Fabrication of L-cysteine stabilized α-FeOOH nanocomposite on porous hydrophilic biochar as an effective adsorbent for Pb2+ removal

Fe Crystalline Phases in Fe/C Composites Modulated the Selective Adsorption of Pb(II) from Industrial Wastewater with Cd(II): An Electronic-Scale Perspective

Fe oxide or Fe0-based materials display weak removal capacity for Pb(II), especially in the presence of Cd(II), and the electronic-scale mechanisms are not reported. In this study, Fe3C(220) modified black carbon (BC) [Fe3C(220)@BC] with high adsorption and selectivity for Pb(II) from industrial wastewater with Cd(II) was developed. The quantitative experiment suggested that Fe species accounted for 80.5-100 and 18.4-33.8% of Pb(II) and Cd(II) removal, respectively. Based on X-ray absorption near-edge structure analysis, 57.3% of adsorbed Pb2+ was reduced to Pb0; however, 61.6% of Cd2+ existed on Fe3C@BC. Density functional theory simulation unraveled that Cd(II) adsorption was attributed to the cation-π interaction with BC, whereas that of Pb(II) was ascribed to the stronger interactions with different Fe phases following the order: Fe3C(220) > Fe0(110) > Fe3O4(311). Crystal orbital bond index and Hamilton population analyses were innovatively applied in the adsorption system and displayed a unique discovery: the stronger Pb(II) adsorption on Fe phases was mediated by a combination of covalent and ionic bonding, whereas ionic bonding was mainly accounted for Cd(II) adsorption. These findings open a new chapter in understanding the functions of different Fe phases in mediating the fate and transport of heavy metals in both natural and engineered systems.

Treatment of aqueous per- and poly-fluoroalkyl substances: A review of biochar adsorbent preparation methods

Per- and poly-fluoroalkyl substances (PFAS) are synthetic chemicals widely used in everyday products, causing elevated concentrations in drinking water and posing a global challenge. While adsorption methods are commonly employed for PFAS removal, the substantial cost and environmental footprint of commercial adsorbents highlight the need for more cost-effective alternatives. Additionally, existing adsorbents exhibit limited effectiveness, particularly against diverse PFAS types, such as short-chain PFAS, necessitating modifications to enhance adsorption capacity. Biochar can be considered a cost-effective and eco-friendly alternative to conventional adsorbents. With abundant feedstocks and favorable physicochemical properties, biochar shows significant potential to be applied as an adsorbent for removing contaminants from water. Despite its effectiveness in adsorbing different inorganic and organic contaminants from water environments, some factors restrict its effective application for PFAS adsorption. These factors are related to the biochar properties, and characteristics of PFAS, as well as water chemistry. Therefore, some modifications have been introduced to overcome these limitations and improve biochar's adsorption capacity. This review explores the preparation conditions, including the pyrolysis process, activation, and modification techniques applied to biochar to enhance its adsorption capacity for different types of PFAS. It addresses critical questions about the adsorption performance of biochar and its composites, mechanisms governing PFAS adsorption, challenges, and future perspectives in this field. The surge in research on biochar for PFAS adsorption indicates a growing interest, making this timely review a valuable resource for future research and an in-depth exploration of biochar's potential in PFAS remediation.

l-Cysteine and barium titanate co-modified enteromorpha biochar as effective peroxymonosulfate activator for atrazine treatment

In this study, pyrolysis and hydrothermal methods were used for Enteromorpha biochar that was co-modified with l-cysteine and barium titanate (LBCBa). It has great environmental tolerance and can remove 93.0 % of atrazine (ATZ, 10 mg·L-1) within 60 mins of ultrasonic treatment. The enhanced hydrophilicity, electron-donating capability, and piezoelectricity of LBCBa are considered to induce excellent performance. The apparent reaction rate of the LBCBa-2/PMS/ATZ system with ultrasonic was 2.87 times that without ultrasonic. The density functional theory points out that, introducing l-cysteine to carbon edges improves the adsorption of ATZ and peroxymonosulfate (PMS), making PMS easier to activate. This work offered unique insights for fabricating effective catalysts and demonstrated the combination of hydrophilic functional groups and piezoelectricity in improving catalytic performance and stability.

