Biochar from extracted marine Chlorella sp. residue for high efficiency adsorption with ultrasonication to remove Cr(VI), Zn(II) and Ni(II)

Synthesis, delineation and technological advancements of algae biochar for sustainable remediation of the emerging pollutants from wastewater-a review

The use of algae for value-added product and biorefining applications is enchanting attention among researchers in recent years due to its remarkable photosynthetic ability, adaptability, and capacity to accumulate lipids and carbohydrates. Algae biomass, based on its low manufacturing costs, is relatively renewable, sustainable, environmentally friendly and economical in comparison with other species. High production rate of algae provides a unique opportunity for its conversion to biochar with excellent physicochemical properties, viz. high surface area and pore volume, high adsorption capacity, abundant functional groups over surface, etc. Despite several potential algal-biochar, a detailed study on its application for removal of emerging contaminants from wastewater is limited. Therefore, this technical review is being carried out to evaluate the specific elimination of inorganic and organic pollutants from wastewater, with a view to assessing adsorption performances of biochar obtained from various algae species. Species-specific adsorption of emerging pollutants from wastewater have been discussed in the present review. The promising methods like pyrolysis, gasification, dry and wet torrefaction for the production of algae biochar are highlighted. The strategies include chemical and structural modifications of algae biochar for the removal of toxic contaminants have also been considered in the current work. The overall aim of this review is to confer about the synthesis, technological advancements, delineation and application of algae biochar for the treatment of wastewater.

Applications of engineered biochar in remediation of heavy metal(loid)s pollution from wastewater: Current perspectives toward sustainable development goals

Environmental pollution of heavy metal(loid)s (HMs) caused adverse impacts, has become one of the emerging concerns and challenges worldwide. Metal(loid)s can pose significant threats to living organisms even when present in trace levels within environmental matrices. Extended exposure to these substances can lead to adverse health consequences in humans. Removing HM-contaminated water and moving toward sustainable development goals (SDGs) is critical. In this mission, biochar has recently gained attention in the environmental sector as a green and alternative material for wastewater removal. This work provides a comprehensive analysis of the remediation of typical HMs by biochars, associated with an understanding of remediation mechanisms, and gives practical solutions for ecologically sustainable. Applying engineered biochar in various fields, especially with nanoscale biochar-aided wastewater treatment approaches, can eliminate hazardous metal(loid) contaminants, highlighting an environmentally friendly and low-cost method. Surface modification of engineered biochar with nanomaterials is a potential strategy that positively influences its sorption capacity to remove contaminants. The research findings highlighted the biochars' ability to adsorb HM ions based on increased specific surface area (SSA), heightened porosity, and forming inner-sphere complexes with oxygen-rich groups. Utilizing biochar modification emerged as a viable approach for addressing lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), and chromium (Cr) pollution in aqueous environments. Most biochars investigated demonstrated a removal efficiency >90 % (Cd, As, Hg) and can reach an impressive 99 % (Pb and Cr). Furthermore, biochar and advanced engineered applications are also considered alternative solutions based on the circular economy.

Research advances on production and application of algal biochar in environmental remediation

Algae, comprising microalgae and macroalgae, have emerged as a promising feedstock for the production of functional biochar. Recently, the application of algal biochar in environmental remediation gains increasing attention. This review summarizes research advancements in the synthesis and application of algal biochar, a versatile and sustainable material for environmental remediation ranging from wastewater treatment to soil improvement. Algal biochar can be prepared by pyrolysis, microwave-assisted pyrolysis, and hydrothermal carbonization. Physical and chemical modifications have proven to be effective for improving biochar properties. Algal biochar is promising for removing diverse pollutants including heavy metals, organic pollutants, and microplastics. The role in soil improvement signifies a sustainable approach to enhancing soil structure, nutrient retention, and microbial activity. Research gaps are identified based on current understanding, necessitating further exploration into variations in biochar characteristics, the performance improvement, large-scale applications, and the long-term evaluation for environmental application. This review provides a better understanding of algal biochar as a sustainable and effective tool in environmental remediation.

Recent insights into mechanism of modified bio-adsorbents for the remediation of environmental pollutants

Rapid industrialization has exacerbated the hazard to health and the environment. Wide spectrums of contaminants pose numerous risks, necessitating their disposal and treatment. There is a need for further remediation methods since pollutant residues cannot be entirely eradicated by traditional treatment techniques. Bio-adsorbents are gaining popularity due to their eco-friendly approach, broad applicability, and improved functional and surface characteristics. Adsorbents that have been modified have improved qualities that aid in their adsorptive nature. Adsorption, ion exchange, chelation, surface precipitation, microbial uptake, physical entrapment, biodegradation, redox reactions, and electrostatic interactions are some of the processes that participate in the removal mechanism of biosorbents. These processes can vary depending on the particular biosorbent and the type of pollutants being targeted. The systematic review focuses on the many modification approaches used to remove environmental contaminants. Different modification or activation strategies can be used depending on the type of bio-adsorbent and pollutant to be remediated. Physical activation procedures such as ultrasonication and pyrolysis are more commonly used to modify bio-adsorbents. Ultrasonication process improves the adsorption efficiency by 15-25%. Acid and alkali modified procedures are the most effective chemical activation strategies for adsorbent modification for pollution removal. Chemical modification increases the removal to around 95-99%. The biological technique involving microbial culture is an emerging field that needs to be investigated further for pollutant removal. A short evaluation of modified adsorbents with multi-pollutant adsorption capability that have been better eliminated throughout the adsorption process has been provided.

Microalgae-based biochar production and applications: A comprehensive review

Biochar, a solid carbonaceous substance synthesized from the thermochemical degradation of biomass, holds significant potential in addressing global challenges such as soil degradation, environmental pollution, and climate change. Its potential as a carbon sequestration agent, together with its versatile applications in soil amendments, pollutant adsorption, and biofuel production, has garnered attention. On the other hand, microalgae, with their outstanding photosynthetic efficiency, adaptability, and ability to accumulate carbohydrates and lipids, have demonstrated potential as emerging feedstock for biochar production. However, despite the significant potential of microalgal biochar, our current understanding of its various aspects, such as the influence of parameters, chemical modifications, and applications, remains limited. Therefore, this review aims to provide a comprehensive analysis of microalgae-based biochar, covering topics such as production techniques, pollutant removal, catalytic applications, soil amendments, and synthesis of carbon quantum dots to bridge the existing knowledge gap in this field.

