Preparation of rice straw-derived biochar for efficient cadmium removal by modification of oxygen-containing functional groups

Insights into the Mechanism of Efficient Cr(VI) Removal from Aqueous Solution by Iron-Rich Wheat Straw Hydrochar: Coupling DFT Calculation with Experiments

Agricultural solid waste has become one of the raw materials for hydrothermal carbon production, promoting resource utilization. This study synthesized two types of ball-milling carbons (Fe-MHBC vs MHBC) with and without FeCl3 modification using wheat straw hydrochars. Cr(VI) adsorption on these two types of ball-milling carbons was investigated. According to Langmuir's maximum adsorption capacity analysis, Fe-MHBC had a capacity of 116.29 mg g-1. The thermodynamic analysis based on isothermal adsorption reveals the spontaneous process of the reaction between the two materials. The adsorption of Cr(VI) on Fe-MHBC exhibited excellent agreement with the pseudo-second-order kinetics model. Furthermore, X-ray photoelectron spectroscopy analysis showed that Fe(II) in the material reduced Cr(VI) when it participated in the reaction. The acidic conditions facilitate the elimination of Cr(VI). The Fe-MHBC has a higher zeta potential, which enhances the electrostatic attraction of Cr(VI) particles. Even with a starting pH of 10, the removal rate can be consistently maintained at over 64%. The adsorption of Cr(VI) was inhibited by various anions and higher ion concentrations. Density functional theory demonstrates that the presence of Fe enhances the adsorption capacity and electron transfer flux of Cr(VI). Fe-MHBC effectively eliminates Cr(VI) by the process of electrostatic adsorption, redox, and complexation reactions. This study demonstrated that hydrochar materials modified by FeCl3 through a ball-milling process show considerable potential as effective adsorbents in the treatment of Cr(VI) pollution, offering a viable and environmentally friendly solution for mitigating this prevalent environmental issue.

Preparation of porous biochar from fusarium wilt-infected banana straw for remediation of cadmium pollution in water bodies

The problem of cadmium pollution and its control is becoming increasingly severe issue in the world. Banana straw is an abundant bio raw material, but its burning or discarding in field not only causes pollution but also spreads fusarium wilt. The objective of this paper is to utilize biochar derived from the wilt-infected banana straw for remediation of Cd(II) pollution while to eliminate the pathogen. The activity of wilt pathogen in biochar was determined by PDA petri dish test. The Cd(II) adsorption of the biochar was determined by batch adsorption experiments. The effects of KOH concentration (0.25, 0.5 and 0.75 M) on the physicochemical characteristics of the biochar were also observed by BET, SEM, FTIR, XRD and XPS. Results showed that pristine banana straw biochar (PBBC) did not harbor any pathogen. The specific surface area (SSA) and Cd(II) adsorption capacity of 0.75 M KOH modified banana straw biochar (MBBC0.75M) were increased by 247.2% and 46.1% compared to that of PBBC, respectively. Cd(II) adsorption by MBBC0.75M was suitable to be described by the pseudo-second-order kinetic model and Freundlich isotherm. After Cd(II) adsorption, the CdCO3 were confirmed by XRD and observed through SEM. The weakness and shift of oxygen-containing functional groups in MBBC0.75M after Cd(II) adsorption implied that those groups were complexed with Cd(II). The results showed that pyrolysis could not only eliminate banana fusarium wilt, but also prepare porous biochar with the wilt-infected banana straw. The porous biochar possessed the potential to adsorb Cd(II) pollutants.

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.

Attempts to obtain clean biochar from hyperaccumulator through pyrolysis: Removal of heavy metals and transformation of phosphorus

The sound disposal of the ensuing heavy metal-rich plants can address the aftermath of phytoremediation. In this study, the first attempt was made to obtain heavy metals-free and phosphorus-rich biochar from phytoremediation residue (PR) by pyrolysis, and the effects of chlorinating agent type, chlorine dosage, and pyrolysis residence time on heavy metal removal, phosphorus (P) transformation, and biochar properties were investigated. The results showed that as chlorine dosage and pyrolysis residence time increased, added polyvinyl chloride (PVC) reduced the concentration of Zn in biochar to one-tenth of that in PR by intensified chlorination, where both Zn concentration (2727.50 mg/kg) and its leaching concentration (29.13 mg/L) met the utilization requirements, in which the acid-base property of biochar plays a key role in heavy metal leaching. Meanwhile, more than 90% of P in PR remained in biochar and the bioavailability of P in biochar enhanced with the decomposition of organic P to inorganic P, where the concentration of plant-availability P (Pnac) expanded from 1878.40 mg/kg in PR to 8454.00 mg/kg in biochar. This study demonstrated that heavy metal hyperaccumulator can be converted into heavy metal-free and phosphorus-rich biochar with promising applications, which provides new perspectives for the treatment of such hazardous wastes.

Evaluation of water management on arsenic methylation and volatilization in arsenic-contaminated soils strengthened by bioaugmentation and biostimulation

Arsenic (As) fate in paddy fields has been one of the most significant current issues due to the strong As accumulation potential of rice plants under flooded conditions. However, no attempt was done to explore As methylation and volatilization under non-flooded conditions. Herein, we investigated the effects of water management on As methylation and volatilization in three arsenic-contaminated soils enhanced by biostimulation with straw-derived organic matter and bioaugmentation with genetic engineered Pseudomonas putida KT2440 (GE P. putida). Under flooded conditions, the application of biochar (BC), rice straw (RS) and their combination (BC+RS) increased total As in porewater. However, these effects were greatly attenuated under non-flooded conditions. Compared with RS amendment alone, the combination of GE P. putida and RS further promoted the As methylation and volatilization, and the promotion percentage under non-flooded conditions were significantly higher than that under flooded conditions. The combined GE P. putida and RS showed the highest efficiency in As methylation (88 µg/L) and volatilization (415.4 µg/(kg·year)) in the non-flooded soil with moderate As contamination. Finally, stepwise multiple linear regression analysis presented that methylated As, DOC and pH in porewater were the most important factors contributing to As volatilization. Overall, our findings suggest that combination of bioaugmentation with GE P. putida and biostimulation with RS/BC+RS is a potential strategy for bioremediation of arsenic-contaminated soils by enhancing As methylation and volatilization under non-flooded conditions.

Application of biochar on soil bioelectrochemical remediation: behind roles, progress, and potential

Bioelectrochemical systems (BESs) that combine electrochemistry with biological methods have gained attention in the remediation of polluted environments, including wastewater, sludge, sediments, and soils. The most attractive advantage of BESs is that the solid electrode is used as an inexhaustible electron acceptor or donor, and biocurrent directly converted from organics can afford the reaction energy of contaminant breakdown, crossing the internal energy barrier of endothermic degradation, which achieves a continuous biodegradation process without the simultaneous use of exogenetic chemicals and bioelectricity recovery. However, soil BESs are hindered by expensive electrode materials, difficult pollutant and electron transfer, low microbial competitive activity, and biocompatibility in contamination remediation. Fortunately, introducing biochar into soil BESs could reveal a high potential in addressing these BES inadequacies. The characteristics of biochar, e.g., conductivity, transferability, high specific surface area, high porosity, large functional groups, and biocompatibility, can improve the performance of soil BESs. In fact, biochar not only carries electrons but also transfers nutrients, pollutants, and even bacteria by facilitating transmission in the bioelectric field of BESs. Consequently, the abilities of biochar make for better functionality of BESs. This review collates information on the roles, application, and progress of biochar in soil BESs, and future prospects are given. It is beneficial for environmental researchers and engineers to extend BES application in environmental remediation and to assist the progress of carbon sequestration and emission reduction based on the inertia of biochar and the blocking of electron flow to form methane.

