Adsorption mechanisms for cadmium from aqueous solutions by oxidant-modified biochar derived from Platanus orientalis Linn leaves

Adsorption of heavy metals by biochar in aqueous solution: A review

Heavy metal pollution (e.g., Cd, Hg, Pb, Cu, Ni, Zn, As and Cr) has become a crucial issue worldwide. Among various remediation strategies, adsorption is widely recognized for its environmental sustainability, cost-effectiveness, and operational simplicity. In this context, biochar has gained significant attention due to its promising adsorption performance. To systematically support adsorption studies, this review compiled essential models for adsorption experiments, including commonly used adsorption kinetics models, isotherm models, and thermodynamic analysis methods. Moreover, we systematically analyzed key factors affecting heavy metal adsorption by biochar, such as its physicochemical properties, environmental pH, temperature, initial concentration, dosage, and the presence of coexisting ions, to identify the conditions that govern adsorption capacity. In addition, the adsorption performance of biochar toward eight significant heavy metals is reviewed in detail, with a focus on elucidating the underlying mechanisms, including complexation, ion exchange, cation-π bonding, electrostatic interactions, and precipitation. Finally, based on identified research gaps and critical challenges, we discuss emerging research tools, including machine learning and advanced surface modifications, to guide the targeted design of biochar materials for enhanced adsorption capacity.

Efficient Removal of Cadmium (Cd2+) from Aqueous Solutions by Chitosan@fig Branch Biochar: Adsorption Performance and Enhanced Complexation

In this study, fig branch biochar (FBB) was modified by chitosan to improve the Cd2+ adsorption performance from an aqueous solution. The surface area, pores, and functional groups of chitosan-modified fig branch biochar (CMFBB) were characterized by DTG-TGA, BET, SEM-EDS, FTIR, and XRD. In addition, the impact of various conditions on the Cd adsorption performance of the biochar was analyzed, including dosage, initial pH, concentration, temperature, reaction time, and coexisting cations. The adsorption kinetics, isotherms, and practical applications were also investigated. Under the optimized conditions including pH 6, adsorbent dose 2 g L-1, reaction time 90 min, Cd2+ concentration 100 mg L-1, and 25 °C, the Cd2+ adsorption capacity was 62.25 mg g-1, representing a 78.26% increase compared to FBB. The adsorption data for Cd2+ by CMFBB were found to be well-described by the pseudo-second-order kinetic and Langmuir isothermal models, indicating that a monolayer chemical adsorption process occurred. The Cd2+ adsorption was a spontaneous endothermic process, and the coexisting cations exerted negligible influence on the adsorption process. After five adsorption-desorption cycles, CMFBB maintained an adsorption efficiency of 92.3%, demonstrating excellent regeneration capability. The quantification of adsorption mechanisms suggested that physical adsorption, cation exchange, precipitation, complexation, and π-π interactions accounted for 3.70, 2.50, 33.04, 58.76, and 2.0% of the total adsorbed Cd2+ in CMFBB, respectively. Compared with FBB, the level of CMFBB increased 26.52% in complexation. This work implies that CMFBB has great potential as an effective absorbent for treating Cd2+ polluted water.

Taloring sawdust derived hydrochar via red mud for cadmium removal: Electron transfer insight and recyclability assessment

Iron modified bio-adsorbents gained a lot of attention recently, especially some iron-contain wastes were employed for fabrication. However, the influence of indigenous impurities in wastes was merely investigated. In this study, red mud (RM), an iron-rich by-product was employed as source to prepare Fe modified hydrochar (RM@HC) by a facile hydrothermal method, and then employed for Cd(II) removal from wastewater. The RM@HC demonstrated excellent adsorption performance with capacity of 598.26 mg/g and maintained with a wide pH range. Further, the removal mechanisms were comprehensively elucidated and calculated, which was attributed to the various interactions include physical adsorption (29.07%), reduction (27.61%), and co-precipitation (25.81%). Moreover, the abundant metal oxides in RM@HC contributed to the removal through co-precipitation by building a highly alkaline environment. This work provided a promising choice for the sustainable reutilization of RM by designing a green bio-adsorbent to remove heavy metals from wastewater.