Mechanochemical synthesis of cysteine-gum acacia intermolecular complex for multiple metal(loid) sequestration from herbal extracts

The ubiquitous heavy metal(loid)s (HMs) contamination has triggered great concern about food safety, while sequestration and separation of trace HMs from herbal extracts still calls for appropriate sorbent materials. In this work, gum acacia was modified by cysteine to form a cysteine-acacia intermolecular complex (Cys-GA complex) via facile mechanochemical synthesis, aiming at capturing multiple HMs simultaneously. Preliminary screening confirms the superiority of Cys-CA complex for both cationic and anionic HMs, and determines an optimum Cys/GA mass ratio of 9:1 to achieve high removal capacities for Pb(II) (938 mg g-1), Cd(II) (834 mg g-1), As(V) (496 mg g-1), and Cr(VI) (647 mg g-1) in simulated aqueous solution. The analysis on HMs-exhausted Cys-GA complex indicates that Pb(II), As(V), and Cr(VI) tend to be removed through chelation, electrostatic attraction, and reduction, while Cd(II) can only be chelated or adsorbed by electrostatic interaction. The batch experiments on commercial herbal (e.g. Panax ginseng, Glycine max, Sophora flavescens, Gardenia jasminoides, Cyclocarya paliurus, and Bamboo leaf) extracts indicate that Cys-GA complex can reduce HMs concentration to attain acceptable level that comply with International Organization for Standardization, with negligible negative effect on its active ingredients. This work provides a practical and convenient strategy to purify HMs-contaminated foods without introducing secondary pollution.

Bio-assembled MgO-coated tea waste biochar efficiently decontaminates phosphate from water and kitchen waste fermentation liquid

Crystal morphology of metal oxides in engineered metal-biochar composites governs the removal of phosphorus (P) from aqueous solutions. Up to our best knowledge, preparation of bio-assembled MgO-coated biochar and its application for the removal of P from solutions and kitchen waste fermentation liquids have not yet been studied. Therefore, in this study, a needle-like MgO particle coated tea waste biochar composite (MTC) was prepared through a novel biological assembly and template elimination process. The produced MTC was used as an adsorbent for removing P from a synthetic solution and real kitchen waste fermentation liquid. The maximum P sorption capacities of the MTC, deduced from the Langmuir model, were 58.80 mg g−1 from the solution at pH 7 and 192.8 mg g−1 from the fermentation liquid at pH 9. The increase of ionic strength (0–0.1 mol L−1 NaNO3) reduced P removal efficiency from 98.53% to 93.01% in the synthetic solution but had no significant impact on P removal from the fermentation liquid. Precipitation of MgHPO4 and Mg(H2PO4)2 (76.5%), ligand exchange (18.0%), and electrostatic attraction (5.5%) were the potential mechanisms for P sorption from the synthetic solution, while struvite formation (57.6%) and ligand exchange (42.2%) governed the sorption of P from the kitchen waste fermentation liquid. Compared to previously reported MgO-biochar composites, MTC had a lower P sorption capacity in phosphate solution but a higher P sorption capacity in fermentation liquid. Therefore, the studied MTC could be used as an effective candidate for the removal of P from aqueous environments, and especially from the fermentation liquids. In the future, it will be necessary to systematically compare the performance of metal-biochar composites with different metal oxide crystal morphology for P removal from different types of wastewater.

Graphical Abstract

Preparation of N-doped cellulose-based hydrothermal carbon using a two-step hydrothermal induction assembly method for the efficient removal of Cr(VI) from wastewater

Cr(VI) pollution is a growing problem that causes the deterioration of the environment and human health. We report the development of an effective adsorbent for the removal of Cr(VI) from wastewater. N-doped cellulose-based hydrothermal carbon (N-CHC) was prepared via a two-step hydrothermal method. The morphology and structural properties of N-CHC were investigated by various techniques. N-CHC has many O and N groups, which are suitable for Cr(VI) adsorption and reduction. Intermittent adsorption experiments showed that N-CHC had an adsorption capacity of 151.05 mg/g for Cr(VI) at pH 2, indicating excellent adsorption performance. Kinetic and thermodynamic analyses indicates that the adsorption of Cr(VI) on N-CHC follows a monolayer uniform adsorption process, which is a spontaneous endothermic process dominated by chemical interaction and limited by diffusion within particles. In a multi-ion system (Pb²⁺, Cd²⁺, Mn⁷⁺, Cl^-, and SO₄^2-), the selectivity of N-CHC toward Cr(VI) was 82.62%. In addition, N-CHC demonstrated excellent reuse performance over seven adsorption-desorption cycles; the Cr(VI) removal rate of N-CHC in 5-20 mg/L wastewater was >99.87%, confirming the potential of N-CHC for large-scale applications. CN/C-OR, pyridinic-N, and pyrrolic-N were found to play a critical role in the adsorption process. This study provides a new technology for Cr(VI) pollution control that could be utilized in large-scale production and other environmental applications.

