and implementation of the circular economy. Characterization of biosorbents, qualitative and quantitative aspects of the adsorption/desorption process and data modeling are examined.

Characterization of Bio-Adsorbents Produced by Hydrothermal Carbonization of Corn Stover: Application on the Adsorption of Acetic Acid from Aqueous Solutions

In this work, the influence of temperature on textural, morphological, and crystalline characterization of bio-adsorbents produced by hydrothermal carbonization (HTC) of corn stover was systematically investigated. HTC was conducted at 175, 200, 225, and 250 °C, 240 min, heating rate of 2.0 °C/min, and biomass-to-H2O proportion of 1:10, using a reactor of 18.927 L. The textural, morphological, crystalline, and elemental characterization of hydro-chars was analyzed by TG/DTG/DTA, SEM, EDX, XRD, BET, and elemental analysis. With increasing process temperature, the carbon content increased and that of oxygen and hydrogen diminished, as indicated by elemental analysis (C, N, H, and S). TG/DTG analysis showed that higher temperatures favor the thermal stability of hydro-chars. The hydro-char obtained at 250 °C presented the highest thermal stability. SEM images of hydro-chars obtained at 175 and 200 °C indicated a rigid and well-organized fiber structure, demonstrating that temperature had almost no effect on the biomass structure. On the other hand, SEM images of hydro-chars obtained at 225 and 250 °C indicated that hydro-char structure consists of agglomerated micro-spheres and heterogeneous structures with nonuniform geometry (fragmentation), indicating that cellulose and hemi-cellulose were decomposed. EDX analysis showed that carbon content of hydro-chars increases and that of oxygen diminish, as process temperature increases. The diffractograms (XRD) identified the occurrence of peaks of higher intensity of graphite (C) as the temperature increased, as well as a decrease of peaks intensity for crystalline cellulose, demonstrating that higher temperatures favor the formation of crystalline-phase graphite (C). The BET analysis showed 4.35 m2/g surface area, pore volume of 0.0186 cm3/g, and average pore width of 17.08 μm. The solid phase product (bio-adsorbent) obtained by hydrothermal processing of corn stover at 250 °C, 240 min, and biomass/H2O proportion of 1:10, was activated chemically with 2.0 M NaOH and 2.0 M HCl solutions to investigate the adsorption of CH3COOH. The influence of initial acetic acid concentrations (1.0, 2.0, 3.0, and 4.0 mg/mL) was investigated. The kinetics of adsorption were investigated at different times (30, 60, 120, 240, 480, and 960 s). The adsorption isotherms showed that chemically activated hydro-chars were able to recover acetic acid from aqueous solutions. In addition, activation of hydro-char with NaOH was more effective than that with HCl.

New insight into the adsorption mechanism of PCP by humic substances with different degrees of humification in the presence of Cr(VI)

Humic substances (HSs) have great retention effects on pentachlorophenol (PCP) migration in subsurface environment, but the adsorption mechanism of PCP by HSs with various aromatic/aliphatic moieties and acidic functional groups in the presence of Cr(VI) is still unclear. In this study, the adsorption mechanism of PCP by undissolved humic acid (HA) and humin (HM) extracted from peat, black soil, lignite and coal was investigated under the presence of Cr(VI). According to the results, HA samples had much lower adsorption capacity for hydrophobic PCP than HM samples due to their higher contents of hydrophilic polar oxygen-containing functional groups. In respect to PCP adsorption mechanism, the molecular unsaturation of HSs associated with humification degree was found to be the determinant instead of polarity. Notably, after reacting with Cr(VI), significant decreasing of PCP adsorption quantities occurred on HSs extracted from lignite and coal with higher degrees of unsaturation (H/C < 0.64), while HSs extracted from peat and black soil with lower degrees of unsaturation (H/C > 0.83) kept almost unchanged, which can be attributed to the much higher reactivity of aromatic domains of HSs for Cr(VI) reduction compared with aliphatic moieties. This indicated that the adsorption mechanism of PCP by HSs with higher and lower degrees of unsaturation might be respectively driven by π-π interaction and hydrophobic interaction. This study highlighted the diverse adsorption mechanisms of PCP on HSs with different degrees of humification, and emphasized the coexisting Cr(VI) only have significant effect on PCP adsorption by HSs with higher humification degrees instead of the lower ones.

Changes and release risk of typical pharmaceuticals and personal care products in sewage sludge during hydrothermal carbonization process

