Removal of cadmium in aqueous solution using wheat straw biochar: effect of minerals and mechanism

Experimental and Theoretical Studies on the Adsorption of Bromocresol Green from Aqueous Solution Using Cucumber Straw Biochar

Biochar prepared from crop straw is an economical method for adsorbing bromocresol green (BCG) from textile industrial wastewater. However, there is limited research on the adsorption mechanism of biochar for the removal of BCG. This study utilized cucumber straw as raw material to prepare biochar with good adsorption potential and characterized its physicochemical properties. Through adsorption experiments, the effects of solution pH, biochar dosage, and initial dye concentration on adsorption performance were examined. The adsorption mechanism of cucumber straw biochar (CBC) for BCG was elucidated at the molecular level using adsorption kinetics, adsorption isotherm models, and density functional theory (DFT) calculations. Results show that the specific surface area of the CBC is 101.58 m2/g, and it has a high degree of carbonization, similar to the structure of graphite crystals. The presence of aromatic rings, -OH groups, and -COOH groups in CBC provides abundant adsorption sites for BCG. The adsorption process of CBC for BCG is influenced by both physical and chemical adsorption, and can be described by the Langmuir isotherm model, indicating a monolayer adsorption process. The theoretical maximum monolayer adsorption capacity (qm) of BCG at 298 K was calculated to be 99.18 mg/g. DFT calculations reveal interactions between BCG and CBC involving electrostatic interactions, van der Waals forces, halogen-π interactions, π-π interactions, and hydrogen bonds. Additionally, the interaction of hydrogen bonds between BCG and the -COOH group of biochar is stronger than that between BCG and the -OH group. These findings provide valuable insights into the preparation and application of efficient organic dye adsorbents.

Chemical aging of biochar-zero-valent iron composites in groundwater: impact on Cd(II) and Cr(VI) co-removal

Biochar (BC), zero-valent iron (ZVI), and their composites are promising materials for use in permeable reactive barriers, although further research is needed to understand how their properties change during long-term aging in groundwater. In this study, BC, ZVI and their composites (4BC-1ZVI) were subjected to the chemical aging tests in five media (deionized water, NaCl, NaHCO3, CaCl2 and a mixture of CaCl2 and NaHCO3 solutions) for 20 days. After treatment, the microscopic analysis and performance tests for the co-removal of Cd(II) and Cr(VI) were carried out. The results indicated that the removal of Cd(II) by aged 4BC-1ZVI followed a pseudo-second-order model, whereas the removal of Cr(VI) was better fitted with a pseudo-first order model. The aging mechanism of 4BC-1ZVI was primarily governed by iron corrosion/passivation, the reduction of soluble components, and the formation of carbonate minerals. Less Fe3O4/ -Fe2O3 was formed during aging in deionized water, NaCl and CaCl2 solutions. The corrosion products, Fe3O4/ -Fe2O3, FeCO3 and -FeOOH, were observed after aging in NaHCO3 and a mixture of NaHCO3 and CaCl2 solutions. The decrease in the soluble components of biochar led to a decrease in cation exchange, while carbonate minerals contributed to Cd(II) precipitation. This work provides insights into the aging processes of BC-ZVI composites for long-term groundwater remediation applications.

Study on the Effect of Magnesium Chloride-Modified Straw Waste Biochar on Acidic Soil Properties

Soil biochar is a kind of organic matter rich in carbon, which is of great significance in soil fertility improvement, fertilizer type innovation and greenhouse gas emission reduction. In this paper, Mg-modified biochar was prepared by thermal cracking using rice straw and corn straw as raw materials. The Mg-modified biochar and unmodified biochar were fully mixed with prepared soil samples at the addition amounts of 0.5% (w/w), 1% (w/w) and 2% (w/w), respectively, and then simulated indoor soil cultivation experiments were carried out. The effects of magnesium ion-modified biochar and non-modified biochar on soil chemical properties and the effects of different amounts of biochar on soil properties were studied. The results showed that the yield of Mg-modified biochar from rice straw and corn straw, prepared by pyrolysis, was 65%, and the ash content was large. The pH of MG-modified corn stalk biochar (MCBC) is weakly basic (8.55), while the pH of MG-modified rice stalk biochar (MRBC) is basic (10.1), and their internal structures are slightly different. After the application of biochar prepared from rice straw and maize stover, soil indicators were determined. Compared to the control, the chemical properties of the treated soil samples were significantly improved, with an increase in soil pH, an increase in the content of effective nutrients, such as fast-acting potassium, fast-acting phosphorus and alkaline dissolved nitrogen, and an increase in the content of the total phosphorus and total nitrogen, as well as an increase in the content of organic matter. The Mg-modified biochar was generally superior to the unmodified biochar in improving soil fertility, at the same addition level. It was also found that the rice-straw biochar performed better than the corn-stover biochar and had a more obvious effect on soil improvement in terms of fast-acting potassium, ammonium nitrogen, nitrate nitrogen, total phosphorus and total nitrogen contents.

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.

Investigating the potential of mineral precipitation in co-pyrolysis biochar: Development of a novel Cd (II) adsorption material utilizing dual solid waste

Ionic cadmium (Cd (II)) in water is a significant threat to ecosystems, the environment, and human health. Research is currently focused on developing efficient adsorption materials to combat Cd (II) pollution in water. One promising solution involves co-pyrolyzing solid residue from anaerobic digestion of food waste with oil-based drill cuttings pyrolysis residue to create a biochar with high organic matter content. This biochar has a lower heavy metal content and leaching toxicity compared to China's national standards, making it both safe and resourceful. It exhibits a high adsorption capacity for Cd (II) in water, reaching up to 47.80 ± 0.37 mg/g. Raising the pyrolysis temperature above 600 °C and increasing the amount of pyrolysis residue beyond 30 % enhances the biochar's adsorption capacity. The adsorption process is primarily driven by mineral precipitation, offering a promising approach for dual waste resource management and reducing heavy metal pollution.

Toxic heavy metal ions contamination in water and their sustainable reduction by eco-friendly methods: isotherms, thermodynamics and kinetics study

Heavy metal ions can be introduced into the water through several point and non-point sources including leather industry, coal mining, agriculture activity and domestic waste. Regrettably, these toxic heavy metals may pose a threat to both humans and animals, particularly when they infiltrate water and soil. Heavy metal poisoning can lead to many health complications, such as liver and renal dysfunction, dermatological difficulties, and potentially even malignancies. To mitigate the risk of heavy metal ion exposure to humans and animals, it is imperative to extract them from places that have been polluted. Several conventional methods such as ion exchange, reverse osmosis, ultrafiltration, membrane filtration and chemical precipitation have been used for the removal of heavy metal ions. However, these methods have high operation costs and generate secondary pollutants during water treatment. Biosorption is an alternative approach to eliminating heavy metals from water that involves employing eco-friendly and cost-effective biomass. This review is focused on the heavy metal ions contamination in the water, biosorption methods for heavy metal removal and mathematical modeling to explain the behaviour of heavy metal adsorption. This review can be helpful to the researchers to design wastewater treatment plants for sustainable wastewater treatment.