Biochar-goethite composites inhibited/enhanced degradation of triphenyl phosphate by activating persulfate: Insights on the mechanism

In this study, the biochar-goethite composites (MBC@FH) were synthesized through co-ball milling and the degradation of triphenyl phosphate (TPhP) was compared in persulfate (PDS) alone system and MBC@FH&PDS systems. The results showed that TPhP can be effectively degraded in PDS alone system and degradation efficiency reached up to 90 % within reaction of 8 h, at a PDS concentration of 10 mM, a reaction temperature of 30 °C and a system pH of 6.12. The obvious degradation can be ascribed to the reactive oxygen species (ROS) generated by self-decompose of PDS, among which ¹O₂, ∙OH and O₂∙^- play a major role in the degradation process. Although 350 °C biochar-goethite composites (MBC35@FH) and 800 °C biochar-goethite composites (MBC80@FH) facilitated PDS activation to produce more ROS, the catalytic degradation of TPhP was different in their systems. The degradation of TPhP was inhibited by MBC35@FH due to its stronger adsorption for TPhP, while MBC80@FH promoted TPhP degradation and degradation efficiency was up to 100 % within 6 h. ¹O₂ and SO₄∙^- played a stronger degradation role than ∙OH and O₂∙^- in above systems. The transformation of Fe species, functional groups (oxygen-containing functional groups, pyrrolic nitrogen) and persistent free radicals (PFRs) on the MBC@FH were involved in the PDS activation to produce ROS. Furthermore, MBC80@FH was more capable of activating PDS than MBC35@FH due to its abundant defect sites, larger specific surface area, more PFRs, higher Fe content and stronger electron transfer capability. In addition, seven possible TPhP intermediates were identified and possible degradation pathways of TPhP were proposed accordingly. This study illustrated that not all metallic carbon catalysts are necessarily beneficial for organic contaminants degradation.

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.

Investigation on the evolution of hydrothermal biochar

The objective of this study was to visualize trends and current research status of hydrothermal biochar research through a bibliometric analysis by using CiteSpace software. The original article data were collected from the Web of Science core database published between 2009 and 2020. A visual analysis network of national co-authored, institutional co-authored and author co-authored articles was created, countries, institutions and authors were classified accordingly. By visualizing the cited literature and journal co-citation networks, the main subject distribution and core journals were identified respectively. By visualizing journal co-citations, the main research content was identified. Further the cluster analysis revealed the key research directions of knowledge structure. Keyword co-occurrence analysis and key occurrence analysis demonstrate current research hotspots and new research frontiers. Through the above analysis, the cooperation and contributions of hydrothermal biochar research at different levels, from researchers to institutions to countries to macro levels, were explored, the disciplinary areas of knowledge and major knowledge sources of hydrothermal biochar were discovered, and the development lineage, current status, hotspots and trends of hydrothermal biochar were clarified. The results obtained from the study can provide a reference for scholars to gain a deeper understanding of hydrothermal biochar.

Efficient Cr(VI) removal from wastewater by D-(+)-xylose based adsorbent: Key roles of three-dimensional porous structures and oxygen groups

Reducing the harm of heavy metals to the environment has been a major scientific challenge. In this study, D-(+)-xylose was used to prepare an adsorbent with rich O groups and three-dimensional porous structures for Cr(VI) adsorption. What's more, the adsorption sites of many oxygen groups in the material were combined with the three-dimensionally connected porous structures, which made the adsorption sites fully in contact with Cr(VI). At the concentration of 300 mg/L, the removal rate of Cr(VI) was 94.50%, 6.4 times that of the non-porous treatment and 9.6 times that of the non-porous and O group treatment. The adsorbent showed a high adsorption capacity (910.10 mg/g) for Cr(VI), and the adsorption model proved that the adsorbent was a multi-molecular layer adsorbent. In addition, the adsorption was controlled by chemical reaction and diffusion, which was also attributed to the three-dimensional porous structure and abundant oxygen groups of the material. XPS and FTIR indicated that four O groups participated in the adsorption reaction (-OH, C-O-C, CO, and C-O), and C-O-C and C-O were the main reaction sites. After treating wastewater from electroplating plants with X-PC, the discharged water met international and domestic discharge standards (Cr(VI) removal rate> 99.90%). This work provides a new idea for the application of sugars in the environment and the design of porous adsorbents.

Adsorption effect and the removal mechanism of silicate composite biochar particles on cadmium in soil

Ordinary biochar has the disadvantages of low strength and fragility, and it is difficult to be separated in heavy metal contaminated soil after the remediation process. In order to realize the recovery and reuse of biochar, we prepared silicate composite biochar (SCB) and the magnetic silicate composite biochar (MSCB) with consistent particle size and high hardness. As well as the passivation effect and mechanism of the material on cadmium in soil was also investigated. The results showed that: (1) The MSCB had good hydraulic properties and strong magnetism, which can be quickly separated from the soil under the condition of external magnetic field. (2) The MSCB can remove 30.32%-38.80% of cadmium in the soil after three times of "application-separation-desorption-reuse", as well as the SCB can remove 28.30%-35.78% of cadmium from the soil. (3) The recovered SCB and MSCB had a certain mass loss, the mass loss rate of the biochar particles was in the range of 2.65%-4.90% after each time of recycling. (4) MSCB mainly immobilized cadmium ions through pore interception, complexation of oxygen-containin/iron-containin functional group and precipitation reaction.