Hydrochars were obtained by hydrothermal carbonization treatment of municipal sewage sludge. Effects of reaction temperature (180-300 °C) and reaction time (2-15 h) on structural characteristics of the hydrochars, and changes and release risk of typical pharmaceuticals and personal care products (PPCPs) in the hydrochars were investigated. Reaction temperature played a more important role than reaction time on hydrochar properties and decarboxylation reaction was the primary process during the converting of sludge to hydrochars. The sludge hydrochars had higher yields, carbon recovery rates, polarity and less aromaticity than biochars. Hydrothermal process reduced PPCPs' load in sludge hydrochars effectively except caffeine and acetaminophen. The hydrochars prepared at intermediate and high temperatures (240 and 300 °C) had higher caffeine concentrations than the original sludge, which can be ascribed to the transformation of N-containing precursors. The highest CaCl₂ extracted caffeine concentration occurred at intermediate temperature of 240 °C (48.1 μg/kg) due to the stronger affinity of caffeine in the high-temperature hydrochars. Caffeine was not detected in hydroxypropyl-β-cyclodextrin (HPCD) extract. Hydrochars prepared at low temperature (180 °C) had a higher acetaminophen concentration than the original sludge, which was attributed to the high thermal stability temperature of acetaminophen. Low- and intermediate-temperature hydrochars had higher CaCl₂ extracted acetaminophen concentrations. The HPCD extracted acetaminophen was low with a range of nd to 6.72 μg/kg. In conclusion, PPCPs are less likely to constitute a limiting factor on the farm application of sludge hydrochar. This study provides theoretical support for the safe application of sludge hydrochar in the farmland.

Enhanced trichloroethylene biodegradation: Roles of biochar-microbial collaboration beyond adsorption

Trichloroethylene (TCE) is a pollutant widely found in groundwater, especially in the heavily contaminated industrial sites. Biological dechlorination method is environmentally friendly and low-cost. However, microorganisms grow slowly and their activity is susceptible to environmental fluctuations. This study used biochar as an additive to promote anaerobic biodegradation of TCE with mixed culture. Results showed that biochar with dose of 0.1-0.4% (w/v) brought a rapid initial decrease of TCE concentration by 39.4-88.8% in 24 h via adsorption mechanism. Biochar produced at 500 °C pyrolysis temperature (BC500) achieved the highest TCE adsorption in comparison to BC300 and BC700. Subsequently, a significantly shortened microbial stagnation phase (from 85 h to 37 h) was observed in the system with the presence of biochar. During the exponential growth phase, BC700 outperformed BC300 and BC500 in terms of TCE degradation efficiency. Electrochemical analysis demonstrated that BC700 possessed the greatest electron transfer capability. Finally, biochar shortened the time for achieving 100% removal of TCE by 54.5-69.7% (from approximate 330 h to 100-150 h). Even at high concentration of TCE (20-30 mg·L^-1) that could lead to serious microbial growth inhibition, the TCE degradation efficiency could be recovered in the presence of BC500. The high-throughput sequencing data revealed that biochar promoted the relative abundance of co-metabolizing dechlorinating microorganisms (Pseudomonas, Burkholderia) in the aqueous solution, and simultaneously led to the selective colonization of reductive dechlorinating microorganisms (Enterobacteriaceae, Clostridium) attached on biochar surface. On the other hand, biochar addition decreased the relative abundance of hydrogen-competing microorganisms, thereby forming an efficient co-metabolism-reductive dechlorination system. These findings allow a better understanding of the promotion mechanism of biochar for microbial dechlorination technology supporting the biochar-assisted bioremediation in practice.

Analysis on the removal of emerging contaminant from aqueous solution using biochar derived from soap nut seeds

For clearing pollutants and emerging contaminants like ciprofloxacin-500mg from wastewaters generated from pharmaceutical industries, soapnut seeds biochar was synthesized and used as an adsorbent for the effective removal process. Tubular furnace operated under nitrogen gas environment was used to synthesize biochar. The batch analysis were carried out successfully to study the removal mechanism and the removal efficiency of the chosen pollutant. The soapnut seeds biochar showed excellent adsorption of ciprofloxacin at pH 6 and temperature 303 K when the dosage was 0.07 g. The Langmuir removal capacity of 33.44 mg/g was received and the Freundlich model provided the best-fits. The ciprofloxacin-500mg adsorption process correlated well with the pseudo-second-order kinetics equation, and the intraparticle diffusion mechanism mainly controlled the process. The characterization of biochar concluded that O-H groups, CO groups, COO^-groups and C-F groups, and π-π interactions, pore-filling effect, and cation exchange interactions played a role in the adsorption process. Therefore, the findings of the present work revealed that soapnut seeds biochar would be an excellent low-cost adsorbent for the removal of ciprofloxacin-500mg from wastewater.

Application of typical artificial carbon materials from biomass in environmental remediation and improvement: A review

Artificial carbon materials (ACMs), notably hydrochar, pyrochar, and artificial humic substances, etc., are considered to be sustainable and eco-friendly materials for environmental remediation and improvement. At present, almost relevant literature mainly focuses on biochar, and it is necessary to systematically summarize and expand studies on ACMs. ACMs are widely used to solve pollution problems in water and soil environments, as well as to remediate and improve soil quality. This review focuses on the following issues: 1. Reveal the synthetic mechanisms and compositional reactions effects of the charring process; 2. Define artificial humus as a novel class of ACMs and discuss the application of environmental remediation and relative enhancement effects; 3. Research the relative mechanisms and significance of ACMs during remediation process, involving removal and fixation of heavy metal ions (HMs)/organic pollutants (OPs), modification of soil physicochemical properties, affecting microbial community effects, and improving fertility for crop growth. Finally, the cost-benefit analysis and security-risk evaluation of ACMs are pointed out.

