Adsorption of Cd(II) from aqueous solutions by rape straw biochar derived from different modification processes

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.

Nano-hydroxyapatite modified biochar: Insights into the dynamic adsorption and performance of lead (II) removal from aqueous solution

Adsorption of lead as Pb(II) using biochar is an environmentally sustainable approach to remediate this kind of pollution affecting wastewater. In this study, rice straw biochar (BC) was modified by combination with nano-hydroxy-apatite (HAP), resulting in a material designated as BC@nHAP, with enhanced adsorption performance. Based on Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses, it was evidenced that, after modification, HAP greatly enhanced surface functional groups (i.e., -COOH and/or -OH) of raw biochar's surface. Batch tests showed that the maximum sorption capacity of BC (63.03 mg g^-1) was improved due to the modification, reaching 335.88 mg g^-1 in BC@nHAP. Pseudo-second order (PSO) kinetics fitted well the adsorption data (R² = 0.99), as well as the Langmuir isotherm model (showing an adsorption value of 335.88 mg g^-1 for q_e). The results of thermodynamic calculations showed that the adsorption was primarily governed by chemisorption process. FTIR spectroscopy and XPS spectrum after adsorption further confirmed that the adsorption mechanisms were ion exchange with Pb²⁺ and surface complexation by -OH and -COOH. In addition, BC@nHAP revealed a brilliant regeneration capability. The maximum adsorption capacity by BC@nHAP was higher than that of raw biochar or other previously reported adsorbents. Therefore, BC@nHAP could be seen as a new sorbent material with high potential for real-scale heavy metal removal from wastewater, and specifically as a capable candidate new sorbent for Pb(II) removal from wastewater, which has clear implications as regard preservation of environmental quality and public health.

Considerations for evaluating innovative stormwater treatment media for removal of dissolved contaminants of concern with focus on biochar

Stormwater from complex land uses is an important contributor of contaminants of concern (COCs) such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), Copper, and Zinc to receiving water bodies. A large portion of these COCs bind to particulate matter in stormwater, which can be removed through filtration by traditional media. However, the remaining dissolved COCs can be significant and require special attention such as engineered treatment measures and media. Biochar is a porous sorbent produced from a variety of organic materials. In the last decade biochar has been gaining attention as a stormwater treatment medium due to low cost compared to activated carbon. However, biochar is not a uniform product and selection of an appropriate biochar for the removal of specific contaminants can be a complex process. Biochars are synthesized from various feedstocks and using different manufacturing approaches, including pyrolysis temperature, impact the biochar properties thus affecting ability to remove stormwater contaminants. The local availability of specific biochar products is another important consideration. An evaluation of proposed stormwater control measure (SCM) media needs to consider the dynamic conditions associated with stormwater and its management, but the passive requirements of the SCM. The media should be able to mitigate flood risks, remove targeted COCs under high flow SCM conditions, and address practical considerations like cost, sourcing, and construction and maintenance. This paper outlines a process for selecting promising candidates for SCM media and evaluating their performance through laboratory tests and field deployment with special attention to unique stormwater considerations.

State-of-the-art of research progress on adsorptive removal of fluoride-contaminated water using biochar-based materials: Practical feasibility through reusability and column transport studies

