Application of co-pyrolysis biochar for the adsorption and immobilization of heavy metals in contaminated environmental substrates

Dual magnetization and amination of cellulosic chains for the efficient adsorption of heavy metals

Compounds functionalized with hydroxyl and amino groups were found to have good potential for the adsorption of different ions. In this work, a new system of cellulosic chains was amended with amine substitutions and bonded to a magnetic core of NiFe2O4@SiO2 to form NiFe2O4@SiO2-cellulose-NH2 system. The prepared sample showed suitable magnetic separation and was characterized via XRD, FT-IR, SEM, EDS, and TGA-DTA analyses. The adsorption potential of NiFe2O4@SiO2-cellulose-NH2 system has been investigated on the heavy metals (Cd, Ni, and Pb) removal from a synthetic wastewater environment. The results show that the magnetic property created by the magnetic core increased the recycling potential of the adsorbent and the magnetic core has a positive effect on the absorption potential of the polymer. The adsorption removal of Cd(II), Ni(II), and Pb(II) ions was studied using NiFe2O4@SiO2-cellulose-NH2 systems in different pH, temperatures, metal ion concentrations, and adsorbent dosages. The maximum adsorption capacities of single heavy metal ions were obtained as 406.44 mg/g (for Cd(II) ions), 411.63 mg/g (for Ni(II) ions), and 414.68 mg/g (for Pb(II) ions) under optimized conditions as pH = 6.5, ion concentration: 500 mg/L, adsorbent dosage: 1.2 g/L and room temperature.

Investigating the carcinogenic and non-carcinogenic health hazards of heavy metal ions in Spinacia oleracea grown in agricultural soil treated with biochar and humic acid

This study addressed the bioaccumulation and human health risk among the consumption of Spinacia oleracea grown in agricultural soil treated with humic acid (189-2310 ppm) and biochars (0.00-5.10%.wt). The biochars came from two local feedstocks of rice-husk (RH) and sugar-beet-pulp (SBP) pyrolyzed at temperatures 300 and 600 °C. Total concentrations of Cu, Cd, and Ni found in both the soil and biomass/biochar exceeded global safety thresholds. The bioaccumulation levels of HMs in spinach leaves varied, with Fe reaching the highest concentration at 765.27 mg kg-1 and Cd having the lowest concentration at 3.31 mg kg-1. Overall, the concentrations of Zn, Cd, Pb, and Ni in spinach leaves exceeded the safety threshold limits, so that its consumption is not recommended. The assessment of hazard quotient (HI) for the HMs indicated potential health hazards for humans (HI > 1) from consuming the edible parts of spinach. The biochar application rates of 4.35%wt and 0.00%.wt resulted in the highest (3.69) and lowest (3.15) HI values, respectively. The cumulative carcinogenic risk (TCR) ranged from 0.0085 to 0.0119, exceeding the cancer risk threshold. Introducing 5.10%wt biomass/biochar resulted in a 36% rise in TCR compared to the control. The utilization of humic acid alongside HMs-polluted biochars results in elevated levels of HMs bioaccumulation exceeding the allowable thresholds in crops (with a maximum increase of 49% at 2000 ppm humic acid in comparison to 189 ppm). Consequently, this raised the HI by 46% and the TCR by 22%. This study demonstrated that the utilization of HMs-polluted biochars could potentially pose supplementary health hazards. Moreover, it is evident that the utilization of HMs-polluted biochars in treating metal-contaminated soil does not effectively stabilize or reduce pollution.

Sustainable Valorisation of Sewage Sludge via Carbon Dioxide-Assisted Pyrolysis

The escalating volume of sewage sludge (SS) generated poses challenges in disposal, given its potential harm to the environment and human health. This study explored sustainable solutions for SS management with a focus on energy recovery. Employing CO2-assisted pyrolysis, we converted SS into flammable gases (H2 and CO; syngas). Single-stage pyrolysis of SS in a CO2 conditions demonstrated that CO2 enhances flammable gas production (especially CO) through gas phase reactions (GPRs) with volatile matter (VM) at temperatures ≥ 520 ˚C. Specifically, the CO2 partially oxidized the VM released from SS and concurrently underwent reduction into CO. To enhance the syngas production at temperatures ≤ 520 ˚C, multi-stage pyrolysis setup with additional heat energy and a Ni/Al2O3 catalyst were utilized. These configurations significantly increased flammable gas production, particularly CO, at temperatures ≤ 520 ˚C. Indeed, the flammable gas yield in the catalytic pyrolysis of SS increased from 200.3 mmol under N2 conditions to 219.2 mmol under CO2 conditions, representing a 4.4-fold increase compared to single-stage pyrolysis under CO2 conditions (50.0 mmol). By integrating a water-gas-shift reaction, the flammable gases produced from CO2-assisted catalytic pyrolysis were expected to have the potential to generate revenue of US$4.04 billion. These findings highlight the effectiveness of employing CO2 in SS pyrolysis as a sustainable and effective approach for treating and valorising SS into valuable energy resources.

A systematic review of biochar aging and the potential eco-environmental risk in heavy metal contaminated soil

Biochar is widely accepted as a green and effective amendment for remediating heavy metals (HMs) contaminated soil, but its long-term efficiency and safety changes with biochar aging in fields. Currently, some reviews have qualitatively summarized biochar aging methods and mechanisms, aging-induced changes in biochar properties, and often ignored the potential eco-environmental risk during biochar aging process. Therefore, this review systematically summarizes the study methods of biochar aging, quantitatively compares the effects of different biochar aging process on its properties, and discusses the potential eco-environmental risk due to biochar aging in HMs contaminated soil. At present, various artificial aging methods (physical aging, chemical aging and biological aging) rather than natural field aging have been applied to study the changes of biochar's properties. Generally, biochar aging increases specific surface area (SSA), pore volume (PV), surface oxygen-containing functional group (OFGs) and O content, while decreases pH, ash, H, C and N content. Chemical aging method has a greater effect on the properties of biochar than other aging methods. In addition, biochar aging may lead to HMs remobilization and produce new types of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs) and colloidal/nano biochar particles, which consequently bring secondary eco-environmental risk. Finally, future research directions are suggested to establish a more accurate assessment method and model on biochar aging behavior and evaluate the environmental safety of aged biochar, in order to promote its wider application for remediating HMs contaminated soil.

Heavy metal immobilization and radish growth improvement using Ca(OH)2-treated cypress biochar in contaminated soil

Heavy metal contamination poses a significant threat to soil quality, plant growth, and food safety, and directly affects multiple UN SDGs. Addressing this issue and offering a remediation solution are vital for human health. One effective approach for immobilizing heavy metals involves impregnating cypress chips with calcium hydroxide (Ca(OH)2) to enhance the chemical adsorption capacity of the resulting woody charcoal. In the present study, un-treated cypress biochar (UCBC) and calcium-treated cypress biochar (TCBC), were introduced into pristine and contaminated soil, at rates of 3, 6, and 9% (w/w). Both BCs were alkaline (UCBC pH: 8.9, TCBC pH: 9.7) with high specific surface area, which improved the soil properties (pH, EC, and OM). Radish (Raphanus sativus) cultivated in pots revealed that both UCBC and TCBC demonstrated significant improvements in growth attributes and heavy metal immobilization compared to the control, with TCBC exhibiting superior effects. The TCBC surface showed highly active nanosized precipitated calcium carbonate particles that were active in immobilizing heavy metals. The application of TCBC at a rate of 9% resulted in a substantial reduction in Zn and Cu uptake by radish roots and shoots. In contaminated soil, Zn uptake by radish roots decreased by 55% (68.3-31.0 mg kg-1), and shoots by 37% (49.3-31.0 mg kg-1); Cu uptake decreased by 40% (38.6-23.2 mg kg-1) in roots and 39% (58.2-35.2 mg kg-1) in shoots. Uptake of Pb was undetectable after TCBC application. Principal component analysis (PCA) highlighted the potential of TCBC over UCBC in reducing heavy metal concentrations and promoting radish growth. Future research should consider the long-term effects and microbial interactions of TCBC application.