Catalytic oxidation of lignite by Pt/TiO2 can enhance cadmium adsorption capacity

Addressing global warming necessitates innovative strategies in fossil fuel management. This study evaluates lignite, a low-rank coal with limited calorific value, exploring applications beyond its use as fuel. Utilizing Pt/TiO2 catalytic oxidation, the research aims to enhance the cadmium adsorption capacity of lignite in wastewater. Lignite, treated with 0.5% Pt/TiO2 at 125 °C for 2 h, demonstrated a threefold increase in cadmium adsorption capacity. Characterization using TGA-DSC confirmed the modification process as exothermic and self-sustainable. Spectroscopic analysis and Boehm titration revealed significant alterations in pore structure, surface area, and oxygen-containing functional groups, emphasizing the effectiveness of catalytic oxidation. Adsorption mechanisms such as complexation, cation exchange, and cation-π interactions were identified, enhancing Cd adsorption. Techniques, including the d-band model, H2-TPR, and O2-TPD, indicated that dissociative adsorption of molecular O2 and the subsequent generation of reactive oxygen species introduced additional oxygen-containing functional groups on the lignite surface. These findings provide essential strategies for the alternative use of lignite in environmental remediation, promoting sustainable resource utilization and enhancing cost-effectiveness in remediation processes. ENVIRONMENTAL IMPLICATION: This study innovates in using lignite to reduce cadmium (Cd) contamination in wastewater. Employing Pt/TiO2 catalytic oxidation, lignite is transformed, enhancing its cadmium adsorption capacity. This process, being exothermic, contributes to decreased energy consumption. The approach not only mitigates the hazardous impacts of cadmium but also aligns with sustainability by reducing greenhouse gas emissions and energy use, showcasing a multifaceted environmental advancement.

Effects of different aging methods on the ability of biochar to adsorb heavy metal cadmium and its physical and chemical properties

The aging process can affect the physical and chemical properties as well as adsorption capacity of biochar. This study focuses on the heavy metal cadmium (Cd) as the research object, and artificially ages biochar prepared from rice straw and corn straw through accelerated freeze-thaw cycles, alternating dry wet cycles, and ultraviolet light treatment, in order to evaluate the effects of different aging conditions on the physical and chemical properties of the two different types of biochar and on their adsorption capacities for Cd. After aging, the pH of rice and corn biochar decreased to varying degrees, respectively. The surface structure was ruptured, the average pore diameter was decreased, and the specific surface area was increased by 27.3%, 21.9%, and 9.8% (rice) and 95.4%, 27.7%, and 13.4% (corn). Ultraviolet light aging has the most significant impact on the elemental content of biochar, and the C content was decreased by 12.4% (rice) and 9.3% (corn). The O content was increased by 11.2% (rice) and 44.1% (corn), and the numbers of O/C, H/C, (O + N)/C, and oxygen-containing functional groups were increased. These results demonstrate that the aging process reduces the degree of aromatization of biochar, while enhancing its polarity and Cd adsorption capacity. Rice straw biochar (RSB) has a greater ability to adsorb Cd than corn straw biochar (CSB). In addition, ultraviolet light aging is particularly effective in increasing heavy metal adsorption.

Adsorption behavior of lead, cadmium, and arsenic on manganese-modified biochar: competition and promotion

Biochar adsorption of heavy metals has been a research hotspot, yet there has been limited reports on the effect of heavy metal interactions on adsorption efficiency in complex systems. In this study, the adsorbent was prepared by pyrolysis of rice straw loaded with manganese (BC-Mn). The interactions of Pb, Cd and As adsorption on BC-Mn were systematically studied. The results of the adsorption isotherms for the binary metal system revealed a competitive adsorption between Pb and Cd, resulting in decreased Pb (from 214.38 mg/g to 148.20 mg/g) and Cd (from 165.73 mg/g to 92.11 mg/g). A notable promotion occurred between As and Cd, showing an increase from 234.93 mg/g to 305.00 mg/g for As and 165.73 mg/g to 313.94 mg/g for Cd. In the ternary metal system, Pb inhibition did not counteract the promotion of Cd and As. Furthermore, the Langmuir isotherm effectively described BC-Mn's adsorption process in monometallic, binary, and ternary metal systems (R2 > 0.9294). Zeta and FTIR analyses revealed simultaneous competition between Pb and Cd for adsorption on BC-Mn's -OH sites. XPS analysis revealed that As adsorption by BC-Mn facilitated the conversion of MnO2 and MnO to MnOOH, resulting in increased hydroxyl radical production on BC-Mn's surface. Simultaneously, Cd combined with the adsorbed As to form ternary Cd-As-Mn complexes, which expedited the removal of Cd. These results help to provide theoretical support as well as technical support for the treatment of Pb-Cd-As contaminated wastewater.

Magnetic biochar enhanced copper immobilization in agricultural lands: Insights from adsorption precipitation and redox

Biochar has exceeded expectations for heavy metal immobilization and has been prepared from widely available sources and inexpensive materials. In this research, coconut shell biochar (CSB), bamboo biochar (BC), magnetic coconut shell charcoal (MCSB), and magnetic bamboo biochar (MBC) were manufactured via co-pyrolysis, and their adsorption properties were tested. The pseudo-secondary (R2 = 0.980-0.985) adsorption kinetic fittings for the four biochas were superior to the pseudo-primary kinetics (R2 = 0.969-0.982). Unmodified biochar adsorption isotherms were more consistent with the Freundlich model, while magnetic biochar fitted Langmuir models better. The maximum adsorption capacity of MCSB for Cu(Ⅱ) reached 371.50 mg g-1. The adsorption mechanisms quantitatively analysis of the biochar indicated that chemical precipitation and ion exchange contributed to the adsorption, in which the magnetic biochar metal-π complexation also enhanced the adsorption. The pot experiment revealed that MCSB (2.0 %DW) significantly enhanced the biomass of lettuce, and facilitated the immobilization of DTPA-Cu (p < 0.05). SEM-EDS, XPS, and FTIR were utilized for morphological characterization and functional group identification, and the increased active adsorption sites (-OH, -COOH, CO, and Fe-O) of MCSB enhanced chemisorption and π-π EDA complexation with Cu(Ⅱ). EEM-PARAFAC and RDA analysis further elucidated that magnetic biochar immobilized copper and reduced biotoxicity (efficiency: 76.12%) by adjusting soil pH, phosphate, and SOM release (negative correlation). The presence of iron oxides (FeOx) promoted in situ adsorption of metallic copper and offered new insights into soil remediation.

Exploring adsorption capacity and mechanisms involved in cadmium removal from aqueous solutions by biochar derived from euhalophyte

Biochar has shown potential as a sorbent for reducing Cd levels in water. Euhalophytes, which thrive in saline-alkali soils containing high concentrations of metal ions and anions, present an intriguing opportunity for producing biochar with inherent metal adsorption properties. This study focused on biochar derived from the euhalophyte Salicornia europaea and aimed to investigate its Cd adsorption capacity through adsorption kinetics and isotherm experiments. The results demonstrated that S. europaea biochar exhibited a high specific surface area, substantial base cation content, and a low negative surface charge, making it a highly effective adsorbent for Cd. The adsorption data fit well with the Langmuir isotherm model, revealing a maximum adsorption capacity of 108.54 mg g-1 at 25 °C. The adsorption process involved both surface adsorption and intraparticle diffusion. The Cd adsorption mechanism on the biochar encompassed precipitation, ion exchange, functional group complexation, and cation-π interactions. Notably, the precipitation of Cd2+ with CO32- in the biochar played a dominant role, accounting for 73.7% of the overall removal mechanism. These findings underscore the potential of euhalophytes such as S. europaea as a promising solution for remediating Cd contamination in aquatic environments.

Biomass based biochar production approaches and its applications in wastewater treatment, machine learning and microbial sensors

Biochar is a stable carbonaceous material derived from various biomass and can be utilized as adsorbents, catalysts and precursors in various environmental applications. This review discusses various feedstock materials and methods of biochar production via traditional as well as modern approaches. Additionally, the biochar characteristics, HTC process, and its modification by employing steam and gas purging, acidic, basic / alkaline and organo-solvent, electro- and magnetic fields have been discussed. The recent biochar applications for real water, wastewater and industrial wastewater for the abstraction of environmental contaminants also reviewed. Moreover, applications in machine learning and microbial sensors were discussed. In the meantime, analyses on commercial and environmental profit, current ecological concerns and the future directions of biochar application have been well presented.

