Microwave assisted hydrothermal preparation of rice straw hydrochars for adsorption of organics and heavy metals

Hydrochar as a bio-based adsorbent for heavy metals removal: A review of production processes, adsorption mechanisms, kinetic models, regeneration and reusability

The spread of heavy metals throughout the ecosystem has extremely endangered human health, animals, plants, and natural resources. Hydrochar has emerged as a promising adsorbent for removal of heavy metals from water and wastewater. Hydrochar, obtained from hydrothermal carbonization of biomass, owns unique physical and chemical properties that are highly potent in capturing heavy metals via surface complexation, electrostatic interactions, and ion exchange mechanisms. This review focuses on removing heavy metals by hydrochar adsorbents from water bodies. The article discusses factors affecting the adsorption capacity of hydrochars, such as contact time, pH, initial metal concentration, temperature, and competing ions. Literature on optimization approaches such as surface modification, composite development, and hybrid systems are reviewed to enlighten mechanisms undertaking the efficiency of hydrochars in heavy metals removal from wastewater. The review also addresses challenges such as hydrochar regeneration and reusability, alongside potential issues related to its disposal and metal leaching. Integration with current water purification methods and the significance of ongoing research and initiatives promoting hydrochar-based technologies were also outlined. The article concludes that combining hydrochar with modern technologies such as nanotechnology and advanced oxidation techniques holds promise for improving heavy metal remediation. Overall, this comprehensive analysis provides valuable insights to guide future studies and foster the development of effective, affordable, and environmentally friendly heavy metal removal technologies to ensure the attainment of safer drinking water for communities worldwide.

Interfacial interaction mechanism between Mn doped highly conjugated biochar and berberine hydrochloride

MnSO4-modified biochar (Mn-BC) was synthesized to remove berberine hydrochloride (BH) from wastewater by utilizing tea waste as raw material and MnSO4 as modifier. Brunel Emmett Taylor (BET) analysis reveals that the specific surface area (SSA) and average pore size (Dave) of Mn-BC are 1.4 and 7 times higher than those of pristine biochar apart, attributing to the dissociation effect can promote the dispersion of MnSO4 in the pores of the biochar. Meanwhile, the doping of Mn not only introduces additional oxygen-containing functional groups (OCFGs), but also modulates the π electron density. Furthermore, Response surface method (RSM) analysis reveals that Mn-BC dosage has the most significant effect on BH removal, followed by BH concentration and pH value. Kinetic and isothermal studies reveal that the BH adsorption process of Mn-BC was mainly dominated by chemical and monolayer adsorption. Meanwhile, density functional theory (DFT) calculations confirm the contribution of Mn doping to the conjugation effect in the adsorption system. Originally proposed Mn-BC is one potentially propitious material to eliminate BH from wastewater, meanwhile this also provides a newfangled conception over the sustainable utilization of tea waste resources.

Recent advances in environmental and agricultural applications of hydrochars: A review

Hydrochar is a carbonaceous material that is generated through the process of hydrothermal carbonization (HTC) from biomass, which has garnered considerable attention in recent years owing to its potential applications in a diverse range of fields, such as environmental remediation and agriculture. Hydrochar is produced from a diverse range of biomass waste materials and retains exceptional properties, including high carbon content, stability, and surface area, making it an optimal candidate for various enviro-agricultural applications. Moreover, it delves into the production process of hydrochar, with explicit emphasis on the optimization of certain properties during the production of hydrochar from bio-waste. Furthermore, the potential of hydrochar as an adsorbent and catalyst support for heavy metals and dyes was extensively explored, along with a soil remediation potential that can improve the physical, chemical and biological properties of soil. This comprehensive review aims to provide a thorough overview of hydrochar with a particular focus on its production, properties, and prospective applications. The significance of hydrochar is accentuated and the growing need for alternative sources of energy and materials that are environmentally sustainable is highlighted in this paper. Besides, the consequence of hydrochar on soil properties such as water-holding capacity, nutrient retention, and total soil porosity, as well as its influence on soil chemical properties such as cation exchange capacity, electrical conductivity, and surface functionality is scrutinized.

Ecological consequences of biochar and hydrochar amendments in soil: assessing environmental impacts and influences

Anthropogenic activities have caused irreversible consequences on our planet, including climate change and environmental pollution. Nevertheless, reducing greenhouse gas (GHG) emissions and capturing carbon can mitigate global warming. Biochar and hydrochar are increasingly used for soil remediation due to their stable adsorption qualities. As soil amendments, these materials improve soil quality and reduce water loss, prevent cracking and shrinkage, and interact with microbial communities, resulting in a promising treatment method for reducing gas emissions from the top layer of soil. However, during long-term studies, contradictory results were found, suggesting that higher biochar application rates led to higher soil CO2 effluxes, biodiversity loss, an increase in invasive species, and changes in nutrient cycling. Hydrochar, generated through hydrothermal carbonization, might be less stable when introduced into the soil, which could lead to heightened GHG emissions due to quicker carbon breakdown and increased microbial activity. On the other hand, biochar, created via pyrolysis, demonstrates stability and can beneficially impact GHG emissions. Biochar could be the preferred red option for carbon sequestration purposes, while hydrochar might be more advantageous for use as a gas adsorbent. This review paper highlights the ecological impact of long-term applications of biochar and hydrochar in soil. In general, using these materials as soil amendments helps establish a sustainable pool of organic carbon, decreasing atmospheric GHG concentration and mitigating the impacts of climate change.

Hydrothermal synthesis of Mg/Al-layered double hydroxide modified water hyacinth hydrochar for remediation of wastewater containing mordant brown dye

A high-performance dye adsorbent of Mg/Al-layered double hydroxide modified water hyacinth hydrochar (MgAl@WH) was synthesized by a simple hydrothermal method. The surface functional groups, elemental composition, crystalline structure, and surface morphology of the prepared (MgAl@WH) were determined using different analytical techniques. The characterization results revealed that the (MgAl@WH) hydrochar surface offered more active adsorption sites, facilitating the mordant brown (anionic dye) adsorption, leading to its superior performance with much higher uptake capability (311.0 mg g-1 at 298 K) than Mg/Al double hydroxide nanosheets (MgAl DLHs, 80.2 mg g-1 at 298 K) and dried water hyacinth (WH, 10.0 mg g-1 at 298 K). The adsorption behavior of MgAL@WH follows the pseudo second order kinetic model (R2 = 0.999) and Langmuir isotherm model (R2 = 0.999). Moreover, MgAl@WH bonded efficiently with mordant brown dye via hydrogen bonding and interlayer anion exchange with monolayer formation. Additionally, the recycling tests revealed that the MgAl@WH can be reused over 10 cycles without significant change in the removal efficiency. Based on the obtained findings, Mg/Al-layered double hydroxide modified water hyacinth hydrochar (MgAl@WH), for its economic and environmental benefits, has recently been used as an efficient adsorbent to remediate industrial wastewater containing anionic dyes.

