Effects and mechanism on cadmium adsorption removal by CaCl2-modified biochar from selenium-rich straw

Adsorptive immobilization of cadmium and lead using unmodified and modified biochar: A review of the advances, synthesis, efficiency and mechanisms

Biochar is an environmentally friendly adsorbent material with excellent adsorption performance due to its extensive pore structure, large specific surface area, and numerous surface functional groups. It is commonly used to treat inorganic and organic pollutants. In recent years, with increasing focus on controlling soil pollution caused by heavy metals such as cadmium (Cd) and lead (Pb), the potential application of biochar has attracted much attention. This review used Citespace to quantitatively analyze the literature on the application of biochar from 2021 to 2024. It then explains the preparation techniques of unmodified and modified biochar and presents the physical and chemical properties and adsorption capacity of different biochar types for Cd and Pb. It also illustrates and compares the preparation process, modification methods, and adsorption mechanisms of biochar. Additionally, it evaluates the impacts of biochar application on heavy metal removal from rice, wheat, and corn, as well as their yields. This article contributes to the identification of the most effective materials and methods for biochar synthesis. It provides suggestions for remediation of soil heavy metal pollution and yield increase.

Adsorption properties and mechanisms of Cd by co-pyrolysis composite material derived from peanut biochar and tailing waste

Cadmium (Cd) contamination in aquatic systems is a widespread environmental issue. In this study, a solid waste iron tailings and biochar hybrid (Fe-TWBC) was successfully synthesized derived from co-pyrolysis of peanut shell and tailing waste (Fe-TW). Characterization analyses showed that the metal oxides from solid waste iron tailings successfully loaded onto the biochar surface, with more functional groups in Fe-TWBC. The Fe-TWBC had a maximum capacity of 95.06 mg·g-1 on Cd2+ adsorption, which was 1.40 times to pristine BC (70.46 mg·g-1) and 2.53 times to Fe-TW (37.51 mg·g-1). The adsorption behavior followed the pseudo-second-order kinetics and Freundlich models. DFT calculations revealed that the O-top of Fe-O group was the most chemically reactive site for Cd2+ adsorption with the high adsorption energy values of - 4.05 eV, short O-Cd bond lengths (2.133 Å), low electrostatic potentials in small blue regions near the O atom of Fe-O group, low energy gap (0.22 eV) and large electrophilic Fukui index (f- = 0.18). Overall, these findings suggest that Fe-TWBC is effective in removing Cd2+ from aqueous solution and promotes the utilization of biowaste and solid waste iron tailing waste for cleaner production.

Comprehensive insights into the application of graphene-based aerogels for metals removal from aqueous media: Surface chemistry, mechanisms, and key features

Efficient removal of heavy metals and other toxic metal pollutants from wastewater is essential to protect human health and the surrounding vulnerable ecosystems. Therefore, significant efforts have been invested in developing practical and sustainable tools to address this issue, including high-performance adsorbents. In this respect, within the last few years, graphene-based aerogels/xerogels/cryogels (GBAs) have emerged and drawn significant attention as excellent materials for removing and recovering harmful and valuable metals from different aqueous media. Such an upward trend is mainly due to the features of the aerogel materials combined with the properties of the graphene derivatives within the aerogel's network, including the GBAs' unique three-dimensional (3D) porous structure, high porosity, low density, large specific surface area, exceptional electron mobility, adjustable and rich surface chemistry, remarkable mechanical features, and tremendous stability. This review offers a comprehensive analysis of the fundamental and practical aspects and phenomena related to the application of GBAs for metals removal. Herein, we cover all types of (bottom-up) synthesized GBAs, including true microporous graphene-based aerogels as well as other 3D graphene-based open-cell interconnected mesoporous and macroporous aerogels, foams, and sponges. Indeed, we provide insights into the fundamental understanding of the GBAs' suitability for such an important application by revealing the mechanisms involved in metals removal and the factors inducing and controlling the highly selective behavior of these distinctive adsorbents. Besides conventional adsorptive pathways, we critically analyzed the ability of GBAs to electrochemically capture metal pollutants (i.e., electrosorption) as well as their efficiency in metals detoxification through reductive mechanisms (i.e., adsorption-reduction-readsorption). We also covered the reusability aspect of graphene aerogels (GAs)-based adsorbents, which is strongly linked to the GBAs' outstanding stability and efficient desorption of captured metals. Furthermore, in view of their numerous practical and environmental benefits, the development and application of magnetically recoverable GAs for metals removal is also highlighted. Moreover, we shed light on the potential practical and scalable implementation of GBAs by evaluating their performance in continuous metals removal processes while highlighting the GBAs' versatility demonstrated by their ability to remove multiple contaminants along with metal pollutants from wastewater media. Finally, this review provides readers with an accessible overview and critical discussion of major recent achievements regarding the development and applications of GAs-based adsorbents for metal ions removal. Along with our recommendations and suggestions for potential future work and new research directions and opportunities, this review aims to serve as a valuable resource for researchers in the field of wastewater treatment and inspire further progress towards developing next-generation high-performance GBAs and expanding their application.