Adsorption characteristics and mechanisms of Cd(II) from wastewater by modified chicken manure biochar

Due to the threat to food supply and human health posed by cadmium-contaminated wastewater, a highly effective adsorbent is under necessary development to remove cadmium from wastewater. In this study, four new types of modified biochars with different modifier concentrations were prepared from chicken manure using K2FeO4 as a modifier, and the modified biochar KFBC1 with the best adsorption effect was obtained through optimal experiments. Various characterization analyses have shown that KFBC1 has a rough surface structure, abundant pore structure, and a large number of functional groups. Additionally, iron oxides are introduced on the surface of the biochar, which provided a favorable condition for the adsorption of Cd(II) in wastewater. The adsorption performance of Cd(II) on the biochar before and after modification was investigated through batch adsorption experiments. The adsorption kinetic model of KFBC1 to Cd(II) in solution was in accordance with the quasi-secondary kinetic model, and the adsorption isothermal model was in accordance with the Langmuir model, with a maximum adsorption capacity of 330.06 mg/g, which was 5.15 fold of pristine BC. Meanwhile, the adsorption rate of Cd(II) by KFBC1 was positively correlated with dosage and pH. Pore adsorption, ion exchange, surface precipitation, interaction with -π electrons, and complexation of oxygen-containing functional groups on the surface were considered as important mechanisms for the removal of Cd(II) by KFBC1. According to the results, KFBC1 is a novel and efficient adsorbent that can be used as a treatment agent for cadmium-contaminated wastewater.

Biomass directional pyrolysis based on element economy to produce high-quality fuels, chemicals, carbon materials - A review

Biomass is regarded as the only carbon-containing renewable energy source and has performed an increasingly important role in the gradual substitution of conventional fossil energy, which also contributes to the goals of carbon neutrality. In the past decade, the academic field has paid much greater attention to the development of biomass pyrolysis technologies. However, most biomass conversion technologies mainly derive from the fossil fuel industry, and it must be noticed that the large element component difference between biomass and traditional fossil fuels. Thus, it's necessary to develop biomass directional pyrolysis technology based on the unique element distribution of biomass for realizing enrichment target element (i.e., element economy). This article provides a broad review of biomass directional pyrolysis to produce high-quality fuels, chemicals, and carbon materials based on element economy. The C (carbon) element economy of biomass pyrolysis is realized by the production of high-performance carbon materials from different carbon sources. For efficient H (hydrogen) element utilization, high-value hydrocarbons could be obtained by the co-pyrolysis or catalytic pyrolysis of biomass and cheap hydrogen source. For improving the O (oxygen) element economy, different from the traditional hydrodeoxygenation (HDO) process, the high content of O in biomass would also become an advantage because biomass is an appropriate raw material for producing oxygenated liquid additives. Based on the N (nitrogen) element economy, the recent studies on preparing N-containing chemicals (or N-rich carbon materials) are reviewed. Moreover, the feasibility of the biomass poly-generation industrialization and the suitable process for different types of target products are also mentioned. Moreover, the enviro-economic assessment of representative biomass pyrolysis technologies is analyzed. Finally, the brief challenges and perspectives of biomass pyrolysis are provided.

Rape straw biochar enhanced Cd immobilization in flooded paddy soil by promoting Fe and sulfur transformation

Cd is normally associated with sulfide and Fe oxides in flooded paddy soil. The mechanisms of biochar enhanced Cd immobilization by promoting Fe transformation and sulfide formation are unclear. Rape straw biochar (RSB) pyrolyzed at 450 °C (LB) and 800 °C (HB) was added to Cd-contaminated paddy soil at 1% (LB1, HB1) and 2% (LB2, HB2) doses. The results showed that Fe/Mn oxide-Cd (Fe/Mn-Cd) and free Fe oxide (Fed) concentrations decreased in the first 12 days and then rose, while Fe2+ in pore water (W-Fe2+) tended to rise first and then fall. The electron transfer rate of soil in the HB2 treatment was 4.9-fold higher than that in the treatment without biochar (CK). Fe oxide reduction was enhanced by RSB, with a maximum increase in W-Fe2+ by 62.1% in HB2 on Day 12. The negative correlation between W-Fe2+ and Fed showed that Fe2+ promoted the reformatted of seconded Fe minerals after Day 12, and the Fed in the HB2 treatments increased by 31.5% in this period. RSB addition also promoted the reformation of poorly crystallized Fe oxide (Feo) by increasing soil pH, which increased by 17.2% and 15.1% on average in the LB2 and HB2 treatments, respectively, compared to CK. Compared to Day 7, the increased rate of Fe/Mn-Cd on Day 30 in RSB was approximately twice that of CK. Compared to the molybdate group, the maximum decrease in CaCl2-Cd was 29.1% in LB2 on Day 12. LB2 increased SO42- and acid-volatile sulfide concentrations by 6.9- and 4.1-fold, respectively, compared to CK. These results suggested that RSB, particularly HB, promoted more Cd adsorption in Fe minerals by increasing Fe hydroxylation and recrystallization processes. LB increased the contribution of sulfide to Cd immobility.

Co-production of spirosiloxane and biochar adsorbent from wheat straw by a low-cost and environment-friendly method

To enhance the value of wheat straw derivatives, wheat straw ash (WSA) was used as a reactant for the first time to synthesize spirocyclic alkoxysilane, an important organosilicon raw material, using an energy-saving and environmentally friendly non-carbon thermal reduction method. After spirocyclic alkoxysilane extraction, the biochar in the wheat straw ash prepared an adsorbent for Cu²⁺. The maximum copper ion adsorption capacity (Q_m) of silica-depleted wheat straw ash (SDWSA) was 31.431nullmg/g, far exceeding those of WSA and similar biomass adsorbents. The effects of the pH, adsorbent dose, and contact time on the adsorption behaviour of the SDWSA for Cu²⁺ adsorption were systematically investigated. The adsorption mechanism of Cu²⁺ by the SDWSA was investigated using the Langmuir, Freundlich, pseudo-first-order kinetic, pseudo-second-order kinetic, and Weber and Morris models by combining the preliminary experimental data and characterization results. The adsorption isotherm and Langmuir equation matched perfectly. The Weber and Morris model can describe the mass-transfer mechanism of Cu²⁺ adsorption by SDWSA. Both film and intraparticle diffusion are rapid control steps. Compared to WSA, SDWSA has a larger specific surface area and a higher content of oxygen-containing functional groups. A large specific surface area provides more adsorption sites. Oxygen-containing functional groups react with Cu²⁺ through electrostatic interactions, surface complexation, and ion exchange, which are the possible adsorption mechanisms for SDWSA. These methods improve the added value of wheat straw derivatives and promote wheat straw ash recovery and centralized treatment. This makes it possible to use the thermal energy of wheat straw and facilitates the treatment of exhaust gases and carbon capture.

Comprehensive study on the hydrochar for adsorption of Cd(II): preparation, characterization, and mechanisms

Hydrothermal carbonization process via converting invasive plants into functional materials may provide a novel strategy to comprehensively control and utilized the exotic invasive plants. In this study, Eupatorium adenophorum was utilized to fabricate the hydrochar via hydrothermal carbonization process, which was further applied to remove Cd(II). The results showed that the hydrochar was a mesoporous material with abundant O-containing functional groups (OFPs) on the surface. The adsorption isotherms were fitted by both the Langmuir and Freundlich models, and the maximum adsorption amount achieved 24.53 mg/g. The adsorption dynamics were governed by surface adsorption and film diffusion. pH and ionic strength can exert a strong influence on the adsorption efficiency. The mechanisms on the adsorption of Cd(II) on the hydrochar concluded the pore-filling effects, electrostatic interactions, ion exchange, precipitation, coordination with π electrons, and surface complexation with the OFPs, such as hydroxyl, carboxylic, phenol, acetyl, and ester groups. Thus, hydrothermal carbonization process may provide a promising technique to fabricate the hydrocar for the treatment of Cd(II), which may facilitate comprehensive control of invasive plants and boost to the carbon neutrality.