Fluoride (F^-) is one of the essential elements found in soil and water released from geogenic sources and several anthropogenic activities. Fluoride causes fluorosis, dental and skeletal growth problems, teeth mottling, and neurological damage due to prolonged consumption, affecting millions worldwide. Adsorption is an extensively implemented technique in water and wastewater treatment for fluoride, with significant potential due to efficiency, cost-effectiveness, ease of operation, and reusability. This review highlights the current state of knowledge for fluoride adsorption using biochar-based materials and the limitations of biochar for fluoride-contaminated groundwater and industrial wastewater treatment. Biochar materials have shown significant adsorption capacities for fluoride under the influence of low pH, biochar dose, initial concentration, temperature, and co-existing ions. Modified biochar possesses various functional groups (-OH, -CC, -C-O, -CONH, -C-OH, X-OH), in which enhanced hydroxyl (-OH) groups onto the surface plays a significant role in fluoride adsorption via electrostatic attraction and ion exchange. Regeneration and reusability of biochar sorbents need to be performed to a greater extent to improve removal efficiency and reusability in field conditions. Furthermore, the present investigation identifies the limitations of biochar materials in treating fluoride-contaminated drinking groundwater and industrial effluents. The fluoride removal using biochar-based materials at an industrial scale for understanding the practical feasibility is yet to be documented. This review work recommend the feasibility of biochar-based materials in column studies for fluoride remediation in the future.

Phytoremediation plants (ramie) and steel smelting wastes for calcium silicate coated-nZVI/biochar production: Environmental risk assessment and efficient As(V) removal mechanisms

Arsenic is one of the most common and harmful pollutants in environment throughout the world, especially in aqueous solutions. In this study, two kinds of industrial solid wastes (Oxide scale (OS) and Blast furnace slag (BFS)) and one kind of phytoremediation plant waste (Ramie stalk) were used to prepare an environmentally friendly, low-cost, and efficient calcium silicate coated nano zero-valent iron (nZVI)/biochar composite (BOS) for As(V) adsorption. The potential environmental risks of BOS and their effects on removal of arsenic ions from aqueous media were investigated. The adsorption mechanism was explored and discussed based on XRD, SEM-EDS, XPS, etc. The results suggested that the environmental risk and heavy metals toxicity in BOS by co-pyrolysis were significantly reduced compared to the original materials, and no additional contaminant was observed in the subsequent experiments. Simultaneously, the BOS showed excellent As(V) removal capacity (>99%) and regenerative properties. The As(V) removal mechanisms are mainly ascribed to the complexation and co-precipitation between Fe and As, and the hydrogen bond between CO functional group of BOS and As. The mechanism of enhanced nZVI activity for As(V) removal was revealed. A protective layer of Ca₂SiO₄ was formed on the surface of nZVI during the co-pyrolysis process to prevent the passivation of nZVI. During the reaction process, the Ca₂SiO₄ covering the nZVI surface would be continuously detached to expose the fresh surface of nZVI, thus providing more redox activity and adsorption sites. This study provides a new way to treat and recycle industrial steel solid wastes and phytoremediation plant wastes, and the produced calcium silicate coated-nZVI/biochar composite is proposed to be a very promising material for practical remediation of As(V)-contaminated water bodies.

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.

Functionalized biochars: Synthesis, characterization, and applications for removing trace elements from water

Biochar (BC) has been recognized as an effective adsorbent to remove trace elements (TEs) from water. However, low surface functionality and small pore size can limit the adsorption ability of pristine biochar. These limitations can be addressed by using functionalized biochars which are developed by physical, chemical, or biological activation of biochar to improve their physico-chemical properties and adsorption efficiency. Despite the large amount of research concerning functionalized biochars in recent decades, to our knowledge, no comprehensive review of this topic has been published. This review focuses solely on the synthesis, characterization, and applications of functionalized/engineered biochars for removing TEs from water. Firstly, we evaluate the synthesis of functionalized biochars by physical, chemical, and biological strategies that yield the desired properties in the final product. The following section describes the characterization of functionalized biochars using various techniques (SEM, TEM, EDS, XRD, XANES/NEXAFS, XPS, FTIR, and Raman spectroscopy). Afterward, the role of functionalized biochars in the adsorption of different TEs from water/wastewater is critically evaluated with an emphasis on the factors affecting sorption efficiency, sorption mechanisms, fate of sorbed TEs from contaminated environments and associated challenges. Finally, we specifically scrutinized the future recommendations and research directions for the application of functionalized biochar. This review serves as a comprehensive resource for the use of functionalized biochar as an emerging environmental material capable of removing TEs from contaminated water/wastewater.

Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO₄^-, CrO₄^2-, and Cr₂O₇^2- ions in aqueous solutions. At low pH (∼1-4), the availability of HCrO₄^- ions attributes the electrostatic forces of attraction due to the available functional groups such as -NH₄⁺, -COOH, and -OH₂⁺, which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr(VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr(VI) removal and other heavy metals are elegantly discussed.

Current trends in production, morphology, and real-world environmental applications of biochar for the promotion of sustainability

Biochar has sparked a lot of interest in recent years for various applications such as contaminant removal, carbon sequestration and soil amendment. However, laboratory scale studies dominates over the real-world applications. In this study, we first enumerated on the various ongoing uses of biochar along with adsorption mechanism; and then identified the most suitable source, and efficient method for biochar production; and finally proposed real-world practical applications of biochar as an additive to control diffuse micropollutant pollution along the highways and railway tracks; as well as the pollution through agricultural runoff. We demonstrated that directly or indirectly, biochar can definitely help to achieve all the 17 Sustainable Development Goals. The study on the practical use of biochar is an emerging area, because a lot of research needs are there to realise the full-potential of biochar with ways of attaining sustainable development goals towards waste, energy and pollution management.

Effect of varying pH and co-existing microcystin-LR on time- and concentration-dependent cadmium sorption by goethite-modified biochar derived from distillers' grains

Cadmium (Cd) is one dangerous and widespread heavy metal that of great environmental concern. To cost-efficiently adsorb aqueous Cd under influence of various factors, this study succeeded in fabricating goethite-modified biochar (GBC) derived from distillers' grains (DGs) for Cd sorption of different concentrations (10-100 mg L^-1) at pH of 3, 6 and 8 with and without microcystin-LR (MC-LR). Sorption kinetics and isotherms data revealed that Cd sorption capacity of GBC and unmodified BC increased as pH elevated from 3 to 6 but stabilized when pH further elevated to 8. Pseudo-second-order and Langmuir models more accurately fitted to sorption data for both BCs, implying monolayer chemisorption of Cd onto BCs. GBC exhibited more robust sorption for each Cd concentration than unmodified BC, with the maximum sorption capacity of around 28 mg g^-1 at neutral and weak alkaline pH. Notably, goethite-modification obviously increased bulk polarity, specific surface area, porosity and surface oxygenic group abundance of BC, thus strongly enhancing Cd sorption by creating more sorption sites mainly via pore-filling, electrostatic attraction, and also via complexation and cation exchange. Co-existing MC-LR of 100 μg L^-1 did not obviously affect Cd sorption by both BCs for most Cd levels at each pH, mostly because sorption mechanisms diverged between MC-LR and Cd to largely avoid their competition for sorption sties. Thus, goethite could modify DG-BC as promising and cost-efficient sorbent for Cd even with co-existing MC-LR, especially at neutral and weak alkaline pH that common in the nature. This study was greatly implicated in modifying and applying DG-BC for Cd immobilization in MC-LR laden waters with various pH circumstances.

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

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

Effects of different modifiers on the sorption and structural properties of biochar derived from wheat stalk