Integrated structural and chemical analyses for HCl-supported hydrochar and their adsorption mechanisms for aqueous sulfachloropyridazine removal

In this study, various HCl-supported hydrochar made from root powder of long-root Eichhornia crassipes were applied to adsorb aqueous sulfachloropyridazine (SCP). Adsorption capacity (q_e μg g^-1) was positively correlated with combined severity-CS. With CS increasing, carbonization degree, hydrophobicity, porosity and isoelectric point of hydrochar increased, but content of polar functional groups decreased. Hydrophobic interaction was important for SCP adsorption. A 24 × 36 peak area table was generated from 24 FT-IR absorbance spectra computed by peak detection algorithm. Afterwards, correlation analysis between q_e μg g^-1 and FT-IR peak area were conducted, indicating that wavenumbers at 555.4, 1227.47, 1374.51, 1604.5, 2901.4/2919.2 and 3514.63 cm^-1 were helpful for SCP adsorption. Further, multivariate linear regression analyses showed that aromatic skeleton and phenolic hydroxyl were the two biggest contributors. Electrostatic attraction did not exist during the SCP adsorption process. Under strong acid condition, protonated amino groups in cationic SCP acting as a hydrogen donator interacted with electron-rich functional groups onto hydrochar by Hydrogen interaction. Under weak acid condition, neutral SCP served as an π electron donor to bond with hydrochar by π-π electron donator-acceptor interaction. This work could guide the functional groups modification strategy of hydrochar to make better use of it in water purification field.

Structural dependent Cr(VI) adsorption and reduction of biochar: hydrochar versus pyrochar

Hydrochar and pyrochar are two typical biochars, and possess different intrinsic structures and chemical properties as well as pollutant removal abilities. However, their structural dependent pollutant removal performances and the related mechanisms are far less studied. In this study, we systematically compared the Cr(VI) removal processes of hydrochar and pyrochar in dark and under simulated sunlight at pH 5.7 ± 0.1, aiming to clarify the structural dependent Cr(VI) removal of biochar. In dark, hydrochar could remove 19.0% of Cr(VI) only via adsorption within 8 h, less than that (23.5%) of pyrochar via both adsorption and indirect solution •O₂^- reduction pathway. Although simulated sunlight irradiation could significantly promote the Cr(VI) reduction performances of both hydrochar and pyrochar, the Cr(VI) reduction percentage (88.1%) of hydrochar via both direct surface electron reduction and indirect solution •O₂^- reduction pathways, was much higher than that (30.2%) of pyrochar only via indirect solution •O₂^- reduction pathway. This different Cr(VI) reduction pathway of hydrochar and pyrochar was arisen from their structural dependent Cr(VI) adsorption models, as revealed by ATR-FTIR characterization and DFT calculation. More phenolic -OH group on hydrochar surface provided abundant sites for Cr(VI) chemical adsorption to form a strong inner-sphere complex, favoring the interfacial electron transfer for the direct surface Cr(VI) reduction. In contrast, more micropores in pyrochar were responsible for the Cr(VI) physical adsorption via intra-particle and boundary layer diffusion, which hampered the surface Cr(VI) direct reduction because of the weak interfacial interaction between Cr(VI) and pyrochar. This study clarifies the influence of surface structure on the Cr(VI) adsorption and reduction pathways of biochar, and also provides an efficient Cr(VI) removal strategy with sunlight and hydrochar.

Efficient adsorption of ciprofloxacin using Ga2S3/S-modified biochar via the high-temperature sulfurization

Ga2S3 and sulfur co-modified biochar (Ga/S-BC) composites were prepared for enhancing the adsorption of ciprofloxacin from sugarcane bagasse via the high-temperature sulfurization. In contrast with sulfur-modified biochar, Ga/S-BC exhibited the better adsorption capacity for ciprofloxacin removal. The increasing Ga content induced to the climbing and then declining adsorption activity of Ga/S-BC. Among these obtained Ga/S-BC composites, optimal 3-Ga/S-BC with a Ga content of 7.40% and surface area of 681.67 m2 g-1 exhibited the superior capacity of 330.21 mg g-1. The adsorption capacity of 3-Ga/S-BC declined to 301.66 mg g-1 after nine cycles. pH and inorganic salts also affected the adsorption capacity of 3-Ga/S-BC for ciprofloxacin removal. The adsorption isotherms of obtained Ga/S-BC composites were well described by Langmuir isotherm, and their adsorption kinetics were well estimated via second-order model. The adsorption performance of 3-Ga/S-BC in ciprofloxacin removal was a physisorption and spontaneous process.