Nano-chlorapatite modification enhancing cadmium(II) adsorption capacity of crop residue biochars

Cadmium (Cd) contamination in rivers or lakes has attracted worldwide concerns. Biochar pyrolyzed form crop residues (CR) could adsorb Cd(II) from aquatic environments, while the removal capacity of single CR biochar is relatively low. Nano-chlorapatite (nClAP) modification can enhance metal scavenging ability, but little is known about the behaviors and mechanisms of Cd(II) adsorption by nClAP-modified CR biochars. In this study, the influences of feedstock type, pyrolysis temperature, nClAP modification and aquatic environments on Cd(II) adsorption of biochars derived from rice (RB) and wheat (WB) husks were investigated comprehensively. Results showed that the pristine RB and WB showed low and similar Cd(II) adsorption capacities, while the rise of pyrolysis temperatures from 300 to 600 °C significantly improved the adsorption capacities. The Cd(II) adsorption of both RB and WB was regarded as monolayer chemical processes controlled by chemical precipitation, surface complexation and cation exchange mechanisms. Moreover, the nClAP modification notably enhanced Cd(II) adsorption capacities from 13.2 to 39.9 mg·g^-1 of pristine biochars to 25.2-60.7 mg·g^-1 of modified biochars attributed to the improved contribution of Cd(II)-phosphate precipitation. Among all biochars, the nClAP-modified RB and WB pyrolyzed at 500 °C had the highest Cd(II) adsorption capacities with 60.7 and 48.3 mg·g^-1, respectively. These biochars could maintain good adsorption performances under the neutral-alkaline (pH 6-8), low ionic strength, high dissolved organic matter and all oxidation-reduction potential conditions. In conclusion, this study reveals the importance of nClAP modification to optimize Cd(II) adsorption of CR biochars, which provides a promising future for its practical application in aquatic Cd(II) scavenging.

Phosphoric acid-activated biochar derived from sunflower seed husk: Selective antibiotic adsorption behavior and mechanism

In recent years, the unnecessary overuse of antibiotics has increased globally, resulting in antibiotic contamination of water, which has become a significant environmental concern. This study aims to examine the adsorption behavior of antibiotics (Tetracycline TC, Ciprofloxacin CIP, Ibuprofen IBP, and Sulfamethoxazole SMX) onto H₃PO₄-activated sunflower seed husk biochar (PSF). The results demonstrated that H₃PO₄ could enhance the specific surface area (378.8 m²/g) and create a mesoporous structure of biochar. The adsorption mechanism was investigated using kinetic models, isotherms, and thermodynamics. The maximum adsorption capacities (q_max) of TC, CIP, SMX, and IBP are 429.3, 361.6, 251.3, and 251.1 mg g^-1, respectively. The adsorption mechanism of antibiotics on PSF was governed by complex mechanisms, including chemisorption, external diffusion, and intraparticle diffusion. This research provides an environmentally friendly method for utilizing one of the agricultural wastes for the removal of a variety of antibiotics from the aquatic environment.

Enhanced removal of heavy metals by α-FeOOH incorporated carboxylated cellulose nanocrystal: synergistic effect and removal mechanism

Simultaneous and highly efficient removal of heavy metal cations and oxyanions is significant for both water and soil remediation, but it remains a major challenge due to the complexity. In this work, a novel hybrid of α-FeOOH incorporated carboxylated cellulose nanocrystal (Fe/CNC) is synthesized via a hydrothermal process, which shows improved α-FeOOH dispersion and heavy metal removal capacity. In single adsorbate system, maximum adsorption capacities toward Pb(II), Cd(II), and As(V) by Fe/CNC reach 126.06, 53.07, and 15.80 mg g^-1, respectively, and the Fe leaching is much lower than that of α-FeOOH. In binary and ternary adsorption systems, simultaneous removal of Pb(II), Cd(II), and As(V) is proved, and the competition and synergy coexist among heavy metals. FTIR and XPS spectra have revealed the synergistic removal mechanism: Pb(II) and Cd(II) are mainly removed by surface complexation with oxygen-containing functional groups on C-CNC and α-FeOOH, and precipitation on the surface of α-FeOOH, while ligand exchange with Fe-OH is responsible for As(V) removal. The soil incubation experiments show that exchangeable and carbonate-bound Pb, Cd, and As are transformed into more stable forms in contaminated soil containing Fe/CNC composites. This work provides a novel composite material for remediation of heavy metal-contaminated environments.

Degradation of organic pollutants from water by biochar-assisted advanced oxidation processes: Mechanisms and applications

Biochar has shown large potential in environmental remediation because of its low cost, large specific surface area, porosity, and high conductivity. Biochar-assisted advanced oxidation processes (BC-AOPs) have recently attracted increasing attention to the remediation of organic pollutants from water. However, the effects of biochar properties on catalytic performance need to be further explored. There are still controversial and knowledge gaps in the reaction mechanisms of BC-AOPs, and regeneration methods of biochar catalysts are lacking. Therefore, it is necessary to systematically review the latest research progress of BC-AOPs in the treatment of organic pollutants in water. In this review, first of all, the effects of biochar properties on catalytic activity are summarized. The biochar properties can be optimized by changing the feedstocks, preparation conditions, and modification methods. Secondly, the catalytic active sites and degradation mechanisms are explored in different BC-AOPs. Different influencing factors on the degradation process are analyzed. Then, the applications of BC-AOPs in environmental remediation and regeneration methods of different biochar catalysts are summarized. Finally, the development prospects and challenges of biochar catalysts in environmental remediation are put forward, and some suggestions for future development are proposed.

Insight into atrazine removal by fallen leaf biochar prepared at different pyrolysis temperatures: Batch experiments, column adsorption and DFT calculations

The environmental pollution caused by atrazine in the agricultural production cannot be ignored. In this study, the fallen leaf biochar (LBC) was prepared at three different temperatures (500 °C, 600 °C, and 700 °C) using a simple pyrolysis method (500 LBC, 600 LBC, and 700 LBC) for atrazine adsorption. Batch experiments showed that the performance of LBC in atrazine adsorption improved with rising pyrolysis temperature, and the highest adsorption amount of 700 LBC reached 84.32 mg g^-1. Kinetic and isotherm models showed that the adsorption behaviors were both monolayer and multilayer chemisorption. The findings of the characterizations (Elemental analysis, BET, XRD, Raman, FT-IR, and XPS) confirmed that the degree of aromatization determined the adsorption capacity of LBC to atrazine, and π-π electron donor-acceptor interaction was the main adsorption mechanism. Density functional theory (DFT) calculations showed that the highly aromatized biochar was more effective for atrazine adsorption, manifested as smaller molecular distances, higher adsorption energies, more stable complex structures, and stronger π-electron conjugation. In the column adsorption experiments, reducing the inlet flow rate or increasing the bed height extended the breakthrough time and exhaustion time of the breakthrough curves, and 700 LBC still showed good adsorption performance after five cycles. Overall, fallen leaf biochar as a reuse product of resource showed good potential for application in atrazine adsorption, which can be used for atrazine-contaminated water remediation.