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.

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.

Fundamental properties and phosphorus transformation mechanism of soybean straw during microwave hydrothermal conversion process

Microwave hydrothermal (MHT) conversion is emerging as a promising technology for the disposal and reutilization of biowastes. This study investigated the fundamental properties and phosphorus transformation mechanism of soybean straw during the MHT conversion process. The oxygen-containing functional groups in soybean straw were stripped, and a trend of dehydration was observed as the temperature increased during the MHT process. Cellulose was identified as the major component of the MHT solid products at high temperature. Glucose and glucuronic acid in the MHT liquid products were gradually converted to formic acid and acetic acid with increasing temperature and holding time. The characteristics of the MHT products directly affected the changes in P speciation and transformation. Most of the P was distributed in liquid products and the impact of holding time was not significant on P distribution at low MHT temperature. With the increase in temperature and holding time, P gradually transferred into the solid products. The proportion of organic phosphorus and soluble inorganic phosphorus in soybean straw was high, and it decreased noticeably after the MHT process. The increase in MHT temperature promoted the conversion of OP and AP into IP and NAIP respectively. P K-edge X-ray absorption near edge structure analysis reveals that Ca5(PO4)3(OH) was the major component of soybean straw and more Ca5(PO4)3(OH) was formed at lower MHT temperature. This study provides fundamental knowledge on the property changes of soybean straw and the transformation of phosphorus during MHT conversion process, which is essential for its disposal and further utilization.

Efficient Photocatalytic Degradation of Congo Red Dye Using Facilely Synthesized and Characterized MgAl2O4 Nanoparticles

The discharge of congo red dye into water sources by factories has been associated with a range of health concerns, such as cancer, redness, skin irritation, and allergic reactions. As a result, this research focused on the cost-effective and straightforward production of MgAl2O4 nanoparticles by using the Pechini sol-gel process. Subsequently, these nanoparticles were employed for the successful photocatalytic decomposition of congo red dye. Moreover, extensive characterization of the fabricated MgAl2O4 nanoparticles was conducted through diverse methodologies, which included Fourier-transform infrared spectroscopy, ultraviolet-visible spectrophotometry, high-resolution transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy (FE-SEM), and powder X-ray diffraction (XRD). Furthermore, the XRD analysis disclosed that the average crystal size of the produced MgAl2O4 nanoparticles is 10.36 nm, and their optical energy gap was determined to be 3.71 eV. The FE-SEM examination unveiled a combination of spherical and disorganized structures with a 0.14 μm average grain size. HR-TEM analysis, in turn, revealed that the fabricated MgAl2O4 nanoparticles were composed of minuscule spherical particles with an average diameter of 8.75 nm. The maximum degradation of 50 mL of congo red dye at a concentration of 25 mg/L reached 99.27% within 80 min at a pH of 3. Additionally, the findings confirmed the consistent decomposition activity toward congo red dye even after four cycles, thereby validating the effectiveness and reusability of the MgAl2O4 nanoparticles that were developed in this study.

Impacts of kaolinite enrichment on biochar and hydrochar characterization, stability, toxicity, and maize germination and growth

In this study, biochar (BC) and hydrochar (HC) composites were synthesized with natural kaolinite clay and their properties, stability, carbon (C) sequestration potential, polycyclic aromatic hydrocarbons (PAHs) toxicity, and impacts on maize germination and growth were explored. Conocarpus waste was pretreated with 0%, 10%, and 20% kaolinite and pyrolyzed to produce BCs (BC, BCK10, and BCK20, respectively), while hydrothermalized to produce HCs (HC, HCK10, and HCK20, respectively). The synthesized materials were characterized using X-ray diffraction, scanning electron microscope analyses, Fourier transform infrared, thermogravimetric analysis, surface area, proximate analyses, and chemical analysis to investigate the distinction in physiochemical and structural characteristics. The BCs showed higher C contents (85.73-92.50%) as compared to HCs (58.81-61.11%). The BCs demonstrated a higher thermal stability, aromaticity, and C sequestration potential than HCs. Kaolinite enriched-BCs showed the highest cation exchange capacity than pristine BC (34.97% higher in BCK10 and 38.04% higher in BCK20 than pristine BC), while surface area was the highest in kaolinite composited HCs (202.8% higher in HCK10 and 190.2% higher in HCK20 than pristine HC). The recalcitrance index (R50) speculated a higher recalcitrance for BC, BCK10, and BCK20 (R50 > 0.7), minimal degradability for HCK10 and HCK20 (0.5 < R50 < 0.7), and higher degradability for biomass and HC (R50 < 0.5). Overall, increasing the kaolinite enrichment percentage significantly enhanced the thermal stability and C sequestration potential of charred materials, which may be attributed to changes in the structural arrangements. The ∑ total PAHs concentration in the synthesized materials were below the USEPA's suggested limits, indicating their safe use as soil amendments. Germination indices reflected positive impacts of synthesized charred materials on maize germination and growth. Therefore, we propose that kaolinite-composited BCs and HCs could be considered as efficient and cost-effective soil amendments for improving plant growth.

Hydrochar-nanoparticle integration for arsenic removal from wastewater: Challenges, possible solutions, and future horizon

Arsenic (As) contamination poses a significant threat to human health, ecosystems, and agriculture, with levels ranging from 12 to 75% attributed to mine waste and stream sediments. This naturally element is abundant in Earth's crust and gets released into the environment through mining and rock processing, causing ≈363 million people to depend on As-contaminated groundwater. To combat this issue, introducing a sustainable hydrochar system has achieved a remarkable removal efficiency of over 92% for arsenic through adsorption. This comprehensive review presents an overview of As contamination in the environment, with a specific focus on its impact on drinking water and wastewater. It delves into the far-reaching effects of As on human health, ecosystems, aquatic systems, and agriculture, while also exploring the effectiveness of existing As treatment systems. Additionally, the study examines the potential of hydrochar as an efficient adsorbent for As removal from water/wastewater, along with other relevant adsorbents and biomass-based preparations of hydrochar. Notably, the fusion of hydrochar with nanoparticle-centric approaches presents a highly promising and environmentally friendly solution for achieving the removal of As from wastewater, exceeding >99% efficiency. This innovative approach holds immense potential for advancing the realms of green chemistry and environmental restoration. Various challenges associated with As contamination and treatment are highlighted, and proposed solutions are discussed. The review emphasizes the urgent need to advance treatment technologies, improve monitoring methods, and enhance regulatory frameworks. Looking outlook, the article underscores the importance of fostering research efforts, raising public awareness, and fostering interdisciplinary collaboration to address this critical environmental issue. Such efforts are vital for UN Sustainable Development Goals, especially clean water and sanitation (Goal 6) and climate action (Goal 13), crucial for global sustainability.