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

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

Biochar-mediated reduction of m-nitrotoluene: Interaction between reduction of m-nitrotoluene and sequestration of contaminants

Biochar is a highly effective adsorbent for nitroaromatic compounds (NACs), and acts as an electron shuttle that mediates the reduction of NACs. Hence, when biochar is used to mediate NAC reduction, adsorption and reduction will occur simultaneously and affect each other. However, the effect of biochar-mediated NAC reduction on sorption remains unknown. Eight biochars with different physicochemical properties were used to adsorb m-nitrotoluene and mediate its reduction. The results showed that the adsorption of m-nitrotoluene onto the various biochars facilitated its reduction, whereas biochar-mediated reduction retarded and weakened contaminant adsorption, which increased the environmental risk posed by m-nitrotoluene. Nevertheless, biochars with a high graphitization degree and developed porosity not only had a great catalytic ability, but also significantly alleviated the negative effect of reduction on adsorption. This was ascribed to the π-π interaction and pore-filling effect, which played more important roles than the hydrophobic effect in adsorbing the reduction product (m-toluidine) onto the studied biochars during reduction. Furthermore, the methanol extraction results indicated that the eight biochars presented significantly stronger sequestration abilities for adsorbed m-toluidine than for adsorbed m-nitrotoluene. This resulted from the hydrogen bonding and the Lewis acid-base effect between m-toluidine and each biochar, which were absent for m-nitrotoluene. These results suggest that biochars with a high graphitization degree and developed porosity are applicable for mediating reduction-enhancing sequestration of NACs, which could be a novel strategy for NAC remediation.

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.

Coupling experiments with calculations to understand the thermodynamics evolution for the sorption of zwitterionic ciprofloxacin on oxidizing-aged pyrogenic chars in the aquatic system

Oxidized aging due to the long-term exposure can significantly alter the sorption of pyrogenic chars (i.e., biochar, BC) towards antibiotics, which determined their fates in natural environments. In this study, the sorption of ciprofloxacin (CIP) on the oxidizing-aged BCs was studied linking the experimental thermodynamics and theoretical calculations. Results revealed that Q₀ of CIP negatively correlated with their average site energies (E_m), while pore-normalized Q₀ on aged BCs were 2-6 folds higher than fresh BCs. From competitive sorption, it is proposed that the transformation of CIP^± to CIP⁺ occurred and the π⁺-π electron donor-acceptor interaction and Coulombic attraction onto the aged BCs played a critical role. These two specific interactions with CIP were thermodynamically improved when aging degree increased and favored the free energies (Δ_aG) of sorption by 2-5 kJ mol^-1. Based on the identified relationship between experimental Δ_OA-ΔG⁰ with E_a through DFT calculations, the contributions of the specific interactions to antibiotic sorption on aged BCs were quantified. This study provided an in-depth understanding of how the aging process affects the sorption of zwitterionic antibiotics on BCs and also possibilities to predict the fate of antibiotics in the presence of BCs over a long-term period.

The effect of organic solvent washing on the structure of hydrochar-based dissolved organic matters and its potential environmental toxicity

With the increased interest in the practical use of hydrochar, concerns about the possible environmental biotoxicity of hydrochar and its released dissolved organic matters (DOM) have grown. As a common method for removing bio-oil on the surface of hydrochar, the effect of organic solvent washing on the properties of hydrochar released DOM remains unclear. In this study, we made a comprehensive comparison of hydrochar properties and molecule structure as well as biotoxicity of DOM released from HC (raw hydrochar) and THC (hydrochar washed by tetrahydrofuran). The results indicated that the mass loss of hydrochar was obvious after tetrahydrofuran (THF) washing, and a decline of H/C atomic ratio and increase of N/C and O/C atomic ratios was observed based on Van Krevelen (VK) diagram. This result was further confirmed by FTIR, ¹³C NMR, and XPS results. Meanwhile, the molecule structure of DOM was shifted to lower molecule weight with higher O-contain compounds after THF extraction due to the demethanation process. However, the biotoxicity experiments indicated that both extracted DOM had no significant impact on germination rate of wheat, and HC-treated sample even exhibited growth superiority. Nevertheless, potential toxicity was observed with the increase of the activity of antioxidant enzymes, and THF washing aggravated the potential oxidative damage through increasing the aromaticity of DOM. Such understanding highlights the importance of evaluating hydrochar and its released DOM before applications, so as to reduce the potential environment biotoxicity.

The contrasting role of minerals in biochars in bisphenol A and sulfamethoxazole sorption

Biochars are one of carbon-rich substances that have attracted enormous attention because of its values in energy storage, carbon sequestration, and environment remediation. Apart from the carbon structure, biochars also contain inherent mineral component and polar functional groups. However, the importance of the inherent minerals to the stability of biochars as well as the sorption of organic compounds remains unclear. In this work, the demineralized treatment by the hydrofluoric acid was employed to remove the inorganic minerals from biochars produced at 300 and 500 °C. The inorganic minerals in biochars were identified and quantified by XRD, XPS and SEM-EDS techniques. Approximately 75% of biochar minerals belonged to the Si- and Al-containing minerals, which connected with carbon skeletons. The impact of these minerals to bisphenol A (BPA) and sulfamethoxazole (SMX) sorption was investigated. The mineral removal decreased BPA sorption but increased SMX sorption. Moreover, the relative contributions of surface adsorption and partition processes were quantified for both compounds through isotherm modeling. The BPA sorption was regulated by the joint effect of adsorption and partition, while more than 82% of the SMX sorption was dominated by the partition process. Such understanding of biochar minerals and carbon structure to the migration of organic contaminants will benefit biochar production and application.