Removal of cadmium in aqueous solutions using a ball milling-assisted one-pot pyrolyzed iron-biochar composite derived from cotton husk

A novel iron-biochar composite adsorbent was produced via ball milling-assisted one-pot pyrolyzed BM-nZVI-BC 800. Characterization proved that nano zero valent iron was successfully embedded in the newly produced biochar, and the nZVI payload was higher than that of traditional one-pot pyrolyzed methods. BM-nZVI-BC 800 provided a high adsorption performance of cadmium reaching 96.40 mg·g^-1 during batch testing. Alkaline conditions were beneficial for cadmium removal of BM-nZVI-BC 800. The pseudo-second-order kinetic model and Langmuir isotherm fitted better, demonstrating that the Cd adsorption on the BM-nZVI-BC 800 was a chemical and surface process. The intraparticle diffusion controlled the adsorption of BM-nZVI-BC 800. The physisorption dominated by high specific surface area and mesoporous structure was the primary mechanism in the removal of cadmium, though electrostatic attraction and complexation also played a secondary role in cadmium adsorption. Compared to adsorbents prepared by more traditional methods, the efficiencies of the ball milling-assisted one-pot pyrolyzed method appears superior.

Remediation of cadmium or arsenic contaminated water and soil by modified biochar: A review

Biochar has a high specific surface area with abundant pore structure and functional groups, which has been widely used in remediation of cadmium or arsenic contaminated water and soil. However, the bottleneck problem of low-efficiency of pristine biochar in remediation of contaminated environments always occurs. Nowadays, the modification of biochar is a feasible way to enhance the performance of biochar. Based on the Web of science™, the research progress of modified biochar and its application in remediation of cadmium or arsenic contaminated water and soil have been systematically summarized in this paper. The main modification strategies of biochar were summarized, and the variation of physicochemical properties of biochar before and after modification were illustrated. The efficiency and key mechanisms of modified biochar for remediation of cadmium or arsenic contaminated water and soil were expounded in detail. Finally, some constructive suggestions were given for the future direction and challenges of modified biochar research.

Adsorption of Hexavalent Chromium Using Activated Carbon Produced from Sargassum ssp.: Comparison between Lab Experiments and Molecular Dynamics Simulations

Adsorption is one of the most successful physicochemical approaches for removing heavy metal contaminants from polluted water. The use of residual biomass for the production of adsorbents has attracted a lot of attention due to its cheap price and environmentally friendly approach. The transformation of Sargassum-an invasive brown macroalga-into activated carbon (AC) via phosphoric acid thermochemical activation was explored in an effort to increase the value of Sargassum seaweed biomass. Several techniques (nitrogen adsorption, pHPZC, Boehm titration, FTIR and XPS) were used to characterize the physicochemical properties of the activated carbons. The SAC600 3/1 was predominantly microporous and mesoporous (39.6% and 60.4%, respectively) and revealed a high specific surface area (1695 m2·g-1). To serve as a comparison element, a commercial reference activated carbon with a large specific surface area (1900 m2·g-1) was also investigated. The influence of several parameters on the adsorption capacity of AC was studied: solution pH, solution temperature, contact time and Cr(VI) concentration. The best adsorption capacities were found at very acid (pH 2) solution pH and at lower temperatures. The adsorption kinetics of SAC600 3/1 fitted well a pseudo-second-order type 1 model and the adsorption isotherm was better described by a Jovanovic-Freundlich isotherm model. Molecular dynamics (MD) simulations confirmed the experimental results and determined that hydroxyl and carboxylate groups are the most influential functional groups in the adsorption process of chromium anions. MD simulations also showed that the addition of MgCl2 to the activated carbon surface before adsorption experiments, slightly increases the adsorption of HCrO4- and CrO42- anions. Finally, this theoretical study was experimentally validated obtaining an increase of 5.6% in chromium uptake.

Recycling of heavy metals and modification of biochar derived from Napier grass using HNO3

The disposal of biomass enriched with heavy metals (HMs) limits the application of phytoextraction. This study investigated the feasibility of obtaining K-rich fertilizer with low risk of HMs and biochar with good application prospect by extracting Napier grass biochar using 15% HNO₃ and separating HMs from the filtrate using 40% KOH. In this study, Napier grass biochar produced at 500 °C showed better potential for utilization owing to its relatively low HM contents, high nutrient contents, and high yield. In fact, 61.26% Cd, 84.22% Zn, and more K were extracted from biochar when the pH was adjusted to 1 using 15% HNO₃. Then, Cd and Zn could be almost separated from the filtrate by adjusting the pH to 10 or more by adding 40% KOH. The Cd content in the biochar was reduced from a low risk level to a no-risk level, and the Zn content in the biochar was reduced from a medium risk level to a low risk level when the pH was adjusted to 1 and 2 by adding 15% HNO₃. The adsorption capacity of biochar to dyes was enhanced when the pH was adjusted to 1 using 15% HNO₃. The cation exchange mechanism endows the biochar with better potential for reuse (for methylene blue). This work provides a safe, efficient, and maneuverable resource allocation method.

Production of biochar from rice straw and its application for wastewater remediation - An overview

The valorization of biochar as a green and low-cost adsorbent provides a sustainable alternative to commercial wastewater treatment technologies that are usually chemical intensive and expensive. This review presents an in-depth analysis focusing on the rice straw-derived biochar (RSB) for removal of various types of contaminants in wastewater remediation. Pyrolysis is to date the most established technology to produce biochar. Subsequently, biochar is upgraded via physical, chemical or hybrid activation/modification techniques to enhance its adsorption capacity and robustness. Thus far, acid-modified RSB is able to remove metal ions and organic compounds, while magnetic biochar and electrochemical deposition have emerged as potential biochar modification techniques. Besides, temperature and pH are the two main parameters that affect the efficiency of contaminants removal by RSB. Lastly, the limitations of RSB in wastewater remediation are elucidated based on the current advancements of the field, and future research directions are proposed.

Biochar raw material selection and application in the food chain: A review

As one of the largest carbon emitters, China promises to achieve carbon emissions neutrality by 2060. Various industries are developing businesses to reduce carbon emissions. As an important greenhouse gas emissions scenario, the reduction of carbon emissions in the food chain can be achieved by preparing the wastes into biochar. The food chain, as one of the sources of biochar, consists of production, processing and consumption, in which many wastes can be transferred into biochar. However, few studies use the food chain as the system to sort out the raw materials of biochar. A systematic review of the food chain application in serving as raw materials for biochar is helpful for further application of such technique, providing supportive information for the development of biochar preparation and wastes treating. In addition, there are many pollution sources in the food production process, such as agricultural contaminated soil and wastewater from livestock and aquatic, that can be treated on-site to achieve the goal of treating wastes with wastes within the food chain. This study focuses on waste resource utilization and pollution remediation in the food chain, summarizing the sources of biochar in the food chain and analyzing the feasibility of using waste in food chain to treat contaminated sites in the food chain and discussing the impacts of the greenhouse gas emissions. This review provides a reference for the resource utilization of waste and pollution reduction in the food chain.