Utilization of organic waste from Chinar leaves as sustainable and eco-friendly adsorbent for fluoride removal

Due to concerns about high water fluoride concentrations and their detrimental consequences on health, particularly dental and skeletal fluorosis, dependable and cost-effective defluoridation techniques are needed. Chinar leaves (Platanus orientalis), a common waste, might be utilized for the production of activated carbon. For Chinar leaf activated carbon (CLAC) manufacturing, two pre-pyrolysis chemical modification procedures were used: acidic HCl (H-activation) and alkaline NaOH (OH-activation). The success of fluoride removal suggests further research and implementation in locations with fluoride-related water quality issues. This study examines how CLAC dosage, fluoride concentration, temperature, pH, and contact exposure effect defluoridation efficiency. The pseudo-second-order non-linear kinetic model and Freundlich non-linear isotherm model with R2 = 0.99 fit the data, resulting in a peak adsorption capacity of 30.3 mg/g for 0.3 g CLAC. In the present work, the adsorption mechanism was regulated by more than intraparticle diffusion. Adsorption occurred spontaneously as exothermic monolayer chemisorption, according to thermodynamic studies. Adsorbent activated with HCl (H-activated) showed promising results, with 73% F- removal efficiency for OH-activated and 91% for H-activated CLAC.

Enhanced Cadmium Adsorption Dynamics in Water and Soil by Polystyrene Microplastics and Biochar

Microplastics (MPs) are prevalent emerging pollutants in soil environments, acting as carriers for other contaminants and facilitating combined pollution along with toxic metals like cadmium (Cd). This interaction increases toxic effects and poses substantial threats to ecosystems and human health. The objective of this study was to investigate the hydrodynamic adsorption of Cd by conducting experiments where polystyrene microplastics (PS) and biochar (BC) coexisted across various particle sizes (10 µm, 20 µm, and 30 µm). Then, soil incubation experiments were set up under conditions of combined pollution, involving various concentrations (0.5 g·kg-1, 5 g·kg-1, 50 g·kg-1) and particle sizes of PS and BC to assess their synergistic effects on the soil environment. The results suggest that the pseudo-second-order kinetic model (R2 = 0.8642) provides a better description of the adsorption dynamics of Cd by PS and BC compared to the pseudo-first-order kinetic model (R2 = 0.7711), with an adsorption saturation time of 400 min. The Cd adsorption process in the presence of PS and BC is more accurately modeled using the Freundlich isotherm (R2 > 0.98), indicating the predominance of multilayer physical adsorption. The coexistence of 10 µm and 20 µm PS particles with BC enhanced Cd absorption, while 30 µm PS particles had an inhibitory effect. In soil incubation experiments, variations in PS particle size increased the exchangeable Cd speciation by 99.52% and decreased the residual speciation by 18.59%. The addition of microplastics notably impacted the exchangeable Cd speciation (p < 0.05), with smaller PS particles leading to more significant increases in the exchangeable content-showing respective increments of 45.90%, 106.96%, and 145.69%. This study contributes to a deeper understanding of the mitigation mechanisms of biochar in the face of combined pollution from microplastics and heavy metals, offering theoretical support and valuable insights for managing such contamination scenarios.