Organoarsenic conversion to As(III) in subcritical hydrothermal reaction of livestock manure

Liquid phase produced by the subcritical hydrothermal liquefaction (HTL) of livestock manure is extensively used in agronomic and environmental applications, but the potential risks caused by inherent pollutants (e.g., roxarsone, ROX) of the livestock manure have not been considered. This study shows that less toxic ROX is completely converted into highly toxic As(III) and As(V) in the HTL reaction with temperature more than 240 °C. Moreover, more than 81.5% of As is distributed in the liquid phase generated by the livestock manure HTL reaction. Notably, the hydrothermal products of livestock manure facilitate the conversion of As(V) to As(III). The resulting hydrochar and aldehydes act as electron donors for As(V) reduction, thus resulting in the formation of As(III). Furthermore, the dissociated As promotes the depolymerization and deoxygenation of the macromolecular compounds to produce more small oxygen-containing compounds such as aldehydes, further boosting the As(V) reduction to As(III). These results indicate that the liquid phase of the livestock manure has potential risks in applications as a fertilizer. Such findings have substantial implications in biomass utilization and redox reactions of envirotechnical and biogeochemical relevance.

Graphitic Carbon Nitride-Based Composite in Advanced Oxidation Processes for Aqueous Organic Pollutants Removal: A Review

In recent decades, a growing number of organic pollutants released have raised worldwide concern. Graphitic carbon nitride (g-C3N4) has drawn increasing attention in environmental pollutants removal thanks to its unique electronic band structure and excellent physicochemical stability. This paper reviews the recent progress of g-C3N4-based composites as catalysts in various advanced oxidation processes (AOPs), including chemical, photochemical, and electrochemical AOPs. Strategies for enhancing catalytic performance such as element-doping, nanostructure design, and heterojunction construction are summarized in detail. The catalytic degradation mechanisms are also discussed briefly.

Investigation on parameters optimization to produce hydrochar without carbohydrate carbon

Owing to its superior energy-saving function and strong potential to be applied as a soil amendment and fertilizer, hydrochar has gained wide attention in recent years. However, hydrochar contains greater amounts of labile fractions than traditional biochar and may exacerbate the short-term greenhouse effect. To lower the risk of greenhouse gas release due to labile fractions, optimize parameters must be determined to produce hydrochar without carbohydrate carbon. In addition, the effects of varying feedstocks and process conditions on hydrochar structure as well as its dissolved organic matter (DOM) must be investigated. Spartina alterniflora and pig manure were used to produce two hydrochars (HSAs and HPMs) and their corresponding DOM samples (DSAs and DPMs) at various production temperatures (T_p) and reaction times (t_r). The carbohydrate vanishing points (CVPs) were 265 °C-1 h, 250 °C-2 h, and 245 °C-4 h for the HSAs and 260 °C-1 h, 250 °C-2 h, and 250 °C-4 h for the HPMs. With the isolation of DOM, 1.09-4.33% organic carbon of the hydrochar was released. The aromaticity of DSAs decreased with increasing T_p and t_r. The molecular weights of the DSAs and DPMs decreased with increasing T_p and t_r. This study uncovered hydrochar's molecular structure as well as the content and properties of its labile fractions. Results can be used to help design specific hydrochars for potential applications, based on the trend of the molecular change under the condition of the studied parameters optimization to produce hydrochar in this study.

Novel oxone treated hydrochar for the removal of Pb(II) and methylene blue (MB) dye from aqueous solutions

This study represents the first ever work on a novel oxone treated hydrochar as an adsorbent for the efficient removal of different contaminants from aqueous solutions. Pine wood hydrochar (HC) was prepared by hydrothermal treatment at 300 °C and oxidized with oxone to produce oxidized pine wood hydrochar (OHC). Different analytical tools such as elemental analysis, FTIR, TGA, FE-SEM, and BET were used for the characterization of the OHC. Conductometric titration of OHC showed a substantial increase from 22 μmol/g to 600 μmol/g in the hydrochar carboxylic content. The OHC sorption performance was assessed by using Pb(II) ions and methylene blue (MB) dye as two models of contaminants. Sorption benchmarks were performed by varying the contaminant initial concentration, time, and temperatures. Sorption kinetic data was fitted well to the pseudo-second order kinetic model with high correlation coefficients (R² > 0.99) and isothermal data was fitted to the Langmuir model. The highest adsorption capacities for MB and Pb(II) were 86.7 mg/g and 46.7 mg/g, respectively. This study proves that oxone treatment could be a potential sustainable oxidation method to tune the hydrochar surface to increase selectivity towards heavy metal ions and dye sorption.