Hydrothermal Ammonia Carbonization of Rice Straw for Hydrochar to Separate Cd(II) and Zn(II) Ions from Aqueous Solution

Hydrochar is considered to be a good adsorbent for the separation of metal ions from aqueous solutions. However, the yield of hydrochar from raw straw is generally low, because the hydrothermal carbonization occurs via dehydration, polymerization, and carbonization. In this work, various hydrochar samples were prepared from rice straw with nitrogen and phosphorus salt; moreover, toilet sewage was used instead of nitrogen, and phosphorus salt and water were used to promote the polymerization and carbonization process. The modified carbon was characterized using XRD, XPS, SEM, and FTIR, and the adsorption capacity was investigated. A significant increase in hydrochar yield was observed when toilet sewage was used as the solvent in the hydrothermal carbonization process. The adsorption capacity of N/P-doped rice straw hydrochar for Cd2+ and Zn2+ metal ions was 1.1-1.4 times higher than that those using the rice straw hydrochar. The Langmuir models and pseudo-second-order models described the metal adsorption processes in both the single and binary-metal systems well. The characterization results showed the contribution of the surface complexation, the electrostatic interaction, the hydrogen bond, and the ion exchange to the extraction of Cd2+ and Zn2+ using N/P-doped rice straw hydrochar.

A review on adsorption of heavy metals from wastewater using carbon nanotube and graphene-based nanomaterials

The rampant rise in world population, industrialization, and urbanization expedite the contamination of water sources. The presence of the non-biodegradable character of heavy metals in waterways badly affects the ecological balance. In this modern era, the unavailability of getting clear water as well as the downturn in water quality is a major concern. Therefore, the effective removal of heavy metals has become much more important than before. In recent years, the attention to better wastewater remediation was directed towards adsorption techniques with novel adsorbents such as carbon nanomaterials. This review paper primarily emphasizes the fundamental concepts, structures, and unique surface properties of novel adsorbents, the harmful effects of various heavy metals, and the adsorption mechanism. This review will give an insight into the current status of research in the realm of sustainable wastewater treatment, applications of carbon nanomaterials, different types of functionalized carbon nanotubes, graphene, graphene oxide, and their adsorption capacity. The importance of MD simulations and density functional theory (DFT) in the elimination of heavy metals from aqueous media is also discussed. In addition to that, the effect of factors on heavy metal adsorption such as electric field and pressure is addressed.

Prediction of heavy metals adsorption by hydrochars and identification of critical factors using machine learning algorithms

Hydrochar has become a popular product for immobilizing heavy metals in water bodies. However, the relationships between the preparation conditions, hydrochar properties, adsorption conditions, heavy metal types, and the maximum adsorption capacity (Q_m) of hydrochar are not adequately explored. Four artificial intelligence models were used in this study to predict the Q_m of hydrochar and identify the key influencing factors. The gradient boosting decision tree (GBDT) showed excellent predictive capability for this study (R²=0.93, RMSE=25.65). Hydrochar properties (37%) controlled heavy metal adsorption. Meanwhile, the optimal hydrochar properties were revealed, including the C, H, N, and O contents of 57.28-78.31%, 3.56-5.61%, 2.01-6.42%, and 20.78-25.37%. Higher hydrothermal temperatures (>220 °C) and longer hydrothermal time (>10 h) lead to the optimal type and density of surface functional groups for heavy metal adsorption, which increased the Q_m values. This study has great potential for instructing industrial applications of hydrochar in treating heavy metal pollution.

Influence of temperature change on the immobilization of soil Pb and Zn by hydrochar: Roles of soil microbial modulation

Considering the potential effect of the ambient temperature on soil microorganisms during heavy metal immobilization by hydrochar, 60 days of soil incubation was conducted to explore the impact of ambient temperature (5, 25, and 35 °C) on the immobilization of Pb and Zn by chitosan-magnetic sawdust hydrochar (CMSH) and magnetic chitosan hydrochar (MCH). The results showed that soil pH was relatively high and total organic carbon (TOC) was slightly lower in the 35 °C treatment. The diethylenetriaminepentaacetic acid (DTPA) available state content decreased significantly with the temperature increasing. Meanwhile, the ratios of stable Pb and Zn in the sequential extraction method proposed by the European Community Bureau of Reference (BCR) gradually increased with increasing temperature. The heatmap based on microbial community showed that elevated temperature not only favored the enrichment of metal-stable phyla, such as Chloroflexi, but was also involved in inhibiting the growth of Firmicutes, Actinobacteriota, and Proteobacteria. Meanwhile, different genera (Fonticella and Bacillus) in the Firmicutes phylum had distinct responses to temperature as well as to heavy metal immobilization effects. Subsequently, redundancy analysis confirmed that Chloroflexi and Fonticella were positively correlated with temperature and stable state metal content, while Actinobacteriota and Bacillus were negatively correlated with temperature and were positively correlated with DTPA available metal content. Moreover, Pb and Zn indicators displayed significant correlations for the dominant genera (R² > 0.8, p < 0.02).

Magnetic seeds promoted high-density sulfonic acid-based hydrochar derived from sugar-rich wastewater for removal of methylene blue

Methylene blue (MB) removal from dyeing wastewater using low-cost bio-derived adsorbent is a significant and challenging field. Herein, magnetic sugar hydrochar (MGHC) precursors derived from sugar-rich wastewater with small particle size and rich oxygen-containing functional groups (OCFGs) are prepared from sugar-rich aqueous solution via Fe salt-modified hydrothermal procedure. The role of Fe₃O₄ nanoparticles formed during the sugar carbonization is to provide numerous magnetic seeds to generate MGHC with core-shell structure, which reduces the particle size of hydrochar. This increases the amount of OCFGs on the surface of MGHC for bonding the sulfonic acid groups. Therefore, sulfonic acid-modified MGHC-SA shows the rapid MB adsorption rate and excellent adsorption capacity. The highest MB capacity is 869.6 mg/g at pH = 11.0 and 298 K. Additionally, the MGHC-SA can be easily recovery by magnet. And the stability of MGHC-SA was also evaluated, no degradation of adsorption performance was observed, even the adsorbent was regenerated 10 times. This study puts forward a promising way to acquire functional groups rich and easy recovery hydrochar from sugar wastewater for MB removal.