Unveiling biochar potential to promote safe crop production in toxic metal(loid) contaminated soil: A meta-analysis

Biochar application emerges as a promising and sustainable solution for the remediation of soils contaminated with potentially toxic metal(loid)s (PTMs), yet its potential to reduce PTM accumulation in crops remains to be fully elucidated. In our study, a hierarchical meta-analysis based on 276 research articles was conducted to quantify the effects of biochar application on crop growth and PTM accumulation. Meanwhile, a machine learning approach was developed to identify the major contributing features. Our findings revealed that biochar application significantly enhanced crop growth, and reduced PTM concentrations in crop tissues, showing a decrease trend of grains (36.1%, 33.6 to 38.6%) > shoots (31.1%, 29.3 to 32.8%) > roots (27.5%, 25.7 to 29.2%). Furthermore, biochar modifications were found to amplify its remediation potential in PTM-contaminated soils. Biochar application was observed to provide favorable conditions for reducing PTM uptake by crops, primarily through decreasing available PTM concentrations and improving overall soil quality. Employing machine learning techniques, we identified biochar properties, such as surface area and C content as a key factor in decreasing PTM bioavailability in soil-crop systems. Furthermore, our study indicated that biochar application could reduce probabilistic health risks associated with of the presence of PTMs in crop grains, thereby contributing to human health protection. These findings highlighted the essential role of biochar in remediating PTM-contaminated lands and offered guidelines for enhancing safe crop production.

Investigating the adsorption potential of char derived from waste latex for methylene blue removal

This study presents the adsorption of methylene blue (MB) dye using latex char derived from pyrolysis of latex gloves. The adsorption process was investigated systematically using Response Surface Methodology (RSM) with a Central Composite Design (CCD). The effects of four key variables, namely pH, time, temperature, and adsorbent dosage, were studied using a factorial design enriched with center points and axial points. Experimental data were analyzed using a second-order polynomial regression model to construct a response surface model, which elucidated the relationship between the variables and MB removal efficiency. The study found that the char obtained at 800 °C exhibited the highest adsorption capacity due to its increased carbonization, expanded surface area, and diverse pore structure. Analysis of Variance (ANOVA) confirmed the significance of the quadratic model, with remarkable agreement between predicted and experimental outcomes. Diagnostic plots validated the model's reliability, while 3D contour graphs illustrated the combined effects of variables on MB removal efficiency. Optimization using DoE software identified optimal conditions resulting in a 99% removal efficiency, which closely matched experimental results. Additionally, adsorption isotherms revealed that the Freundlich model best described the adsorption behavior, indicating heterogeneous surface adsorption with multilayer adsorption. This comprehensive study provides valuable insights into the adsorption process of MB dye using latex char, with implications for wastewater treatment and environmental remediation.

Amino-modified upcycled biochar achieves selective chromium removal in complex aqueous matrices

Chromium pollution of groundwater sources is a growing global issue, which correlates with various anthropogenic activities. Remediation of both the Cr(VI) and Cr(III), via adsorption technologies, has been championed in recent years due to ease of use, minimal energy requirements, and the potential to serve as a highly sustainable remediation technology. In the present study, a biochar sorbent sourced from pineapple skins, allowed for the upcycling of agricultural waste into water purification technology. The biochar material was chemically modified, through a green amination method, to produce an efficient and selective adsorbent for the removal of both Cr(VI) and Cr(III) from complex aqueous matrices. From FTIR analysis it was evident that the chemical modification introduced new C-N and N-H bonds observed in the modified biochar along with a depletion of N-O and C-H bonds found in the pristine biochar. The amino modified biochar was found to spontaneously adsorb both forms of chromium at room temperature, with binding capacities of 46.5 mg/g of Cr(VI) and 27.1 mg/g of Cr(III). Interference studies, conducted in complex matrices, showed no change in adsorption capacity for Cr(VI) in matrices containing up to 3,000× the concentration of interfering ions. Finally, Cr(III) removal was synergized to 100% adsorption at interfering ions concentrations up to 330× of the analyte, which were suppressed at higher interference concentrations. Considering such performance, the amino modified biochar achieved selective removal for both forms of chromium, showing great potential for utilization in complex chromium pollution sources.

Cadmium immobilization in soil using phosphate modified biochar derived from wheat straw

The phosphate-modified biochar (BC) immobilizes cadmium (Cd), yet little is known about how phosphate species affect Cd detoxification in contaminated soils. We developed phosphate-modified biochar through the pyrolysis of wheat straw impregnated with three types of phosphate: mono‑potassium phosphate (MKP), dipotassium hydrogen phosphate (DKP), and tripotassium phosphate (TKP). The Cd adsorption mechanism of modified biochar was investigated by biochar characterization, adsorption performance evaluation, and soil incubation tests. The results demonstrated that the efficiency of biochar in immobilizing Cd2+ followed the order: TKP-BC > DKP-BC > MKP-BC. The TKP-BC had the highest orthophosphate content, the fastest adsorption rate, and the largest adsorption capacity (Langmuir) of 257.28 mg/g, which is 6.31 times higher than that of the unmodified BC (CK). In contrast, pyrophosphate was predominant in MKP-BC and DKP-BC. The primary adsorption mechanism for Cd2+ was precipitation, followed by cation exchange, as evidenced by the formation of CdP minerals on the BC surface, and an increase of K+ in solution (compared to water-soluble K+) and a decrease of K+ in the biochar during adsorption. Desorption of Cd from the TKP-BC after adsorption was 9.77 %-12.39 % at a pH of 5-9, much lower than that of CK. The soil incubation test showed the diethylenetriaminepentaacetic acid extracted Cd of TKP-BC, MKP-BC, and DKP-BC was reduced by 67.93 %, 18.41 % and 31.30 % over CK, respectively. Using the planar optodes technique, we also found that TKP-BC had the longest effect enhancing in situ soil pH. This study provides a theoretical basis for developing heavy metal pollution control technology using green remediation materials and offers insights into the remediation mechanisms.

Machine learning-based prediction and experimental validation of heavy metal adsorption capacity of bentonite

As a natural adsorbent material, bentonite is widely used in the field of heavy metal adsorption. The heavy metal adsorption capacity of bentonite varies significantly in studies due to the differences in the properties of bentonite, solution, and heavy metal. To achieve accurate predictions of bentonite's heavy metal adsorption capacity, this study employed six machine learning (ML) regression algorithms to investigate the adsorption characteristics of bentonite. Finally, an eXtreme Gradient Boosting Regression (XGB) model with outstanding predictive performance was constructed. Explanation analysis of the XGB model further reveal the importance and influence manner of each input feature in predicting the heavy metal adsorption capacity of bentonite. The feature categories influencing heavy metal adsorption capacity were ranked in order of importance as adsorption conditions > bentonite properties > heavy metal properties. Furthermore, a web-based graphical user interface (GUI) software was developed, facilitating researchers and engineers to conveniently use the XGB model for predicting the heavy metal adsorption capacity of bentonite. This study provides new insights into the adsorption behaviors of bentonite for heavy metals, offering guidance and support for enhancing its application efficiency and addressing heavy metal pollution remediation.

Adsorption of Cadmium and Lead Capacity and Environmental Stability of Magnesium-Modified High-Sulfur Hydrochar: Greenly Utilizing Chicken Feather

Chicken feathers represent a viable material for producing biochar adsorbents. Traditional slow pyrolysis methods often result in sulfur element losses from chicken feathers, whereas hydrothermal reactions generate substantial amounts of nutrient-rich hydrothermal liquor. Magnesium-modified high-sulfur hydrochar MWF was synthesized through magnesium modification, achieving a S content of 3.68%. The maximum equilibrium adsorption amounts of MWF for Cd2+ and Pb2+ were 25.12 mg·g-1 and 70.41 mg·g-1, respectively, representing 4.00 times and 2.75 times of WF. Magnesium modification elevated the sulfur content, pH, ash content, and electronegativity of MWF. The primary mechanisms behind MWF's adsorption of Cd2+ and Pb2+ involve magnesium ion exchange and complexation with C=O/O=C-O, quaternary N, and S functional groups. MWF maintains robust stability and antioxidative properties, even with low aromaticity levels. Given the lower energy consumption during hydrochar production, MWF offers notable carbon sequestration benefits. The hydrothermal solution derived from MWF is nutrient-rich. Following supplementation with inorganic fertilizer, the hydrothermal solution of MWF significantly enhanced bok choy growth compared to the control group. In general, adopting magnesium-modified hydrothermal reactions to produce hydrochar and converting the resultant hydrothermal solution into water-soluble fertilizer proves a viable strategy for the eco-friendly utilization of chicken feathers. This approach carries substantial value for heavy metal remediation and agricultural practices.