Comparative transcriptome reveals lignin biosynthesis being the key molecular pathway regulating oilseed rape growth treated by SiO2 NPs and biochar

Biochar and SiO2 NPs are effective soil conditioners, but the impacts and mechanisms of combined application in oilseed rape are not yet clear. Therefore, an experiment was designed to investigate oilseed rape growth, physiological indexes, and transcriptome sequencing under four treatments: control (CK), Platanus orientalis L. leaf biochar (B), SiO2 NPs (S), and BS. Our results showed that B, S and BS treatments all promoted the root growth, root activity and biomass of oilseed rape, especially the root length and fresh weight in BS, which were increased by 77.48% and 279.07%, respectively. Moreover, the three-dimensional fluorescence spectra of B and BS were similar, and the tyrosine-like substance proportion in B, S and BS increased from 7.8 to 9.4%, 10.2% and 19.5%, respectively. In transcriptome analysis, there were 10,280 differentially expressed genes (DEGs) shared in B and BS, 3431 DEGs shared in S and BS, and 2815 DEGs shared in B, S and BS. We also found that B, S and BS all regulated oilseed rape growth by inducing the lignin biosynthesis and the relevant genes encoding BBE-like, BGL, UDP in the phenylpropanoid biosynthesis pathway. The results provide gene regulation associated with the phenylpropanoid biosynthesis applying the biochar and SiO2 NPs, which can be used to increase biomass.

Recycling alkali lignin-derived biochar with adsorbed cadmium into cost-effective CdS/C photocatalyst for methylene blue removal

Cadmium (Cd)-enriched adsorbents wastes possess great environmental risk due to their large-scale accumulation and toxicity in the natural environment. Recycling spent Cd-enriched adsorbents into efficient catalysts for advanced applications could address the environmental issues and attain the carbon neutral goal. Herein, a facile strategy is developed for the first time to reutilize the alkali lignin (AL)-derived biochar (ALB) absorbed with Cd into cadmium sulphide (CdS)/C composite for the efficient methylene blue (MB) removal. The ALB is initially treated with Cd-containing solution, then the recycling ALB samples with adsorbed Cd are converted to the final CdS/C composite using NaS2 as the sulphurizing reagent for vulcanization reaction. The optimal ALB400 demonstrates a high adsorption capacity of 576.0 mg g-1 for Cd removal. Then the converted CdS/C composite shows an efficient MB removal efficiency of 94%. The photodegradation mechanism is mainly attributed to carbon components in the CdS/C composite as electron acceptor promoting the separation of photoelectrons/holes and slowing down the abrasion of CdS particles. The enhanced charge transfer and contact between the carrier and the active site thus improves the removal performance and reusability. This work not only develops a method for removing Cd from wastewater effectively and achieving the waste resource utilization but also further offers a significant guidance to use other kinds of spent heavy metal removal adsorbents for the construction of low-cost and high value-added functional materials.

Sulfur-functionalized sawdust biochar for enhanced cadmium adsorption and environmental remediation: A multidisciplinary approach and density functional theory insights

Pristine biochar typically exhibits limited capacity for heavy metal adsorption due to its inadequate pore development and insufficient surface functionality. This study introduces an innovative chemical strategy to enhance the surface of sawdust biochar with sulfur-based functional groups (C=S, C-S, S-S, S2-, S-H, -SO32-, -SO42-) to significantly improve cadmium (Cd) adsorption. Sulfur-doping using H2SO4, Na2S, and Na2S2O3 markedly increased the sulfur content from 0.11% (pristine) to 2.81% (H2SO4), 0.57% (Na2S), and 13.27% (Na2S2O3). Characterization techniques such as SEM-EDS, FTIR, and XPS confirmed the successful incorporation of sulfur moieties and additional oxygen-containing groups, improving surface functionality. The Cd adsorption capacity of S-modified biochar increased by 4.8-9.0 times compared to pristine biochar, with peak values of 39.38, 20.84, and 34.14 mg g-1 for H2SO4, Na2S, and Na2S2O3-modified biochar, respectively. The equilibrium time was significantly reduced from 4 h (pristine) to 5-10 min (S-modified). The enhanced Cd adsorption was attributed to the synergistic interplay of electrostatic attraction, cadmium-π electron interactions, complexation, and ion exchange mechanisms, facilitated by the presence of oxygen and sulfur functional groups. Density Functional Theory (DFT) calculations showed that sulfur doping modulated the electronic properties of the biochar-Cd systems, narrowing the band gap and enhancing the Cd-O bonds, thereby improving the Cd adsorption performance. Additionally, the binding energies of the S-modified biochar-Cd complex were found to be more stable compared to those before Cd adsorption. This study demonstrates that both oxygen and sulfur-functionalized sawdust biochar is an effective and eco-friendly adsorbent for Cd removal, highlighting the significance of tailored surface modifications to augment biochar's reactivity and affinity towards specific contaminants. The developed material offers a sustainable and scalable solution for Cd removal from aqueous environments, contributing to advanced water treatment technologies and environmental remediation strategies.