The magic of algae-based biochar: advantages, preparation, and applications

Compared with other biomass sources, the use of algae as a raw material to prepare biochar (BC) has important advantages including safety, high yield and economy. The protein content of algae cells is as high as 3.2 mg DCW/L, and the graphitic-N and N-O functional groups generated by the pyrolysis of proteins could effectively activate free radicals. Combined with the generated pore structure, the electron transfer/exchange capability was enhanced, which is conducive to improving its catalytic performance. Algae as a natural N source, the manuscript analyzed the surface properties and physicochemical properties of algae-based BC, and investigated its degradation effect on organic/inorganic pollutants in wastewater. Subsequently, the effect of nitrogen-doped BC on the adsorption/catalysis capacity was discussed. Finally, the directed preparation of algae-based BC applied in different scenarios was summarized. Algae-based BC has the property of N doping, which broadens its application efficiency in the environmental field. Overall, this manuscript reviews how to achieve efficient utilization of algae-based BC in wastewater.

Exploitation of lignocellulosic-based biomass biorefinery: A critical review of renewable bioresource, sustainability and economic views

Urbanization has driven the demand for fossil fuels, however, the overly exploited resource has caused severe damage on environmental pollution. Biorefining using abundant lignocellulosic biomass is an emerging strategy to replace traditional fossil fuels. Value-added lignin biomass reduces the waste pollution in the environment and provides a green path of conversion to obtain renewable resources. The technology is designed to produce biofuels, biomaterials and value-added products from lignocellulosic biomass. In the biorefinery process, the pretreatment step is required to reduce the recalcitrant structure of lignocellulose biomass and improve the enzymatic digestion. There is still a gap in the full and deep understanding of the biorefinery process including the pretreatment process, thus it is necessary to provide optimized and adapted biorefinery solutions to cope with the conversion process in different biorefineries to further provide efficiency in industrial applications. Current research progress on value-added applications of lignocellulosic biomass still stagnates at the biofuel phase, and there is a lack of comprehensive discussion of emerging potential applications. This review article explores the advantages, disadvantages and properties of pretreatment methods including physical, chemical, physico-chemical and biological pretreatment methods. Value-added bioproducts produced from lignocellulosic biomass were comprehensively evaluated in terms of encompassing biochemical products , cosmetics, pharmaceuticals, potent functional materials from cellulose and lignin, waste management alternatives, multifunctional carbon materials and eco-friendly products. This review article critically identifies research-related to sustainability of lignocellulosic biomass to promote the development of green chemistry and to facilitate the refinement of high-value, environmentally-friendly materials. In addition, to align commercialized practice of lignocellulosic biomass application towards the 21st century, this paper provides a comprehensive analysis of lignocellulosic biomass biorefining and the utilization of biorefinery green technologies is further analyzed as being considered sustainable, including having potential benefits in terms of environmental, economic and social impacts. This facilitates sustainability options for biorefinery processes by providing policy makers with intuitive evaluation and guidance.

Biochar production and its environmental applications: Recent developments and machine learning insights

Biochar production through thermochemical processing is a sustainable biomass conversion and waste management approach. However, commercializing biochar faces challenges requiring further research and development to maximize its potential for addressing environmental concerns and promoting sustainable resource management. This comprehensive review presents the state-of-the-art in biochar production, emphasizing quantitative yield and qualitative properties with varying feedstocks. It discusses the technology readiness level and commercialization status of different production strategies, highlighting their environmental and economic impacts. The review focuses on integrating machine learning algorithms for process control and optimization in biochar production, improving efficiency. Additionally, it explores biochar's environmental applications, including soil amendment, carbon sequestration, and wastewater treatment, showcasing recent advancements and case studies. Advances in biochar technologies and their environmental benefits in various sectors are discussed herein.

Optimization of biochar production from greywater grown polyculture microalgae using microwave pyrolysis

Biochar was produced from polyculture microalgae cultivated in greywater using microwave pyrolysis. The highest biochar yield and fixed carbon content of 49.9% and 68.7% were obtained at microwave power (P) of 800 W and reaction time (T) of 8.6 min. The developed quadratic models, 166.96 - 0.23P - 3.87 T - 3.49 x10-3PT + 1.73 x10-4P2 + 0.13 T2 and - 73.79 + 0.29P + 1.86 T - 1.80 x10-4P2 could predict biochar yield and fixed carbon content respectively with errors of 6.2 and 7.9%. The volatile matter (VM), fixed carbon (FC), and high heating value (HHV) of the biomass were 69.2%. 23.4% and 17.6 MJ/Kg, respectively. VM, FC, and HHV for biochar obtained at optimum conditions were 20.2%, 68.7%, and 28.3 MJ/Kg, respectively. The process had a net positive energy balance of 11.32 MJ/Kg and energy efficiency of 1.76. This study paves the way for biochar production from greywater-grown microalgae, contributing to waste valorization and energy sustainability.

Biochar production from microalgae: a new sustainable approach to wastewater treatment based on a circular economy

The generation of wastewater due to human activities are the main responsible for environmental problems. These problems are caused by the large amount of organic and inorganic pollutants related to the presence of pesticides, metals, pathogens, drugs and dyes. The photosynthetic treatment of effluents emerges as a sustainable and low-cost alternative for developing wastewater treatment systems based on a circular economy. Chemical compounds present in wastewater can be recovered and reused as a source of nutrients in microalgae cultivation to produce value-added bioproducts. The microalgal biomass produced in the cultivation with effluents has the potential to produce biochar. Biochar is carbon-rich charcoal that can be obtained by converting microalgae biomass through thermal decomposition of organic raw material under limited oxygen supply conditions. Pyrolysis, torrefaction, and hydrothermal carbonization are processes used for biochar synthesis. The application of microalgal biochar as an adsorbent material to remove several compounds present in effluents is an effective and fast treatment. This effectiveness is usually related to the unique physicochemical characteristics of the biochar, such as the presence of functional groups, ion exchange capacity, thermal stability, and high surface area, volume, and pore area. In addition, biochar can be reused in the adsorption process or applied in agriculture for soil correction. In this context, this review article describes the production, characterization, and use of microalgae biochar through a sustainable approach to wastewater treatment, emphasizing its potential in the circular economy. In addition, the article approaches the potential of microalgal biochar as an adsorbent material and its reuse after the adsorption of contaminants, as well as highlights the challenges and future perspectives on this topic.