Adsorption of heavy metal onto biomass-derived activated carbon: review

Due to the rapid development of the social economy and the massive increase in population, human beings continue to undertake processing, and commercial manufacturing activities of heavy metals, which has caused serious damage to the environment and human health. Heavy metals lead to serious environmental problems such as soil contamination and water pollution. Human health and the living environment are closely affected by the handling of heavy metals. Researchers must find several simple, economical and practical methods to adsorb heavy metals. Adsorption technology has been recognized as an efficient and economic strategy, exhibiting the advantages of recovering and reusing adsorbents. Biomass-derived activated carbon adsorbents offer large adjustable specific surface area, hierarchically porous structure, strong adsorption capacity, and excellent high economic applicability. This paper focuses on reviewing the preparation methods of biomass-derived activated carbon in the past five years. The application of representative biomass-derived activated carbon in the adsorption of heavy metals preferentially was described to optimize the critical parameters of the activation type of samples and process conditions. The key factors of the adsorbent, the physicochemical properties of the heavy metals, and the adsorption conditions affecting the adsorption of heavy metals are highlighted. In addition, the challenges faced by biomass-derived activated carbon are also discussed.

Biomass-Based Hydrothermal Carbons for the Contaminants Removal of Wastewater: A Mini-Review

The preparation of adsorbents with eco-friendly and high-efficiency characteristics is an important approach for pollutant removal, and can relieve the pressure of water shortage and environmental pollution. In recent studies, much attention has been paid to the potential of hydrothermal carbonization (HTC) from biomass, such as cellulose, hemicellulose, lignin, and agricultural waste for the preparation of adsorbents. Hereby, this paper summarizes the state of research on carbon adsorbents developed from various sources with HTC. The reaction mechanism of HTC, the different products, the modification of hydrochar to obtain activated carbon, and the treatment of heavy metal pollution and organic dyes from wastewater are reviewed. The maximum adsorption capacity of carbon from different biomass sources was also evaluated.

Treatment of fluorinated wastewater with chitosan modified activated sludge lysis ash

Preparation of a novel environmentally friendly and cost-effective composite adsorbent for fluoride removal is presented in this work. An activated sludge lysis ash/chitosan (ASLA/C) composite adsorbent was synthesised using an in situ coprecipitation method, and the removal effect of the material was analysed by static adsorption, isothermal adsorption and kinetic adsorption tests. Langmuir model could better describe the adsorption process and the adsorption was in accordance with the kinetic equation of the pseudo-second-order kinetics reaction. The values of adsorption thermodynamic and kinetic parameters were indicated that the adsorption of fluoride ions is a spontaneous, heat-absorbing entropic process, and the reaction was carried out by a combination of mechanisms, such as electrostatic adsorption, ion exchange, surface complexation and hydrogen bonding. The experimental results indicated that ASLA/C can be used as a cheap and readily available alternative efficient adsorbent where the maximum fluorinate absorption was observed with 7.714 mg g^-1, while solving the problem of waste from activated sludge lysis disposal and realizing the environmental benefits of waste.

Transformation of Residual Açai Fruit (Euterpe oleracea) Seeds into Porous Adsorbent for Efficient Removal of 2,4-Dichlorophenoxyacetic Acid Herbicide from Waters

Brazil's production and consumption of açai pulp (Euterpe oleracea) occur on a large scale. Most of the fruit is formed by the pit, which generates countless tons of residual biomass. A new purpose for this biomass, making its consumption highly sustainable, was presented in this study, where activated carbon (AC) was produced with zinc chloride for later use as an adsorbent. AC carbon formed by carbon and with a yield of 28 % was satisfactorily used as an adsorbent in removing the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Removal efficiency was due to the highly porous surface (Vp = 0.467 cm3 g-1; Dp = 1.126 nm) and good surface área (SBET = 920.56 m2 g-1). The equilibrium data fit the Sips heterogeneous and homogeneous surface model better. It was observed that the increase in temperature favored adsorption, reaching a maximum experimental capacity of 218 mg g-1 at 328 K. The thermodynamic behavior indicated a spontaneous, favorable, and endothermic behavior. The magnitude of the enthalpy of adsorption was in agreement with the physical adsorption. Regardless of the herbicide concentration, the adsorbent displayed fast kinetics, reaching equilibrium within 120 min. The linear driving force (LDF) model provided a strong statistical match to the kinetic curves. AC with zinc chloride (ZnCl2), created from leftover açai biomass, is a potential alternative as an adsorbent for treating effluents containing 2,4-D.

A comprehensive review of capacitive deionization technology with biochar-based electrodes: Biochar-based electrode preparation, deionization mechanism and applications

The recently developed techniques for desalination and wastewater treatment are costly and unsustainable. Therefore, a cost-effective and sustainable approach is essential to achieve desalination through wastewater treatment. Capacitive deionization (CDI), an electrochemical desalination technology, has been developed as a novel water treatment technology with great potential. The electrode material is one of the key factors that promotes the development of CDI technology and broadens the scope of CDI applications. Biochar-based electrode materials have attracted increasing attention from researchers because of their advantages, such as environmentally friendly, economical, and renewable properties. This paper reviews the methods for preparing biochar-based electrode materials and elaborates on the mechanism of CDI ion storage. We then summarize the applications of CDI technology in water treatment, analyze the mechanism of pollutant removal and resource recovery, and discuss the applicability of different CDI configurations, including hybrid CDI systems. In addition, the paper notes that environmentally friendly green activators that facilitate the development of pore structure should be developed more often to avoid the adverse environmental impact. The development of ion-selective electrode materials should be enhanced and it is necessary to comprehensively assess the impact of heteroatoms on selective ion removal and CDI performance. Electrooxidation of organic pollutants should be further promoted to achieve organic degradation by extending to redox reactions.