Enhanced Pb immobilization by CaO/MgO-modified soybean residue (okara) in phosphate mining wasteland soil: Mechanism and microbial community structure

Lead (Pb) contamination is an inevitable consequence of phosphate mining, necessitating the development of effective remediation strategies. This study investigated the use of CaO/MgO-modified okara (CMS) as an eco-friendly approach to remediate Pb-contaminated soils from phosphate mining wastelands. In the present study, following 30 d of CMS application, the exchangeable Pb content was significantly decreased to 10.46%, with the majority of Pb transforming into more stable forms: carbonate-bound Pb (56.44%), Fe/Mn oxide-bound Pb (11.03%), and organic-bound Pb (19.58%). Additionally, the treatment led to a substantial enhancement in total phosphorus, available phosphorus, ammonium, and soil organic matter, thereby improving soil fertility. The microbial community structure was also significantly influenced by CMS, with a notable increase in Firmicutes to 45%. Key genera within the microbial community included Azospirillum, Pseudoxanthomonas, Sphingomonas, and Microvirga, with Pseudoxanthomonas and Massilia being the main differential species. These genera were significantly positively correlated, contributing to the maintenance of microbial community homeostasis and promoting the production of CO32- and PO43-, which further accelerated Pb immobilization. The results indicate that CMS is an effective amendment for Pb immobilization in contaminated soils, enhancing soil fertility and modulating the microbial community to promote Pb stabilization. This provides valuable insights into the ecological remediation of Pb-contaminated soils and water bodies, highlighting the potential of waste reuse in environmental management.

Magnesium polypeptide combined with microbially induced calcite precipitation for remediation of lead contamination in phosphate mining wasteland soil

Soil Pb contamination is inevitable, as a result of phosphate mining. It is essential to explore more effective Pb remediation approaches in phosphate mining wasteland soil to ensure their viability for a gradual return of soil quality for cultivation. In this study, a Pb-resistant urease-producing bacterium, Serratia marcescens W1Z1, was screened for remediation using microbially induced carbonate precipitation (MICP). Magnesium polypeptide (MP) was prepared from soybean meal residue, and the combined remediation of Pb contamination with MP and MICP in phosphate mining wasteland soil was studied. Remediation of Pb using a combination of MP with MICP strain W1Z1 (WM treatment) was the most effective, with the least exchangeable Pb at 30.37% and the most carbonate-bound Pb at 40.82%, compared to the other treatments, with a pH increase of 8.38. According to the community analysis, MP moderated the damage to microbial abundance and diversity caused by MICP. Total nitrogen (TN) was positively correlated with Firmicutes, pH, and carbonate-bound Pb. Serratia inoculated with strain W1Z1 were positively correlated with bacteria belonging to the Firmicutes phylum and negatively correlated with bacteria belonging to Proteobacteria. The available phosphate (AP) in the phosphate mining wasteland soil could encapsulate the precipitated Pb by ion exchange with carbonate, making it more stable. Combined MP-MICP remediation of Pb contamination in phosphate mining wasteland soil was effective and improved the soil microenvironment.

Microalgae realizes self N-doped biochar for heavy metal polluted sediment remediation

Heavy metal contamination in sediment has become a significant global environmental challenge. Numerous studies have demonstrated the effectiveness of modified biochar to solve heavy metal contamination in sediment. However, the modification process with complex methods and expensive modifiers prevented its large-scale application. In this study, an N self-doped biochar was obtained by pyrolysis of Spirulina sp. (SBC). Meanwhile, the K2CO3 impregnation method was utilized to prepare Spirulina sp. biochar (KSBC), which demonstrated a higher specific surface area (874 m2/g) and richer O, N functional groups. The adsorption capacity of KSBC550-120 for Cu (Ⅱ), Zn (Ⅱ), and Cd (Ⅱ) was 57.9 ± 0.3 mg/g, 43.6 ± 0.7 mg/g, and 63.9 ± 0.6 mg/g, respectively. The adsorption process is primarily governed by chemical processes, mainly through ion exchange, surface complexation, dissolution-precipitation, electrostatic interactions, adsorption-reduction, and cation-π interactions. Moreover, utilizing KSBC550-120 for mixing or capping effectively reduced heavy metal concentrations in both the overlying and pore water of the sediments. 1.0 wt% KSBC550-120 with capping treatment significantly reduced the release of heavy metals from the sediment by 80.3-91.9%. This study provides effective theoretical support for re-utilizing waste algal residues and remediation of the heavy metal-contaminated river and lake sediments using microalgae biochar.