Improvements in the biochemical responses and Pb and Ni phytoremediation of lavender (Lavandula angustifolia L.) plants through Funneliformis mosseae inoculation

BACKGROUND

Heavy metals (HMs) phytoremediation is a well-recognized protocol to remove toxic elements from the soil. As known, arbuscular mycorrhizal fungi (AMF) enhance the plants' growth responses. The idea of the present study was to assay the response of lavender plants to HMs stress under AMF inoculation. We hypothesized that mycorrhiza will enhance the phytoremediation and simultaneously reduce the harmful effects of heavy HMs. So, lavender (Lavandula angustifolia L.) plants were inoculated with AMF (0 and 5 g Kg^-1 soil) under Pb [150 and 225 mg kg^-1 soil from Pb (NO₃)₂] and Ni [220 and 330 mg kg^-1 soil from Ni (NO₃)₂] pollution, in the greenhouse conditions. The control treatment was plants not treated with AMF and HMs. Doing this, the root colonization, HMs uptake, enzymatic and non-enzymatic antioxidants pool, MDA, proline, total phenolics (TPC), flavonoids (TFC), anthocyanins, and essential oil (EO) components were evaluated.

RESULTS

According to the findings, the AMF inoculation enhanced shoot and root Pb and Ni content, antioxidant enzymes activity, the total antioxidant activity by DPPH and FRAP methods, TPC, TFC, anthocyanins, and H₂O₂ content in the lavender plants subjected to Pb and Ni stress. Moreover, the highest (28.91%) and the least (15.81%) percentages of borneol were identified in the lavender plants subjected to AMF under 150 mg kg^-1 of Pb and the control plants without AMF application, respectively. Furthermore, the top 1,8-cineole (12.75%) content was recorded in AMF-inoculated plants.

CONCLUSIONS

The overall results verify that AMF inoculation can be a reliable methodology to enhance the phytoremediation of Pb and Ni by lavender plants while maintaining reliable growth potential. The treatments improved the main EO constituents content, especially under moderate HMs stress conditions. With more detailed studies, the results will be advisable for the extension section for the phytoremediation of polluted soils.

Application of covalent organic frameworks and metal–organic frameworks nanomaterials in organic/inorganic pollutants removal from solutions through sorption-catalysis strategies

With the fast development of agriculture, industrialization and urbanization, large amounts of different (in)organic pollutants are inevitably discharged into the ecosystems. The efficient decontamination of the (in)organic contaminants is crucial to human health and ecosystem pollution remediation. Covalent organic frameworks (COFs) and metal–organic frameworks (MOFs) have attracted multidisciplinary research interests because of their outstanding physicochemical properties like high stability, large surface areas, high sorption capacity or catalytic activity. In this review, we summarized the recent works about the elimination/extraction of organic pollutants, heavy metal ions, and radionuclides by MOFs and COFs nanomaterials through the sorption-catalytic degradation for organic chemicals and sorption-catalytic reduction-precipitation-extraction for metals or radionuclides. The interactions between the (in)organic pollutants and COFs/MOFs nanomaterials at the molecular level were discussed from the density functional theory calculation and spectroscopy analysis. The sorption of organic chemicals was mainly dominated by electrostatic attraction, π-π interaction, surface complexation and H-bonding interaction, whereas the sorption of radionuclides and metal ions was mainly attributed to surface complexation, ion exchange, reduction and incorporation reactions. The porous structures, surface functional groups, and active sites were important for the sorption ability and selectivity. The doping or co-doping of metal/nonmetal, or the incorporation with other materials could change the visible light harvest and the generation/separation of electrons/holes (e−/h+) pairs, thereby enhanced the photocatalytic activity. The challenges for the possible application of COFs/MOFs nanomaterials in the elimination of pollutants from water were described in the end.

Removal of heavy metals from aqueous solutions by high performance capacitive deionization process using biochar derived from Sargassum hemiphyllum

Capacitive deionization (CDI) has been considered as an efficient, energy-saving and environmental friendly technology for water treatment. For the practical application of CDI, high-performance electrode materials beyond standard activated carbon should be developed. In this study, biochar derived from brown algae Sargassum hemiphyllum prepared by pyrolysis at 300-700 °C and then used as the CDI electrode to remove Cu(II) from aqueous solutions. According to the findings, the optimal pyrolysis temperature was 700 °C, and the electrosorption capacity of BAB700 was 75-120 mg·g-1 at an applied voltage of 1.2 V across wide range of initial pH, temperatures and ion types. Moreover, BAB700 also exhibited outstanding ability to electrosorb other heavy metals (Zn(II), Ni(II), and Cd(II)). In addition, the BAB700 retained the Cu(II) removal efficiency of 70 % in 10 cycles. Cu(II) in actual water is completely eliminated with great reproducibility, resulting in a high degree of applicability for water treatment.

Effective Usage of Biochar and Microorganisms for the Removal of Heavy Metal Ions and Pesticides

The bioremediation of heavy metal ions and pesticides is both cost-effective and environmentally friendly. Microbial remediation is considered superior to conventional abiotic remediation processes, due to its cost-effectiveness, decrement of biological and chemical sludge, selectivity toward specific metal ions, and high removal efficiency in dilute effluents. Immobilization technology using biochar as a carrier is one important approach for advancing microbial remediation. This article provides an overview of biochar-based materials, including their design and production strategies, physicochemical properties, and applications as adsorbents and support for microorganisms. Microorganisms that can cope with the various heavy metal ions and/or pesticides that enter the environment are also outlined in this review. Pesticide and heavy metal bioremediation can be influenced by microbial activity, pollutant bioavailability, and environmental factors, such as pH and temperature. Furthermore, by elucidating the interaction mechanisms, this paper summarizes the microbe-mediated remediation of heavy metals and pesticides. In this review, we also compile and discuss those works focusing on the study of various bioremediation strategies utilizing biochar and microorganisms and how the immobilized bacteria on biochar contribute to the improvement of bioremediation strategies. There is also a summary of the sources and harmful effects of pesticides and heavy metals. Finally, based on the research described above, this study outlines the future scope of this field.