Characterization and nutritional value of hydrothermal liquid products from distillers grains

Hydrothermal liquid products (HLPs) produced by hydrothermal treatment (HTT) contain a large amount of nitrogen, phosphorus and other substances, while the environmental problems caused by arbitrary discharge. This work explored the effects of temperature, reaction time and solid-liquid ratio on the chemistry of HLPs of two different distillers grains, with a focus on nutrient composition. Increased HTT temperature was related to increased HLPs pH, dissolved organic carbon content, and aromaticity, and decreased electrical conductivity. Maximum nutrient extraction efficiencies observed for NH₄⁺-N, NO₃^--N and PO₄^3- were 92.0, 89.9, and 94.3%, respectively. Response surface methodology showed that the release of nutrient extraction efficiency was the greatest at the hydrothermal treatment of 200 °C for 1 h, and using a solid/liquid ratio of 10%. Comparative studies, the nutritional value of HLPs are appropriate for use as an agricultural fertilizer, and its use as a substitute for synthetic fertilizers could increase the sustainability and profitability of farming.

Adsorption Characteristics and Mechanisms of Fe-Mn Oxide Modified Biochar for Pb(II) in Wastewater

This study prepared iron-manganese oxide-modified biochar (FM-BC) by impregnating rice straw biochar (BC) with a mixed solution of ferric nitrate and potassium permanganate. The effects of pH, FM-BC dosage, interference of coexisting ions, adsorption time, incipient Pb(II) concentration, and temperature on the adsorption of Pb(II) by FM-BC were investigated. Moreover, the Pb(II) adsorption mechanism of FM-BC was analyzed using a series of characterization techniques. The results showed that the Fe-Mn oxide composite modification significantly promoted the physical and chemical functions of the biochar surface and the adsorption capacity of Pb(II). The specific surface area of FM-BC was 18.20 times larger than that of BC, and the maximum Pb(II) adsorption capacity reached 165.88 mg/g. Adsorption kinetic tests showed that the adsorption of Pb(II) by FM-BC was based on the pseudo-second-order kinetic model, which indicated that the adsorption process was mainly governed by chemical adsorption. The isothermal adsorption of Pb(II) by FM-BC conformed to the Langmuir model, indicating that the adsorption process was spontaneous and endothermic. Characterization analyses (Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy) showed that the adsorption mechanism of Pb(II) by FM-BC was mainly via electrostatic adsorption, chemical precipitation, complexation, ion exchange, and the transformation of Mn₂O₃ into MnO₂. Therefore, FM-BC is a promising adsorbent for Pb(II) removal from wastewater.

Agricultural waste-derived biochars from co-hydrothermal gasification of rice husk and chicken manure and their adsorption performance for dimethoate

This study aimed to combine energy utilisation of agricultural wastes with the dimethoate (DT) adsorption from agricultural wastewater via hydrogen and biochar production using co-hydrothermal gasification (CHTG). The gasification behaviour after CHTG of five ratios of rice husk (RH) and chicken manure (CM) and the corresponding adsorption performance of biochars on DT were evaluated. The results demonstrated that the feedstock of 3RH+ 1CM achieved the maximum gas yield and hydrogen gasification efficiency (HGE), and the highest adsorption capacity of the derived biochars was 3.57 mg g^-1. Surface characterisation and elemental analysis showed that the biochar derived under different C/N ratios varied considerably. The results of the isotherm and kinetic simulation showed that the Langmuir model and pseudo-first-order model best fitted the experimental data. The superior performance of agricultural waste-derived biochars (AWB) over five cycles of regeneration and adsorption indicated that AWB is a green and stable adsorption material for farmland tailwater. In addition, the degradation pathway of DT during hydrothermal gasification (HTG) regeneration of the spent adsorbent was comprehensively discussed. The CHTG treatment enhanced the yield of gaseous products from RH and CM and produced AWBs with high adsorption capacities for DT. This provides a green and efficient technology for resource utilisation of agricultural waste and treatment of agricultural wastewater using pesticide residues.

A novel preparation process of straw-based iron material for enhanced persulfate activation of reactive black 5 degradation

In this study, a new straw-iron composite material (ST@Fe) was synthesized through impregnation and freeze-drying process for persulfate (PS) activation to degrade reactive black 5 (RB5). Scanning electron microscope, Brunauer-Emmett-Teller, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy confirmed that straw owns huge pore structure and varieties of organic functional groups, including hydroxyl carboxyl groups, which could effectively adsorb and complex iron ions. The interaction between the active iron particles in ST@Fe and straw generated Fe²⁺ for PS activation, effectively degrading over 94.80% of RB5 at an initial concentration of 20 ppm in 100 min with a specific degradation capacity of 18.97 min^-1 per unit of iron ions. ST@Fe/PS system demonstrated high tolerance in a wide initial pH range, which could gradually attack the RB5 molecular structure and significantly reduce the mineralization of water. Quenching experiments and electron paramagnetic resonance demonstrated the efficient generation of ROS including sulfate radicals, hydroxyl radicals, and singlet oxygen, and confirmed the dominance of sulfate radicals in the degradation process. The continuous degradation capacity and reusability of ST@Fe were also evaluated, which proved that the contaminant could be effectively degraded even after multiple cycles in the simulated textile wastewater, indicating its potential for use in practical remediation. This work provided a new method for the preparation of modified functional materials for the degradation of organic pollutants in textile wastewater and posed a novel strategy for the utilization of waste biomass.

Structural, inorganic, and adsorptive properties of hydrochars obtained by hydrothermal carbonization of coffee waste

The hydrothermal carbonization process is a suitable process for the conversion of potentially harmful lignocellulosic waste into hydrochars. Defective coffee beans were the precursor raw material for hydrochar synthesis. Reactions were performed in a high-pressure reactor at 150, 200, and 250 °C, in autogenous pressure, for 40 min. Hydrochars were recovered by filtration and characterized by energy dispersive X-ray fluorescence spectroscopy, UV-Vis spectrophotometry, attenuated total reflection Fourier-transform infrared spectroscopy, differential thermal analysis, and scanning electron microscopy. Methylene blue adsorption tests were performed and analyzed by Langmuir and Freundlich adsorption isotherms. Adsorption mechanisms were investigated by computational calculations at DFT level. Results suggest that hydrochars from defective coffee beans can be applied as technological resources in the agronomic and environmental fields due to their inorganic composition, mainly to high magnesium content, the structural characteristics of porosity, biodegradation control, soil carbon-fixation and adsorption capacity. Important adsorption processes are caused by the development of oxygenated functional groups on the hydrochar surface.

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.