Adsorption Efficiency of Cadmium (II) by Different Alkali-Activated Materials

The objective of this study was to demonstrate the potential utilization of fly ash (FA), wood ash (WA), and metakaolin (MK) in developing new alkali-activated materials (AAMs) for the removal of cadmium ions from waste water. The synthesis of AAMs involved the dissolution of solid precursors, FA, WA, and MK, by a liquid activator (Na2SiO3 and NaOH). In concentrated solutions of the activator, the formation of an aluminosilicate gel structure occurred. DRIFT spectroscopy of the AAMs indicated main vibration bands between 1036 cm-1 and 996 cm-1, corresponding to Si-O-Si/Si-O-Al bands. Shifting vibration bands were seen at 1028 cm-1 to 1021 cm-1, indicating that the Si-O-Si/Si-O-Al bond is elongating, and the bond angle is decreasing. Based on the X-ray diffraction results, alkali-activated samples consist of an amorphous phase and residual mineral phases. The characteristic "hump" of an amorphous phase in the range from 20 to 40° 2θ was observed in FA and in all AWAFA samples. By the XRD patterns of the AAMs obtained by the activation of a solid three-component system, a new crystalline phase, gehlenite, was identified. The efficiency of AAMs in removing cadmium ions from aqueous solutions was tested under various conditions. The highest values of adsorption capacity, 64.76 mg/g (AWAFA6), 67.02 mg/g (AWAFAMK6), and 72.84 mg/g mg/g (AWAMK6), were obtained for materials activated with a 6 M NaOH solution in the alkali activator. The Langmuir adsorption isotherm and pseudo-second kinetic order provided the best fit for all investigated AAMs.

Recent Progresses in Pyrolysis of Plastic Packaging Wastes and Biomass Materials for Conversion of High-Value Carbons: A Review

The recycling of plastic packaging wastes helps to alleviate the problems of white pollution and resource shortage. It is very necessary to develop high-value conversion technologies for plastic packaging wastes. To our knowledge, carbon materials with excellent properties have been widely used in energy storage, adsorption, water treatment, aerospace and functional packaging, and so on. Waste plastic packaging and biomass materials are excellent precursor materials of carbon materials due to their rich sources and high carbon content. Thus, the conversion from waste plastic packaging and biomass materials to carbon materials attracts much attention. However, closely related reviews are lacking up to now. In this work, the pyrolysis routes of the pyrolysis of plastic packaging wastes and biomass materials for conversion to high-value carbons and the influence factors were analyzed. Additionally, the applications of these obtained carbons were summarized. Furthermore, the limitations of the current pyrolysis technology are put forward and the research prospects are forecasted. Therefore, this review can provide a useful reference and guide for the research on the pyrolysis of plastic packaging wastes and biomass materials and the conversion to high-value carbon.

Cellulose de-polymerization is selective for bioethanol refinery and multi-functional biochar assembly using brittle stalk of corn mutant

As lignocellulose recalcitrance principally restricts for a cost-effective conversion into biofuels and bioproducts, this study re-selected the brittle stalk of corn mutant by MuDR-transposon insertion, and detected much reduced cellulose polymerization and crystallinity. Using recyclable CaO chemical for biomass pretreatment, we determined a consistently enhanced enzymatic saccharification of pretreated corn brittle stalk for higher-yield bioethanol conversion. Furthermore, the enzyme-undigestible lignocellulose was treated with two-step thermal-chemical processes via FeCl2 catalysis and KOH activation to generate the biochar with significantly raised adsorption capacities with two industry dyes (methylene blue and Congo red). However, the desirable biochar was attained from one-step KOH treatment with the entire brittle stalk, which was characterized as the highly-porous nanocarbon that is of the largest specific surface area at 1697.34 m2/g and 2-fold higher dyes adsorption. Notably, this nanocarbon enabled to eliminate the most toxic compounds released from CaO pretreatment and enzymatic hydrolysis, and also showed much improved electrochemical performance with specific capacitance at 205 F/g. Hence, this work has raised a mechanism model to interpret how the recalcitrance-reduced lignocellulose is convertible for high-yield bioethanol and multiple-function biochar with high performance.

Revolutionizing soil heavy metal remediation: Cutting-edge innovations in plant disposal technology

Soil contamination with heavy metals has emerged as a global environmental threat, compromising agricultural productivity, ecosystem integrity, and human health. Conventional remediation techniques often fall short due to high costs, operational complexities, and environmental drawbacks. Plant-based disposal technologies, including biochar, phytometallurgy, and phrolysis, have emerged as promising solutions in this regard. Grounded in a novel experimental framework, biochar is studied for its dual role as soil amendment and metal adsorbent, while phytometallurgy is explored for its potential in resource recovery and economic benefits derived from harvested metal-rich plant biomass. Pyrolysis, in turn, is assessed for transforming contaminated biomass into value-added products, thereby minimizing waste. These plant disposal technologies create a circular model of remediation and resource utilization that holds the potential for application in large-scale soil recovery projects, development of environmentally friendly agro-industries, and advancement in sustainable waste management practices. This review mainly discussed cutting-edge plant disposal technologies-biochar application, phytometallurgy, and pyrolysis-as revolutionary approaches to soil heavy metal remediation. The efficacy, cost-effectiveness, and environmental impact of these innovative technologies are especially evaluated in comparison with traditional methods. The success of these applications could signal a paradigm shift in how we approach both environmental remediation and resource recovery, with profound implications for sustainable development and circular economy strategies.

Petroleum-contaminated soil bioremediation and microbial community succession induced by application of co-pyrolysis biochar amendment: An investigation of performances and mechanisms

This study aimed to remediate petroleum-contaminated soil using co-pyrolysis biochar derived from rice husk and cellulose. Rice husk and cellulose were mixed in various weight ratios (0:1, 1:0, 1:1, 1:3 and 3:1) and pyrolyzed under 500 °C. These biochar variants were labeled as R0C1, R1C0, R1C1, R1C3 and R3C1, respectively. Notably, the specific surface area and carbon content of the co- pyrolysis biochar increased, potentially promoting the growth and colonization of soil microorganisms. On the 60th day, the microbial control group achieved a 46.69% removal of pollutants, while the addition of R0C1, R1C0, R1C3, R1C1 and R3C1 resulted in removals of 70.56%, 67.01%, 67.62%, 68.74% and 67.30%, respectively. In contrast, the highest efficiency observed in the abiotic treatment group was only 24.12%. This suggested that the removal of petroleum pollutants was an outcome of the collaborative influence of co-pyrolysis biochar and soil microorganisms. Furthermore, the abundance of Proteobacteria, renowned for its petroleum degradation capability, obviously increased in the treatment group with the addition of co-pyrolysis biochar. This demonstrated that co-pyrolysis biochar could notably stimulate the growth of functionally associated microorganisms. This research confirmed the promising application of co-pyrolysis biochar in the remediation of petroleum-contaminated soil.

Remediation of biochar-supported effective microorganisms and microplastics on multiple forms of heavy metals in eutrophic lake

In mineral-rich areas, eutrophic lakes are at risk of HMs pollution. However, few papers focused on the repair of HMs in eutrophic environment. Our study analyzed multiple forms of HMs, pore structure and microbial responses in the water-sediment system of eutrophic lake treated with biochar, Effective Microorganisms (EMs) or/and microplastics (MPs). As biochar provided an ideal carrier for EMs, the remediation of biochar-supported EMs (BE) achieved the greatest repairment that improved the bacterial indexes and greatly decreased the most HMs in various forms across the water-sediment system, and it also reduced metal mobility, bioavailability and ecological risk. The addition of aged MPs (MP) stimulated the microbial activity and significantly reduced the HMs levels in different forms due to the adsorption of biofilms/EPS adhered on MPs, but it increased metals mobility and ecological risks. The strong adsorption and high mobility of aged MPs would increase enrichment of HMs and cause serious ecological hazards. The incorporation of BE and MP (MBE) also greatly reduced the HMs in full forms, which was primarily ascribed to the adsorption of superfluous biofilms/EPS, but it distinctly depressed the microbial activity. The single addition of biochar and EMs resulted in the inability of HMs to be adsorbed due to the preferentially adsorption of dissolved nutrients and the absence of effective carrier, respectively. In the remediation cases, the remarkable removal of HMs was principally accomplished by the adsorption of HMs with molecular weight below 100 kDa, especially 3 kDa ∼100 kDa, which had higher specific surfaces and abundant active matters, resulting in higher adsorption onto biofilms/EPS.