Iron and silicon modified biochar for enhancing cadmium removal from water: Unveiling the crucial role of iron-induced silicon dissolution

The interaction mechanisms of silicon (Si) and active ingredient iron (Fe) on cadmium (Cd) removal are still unknown. Herein, the Fe/Si modified biochar (Fe/Si-BC) was synthesized to enhance Cd removal by pre-immersion of Fe and ball milling loading of Si. Detailed characterizations indicated that Fe and Si were successfully introduced into Fe/Si-BC, resulting in the formation of a new metallic silicate (Ca2.87Fe0.13(SiO3)2). The maximum Cd adsorption capacity of Fe/Si-BC (31.66 mg g-1) was 3.6 times and 2.5 times higher than that of Fe-BC (8.89 mg g-1) and Si-BC (11.03 mg g-1), respectively, deriving from an enhancement of Si dissolution induced by Fe introduction. The dissolved Si could capture and combine Cd to form CdSiO3 precipitation, which was strongly supported by the random forest regression and correlation between dissolved Si content and Cd adsorption capacity. This study advances the mechanistic insights into synergistic functions of Si and Fe in Cd removal.

Biochar as a partner of plants and beneficial microorganisms to assist in-situ bioremediation of heavy metal contaminated soil

Synergistic remediation of heavy metal (HM) contaminated soil using beneficial microorganisms (BM) and plants is a common and effective in situ bioremediation method. However, the shortcomings of this approach are the low colonisation of BM under high levels of heavy metal stress (HMS) and the poor state of plant growth. Previous studies have overlooked the potential of biochar to mitigate the above problems and aid in-situ remediation. Therefore, this paper describes the characteristics and physicochemical properties of biochar. It is proposed that biochar enhances plant resistance to HMS and aids in situ bioremediation by increasing colonisation of BM and HM stability. On this basis, the paper focuses on the following possible mechanisms: specific biochar-derived organic matter regulates the transport of HMs in plants and promotes mycorrhizal colonisation via the abscisic acid signalling pathway and the karrikin signalling pathway; promotes the growth-promoting pathway of indole-3-acetic acid and increases expression of the nodule-initiating gene NIN; improvement of soil HM stability by ion exchange, electrostatic adsorption, redox and complex precipitation mechanisms. And this paper summarizes guidelines on how to use biochar-assisted remediation based on current research for reference. Finally, the paper identifies research gaps in biochar in the direction of promoting beneficial microbial symbiotic mechanisms, recognition and function of organic molecules, and factors affecting practical applications.

An efficient and simple approach to remove Cd(II) in aqueous solution by using rice straw biochar: performance and mechanisms

In recent years, cadmium pollution in water environment has become an environmental problem that could not be ignored. As a porous carbon rich solid material, biochar is an environment-friendly new material because of its ultra-high adsorption capacity and strong chemical stability. In this study, rice straw biochar (RS-Biochar) was successfully prepared at different temperatures for removal of Cd(II) from aqueous solution. Through a series of characterization and adsorption experiments, the adsorption principle of Cd(II) by RS-Biochar was deeply studied. The results showed that RS-Biochar prepared at 600 °C (BioC600) has high specific surface area (232.6 m2/g) and shows high Cd(II) removal rate of 91.23% with the maximum Cd(II) adsorption capacity of 8.62 mg/g. The Langmuir model fit well to describe the adsorption process of Cd(II) on the BioC600. The mechanism analysis showed that hydroxyl and carboxyl groups on the biochar surface were concerned in the removal of Cd(II). The formation of CdCO3 in the adsorption process was also be proven. Importantly, RS-Biochar could be conveniently produced with needed scale, displaying a promising approach for remediating Cd(II)-contaminated water environment and a huge application potential.