Recent advances of thermochemical conversion processes for biorefinery

Lignocellulosic biomass is one of the most promising renewable resources and can replace fossil fuels via various biorefinery processes. Through this study, we addressed and analyzed recent advances in the thermochemical conversion of various lignocellulosic biomasses. We summarized the operation conditions and results related to each thermochemical conversion processes such as pyrolysis (torrefaction), hydrothermal treatment, gasification and combustion. This review indicates that using thermochemical conversion processes in biorefineries is techno-economically feasible, easy, and effective compared with biological processes. The challenges experienced in thermochemical conversion processes are also presented in this study for better understanding the future of thermochemical conversion processes for biorefinery. With the aid of artificial intelligence and machine learning, we can reduce time-consumption and experimental work for bio-oil production and syngas production processes.

Removal of Copper, Nickel, and Zinc Ions from an Aqueous Solution through Electrochemical and Nanofiltration Membrane Processes

Heavy metal contamination in water is a major health concern, directly related to rapid growth in industrialization, urbanization, and modernization in agriculture. Keeping this in view, the present study has attempted to develop models for the process optimization of nanofiltration (NF) membrane and electrocoagulation (EC) processes for the removal of copper, nickel, and zinc from an aqueous solution, employing the response surface methodology (RSM). The variable factors were feed concentration, temperature, pH, and pressure for the NF membrane process; and time, solution pH, feed concentration, and current for the EC process, respectively. The central composite design (CCD), the most commonly used fractional factorial design, was employed to plan the experiments. RSM models were statistically analyzed using analysis of variance (ANOVA). For the NF membrane, the rejection of Zn, Ni, and Cu was observed as 98.64%, 90.54%, and 99.79% respectively; while the removal of these through the EC process was observed as 99.81%, 99.99%, and 99.98%, respectively. The above findings and a comparison with the conventional precipitation and adsorption processes apparently indicate an advantage in employing the NF and EC processes. Further, between the two, the EC process emerged as more efficient than the NF process for the removal of the studied metals.

Low Temperature One-Pot Hydrothermal Carbonization of Corn Straw into Hydrochar for Adsorbing Cadmium (II) in Wastewater

Corn straw, a typical agricultural waste, was directly converted into hydrochar with a yield of 77.56% by hydrothermal carbonization at 140–230 °C for 2 h with a solid–liquid ratio of 1:20. The morphology and surface properties were characterized by elemental analysis, specific surface area and pore size analysis and Fourier transform infrared spectroscopy. The results showed that with the increase of hydrothermal reaction temperature, some physical and chemical properties such as the increase of hydrocarbon content, crystallinity, and specific surface area of hydrochar changed significantly. A series of chemical reactions such as dehydration, decarboxylation, and aromatization occurred in the hydrothermal carbonization process so that the prepared hydrochar had rich oxygen-containing functional groups (-HO, C-O-C, C=O) and unique porous structure made the hydrochar prepared at 170 °C had the best removal effect on Cd2+ in solution (5.84 mg/g). These specific conditions could remove Cd2+ and greatly improve the adsorption performance. The pseudo-second-order kinetic model and Freundlich isotherm model could better describe the adsorption behavior of Cd2+. Therefore, corn straw hydrochar as a potential adsorbent for removing Cd2+ from water.

A review on adsorptive separation of toxic metals from aquatic system using biochar produced from agro-waste

Water is a basic and significant asset for living beings. Water assets are progressively diminishing due to huge populace development, industrial activities, urbanization and rural exercises. Few heavy metals include zinc, copper, lead, nickel, cadmium and so forth can easily transfer into the water system either direct or indirect activities of electroplating, mining, tannery, painting, fertilizer industries and so forth. The different treatment techniques have been utilized to eliminate the heavy metals from aquatic system, which includes coagulation/flocculation, precipitation, membrane filtration, oxidation, flotation, ion exchange, photo catalysis and adsorption. The adsorption technique is a better option than other techniques because it can eliminate heavy metals even at lower metal ions concentration, simplicity and better regeneration behavior. Agricultural wastes are low-cost biosorbent and typically containing cellulose have the ability to absorb a variety of contaminants. It is important to note that almost all agro wastes are no longer used in their original form but are instead processed in a variety of techniques to improve the adsorption capacity of the substance. The wide range of adsorption capacities for agro waste materials were observed and almost more than 99% removal of toxic pollutants from aquatic systems were achieved using modified agro-waste materials. The present review aims at the water pollution due to heavy metals, as well as various heavy metal removal treatment procedures. The primary objectives of this research is to include an overview of adsorption and various agriculture based adsorbents and its comparison in heavy metal removal.

Microwave-assisted catalytic hydrothermal carbonization of Laminaria japonica for hydrochars catalyzed and activated by potassium compounds

There are limited investigations describing preparation and application of alga-based hydrochars via microwave-assisted catalytic hydrothermal carbonization (MA-CHTC). Therefore, hydrochars were successfully prepared from macroalgae biomass Laminaria japonica impregnated with KH2PO4, KCl, K2CO3, and KOH as acidic, neutral salt, and alkaline catalysts, respectively, via the MA-CHTC. Comprehensive characterization of physicochemical properties of the hydrochars, including yields, elemental and phase composition, specific surface areas, functional groups, and morphology, confirmed different catalytic effects of these catalysts on hydrochar formation. Adsorption kinetics and isotherms of Pb(II) revealed significant improvement of adsorption capacities for Pb(II) due to synergetic chemical activation of the spiked catalysts. Therefore, the synergetic catalytic effects and chemical activation is benefic for tailored design of engineered hydrochars with different properties for special application through selection of catalysts during the MA-CHTC process.

Optimized production, Pb(II) adsorption and characterization of alkali modified hydrochar from sugarcane bagasse

Today, sugarcane bagasse (SB) is used for bioethanol and biodiesel production, energy generation, and adsorbent synthesis. The goal of this project is to determine the optimized conditions for producing adsorbent from sugarcane bagasse using hydrothermal carbonization (HTC) and KOH activation. To optimize process parameters such as reaction temperature, residence time, ZnCl2/SB mixing ratios, and water/SB mixing ratios, response surface methodology was used. The results revealed that the optimum modified adsorption occurred at 180 °C, 11.5 h, a water to biomass ratio of (5:1), and a ZnCl2 to precursor ratio of (3.5:1). The physicochemical features of optimum activated hydrochar were investigated, as well as batch adsorption experiments. The pseudo-second-order kinetic model and the Langmuir isotherm model were found to fit the experimental results in batch adsorption studies [[Formula: see text] (mg/g)]. Thermodynamic experiments further confirmed the spontaneous and exothermic adsorption mechanism.