Green modification of biochar with poly(aspartic acid) enhances the remediation of Cd and Pb in water and soil

Biochar is a promising adsorbent for the remediation of heavy metals in water and soil. However, pristine biochar has a limited adsorption capacity for heavy metals, which restricts its application in the field of heavy metal immobilization. In the present study, the acidic amino acid-modified biochar was prepared, and its adsorption properties for cadmium (Cd) and lead (Pb) in aqueous solution were investigated. The results showed that poly(aspartic acid)-modified biochar (PASP-BC) was more effective in removing Cd(II) from water compared to biochar modified with poly(glutamic acid) (PGA-BC), aspartic acid (ASP-BC), and glutamic acid (GA-BC). The calculated maximum adsorption capacities, derived from Langmuir fitting parameters, for Cd(II) and Pb(II) by PASP-BC were 44.2 mg/g and 126.1 mg/g, respectively, which were 3.78 and 2.70 times higher than that for pristine BC. Based on the results from Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses, ion exchange, complexation, and electrostatic adsorption were identified as the mechanisms for Cd(II) and Pb(II) adsorption by PASP-BC. The results of the Toxicity Characteristic Leaching Procedure (TCLP) showed that PASP-BC effectively reduced the leachability of Cd and Pb by 91.2% and 84.7%, respectively, at a dosage of 3%. The pot experiment demonstrated that PASP-BC significantly reduced the bioavailability of heavy metals in Triticum aestivum L The maximum reduction in Cd and Pb content in roots was 76.3% and 72.6% when 3% PASP-BC was applied. Importantly, the application of PASP-BC decreased the translocation factor of heavy metals in wheat. Therefore, the green modification of biochar with poly(aspartic acid) has great potential in the heavy metals removal from wastewater and remediation in contaminated soil.

Adsorption characteristics and mechanism of novel ink melanin composite modified chitosan for Cd(II) in water

In this study, chitosan (CS), carboxymethyl chitosan (CMCS), and chitosan quaternary ammonium salt (HACC) were successfully loaded with ink melanin (ME) as efficient adsorbents for Cd(II) removal. The results of batch adsorption experiments and structural characterization showed that the modified CS loaded with ME improved the adsorption capacity of the composites for Cd(II). The pseudo-second-order kinetic and Langmuir equations were better suited to describe the batch adsorption experiments. The adsorption of Cd(II) was chemisorption with desirable adsorption effect when the concentration of the three composites was 0.5 mg/mL and the pH value was neutral. Among them, HACC-ME demonstrated remarkable Cd(II) adsorption performance (107.18 mg/g) and sustained an 85 % efficiency in Cd(II) removal over five adsorption-desorption cycles. Ion exchange, complexation, electrostatic attraction, and hydrophobic interaction were the primary mechanisms for Cd(II) removal. Overall, HACC-ME could be employed as a low-cost and highly efficient new natural adsorbent material for the removal of Cd(II) ions from wastewater. These findings illuminate pathways for the development of efficient and novel natural adsorbent materials for environmental cleanup purposes.

Silicone-modified black peanut shell (BPS) biochar adsorbents: Preparation and their adsorptions for copper(II) from water

Novel silicone-modified biochar adsorbents (BPS-MBCs) were prepared by utilizing waste black peanut shell (BPS) as a raw biochar and gamma-amino-propyl triethoxysilane (silicone) as an inorganic modifier. The novelty of this work is that the incorporation of silicone into BPS can rise the specific surface area and porosity of BPS-MBCs and elevate their adsorptions for copper (II). Sorption kinetics data for copper (II) were molded using five kinetic equations [i.e. Lagergren 1st-order and 2nd-order, intraparticle diffusion (IN-D), Elovich, and Diffusion-chemisorption]. The equilibrium adsorption data for copper (II) were analyzed using two-parameter isotherm equations [i.e. Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin] and three-parameter Sips, Redlich-Peterson and Toth isotherm models. It was validated that copper (II) sorption on BPS-MBCs matched better with pseudo-2nd-order kinetic, Diffusion-chemisorption and Langmuir isotherm models. The maximal qmLan of BPS-MBC-400 was near 284 mg/g at 45 °C. By multi-phase fitting of IN-D modelling, intra-particle diffusion coefficient (kin-d) and diffusion coefficient of external mass-transfer (DEx-Di) for copper (II) were calculated. The low sorption energy from Temkin and mean free energy from D-R modellings implied that copper (II) sorption was initiated by weak non-covalent bond interactions. Thermodynamic parameters indicated that copper (II) on BPS-MBCs was an endothermic and spontaneous process. Recycling of BPS-MBC-400 for copper (II) suggested it has excellent reusability. The major mechanism of copper (II) on BPS-MBCs is possibly comprised of multiple processes, such as physical adsorption (electrostatic attraction), chemical adsorption (adsorption from functional groups, chelation, and ion exchange) and diffusion-chemisorption. Based on these findings, it is expects that BPS-MBCs are promising sorbents for copper (II) eradication from Cu(II)-including wastewater.