Pretreatment of straw using filamentous fungi improves the remediation effect of straw biochar on bivalent cadmium contaminated soil

Carbonized products of waste agricultural straws used for soil remediation can reduce impact of heavy metals on soil ecology and crop growth. Here, we demonstrated straw fermentation residues to be suitable for preparation of soil remediation agents by pyrolysis. Lignocellulose degradability of filamentous fungi during fermentation was found to significantly enhance properties of biochar for cadmium (Cd (II))-contaminated paddy soil remediation. Obtained biochars were indicated to have rich oxygen-containing groups, thus showing enhanced removal ability of Cd (II). Adsorption capacity of biochar (BaWS) prepared from wheat straw, which has been fermented by Trichoderma asperellum T-1, reached 105.9 mg g^-1, 372.8% higher than that from natural wheat straw (BWS). Fermentation of straws by Trichoderma reesei QM6a can also improve the adsorption performance of biochar, but the effect is much weaker. The content of bioavailable Cd (II) in paddy soil reduced 83.7% within 15 days after addition of 1% BaWS. Significantly, adding 1% BaWS had better effect on increasing soil pH and removing available Cd (II) , than adding 3% BWS. These results suggest that the used dosage of microbial pretreated straw biochar for the remediation of Cd (II)-contaminated paddy soil was only 1/3 of that of conventional biochar. The enhanced property of biochar was attributed to deconstruction of straws by filamentous fungi before being pyrolyzed. Thus, fermented straws were indicated more suitable for the preparation of biochar used as effective soil remediation agents.

Competitive adsorption, immobilization, and desorption risks of Cd, Ni, and Cu in saturated-unsaturated soils by biochar under combined aging

This study investigated saturated-unsaturated soils, which were closer to the actual field conditions than traditional batch and column experiments with large water-soil ratios. The competitive adsorption, immobilization, and desorption of Cd, Ni, and Cu in soils treated with original and KMnO₄-modified biochars were investigated under combined aging. Moreover, the employment of a three-layer mesh method enabled the independent analysis of heavy metals on biochar and soil during aging. The results showed that the order of biochar adsorption capacities was Cd > Cu > Ni in tested soils, and competing with Ni and Cu enhanced the Cd adsorption on biochars. Cd desorbed most with the CaCl₂ solution while Ni and Cu desorbed most with citric acid. Modified biochar had improved immobilization effects compared to original biochar, and maintained the most stable remediation effects. The maximum variations in the stable Cd fraction during aging were 7.21%, 13.26%, and 14.71% for modified biochar, original biochar, and CK, respectively. However, for Ni and Cu, the biochar application reduced the residual fraction and increased desorption by citric acid. However, the stable fractions of Ni and Cu remained dominant, accounting for 83.28-97.85% and 86.31-98.96%, respectively.

Cadmium long-term immobilization by biochar and potential risks in soils with different pH under combined aging

Cd long-term immobilization by biochar and potential risk in soils with different pH were quantified under a combined artificial aging, which simulated five years of aging in the field based on local climate. Two biochars (original and KMnO₄-modified) and five soils with different pH were tested, and an improved three-layer mesh method was employed in this study. Five aging cycles were carried out (Cycle 1-Cycle 5), and each aging cycle quantitatively simulated 1 year of natural aging. As the aging time increased, Cd leaching loss in all soils gradually increased from Cycle 1 to Cycle 5; for relatively stable Cd fraction, it decreased firstly and then stabilized in acidic and neutral soils (S1-S4), while it decreased firstly and then increased in alkaline soil (S5). Biochars significantly promoted Cd immobilization in strongly acidic soil (S1) by increasing relatively stable fractions and decreasing leaching loss. For weakly acidic and neutral soils (S2-S4), although biochars still had positive effects, the immobilization effects were weakened to certain extents compared with S1. The percentage of Cd leaching loss decreased by 19.12% in strongly acidic soil (S1) and by 1.12-11.35% in weakly acidic and neutral soils (S2-S4) after modified biochar treatment. For alkaline soil (S5), the application of biochars had negative effects on Cd immobilization by decreasing relatively stable fractions and increasing leaching loss, and posed risks to the environment. For strongly acidic soil (S1) and weakly acidic and neutral soils (S2-S4), the percentages of relatively stable fractions increased from 6.09-19.93% to 24.98-36.70% after modified biochar treatment. However, for alkaline soil, the percentage of relatively stable fractions decreased from 55.27% to 53.93% after biochar treatment. The more acidic the soil, the more effective the Cd immobilization by biochar. Biochars with high pH level are not suitable for the remediation of alkaline Cd contaminated soil.

Mechanism underlying how a chitosan-based phosphorus adsorbent alleviates cadmium-induced oxidative stress in Bidens pilosa L. and its impact on soil microbial communities: A field study

In the present study, field experiments were conducted in Side village, Yangshuo, Guilin, Guangxi Province, China, using four C-BPA application levels (control (0 mg m^-2), T1 (100 mg m^-2), T2 (200 mg m^-2) and T3 (400 mg m^-2)) to clarify the mechanism by which a chitosan-based phosphorus adsorbent (C-BPA) applied as a passivator helps Bidens pilosa L. (B. pilosa L.) alleviate cadmium (Cd)-induced oxidative stress in Cd-contaminated soil. In the aqueous phase, C-BPA successfully adsorbed Cd²⁺ on the surface primarily via ion exchange, and C-BPA has potential Cd²⁺ adsorption capacity, enabling its use as a passivator in real Cd-contaminated environments. In Cd-contaminated soils, under C-BPA application at the T3 level, the pH value increased by 11.2%, and the acid-soluble form of Cd decreased by 26.5%. Additionally, the application of C-BPA improved the rhizosphere soil environment and impacted the soil microbial community diversity and structure. Among soil microbes, the soil fungal community was more sensitive than bacteria to C-BPA application. Dehydrogenase, acetic acid, soil pH and Eurotiomycetes or Dothideomycetes significantly impacted Cd accumulation in the leaves of B. pilosa L.; Cd accumulation in leaves was decreased by 68.1% under C-BPA application at the T3 level. Additionally, the variation of increased catalase (CAT) and peroxidase (POD) jointly promoted plant growth; the plant weight was increased by 112.7% under the C-BPA application at the T3 level. Notably, the production of CAT and POD by B. pilosa L. was more effective than the synthesis of glutathione (GSH) in helping B. pilosa L. eliminate excess reactive oxygen species (ROS). Therefore, our findings demonstrated that the application of C-BPA to Cd-contaminated soil can greatly improve the rhizosphere soil environment, help B. pilosa L. eliminate ROS and promote plant growth.

Effects of competitive adsorption with Ni(II) and Cu(II) on the adsorption of Cd(II) by modified biochar co-aged with acidic soil

To investigate the effects of competitive adsorption with Ni(II) and Cu(II) on the adsorption of Cd(II) by modified biochar co-aged with acidic soil, four biochars were employed in this study, namely original biochar, KMnO₄-modified biochar and two aged biochars which co-aged with an acidic soil using above biochars under freeze-thaw cycling and dry-wet cycling for 54 days simulating 6 years of natural aging. The results showed that biochar adsorption capacities of three heavy metal ions were in the order of Cd(II) > Cu(II) > Ni(II) in the single system while Cu(II) > Cd(II) > Ni(II) in binary and ternary systems. Modification improved biochar adsorption capacity of Cd(II), but competitive adsorption with Ni(II) and Cu(II) weakened the improvement of modification on adsorption performance of modified biochar in binary and ternary systems. The Q_MBC/Q_BC of Cd(II) (Q_MBC and Q_BC are the adsorption capacities of heavy metals by modified and original biochars) decreased from 231.57% (single system) to 216.67%∼219.41% (binary system) and further decreased to 207.74% (ternary system). Co-aging with soil weakened the adsorption capacities of biochars for Cd(II), even worse, competition aggravated this negative effect of co-aging. The Q_AMBC/Q_MBC of Cd(II) (Q_AMBC is the adsorption capacities of heavy metals by aged modified biochar) decreased from 65.41% (single system) to 14.43%∼19.46% (binary and ternary systems). Therefore, the impact of competition should be fully considered when evaluating Cd long-term remediation effects of modified biochar in Cd polluted soils accompanied with other heavy metals.