Adsorption of Pb(II) from water by treatment with an O-hydroxyphenyl thiourea-modified chitosan

An O-hydroxyphenyl thiourea-modified chitosan (OTCS) with excellent Pb(II) adsorption performance and selectivity was prepared as an adsorbent. The structure and morphology of the adsorbent were systematically investigated by SEM, BET, FTIR, EDX, zeta-potential measurements, XPS and XRD. The impacts of the initial Pb(II) concentration, reaction time, temperature, pH value, and coexisting ions were explored. At pH 7 and 303 K, the maximal adsorption capacity of OTCS for Pb(II) was 208.33 mg/g, which was greater than those of other adsorbing materials reported in the literature. The metal ion adsorption kinetics and isotherm models were found to obey pseudo-second-order kinetics and the Langmuir isothermal model, indicating that the adsorption process was monolayer chemisorption. The adsorption process could proceed spontaneously, and the thermodynamic results revealed that the adsorption mechanism was an endothermic reaction. The ion exchange and chelation between the sulfur, nitrogen and oxygen groups on the adsorbent and lead ions endowed the material with excellent adsorption properties. Significantly, OTCS showed excellent selectivity toward Pb(II). Therefore, the adsorbent OTCS is expected to promote the wider application of chitosan in the adsorption of Pb(II).

Water-based ferrofluid with tunable stability and its significance in nuclear wastewater treatment

The treatment of nuclear wastewater is one of the most urgent and arduous tasks currently, but traditional adsorption materials are significantly limited in practice due to their high demands on auxiliary operations (e.g., shaking or centrifugation) caused by poor stability or recyclability. To tackle this challenge, a water-based ferrofluid composed of magnetic nanoparticles grown in polyethylenimine branches is reported and applied to nuclear wastewater treatment. It is demonstrated that the ferrofluid can keep stable spontaneously in a wide pH range (3-11) out of their ultra-small size, strong electropositivity as well as high charge buffering capacity to achieve shaker-free adsorption, and can be magnetically separated after the neutralization of their positive charge to achieve convenient recycle. Meanwhile, it is found that the ferrofluid shows wide pH/adsorbate applicability and strong ion selectivity in radionuclides absorption. Furthermore, it is anticipated to achieve maximum adsorption capacities for U(VI), Sr(II) and Co(II) as high as 331.5, 427.8 and 759.6 mg/g, respectively. With these characteristics, this ferrofluid outperforms other reported adsorbents. In conclusion, this work provides a practical and effective radioactive wastewater treatment strategy, and enlightens the development of materials for other applications facing the dilemma of incompatible stability and recyclability.

Evaluating the remediation potential of MgFe2O4-montmorillonite and its co-application with biochar on heavy metal-contaminated soils

In this work, a novel and efficient magnesium ferrite-modified montmorillonite (MgFe2O4-MMT) compound was prepared. MgFe2O4-MMT and biochar were mixed at 0:10, 1:9, 3:7, 4:6, and 10:0 w/w combinations and were used for heavy metal immobilization in soil polluted with multiple heavy metals. MgFe2O4-MMT can significantly increase soil alkalinity, and it exhibited the most optimal effect in immobilization of heavy metals in soil. The amounts of Cd, Pb, Cu, and Zn that were extracted by the toxicity characteristic leaching procedure (TCLP) decreased by 58.4%, 50.3%, 42.9%, and 24.7%, respectively. MgFe2O4-MMT can immobilize heavy metals through electrostatic interactions and cation exchange processes. Although, the immobilization of potentially toxic elements by MgFe2O4-MMT and biochar was inferior to that by MgFe2O4-MMT. The combined application of MgFe2O4-MMT and biochar dramatically increased the diversity and richness of the soil bacterial community. The Chao1 index for M3B7 treatment group was 1.7 and 1.2 times higher than that for the control and MgFe2O4-MMT treatment groups, respectively. The combination of biochar and MgFe2O4-MMT might be a cost-effective and ecological remediation approach for mild Pb and Cd contamination.

A state-of-the-art review on modeling the biochar effect: Guidelines for beginners

Biochar has been widely advocated due to its special properties and sustainability for agriculture soil amendment. The influencing mechanism of biochar on soil properties is a key aspect of quantifying and predicting its benefits and trade-offs. The contribution of biochar to both environmental and agricultural benefits has been deeply discussed and extensively reviewed, but few reviews have focused on modeling biochar effects. An overview of recent advances in biochar modeling is illustrated and approaches classified in this paper. Applications of a machine learning model, a deterministic model, and a numerical model to biochar are categorized and summarized. A discussion of the advantages and disadvantages of each model and a comparison among them are also provided. Finally, this paper gives many suggestions on narrowing the knowledge gap to advance biochar modeling. Further study of biochar modeling in management planning and design and application of the model results in agricultural systems will help accelerate the expansion of biochar's application scale and encourage the efficient utilization of waste in agricultural systems.