Adsorption of Pb(II) by UV-aged microplastics and cotransport in homogeneous and heterogeneous porous media

To investigate the adsorption effects of aged microplastics (MPs) on Pb(II) and their co-transport properties in homogeneous (quartz sand) and heterogeneous (quartz sand with apple branches biochar) porous media, we explored the co-transport of UV-irradiated aged MPs and coexisting Pb(II) along with their interaction mechanisms. The UV aging process increased the binding sites and electronegativity of the aged MPs' surface, enhancing its adsorption capacity for Pb(II). Aged MPs significantly improved Pb(II) transport through homogeneous media, while Pb(II) hindered the transport of aged MPs by reducing electrostatic repulsion between these particles and the quartz sand. When biochar, with its loose and porous structure, was used as a porous medium, it effectively inhibited the transport capacity of both contaminants. In addition, since the aged MPs cannot penetrate the column, a portion of Pb(II) adsorbed by the aged MPs will be co-deposited with the aged MPs, hindering Pb(II) transport to a greater extent. The transport experiments were simulated and interpreted using two-point kinetic modeling and the DLVO theory. The study results elucidate disparities in the capacity of MPs and aged MPs to transport Pb(II), underscoring the potential of biochar application as an effective strategy to impede the dispersion of composite environmental pollutants.

Trends in phytoremediation of heavy metals-contaminated soils: A Web of science and CiteSpace bibliometric analysis

Heavy metals pollution in soils is an urgent environmental issue worldwide. Phytoremediation is a green and eco-friendly way of remediating heavy metals. However, a systematic overview of this field is limited, and little is known about future development trends. Therefore, we used CiteSpace software to conduct bibliometric and visual analyses of published literature in the field of phytoremediation of heavy metals in soils from the Web of Science core collection and identified research hotspots and development trends in this field. Researchers are paying increased attention to phytoremediation of heavy metals in soils, especially environmental researchers. A total of 121 countries or regions, 3790 institutions, 4091 funded organisations and 15,482 authors have participated in research in this area. China, India, and Pakistan are the largest contributors. There has been extensive cooperation between countries, institutions, and authors worldwide, but there is a lack of cooperation among top authors. 'Calcareous soil', 'Co-contaminated soil' and 'Metal availability' are the most intensively investigated topics. 'EDTA', 'Plant growth-promoting Rhizobacteria', 'Photosynthesis', 'Biochar' and 'Phytoextraction' are research hotspots in this field. In addition, more and more researchers are beginning to pay attention to research on co-contaminated soil, metal availability, chelating agents, and microbial-assisted phytoremediation. In summary, bibliometric, and visual analyses in the field of phytoremediation of heavy metals in soils identifies probable directions for future research and provides a resource through which to better understand this rapidly advancing subject.

Adsorption and reduction of Cr(VI) by N, S co-doped porous carbon from sewage sludge and low-rank coal: Combining experiments and theoretical calculations

Herein, a novel N, S co-doped porous carbon (S5C5-AC) for Cr(VI) removal was prepared by co-hydrothermal carbonization (HTC) of sewage sludge (SS) and low-rank coal (LC) combining with KOH modification. The results showed that S5C5-AC had excellent adsorption performance on Cr(VI), and lower pH value, higher initial concentration and longer contact time were beneficial for Cr(VI) adsorption. The adsorption kinetics and isotherms revealed that Cr(VI) adsorption by S5C5-AC was homogeneous and dominated by chemisorption. The adsorption isotherm showed that the maximum equilibrium adsorption capacity of S5C5-AC for Cr(VI) was 382.04 mg/g at 25 °C. Furthermore, the results showed that the main mechanisms for Cr(VI) removal were the pore filling, electrostatic interaction and reduction. Moreover, the electron transfer mechanism during the adsorption and reduction process was further explored at the molecular and electronic levels by density functional theory (DFT) and front orbital theory (FOT) simulations. The analysis of DFT and FOT indicated that the synergistic effect between S and N functional groups was exhibited during the Cr(VI) removal process. Considering the existence of synergistic effects between N and S functional groups during adsorption, the S and N content and form were modified collaboratively. Increasing the relative content of pyrrolic N may be the most effective pathway for improving removal performance. Besides that, S5C5-AC exhibited excellent adsorption capacity over a high coexisting ion concentration range and various actual water bodies and regeneration performance, which indicated that S5C5-AC had promising potential for the remediation of wastewater in industrial applications.

Biochar application for the remediation of soil contaminated with potentially toxic elements: Current situation and challenges

Recently, biochar has garnered extensive attention in the remediation of soils contaminated with potentially toxic elements (PTEs) owing to its exceptional adsorption properties and straightforward operation. Most researchers have primarily concentrated on the effects, mechanisms, impact factors, and risks of biochar in remediation of PTEs. However, concerns about the long-term safety and impact of biochar have restricted its application. This review aims to establish a basis for the large-scale popularization of biochar for remediating PTEs-contaminated soil based on a review of interactive mechanisms between soil, PTEs and biochar, as well as the current situation of biochar for remediation in PTEs scenarios. Biochar can directly interact with PTEs or indirectly with soil components, influencing the bioavailability, mobility, and toxicity of PTEs. The efficacy of biochar in remediation varies depending on biomass feedstock, pyrolysis temperature, type of PTEs, and application rate. Compared to pristine biochar, modified biochar offers feasible solutions for tailoring specialized biochar suited to specific PTEs-contaminated soil. Main challenges limiting the applications of biochar are overdose and potential risks. The used biochar is separated from the soil that not only actually removes PTEs, but also mitigates the negative long-term effects of biochar. A sustainable remediation technology is advocated that enables the recovery and regeneration (95.0-95.6%) of biochar from the soil and the removal of PTEs (the removal rate of Cd is more than 20%) from the soil. Finally, future research directions are suggested to augment the environmental safety of biochar and promote its wider application.

Acid treatment for enhancing Hg0 removal efficiency of chlorine-loaded biochar: mechanism and kinetic analysis

Adsorbents modified solely with chlorine have limited effectiveness in removing mercury at high temperatures. This study aims to investigate the influence of various acid (HNO3, H2SO4, and H2O2) loadings on the removal efficiency of mercury from NH4Cl-modified adsorbents. The objective is to develop rice straw carbon adsorbents that are both more efficient and cost-effective. The experiments were conducted on a fixed bed experimental platform, with SEM and BET to observe the physical property changes of the modified char samples. XPS analysis was employed to analyze the effects of oxygen, chlorine, and sulfur functional groups. Additionally, a kinetic model was used to investigate the interaction mechanism between the adsorbent and mercury. The findings demonstrate that co-modification surpasses the use of NH4Cl alone, with the combination of NH4Cl and HNO3 yielding the best results. Co-modification enhances the development of a more refined and compact pore structure on the char surface, promoting the physical adsorption of mercury. Moreover, an increased presence of chlorine and oxygen functional groups is observed on the char surface, particularly in the NH4Cl and HNO3 co-modified samples, further enhancing the chemical adsorption capacity of the char. The results from the kinetic analysis support this conclusion. Furthermore, the adsorption process of Hg0 relies on both external mass transfer and chemical adsorption, with the chemical adsorption process playing a more significant role as the controlling factor.

Polyethylenimine Grafted onto Nano-NiFe2O4@SiO2 for the Removal of CrO42-, Ni2+, and Pb2+ Ions from Aqueous Solutions

Polyethyleneimine (PEI) has been reported to have good potential for the adsorption of metal ions. In this work, PEI was covalently bound to NiFe2O4@SiO2 nanoparticles to form the new adsorbent NiFe2O4@SiO2-PEI. The material allowed for magnetic separation and was characterized via powder X-ray diffraction (PXRD), showing the pattern of the NiFe2O4 core and an amorphous shell. Field emission scanning electron microscopy (FE-SEM) showed irregular shaped particles with sizes ranging from 50 to 100 nm, and energy-dispersive X-ray spectroscopy (EDX) showed high C and N contents of 36 and 39%, respectively. This large amount of PEI in the materials was confirmed by thermogravimetry-differential thermal analysis (TGA-DTA), showing a mass loss of about 80%. Fourier-transform IR spectroscopy (FT-IR) showed characteristic resonances of PEI dominating the spectrum. The adsorption of CrO42-, Ni2+, and Pb2+ ions from aqueous solutions was studied at different pH, temperatures, metal ion concentrations, and adsorbent dosages. The maximum adsorption capacities of 149.3, 156.7, and 161.3 mg/g were obtained for CrO42-, Ni2+, and Pb2+, respectively, under optimum conditions using 0.075 g of the adsorbent material at a 250 mg/L ion concentration, pH = 6.5, and room temperature.