Recent progress in microalgae-derived biochar for the treatment of textile industry wastewater

Textile industry utilize a massive amount of dyes for coloring. The dye-containing effluent is released into wastewater along with heavy metals that are part of dye structure. The treatment of textile industry wastewater using conventional techniques (coagulation, membrane technique, electrolysis ion exchange, etc.) is uneconomical and less efficient (for a low concentration of pollutants). Moreover, most of these techniques produce toxic sludge, making them less environmentally friendly. Algae base industry is growing for food, cosmetics and energy needs. Algae biomass in unique compared to lignocellulosic biomass due to presence of various functional group on its surface and presence of various cations. These two characteristics are unique for biochar as a tool for environmental decontamination. Algae biomass contain functional groups and cations that can be effective for removal of organic contaminants (dyes) and heavy metals. Algae can be micro and macro and both have entirely different biomass composition which will lead to a synthesis of different biochar even under same synthesis process. This study reviews the recent progress in the development of an economically viable and eco-friendly approach for textile industry wastewater using algae biomass-derived absorbents. The strategy employed microalgal biochar to remove organic pollutants (dyes) and heavy metals from textile effluents by biosorption. This article discusses different methods for preparing algal biochar (pyrolysis, hydrothermal carbonization and torrefaction), and the adsorption capacity of biochar for dyes and heavy metals. Work on hydrothermal carbonization and torrefaction of microalgal biomass for biochar is limited. Variation in structural and functional groups changes on biochar compared to original microalgal biomass are profound in contract with lignocellulosic biomass. Existing Challenges, future goals, and the development of these technologies at the pilot level are also discussed.

Magnetic biochar derived from macroalgal Sargassum hemiphyllum for highly efficient adsorption of Cu(II): Influencing factors and reusability

In this study, the brown algae Sargassum Hemiphyllum was used as a carbon source for synthesis of magnetic porous biochar via pyrolyzing at high temperature and and doping iron oxide particles (Fe-BAB). Cu (II) species were removed from aqueous solutions using Fe-BAB under various conditions. Fe-BAB demonstrated superior Cu (II) adsorption (105.3 mg g-1) compared to other biochars. On the surface of Fe-BAB, there are several oxygen-containing functional groups, such as -COOH and -OH, which are likely responsible for the excellent heavy metal removal performance. By utilizing magnet, the Fe-BAB can be conveniently separated from the solution and ready for further usage. Multi-adsorption mechanisms were responsible for Cu adsorption on Fe-BAB. Using the magnetic algal biochar for heavy metal removal is feasible due to its high adsorption efficiency and simplicity of separation.

Ultrasonication expedited As(III) adsorption onto chitosan impregnated Ni-Fe layered double hydroxide biosorbent: Optimization studies and artificial intelligence modelling

Chitosan intercalated Ni-Fe layered double hydroxide (Ni-Fe LDH/Ch), prepared by co-precipitation was examined for adsorptive elimination of arsenic (III). Energy Dispersive X-ray analysis, X-ray diffraction, Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy, and Dynamic Light Scattering validated the successful synthesis of the composite with enhanced adsorption sites. Maximal As(III) removal was obtained at adsorbent dose 1 gL^-1, pH 7, ultrasonication time 30 min, temperature 298 K, and initial arsenic concentration 50 mgL^-1. The experimentally obtained values fit the Langmuir isotherm and pseudo-second-order dynamics well (R² > 0.98), while thermodynamic evaluation confirmed exothermic and spontaneous reaction (ΔG = -8.13 kJ mol^-1). Further, adaptive neuro-fuzzy inference system and artificial neural network successfully predicted As(III) removal percentage with a high correlation coefficient (R² > 0.94) and low statistical errors (MSE< 0.002, AARE< 0.063). The prepared material successfully brought down arsenic level by 62% in a natural water sample and showed good reusability up to 5 consecutive treatment cycles. The results recommended that Ni-Fe LDH/Ch has ample potential for arsenic remediation, and further investigations can be carried out for large-scale applications.

Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO₄^-, CrO₄^2-, and Cr₂O₇^2- ions in aqueous solutions. At low pH (∼1-4), the availability of HCrO₄^- ions attributes the electrostatic forces of attraction due to the available functional groups such as -NH₄⁺, -COOH, and -OH₂⁺, which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr(VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr(VI) removal and other heavy metals are elegantly discussed.

Fabrication of Nano Iron Oxide–Modified Biochar from Co-Hydrothermal Carbonization of Microalgae and Fe(II) Salt for Efficient Removal of Rhodamine B

Dye adsorption by magnetic modified biochar has now received growing interest due to its excellent adsorption performance and facile separation for recycling. In this study, nano iron oxide–modified biochar was fabricated via the successive hydrothermal-pyrolyzing method using Chlorella vulgaris (Cv) and FeSO₄·7H₂O as raw materials, and its adsorption on Rhodamine B (RhB) in aqueous solution was studied. Multiple techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), vibrating sample magnetometry (VSM) and X-ray photoelectron spectroscopy (XPS) were employed to comprehensively characterize the structure, morphology and physicochemical properties of the adsorbent. The as-synthesized nano iron oxide–modified biochar (CBC-Fe(II)) exhibited a large surface area (527.6 m²/g) and high magnetic saturation value (13.7 emu/g) to facilitate magnetic separation. Compared with CBC and CBC-Fe(III), CBC-Fe(II) exhibited superior adsorption ability towards RhB in aqueous solution, with a maximum adsorption capacity of 286.4 mg/g. The adsorption process of RhB onto CBC-Fe(II) was well described by the pseudo-second-order kinetic model and Langmuir isotherm model, indicating monolayer chemisorption behaviors for the adsorption system. Facile preparation, great adsorption performance and magnetic recovery properties endow CBC-Fe(II) to be a promising adsorbent for dye removal.