Efficient Adsorption of Nitrogen and Phosphorus in Wastewater by Biochar

Nitrogen and phosphorus play essential roles in ecosystems and organisms. However, with the development of industry and agriculture in recent years, excessive N and P have flowed into water bodies, leading to eutrophication, algal proliferation, and red tides, which are harmful to aquatic organisms. Biochar has a high specific surface area, abundant functional groups, and porous structure, which can effectively adsorb nitrogen and phosphorus in water, thus reducing environmental pollution, achieving the reusability of elements. This article provides an overview of the preparation of biochar, modification methods of biochar, advancements in the adsorption of nitrogen and phosphorus by biochar, factors influencing the adsorption of nitrogen and phosphorus in water by biochar, as well as reusability and adsorption mechanisms. Furthermore, the difficulties encountered and future research directions regarding the adsorption of nitrogen and phosphorus by biochar were proposed, providing references for the future application of biochar in nitrogen and phosphorus adsorption.

Effects of Pyrolysis Temperature and Acid-Base Pre-Treatment on the Synthesis of Biochar-Based Slow-Release Selenium Fertilizer and Its Release in Soil

Plant-derived selenium is an important source of selenium (Se) for humans, which, however, has been restricted by a low content of Se in soil. Traditional Se fertilizers have tended to result in low selenium utilization. Thus, it was necessary to develop a new slow-release material to control Se fertilizer release. In this study, biochar pyrolyzed at 300 °C and 800 °C was cross-linked with polyethyleneimine (PEI) after being treated with HNO3 or NaOH (which were labeled Acid-W300, Acid-W800, Alkali-W300, and Alkali-W800). The results showed that the maximum adsorption capacities of Acid-W300, Alkali-W300, Acid-W800, and Alkali-W800 were 329.16 mg/g, 321.93 mg/g, 315.04 mg/g, and 344.33 mg/g, respectively. Among them, Acid-W800 and Alkali-W800 were mainly imine- and amide-bonded with SO32-, while Acid-W300 and Alkali-W300 were loaded with SO32- by forming the C-Se bonding as well as through imine- and amide-bonding. The release of four biochar-based selenium fertilizers in the red soil and brown soil extracts conformed to the pseudo-second-order kinetic model. The release rate and release amount of four biochar-based selenium fertilizers in the red soil extract were higher than those in the brown soil extract. Alkali-W800-Se had a higher proportion of Se-exchangeable release, accounting for 87.5% of the total loaded selenium, while Acid-W300-Se had the lowest proportion at 62.2%. However, the Se releases of Alkali-W800-Se were more than 42.49% and 37.67% of the total Se-loading capacity during 5 days of continuous red soil extraction and brown soil extraction, respectively. Acid-W300-Se released less than 20% of the total Se-loading capacity. Thus, Acid-W300-Se was the recommended slow-release Se fertilizer in red soil and brown soil.

Rapid and efficient removal of multiple heavy metals from diverse types of water using magnetic biochars derived from antibiotic fermentation residue

Pyrolysis is a promising method to treat antibiotic fermentation residue (AFR), a hazardous waste in China, with the benefits of detoxification and resource recycling. However, the application of the AFR-derived biochar has been limited yet, restricting the use of pyrolysis to treat AFR. Herein, for the first time, we reported the use of magnetic biochars derived from vancomycin fermentation residue to rapidly and efficiently co-adsorb multiple heavy metals from diverse types of water with complex matrices. The biochar prepared at 700 °C (labeled as VBC700) exhibited high affinity and selectivity for multiple heavy metals, especially for Ag(I), Hg(II), Pb(II), and Cu(II). The kinetics for Ag(I), Hg(II), and Pb(II) were ultrafast with an equilibrium time of only 5 min, while those for Cu(II) were relatively slower. The maximum adsorption capacity calculated from the Langmuir model for Ag(I), Hg(II), Pb(II), and Cu(II) reached 177.4, 105.9, 387.1, 124.5 mg/g, respectively, which were superior to much previously reported adsorbents. Impressively, Na(I), K(I), Ca(II), Mg(II), and salinity did not affect the capture of these heavy metals, and thus >99% of Ag(I), Pb(II), and Cu(II) were concurrently removed from complex water matrices including seawater, which has rarely been reported before. Furthermore, VBC700 remained high adsorption performance at pH ≥ 3. The adsorption mechanisms included ion exchange, precipitation, and inner-sphere complexation. Overall, the results demonstrate that VBC700 would be an excellent adsorbent to co-capture multiple heavy metals from diverse types of water, highlighting the feasibility of using pyrolysis to achieve a win-win goal for AFR management and heavy metal pollution control.