Recyclable nitrogen-doped biochar via low-temperature pyrolysis for enhanced lead(II) removal

Facile and low-cost preparation are essential in the conversation of agricultural waste into biochar. In this work, nitrogen-doped biochar (NBC-350-0.1) was prepared by thermal decomposition of urea (urea/biochar = 0.1:1 mass ratio) at a low temperature of 350 °C. NBC-350-0.1 showed good performance for Pb(II) removal with the maximum adsorption capacity of 130.87 mg g^-1 at 25 °C, which was five times that of pristine biochar (BC). Adsorption kinetics, isotherms and thermodynamics studies indicated that the adsorption of Pb(II) by NBC-350-0.1 or BC was the homogeneous monolayer adsorption with chemical action as the rate-limiting step, and was accompanied by spontaneous endothermic. Further analysis showed that the removal of Pb(II) on NBC-350-0.1 and BC depended on the complexation with unsaturated carbon bonds and ion exchange with Ca(II). Moreover, graphitic- and pyridinic-N in NBC-350-0.1 exerted a key part in the adsorption of Pb(II). NBC-350-0.1 regenerated by NaOH exhibited excellent recycling performance keeping the original removal efficiency at 84% after five cycles. In addition, this N doping method is suitable for improving the performance of coffee grounds, sawdust, and bagasse biochar. These results would provide an idea for obtaining recyclable N-doped biochar to treat the Pb(II) polluted wastewater.

A State-of-the-Art Review on Innovative Geopolymer Composites Designed for Water and Wastewater Treatment

There is nothing more fundamental than clean potable water for living beings next to air. On the other hand, wastewater management is cropping up as a challenging task day-by-day due to lots of new additions of novel pollutants as well as the development of infrastructures and regulations that could not maintain its pace with the burgeoning escalation of populace and urbanizations. Therefore, momentous approaches must be sought-after to reclaim fresh water from wastewaters in order to address this great societal challenge. One of the routes is to clean wastewater through treatment processes using diverse adsorbents. However, most of them are unsustainable and quite costly e.g. activated carbon adsorbents, etc. Quite recently, innovative, sustainable, durable, affordable, user and eco-benevolent Geopolymer composites have been brought into play to serve the purpose as a pretty novel subject matter since they can be manufactured by a simple process of Geopolymerization at low temperature, lower energy with mitigated carbon footprints and marvellously, exhibit outstanding properties of physical and chemical stability, ion-exchange, dielectric characteristics, etc., with a porous structure and of course lucrative too because of the incorporation of wastes with them, which is in harmony with the goal to transit from linear to circular economy, i.e., "one's waste is the treasure for another". For these reasons, nowadays, this ground-breaking inorganic class of amorphous alumina-silicate materials are drawing the attention of the world researchers for designing them as adsorbents for water and wastewater treatment where the chemical nature and structure of the materials have a great impact on their adsorption competence. The aim of the current most recent state-of-the-art and scientometric review is to comprehend and assess thoroughly the advancements in geo-synthesis, properties and applications of geopolymer composites designed for the elimination of hazardous contaminants viz., heavy metal ions, dyes, etc. The adsorption mechanisms and effects of various environmental conditions on adsorption efficiency are also taken into account for review of the importance of Geopolymers as most recent adsorbents to get rid of the death-defying and toxic pollutants from wastewater with a view to obtaining reclaimed potable and sparkling water for reuse offering to trim down the massive crisis of scarcity of water promoting sustainable water and wastewater treatment for greener environments. The appraisal is made on the performance estimation of Geopolymers for water and wastewater treatment along with the three-dimensional printed components are characterized for mechanical, physical and chemical attributes, permeability and Ammonium (NH4+) ion removal competence of Geopolymer composites as alternative adsorbents for sequestration of an assortment of contaminants during wastewater treatment.

Synthesis of hydrochar supported zero-valent iron composites through hydrothermal carbonization of granatum and zero-valent iron: potential applications for Pb2+ removal

In the present investigation, a one-step synthesis of hydrochar (HC) supported zero-valent iron (ZVI) was performed through hydrothermal carbonization (HTC) of granatum and ZVI. According to XRD, XPS, and FTIR data, ZVI was evenly distributed on the surface of the hydrochar. In addition, the external ZVI oxide layer and the functional groups present in the hydrochar remained on the surface of the HC/ZVI composites after HTC treatment. The surface area of the HC/ZVI composites was between 31.11 and 44.16 m²/g. These numbers were higher than those obtained for hydrochar (20.36 m²/g) and ZVI (12.14 m²/g) separately. The Pb²⁺ adsorption capacity of hydrochar and ZVI was 28.64 and 192.44 mg/g, respectively (25 °C, pH = 6.05, Pb²⁺ concentration of 200 mg/L with 0.05 g HC and 0.01 g ZVI). In addition, the adsorption capacity of the composites was between 49.63 and 88.09 mg/g. The data obtained for Pb²⁺ removal by the samples used in this experiments fitted well the pseudo-second-order kinetics and Langmuir isotherm models. Therefore, hydrochar may represent a promising supporting material for the synthesis of ZVI composites.