Study on the mechanism of biochar loaded typical microalgae Chlorella removal of cadmium

Coconut shell activated carbon (Csac) is one of the most widely used materials to remove cadmium (Cd) from contaminated water. A large diversity of microorganisms exists in various aquatic systems and may aid Cd removal by Csac. In this study, we explored the reactions of Csac with microalgae (Chlorella) in Cd-containing media. The results of scanning electron microscope (SEM) imaging, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), superconducting pulse-Fourier transform nuclear magnetic resonance (pulse-FT NMR) and X-ray photoelectron spectroscopy (XPS) indicated that Chlorella could adhere in the micropores of Csac formed Csac@Chlorella composite adsorbent loading Chlorella. Furthermore, the composite adsorbent surface had abundant functional groups such -COOH, -OH and C-O-C, which served as active sites during the adsorption process. Compared with Csac, Csac@Chlorella had an enhanced Cd adsorption capacity evidently. The results showed that pH 8, 0.2 g Csac, OD₆₈₀ of 0.1 for Chlorella were optimal conditions for maximum Cd adsorption capacity within one hour contact time. Furthermore, the Cd adsorption process was well described by the pseudo-second-order and Langmuir adsorption isotherm models. The models revealed that the adsorption process was mainly based on chemical adsorption of a single molecular layer, accompanied by electrostatic attraction, complexation and intracellular adsorption, amongst other parameters. Collectively, the findings illustrate that the microalgae (Chlorella)-Csac-Cd interaction is complex and will thus have immense interest to a broad range of biological, environmental, and geoscience communities.

Microalgal-based biochar in wastewater remediation: Its synthesis, characterization and applications

Microalgae are drawing attentions among researchers for their biorefinery use or value-added products. The high production rate of biomasses produced are attractive for conversion into volatile biochar. Torrefaction, pyrolysis and hydrothermal carbonization are the recommended thermochemical conversion techniques that could produce microalgal-based biochar with desirable physiochemical properties such as high surface area and pore volume, abundant surface functional groups, as well as functionality such as high adsorption capacity. The characterizations of the biochar significantly influence the mechanisms in adsorption of pollutants from wastewaters. Specific adsorption of the organic and inorganic pollutants from the effluent are reviewed to examine the adsorption capacity and efficiency of biochar derived from different microalgae species. Last but not least, future remarks over the challenges and improvements are discussed accordingly. Overall, this review would discuss the synthesis, characterization and application of the microalgal-based biochar in wastewater.

Algae-mediated antibiotic wastewater treatment: A critical review

The existence of continually increasing concentrations of antibiotics in the environment is a serious potential hazard due to their toxicity and persistence. Unfortunately, conventional treatment techniques, such as those utilized in wastewater treatment plants, are not efficient for the treatment of wastewater containing antibiotic. Recently, algae-based technologies have been found to be a sustainable and promising technique for antibiotic removal. Therefore, this review aims to provide a critical summary of algae-based technologies and their important role in antibiotic wastewater treatment. Algal removal mechanisms including bioadsorption, bioaccumulation, and biodegradation are discussed in detail, with using algae-bacteria consortia for antibiotic treatment, integration of algae with other microorganisms (fungi and multiple algal species), hybrid algae-based treatment and constructed wetlands, and the factors affecting algal antibiotic degradation comprehensively described and assessed. In addition, the use of algae as a precursor for the production of biochar is highlighted, along with the modification of biochar with other materials to improve its antibiotic removal capacity and hybrid algae-based treatment with advanced oxidation processes. Furthermore, recent novel approaches for enhancing antibiotic removal, such as the use of genetic engineering to enhance the antibiotic degradation capacity of algae and the integration of algal antibiotic removal with bioelectrochemical systems are discussed. Finally, some based on the critical review, key future research perspectives are proposed. Overall, this review systematically presents the current progress in algae-mediated antibiotic removal technologies, providing some novel insights for improved alleviation of antibiotic pollution in aquatic environments.

Dithiocarbamate modification of activated carbon for the efficient removal of Pb(ii), Cd(ii), and Cu(ii) from wastewater

Proposed adsorption mechanisms: ion exchange and chelation.

Hydrothermal Carbonization of Residual Algal Biomass for Production of Hydrochar as a Biobased Metal Adsorbent

Conversion of residual algal biomass to value-added products is essential for enhancing the economics of algae cultivation. Algal hydrochar produced via hydrothermal carbonization of lipid-extracted Picochlorum oculatum is a material rich in oxygen functional groups and carbon (up to 67.3%) and hence a promising candidate for remediation of wastewaters. The hydrothermal carbonization conditions were optimized and the adsorption capacity of the hydrochar was tested for metal removal. By the end of the remediation process, cumulative removal of Al3+, Cu2+, Fe2+, Mg2+, Mn2+, and Pb2+ reached 89, 98, 75, 88, 75, and 100%, respectively. The adsorption of all metals was found to follow pseudo second-order kinetics and the Langmuir isotherm. Overall, when hydrothermal carbonization is applied to lipid-extracted algae, it generates a promising biobased adsorbent with value-added potential in metal remediation.

Cultivation and Biorefinery of Microalgae (Chlorella sp.) for Producing Biofuels and Other Byproducts: A Review

Microalgae-based carbon dioxide (CO2) biofixation and biorefinery are the most efficient methods of biological CO2 reduction and reutilization. The diversification and high-value byproducts of microalgal biomass, known as microalgae-based biorefinery, are considered the most promising platforms for the sustainable development of energy and the environment, in addition to the improvement and integration of microalgal cultivation, scale-up, harvest, and extraction technologies. In this review, the factors influencing CO2 biofixation by microalgae, including microalgal strains, flue gas, wastewater, light, pH, temperature, and microalgae cultivation systems are summarized. Moreover, the biorefinery of Chlorella biomass for producing biofuels and its byproducts, such as fine chemicals, feed additives, and high-value products, are also discussed. The technical and economic assessments (TEAs) and life cycle assessments (LCAs) are introduced to evaluate the sustainability of microalgae CO2 fixation technology. This review provides detailed insights on the adjusted factors of microalgal cultivation to establish sustainable biological CO2 fixation technology, and the diversified applications of microalgal biomass in biorefinery. The economic and environmental sustainability, and the limitations and needs of microalgal CO2 fixation, are discussed. Finally, future research directions are provided for CO2 reduction by microalgae.

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.