Remediation materials for the immobilization of hexavalent chromium in contaminated soil: Preparation, applications, and mechanisms

Hexavalent chromium is a toxic metal that can induce severe chromium contamination of soil, posing a potential risk to human health and ecosystems. In recent years, the immobilization of Cr(VI) using remediation materials including inorganic materials, organic materials, microbial agents, and composites has exhibited great potential in remediating Cr(VI)-contaminated soil owing to the environmental-friendliness, short period, simple operation, low cost, applicability on an industrial scale, and high efficiency of these materials. Therefore, a systematical summary of the current progress on various remediation materials is essential. This work introduces the production (sources) of remediation materials and examines their characteristics in detail. Additionally, a critical summary of recent research on the utilization of remediation materials for the stabilization of Cr(VI) in the soil is provided, together with an evaluation of their remediation efficiencies toward Cr(VI). The influences of remediation material applications on soil physicochemical properties, microbial community structure, and plant growth are summarized. The immobilization mechanisms of remediation materials toward Cr(VI) in the soil are illuminated. Importantly, this study evaluates the feasibility of each remediation material application for Cr(VI) remediation. The latest knowledge on the development of remediation materials for the immobilization of Cr(VI) in the soil is also presented. Overall, this review will provide a reference for the development of remediation materials and their application in remediating Cr(VI)-contaminated soil.

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

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

Selective removal of Pb(II) from yellow rice wine using magnetic carbon-based adsorbent

BACKGROUND

The non-distilled property and prolonged production period of yellow rice wine have significantly increased the metal residue problem, posing a threat to human health. In this study, a magnetic carbon-based adsorbent, named magnetic nitrogen-doped carbon (M-NC), was developed for the selective removal of lead(II) (Pb(II)) from yellow rice wine.

RESULTS

The results showed that the uniformly structured M-NC could be easily separated from the solution, exhibiting a high Pb(II) adsorption capacity of 121.86 mg g-1 . The proposed adsorption treatment showed significant Pb(II) removal efficiencies (91.42-98.90%) for yellow rice wines in 15 min without affecting their taste, odor, and physicochemical characteristics of the wines. The adsorption mechanism studied by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) analyses indicated that the selective removal of Pb(II) could be attributed to the electrostatic interaction and covalent interaction between the empty orbital of Pb(II) and the π electrons of the N species on M-NC. Additionally, the M-NC showed no significant cytotoxicity on the Caco-2 cell lines.

CONCLUSION

Selective removal of Pb(II) from yellow rice wine was achieved using magnetic carbon-based adsorbent. This facile and recyclable adsorption operation could potentially address the challenge of toxic metal pollution in liquid foods. © 2023 Society of Chemical Industry.

All-in-one strategy to prepare molded biochar with magnetism from sewage sludge for high-efficiency removal of Cd(Ⅱ)

Biochar in powder could lead to the separation difficulties after using and easy dispersion by wind with non-necessary consumption during the practical application. The current method for preparing molded biochar is multi-step, tedious, and required exogenous reagents. Moreover, the dehydration of sewage sludge with high water content (>85%) causes expensive production cost, limiting its secondary utilization. Therefore, an "all-in-one" strategy was developed to prepare molded biochar with magnetism by using sewage sludge as endogenetic binder, water source, carbon source, as well as magnetic source, and biomass wastes as water moderator and pore-forming agent. The molded biochar showed high removal capacity towards Cd(Ⅱ) of 456.2 mg/g, which was 6 times higher than the commercial activated carbon in powder (69.1 mg/g). The excellent removal performance of the molded biochar was in linear correlation the O/C ratio (R² =0.855), resulting in the complexation with Cd(Ⅱ). DFT calculations indicated the amounts and species of oxygen changed the electron distribution and electron-donation properties of biochar for Cd(Ⅱ). Moreover, the Na⁺ exchanges with Cd(Ⅱ) were also an important removal mechanism. This study provided a novel synthesis strategy for the molded biochar with both high particle density and high adsorption capability.

Effect of iron impregnation ratio on the properties and adsorption of KOH activated biochar for removal of tetracycline and heavy metals

The effect of iron impregnation ratio on magnetic biochars (MBCs) prepared by biomass pyrolysis accompanied by KOH activation has been less reported. In this study, MBCs were produced by one-step pyrolysis/KOH-activation of walnut shell, rice husk and cornstalk with different impregnation ratios (0.3-0.6). The properties, adsorption capacity and cycling performance for Pb(II), Cd(II) and tetracycline of MBCs were determined. MBCs prepared with low impregnation ratio (0.3) showed stronger adsorption capacity on tetracycline. The adsorption capacity of WS-0.3 toward tetracycline was up to 405.01 mg g^-1, while that of WS-0.6 was only 213.81 mg g^-1. It is noteworthy that rice husk and cornstalk biochar with an impregnation ratio of 0.6 were more effective in removing Pb(II) and Cd(II), and the content of Fe⁰ crystals on surface strengthened the ion exchange and chemical precipitation. This work highlights that the impregnation ratio should be changed according to the actual application scenarios of MBC.

Functional biochar fabricated from red mud and walnut shell for phosphorus wastewater treatment: Role of minerals

A novel functional biochar (BC) was prepared from industrial waste red mud (RM) and low-cost walnut shell by one facile-step pyrolysis method to adsorb phosphorus (P) in wastewater. The preparation conditions for RM-BC were optimized using Response Surface Methodology. The adsorption characteristics of P were investigated in batch mode experiments, while a variety of techniques were used to characterize RM-BC composites. The impact of key minerals (hematite, quartz, and calcite) in RM on the P removal efficiency of the RM-BC composite was studied. The results showed that RM-BC composite produced at 320 °C for 58 min, with a 1:1 mass ratio of walnut shell and RM, had a maximum P sorption capacity of 15.48 mg g^-1, which was more than double that of the raw BC. The removal of P from water was found to be facilitated significantly by hematite, which forms Fe-O-P bonds, undergoes surface precipitation, and exchanges ligands. This research provides evidence for the effectiveness of RM-BC in treating P in water, laying the foundation for future scaling-up trials.

Enhancing lead immobilization by biochar: Creation of "surface barrier" via bio-treatment

The long-term effectiveness of heavy metal immobilization is always a concern. This study proposes a completely novel approach to enhance the stability of heavy metals by combined biochar and microbial induced carbonate precipitation (MICP) technology, to create a "surface barrier" of CaCO₃ layer on biochar after lead (Pb²⁺) immobilization. Aqueous sorption studies and chemical and micro-structure tests were used to verify the feasibility. Rice straw biochar (RSB700) was produced at 700 °C, which shows high immobilization capacity of Pb²⁺ (maximum of 118 mg g^-1). But the stable fraction only accounts for 4.8% of the total immobilized Pb²⁺ on biochar. After MICP treatment, the stable fraction of Pb²⁺ significantly increased to a maximum of 92.5%. Microstructural tests confirm the formation of CaCO₃ layer on biochar. The CaCO₃ species are predominantly calcite and vaterite. Higher Ca²⁺ and urea concentrations in cementation solution resulted in higher CaCO₃ yield but lower Ca²⁺ utilization efficiency. The main mechanism of the "surface barrier" to enhance Pb²⁺ stability on biochar was likely the encapsulation effect: it physically blocked the contact between acids and Pb²⁺ on biochar, and chemically buffer the acidic attack from the environment. The performance of the "surface barrier" depends on both the yield of CaCO₃ and their distribution uniformity on biochar's surface. This study shed lights on the potential application of the "surface barrier" strategy combining biochar and MICP technologies for enhanced heavy metal immobilization.