Biochar based sorptive remediation of steroidal estrogen contaminated aqueous systems: A critical review

Remediation of steroidal estrogens from aqueous ecosystems is of prevailing concern due to their potential impact on organisms even at trace concentrations. Biochar (BC) is capable of estrogen removal due to its rich porosity and surface functionality. The presented review emphasizes on the adsorption mechanisms, isotherms, kinetics, ionic strength and the effect of matrix components associated with the removal of steroidal estrogens. The dominant sorption mechanisms reported for estrogen were π-π electron donor-acceptor interactions and hydrogen bonding. Natural organic matter and ionic species were seen to influence the hydrophobicity of the estrogen in multiple ways. Zinc activation and magnetization of the BC increased the surface area and surface functionalities leading to high adsorption capacities. The contribution by persistent free radicals and the arene network of BC have promoted the catalytic degradation of adsorbates via electron transfer mechanisms. The presence of surface functional groups and the redox activity of BC facilitates the bacterial degradation of estrogens. The sorptive removal of estrogens from aqueous systems has been minimally reviewed as a part of a collective evaluation of micropollutants. However, to the best of our knowledge, a critique focusing specifically and comprehensively on BC-based removal of steroidal estrogens does not exist. The presented review is a critical assessment of the existing literature on BC based steroidal estrogen adsorption and attempts to converge the scattered knowledge regarding its mechanistic interpretations. Sorption studies using natural water matrices containing residue level concentrations, and dynamic sorption experiments can be identified as future research directions.

Hydrochar did not reduce rice paddy NH3 volatilization compared to pyrochar in a soil column experiment

Pyrochar (PC) is always with high pH value, and improper application might increase rice paddy ammonia volatilization (PAV), which is the main nitrogen loss through air during rice production. Differently, hydrochar (HC) takes the advantages of high productive rate and always with lower pH value compared with PC. However, effect pattern and mechanism of HC on PAV are still unclear. In the present study, soil column experiments were conducted to investigate the effect of PC and HC application on PAV. In total, treatments with four types of biochar (WPC, SPC, WHC and SHC, i.e., PC and HC prepared with wheat straw and sawdust, respectively) and two application rates (0.5% and 1.5%, w/w) were set up and non-biochar application was used as control. Results showed that, application of HC with low pH value could not reduce PAV compared with PC. Total PAV increased significantly as the increase of HC application rate (especially for WHC). The increment of PAV under high rate HC application might be due to the strong buffer capacity of soil, the aging of biochar, the high nitrogen from HC. The results indicated that HC should be pretreatment before utilization in agricultural environment considering PAV reduction.

Removal of ochratoxin A from liquid media using novel low-cost biosorbents

Ground shells of almonds (ALM), hazelnuts (HAZ), walnuts (WAL), and chestnuts (CHE), coconut fiber (COC), spent coffee grounds (COF), and clementine peel (CLE) were used to remove ochratoxin A (OTA) from both water and an ethanol/water mixture (14:86, v/v). Other very efficient adsorbents like wood biochar (BC) and hydrochar (HC) and a humic acid (HA) were also adopted as a comparison. In batch experiments, sorption of OTA from water followed the trend BC (100% removed) > HA > CLE > COC > HC > COF > ALM > HAZ > CHE > WAL (8% removed), whereas sorption of OTA from ethanol/water mixture (14:86, v/v) onto only the raw materials was COC (54% removed) > CLE > HAZ > ALM > COF > CHE > WAL (0.4% removed). The desorption of the toxin from all materials in water was rather low. Afterwards, sorption kinetics and isotherms of OTA onto CLE, COC, and COF were performed. The three materials adsorbed OTA in about 2 h according to a pseudo-second-order kinetic model, thus indicating the occurrence of a chemisorption mechanism. Equilibrium sorption data of OTA onto CLE followed preferentially the Freundlich model, whereas those on COC and COF fitted well both Freundlich and Langmuir isotherms (r > 0.996). The values of Freundlich sorption constants, KFads, for CLE, COC, and COF were 313, 202, and 98 L kg-1, respectively. OTA desorption from each of the three materials showed hysteretic effects. Overall findings of this work suggest that raw plant wastes could be effectively used as biosorbents to abate the level of OTA in liquid media.

One-pot synthesis and characterization of engineered hydrochar by hydrothermal carbonization of biomass with ZnCl2

Hydrochar, the product of hydrothermal carbonization of biomass, is a sustainable alternative to other carbonaceous environmental sorbents. However, its use has been limited due to its low surface area. A one-pot biomass/metal salt co-hydrothermal synthesis method might improve its sorptive properties while retaining its efficient production characteristic. Thus, bamboo sawdust and zinc chloride (ZnCl₂) were combined in a hydrothermal reactor (200 °C, 7 h) for preparing modified hydrochar. Compared to the non-modified hydrochar, the hydrochar produced with the addition of ZnCl₂ during hydrothermal treatment was more fully carbonized (C content increased from 54% to 64%), of higher surface area after acid washing (30 versus 1.7 m² g^-1), and enriched in O-containing functional groups and of greater aromaticity (according to FTIR and XRD analysis). Because of these improved properties, Methylene blue adsorption capacity of the modified hydrochar increased by nearly 90% and by 257% after it was rinsed with acid. This study highlights the potential of this one-pot co-hydrothermal treatment of biomass in presence of metal salt to provide a simple and effective hydrochar with properties suitable for environmental remediation and water treatment.