Biosorption of heavy metal ions by green alga Neochloris oleoabundans: Effects of metal ion properties and cell wall structure

Effects of metal ion proprieties and the cell wall structure of green alga Neochloris oleoabundans were investigated on five strategically selected heavy metal ions, Pb(II), Hg(II), Zn(II), Cd(II) and Cu(II). The biosorption of these ions were energy-independent and spontaneous Langmuir adsorption. The adsorption capacities of Pb(II), Hg(II), Zn(II), Cd(II) and Cu(II) were determined to be 1.03, 0.91, 1.20, 0.65 and 1.23 mmol/g, respectively. Data suggest that peptide-containing molecules and non-cellulosic polysaccharides on cell wall were the primary sites of adsorption. Ion Pb(II) showed the strongest inhibitive effects on the adsorption of other metal ions on cells in binaries, corresponding to its large affinity to the biosorbents, which was next only to that of Cu(II). A linear relation was established for the first time between the adsorption capacity and the impact factor, which is defined in this paper as the electronegativity of a metal ion normalized by its atomic radius. In other words, adsorption capacity of N. oleoabundans biomass to the tested two-valence metal ions is proportional to the electronegativity and inversely proportional to the radius of the metal ions. Cell aggregation was caused by the addition of Cu(II), which exhibited distinctive adsorption behaviors than other metal ions.

Potential application of Allium Cepa seeds as a novel biosorbent for efficient biosorption of heavy metals ions from aqueous solution

Abatement of pollutants i.e. heavy metals by using green biomaterials is an emerging area of interest due to its cost-effective and renewability. In the present study, the potential of Alium Cepa seed biomass (ACSB) as a novel biosorbent for the adsorption of Cr(VI), Cd(II), Zn(II), Cu(II) and Pb(II) was investigated. The FTIR spectrum of ACSB confirmed a presence of surface OH bond, an essential functional group for metal uptake. Biosorption factors such as pH (2-10), time (15-190 min), dosage (1-5 g/L) and initial metal concentration (50-200 mg/L) were optimized at the ambient conditions. The equilibrium adsorption time was obtained at 90 min for Cd(II), Cu(II) and Pb(II), as well as 120 min for Cr(VI) and Zn(II), respectively, for the mentioned metal ions removal. The maximum removal efficiency was obtained at 4 g/L of ASCB for 50 mg/L adsorbate and a neutral pH. Under this condition, the maximum uptake was 0.67, 1.50, 1.68, 1.03 and 1.75 mg/L for Cr(VI), Cd(II), Zn(II), Cu(II) and Pb(II), respectively. Monolayer biosorption was determined for the studied heavy metals. The removal of the metal ions by ACSB followed a pseudo 2nd order sorption kinetics. The results suggested that ACSB is more suitable to remove (99%) Pb(II), Cu(II), Cd(II) as compared to Zn(II) and Cr(VI).

(3-Aminopropyl)triethoxysilane and iron rice straw biochar composites for the sorption of Cr (VI) and Zn (II) using the extract of heavy metals contaminated soil

Heavy metals like Cr (VI), when released into the environment, pose a serious threat to animal and human health. In this study, iron and (3-Aminopropyl)triethoxysilane (APTES) biochar composites were prepared from the biochar, which was produced through the pyrolysis of rice straw at 400 and 600 °C, using the chemical processes with an aim that the doping of pristine biochar structure with the Fe and NH₂ radicals would enhance the removal of Cr (VI) and Zn (II) adsorption in both aqueous solution and soil. Both biochar composites were mixed at a rate of 3% (w/w) with the mine soil for the soil incubation test, and after completion of the test, a soil fertility index (SFI) was calculated. Results showed that both iron and APTES biochar composites followed the Langmuir-Freundlich isotherm showing the maximum removal capacity of 100.59 mg/g for Cr (VI) by APTES/SiBC 600 and maximum adsorption capacity of 83.92 mg/g for Zn²⁺ by Fe/BC 400. The SFI of the mine-soil amended with both Fe and APTES biochar composites were 16.67 and 13.04%, respectively higher than the controlled study. The mitotic index of the A. cepa cells that grew up in the soil amended with Fe/BC and APTES/SiBC were 40.47 and 44.45%, respectively, higher than the controlled study. The results indicated that the incorporation of the Fe and APTES biochar composites in the soil effectively reduced the metal toxicity and improved the soil physicochemical properties. This study opens up the prospects of using biochar composites in contaminated soil and water treatments.

Magnesium-zinc ferrites as magnetic adsorbents for Cr(VI) and Ni(II) ions removal: Cation distribution and antistructure modeling

The magnesium-zinc ferrites Mg_1-xZn_xFe₂O₄ (x = 0…1) were studied as magnetic sorbents for environmental applications. Low-temperature Mössbauer spectroscopy was used to determine the distribution of magnesium and ferric ions in the spinel crystal lattice. The influence of Zn content on magnetic parameters was investigated on the basis of VSM data. As the molar ratio of zinc to magnesium increases from 0 to 1, the pH_PZC value decreases from 10.5 to 8.9. Langmuir and Freundlich models were used to check whether single-layer or multi-layer adsorption occurs. The adsorption of Cr(VI) and Ni(II) ions is well fitted by the Langmuir equation. To check the physical or chemical nature of the sorption process, the Dubinin-Radushkevich equation was used. It was found that the processes of adsorption of Cr(VI) and Ni(II) ions are of a chemical nature. The best Cr(VI) ion adsorption capacity was found for the Mg_0·2Zn_0·8Fe₂O₄ sample (q_e = 30.49 mg/g). The percentage of heavy metal removal by the mixed ferrite samples increases with increasing zinc content. The most effective sorbent for Ni(II) removal is the Mg_0·4Zn_0·6Fe₂O₄ sample (93.2%). Modeling the antistructure provides deeper insight into the mechanism of heavy metal adsorption. The obtained magnesium-zinc ferrites are promising magnetic adsorbents for removing chromate and nickel ions from the environment.

Ultrasonic assisted agro waste biomass for rapid removal of Cd(II) ions from aquatic environment: Mechanism and modelling analysis

In this research, dragon fruit peel, an agro-waste was used to prepare the new adsorbent for the removal of Cd(II) ions from the aquatic environment. The characterization techniques of SEM, FTIR and EDX for the prepared materials have been studied. The influential parameters for Cd(II) ions were experimented and identified the probable conditions for the maximum adsorption of Cd(II) ions. The investigations on isotherms, kinetics, and thermodynamics for Cd(II) ions removal were examined. Adsorption isotherm data was well discussed with Langmuir model based on the obtained good correlation coefficient and error values. Moreover, it follows the pseudo-first-order and exothermic process. The values of monolayer adsorption capacity of surface-modified dragon fruit peel (SMDFP) and ultrasonic-assisted dragon fruit peel (UADFP) was determined to be 7.469 and 24.76 mg/g at an equilibrium condition, respectively. This study exposed that ultrasonic-assisted dragon fruit peel can be a suitable adsorbent for Cd(II) ions removal from the water environment.