The importance of presoaking to improve the efficiency of MgCl2-modified and non-modified biochar in the adsorption of cadmium

Investigating the effect of presoaking, as one of the most important physical factors affecting the adsorption behavior of biochar, on the adsorption of heavy metals by modified or non-modified biochar and presoaking mechanism is still an open issue. In this study, the water presoaking effect on the kinetics of cadmium (Cd) adsorption by rice husk biochar (produced at 450 °C, B1, and at 600 °C, B2) and the rice husk biochar modified with magnesium chloride (B1 modified with MgCl₂, MB1, and B2 modified with MgCl₂, MB2) was investigated. Furthermore, the effect of pH (2, 5, and 6), temperature (15, 25, and 35 °C), and biochar particle size (100 and 500 µm) on the kinetics of Cd adsorption was also investigated. Results revealed that the content of Cd adsorbed by the presoaked biochar was significantly higher than that by the non-presoaked biochar. The highest Cd adsorption capacity of MB2 and MB1 was 98.4 and 97.6 mg g^-1, respectively, which was much better than that of B1 (7.6 mg g^-1) and B2 (7.5 mg g^-1). The modeling of kinetics results showed that in all cases pseudo-second-order model was well-fitted (R²>0.99) with Cd adsorption data. The results also indicated that the highest Cd adsorption values were observed at pH 6 in presoaked MB1 with size of 100 µm as well as at the temperature of 35 °C in presoaked MB2, indicating the optimum conditions for this process. The presoaking process was not affected by biochar size and pH, and the difference in adsorbed Cd content between presoaked biochars and non-presoaked ones was also similar. However, the temperature had a negative effect on presoaking. The presoaking process decreased micropores (<10 µm) in the biochars but had no effect on biochar hydrophobicity. Therefore, presoaking, which could significantly increase Cd adsorption and reduce equilibrium time by reducing the micropores of biochars, is suggested as an effective strategy for improving the efficiency of modified biochars or non-modified ones in the adsorption of contaminants (Cd) from aquatic media.

Selenite reduced uptake/translocation of cadmium via regulation of assembles and interactions of pectins, hemicelluloses, lignins, callose and Casparian strips in rice roots

Selenium (Se) can reduce cadmium (Cd) uptake/translocation via regulating pectins, hemicelluloses and lignins of plant root cell walls, but the detailed molecular mechanisms are not clear. In this study, six hydroponic experiments were set up to explore the relationships of uptake/translocation inhibition of Cd by selenite (Se(IV)) with cell wall component (CWC) synthesis and/or interactions. Cd and Se was supplied (alone or combinedly) at 1.0 mg L^-1 and 0.5 mg L^-1, respectively, with the treatment without Cd and Se as the control. When compared to the Cd1 treatment, the Se0.5Cd1 treatment 1) significantly increased total sugar concentrations in pectins, hemicelluloses and callose, suggesting an enhanced capacity of binding Cd or blocking Cd translocation; 2) stimulated the deposition of Casparian strips (CS) in root endodermis and exodermis to block Cd translocation; 3) stimulated the release of C-O-C (-OH^- or -O^-) and CO (carboxyl, carbonyl, or amide) to combine Cd; 4) regulated differential expression genes (DEGs) and metabolites (DMs) correlated with synthesis and/or interactions of CWSs to affect cell wall net structure to affect root cell division, subsequent root morphology and finally elemental uptake; and 5) stimulated de-methylesterification of pectins via reducing expression abundances of many DMs and DEGs in the Yang Cycle to reduce supply of methyls to homogalacturonan, and regulated gene expressions of pectin methylesterase to release carboxyls to combine Cd; and 6) down-regulated gene expressions associated with Cd uptake/translocation.

Use of additives to improve collective biogas plant performances: A comprehensive review

Nowadays, anaerobic digestion (AD) is being increasingly encouraged to increase the production of biogas and thus of biomethane. Due to the high diversity among feedstocks used, the variability of operating parameters and the size of collective biogas plants, different incidents and limitations may occur (e.g., inhibitions, foaming, complex rheology). To improve performance and overcome these limitations, several additives can be used. This literature review aims to summarize the effects of the addition of various additives in co-digestion continuous or semi-continuous reactors to fit as much as possible with collective biogas plant challenges. The addition of (i) microbial strains or consortia, (ii) enzymes and (iii) inorganic additives (trace elements, carbon-based materials) in digester is analyzed and discussed. Several challenges associated with the use of additives for AD process at collective biogas plant scale requiring further research work are highlighted: elucidation of mechanisms, dosage and combination of additives, environmental assessment, economic feasibility, etc.

Heavy Metal Contamination in the Aquatic Ecosystem: Toxicity and Its Remediation Using Eco-Friendly Approaches

Urbanization and industrialization are responsible for environmental contamination in the air, water, and soil. These activities also generate large amounts of heavy metal ions in the environment, and these contaminants cause various types of health issues in humans and other animals. Hexavalent chromium, lead, and cadmium are toxic heavy metal ions that come into the environment through several industrial processes, such as tanning, electroplating, coal mining, agricultural activities, the steel industry, and chrome plating. Several physical and chemical methods are generally used for the heavy metal decontamination of wastewater. These methods have some disadvantages, including the generation of secondary toxic sludge and high operational costs. Hence, there is a need to develop a cost-effective and eco-friendly method for the removal of heavy metal ions from polluted areas. Biological methods are generally considered eco-friendly and cost-effective. This review focuses on heavy metal contamination, its toxicity, and eco-friendly approaches for the removal of heavy metals from contaminated sites.

Effects of modification and co-aging with soils on Cd(II) adsorption behaviors and quantitative mechanisms by biochar

In this study, original and two KMnO₄-modified rice straw biochars (pre- and postmodification) were prepared, which were all pyrolysed at 400 °C. Premodified biochar had the largest Cd adsorption capacity, strongest acid and solute buffering capacity, which benefited from the increase of carbonate content, specific surface area, and the emergence of Mn(II) and MnO_x through modification. Original and premodified biochars were then conducted four types of aging process by an improved three-layer mesh method, namely, aging without soil and co-aging with acid (pH = 5.00), neutral (pH = 7.00), and alkaline (pH = 8.30) soils. The adsorption capacities of modified biochar were always larger than those of original biochar after aging processes. After four aging processes, Cd(II) adsorption capacities were basically in the order of aged biochar without soil > biochar co-aged with alkaline soil > biochar co-aged with neutral soil > biochar co-aged with acid soil, and KMnO₄-modified biochar was always better than original biochar after co-aging with soils. The dominant adsorption mechanism of original and premodified biochars (fresh and aged) for Cd(II) was all the precipitation and adsorption with minerals (accounted for 58.55 ~ 85.55%). In this study, we highlighted that biochar remediation for Cd should be evaluated by co-aging with soil instead of aging without soil participation.