Hierarchical porous biochar from plant-based biomass through selectively removing lignin carbon from biochar for enhanced removal of toluene

This study proposed a simple and green air oxidation (AO) method to prepare hierarchical porous biochar by selectively removing lignin carbon from biochar after the pyrolysis of plant-based biomass, based on the fact that the thermal decomposition temperature in air between lignin carbon and cellulose/hemicellulose carbon was different. Three kinds of biomass with different lignocellulose contents were used, including walnut shell, cypress sawdust and rice straw. The results found that AO treatment could effectively improve the pore structure of the three biochar. The specific surface area of WCO-4, CCO-4 and RCO-4 was 555.0 m²/g, 418.7 m²/g and 291.9 m²/g, respectively, which was significantly higher than those of WC (319.5 m²/g), CC (381.7 m²/g) and RC (69.6 m²/g), respectively. Among these, walnut shell biochar with air oxidation (WCO) had higher surface area of 555.0 m²/g and mesopore volume of 0.116 cm³/g, this was related to its high content of lignin, which could facilitate the formation of mesopores by AO treatment with high selectivity. The toluene adsorption capacity of WCO reached 132.9 mg/g, which increased by 223.4% from that without AO treatment. The kinetics study indicated that the diffusion rates of toluene molecule were improved due to the increased mesopores volume of biochar and micropores also play an important role in the adsorption of toluene. The results demonstrate that AO treatment is a promising method to develop hierarchical porous structure for lignocellulose-rich plant-based biomass with low cost and environmental-friendly, which greatly enhanced the toluene adsorption capacity.

Goethite/biochar-activated peroxymonosulfate enhances tetracycline degradation: Inherent roles of radical and non-radical processes

While biochar supported iron materials have been widely studied in advanced oxidation processes (AOPs), little is known about the effect and mechanism of goethite/biochar in sulfate radical (SO_4^-) based AOPs. Herein, a novel goethite/biochar composite was applied as peroxymonosulfate (PMS) activator for tetracycline (TC) degradation in the water. The superior catalytic efficiency of goethite/biochar was achieved through radical (OH and SO₄^-) and non-radical (¹O₂) processes according to the radicals quenching experiments and electron paramagnetic resonance analysis. Carbonyl group and Fe species were the main active sites on the surface of goethite/biochar, which was demonstrated by combining Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and reaction kinetic experiments. Furthermore, nine main by-products of TC degradation were detected by liquid chromatography-mass spectrometry and the reasonable degradation pathway was proposed according to the molecular structure analysis. Overall, the goethite/biochar materials could be applied to activate PMS for TC degradation, and this study will benefit the application of iron/biochar materials in practical water treatment.

Preparation and characterization of boron-doped corn straw biochar: Fe (Ⅱ) removal equilibrium and kinetics

Nowadays, iron ions as a ubiquitous heavy metal pollutant are gradually concerned and the convenient and quick removal of excessive iron ions in groundwater has become a major challenge for the safety of drinking water. In this study, boron-doped biochar (B-BC) was successfully prepared at various preparation conditions with the addition of boric acid. The as-prepared material has a more developed pore structure and a larger specific surface area (up to 897.97 m²/g). A series of characterization results shows that boric acid effectively activates biochar, and boron atoms are successfully doped on biochar. Compared with the ratio of raw materials, the pyrolysis temperature has a greater influence on the amount of boron doping. Based on Langmuir model, the maximum adsorption capacity of 800B-BC_1:2 at 25 °C, 40 °C, 55 °C are 50.02 mg/g, 95.09 mg/g, 132.78 mg/g, respectively. Pseudo-second-order kinetic model can better describe the adsorption process, the adsorption process is mainly chemical adsorption. Chemical complexation, ions exchange, and co-precipitation may be the main mechanisms for Fe²⁺ removal.

Fe2+/H2O2-Strengite method with the enhanced settlement for phosphorus removal and recovery from pharmaceutical effluents