Non-domestic wastewater treatment with fungal/bacterial consortium followed by Chlorella sp., and thermal conversion of the generated sludge

Liquid waste from biological stains is considered non-domestic wastewater difficult to treat, generating high environmental impact. Therefore, the objective of this work was to carry out secondary and tertiary treatment of these effluents at a pilot scale, using a fungal/bacterial consortium followed by Chorella sp., for 15 days. In addition, to obtain an adsorbent material for Malachite Green dye removal, sludge generated in the plant and pine bark co-pyrolysis was performed. For microalgae isolation and selection of the Chlorophyceae class, Chlorococcales order, and Chorella sp. genus Winogradsky columns were employed. After 15 days of pilot plant treatment, removal percentages of 91 ± 2%, 90 ± 4% and 17 ± 2% were obtained for Colour Units, Chemical Oxygen Demand and Nitrates, respectively. Two types of class II biochar (BC₅₀₀ and BC₇₀₀) and one of class III (BC₃₀₀) were produced. The highest value for Fixed carbon (FC) was obtained at 300 °C (27.3 ± 3%), decreasing as the temperature increased by 25.9 ± 5% and 24.8 ± 2%, for BC₅₀₀ and BC₇₀₀, respectively. Biochar yield was 62.1 ± 3%, 46.3 ± 4% and 31.6 ± 3% for BC₃₀₀, BC₅₀₀ and BC₇₀₀, respectively. Finally, BC₅₀₀ and BC₇₀₀ biochar efficiently adsorbed Malachite Green obtaining qe values of 0.290 ± 0.032, 0.281 ± 0.015, 0.186 ± 0.009 and 0.191 ± 0.012 mg g^-1 at pH values of 4.0 and 8.0 ± 0.2, respectively. Pseudo-second order model demonstrated a chemical adsorption took place, which was influenced by pH.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02780-1.

Application of biochars in the remediation of chromium contamination: Fabrication, mechanisms, and interfering species

Chromium (Cr) is one of the most toxic pollutants that has accumulated in terrestrial and aqueous systems, posing serious risks towards living beings on a worldwide scale. The immobilization, removal, and detoxification of active Cr from natural environment can be accomplished using multiple advanced materials. Biochar, a carbonaceous pyrolytic product made from biomass waste, is considered as a promising material for the elimination of Cr contamination. The preparation and properties of biochar as well as its remediation process for Cr ions have been well investigated. However, the distinct correlation of the manufacturing, characteristics, and mechanisms involved in the remediation of Cr contamination by various designed biochars is not summarized. Herein, this review provides information about the production, modification, and characteristics of biochars along with their corresponding effects on Cr stabilization. Biochar could be modified via physical, hybrid, chemical, and biological methods. The remediating mechanisms of Cr contamination using biochars involve adsorption, reduction, electron shuttle, and photocatalysis. Moreover, the coexisting ions and organic pollutants change the pattern of the remediating process of biochar in actual Cr contaminated water and soil. Finally, the present limitations and future perspectives are proposed.

Adsorptive separation of toxic metals from aquatic environment using agro waste biochar: Application in electroplating industrial wastewater

In this research, raw jujube seeds (RJS) treated with sulphuric acid followed by ultrasonic treatment such as ultrasonic assisted jujube seeds (UAJS) based biochar have been experimented as a viable material for treating Zn(II) and Pb(II) contaminated water. The adsorption ability of UAJS was compared with RJS through Langmuir adsorption capacity. The produced adsorbents were analysed by using BET surface area and thermogravimetric analyses. The removal kinetics, isotherms and thermodynamic behaviours of metal ions adsorption by UAJS were studied. Adsorption equilibrium data were analysed using various equilibrium models and Freundlich isotherm was appropriate towards explain the adsorption characteristics. UAJS Langmuir capacity of 221.1 mg/g and 119.8 mg/g were obtained for Zn(II) ions and Pb(II) ions, respectively. The results observed that UAJS holds higher capacity as compared with RJS. The pseudo-first order model was relevant to address adsorption behaviour. The mechanism on the separation of metal ions by UAJS was tested using diffusion and Boyd models. The mechanism outcomes observed that the internal and external diffusion controlled the separation process. The thermodynamic results explain the separation process was viable, exothermic and natural. The electroplating industrial wastewater was also treated with UAJS biochar to remove the metal ions such as copper, nickel, chromium and zinc ions from wastewater. Desorption process showed that 0.1 N HCl provide the good results as compared with other desorbing agents. The adsorbent property is not lost till the maximum of 5 adsorption/desorption cycles. The produced UAJS can be a better adsorbent for treating the heavy metal polluted wastewater.

Conversion of tannery solid waste to an adsorbent for high-efficiency dye removal from tannery wastewater: A road to circular utilization

The high value-added use of tannery solid waste and elimination of tannery liquid waste in the leather-making industry have attracted widespread attention. In this study, a MgO-doped biochar (MgO/BC) adsorbent was successfully prepared by utilizing tannery solid waste (i.e., non-tanned hide wastes) as the biomass material for dye removal from tannery wastewater. Characterization results indicated that MgO was uniformly embedded into the porous BC structure. The adsorption capacity of acid orange II by MgO/BC reached up to 448.4 mg g^-1, which drastically exceeded the pure BC and other reported adsorbents. The adsorption behavior of acid orange II by MgO/BC matched nicely with Langmuir isotherm and pseudo-second-order kinetic model. This satisfactory adsorption capacity of MgO/BC for acid orange II was mainly due to the large specific surface area and the enhanced electrostatic interaction. According to the BET, zeta potential and XPS analysis, the possible mechanism towards acid orange II removal was attributed to the pore filling, surface complexation, electrostatic attraction and π-π interaction. In addition, MgO/BC showed the efficient removal towards anionic dyes from actual tannery wastewater. This work could provide guidance for the value-added utilization of tannery solid waste and a practical way to remove dyes from tannery wastewater.