Modified chicken manure biochar enhanced the adsorption for Cd2+ in aqueous and immobilization of Cd in contaminated agricultural soil

Biochar is thought to be good sorbent for heavy metal and exploring ways to increase the efficiency of heavy metal adsorption by biochar is of great importance. Chicken manure biochar was modified with sulfur, hydroxyapatite and MnFe₂O₄ respectively. The properties and composition of the pristine and modified biochar was characterized. The pH and ash content of biochar was significantly increased after modification. Energy dispersive spectroscopy results showed that biochar modified with sulfur, hydroxyapatite and MnFe₂O₄ was successfully loaded on S, Ca/P and Fe/Mn respectively. The adsorption kinetic of Cd²⁺ absorption by pristine and modified biochar was better fitted by the pseudo second-order kinetic model, suggesting that the adsorption of Cd²⁺ on biochar followed the process of chemisorption. The Cd²⁺ adsorption isotherms of sulfur modified chicken manure biochar (SCMB), hydroxyapatite modified chicken manure biochar (HCMB) and MnFe₂O₄ modified chicken manure biochar (FMCMB) was better fitted by Freundlich model, while the Cd²⁺ adsorption by pristine chicken manure biochar (CMB) was well fitted by Langmuir model. The maximum Cd²⁺ adsorption capacity of SCMB, HCMB, FMCMB and CMB was 188.20, 111.53, 109.94 and 19.65 mg·g^-1 respectively. Quantitative analysis of Cd²⁺ adsorption mechanism by biochar showed that the contribution of ion exchange for Cd²⁺ adsorption of CMB accounted for 58 %, while SCMB, HCMB and FMCMB decreased to only 12 %, 8 % and 4 % respectively. Meanwhile, the contribution of precipitation, complexion and metal-Cπ coordination for Cd²⁺ adsorption increased after modification. Pot experiment showed that application of SCMB significantly increased soil pH value, decreased the bioavailable Cd in soil and Cd uptake by brassica chinensis shoots, suggesting that SCMB can be a potential material for the safety use of Cd contaminated agricultural soil.

Comparison of cadmium adsorption by hydrochar and pyrochar derived from Napier grass

Biochar (e.g. pyrochar and hydrochar) is considered a promising adsorbent for Cd removal from aqueous solution. Considering the vastly different physicochemical properties between pyrochar and hydrochar, the Cd²⁺ sorption capacity and mechanisms of pyrochars and hydrochars should be comparatively determined to guide the production and application of biochar. In this study, the hydrochars and pyrochars were prepared from Napier grass by hydrothermal carbonization (200 and 240 °C) and pyrolysis (300 and 500 °C), respectively, and the physicochemical properties and Cd²⁺ sorption performances of biochars were systematically determined. The pyrochars had higher pH and ash content as well as better stability, while the hydrochars showed more oxygen-containing functional groups (OFGs) and greater energy density. The pseudo second order kinetic model best fitted the Cd²⁺ sorption kinetics data of biochars, and the isotherm data of pyrochar and hydrochar were well described by Langmuir and Freundlich models, respectively. In comparison with hydrochar, the pyrochar exhibited better Cd²⁺ sorption capacity (up to 71.47 mg/g). With increasing production temperature, the Cd²⁺ sorption capacity of pyrochar elevated, while the reduction was found for hydrochar. The mineral interaction, complexation with surface OFGs, and coordination with π electron were considered the main mechanisms of Cd²⁺ removal by biochars. The minerals interaction and the complexation with OFGs was the dominant mechanism of Cd²⁺ removal by pyrochars and hydrochars, respectively. Therefore, the preparation technique and temperature have significant impacts on the sorption capacity and mechanisms of biochar, and pyrochar has better potential for Cd²⁺ removal than the congenetic hydrochar.

Reduced cadmium (Cd) accumulation in lettuce plants by applying KMnO4 modified water hyacinth biochar

In this study, water hyacinth was adopted to prepare biochar followed by modification using KMnO₄. And the modified biochars were applied in Cd contaminated soil, exploring the effects of water hyacinth biochar on lettuce growth, Cd enrichment, soil enzyme activities and microbial changes by pot experiments. Modified biochar application significantly reduced the Cd accumulation in lettuce shoots and roots. Compared to the control, the application of water hyacinth biochar at 1% rate resulted in significant reduction of Cd contents by 40.7% and 33.7% in the shoots and roots of lettuce. Also, the reduction was 33.3% and 20.8% compared with the application rate of unmodified biochar. With the increase of biochar application, the amount of Cd was absorbed by lettuce shoots and roots showing significant reduction of plant Cd accumulation in response to the biochar application rate. Additionally, the lowest available Cd concentration in soil (1.34 mg kg⁻¹) was obtained with the application of modified biochar at 1% rate, which might be the main reason for the lower Cd concentration in lettuce shoot and root parts. Furthermore, structural analysis showed that Cd was fixed on the modified biochar, in a passivated state, by larger specific surface area, more active sites and more stable covalent binding complexes leading to a strong decrease of the available Cd in the soil. Moreover, it was concluded that the increment of the enzyme activities in the soil was up to 2.51 times significantly following the application of modified water hyacinth biochar with 3% amount. Lastly, 16sRNA sequencing showed that biochar addition may lead to changes of microbial structure and abundance in soil.

Marked changes in biochar's ability to directly immobilize Cd in soil with aging: implication for biochar remediation of Cd-contaminated soil

To investigate the change in biochar's ability to directly immobilize Cd in soil, a successive wheat cultivation experiment was conducted. Three biochars with different Cd adsorption mechanisms were added to the soils, and a mesh bag was used to separate the soil particles (> 1 μm) from the biochar. The results showed that the ash contents and anionic contents (CO₃^2- and PO₄^3-) of the biochar decreased with the cultivation time, while the oxygen-containing functional group content and CEC of the biochar increased. As a result, the Cd concentration on biochar decreased, by 68.9% for WBC300, while unstable Cd species (acid soluble and reducible fraction of Cd) on biochar increased with successive cultivation, increasing from 3 to 17% for WBC300 in FS. Correspondingly, the ability of biochar to inhibit Cd accumulation in wheat decreased. The results of this study illustrated that the ability of biochar to directly immobilize Cd in soil is not permanent; it gradually decreases with aging in soil. The adsorption mechanism of Cd on biochar changed from precipitation to complexation, and ion exchange processes could be the main reason.

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.

The mechanistic insights into the leaching behaviors of potentially toxic elements from the indigenous zinc smelting slags under the slag dumping site scenario

Since lager quantities of the zinc (Zn) smelting slags were traditionally dumped at the indigenous Zn smelting sites, the release characterization of potentially toxic elements (PTEs) from the Zn smelting slags under various environmental conditions were of great significance for an environmental risk analysis. The acidification of the Zn smelting slags to pH= 4 and 6 would result in the leaching concentrations of Cd and Mn exceeding the fourth-class standard of surface water quality standard in China (GB3838-2002). Notably, most metals exhibited an amphoteric leaching pattern, where the highest leached concentrations of As, Cd, Cu, Mn, Pb, and Zn were 4.15, 4.21, 140.0, 78.1, 156.9 and 477.0 mg/L, respectively. In addition, the highest release of toxic metals within 96 h reached 0.17 % of As, 3.50 % of Cd, 2.77 % of Cu, 6.92 % of Mn, 0.13 % of Pb, and 2.57 % of Zn, respectively. The combined results of various characterization techniques suggested that the PTEs remobilization effected by rhizosphere-like organic acids were mainly controlled by the precipitation of newly formed Fe, Mn and Al (hydr) oxides and the complexation of organic ligands. The present study results could provide valuable insights into the long-term leaching behaviors of PTEs from the Zn smelting slags to reduce ecological hazard.

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.

The Mechanism of Cu2+ Sorption by Rice Straw Biochar and Its Sorption-Desorption Capacity to Cu2+ in Soil

Copper (Cu) pollution in soils has received considerable research attention globally, and biochar has been widely used as an adsorbent for soil pollution of Cu. However, most of the studies focused on the adsorption capacity of biochar, the bioavailability of Cu absorbed by biochar remains unclear. In this work, rice straw biomass was pyrolyzed under oxygen-limited conditions at 400°C (BC400) and 600°C (BC600), their apparent structure, group characteristics, and basic physical and chemical properties were determined. The isothermal and kinetics adsorption of Cu by BC400 and BC600 were analyzed. A pot experiment was used to evaluate the passivation of Cu in the soil by biochar and the bioavailability of Cu adsorbed by biochar in the soil. The smooth surfaces of BC400 evolved into more rough surfaces for BC600, and both types of surfaces may give active sorption sites for Cu, according to SEM pictures. FTIR analysis suggested that BC600 is endowed with more condensed aromatic carbon structures and more available polar functional groups. The adsorption processes of Cu²⁺ by biochar were better fitted Langmuir equation and pseudo-second-order kinetic model. The adsorption isotherms showed monolayer adsorption of Cu²⁺ on biochar. The maximum adsorption capacities of BC600 and BC400 on Cu²⁺ were 43.75 and 30.70 mg g^-1, respectively. Moreover, the pot experiment showed that BC400 and BC600 not only have a strong "passivation" effect on Cu in soil but also prevent the release of adsorbed Cu. Overall, more aromatic carbon structure, more polar functional groups, and higher pH are associated with BC600's increased Cu immobilization ability in soil.