Tuning active sites on biochars for remediation of mercury-contaminated soil: A comprehensive review

Mercury (Hg) contamination is acknowledged as a global issue and has generated concerns globally due to its toxicity and persistence. Tunable surface-active sites (SASs) are one of the key features of efficient BCs for Hg remediation, and detailed documentation of their interactions with metal ions in soil medium is essential to support the applications of functionalized BC for Hg remediation. Although a specific active site exhibits identical behavior during the adsorption process, a systematic documentation of their syntheses and interactions with various metal ions in soil medium is crucial to promote the applications of functionalized biochars in Hg remediation. Hence, we summarized the BC's impact on Hg mobility in soils and discussed the potential mechanisms and role of various SASs of BC for Hg remediation, including oxygen-, nitrogen-, sulfur-, and X (chlorine, bromine, iodine)- functional groups (FGs), surface area, pores and pH. The review also categorized synthesis routes to introduce oxygen, nitrogen, and sulfur to BC surfaces to enhance their Hg adsorptive properties. Last but not the least, the direct mechanisms (e.g., Hg- BC binding) and indirect mechanisms (i.e., BC has a significant impact on the cycling of sulfur and thus the Hg-soil binding) that can be used to explain the adverse effects of BC on plants and microorganisms, as well as other related consequences and risk reduction strategies were highlighted. The future perspective will focus on functional BC for multiple heavy metal remediation and other potential applications; hence, future work should focus on designing intelligent/artificial BC for multiple purposes.

Improvement of the specific surface area of biochar by calcium-precipitated nanoparticles synthesized by microbial induction as a template skeleton: Removal mechanism of tetracycline in water

The template method is an effective means to improve the specific surface area and porosity of biochar, but the synthesis of template agents and the way they are integrated with biomass materials still need further development. Therefore, the free Pseudomonas sp. Y1 was used to synthesize calcium-precipitated nanoparticles (CPN) on sludge as a fused template skeleton to enlarge the surface area of sludge biochar facilitating the adsorption of tetracycline (TC) in this work. The modified biochar (FBC) showed excellent specific surface area (448.55 m2 g-1) and porosity (0.0053 cm³ g-1), stable morphological structure, abundant active functional groups, and appreciable adsorption capacity (65.43 mg g-1) based on several characterization and adsorption experiments. Moreover, the adsorption model postulated that the removal of TC is mainly a chemisorption-based heat-trapping, disordered multilayer interaction. In detail, this process involved the joint contribution from electrostatic interactions, ligand exchange, hydrogen bonding, π-π bonding, complexation, and pore filling. Meanwhile, the adaptability and stability of FBC were examined by pH and coexisting substances. This template skeleton induced by microorganisms can provide new insight into the modification of biochar with the template method.

Mechanochemical synthesis of calcium-biochar for decontamination of arsenic-containing acid mine drainage

Ca-biochar is an efficient material for As(III)-containing acid mine drainage (AMD) decontamination, while it is challenging to fabricate Ca-biochar with oyster shell waste as the Ca source due to its complex structure. Herein, a mechanochemical method was proposed to activate oyster shell waste and wood waste for Ca-biochar design and production, and its efficacy and relevant mechanisms for AMD detoxification were evaluated. The smaller size Ca-biochar produced by the medium-speed ball milling showed a higher As(III) removal (74.0 %) compared to high-speed ball milling (60.9 %), attributed to the formation of finer Ca(OH)2 while avoiding particle aggregation, which could release more Ca (89.0 mg/g) and alkalinity for the co-precipitation of As. Meanwhile, wood-based biochar substrate served as a platform for co-precipitation, and its surface functionality supported the oxidative immobilization of As. This study presents a promising route for upcycling food and wood waste to produce Ca-biochar for AMD decontamination.