Investigation of the Potential of Repurposing Medium-Density Fiberboard Waste as an Adsorbent for Heavy Metal Ion Removal

Medium-density fiberboard (MDF) waste generation has increased steadily over the past decades, and therefore, the investigation of novel methods to recycle this waste is very important. The potential of repurposing MDF waste as an adsorbent for the treatment of Cd(II), Cu(II), Pb(II), and Zn(II) ions in water was investigated using MDF offcuts. The highest adsorption potential in single-metal ion solution systems was observed for Pb(II) ions. The experimental data of Pb(II) ions fit well with the Freundlich isotherm and pseudo-second-order kinetic models. Complexation and electrostatic interactions were identified as the adsorption mechanisms. The adsorption behavior of multi-metal ion adsorption systems was investigated by introducing Cd(II) ions as a competitive metal ion. The presence of the Cd(II) ions reduced the adsorption potential of Pb(II) ions, yet the preference for the Pb(II) ions remained. Regeneration studies were performed by using 0.1 M HCl as a regeneration agent for both systems. Even though a significant amount of adsorbed metal ions were recovered, the adsorption potential of the MDF was reduced in the subsequent adsorption cycles. Based on these results, MDF fines have the potential to be used as an economical adsorbent for remediation of wastewater containing heavy metal ions.

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.

Recent progress in the production and application of biochar and its composite in environmental biodegradation

Over the past few decades, extensive research has been conducted to develop cost-effective and high-quality biochar for environmental biodegradation purposes. Pyrolysis has emerged as a promising method for recovering biochar from biomass and waste materials. This study provides an overview of the current state-of-the-art biochar production technology, including the advancements and biochar applications in organic pollutants remediation, particularly wastewater treatment. Substantial progress has been made in biochar production through advanced thermochemical technologies. Moreover, the review underscores the importance of understanding the kinetics of pollutant degradation using biochar to maximize its synergies for potential environmental biodegradation. Finally, the study identifies the technological gaps and outlines future research advancements in biochar production and its applications for environmental biodegradation.

Efficient removal of both heavy metal ion and dyes from wastewater using magnetic response adsorbent of block polymer brush-grafted N-doped biochar

The recovery of biomass from agricultural and forestry waste could realize effective utilization of waste and synthesis of novel adsorbent. Herein, porous biochar was prepared from waste ginkgo biloba leaves and modified by Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT). And the prepared adsorbent exhibited excellent adsorption capacity owing to its abundant functional groups and porous structure. In addition, the adsorption capacities of the prepared adsorbent for Malachite Green (MG), Amaranth (AM) and Cr (Ⅵ) were 422.59, 373.75 and 368.82 mg/g, respectively, surpassing those of many previously reported materials. Subsequently, the influence of various factors on adsorption performance was studied. The results showed that adsorption of MG, AM and Cr (Ⅵ) on adsorbent followed pseudo-second-order and Langmuir models and the adsorbent also displayed excellent cycling performance. The experimental results of application in various water samples showed that the adsorbent had outstanding adsorption performance in real water samples, further proving that the adsorbent had wide application and practicability. Finally, a simple adsorption column was used for filtration experiments to simulate industrial application. The results were exhibited that the adsorbent had great potential in treating wastewater containing MG, AM and Cr (Ⅵ).