Application of biochar for minewater remediation: Effect of scaling up production on performance under laboratory and field conditions

Metals discharged from abandoned mines are a major source of pollution in many parts of the world. As a result, there is a growing need for suitable low-cost remediation methods. While a large literature base exists demonstrating the efficacy of biochar to remove metals from solution, most studies are confined to the laboratory. This study examines the effects on the biochar quality when scaling up production from laboratory to pilot scale. Pilot scale biochars were produced using a 600 kg batch pyrolysis reactor, these chars were then deployed in the field using a series of 100 mm × 1200 mm cylindrical treatment cells installed at the point of discharge from an abandoned mine site. Most biochars produced at a pilot removed more zinc under laboratory conditions, however all of the biochars showed a reduced performance when tested in the field, this ranged from a 14% to an 85% reduction depending on the biochar.

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.

Gamma-Ray-Induced Structural Transformation of GQDs towards the Improvement of Their Optical Properties, Monitoring of Selected Toxic Compounds, and Photo-Induced Effects on Bacterial Strains

Structural modification of different carbon-based nanomaterials is often necessary to improve their morphology and optical properties, particularly the incorporation of N-atoms in graphene quantum dots (GQDs). Here, a clean, simple, one-step, and eco-friendly method for N-doping of GQDs using gamma irradiation is reported. GQDs were irradiated in the presence of the different ethylenediamine (EDA) amounts (1 g, 5 g, and 10 g) and the highest % of N was detected in the presence of 10 g. N-doped GQDs emitted strong, blue photoluminescence (PL). Photoluminescence quantum yield was increased from 1.45, as obtained for non-irradiated dots, to 7.24% for those irradiated in the presence of 1 g of EDA. Modified GQDs were investigated as a PL probe for the detection of insecticide Carbofuran (2,2-Dimethyl-2,3-dihydro-1-benzofuran-7-yl methylcarbamate) and herbicide Amitrole (3-amino-1,2,4-triazole). The limit of detection was 5.4 μmol L^-1 for Carbofuran. For the first time, Amitrole was detected by GQDs in a turn-off/turn-on mechanism using Pd(II) ions as a quenching agent. First, Pd(II) ions were quenched (turn-off) PL of GQDs, while after Amitrole addition, PL was recovered linearly with Amitrole concentration (turn-on). LOD was 2.03 μmol L^-1. These results suggest that modified GQDs can be used as an efficient new material for Carbofuran and Amitrole detection. Furthermore, the phototoxicity of dots was investigated on both Gram-positive and Gram-negative bacterial strains. When bacterial cells were exposed to different GQD concentrations and illuminated with light of 470 nm wavelength, the toxic effects were not observed.

A review on the production of P-enriched hydro/bio-char from solid waste: Transformation of P and applications of hydro/bio-char

Phosphorus (P) is a necessary element for plant growth and animal health. Most P utilized by anthropogenic activities is released within the generation of various solid wastes such as sewage sludge, animal manure, and wetland plant, which increase the risk of water contamination. (Hydro)thermal treatment could be employed for solid waste treatment with the production of value-added hydro/bio-char, and the behavior of P during the thermochemical treatment process is critical for the further utilization of hydro/bio-char. This study provides a systematic review of the migration and transformation mechanisms of P during thermochemical treatment of various solid wastes, and special emphasis is given to the potential applications of P-enriched hydro/bio-char. Future challenges and perspectives in the thermal treatment of P-enriched solid waste are presented as well. The distribution and speciation of P were affected by feedstock properties, thermal technique, and reaction conditions, correspondingly affecting hydro/bio-char applications. The derived P-enriched hydro/bio-char was mainly applied as an agricultural soil amendment, P recovery source, and heavy metal sorbent, which could be adjusted by varying treatment process parameters. Additionally, potentially toxic substances, such as heavy metals in the solid waste, should be addressed during the production and application of hydro/bio-char. Overall, the production of P-enriched hydro/bio-char from solid waste is a promising route to simultaneously achieve P reclamation and solid waste treatment.

Removal of Cr(VI) from Wastewater Using Graphene Oxide Chitosan Microspheres Modified with α-FeO(OH)

Graphene oxide and chitosan microspheres modified with α−FeO(OH) (α−FeO(OH)/GOCS) are prepared and utilized to investigate the performance and mechanism for Cr(VI) removal from aqueous solutions and the possibility of Fe secondary pollution. Batch experiments were carried out to identify the effects of pH, mass, and volume ratio (m/v), coexisting ions, time (t), temperature (T), and Cr(VI) initial concentration (C0) on Cr(VI) removal, and to evaluate adsorption kinetics, equilibrium isotherm, and thermodynamics, as well as the possibility of Fe secondary pollution. The results showed that Cr(VI) adsorption increased with C0, t, and T but decreased with increasing pH and m/v. Coexisting ions inhibited Cr(VI) adsorption, and this inhibition increased with increasing concentration. The influence degrees of anions and cations on the Cr(VI) adsorption in descending order were SO42− > PO42− > NO3− > Cl− and Ca2+ > Mg2+ > Mn2+, respectively. The equilibrium adsorption capacity of Cr(VI) was the highest at 24.16 mg/g, and the removal rate was 97.69% under pH = 3, m/v = 1.0 g/L, T = 298.15 K, and C0 = 25 mg/L. Cr(VI) adsorption was well fitted to a pseudo-second-order kinetic model and was spontaneous and endothermic. The best fit of Cr(VI) adsorption with the Langmuir and Sips models indicated that it was a monolayer and heterogeneous adsorption. The fitted maximum adsorption capacity was 63.19 mg/g using the Sips model under 308.15 K. Cr(VI) removal mainly included electrostatic attraction between Cr(VI) oxyanions with surface Fe−OH2+, and the adsorbed Cr(VI) was partially reduced to Cr(III) and then precipitated on the surface. In addition, there was no Fe secondary pollution during Cr(VI) adsorption.

Effect of oxidative aging of biochar on relative distribution of competitive adsorption mechanism of Cd2+ and Pb2

In this study, aged biochar (CCB350 and CCB650) were obtained from pyrolysis of corn stalk biochar (CB350 and CB650) at the degree of 350 °C and 650 °C by artificial oxidation with hydrogen peroxide (H₂O₂). Also, the mechanism of Pb²⁺ and Cd²⁺ on fresh and aged biochars was analyzed qualitatively and quantitatively by batch adsorption experiments combined with characterization. The adsorption isotherm results showed that aging treatment decreased the adsorption capacity of Pb²⁺ and Cd²⁺ and inhibited the competitive adsorption behavior of heavy metals. In the single-metal system, precipitation and cation exchange were considered as the main adsorption mechanisms for CB350 and CB650, with a ratio of 40.07–48.23% and 38.04–57.19%, respectively. Competition between Pb²⁺ and Cd²⁺ increased the relative contribution of mineral precipitation, but decreased the contribution of cation exchange mechanism. Aging resulted in the rise of the contribution of surface complexation to the adsorption of Pb²⁺ and Cd²⁺ on biochars, especially in low-temperature biochars, but weakened the contribution of mineral precipitation to the adsorption. Further, the contribution of other adsorption mechanisms was significantly enhanced for high-temperature aged biochars. These results are important to evaluate its long-term application prospects in the natural environment.