Difference in characteristics and nutrient retention between biochars produced in nitrogen-flow and air-limitation atmospheres

The different effects of nitrogen-flow (NF) and air-limitation (AL) pyrolysis on the characteristics and nutrient retention of biochars (BCs) are unclear. Hence, in this study, BCs derived from bamboo, corn straw, and wheat straw were produced in AL and NF atmospheres at various temperatures (300-750 °C), and their different characteristics and nutrient retention rates were compared systematically. Nitrogen-flow pyrolysis facilitates C retention and graphitic C formation, and AL pyrolysis improves the polarity and supports the formation of oxygen-containing groups. With increasing pyrolysis temperature, C retention and graphitic C formation in BCs derived from AL pyrolysis decreases more significantly compared with BCs from NF pyrolysis. At 750 °C, the polarity and oxygen-containing groups of BCs derived from AL pyrolysis increase, whereas those from BCs derived from NF pyrolysis decrease. The observations are attributable to the AL and high-temperature-enhanced oxidization and gasification of C. An AL atmosphere with a higher pyrolysis temperature supports porosity and results in a larger specific surface area. Although pyrolysis temperature and atmosphere have negligible effects on nutrient retention, a low pyrolysis temperature facilitates the formation of water-soluble Ca, Mg, and P, and AL pyrolysis facilitates the formation of water-soluble P because the high pyrolysis temperature improves the pH and mineral stability of BCs, and air limitation facilitates the oxidation of organic P into PO₄ ^3- . This study provides a reference for selecting AL or NF pyrolysis based on various pyrolysis temperatures to produce BCs and applying these in C sequestration, contaminant sorption, and soil quantity improvement.

Synthesis and fabrication of g-C3N4-based materials and their application in elimination of pollutants

With the fast development of industrial and human activity, large amounts of persistent organic pollutants, heavy metal ions and radionuclides are released into the natural environment, which results in environmental pollution. The efficient elimination of the natural environment is crucial for the protection of environment to against the pollutants' toxicity to human beings and living organisms. Graphitic carbon nitride (g-C₃N₄) has drawn multidisciplinary attention especially in environmental pollutants' cleanup due to its special physicochemical properties. In this review, we summarized the recent works about the synthesis of g-C₃N₄, element-doping, structure modification of g-C₃N₄ and g-C₃N₄-based materials, and their application in the sorption, photocatalytic degradation and reduction-solidification of persistent organic pollutants and heavy metal ions. The interaction mechanisms were discussed from advanced spectroscopic analysis and computational approaches at molecular level. The challenges and future perspectives of g-C₃N₄-based materials' application in environmental pollution management are presented in the end. This review highlights the real applications of g-C₃N₄-based materials as adsorbents or photocatalysts in the adsorption-reduction-solidification of metal ions or photocatalytic degradation of organic pollutants. The contents are helpful for the undergraduate students to understand the recent works in the elimination of organic/inorganic pollutants in their pollution management.

Effects of feedstock and inherent mineral components on oxidation resistance of biochars

Chemical stability assessment of biochar has been universally used to indicate its potential of long-term carbon sequestration. The comparative study on oxidation resistance of biochars from diverse series of feedstock is relatively limited, as well as the effects of endogenous minerals on biochar stability. Herein, oxidation resistance of biochars from peanut shell, bamboo, saw dust, reed stalk, furfural residues, seaweed degumming residues and Enteromorpha prolifera at 500 °C (PS500, BB500, SD500, RS500, FR500, SR500 and EP500) was examined by the treatments of H₂O₂, K₂Cr₂O₇ and thermogravimetric analysis (TGA). Under H₂O₂ or K₂Cr₂O₇ condition, C loss of algae-derived biochars (SR500 and EP500) was extremely greater than that of other biochars due to higher content of labile carbon components. PS500, BB500, SD500, RS500 and FR500 characterized with similar properties in carbon fraction, but they exhibited different ability to resist oxidation. The mineral fraction of biochars (e.g., content and species) varied with the feedstock, which played complex effects on the oxidation resistance. The mineral decomposition (e.g., CaCO₃) in EP500 and SR500 above 500 °C influenced the analysis of biochar stability by TGA. After acid-washing, EP500 and SR500 showed weaker thermal oxidation resistance, agreed with the results of H₂O₂ and K₂Cr₂O₇ oxidation. The oxidation resistance of biochars was correlated better with O/C ratio, implying that O/C ratio was more robust indicator than other indexes (e.g., H/C ratio and the ratio of D band to G band of Raman). The FTIR, Raman and XPS results further demonstrated the elimination of aliphatics and amorphous aromatics and/or the carboxylation/carbonylation of aromatic structures by H₂O₂ and K₂Cr₂O₇. These findings are useful for better understanding the impacts of feedstock and inherent minerals on the oxidation resistance of biochars.