Simultaneous adsorption of ciprofloxacin and Cu2+ using Fe and N co-doped biochar: Competition and selective separation

The treatment of combined antibiotics and heavy metals pollution is a critical challenge. Herein, iron and nitrogen co-doped biochar (Fe/N-BC) was synthesized using rape straw as precursor, and applied for the adsorption of ciprofloxacin (CIP) and Cu2+ in single and binary systems. The qmax for CIP and Cu2+ were 46.45 mg g-1 and 30.77 mg g-1, respectively. Adsorption decreased in a binary matrix, indicating that there was a competitive effect between CIP and Cu2+, which might be due to CIP and Cu2+ sharing similar active adsorption sites on Fe/N-BC. Interestingly, CIP and Cu2+ co-adsorption was a pH-dependent process. Fe/N-BC has potential to highly selectively separate CIP/Cu2+ from mixed solutions through adjusting pH values. Furthermore, adsorption mechanisms were systematically investigated in this research. This research could help to provide a deeper understanding of the synchronously removing specific antibiotics and heavy metals by biochar adsorbents.

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.

Synergistic adsorption and oxidation of trivalent antimony from groundwater using biochar supported magnesium ferrite: Performances and mechanisms

Antimony (Sb) pollution is considered an environmental problem, since Sb is toxic and carcinogenic to humans. Here, a novel biochar supported magnesium ferrite (BC@MF) was adopted for Sb(III) removal from groundwater. The maximum adsorption capacity was 77.44 mg g^-1. Together with characterization, batch experiments, kinetics, isotherms, and thermodynamic analyses suggested that inner-sphere complexation, H-bonding, and electrostatic interactions were the primary mechanisms. C-C/CC, C-O, and O-CO groups and Fe/Mg oxides might have acted as adsorption sites. The adsorbed Sb(III) was oxidized to Sb(V). The generation of reactive oxygen species, iron redox reaction, and oxidizing functional groups all contributed to Sb(III) oxidation. Furthermore, the fixed-bed column system demonstrated a satisfactory Sb removal performance; BC@MF could treat ∼6060 BV of simulated Sb-polluted groundwater. This research provides a promising approach to sufficiently remove Sb(III) from contaminated groundwater, providing new insights for the development of innovative strategies for heavy metal removal.

NaHCO3 activated sludge-derived biochar by KMnO4 modification for Cd(II) removal from aqueous solutions

The surface flat pristine biochar provides limited adsorption sites for Cd(II) adsorption. To address this issue, a novel sludge-derived biochar (MNBC) was prepared by NaHCO₃ activation and KMnO₄ modification. The batch adsorption experiments illustrated that the maximum adsorption capacity of MNBC was twice that of pristine biochar and reached equilibrium more quickly. The pseudo-second order and Langmuir model were more suitable for analyzing the Cd(II) adsorption process on MNBC. Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^- and NO^-₃ had no effect on the Cd(II) removal. Cu²⁺ and Pb²⁺ inhibited the Cd(II) removal, while PO^3-₄ and humic acid (HA) promoted it. After 5 repeated experiments, the Cd(II) removal efficiency on MNBC was 90.24%. The Cd(II) removal efficiency of MNBC in different actual water bodies was over 98%. Furthermore, MNBC owned excellent Cd(II) adsorption performance in fixed bed experiments, and the effective treatment capacity was 450 BV. The co-precipitation, complexation, ion exchange and Cd(II)-π interaction were involved in Cd(II) removal mechanism. XPS analysis showed that NaHCO₃ activation and KMnO₄ modification enhanced the complexation ability of MNBC to Cd(II). The results suggested that MNBC can be used as an effective adsorbent for treating of Cd-contaminated wastewater.

Performance and mechanisms for Cd(II) and As(III) simultaneous adsorption by goethite-loaded montmorillonite in aqueous solution and soil

A series of goethite-modified montmorillonite (GMt) materials was synthesized for the amelioration of cationic cadmium (Cd) and anionic arsenic (As) complex contaminants in soil and water bodies. The results showed that goethite (Gt) was successfully loaded onto the surface of montmorillonite (Mt), which possessed more functional groups (such as Fe-O, and Fe-OH) and a larger specific surface area. GMt-0.5 (Mt loaded with Gt at a ratio of 0.5:1) showed the highest adsorption capacity for Cd(II) and As(III) with the maximum of 50.61 mg/g and 57.58 mg/g, respectively. The removal rate of Cd(II) was highly pH dependent, while the removal rate of As(III) showed little dependence on pH. The goethite on montmorillonite might contribute to the formation of surface complexes with As(III) and oxidation of As(III) to As(V). In the binary system, both, synergistic and competitive adsorption existed simultaneously. Importantly, in the binary system, the removal of As(III) was more favorable because of the electrostatic interaction, formation of a ternary complex, and co-precipitation. In addition, the amendment of GMt-0.5 significantly reduced the availability of Cd and As in the soil. This study suggests that GMt-0.5 is a promising candidate for the simultaneous immobilization of metal (loid)s in both, aqueous solution and mine soil.