A review on arsenic in the environment: bio-accumulation, remediation, and disposal

Arsenic is a widespread serious environmental pollutant as a food chain contaminant and non-threshold carcinogen. Arsenic transfer through the crops-soil-water system and animals is one of the most important pathways of human exposure and a measure of phytoremediation. Exposure occurs primarily from the consumption of contaminated water and foods. Various chemical technologies are utilized for As removal from contaminated water and soil, but they are very costly and difficult for large-scale cleaning of water and soil. In contrast, phytoremediation utilizes green plants to remove As from a contaminated environment. A large number of terrestrial and aquatic weed flora have been identified so far for their hyper metal removal capacity. In the panorama presented herein, the latest state of the art on methods of bioaccumulation, transfer mechanism of As through plants and animals, and remediation that encompass the use of physicochemical and biological processes, i.e., microbes, mosses, lichens, ferns, algae, and macrophytes have been assessed. Since these bioremediation approaches for the clean-up of this contaminant are still at the initial experimental stages, some have not been recognized at full scale. Nonetheless, extensive research on these primitive plants as bio-accumulators can be instrumental in controlling arsenic exposure and rehabilitation and may result in major progress to solve the problem on a worldwide scale.

Preparation and application of biochar from co-pyrolysis of different feedstocks for immobilization of heavy metals in contaminated soil

Co-pyrolysis is a potentially effective method for both biomass waste management and multi-functional biochar-based product design. It involves the thermochemical decomposition of biomass waste under anoxic conditions, which can reduce the cost of disposal and produce biochar with beneficial properties. Herein, this study aimed to investigate the properties and environmental applications of biochar from single- and mixed- feedstocks of wheat straw, rice husk, pig manure, and oyster shell at 450 ℃, respectively. A pot experiment with Chinese cabbage was carried out to compare the effects of biochars with limestone on soil Cd and Pb immobilization at two harvest periods. The results indicated that co-pyrolysis of various biomasses exhibited synthetic effects on promoting the calorific value of syngas and enhancing the quality of produced biochar. The pot experiment revealed a significant promotion on soil pH, soil organic matter, cation exchange capacity, and soluble Ca, which consequently reduced Cd and Pb availability. In contrast with limestone treatment, soil amendment with single biomass-derived and co-pyrolysis-derived (COPB) biochars had a significant positive impact on soil fertility and microbial biomass. Application of COPB at a 0.5% dosage consistently and most effectively enhanced the shoot biomass, increased leaf Vitamin C content but reduced leaf content of nitrate and heavy metals in both harvests. Using COPB for soil remediation would be financially visible due to the enhancement of crop yield. Therefore, this study proposes a strategy for targeted enhancement of the functions of biochar derived from co-pyrolysis of selected biomass waste for soil remediation and agricultural production.

Review on arsenic environment behaviors in aqueous solution and soil

Arsenic pollution in environment has always been an important environmental problem that has attracted wide attention in recent years. Adsorption is one of the main methods of treatment for arsenic in the aqueous solution and soil because of the advantages of high efficiency, low cost and wide application. Firstly, this report summarizes the commonly and widely used adsorbent materials such as metal-organic frameworks, layered bimetallic hydroxides, chitosan, biochar and their derivatives. The adsorption effects and mechanisms of these materials are further discussed, and the application prospects of these adsorbents are considered. Meanwhile, the gaps and deficiencies in the study of adsorption mechanism was pointed out. Then, this study comprehensively evaluated the effects of various factors on arsenic transport, including (i) the effects of pH and redox potential on the existing form of As; (ii) complexation mechanism of dissolved organic matter and As; (iii) factors affecting the plant enrichment of As. Finally, the latest scientific researches on microbial remediation of arsenic and the mechanisms were summarized. The review finally enlightens the subsequent development of more efficient and practical adsorption material.

Chemical speciation determines combined cytotoxicity: Examples of biochar and arsenic/chromium

As both electron donors and acceptors, biochars (BCs) may interact with multivalent metal ions in the environment, causing changes in ionic valence states and resulting in unknown combined toxicity. Therefore, we systematically investigated the interaction between BCs and Cr (Cr(III) & Cr(VI)) or As (As(III) & As(V)) and their combined cytotoxicity in human colorectal mucosal (FHC) cells. Our results suggest that the redox-induced valence state change is a critical factor in the combined cytotoxicity of BCs with Cr/As. Specifically, when Cr(VI) was adsorbed on BCs, 86.4 % of Cr(VI) was reduced to Cr(III). In contrast, As(III) was partially oxidized to As(V) with a ratio of 37.2 %, thus reaching a reaction equilibrium. Meanwhile, only As(V) was released in the cell, which could cause more As(III) to be oxidized. As both Cr(III) and As(V) are less toxic than their corresponding counterparts Cr(VI) and As(III), different redox interactions between BCs and Cr/As and release profiles between BCs and Cr/As together lead to reduced combined cytotoxicity of BP-BC-Cr(VI) and BP-BC-As(III). It suggests that the valence state changes of metal ions due to redox effects is one of the parameters to be focused on when studying the combined toxicity of complexes of BCs with different heavy metal ions.

Synthesis of zeolite-doped polyaniline composite for photocatalytic degradation of methylene blue from aqueous solution

The generation of wastewater has increased rapidly with the expansion of industries, hence, posing a risk to human health and the environment. The development of novel materials and technologies for textile wastewater treatment is constantly evolving. In this work, the photocatalytic degradation of methylene blue employing ZSM-5 zeolite-doped polyaniline composites is presented. To fabricate ZSM-5-based polyaniline (PANI) composites, the simple approach of in situ oxidative polymerization has been adopted. Different weight ratios of ZSM-5 have been used for the synthesis, and samples have been labelled as PAZe-1, PAZe-5, and PAZe-10. Different characterization techniques were used to characterize the prepared composites, including field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and thermo-gravimetry analysis (TGA). The photocatalytic performance of polyaniline, ZSM-5, and their composites was assessed by monitoring the degradation of methylene blue in the presence of visible light. Degradation results of the polyaniline-doped composites were found to be better than that of the polyaniline alone. When the photocatalytic efficiencies of different composites were compared, the PAZe-1 showed the best performance, with 99.9% degradation efficiency after 210 min of irradiation, while PANI, PAZe-5, PAZe-10, and ZSM-5 show 38%, 82%, 71%, and 99% removal efficiency. Apart from methylene blue, the composite PAZe-1 was further explored for the degradation of other organic pollutants such as methyl orange, chlorpyrifos, 2,4-dichlorophenoxy acetic acid, and p-nitroaniline. To determine the reactive species involved in the photocatalysis mechanism, scavenger studies were performed.

Immobilization effect of heavy metals in biochar via the copyrolysis of sewage sludge and apple branches

The excess sludge produced by sewage treatment plants can be recycled into energy through pyrolysis, and the byproduct biochar can be used for soil remediation. However, the heavy metals in sludge are retained in biochar after pyrolysis and may cause secondary pollution during its soil application. Herein, a fast copyrolysis method of activated sludge (AS) and apple branches (AT) was proposed to immobilize heavy metals while improving bio-oil yield. The results showed that the heavy metal release from the copyrolyzed biochar was markedly reduced compared with that from the biochar produced through the pyrolysis of AS alone (78% for Cr and 28% for Pb). The kinetic behavior of ion release from different biochars could be described by a first-order kinetic model. The excellent fixation of heavy metals was attributed to complexation by abundant oxygen-containing surface functional groups (-O-, =O, and -CHO) that were mainly donated by AT. Furthermore, high-temperature pyrolysis was conducive to the fixation of metals, and the release of Pb²⁺ and Cr³⁺ from the biochar pyrolyzed at 600 °C was approximately 2/3 and 1/10 of that from the biochar pyrolyzed at 400 °C, respectively. A growth experiment on Staphylococcus aureus and Escherichia coli revealed that the toxicity of the copyrolyzed biochar was greatly reduced. This work can provide a method for heavy metal fixation and simultaneous resource recovery from organic wastes.

Biochar aerogel enhanced remediation performances for heavy oil-contaminated soil through biostimulation strategy

Petroleum leakages can seriously damage the soil environment and cause a persistent harm to human health, due to the release of heavy oil pollutants with a high viscosity and high molecular weight. In this paper, biochar aerogel materials were successfully prepared under 600, 700 and 800 ℃ (accordingly labeled as 600-aerogel, 700-aerogel and 800-aerogel) with green, sustainable and abundant sisal leaves as raw materials for the remediation of heavy oil-contaminated soil. The remediation performances of biochar aerogel supplement for heavy oil-contaminated soil were investigated, while microbial abundance and community structure were characterized. The degradation efficiency of 600-aerogel, 700-aerogel and 800-aerogel treatments was accordingly 80.69%, 86.04% and 86.62% after 60 days. Apart from adsorption behavior, biostimulation strengthened the degradation efficiency, according to findings from first-order degradation kinetics. Biochar aerogel supplement basically increased genera microbial abundance for Sinomonas, Streptomyces, Sphingomonas and Massilia with petroleum degradation abilities through microorganisms' biostimulation. Sinomonas as the dominant genus with the highest abundance probably contributed much higher capacities to heavy oil degradation. This study can provide an inspiring reference for the development of green carbon-based materials to be applied in heavy oil-contaminated soils through biostimulation mechanisms.