Removal of tetracycline with aluminum boride carbide and boehmite particles decorated biochar derived from algae

For the first time, using aluminum-boron electrodes in the electrocoagulation cell for harvesting the cultivated Chlorella microalgae and then performing a hydrothermal process of producing biofuel, mesoporous biochar was produced with an average pore diameter of 11.62 nm, a high specific surface area of 126.4 m²/g and a total pore volume of 0.55 cm³/g. Based on the chemical characterization, aluminum boride carbide (Al₃B₄₈C₂) and boehmite [Al₂(OOH)₂] were identified in the biochar composition so that 7.17 wt% Al and 16.67 wt% B were measured on the biochar surface by EDS analysis. As the by-product of hydrothermal converting microalgae Chlorella into biofuel, the residual biochar was innovatively used to separate tetracycline from aqueous solutions. The nonlinear form of the Freundlich model fitted theadsorption equilibrium data well with the least error function value explained by the intraparticle diffusion model. The maximum adsorption capacity of 25.94 mg/g was obtained through endothermic physical adsorption.

Corn stalk-based activated carbon synthesized by a novel activation method for high-performance adsorption of hexavalent chromium in aqueous solutions

A simple activation method involving treatment with KOH solution was used to synthesize activated carbon (ACs) with micro-meso pores from the agricultural waste of corn stalks. The activation reagent, KOH solution, was easily separated for recycling by centrifugation from the pre-treated corn stalks, and the pollution in the carbonization process was greatly reduced. The morphology and structure of the ACs were characterized by SEM, TEM, N₂ adsorption, XRD, FT-IR and Raman analysis. The prepared carbon was applied as an adsorbent for the removal of Cr(VI) in a batch adsorption process. The effect of the concentration of KOH solution on the structure, morphology and Cr(VI) adsorption performance of the synthesized ACs was investigated. The characterization results revealed that some functional groups in the corn stalks were removed by pretreatment with KOH solution and micro-meso porous structures were generated. The ACs showed high adsorption performance for Cr(VI), and the maximum adsorption ability of the ACs prepared by activation with 4% KOH solution reached 89.5 mg g^-1 at an adsorbent dosage of 2.5 g·L^-1 and pH value of 4.5.

Biochar derived from red algae for efficient remediation of 4-nonylphenol from marine sediments

4-Nonylphenol (4-NP), a phenolic endocrine disruptor chemical (EDC), is known to have high toxicity to aquatic organisms and humans. The remediation of 4-NP-contaminated marine sediments was studied using red algae-based biochar (RAB) thermochemically synthesized from Agardhiella subulata with simple pyrolysis process under different temperatures of 300-900 °C in CO₂ atmosphere. The RAB was characterized by XRD, Raman, FTIR spectroscopy, and zeta potential measurements. The calcium in RAB efficiently activated sodium percarbonate (SPC) to generate reactive radicals for the catalytic degradation of 4-NP at pH 9.0. The oxygen-containing functional groups reacted with H₂O₂, which increased the generation of reactive radicals under alkaline pH condition. Ca²⁺ ion was the active species responsible for 4-NP degradation. CaO/CaCO₃ on RAB surface enhanced direct electron transfer, increased HO production, and 4-NP degradation in marine sediments. Langmuir‒Hinshelwood type kinetics well described the 4-NP degradation process. Remediation of contaminated sediments using RAB could be a sustainable approach toward closed-loop biomass cycling in the degradation of 4-NP contaminants.

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

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

Bio-reduction and synchronous removal of hexavalent chromium from aqueous solutions using novel microbial cell/algal-derived biochar particles: Turning an environmental problem into an opportunity

In China, Dolichospermum flos-aquae is one of the most prevalent bloom-forming cyanobacteria and thus a major challenge for the concerned catchment area. To solve this problem and turn it into an opportunity for heavy metal remediation, we investigated the potential of D. flos-aquae for production algal biochar, and constructed a microbe-algal biochar composite. The microbe-biochar composite (biochar immobilized Proteus mirabilis PC801) showed superior hexavalent chromium removal capacity. It produced 100% Cr(VI) (150 mg/L) removal efficiency, with 87.7% total Cr immobilized in/on the particles and only 12.3% Cr(III) left in solution. Furthermore, Scanning electron microscopy-energy dispersive spectroscopy and antioxidase activity results showed that Cr(VI) reduction mainly occurred outside the cells, and the biochar can effectively protect P. mirabilis YC801 from the direct toxicity of chromium, thereby promoting the removal efficiency. Overall, this study provides a promising approach by utilizing this harmful algae for the bio-remediation of Cr(VI)-contaminated groundwater in practical application.

Structural analysis and heavy metal adsorption of N-doped biochar from hydrothermal carbonization of Camellia sinensis waste

N-doped biochar as adsorption material for heavy metal removal has attracted increasing concern in environmental application due to its unique features. Here, N-doped biochar was prepared by hydrothermal carbonization of Camellia sinensis branch waste using KOH/NH₄Cl at 120-280 °C for 2 h under 0.4-6.5 MPa, followed by structural analysis. The results showed that the highest N content determined by elemental analysis could reach up to 6.18% in biochar, and the major N species were involved in graphitic N, pyrrolic N, and pyridinic N. Interestingly, these N-doped biochar exhibited the effective adsorption ability of Cu²⁺, Pb²⁺, Zn²⁺, and Cr⁶⁺. The batch adsorption behavior had a better adjustment to the pseudo-second-order kinetic and the Langmuir adsorption isotherm models. In brief, the present results are attributed to develop low-cost adsorbent for removing heavy metal ions.

Hexavalent chromium removal from water by microalgal-based materials: Adsorption, desorption and recovery studies

The current study presents a comprehensive comparison towards the potential of different microalgal-based materials for the removal of hexavalent chromium (Cr(VI)) from water. Among the tested materials, microalgal biochar showed the highest removal efficiency (100%) of Cr(VI). The highest monolayer estimated adsorption capacities were 23.98, 25.19 and 24.27 mg/g at 5, 22 and 35 °C, respectively. Experimental data showed good compliance with pseudo-second-order kinetic model. The results of continuous column studies showed that the column removal efficiency increased from 52.33 to 57.58% by increasing the adsorbent dose from 0.125 to 0.200 g. Desorption efficiency of Cr(VI) by 0.1 M NaOH was increased from 51.16 to 59.41% by sonication bath as compared to roller shaker. More than 97% of desorbed Cr(VI) was recovered in less than 10 min by BaCl₂. This study shows that non-living microalga materials are more effective than living cells in the removal and recovery of Cr(VI) from water.