Enhanced cadmium removal by biochar and iron oxides composite: Material interactions and pore structure

The combination of biochar (BC) and iron minerals improves their pollutant adsorption capacity. However, little is known about the reactivity of BC-iron mineral composites regarding their interaction and change in the pore structure. In this study, the mechanism of cadmium (Cd) adsorption by BC-iron oxide composites, such as BC combined with ferrihydrite (FH) or goethite (GT), was explored. The synergistic effect of the BC-FH composite significantly improved its Cd adsorption capacity. The adsorption efficiencies of BC-FH and BC-GT increased by 15.0% and 10.8%, respectively, compared with that of uncombined BC, FH, and GT. The strong Cd adsorption by BC-FH was attributed to stable interactions and stereoscopic pore filling between BC and FH. The scanning electron microscopy results showed that FH particles entered the BC pores, whereas GT particles were loaded onto the BC surface. FTIR spectroscopy showed that GT covered a larger area of the BC surface than FH. After loading FH and GT, BC porosities decreased by 9.3% and 4.1%, respectively. Quantum chemical calculations and independent gradient mode analysis showed that van der Waals interactions, H-bonds, and covalent-like interactions maintained stability between iron minerals and BC. Additionally, humic acid increased the agglomeration of iron oxides and formed larger particles, causing additional aggregates to load onto the BC surface instead of entering the BC pores. Our results provide theoretical support to reveal the interfacial behavior of BC-iron mineral composites in soil and provide a reference for field applications of these materials for pollution control and environmental remediation.

Multicomponent adsorption of heavy metals onto biogenic hydroxyapatite: Surface functional groups and inorganic mineral facilitating stable adsorption of Pb(Ⅱ)

Due to the coexistence of various heavy metals in the contaminated environment, it is essential to comprehensively study the multicomponent adsorption of heavy metals in order to tackle these combined pollutants. Herein, the adsorption processes of Pb(Ⅱ), Cu(Ⅱ) and Cd(Ⅱ) by biogenic hydroxyapatite (BHAp) were investigated in single and multicomponent systems. The maximum adsorption capacity for Pb(Ⅱ), Cu(Ⅱ) and Cd(Ⅱ) by BHAp in single system reached 311.16, 82.05 and 92.54 mg g^-1, respectively, while adsorption capacity for Cu(Ⅱ) and Cd(Ⅱ) in multicomponent system decreased more obviously than that of Pb(Ⅱ). Furthermore, the stability of Cu(Ⅱ) and Cd(Ⅱ) adsorbed on BHAp was indeed influenced in multicomponent system. By means of the characterization analysis, it was found that ion exchange was more instrumental in the adsorption processes of Cu(Ⅱ) and Cd(Ⅱ) in single system than in multicomponent system. Significantly, it was observed that the proportion of generally stable Pb(II) adsorbed on BHAp exceeded 95% in both single and multicomponent systems. This result might be due to the in-site growth of stable crystals of Pb_xCa_10-x(PO₄)₆(OH)₂, which was synergistically induced by surface functional groups and inorganic mineral of BHAp, and was unaffected by the coexistence of Cu(Ⅱ) and Cd(Ⅱ).

Chemical modification of biochars as a method to improve its surface properties and efficiency in removing xenobiotics from aqueous media

Biochar (BC) is a carbonaceous material produced by pyrolysis of biomass, applied in various areas such as water purification, fuel production, soil amendment, etc. Many types of BC are characterized by insufficient textural parameters or poor surface chemistry, and hence by low adsorption capacity. This makes innovative chemical methods increasing BC ability to remove xenobiotics from aquatic environments highly needed. Many of them have already been described in the literature. This review presents them in detail and evaluates their effectiveness in improving textural parameters, surface chemistry, and adsorption capacity of BC.

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.