Remediation of Pb(II) and Cd(II) in polluted waters with calcium thioglycolate-modified straw biochar

The pollution of water bodies by heavy metals (HMs) such as Pb(II) and Cd(II) poses a serious environmental risk. Herein, rice straw biochar (RBC) modified with calcium thioglycolate was used to remove Pb(II) and Cd(II) from aqueous solutions. The adsorption performance of the modified biochar was investigated via adsorption kinetics and isotherm model fitting. Furthermore, scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to elucidate the modification and adsorption mechanisms. The results showed that the modification process loaded sulfur-containing functional groups, calcium carbonate, and calcium oxalate crystals on the biochar surface, considerably enhancing its complexation performance and ion-exchange capacity. The equilibrium adsorption amounts for Pb(II) and Cd(II) reached 124.92 and 65.44 mg g-1 in unary systems, respectively; they reached 121.34 and 39.43 mg g-1 in a binary Pb(II) and Cd(II), respectively. Moreover, the optimal adsorption conditions were as follows: pH = 6, temperature = 25 °C, dosage = 0.8 g L-1, and contact time = 2 h. In the binary Pb(II) and Cd(II) system, the adsorption process obeyed the Langmuir competitive adsorption model, which means that one adsorption site on the modified biochar was effective for only one heavy-metal ion, and the modified biochar was more selective for Pb(II) than for Cd(II). The adsorption mechanism, which was dominated by chemisorption, mainly involved complexation, precipitation, ion exchange, and cation-π interactions. Meanwhile, adsorption and desorption experiments indicated that the modified biochar exhibited satisfactory recycling performance, demonstrating its feasibility as an inexpensive and efficient heavy-metal adsorbent for polluted water.

Fluorinated Artificial Solid-Electrolyte-Interphase Layer for Long-Life Sodium Metal Batteries

Sodium metal batteries have garnered significant attention due to their high theoretical specific capacity, cost effectiveness, and abundant availability. However, the propensity for dendritic sodium formation, stemming from the highly reactive nature of the sodium metal surface, poses safety concerns, and the uncontrollable formation of the solid-electrolyte interphase (SEI) leads to large cell impedance and battery failures. In this study, we present a novel approach where we have successfully developed a stable fluorinated artificial SEI layer on the sodium metal surface by employing various weight percentages of tin fluoride in a dimethyl carbonate solution, utilizing a convenient, cost-effective, and single-step method. The resulting fluoride-rich protective layer effectively stabilized the Na metal surfaces and significantly enhanced cycling stability. The engineered artificial SEI layer demonstrated an enhanced lifetime of Na metal symmetric cells of over 3.5 times, over 700 h at the current density of 0.25 mA/cm2, in cycling performance compared to the untreated sodium, which is attributed to the suppression of dendrite formation and the reduction of undesired SEI formation during high-current cycling.

The Physicochemical Characteristics and Heavy Metal Retention Capability of Black Liquor Lignin-Based Biochars

Due to its high carbon content, lignin, particularly for lignin-containing solid waste, is considered an excellent raw material for the preparation of carbon materials like biochar. To produce high-quality lignin-based biochar (LGBCs), lignin extracted from black liquor was employed to prepare biochar at various pyrolysis temperatures (300~600 °C). The physicochemical properties of LGBCs were assessed using scanning electron microscopy, N2 adsorption/desorption, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction. Furthermore, the adsorption capability and potential mechanism of LGBCs in removing Cd(II) were investigated as well. The results indicate that LGBCs produced at higher pyrolysis temperatures exhibit rougher surfaces and more developed pore structures, which facilitate the exposure of numerous active adsorption sites. The adsorption of Cd(II) by LGBCs generally follows the order of LG-300C < LG-400C < LG-500C < LG-600C. According to the Langmuir adsorption isotherm model, the theoretical maximum adsorption capacity of LG-600C for Cd(II) is calculated to be 18.54 mg/g. Adsorption mechanism analysis reveals that the complexation interaction, dependent on the surface functional groups, plays a crucial role in the adsorption of Cd(II) by LGBCs prepared at higher pyrolysis temperatures. This study demonstrates that, by controlling the pyrolysis temperature during biochar preparation, high-quality lignin-based biochar can be readily obtained.