Phosphorus excessively discharged into the water body is a primary cause of eutrophication, but phosphorus resource is limited and non-renewable. If phosphorus could be recovered from wastewaters, it can not only prevent phosphorus pollution but also achieve the recycling of phosphorus resources. This work proposed a novel strategy, Fe²⁺/H₂O₂-strengite method with the enhanced settlement, for phosphorus removal and recovery from pharmaceutical wastewater containing organic phosphorus (OP). In this scheme, OP could be converted into inorganic phosphorus (IP) in the Fe²⁺/H₂O₂ oxidation system, and then IP was recovered in the strengite system. This approach possessed the advantages of simple operation, high efficiency and valuable recovery products, besides, reducing the consumption of reagents, and hardly resulting in secondary pollution. Sixty cycles of phosphorus removal and recovery experiments were conducted, in which pH value was 4 and the initial molar ratio of Fe/P was 1.5. This process achieved a satisfactory and steady phosphorus removal performance, with soluble phosphate and total phosphorus removal efficiencies of 95.3% ± 1.7% and 91.4% ± 2.5%, respectively, and phosphorus was recovered. Possible mechanisms involved: the formation of amorphous strengite (FePO₄·2H₂O) analogue, the adsorption of hydrous ferric oxide (HFO) to phosphorus, and the flocculation of ferric salts. Besides, the presence of quartz as carriers could enhance the settling efficiency of products. Also, via various characterizations, products included amorphous strengite analogue and goethite mixed with phosphorus. This work provided an effective method to reduce OP pollution and recover phosphorus, and supplied thoughts for the treatment of refractory pollutants and the recycling of limited resources.

Sustainable advances on phosphorus utilization in soil via addition of biochar and humic substances

The intervention of human in phosphorus pool seems to be a vicious circle. The rapid population growth leads to the global food shortage, which leads to the massive use of phosphate fertilizer and the continuous exploitation of phosphate rocks. With the massive loss and fixation of phosphate fertilizer in the soil, the unavailable phosphorus in the soil becomes superfluous, while the phosphate mineral resources turn to scarce. Interestingly, exogenous carbonaceous materials, notably, biochar and humic substances, have been widely used as soil conditioners in agricultural production up to date, among other actions to interfere with the balance between the different phosphate species, which offer effective roles for increasing soil available phosphorus. This article reviews the regulation mechanisms of biochar and humic substances on phosphorus availability and circulation, including improving soil physicochemical characteristics, regulating microbial community structure, and directly interacting with phosphorus to affect the fate of phosphorus in soil. Finally, the prospects for future research directions are made, and it is hoped that the review of this article can arouse people's attention to the current plight of agricultural production and provide some methods for improving the efficiency of phosphate fertilizer use in the future.

Preparation of magnetic biochar and its application in catalytic degradation of organic pollutants: A review

In recent years, magnetic biochar (MBC) has been greatly concerned because of its magnetic separation characteristics, and has been successfully used as a catalyst in the catalytic degradation of organic pollutants. However, there is currently a lack of a more systematic summary of MBC preparation methods, and no detailed overview of the catalytic mechanism of MBC catalysts for the degradation of organic pollutants. Therefore, we carry out this work to fill the above gaps. At first, we summarize the raw materials, preparation methods, and types of MBC in detail, and emphasize the MBC prepared by iron-containing sludge. Then, the catalytic mechanisms of MBC in peroxydisulfate, peroxymonosulfate, Fenton-like, photocatalysis, and NaBH₄ systems are carefully summarized, highlighting the contribution of various parts of MBC in catalysis. The degradation efficiency of organic pollutants in the above systems is evaluated. Finally, the stability and reusability of MBC catalysts are evaluated. In conclusion, this review contributes a meager force to the future development of MBC.

Critical review on soil phosphorus migration and transformation under freezing-thawing cycles and typical regulatory measurements

Freezing-thawing period plays an important role in the soil nutrient cycling. The frequency of freezing-thawing cycles (FTCs) can directly affect the supply of effective soil nutrients, further influences the growth and development of crops. Phosphorus is one of the essential nutrients for crop growth, and almost no compounds in gas form in nature, which is non-renewable resources. In modern agricultural production, phosphorus required by plants is mainly from the soil, but the utilization rate of phosphorus fertilizer in soil is generally only 10%-25%. Therefore, it is of great significance to study phosphorus migration and transformation behavior of soil in the non-growth period and related interfacial processes for improving the utilization efficiency of phosphorus fertilizer, increasing crop yield, reducing excessive application of phosphorus fertilizer, and subsiding environmental pollution. This paper systematically concludes key interfacial process of soil phosphorus in freezing-thawing soil system and relative mechanisms describing migration and transformation behavior of soil phosphorus. Besides, it summarizes the mediating effects of widely used soil conditioner on phosphorus cycling. The results show that freezing- thawing will destroy the structure of the soil, causing phosphorus to migrate along with runoff, soil water and heat movement. It also affects the types of microorganisms, the activity of microbial communities and the oxidation-reduction reaction of related minerals, making the phosphorus in soil from an unstable form to an active form. Biochar and humic substances can improve the physical and chemical properties of the soil, and have favorable effects on soil during freezing-thawing period. This review has important significance for the rational utilization of existing phosphorus resources, the maintenance of soil phosphorus cycle balance and the sustainable development of agriculture, meanwhile, has guiding significance for the reasonable utilization of agricultural wastes.