Biochar derived carbonaceous material for various environmental applications: Systematic review

Biochar is the solid material produced from the carbonization of organic feedstock biomass. This material has several unique characteristics such as greater carbon content, good electrical conductivity, high stability and large surface area, which can be applied in several research areas such as generation of power and wastewater treatment. In connection with this, recently, the investigations on biochar significantly focus on the removal of toxic heavy metals since the biochar material is easily available and environmentally friendly. According to an environmental analytical device, biochar-derived carbonaceous material has been additionally applied to the synthesis of an effective, sensitive, and low-cost electrochemical sensor. Biochar with an assessment of electrochemical properties has engaged with different redox reactions in water. In this survey, electrochemical ways of behaving of biochar in light of the electrochemical structures were analytically compiled as well as the impact from biomass sources and manufacturing process including carbonization strategies, pre-treatment/changed techniques. This review emphasizes the various synthesis methods of biochar form organic feedstock, properties and different modulations of biochar for the bioremediation of heavy metals. This review study emphasizes the utilization of biochar as sensing platform and supercapacitor for electrode fabrication in electrochemical biosensor to enhance the remediation of toxic contaminants from water streams and by switching the less ecological traditional materials. Brief information on the techniques employed for packaging biochar as carbon electrode is summarized. Scope in the aspect of environmental concern of biochar, future challenges and prospects are proposed in detail.

Efficient removal of cadmium by salts modified-biochar: Performance assessment, theoretical calculation, and quantitative mechanism analysis

Modified biochar is a feasible adsorbent to solve cadmium pollution in water. However, few studies could elucidate the mechanism of cadmium adsorption by biochar from a molecular perspective. Furthermore, traditional modification methods are costly and have the risk of secondary contamination. Hence, several environmentally friendly sodium salts were used to modify the water chestnut shell-based biochar and employ it in the Cd²⁺ adsorption in this work. The modification of sodium salt could effectively improve the specific surface area and aromaticity of biochar. Na₃PO₄ modified biochar exhibited the highest Cd²⁺ adsorption capacity (112.78 mg/g). The adsorption of Cd²⁺ onto biochar was an endothermic, monolayer, chemisorption process accompanied by intraparticle diffusion. Microscopically, the enhancement of aromatization after modification made Cd²⁺ more likely to interact with the regions rich in π electrons and lone pair electrons. This study provided a new research perspective and application guidance for heavy metal adsorption on biochar.

Algae, biochar and bacteria for acid mine drainage (AMD) remediation: A review

Acid mine drainage (AMD) is a global issue and causes harmful environmental impacts. AMD has high acidity and contains a high concentration of heavy metals and metalloids, making it toxic to plants, animals, and humans. Traditional treatments for AMD have been widely used for a long time. Nevertheless, some limitations, such as low efficacy and secondary contamination, have led them to be replaced by other methods such as bio-based AMD treatments. This study reviewed three bio-based treatment methods using algae, biochar, and bacteria that can be used separately and potentially in combination for effective and sustainable AMD treatment to identify the removal mechanisms and essential parameters affecting AMD treatment. All bio-based methods, when applied as a single process and in combination (e.g. algae-biochar and algae-bacteria), were identified as effective treatments for AMD. Also, all these bio-based methods were found to be affected by some parameters (e.g. pH, temperature, biomass concentration and initial metal concentration) when removing heavy metals from AMD. However, we did not identify any research focusing on the combination of algae-biochar-bacteria as a consortium for AMD treatment. Therefore, due to the excellent performance in AMD treatment of algae, biochar and bacteria and the potential synergism among them, this review provides new insight and discusses the feasibility of a combination of algae-biochar-bacteria for AMD treatment.

Modified Polymeric Biosorbents from Rumex acetosella for the Removal of Heavy Metals in Wastewater

The contamination of water resources by effluents from various industries often contains heavy metals, which cause irreversible damage to the environment and health. The objective was to evaluate different biosorbents from the weed Rumex acetosella to remove metal cations in wastewater. Drying, grinding and sieving of the stems was carried out to obtain the biomass, retaining the fractions of 250 to 500 µm and 500 to 750 µm, which served to obtain the biosorbents in natura (unmodified), acidic, alkaline, and mixed. Proximal analysis, PZC, TOC, removal capacity, influence of pH, functional groups, thermal analysis, structural characteristics, adsorption isotherms, and kinetic study were evaluated. The 250 µm mixed treatment was the one that presented the highest removal percentages, mainly due to the OH, NH, -C-H, COOH, and C-O functional groups achieving the removal of up to 96.14% of lead, 36.30% of zinc, 34.10% of cadmium and 32.50% of arsenic. For contact times of 120 min and an optimum pH of 5.0, a loss of cellulose mass of 59% at 328 °C and a change in the surface of the material were also observed, which allowed for obtaining a topography with greater chelating capacity, and the Langmuir and pseudo-second order models were better fitted to the adsorption data. The new biosorbents could be used in wastewater treatment economically and efficiently.