Simultaneous Removal of Cu2+, Cd2+ and Pb2+ by Modified Wheat Straw Biochar from Aqueous Solution: Preparation, Characterization and Adsorption Mechanism

As an eco-friendly and efficient adsorbent for removal of potential toxic metals from aqueous solution, biochar has received widespread attention. In the present study, wheat straw biochar (BC) and corresponding modified biochar (HNC) were used to remove Cu²⁺, Cd²⁺ and Pb²⁺ from an aqueous solution. The influence of the environment factors on metals adsorption and adsorption mechanism were discussed in detail. The results showed that the HNC had porous structures and owned ample functional groups (-OH, -COOH and C-N groups) compared with the BC. In the single system, the adsorption capacities of HNC for Cu²⁺, Cd²⁺ and Pb²⁺ at a pH of 5.5 were 18.36, 22.83 and 49.38 mg/g, which were 76.89%, 164.36% and 22.75% higher than that of the BC, respectively. In addition, the adsorption process of Cu²⁺ and Cd²⁺ on BC and HNC fitted to the Langmuir isotherm model and pseudo-second-order kinetics, but the adsorption of Pb²⁺ on BC and HNC fitted to the Langmuir isotherm model and pseudo-first-order kinetics. Adsorption isotherms indicated that the adsorption of Cu²⁺, Cd²⁺ and Pb²⁺ by BC and HNC was a spontaneous endothermic process. The competitive adsorption of mixed metal ions (Cu²⁺, Cd²⁺ and Pb²⁺) revealed that HNC was more preferential to adsorb Cu²⁺ compared with Cd²⁺ and Pb²⁺. Furthermore, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses revealed that the main adsorption mechanisms were surface complexation and precipitation, and the adsorbed Cu²⁺, Cd²⁺ and Pb²⁺ on HNC mainly exist as CuO, Cd(OH)₂, Pb₃O₄ and Pb(OH)₂.

Enhancing Cd(II) adsorption on rice straw biochar by modification of iron and manganese oxides

Metal oxide-modified biochar showed excellent adsorption performance in wastewater treatment. Iron nitrate and potassium permanganate were oxidative modifiers through which oxygen-containing groups and iron-manganese oxides could be introduced into biochar. In this study, iron-manganese (Fe-Mn) oxide-modified biochar (BC-FM) was synthesized using rice straw biochar, and the adsorption process, removal effect, and the mechanism of cadmium (Cd) adsorption on BC-FM in wastewater treatment were explored through batch adsorption experiments and characterization (SEM, BET, FTIR, XRD, and XPS). Adsorption kinetics showed that the maximum adsorption capacity of BC-FM for Cd(II) was 120.77 mg/g at 298 K, which was approximately 1.5-10 times the amount of adsorption capacity for Cd(II) by potassium-modified or manganese-modified biochar as mentioned in the literature. The Cd(II) adsorption of BC-FM was well fit by the pseudo-second-order adsorption and Langmuir models, and it was a spontaneous and endothermic process. Adsorption was mainly controlled via a chemical adsorption mechanism. Moreover, BC-FM could maintain a Cd removal rate of approximately 50% even when reused three times. Cd(II) capture by BC-FM was facilitated by coprecipitation, surface complexation, electrostatic attraction, and cation-π interaction. Additionally, the loaded Fe-Mn oxides also played an important role in the removal of Cd(II) by redox reaction and ion exchange in BC-FM. The results suggested that BC-FM could be used as an efficient adsorbent for treating Cd-contaminated wastewater.

Effects of rice straw biochar on methanogenic bacteria and metabolic function in anaerobic digestion

Anaerobic digestion technology mitigates agricultural organic waste pollution, thereby alleviating the energy crisis. Biochar materials increase the utilisation rate of biomass resources and promote the enrichment and growth of microorganisms. Biochar is an effective exogenous additive that stabilises the anaerobic digestion, improves anaerobic digestion efficiency and gas production. Herein, biochar materials were prepared from rice straw utilising the sequencing batch anaerobic digestion process. The biochar microstructure was characterised by scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis, and microbial succession and metabolic pathways were analysed using 16S rRNA sequencing to reveal the molecular mechanisms. Rice straw biochar addition increased gas production during anaerobic fermentation. SEM revealed that numerous cocci and microbacteria became agglomerated and attached to the surface and pores of biochar, which was revealed by BET analysis to be a good habitat for microorganisms. After anaerobic digestion, the specific surface area and total pore volume of biochar decreased. 16S rRNA gene sequencing showed that biochar affected the abundance of certain bacteria and archaea. Biochar had no obvious effect on the function of bacterial flora but inhibited carbohydrate metabolism by bacteria and glycan biosynthesis and metabolism by archaea in the anaerobic fermentation system while promoting lipid metabolism by archaea. Biochar addition inhibited acetic acid production in the anaerobic fermentation system and promoted methane production based on hydrogen and carbon dioxide levels.

Chitosan-modified biochar: Preparation, modifications, mechanisms and applications

The chitosan-modified biochar composite, as a carbohydrate polymer, has received increasing attention and becomes a research hotspot. It is a promising impurity adsorption material, which has potential application value in the agricultural environment fields such as soil improvement and sewage purification. The composite can combine the advantages of biochar with chitosan, and the resulting composite usually exhibits a great improvement in its surface functional groups, adsorption sites, stability, and adsorption properties. In addition, compared to other adsorbents, the composite truly achieves the concept of "waste control by waste". In this paper, the preparation method, composite classification, adsorption mechanism, and models of biochar modified by chitosan are introduced, meanwhile, we also review and summarize their effects on the decontamination of wastewater and soil. In addition to common heavy metal ions, we also review the adsorption and removal of some other organic/inorganic pollutants, including (1) drug residues; (2) dyes; (3) phosphates; (4) radionuclides; (5) perfluorochemicals, etc. Moreover, challenges and prospects for the composite are presented and further studies are called for the chitosan-biochar composite. We believe that the composite will lead to further achievements in the field of environmental remediation.

Biochar from constructed wetland biomass waste: A review of its potential and challenges

Constructed wetland is considered a promising approach for water remediation due to its high efficiency, low operation costs, and ecological benefits, but the large amounts of wetland plant biomass need to be properly harvested and utilized. Recently, wetland plant derived biochar has drawn extensive attention owing to its application potential. This paper provides an updated review on the production and characteristics of wetland plant derived biochar, and its utilization in soil improvement, carbon sequestration, environmental remediation, and energy production. In comparison to hydrothermal carbonization and gasification, pyrolysis is a more common technique to convert wetland plant to biochar. Characteristics of wetland plant biochars varied with plant species, growth environment of plant, and preparation conditions. Wetland plant biochar could be a qualified soil amendment owing to its abundant nutrients. Notably, wetland plant biochar exhibited considerable sorption capacity for various inorganic and organic contaminants. However, the potentially toxic substances (e.g. heavy metal and polycyclic aromatic hydrocarbons) retained in wetland plant biochar should be noticed before large-scale application. To overcome the drawbacks from the scattered distribution, limited productivity, and seasonal operation of constructed wetlands, the economic feasibility of wetland plant biochar production system could be improved via using mobile pyrolysis unit, utilizing local waste heat, and exploiting all the byproducts. Future challenges in the production and application of wetland plant derived biochar include the continuous supply of feedstock and proper handling of potentially hazardous components in the biochar.