Rice husk hydrochars from metal chloride-assisted hydrothermal carbonization as biosorbents of organics from aqueous solution

Hydrochar a carbon-rich material resulting from hydrothermal carbonization of biomass, has received substantial attention because of its potential application in various areas such as carbon sequestration, bioenergy production and environmental amelioration. A series of hydrochars were prepared by metal chloride-assisted hydrothermal carbonization of rice husk and characterized by elemental analysis, zeta potential, X-ray diffraction, Brunauer-Emmett-Teller measurements, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and scanning electron microscopy. The results reveal that the prepared hydrochars have carbon contents ranging from 45.01 to 58.71%, BET specific areas between 13.23 and 45.97 m2/g, and rich O-containing functional groups on the surfaces. The metal chlorides added in the feedwater could improve the degree of carbonization and show significant effects on the physical, chemical and adsorption properties of the hydrochars. The adsorption of the selected organics on the hydrochars is a spontaneous and physisorption-dominated process. The hydrochars possess larger adsorption capacities for 2-naphthol than for berberine hydrochloride and Congo red, and the modeling maximum adsorption capacities of 2-naphthol are in the range of 170.1-2680 mg/g. The adsorption equilibrium could be accomplished in 10, 40 and 30 min for 2-naphthol, berberine hydrochloride and Congo red, respectively. These results suggest metal chloride-assisted hydrothermal carbonization a promising method for converting biomass waste into effective adsorbents for wastewater treatment.

Single-stage production of miscanthus hydrochar at low severity conditions and application as adsorbent of copper and ammonium ions

In the framework of bio-circular economy, miscanthus biomass was valorized through a single-stage, low severity hydrothermal carbonization process. The produced hydrochars were characterized using elemental and spectroscopic methodologies. It was determined that as the temperature increased so did the C content (47.9 and 68.9% for the samples prepared at 180 and 260 °C, respectively), whereas the O content decreased (from 44.2 to 25.5%, respectively). The adsorption behaviour of the hydrochars was investigated in the adsorption of Cu²⁺ and NH₄⁺ and MIS-180 was determined as the optimum sample, achieving q_max values of 310 and 71 mg g^-1, respectively. Isotherm and kinetic analysis indicated the higher number of O-containing functional groups of MIS-180 as the main reason for its higher adsorption capacities. Furthermore, Cu²⁺ adsorption followed the 2nd-order kinetic model, whereas NH₄⁺ adsorption followed the 1st-order kinetic model, due to the different mechanisms involved, inner-sphere and outer-sphere complex formation, respectively.

Characterization Techniques as Supporting Tools for the Interpretation of Biochar Adsorption Efficiency in Water Treatment: A Critical Review

Over the past decade, biochar (BC) has received significant attention in many environmental applications, including water purification, since it is available as a low-cost by-product of the energetic valorisation of biomass. Biochar has many intrinsic characteristics, including its porous structure, which is similar to that of activated carbon (AC), which is the most widely used sorbent in water treatment. The physicochemical and performance characteristics of BCs are usually non-homogenously investigated, with several studies only evaluating limited parameters, depending on the individual perspective of the author. Within this review, we have taken an innovative approach to critically survey the methodologies that are generally used to characterize BCs and ACs to propose a comprehensive and ready-to-use database of protocols. Discussion about the parameters of chars that are usually correlated with adsorption performance in water purification is proposed, and we will also consider the physicochemical properties of pollutants (i.e., K_ow). Uniquely, an adsorption efficiency index BC/AC is presented and discussed, which is accompanied by an economic perspective. According to our survey, non-homogeneous characterization approaches limit the understanding of the correlations between the pollutants to be removed and the physicochemical features of BCs. Moreover, the investigations of BC as an adsorption medium necessitate dedicated parallel studies to compare BC characteristics and performances with those of ACs.

Enhanced adsorption of Cu(II) and Zn(II) from aqueous solution by polyethyleneimine modified straw hydrochar

Heavy metals removal from aqueous phase by adsorption technique has recently attracted a considerable interest. Although various adsorbing materials have been developed, introducing more functional groups is considered as the most efficient way to promote the adsorption capacity of the selected adsorbent. However, this approach is usually limited in costly modification precursor and unguaranteed loading efficacy. In this study, waste corn straw was converted to adsorbent precursor by hydrothermal carbonization. The obtained hydrochar (HC) was chemically activated before being modified by polyethyleneimine (PEI). Multiple analysis methods including Scanning Electron Microscopy, Fourier Transform Infrared analysis, and X-ray Photoelectron Spectroscopy analysis verified the alkali activated hydrochar (alkali-HC) was more efficacy to enhance PEI grafting than acid activation. Based on this, the modified HC materials obtained a better adsorption performance. The sorption process of Cu(II) and Zn(II) on the acid-PEI-HC, alkali-PEI-HC, and pristine HC fitted the pseudo second order kinetic and Freundlich model well, and was dominated by chemisorption. Among these adsorbents, the adsorption capacity of alkali-PEI-HC to metal ions was the maximum, which was 207.6 mg/g to Zn(II) and 56.1 mg/g to Cu(II) at 298 K. Regeneration tests showed a result of no less than 60% of its removal capacity was achieved after five cycles. Therefore, alkali-PEI-HC performed as a promising composite sorbent for metal ions. In addition, the study described here has provided a new basis for the utilization of hydrochar (1.08 kWh kg^-1) derived from agricultural resources as a promising adsorbent precursor.

Mechanism and industrial application feasibility analysis on microwave-assisted rapid synthesis of amino-carboxyl functionalized cellulose for enhanced heavy metal removal

The study presented the successful microwave-assisted (MW-assisted) preparation of a novel adsorbent derived from rice straw (RS_MW-AC) and explored its adsorption performance toward heavy metal ions from water. The RS_MW-AC was rapidly synthesized through pretreatment and one step grafting via the MW-assisted approach. The quantitative predictive correlations between target performance of RS_MW-AC and process parameters were obtained through the response surface methodology (RSM). Meanwhile, the optimal preparation process conditions were determined: NaOH solution concentration, 20%; MW irradiation temperature for pretreatment, 100 and 150 °C; MW irradiation time for pretreatment and grafting, 10 and 60 min; EDTAD-RS mass ratio, 3. The RS_MW-AC showed a good adsorption of different heavy metal ions from water (152.39, 55.46, 52.91, 35.60 and 20.11 mg g^-1 for Pb(Ⅱ), Mn(Ⅱ), Cd(Ⅱ), Cu(Ⅱ) and Ni(Ⅱ), respectively). The adsorption behaviors followed the Langmuir model and pseudo second-order kinetics model with a highly significant correlation. Also of note was that amino and carboxyl groups were successfully introduced on the rice straw based on characterization results. Furthermore, preparation mechanism was explored to reveal reasons why microwave irradiation could accelerate the preparation of the adsorbent; its adsorption process was dominated by electrostatic attraction and chelation. Finally, the study made the industrial application feasibility analysis of MW-assisted approach used for pretreatment and graft reaction of agro-waste biomass.