Mitigation of arsenic accumulation in arugula (Eruca sativa Mill.) using Fe/Al/Zn impregnated biochar composites

Arsenic (As) contamination of aquatic and soil environments is a global concern, highlighting the importance of As removal via high-efficiency and low-cost removal technologies. In the present study, novel trimetallic biochar was developed through pyrolyzing corn straw impregnated with inexpensive metal Fe/Al/Zn (hydr)oxides. The results of SEM, FTIR, and XRD verified the formation of metal oxyhydroxides on the surface of the modified biochars, and the modification increased the specific surface area (SSA), total pore volume (TPV), and surface charge of the Fe/Al/Zn (hydr)oxides modified biochar (FAZ-CB). Compared with the original biochar, higher sorption rates and capacities was observed for the FAZ-CB. The maximum As (V) adsorption capacities of FAZ-CB reached 82.9 mg g^-1. A pot experiment showed that application of FAZ-CB decreased bioavailable As fractions in the red soil significantly reduced the uptake of As by arugula in edible part and root (42.6 and 56.8%, respectively). The present study demonstrated the superiority of FAZ-CB in the As(V) immobilization in red soil, suggesting that it is a promising candidate for practical application for As immobilization. Therefore, FAZ-CB can be used as a promising functionalized biochar to remediate As contaminated red soil.

Porous biochar-nanoscale zero-valent iron composites: Synthesis, characterization and application for lead ion removal

Nano-zero-valent iron has been used in combination with a variety of support carriers to remove heavy metals in solution. However, pre-treatment of the carrier can reflect a better synergistic effect and thus achieve high heavy metal removal capabilities. In this study, the hydrophilic biochar obtained by an acid ammonium persulfate oxidation has an adsorption capacity of up to 135.4 mg g^-1 for Pb²⁺ (25 °C, pH = 6 with adsorbent amount of 10 mg and Pb²⁺ concentration of 50 mg L^-1). Due to the strong Fe-C-O covalent bond, nZVI increases the binding force with the carbon matrix. Benefitting from the high specific surface area, porous structure and rich oxygen-containing functional groups, the resultant nZVI-HPB samples are favourable for Pb²⁺ diffusion and adsorption, exhibiting maximum adsorption capacity of 480.9 mg g^-1 (pH = 6, 25 °C with adsorbent amount of 10 mg and Pb²⁺ concentration of 200 mg L^-1). The multiple interaction mechanisms in the Pb²⁺ removal process such as the reduction reaction, complexation and co-precipitation proceed simultaneously are concluded by the analyses of Fourier-Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) spectra.

Unravelling the Environmental Application of Biochar as Low-Cost Biosorbent: A Review

In this age, a key target for enhancing the competitiveness of the chemical, environmental and biotechnology industries is to manufacture high-value products more efficiently and especially with significantly reduced environmental impact. Under this premise, the conversion of biomass waste to a high-value added product, biochar, is an interesting approach under the circular economy principles. Thus, the improvements in the biochar production and its new and innovative uses are hot points of interest, which are the focus of vast efforts of the scientific community. Biochar has been recognized as a material of great potential, and its use as an adsorbent is becoming a reliable strategy for the removal of pollutants of different streams, according to its high adsorption capacity and potential to eliminate recalcitrant compounds. In this review, a succinct overview of current actions developed to improve the adsorption capability of biochar, mainly of heavy metal and organic pollutants (dyes, pharmaceuticals and personal care products), is summarized and discussed, and the principal adsorption mechanisms are described. The feedstock and the production procedure are revealed as key factors that provide the appropriate physicochemical characteristics for the good performance of biochar as an adsorbent. In addition, the modification of the biochar by the different described approaches proved their feasibility and became a good strategy for the design of selective adsorbents. In the last part of this review, the novel prospects in the regeneration of the biochar are presented in order to achieve a clean technology for alleviating the water pollution challenge.

Adsorption behaviors on trace Pb2+ from water of biochar adsorbents from konjac starch

Biochar adsorbents used to treat different heavy metals in water are efficient and low-cost. Appropriate raw materials, excellent selectivity and detailed adsorption mechanism are of important for research on biochar adsorbents. In this work, konjac starch was dispersed in polyvinylpyrrolidone (PVP) solution to prepare different sizes hydrophilic carbon spheres (HCSs) by hydrothermal synthesis method. Adsorption kinetics of the HCSs towards Pb2+ is described perfectly by the pseudo-second-order equation. With the temperature increasing, adsorption thermodynamics are more consistent with the Freundlich model. The calculated ΔG, ΔH and ΔS shows the adsorption of the HCSs towards Pb2+ is a spontaneous, endothermic and entropy increase process. In addition, HCSs have excellent selectivity for the adsorption of Pb2+ and Cu2+. HCSs prepared from konjac starch make full use of natural biomass resources, they can be used as a potential adsorbent material in treatment on heavy metal ion from water field.