A composite amendment benefits rice (Oryza sativa L.) safety and production in cadmium-contaminated soils by unique characteristics after oxidation modification

In-situ immobilization is an effective strategy for Cd remediation and food safety, while some modifications are necessary to improve immobilization efficiency. In this study, a composite amendment (RFW) derived from rice straw biochar (RSB), fly ash (FA), and white marble (WM) was modified by oxidization (RFW-O) and pyrolysis (RFW-P). The RFW-O showed stronger Cd²⁺ sorption ability than RFW and RFW-P due to larger BET surface area and more oxygen containing-functional groups. Complexation and iron exchange were the two main processes of Cd²⁺ sorption on RFW-O. As a result, the application of RFW-O significantly reduced Cd availability in soils by 10.11-26.24% along with increased soil pH. It was found to be optimal to apply the RFW-O at a dosage of 2.5 wt% for 15 days before transplantation. After RFW-O application, Cd concentrations in brown rice decreased by 40.49% and 41.59% for pot and field experiment, respectively, and were less than 0.2 mg kg^-1. The catalase, dehydrogenase, acid phosphatase and alkaline phosphatase activities in soils increased significantly. Moreover, RFW-O showed no significant effect on rice yield and quality. The RFW-O is thereby considered to be an ideal amendment for in-situ immobilization of Cd-contaminated soils for rice safety and production in practice.

Integrated glycolysis and pyrolysis process for multiple utilization and cadmium collection of hyperaccumulator Sedum alfredii

Phytoremediation is a cost-effective and environmentally-friendly method to treat cadmium (Cd) contaminated soils, however, there is still a lack of safe disposal methods of harvested hyperaccumulators. In this study, by integrating glycolysis and pyrolysis, we investigated the possibility of bioproduct production and Cd collection from the hyperaccumulator Sedum alfredii. By means of acid-alkali pretreatment, the degree of cellulose polymerization was reduced by 36.24% while the surface accessibility was increased by 115.80%, resulting in a bioethanol yield of 9.29%. Meanwhile, 99.22% of total Cd of biomass could be reclaimed by collecting H₂SO₄-pretreatment waste. The saccharification residue was subsequently modified by NaOH-pretreatment-filtrate and converted into biochar at 500 °C which possessed a maximum Cd²⁺ sorption capacity of 60.52 mg g^-1 based on the Langmuir model. Furthermore, sustainability analysis indicated that the economic input of this process is acceptable when considering its good environmental benefits. Taken together, our study provides a strategy for simultaneous bioethanol and biochar production during Cd collection from the hyperaccumulator S. alfredii, which could be a promising alternative for the suitable treatment of metal-enriched plants.

Applications of Biochar and Modified Biochar in Heavy Metal Contaminated Soil: A Descriptive Review

Given that the problem of contaminated soil continues to grow, the development of effective control and remediation measures has become imperative, especially for heavy-metal-contaminated soil. Biochar and modified biochar are eco-friendly and cost-effective remediation materials that are widely used in the remediation of contaminated soil. This review provides an overview of the different raw materials used in the preparation of biochar as well as the modification of biochar using various synthesis methods, highlighting their differences and providing recommendations for biochar and modified biochar as applied toward ameliorating pollution in soil contaminated by heavy metals. We also explore the effects of the physicochemical properties of raw materials, pyrolysis temperature, additives, and modification methods on the properties of the resulting biochar and modified biochar, and systematically present the types of soil and operating factors for repair. Moreover, the mechanisms involved in remediation of heavy-metal-contaminated soil by biochar and modified biochar are outlined in detail, and include adsorption, complexation, precipitation, ion exchange, and electrostatic attractions. Finally, the corresponding monitoring technologies after remediation are illustrated. Future directions for studies on biochar and modified biochar in the remediation of contaminated soil are also proposed to support the development of green environmental protection materials, simple preparation methods, and effective follow-up monitoring techniques.

An efficient, economical, and easy mass production biochar supported zero-valent iron composite derived from direct-reduction natural goethite for Cu(II) and Cr(VI) remove

In this study, a novel biochar-supported zero-valent iron (ZVI) composite was synthesised by a one-pot co-pyrolysis reduction method, and was used to remove Cu(II) and Cr(VI). The raw materials for the composite were derived from natural bagasse/straw and goethite. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, Fourier-transform infrared (FTIR) spectroscopy, thermogravimetry (TG), and Brunauer-Emmett-Teller (BET) analysis were used to characterise the biochar and biochar-supported ZVI composites. Batch removal experiments on the effects of the initial pH and citric acid concentrations were performed as well as kinetic studies and isotherm experiments. The composite materials showed better Cu(II) and Cr(VI) removal performance than single biochar and mineral. The removal of Cu(II) and Cr(VI) is pH-dependent, and proceeds via heterogeneous multilayer chemisorption. Electrochemical analysis revealed that straw biochar-supported ZVI composite exhibited greater electrical conductivity and electron transfer rate than pure biochar and ZVI. FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) elucidated the uptake mechanism, showing that Cu(II) and Cr(VI) were easily adsorbed onto the biochar surface and were then reduced by ZVI. These results indicate that biochar-supported ZVI composite is effective for heavy metal remediation, which is economical, environment-friendly, and suitable for mass production.

Phycoremediation of contaminated water by cadmium (Cd) using two cyanobacterial strains (Trichormus variabilis and Nostoc muscorum)

Background

Water pollution with heavy metals is a severe dilemma that concerns the whole world related to its risk to natural ecosystems and human health. The main objective was to evaluate the removal efficiency of Cd of various concentrations from contaminated aqueous solution by use of two cyanobacterial strains (Nostoc muscorum and Trichormus variabilis). For this purpose, a specially designed laboratory pilot-scale experiment was conducted using these two cyanobacterial strains on four different initial concentrations of Cd (0, 0.5, 1.0 and 2.0 mg L−1) for 21 days.

Results

N. muscorum was more efficient than T. variabilis for removing Cd (II), with the optimum value of residual Cd of 0.033 mg L−1 achieved by N. muscorum after 21 days with initial concentration of 0.5 mg L−1, translating to removal efficiency of 93.4%, while the residual Cd (II) achieved by T. variabilis under the same conditions was 0.054 mg L−1 (89.13% removal efficiency). Algal growth parameters and photosynthetic pigments were estimated for both cyanobacterial strains throughout the incubation period.

Conclusions

High Cd concentration had a more toxic impact on algal growth. The outcomes of this study will help to produce treated water that could be reused in agrarian activities.

Nitric acid-modified hydrochar enhance Cd2+ sorption capacity and reduce the Cd2+ accumulation in rice

Remediating the agricultural soil polluted by cadmium (Cd) is a serious issue in China. Hydrochar showed its potential to purify Cd-contaminated water and improve Cd-contaminated soil due to its vast amounts of macro- and microporous structures. In this study, three concentration gradients of nitric acid (HNO₃, mass fraction: 5%, 10%, 15%) were implemented to age pristine wheat straw hydrochar (N0-HC) aiming to improve surface physiochemical properties. Four HNO₃-aging hydrochars named N0-HC, N5-HC, N10-HC, N15-HC were used to both remove Cd²⁺ from aqueous solution and improve soil properties. Results showed that HNO₃-aging significantly improved the Cd²⁺ adsorption capacity by 1.9-9.9 folds compared to crude hydrochar due to the increased specific surface area (by 1.5-6.5 folds) and oxygen-containing functional group abundance (by 4.5-22.1%). Besides, initial solution pH of 8 or environmental temperature of 318.15 K performed the best Cd²⁺ adsorption capacity. Furthermore, the process of Cd²⁺ adsorption was fitted best to pseudo-second-order (R² = 0.95) and Langmuir models (R² = 0.98), respectively. Nanjing 46 (Oryza sativa L) and HNO₃-aging hydrochars were furtherly applied into Cd-contaminated paddy soil to investigate the mitigation of Cd translation from soil to rice. N15-HC-1% (w/w) performed the best effect on reducing cadmium accumulation in various parts of rice plants. Overall, this research provided an approach to improve hydrochar capacity to remove Cd²⁺ from aqueous solution and mitigate Cd translation from soil to rice.