Bioleaching and immobilizing of copper and zinc using endophytes coupled with biochar-hydroxyapatite: Bipolar remediation for heavy metals contaminated mining soils

Copper and zinc are toxic heavy metals in soils that require development of feasible strategies for remediation of contaminated soils around the mine areas. In this study, the processing conditions and mechanisms of immobilization and bioleaching for remediation of highly contaminated soils with heavy metals are investigated. Soil remediation is carried out using a bioleaching-immobilization bipolar method. The results show that LSE03 bacteria provide efficient leaching result and immobilization on Cu²⁺ and Zn²⁺. Among the bacterial metabolites, cis, cis-muconic acid and isovaleric acid play major roles in the bioleaching process. The bacterial extracellular polymeric substances are rich in a variety of organic acids that show a significant decrease in content after the adsorption process, indicating that all of these substances are involved in the binding of heavy metals. Characterization of the endophytes and immobilizing agents with FTIR, TEM-mapping, and XPS techniques reveal the ability of both bacteria and composites to adsorb Cu-Zn as well as the main functional groups of -OH, -COOH, -PO₄^3-, and -NH. According to the heavy metals species analyses, competitive adsorption experiments, and bioleaching desorption experiments, it is planned to carry out the bipolar remediation of contaminated soil through immobilization followed by bioleaching process. After bipolar remediation processing, 97.923% and 96.387% of available Cu and Zn are respectively removed. Soils fertility significantly increases in all cases. Our study provides a green, practical, and environmentally friendly treatment method for soils contaminated with high concentrations of heavy metals.

Insights into the effects of tea pruning litter biochar on major micronutrients (Cu, Mn, and Zn) pathway from soil to tea plant: An environmental armour

A field study was conducted from 0 to 360 days to investigate the effect of tea pruning litter biochar (TPLBC) on the accumulation of major micronutrients (copper: Cu, manganese: Mn, and zinc: Zn) in soil, their uptake by tea plant (clone: S.3 A/3) and level of contamination in soil due to TPLBC. To evaluate the level of contamination due to TPLBC, a soil pollution assessment was carried out using the geo-accumulation index (I_geo), enrichment factor (EF), contamination factor (CF), potential ecological risk factor (PERF), individual contamination factor (ICF), and risk assessment code (RAC). The total content of Cu, Mn, and Zn gradually increased with increasing doses of TPLBC at 0D, and then decreased with time. The fractionation of the three micronutrients in soil changed after the application of TPLBC. The contamination risk assessment of soil for Cu, Mn, and Zn based on the I_geo, EF, CF, PERF,ICF, and RAC suggested that the application of TPLBC does not have any adverse effect on soil. Except for Mn, the bioconcentration and translocation factors were less than one for Cu and Zn. Results from this study revealed that the application of 400 kg TPLBC ha^-1 is significantly better than the other treatments for Cu, Mn, and Zn at a 5% level of significance.

The recent development of inverse vulcanized polysulfide as an alternative adsorbent for heavy metal removal in wastewater

Inverse vulcanized polysulfides have been used as low-cost and effective adsorbents to remediate heavy metals in wastewater. Inverse vulcanization introduces sustainable polysulfide synthesis by solving the rapid desulfurization problem of unstable polysulfides, and provides superior performance compared to conventional commercial adsorbents. The review discussed the brief applications of the inverse vulcanized polysulfides to remove heavy metal wastewater and emphasized the modified synthesis processes for enhanced uptake ratios. The characteristics of polysulfide adsorbents, which play a vital role during the removal process are highlighted with a proper discussion of the interaction between metal ions and polysulfides. The review paper concludes with remarks on the future outlook of these low-cost adsorbents with high selectivity to heavy metals. These polysulfide adsorbents can be prepared using a wide variety of crosslinker monomers including organic hydrocarbons, cooking oils, and agro-based waste materials. They have shown good surface area and excellent metal-binding capabilities compared to the commercially available adsorbents. Proper postmodification processes have enabled the benefits of repetitive uses of the polysulfide adsorbents. The improved surface area obtained by appropriate choice of crosslinkers, modified synthesis techniques, and regeneration through post-modification has made inverse vulcanized polysulfides capable of removing.

Microwave biochar produced with activated carbon catalyst: Characterization and adsorption of heavy metals

Novel microwave biochar derived from wheat straw (WS) using a range of power levels, with activated carbon catalyst as microwave absorber, was produced, characterized and tested as adsorbent of three heavy metals (Pb²⁺, Cd²⁺, and Cu²⁺). The microwave biochar with the greatest specific surface area (156.09 m² g^-1) and total pore volume (0.0790 cm³ g^-1) were produced at 600 W (WS600) and 500 W (WS500) power level, respectively. Maximum adsorption capacities of WS500 to Pb²⁺, Cd²⁺ and Cu²⁺ were 139.44 mg g^-1, 52.92 mg g^-1, and 31.25 mg g^-1, respectively. Optimal pH value for heavy metal removal was at range of 5-6, and Pb²⁺ showed the strongest affinity in competitive adsorption experiments. The adsorption data were fitted better by pseudo-second-order model and Langmuir isotherm, indicating that adsorption process was mainly explained by monolayer adsorption, and chemical adsorption occupied important role. The predominant adsorption mechanisms of heavy metals on microwave pyrolysis biochar included complexation with oxygen-containing functional groups (i.e., carboxylic acid CO and -OH) and precipitation with carbonate. In addition, reused WS600 maintained 76.17% and 96.07% of their initial adsorption capacity for Cu²⁺ and Cd²⁺, respectively. These results suggest that microwave biochar produced with activated carbon catalyst has excellent potential for efficient use in the removal of heavy metals from waste water.

Divergent responses of earthworms (Eisenia fetida) in sandy loam and clay soils to cerium dioxide nanoparticles

The reported biological effects of cerium dioxide nanoparticles (nCeO₂) in soils range from toxic to protective. However, divergences of nCeO₂ toxicity in soils of different textures are not known. In this study, the availability of nCeO₂ on earthworms (Eisenia fetida) in sandy loam soils and clay soils was discussed, and the biological effects of nCeO₂ (0-1000 mg/kg) on earthworms in two soils were investigated. The results showed the bioaccumulation and biological effects of Ce on earthworms in the two soils were inconsistent. The European Community Bureau of Reference (BCR) sequential extraction revealed that the major portions of Ce in both soils were in the residual form (98-99%), and the acid-soluble Ce fraction was greater in clay soils. However, nCeO₂ was more toxic to earthworms in sandy loam soils than that in clay soils as assessed by earthworm biomass, morphology, and antioxidative damage. Thus, the high ecological risk of nCeO₂ in sandy loam soils with higher pH and lower clay contents needs to be avoided, being used in agriculture to improve both crop yield and quality.

Role of biochar-based free radicals in immobilization and speciation of metals in the contaminated soil-plant environment

The structure of biochar produced at various pyrolysis temperatures influences metal geochemical behavior. Here, the impact of wheat straw-derived biochar (300, 500, and 700 °C) on the immobilization and transformation of metals in the contaminated soil-plant system was assessed. The findings of the sequential extraction revealed that biochar additives had a substantial influence on the speciation of Cr, Ni, Pb, and Zn in the contaminated soil. The lowest F1 (exchangeable and soluble fraction) + F2 (carbonate fraction) accounted for Cr (44%) in WB-300, Ni (43.87%) in WB-500, Pb (43.79%), and Zn (49.78%) in WB-700 with applied amendments of their total amounts. The characterization results indicated that high pyrolysis temperatures (300-700 °C) increased the carbon-containing groups with the potential to adsorb metals from the soil-plant environment. The bioconcentration and translocation factors (BCF and TF) were less than 1, indicating that metal concentration was restricted to maize roots and translocation to shoots. Reactive oxygen species (ROS) intracellularly influence metal interactions with plants. Electron paramagnetic resonance (EPR) was performed to determine hydroxyl radical generation (•OH) in plant segments to assess the dominance of free radicals (FRs). Consequently, the formation of •OH significantly depends on the pyrolysis temperature and the interaction with a contaminated soil-plant environment. Thus, metal transformation can be effectively decreased in the soil-plant environment by applying WB amendments.