Facile modification of hydrochar derived from cotton straw with excellent sorption performance for antibiotics: Coupling DFT simulations with experiments

This study aimed to investigate the sorption of tetracycline (TC) and norfloxacin (NOR) by modified cotton straw hydrochars (CSHC), which would enable the agricultural waste to be processed and recycled. Three kinds of hydrochars were prepared by H₂SO₄, KOH and KMnO₄ modification, showed obvious differences in structures and surface functional groups. The sorption processes contain film diffusion, intraparticle diffusion, and equilibrium. The interaction mechanism between hydrochar and antibiotics include π-π stacking, hydrogen bond, and electrostatic interaction. KMnO₄-modified hydrochar had the largest sorption capacity for TC (58.09 mg/g), while H₂SO₄-modified hydrochar had the largest sorption capacity for NOR (49.64 mg/g). Density functional calculations (DFT) results confirmed that the sorption capacity between hydrochar (HC) and TC was larger than that between HC and NOR. During the sorption process, the TC and NOR were regarded as electron acceptor and electron donor. Generally, CSHC-KMnO₄ and CSHC-H₂SO₄ may be simply prepared and have the potential to eliminate antibiotics from water.

EDTAD-modified cassava stalks loaded with Fe3O4: highly efficient removal of Pb2+ and Zn2+ from aqueous solution

In this study, a novel magnetic cassava stalk composite (M-EMCS) was prepared through modification with ethylenediamine tetraacetic anhydride (EDTAD) and loading of Fe₃O₄. The surface morphology, molecular structure, and magnetic characteristics of the composite were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), vibrating-sample magnetometer (VSM), and X-ray diffraction (XRD). It was shown that EDTAD and Fe₃O₄ were successfully modified and loaded in cassava straw (CS), respectively. The capacity of M-EMCS to absorb heavy metals under different influencing factors was tested by atomic absorption spectroscopy. The adsorption processes of both Pb²⁺ and Zn²⁺ were suitably described by second-order kinetic models and Langmuir models, indicating monolayer chemisorption. M-EMCS had high adsorption rates and adsorption capacities for these two metal ions. The adsorption of Pb²⁺ and Zn²⁺ reached a plateau after 10 min, and the adsorption capacity of Pb²⁺ (163.93 mg/g) was higher than that of Zn²⁺ (84.74 mg/g). Thermodynamic analysis showed that the adsorption of two metals by M-EMCS was spontaneous, endothermic, and irreversible. XPS analysis showed that M-EMCS mainly removes Pb²⁺ and Zn²⁺ through ion exchange, chelation, and redox. Graphical abstract.

Biochars as media for air pollution control systems: Contaminant removal, applications and future research directions

Biochars are low-cost and renewable biomaterials with several applications, including soil amendment, mitigation of greenhouse gas emissions, and removal of both inorganic and organic contaminants in aqueous systems. An increasing body of recent evidence indicates that biochars can also remove gaseous chemical contaminants, such as those occurring in industrial flue gases. However, unlike other applications such as in agroecosystems, soil amendments, and aquatic systems, comprehensive reviews on biochar applications in the field of air pollution control are still lacking. The current paper examined existing evidence to understand the nature of contaminants, particularly the gaseous ones, potential applications, constraints, and future research needs pertaining to biochar applications in air pollution control. The preparation of biochars and their functionalized derivatives, and the properties influencing their capacity to remove gaseous contaminants are summarized. The removal capacity and mechanisms of various organic and inorganic gaseous contaminants by biochars are discussed. Evidence shows that biochars effectively remove metal vapours, particularly elemental mercury (Hg⁰), acidic gases (H₂S, SO₂, CO₂), ozone, nitrogen oxides (NO_x), and organic contaminants including aromatic compounds, volatile organic compounds, and odorous substances. The mechanisms for the removal of gaseous contaminants, including; adsorption, precipitation, and size exclusion were presented. Potential industrial application domains include remediation of gaseous emissions from incinerators, waste-to-energy systems, kilns, biomass and coal-fired boilers/cookers, cremation, smelters, wastewater treatment, and agricultural production systems including livestock husbandry. These industrial applications, coupled with the renewable, low-cost and sustainable nature of biochars, point to opportunities to further develop and scale up the biochar technology in the air pollution control industry. However, the biochar-based air filter technology still faces several challenges, largely stemming from constraints and several knowledge gaps, which were highlighted. Hence, further research is required to address these constraints and knowledge gaps before the benefits of the biochar-based air filters are realized.

KOH-activated porous biochar with high specific surface area for adsorptive removal of chromium (VI) and naphthalene from water: Affecting factors, mechanisms and reusability exploration

Herein, a high-performance porous biochar described as PBC_KOH was successfully synthesized by two-step pyrolysis of corn straw with chemical activation of KOH, and was employed for the elimination of Cr(VI) and naphthalene (NAP) from water. Benefiting from KOH activation, the PBC_KOH was found to possess huge specific surface area of 2183.80 m²/g and many well-developed micropores with average particle size of 2.75 nm and main pore diameters distribution from 1 to 2 nm. The PBC_KOH presented an excellent adsorption performance with a theoretical monolayer uptake of 116.97 mg/g for Cr(VI) and a heterogeneous adsorption capacity of 450.43 mg/g for NAP. The uptake equilibrium was attained within about 120 min for Cr(VI), while about 180 min for NAP following avrami fractional-order model, revealing the existence of multiple kinetics during the adsorption. The thermodynamic results showed that the uptake of both Cr(VI) and NAP occurred spontaneously (-ΔG°), while in an endothermic nature for Cr(VI) (+ΔH°) and an exothermic characteristic for NAP (-ΔH°) with different randomness. Furthermore, the PBC_KOH was believed to enhance the Cr(VI) adsorption mainly through the combination of electrostatic attraction, complexation, ion exchange and reduction action, while achieving the high NAP uptake by pore filling and π-π stacking interactions.