Chemical aging of hydrochar improves the Cd2+ adsorption capacity from aqueous solution

Hydrochar (HC) serves as a promising adsorbent to remove the cadmium from aqueous solution due to porous structure. The chemical aging method is an efficient and easy-operated approach to improve the adsorption capacity of HC. In this study, four chemical aging hydrochars (CAHCs) were obtained by using nitric acid (HNO₃) with mass fractions of 5% (N5-HC), 10% (N10-HC), and 15% (N15-HC) to age the pristine HC (N0-HC) and remove the Cd²⁺ from the aqueous solution. The results displayed that the N15-HC adsorption capacity was 19.99 mg g^-1 (initial Cd²⁺ concentration was 50 mg L^-1), which increased by 7.4 folds compared to N0-HC. After chemical aging, the specific surface area and oxygen-containing functional groups of CAHCs were increased, which contributed to combination with Cd²⁺ by physical adsorption and surface complexation. Moreover, ion exchange also occurred during the adsorption process of Cd²⁺. These findings have important implications for wastewater treatment to transform the forestry waste into a valuable adsorbent for Cd²⁺ removal from water.

Boosted Cr(VI) sorption coupled reduction from aqueous solution using quaternized algal/alginate@PEI beads

APEI beads (algal/alginate-PEI) were quaternized for enhancing the sorption of Cr(VI) (Q-APEI). The readily reduction of Cr(VI) into Cr(III) in acidic solution and in the presence of organic material constitute an additional phenomenon to be taken into account for the removal of Cr(VI) by Q-APEI. The optimal pH value for both the sorption and reduction of Cr(VI) was close to 2. The sorption isotherm was well described by the Sips model in batch system; the experimental maximum Cr(VI) sorption capacity of Q-APEI was 334 mg Cr(VI) g^-1, including a reduction yield close to 25%. The pseudo-second-order kinetic model (PSORE) and the Yan model fit the uptake kinetics and breakthrough curves, in a fixed-bed system with circulation or single-path modes, respectively. The mechanism of reduction-assisted sorption allows boosting the global removal of chromate. Furthermore, the testing of Cr(VI) for three successive sorption and desorption cycles shows the remarkable stability of the sorbent for Cr(VI) removal. The Cr(VI) sorption coupled reduction mechanism and interactions between the sorbent and Cr(VI) were further explained using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS).

A Review on Current Status of Biochar Uses in Agriculture

In a time when climate change increases desertification and drought globally, novel and effective solutions are required in order to continue food production for the world's increasing population. Synthetic fertilizers have been long used to improve the productivity of agricultural soils, part of which leaches into the environment and emits greenhouse gasses (GHG). Some fundamental challenges within agricultural practices include the improvement of water retention and microbiota in soils, as well as boosting the efficiency of fertilizers. Biochar is a nutrient rich material produced from biomass, gaining attention for soil amendment purposes, improving crop yields as well as for carbon sequestration. This study summarizes the potential benefits of biochar applications, placing emphasis on its application in the agricultural sector. It seems biochar used for soil amendment improves nutrient density of soils, water holding capacity, reduces fertilizer requirements, enhances soil microbiota, and increases crop yields. Additionally, biochar usage has many environmental benefits, economic benefits, and a potential role to play in carbon credit systems. Biochar (also known as biocarbon) may hold the answer to these fundamental requirements.

New insights into ball milling effects on MgAl-LDHs exfoliation on biochar support: A case study for cadmium adsorption

Ball milling (BM) as a solvent-free technology has been widely used to tailor the biochar-based adsorbents with high porosity and well dispersion for enhancing their environmental applications. In this study, the ball-milled layered double hydroxides (LDHs) biochar composite (B-LDHs-BC) was successfully fabricated with BM method for Cd(II) adsorption and the BM effects on the LDHs-BC structure-performance relationships were investigated. The solid-state characterization demonstrated the LDHs were successfully exfoliated by BM on the B-LDHs-BC surface which was identified by the enlarged basal spacing and reduced crystallite size of the LDHs. Although the BET surface area of B-LDHs-BC (226 m²/g) was slightly lower than the ball-milled BC, the B-LDHs-BC had more O-containing functional groups and higher adsorption capacity (119 mg/g). The kinetics experiments indicated that the Cd(II) removal by B-LDHs-BC was through both the physical and chemical adsorption processes, and the liquid membrane diffusion was the rate-controlling step. The positive BM effects mainly induced more abundant acidic functional groups and active adsorption sites, and thus enhanced Cd(II) performance of B-LDHs-BC. This work demonstrated a facile solvent-free method for production of the exfoliated LDHs modified BC composite, and also well illustrated the BM effects which can extend their practical use in environment.

Enhancement of Cd(II) adsorption by rice straw biochar through oxidant and acid modifications

To develop high-efficient biochar adsorbents, the effects and mechanisms of oxidant modification and acid modification on Cd(II) adsorption by rice straw biochar were investigated. Three rice straws from Langxi in Anhui Province, Yingtan in Jiangxi Province, and Lianyungang in Jiangsu Province were collected to prepare biochars by anaerobic pyrolysis in a muffle furnace. Rice straw biochars were modified by 15% H₂O₂ and 1:1 HNO₃/H₂SO₄ mixed acid, respectively, to obtain modified biochars. The untreated rice straw biochar and HCl-treated rice straw biochar with carbonate removed were used as controls. The functional groups on the surfaces of the biochars were qualitatively and quantitatively determined by Fourier transform infrared spectra and Boehm titration, respectively. The adsorption and desorption of Cd(II) onto and from the biochars and modified biochars were measured under various pH conditions. The results showed that oxidant modification with 15% H₂O₂ and acid modification with 1:1 HNO₃/H₂SO₄ significantly increased the number of carboxyl functional groups on the surfaces of the biochars, and acid modification was more effective than oxidant modification in amplifying carboxyl functional groups on the surfaces of the biochars. The increase of surface functional groups effectively enhanced the specific adsorption of Cd(II) on the modified biochars. Therefore, both oxidant modification and acid modification enhanced the adsorption of Cd(II) on the biochars through increasing functional groups on the surfaces of the biochars.

Pristine and Magnetic Kenaf Fiber Biochar for Cd2+ Adsorption from Aqueous Solution

Development of strategies for removing heavy metals from aquatic environments is in high demand. Cadmium is one of the most dangerous metals in the environment, even under extremely low quantities. In this study, kenaf and magnetic biochar composite were prepared for the adsorption of Cd2+. The synthesized biochar was characterized using (a vibrating-sample magnetometer VSM), Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The adsorption batch study was carried out to investigate the influence of pH, kinetics, isotherm, and thermodynamics on Cd2+ adsorption. The characterization results demonstrated that the biochar contained iron particles that help in improving the textural properties (i.e., surface area and pore volume), increasing the number of oxygen-containing groups, and forming inner-sphere complexes with oxygen-containing groups. The adsorption study results show that optimum adsorption was achieved under pH 5-6. An increase in initial ion concentration and solution temperature resulted in increased adsorption capacity. Surface modification of biochar using iron oxide for imposing magnetic property allowed for easy separation by external magnet and regeneration. The magnetic biochar composite also showed a higher affinity to Cd2+ than the pristine biochar. The adsorption data fit well with the pseudo-second-order and the Langmuir isotherm, with the maximum adsorption capacity of 47.90 mg/g.