Study on the regulation mechanism of cadmium adsorption system mediated by extraneous dissolved organic matter

Adsorption of biochar on heavy metals is one of the hot spots in the application of biochar. However, the mediation of existing extraneous substances in the environment, such as dissolved organic matter (DOM), could regulate and affect the heavy metals adsorption process on biochar. In our study, we mainly focus on the regulation mechanism of modified biochar on the adsorption process of cadmium mediated by exogenous DOM. The modification significantly changed the functional groups composition on biochar, thus improving the adsorption capacity of cadmium on biochar. In the adsorption system concerned, the combination was formed between DOM and cadmium to a certain extent. The combination had a certain correlation with the influence on the adsorption capacity of cadmium onto biochar in the system.

Sustainable remediation of hazardous environmental pollutants using biochar-based nanohybrid materials

Biochar is a well-known carbon material with diversified functionalities and excellent physicochemical characteristics with high wastewater treatment potential. This review aims to summarize recent advancements in the development of biochar and biochar-based nanohybrid materials as a potential tool for the removal of harmful organic compounds such as synthetic dyes/effluents. The formation of biochar using pyrolysis of renewable feedstocks and their applications in various industries are explained hereafter. The characteristics and construction of biochar-based hybrid materials are explained in detail. Diversity of feedstocks, including municipal wastes, industrial byproducts, agricultural, and forestry residues, endows different biochar types with a wide structural variety. The production of cost-effective biochar drives the interest in manipulating biochars and induces desire functionality using nanoscale reinforcements. Various types of biochars, such as magnetic biochar, layered nanomaterial coated biochar, nanometallic oxide composites, chemically and physically functionalized biochar, have been produced. With the aid of nanomaterial, hybrid biochar exhibits a high potential to remove toxic contaminants. Depending upon biochar type, dyes/effluents can be removed via different mechanisms, including the Fenton process, photocatalytic degradation, π-π interaction, electrostatic interaction, and physical adsorption. In conclusion, desired physicochemical features, and tunable surface properties of biochar present high potential material in removing organic dyes and other effluents. The blended biochar with different materials/nanomaterials endows broader development and multi-functional opportunities for treating dyes/effluents.

Analysis of the Cd(II) Adsorption Performance and Mechanisms by Soybean Root Biochar: Effect of Pyrolysis Temperatures

As one of the most harmful environmental pollutants, cadmium (Cd) has arisen much interest, and many researches have been carried out to study the adsorption of heavy metals by biochar, but the mechanisms were poorly explored and the roles components in biochar played are still indistinct. In this study, we evaluated the adsorption capacities and mechanisms of soybean root biochar pyrolyzed at four different temperatures. The results indicate the biochar properties are significantly determined by pyrolysis temperature, which affects the removal mechanisms of Cd(II) consequently. Microstructure characteristics and mechanism analysis further suggest that Cd(II)-π interactions and sulfur-containing functional groups are the main mechanisms of Cd(II) adsorption. This work shows a new perspective to explain the adsorption mechanisms onto biochar adsorbents and has a benefit for the exploitation of economical and effective adsorbents for Cd(II) removal based on biochars.

Double-edged effects of polyvinyl chloride addition on heavy metal separation and biochar production during pyrolysis of Cd/Zn hyperaccumulator

Pyrolysis is a promising technique to achieve the sustainable utilization of heavy metal hyperaccumulator derived from phytoremediation of contaminated soils. To investigate the feasibility of synergistic treatment of hyperaccumulator and plastic waste (i.e. polyvinyl chloride, PVC), co-pyrolysis of Sedum alfredii and different mass percentages of PVC (5-25 wt%) was conducted at 300-900 °C in the present study. High pyrolysis temperature and low PVC addition amount (5 wt%) effectively promoted the volatilization of Cd and Zn from S. alfredii, while high PVC addition amount (15 wt% and 25 wt%) caused a significant suppression effect at insufficient pyrolysis temperatures. After PVC addition, the yields of biochar increased by 5.18-37.19% as compared with the theoretical values. However, the concentrations of Cd and Zn leached from biochar significantly elevated with increasing PVC addition amount, indicating that the addition of PVC improved the mobility of Cd and Zn in biochar. Moreover, S. alfredii derived biochars showed considerable sorption capacity for Cd (87.6-198.3 mg/g). These results imply that the addition of PVC has double-edged effects on heavy metal separation and biochar production during pyrolysis of Cd/Zn hyperaccumulator, and low PVC addition amount and sufficient pyrolysis temperature are beneficial for the further utilization of biochar.

Characterization of pruned tea branch biochar and the mechanisms underlying its adsorption for cadmium in aqueous solution

In the present study, discarded pruned tea branch was used to prepare a new biochar, and the physicochemical properties and adsorption characteristics were investigated by characterization and batch experiments. With increasing pyrolysis temperature from 400 to 800 °C, the yield, specific surface area, and acidic functional groups had significant differences. The optimum adsorption conditions were determined as pH = 6 and dosage of 2 g L-1. The pseudo-second-order kinetic and Langmuir isothermal model could fit well to the adsorption data, which showed that the adsorption process was dominated by monolayer chemical adsorption. The highest adsorption property (74.04 mg g-1) was obtained by the pyrolysis of tea branch biochar (TBB) at 700 °C owing to the adsorption mechanisms, including surface complexation, precipitation, metal ion exchange, and Cd2+-π interaction. After five cycles of desorption, biochar still showed superior adsorption (80%). Hence, the TBB acted as a regenerable adsorbent for treating Cd2+-containing wastewater.

Adsorption characteristics and the removal mechanism of two novel Fe-Zn composite modified biochar for Cd(II) in water

In this study, adsorbents (Fe/Zn-RBC and Fe/Zn-DBC) for the removal of Cd(II) in water were successfully prepared by iron/zinc composite modified biochar derived from the branches of Robinia pseudoacacia biochar (RBC) and durian shells biochar (DBC). The results revealed that the iron and zinc ions were successfully loaded onto the biochar. The adsorption data of Cd(II) on Fe/Zn-BC conformed to the models of pseudo-second-order kinetic, Langmuir isothermal, and Redlich-Paterson. According to the results of batch experiments, the maximum sorption capacities of Fe/Zn-RBC and Fe/Zn-DBC for Cd(II) were approximately five times and three times higher than RBC and DBC, respectively. As the most dominant adsorption mechanisms, Cd(II) and CO₃^2-, Fe-O, Zn-O, and oxygen-containing functional groups on the Fe/Zn-BC surfaces precipitated CdCO₃, Cd(OH)₂, and CdO. Therefore, Fe/Zn-BC is an excellent adsorbent that removes Cd(II) from aqueous solutions, and also can be used in waste resource utilization, which has potential applications prospects.

The mechanism transformation of ramie biochar's cadmium adsorption by aging

Aging is inevitable when biochar uses for remediate Cadmium (Cd) pollution, but the variation of adsorption mechanism remains unclear. This study uses ramie residue to prepare fresh biochar, and adopt it with acidification and oxidation to simulate the aging process. The difference of physicochemical properties between fresh and aged biochar are studied through microstructure. Then, two kinds of biochar are making adsorption experiments in Cd solution for analyzing their adsorption mechanism. The results show that, both chemisorption and physisorption are exist, chemisorption and physisorption is the predominant way of fresh biochar and aged biochar respectively. Cation exchange is important but weaker in aged biochar than fresh biochar. Carboxyl plays a leading role in complexation of fresh biochar and hydroxy in aged biochar. Coprecipitation and cation-π mechanism impair apparently in aged biochar. This study indicates that aging change ramie biochar's main adsorption mechanism and the primary chemisorption way.