Nano Modifications of Biochar to Enhance Heavy Metal Adsorption from Wastewaters: A Review

Biochar (BC) is a carbon-rich material that can be obtained by thermal decomposition of agricultural solid waste under oxygen-limited conditions. It has received increasing attention as a cost-effective sorbent to treat metal-contaminated water due to attributes such as high porosity and the presence of various functional groups. The heavy metal (HM) sorption and removal capacity of BC can be enhanced by developing novel biochar nanohybrids (BNHs) that can be produced via surface modification of BC with nanomaterials. Loading of nanomaterials on the biochar surface can improve its physicochemical properties through alterations in the functional group profile, porosity, and availability of active sites on the BC surface which can enhance the HM adsorption ability. This manuscript provides information on preparation of nano-based biochar hybrids emanating from the type of modifying agent for the removal of different HM ions from wastewaters, and the underlying mechanisms have been discussed. Further, this compilation discusses published literature depicting the influence of different processes of preparation on the physicochemical properties and adsorption capacity of nanobiochar hybrids. The potential risks of BNHs have been reviewed to effectively avoid the possible harmful impacts on the environment, and future research directions have been proposed.

Biochar application for greenhouse gas mitigation, contaminants immobilization and soil fertility enhancement: A state-of-the-art review

Rising global temperature, pollution load, and energy crises are serious problems, recently facing the world. Scientists around the world are ambitious to find eco-friendly and cost-effective routes for resolving these problems. Biochar has emerged as an agent for environmental remediation and has proven to be the effective sorbent to inorganic and organic pollutants in water and soil. Endowed with unique attributes such as porous structure, larger specific surface area (SSA), abundant surface functional groups, better cation exchange capacity (CEC), strong adsorption capacity, high environmental stability, embedded minerals, and micronutrients, biochar is presented as a promising material for environmental management, reduction in greenhouse gases (GHGs) emissions, soil management, and soil fertility enhancement. Therefore, the current review covers the influence of key factors (pyrolysis temperature, retention time, gas flow rate, and reactor design) on the production yield and property of biochar. Furthermore, this review emphasizes the diverse application of biochar such as waste management, construction material, adsorptive removal of petroleum and oil from aqueous media, immobilization of contaminants, carbon sequestration, and their role in climate change mitigation, soil conditioner, along with opportunities and challenges. Finally, this review discusses the evaluation of biochar standardization by different international agencies and their economic perspective.

Effects of synthesis temperature on ε-MnO2 microstructures and performance: Selective adsorption of heavy metals and the mechanism onto (100) facet compared with (001)

The heavy-metal adsorbent ε-MnO₂ was produced through a simple, one-step oxidation-reduction reaction at three different synthesis temperatures (25 °C, 50 °C and 75 °C) and their morphology and chemical-physical properties were compared. Of the three materials, MnO₂-25 had the largest specific surface area and the highest surface hydroxyl concentration. Its optimal performance was demonstrated by batch adsorption experiments with Pb²⁺, Cd²⁺ and Cu²⁺. Of the three metals, Pb²⁺ was adsorbed best (339.15 mg/g), followed by Cd²⁺ (107.50 mg/g) and Cu²⁺ (86.30 mg/g). When all three metals were present, Pb²⁺ was still absorbed best but now more Cu²⁺ was adsorbed than Cd²⁺. In order to explore the mechanism for the inconsistent adsorption order of Cd²⁺ and Cu²⁺ in single and competitive adsorption, we combined experimental data with density functional theory (DFT) calculations to elucidate the distinct adsorption nature of MnO₂-25 towards these three metals. This revealed that the adsorption affinity of the (100) facet was superior to (001), and since the surface complexes were also more stable on (100), this facet was most likely determining the adsorption order for the single metals. When the metals were present in combination, Pb²⁺ preferentially occupied the active adsorption sites of (100), forcing Cu²⁺ to be adsorbed on the (001) facet where Cd²⁺ was only poorly bound. Thus, the adsorption behavior was affected by MnO₂-25 surface chemistry at a molecular scale. This study provides an in-depth understanding of the adsorption mechanisms of the heavy metals on this adsorbent and offers theoretical guidance for production of adsorbent with improved removal efficiency.

Biochar for the removal of contaminants from soil and water: a review

Biochar shows significant potential to serve as a globally applicable material to remediate water and soil owing to the extensive availability of feedstocks and conducive physio-chemical surface characteristics. This review aims to highlight biochar production technologies, characteristics of biochar, and the latest advancements in immobilizing and eliminating heavy metal ions and organic pollutants in soil and water. Pyrolysis temperature, heat transfer rate, residence time, and type of feedstock are critical influential parameters. Biochar’s efficacy in managing contaminants relies on the pore size distribution, surface groups, and ion-exchange capacity. The molecular composition and physical architecture of biochar may be crucial when practically applied to water and soil. In general, biochar produced at relatively high pyrolysis temperatures can effectively manage organic pollutants via increasing surface area, hydrophobicity and microporosity. Biochar generated at lower temperatures is deemed to be more suitable for removing polar organic and inorganic pollutants through oxygen-containing functional groups, precipitation and electrostatic attraction. This review also presents the existing obstacles and future research direction related to biochar-based materials in immobilizing organic contaminants and heavy metal ions in effluents and soil.

Graphical Abstract

Polyethylenimine-grafted nitrogen-doping magnetic biochar for efficient Cr(VI) decontamination: Insights into synthesis and adsorption mechanisms

Herein, polyethylenimine (PEI)-grafted nitrogen (N)-doping magnetic biochar (PEI_MW@MNBC_BM) was synthesized, and characterization results showed that the microwave-assisted PEI grafting and ball milling-assisted N doping introduced abundant amino, pyridine N and pyrrole N structures onto biochar, which possessed high affinity to Cr(VI) in the anion form. The as-prepared PEI_MW@MNBC_BM displayed pH-dependence adsorption performance and high tolerance to co-existing ions with maximum uptake capacity of Cr(VI) identified as 183.02 mg/g. Furthermore, PEI_MW@MNBC_BM could bind Cr(VI) through electrostatic attraction, complexion, precipitation, reduction and pore filling. Especially, effective reduction of Cr(VI) was ascribed to cooperative electron transfer of partial oxygen-containing functional groups, intramolecular pyridine/pyrrole N, protonated amino and Fe²⁺ on the adsorbent, while oxygen-containing and amino functional groups from N-doping biochar and PEI synergistically complexed Cr(III) via providing lone pair electrons to form coordinate bonds. Furthermore, the stable precipitation was formed between Fe³⁺ and Cr(III). Additionally, the Cr(VI) elimination efficiency could maintain 95.83% even after four adsorption-desorption cycles, suggesting PEI_MW@MNBC_BM as a high-performance adsorbent for Cr(VI) contaminated water remediation.

Application of chitosan- and alginate-modified biochars in promoting the resistance to paddy soil acidification and immobilization of soil cadmium

To develop more green, practical and efficient biochar amendments for acidic soils, chitosan-modified biochar (CRB) and alginate-modified biochar (ARB) were prepared, and their effects on promoting soil pH buffering capacity (pHBC) and immobilizing cadmium (Cd) in the paddy soils were investigated through indoor incubation experiments. The results of Fourier transform infrared spectroscopy and Boehm titration indicated that the introduction of chitosan and sodium alginate effectively amplified the functional groups of the biochar, and improved acid buffering capacity of the biochar. Since there was a plateau region between pH 4.5 and 5.5 in acid-base titration curve of the CRB, adding this biochar to acidic paddy soils apparently improved the pHBC and enhanced the acidification resistance of the paddy soils. The addition of ARB enhanced the reduction reactions during submerging and weakened the oxidation reactions during draining, thus retarded the decline of paddy soil pH during drainage. Furthermore, the pH of the paddy soils with ARB addition was higher at the end of draining, which reduced the activity of soil Cd. Considering the environmental sustainability of chitosan and sodium alginate and convenience of preparation method, biochars modified with these two materials provided alternatives for acidic paddy soil amelioration and heavy metal immobilization. However, the additional experiments should be conducted under field conditions to confirm practical application effects in the future.