Ciprofloxacin adsorption by biochar derived from co-pyrolysis of sewage sludge and bamboo waste

Antibiotics residues in aqueous environment and sewage sludge accumulation have become serious environmental issues. The aim of this study is to investigate the potential of ciprofloxacin (CIP) removal by sludge-based biochar prepared from co-pyrolysis of sewage sludge and bamboo waste (BW). The stability and environmental risk of heavy metals (HMs) in the biochar were further investigated to evaluate potential risks for biochar utilization. Results showed that BW was an outstanding additive to prepare co-pyrolyzed biochar from sludge. A higher CIP removal rate (95%) of BW-sludge biochar (SBC) was obtained under initial CIP concentration of 10 mg/L, and its maximum adsorption capacity was 62.48 mg/g which was calculated from the Langmuir model. The pseudo-second-order and Freundlich model also well fit the CIP adsorption process, indicating a chemical and multilayer adsorption of CIP on a heterogeneous surface of biochar. Adsorption mechanism analysis indicated that the diverse functional groups and Fe species in biochar probably were the dominant factors in the adsorption of CIP. The π-π interaction, H-bond, ion exchange, and Fe-complexation might be the main interactions between the functional species and CIP molecules. Besides, HMs, especially the Cr, Cd, and As, were well immobilized in SBC compared with pure sludge biochar. This work suggested that sludge-based biochar, especially the co-pyrolyzed SBC, could be a potential adsorbent for CIP removal from aqueous solutions.

Thermal oxidation activation of hydrochar for tetracycline adsorption: the role of oxygen concentration and temperature

Poplar hydrochar (RHC) was activated by thermal oxidation (TA-O) in air at 300 °C (O300) and in air + N2 (0.5% O2) at 500 and 700 °C (O500 and O700), respectively, and in N2 at 300-700 °C (N300-N700) as control. Samples characterized by various methods were used to analyze their effect on tetracycline adsorption. The results showed that TA-O greatly increased adsorption capacity qe, 100 (mg·g-1, C0 = 100 mg·L-1) from 6.29 for RHC to 33.32, 96.23 and 60.90 for O300, O500 and O700, respectively. The O300 increased carboxyl and aromaticity whereas little influenced on porosity. The O500, with the highest SBET and Smicro, enhanced adsorption probably by micropore filling and π-π interactions. The O700 fused micropore into mesopore but decreased the SBET, Smicro and qe, 100. Thus, thermal oxidation at 500 °C and 0.5% O2 is recommended for hydrochar activation to absorb tetracycline.

Molecular and microbial insights towards understanding the effects of hydrochar on methane emission from paddy soil

Directly returning rice straw to the paddy soil would significantly stimulate methane emission, and hydrochar has potential to be used as soil conditioner. However, the effects of hydrochar on the methane emission from paddy soil and the related mechanisms are still unclear. In the present study, straw-based hydrochar obtained at 200 °C (HC200), 250 °C (HC250) and 300 °C (HC300) and hydrochar after removal of bio-oil at these temperatures (CHC200, CHC250, and CHC300) were prepared and added to the paddy soil. The application of HC200, HC250 and HC300 resulted in the enhanced methane production compared to the control, showing 4.3, 1.6 and 1.5-fold higher methane production, respectively. It was related to the large amount of dissolved organic matter (DOM) released from hydrochar. Excitation-emission matrix fluorescence spectroscopy with parallel factor analysis (EEM-PARAFAC) showed that the hydrochar-derived DOM mainly included humic-like, phenolic and less aromatic structures, and with the increase of hydrothermal temperature, the content of humic-like substances and phenols increased, while biodegradable organics decreased. This was consistent with the maximum methane production by HC200. After incubation, there was no low-aromatic structures observed in the soil leachate, and the residual organics were mainly humus. The EEM-PARAFAC results were supported by compositional characterization of soil leachate by high-resolution mass spectrometry, and the refractory organics released from hydrochar was mainly lignins or (CRAM)-like structures in the range of H/C = 0.8-1.6 and O/C = 0.1-0.5. The organics dissolved from the washed hydrochar was significantly reduced, and some washed hydrochar (CHC250 and CHC300) even inhibited methane emission possibly due to their ability to adsorb organics. Microbial analysis further showed that the increased methane production resulted from hydrochar was associated with the enrichment of Janibacter, Anaeromyxobacter, Anaerolinea and Sporacetigenium. This present study provided a better understanding to the effect of hydrochar on methanogenesis in paddy soil.

Hydrochars from pinewood for adsorption and nonradical catalysis of bisphenols

In the present study, hydrochars (HCs) were prepared from pinewood biomass by high-temperature pyrolysis and applied as environmental-friendly adsorbents and catalysts in the removal of bisphenol F (BPF) and bisphenol S (BPS) from water. It was found that the structural oxygen defects on hydrochars not only enhance the specific surface area for adsorption of the bisphenols, but also function as an electron conductor for molecular oxygen activation in nonradical pathways. The hydrochar pyrolyzed at 800 °C (HC-800) showed the superior adsorption and catalytic performances toward BPS and BPF removals in a wide pH range, and the removal efficiencies were hardly inhibited by the coexistent inorganic anions and humic acid. Particularly, the nonradical reaction is the dominated catalytic oxidation process in a H₂O₂-HC-800 system, different from the traditional radical-based process with persistent free radicals on hydrochars derived from low-temperature pyrolysis. This study provides a novel route toward the efficient removal of endocrine disrupting compounds via the synergistic adsorption and nonradical catalysis.