Rapid Separation and Efficient Removal of Cd Based on Enhancing Surface Precipitation by Carbonate-Modified Biochar

The high buoyancy of biochar makes its application difficult in Cd removal. In this paper, the content of minerals was increased by modifying carbonate on the biochar surface using the vacuum impregnation method. Enhancing surface precipitation between minerals and Cd introduced a correspondingly great number of Cd precipitates on the biochar surface, leading to the rapid precipitation and separation of buoyant biochar. The physical and chemical properties of carbonate-modified biochar and the adsorption mechanism of Cd were comprehensively studied by jar tests, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results showed that the adsorption of Cd by carbonate-modified biochar was controlled by multiple mechanisms, including surface precipitation, surface complexation, and Cd-π interaction. Surface precipitation dominated the removal of Cd. The contributions of Cd removal mechanisms indicated that the contribution proportions of minerals increased from 89.73 to 97.9% when the pyrolysis temperature increased from 300 to 600 °C, while the contribution proportions of Cd-π binding decreased from 9.99 to 2.08%. Meanwhile, oxygen functional groups have only a marginal effect on Cd adsorption. Besides, the results revealed that the higher surface hydrophobicity and the lower polarity were conducive to biochar separation from water. The Cd removal method can provide efficient adsorption and rapid separation, making it possible to use biochar in water treatment.

High-efficiency removal capacities and quantitative adsorption mechanisms of Cd2+ by thermally modified biochars derived from different feedstocks

Adsorption characteristics of Cd²⁺ on the three biochars modified by pyrolysis and calcination were investigated that were derived from rice straw (TRSB), chicken manure (TCMB) and sewage sludge (TSSB). The pH effect, adsorption kinetics, isotherms and thermodynamics, and desorption were determined, and qualitative analysis of adsorption mechanisms was performed by SEM, XRD, FTIR and XPS. Maximum adsorption capacities reached 177.28, 96.03 and 74.04 mg/g for TCMB, TRSB and TSSB, respectively, which were higher than that of many previously reported biochars. Even after five adsorption-desorption cycles, TCMB showed the strongest reusability without losing significantly adsorption capacity. This suggested that thermally modified biochars, particularly TCMB, could be a preferred adsorbent for Cd²⁺. Relative distribution of adsorption mechanisms was examined by direct and indirect calculation, in which the precipitation and cation-exchange dominated the whole chemisorption process, jointly accounting for 84% (TRSB) to 95% (TCMB) of total adsorption. While the complexation was of minor importance in total adsorption accounting for 5%-16%. The relationship of each mechanism with the properties of biochar was also discussed. These provided new insights on the adsorption effectiveness and mechanisms for Cd²⁺ in the aqueous solution that was critical for evaluating the application of modified biochars.

Capacity and potential mechanisms of Cd(II) adsorption from aqueous solution by blue algae-derived biochars

The removal of potentially toxic metals by biochars is currently a popular and salutary method. In this study, we combined the advantages of blue algae (Microcystic) and pyrolysis technology to produce a late-model biochar. Moreover, the adsorption capacity and potential mechanisms of blue algae-derived biochars for the removal of cadmium (Cd) from aqueous solution were evaluated in comparison with the adsorption capacity and potential mechanisms of corn straw-derived biochar (CSBC) and rice husk-derived biochar (RHBC). Batch adsorption experiments were used to explore the adsorption performance of biochars, and a wide range of characterization techniques were employed: scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and zeta potential analysis. The results showed that the adsorption isotherms could be described well by the Langmuir model and that the pseudo-second-order model fit the Cd(II) adsorption kinetics best, indicating that the process was monolayer and controlled by chemisorption. Moreover, the Cd(II) removal capacity of optimal blue algae-derived biochar (BC600-2) (135.7 mg g^-1) was 85.9% and 66.9% higher than the removal capacity of CSBC and RHBC, respectively. In addition, the results of the characterization methods showed that precipitation with minerals was the primary mechanism, accounting for 68.7-89.5% of the capacity. Overall, blue algae-derived biochars, as a product from freshwater biowaste, may be a novel and potentially valuable adsorbent for Cd(II) removal.

Preparation and application of a novel biochar-supported red mud catalyst：Active sites and catalytic mechanism

A novel catalyst RM-BC_(HP) was synthesized by hydrothermal treatment and pyrolysis (800 ℃) using red mud and coconut shells. Influence of different preparation conditions on catalyst performance was explored. SEM showed that RM-BC_(HP) was porous and RM was successfully loaded on the outside surface and inside the pores of BC. XRD revealed that Fe₂O₃ in RM was reduced to Fe⁰ and Fe₃O₄ in the pyrolysis process, in which pyrolysis temperature and addition ratio of coconut shells were critical. TGA-MS, FT-IR and XPS were also applied to character the catalyst. 100% of AO7 was removed within 30 min with conditions of 2 mM PS, 50 mg/L AO7 and 0.5 g/L RM-BC_(HP), and the Fe leaching was negligible. High removal rate was obtained in tap, river, and lake water. RM-BC_(HP)/PS system also exhibited excellent degradation performance for other dyes (MB, MG and RhB) and antibiotics (TC, OTC and CTC). The mechanism studies demonstrated that PS was mainly activated by Fe⁰ and Fe²⁺ in RM-BC_(HP) to produce different radicals, then ¹O₂ was generated by the reactions among these radicals to degrade AO7. Finally, nine intermediate products of AO7 were identified by FT-ICR-MS and a probable degradation pathway was proposed.

Modeling and Optimization of Biochar Based Adsorbent Derived from Kenaf Using Response Surface Methodology on Adsorption of Cd2+

Cadmium is one of the most hazardous metals in the environment, even when present at very low concentrations. This study reports the systematic development of Kenaf fiber biochar as an adsorbent for the removal of cadmium (Cd) (II) ions from water. The adsorbent development was aided by an optimization tool. Activated biochar was prepared using the physicochemical activation method, consisting of pre-impregnation with NaOH and nitrogen (N2) pyrolysis. The influence of the preparation parameters—namely, chemical impregnation (NaOH: KF), pyrolysis temperature, and pyrolysis time on biochar yield, removal rate, and the adsorption capacity of Cd (II) ions—was investigated. From the experimental data, some quadratic correlation models were developed according to the central composite design. All models demonstrated a good fit with the experimental data. The experimental results revealed that the pyrolysis temperature and heating time were the main factors that affected the yield of biochar and had a positive effect on the Cd (II) ions’ removal rate and adsorption capacity. The impregnation ratio also showed a positive effect on the specific surface area of the biochar, removal rate, and adsorption capacity of cadmium, with a negligible effect on the biochar yield. The optimal biochar-based adsorbent was obtained under the following conditions: 550 °C of pyrolysis temperature, 180 min of heating time, and a 1:1 NaOH impregnation ratio. The optimum adsorbent showed 28.60% biochar yield, 69.82% Cd (II) ions removal, 23.48 mg/g of adsorption capacity, and 160.44 m2/g of biochar-specific area.