Preparation of magnetic hydrochar derived from iron-rich Phytolacca acinosa Roxb. for Cd removal

Considering that hyperaccumulators can accumulate high concentrations of iron salt, they can successfully obtain magnetic hydrochar from iron-rich hyperaccumulators. In this study, iron-rich biomass was obtained by irrigating Phytolacca acinosa Roxb. using iron salt. Magnetic nano-Fe₃O₄ hydrochar was prepared from iron-rich Phytolacca acinosa Roxb. via hydrothermal carbonization to remove Cd. The characterization results showed that the synthesized magnetic nanoparticles had an average size of 2.62 ± 0.56 nm and N elements were doped into magnetic nano-Fe₃O₄ hydrochar with abundant oxygenic groups. Cd adsorption on magnetic nano-Fe₃O₄ hydrochar was better fitted using the Langmuir isotherm and the pseudo-second-order kinetic model. The maximum adsorption capacity was 246.6 mg g^-1 of Cd. The research confirmed that Cd adsorption was controlled by multiple mechanisms from the jar test, transmission electron microscopy mapping, scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. CdCO₃ crystals can be formed after adsorption, indicating that surface precipitation played an important role in Cd adsorption. The abundance of O atoms and the doping of N atoms on the hydrochar surface were conducive to Cd adsorption, indicating that the mechanisms were related to surface complexation and electrostatic attraction. In addition, the significant decrease in Na⁺ content after Cd adsorption illustrated that ion exchange had a non-negligible effect on Cd adsorption. This study not only provides a strategy for preparing magnetic nano-Fe₃O₄ hydrochar derived from iron-rich plants but also verifies multiple Cd adsorption mechanisms using magnetic nano-Fe₃O₄ hydrochar.

Removal of Cd from aqueous solution by chitosan coated MgO-biochar and its in-situ remediation of Cd-contaminated soil

In this study, magnesium oxide biochar-chitosan composite (MgO-BCR-W) prepared through modification with MgCl₂ and chitosan was investigated as an adsorbent for the removal of Cd from the aquatic and soil environment. Kinetic and thermodynamics revealed that the Cd(II) adsorption onto MgO-BCR-W was well fitted by pseudo-second-order and the Langmuir adsorption isotherm. The adsorption capacities of rice husk biochar (BCR) and MgO-BCR-W for Cd(II) reached 11.09 mg/g and 59.66 mg/g, respectively. Attractively, the computed values of R_L ranged between 0 and 1, suggesting that the adsorption of Cd(II) onto MgO-BCR-W is favourable. Characterisations of the adsorbents revealed that the synergistic effect of surface complexation and precipitation mechanisms played a major role in the removal of Cd. In soil incubation experiment, the addition of MgO-BCR-W at the level of 2% was most effective in Cd stabilization compared to the control, which reduced the content of bioavailable Cd by 22.32%. Furthermore, it reduced the acid extractable Cd by 24.77%, while increased the residual Cd content by 22.24%. The results demonstrated that MgO-BCR-W could be used as an effective and eco-friendly adsorbent for Cd remediation in both water and soil environment.

Adsorption characteristics and mechanisms of Pb2+ and Cd2+ by a new agricultural waste-Caragana korshinskii biomass derived biochar

In order to explore the comprehensive utilisation and recycling technology of Caragana korshinskii resources, a new agricultural biomass waste, 15 kinds of Caragana korshinskii biochar (CB) were prepared by controlling the pyrolysis temperature and time at the anaerobic environment. Moreover, we pay more attention to deriving the adsorption mechanisms and exploring the difference in adsorption characteristics of Pb²⁺ and Cd²⁺. The optimal preparation conditions and the batch adsorption experiments were evaluated, and the adsorption characteristics and mechanisms were discussed using 8 theoretical adsorption models and multiple characterisation methods. The results showed that the CB prepared at 650 °C for 3 h presented the best performance. The Langmuir and Freundlich models can well simulate the isotherm adsorption process of CB for Pb²⁺ and Cd²⁺, respectively. The adsorption kinetics of CB for Pb²⁺ and Cd²⁺ were best fitted by the pseudo-second-order model. The adsorption equilibrium for Pb²⁺ and Cd²⁺ was reached within 3 h, and their maximum adsorption capacity reached 220.94 mg g^-1 and 42.43 mg g^-1, respectively. In addition, the best addition amount was 3 g L^-1 and 2.2 g L^-1 for Pb²⁺ and Cd²⁺, respectively. The optimum pH range was 3-6 for Pb²⁺ and 6-7.5 for Cd²⁺. The adsorption mechanisms of CB for Pb²⁺ and Cd²⁺ were physicochemical composite adsorption processes, mainly including physical sorption on surface sites, intraparticle diffusion, electrostatic adsorption, ion/ligand exchange, cationic-π interactions, surface complexation and precipitation. Furthermore, the ash of CB also presented a positive effect on the adsorption of Pb²⁺. Compared with other cellulose- and lignin-based biomass materials, CB showed low cost and efficient performance without complicated modification conditions. Therefore, this study demonstrates that CB is a promising raw material in water pollution control to immobilise heavy metals.

Enhanced electro-Fenton degradation of sulfonamides using the N, S co-doped cathode: Mechanism for H2O2 formation and pollutants decay

Facing low reactivity/selectivity of oxygen reduction reaction (ORR) in electro-Fenton (EF), N, S atoms were introduced into carbon-based cathode. "End-on" O₂ adsorption was achieved by adjusting electronic nature via N doping, while *OOH binding capability was tuned by spin density variation via S doping. Results showed the optimized N, S co-doped cathode presented a 42.47% improvement of H₂O₂ accumulation (7.95 ± 0.02 mg L^-1 cm^-2). According to density functional theory (DFT), N, S co-doped structure favored the "end-on" O₂ adsorption as adsorption energy dropped to - 2.24 eV. Moreover, O-O/C-O bond lengths variation proved a possibility for *OOH desorption. The elaborated cathode was used in EF for sulfonamides (SAs) decay. A 100% removal rate of sulfadiazine (SDZ), sulfathiazole (STZ) and sulfadimethoxine (SDM) was achieved within 60 min, among which SDZ tended to be degraded easily. Because the absolute hardness (η) of those pollutants is ranked as follows: η_SDM> η_STZ> η_SDZ. Degradation pathways were proposed based on the detected byproducts, along with toxicity was evaluated by ecological structure-activity relationship (ECOSAR) program. Results showed that toxic intermediates generated were reduced or even disappeared. EF with N, S co-doped cathode provides a promising process for antibiotics wastewater treatment.

Cd(II) and Pb(II) Adsorption Using a Composite Obtained from Moringa oleifera Lam. Cellulose Nanofibrils Impregnated with Iron Nanoparticles

This work informs on the green synthesis of a novel adsorbent and its adsorption capacity. The adsorbent was synthesized by the combination of iron nanoparticles and cellulose nanofibers (FeNPs/NFCs). Cellulose nanofibers (NFCs) were obtained from Moringa (Moringa oleifera Lam.) by a pulping Kraft process, acid hydrolysis, and ultrasonic methods. The adsorption method has advantages such as high heavy metal removal in water treatment. Therefore, cadmium (Cd) and lead (Pb) adsorption with FeNP/NFC from aqueous solutions in batch systems was investigated. The kinetic, isotherm, and thermodynamic parameters, as well as the adsorption capacities of FeNP/NFC in each system at different temperatures, were evaluated. The adsorption kinetic data were fitted to mathematical models, so the pseudo-second-order kinetic model described both Cd and Pb. The kinetic rate constant (K2), was higher for Cd than for Pb, indicating that the metal adsorption was very fast. The adsorption isotherm data were best described by the Langmuir–Freundlich model for Pb multilayer adsorption. The Langmuir model described Cd monolayer sorption. However, experimental maximum adsorption capacities (qe exp) for Cd (>12 mg/g) were lower than those for Pb (>80 mg/g). In conclusion, iron nanoparticles on the FeNP/NFC composite improved Cd and Pb selectivity during adsorption processes, indicating the process’ spontaneous and exothermic nature.