Nitrobenzene is a widespread contaminant in water. Biochar (BC) is a promising material for removing organic pollutants, but the adsorption capacity of pristine BC is low. Chemical modification is often used to improve the adsorption performance, but information on the sorption of nitrobenzene by modified BC is rare. In this study, BCs pyrolyzed at 300, 500, and 700 °C were modified by hydrochloric acid (HCl), sulfuric acid (H2SO4), sodium hydroxide (NaOH), hydrogen peroxide (H2O2), and nitric acid (HNO3), respectively. The properties, nitrobenzene sorption behaviors, and sorption mechanisms of different BCs were analyzed. The results showed that chemical modification decreased the sorption of nitrobenzene on BCs pyrolyzed at 300 °C, possibly due to the loss of the partition phase and the increase in polarity after modification. Regarding BCs pyrolyzed at 500 and 700 °C, the NaOH and HCl modifications significantly increased the sorption capacity by 19% and 60%, 18%, and 41%, respectively, possibly due to the increase in surface area, available pores, and aromaticity, while HNO3 modification decreased the sorption capacity by 41% and 31%. Two reasons were probably responsible for the decrease: one was the decrease in surface area after HNO3 modification due to the destruction of pore walls and the continuity of holes; the other was the strong repulsion between the nitro groups formed on the surface of BC and the nitro groups of nitrobenzene that drove nitrobenzene molecules away from the surface. A principal component-based comprehensive evaluation of the BC properties, which were significantly correlated with the sorption isotherm parameters, was used to evaluate the nitrobenzene sorption performance of the modified BC. Overall, BC pyrolyzed at 700 °C modified with NaOH or HCl were proposed as effective sorption materials for the removal of nitrobenzene in environment, which also provided a chemical modified method of biochar derived from agricultural waste.

Enhanced adsorption performance of sulfamethoxazole and tetracycline in aqueous solutions by MgFe2O4-magnetic biochar

Biochar has been reported as an excellent adsorbent for antibiotics, but the application faces the challenges of complicated separation. Here, MgFe₂O₄-magnetic biochars (MBCs) derived from corncob were synthesized at 300 °C to remove sulfamethoxazole (SMX) and tetracycline (TC) simultaneously. The characteristics of MBC300 had a high magnetic intensity. MBC300 had the maximum adsorption capacity of SMX with 50.75 mg/g and the high adsorption amount of TC with 120.36 mg/g respectively, which were 4.49 and 6.48 times those of BC300. MBC300 had the advantage of energy conservation compared with MBC450 and MBC600. The better fitting kinetics and isotherms indicated that the SMX and TC sorption onto MBC300 were governed by chemisorption. FTIR and XPS analyses confirmed that the SMX sorption onto MBC300 was dominated by polar interactions and π-π electron donor-acceptor interactions (π-π EDA). Furthermore, the TC sorption was involved in pore filling, π-π EDA, H-bonds, and surface complexation. MBC300 presented effective adsorption of SMX and TC over a wide range of pH. The competition between antibiotics and coexisting pollutants of dissolved organic matter (DOM), Ca²⁺, CO₃^2-, and PO₄^3- significantly inhibited the sorption. The results indicate that MBC300 is an effective and promising adsorbent to treat SMX and TC simultaneously.

Surface Modification of Biochar for Dye Removal from Wastewater

Nowadays, biochar is being studied to a great degree because of its potential for carbon sequestration, soil improvement, climate change mitigation, catalysis, wastewater treatment, energy storage, and waste management. The present review emphasizes on the utilization of biochar and biochar-based nanocomposites to play a key role in decontaminating dyes from wastewater. Numerous trials are underway to synthesize functionalized, surface engineered biochar-based nanocomposites that can sufficiently remove dye-contaminated wastewater. The removal of dyes from wastewater via natural and modified biochar follows numerous mechanisms such as precipitation, surface complexation, ion exchange, cation–π interactions, and electrostatic attraction. Further, biochar production and modification promote good adsorption capacity for dye removal owing to the properties tailored from the production stage and linked with specific adsorption mechanisms such as hydrophobic and electrostatic interactions. Meanwhile, a framework for artificial neural networking and machine learning to model the dye removal efficiency of biochar from wastewater is proposed even though such studies are still in their infancy stage. The present review article recommends that smart technologies for modelling and forecasting the potential of such modification of biochar should be included for their proper applications.

Effect of Pyrolysis Temperature on Removal Efficiency and Mechanisms of Hg(II), Cd(II), and Pb (II) by Maize Straw Biochar

Pyrolysis temperature significantly affects the properties of biochar, which in turn can affect the removal of heavy metal ions and the underlying mechanism. In this work, biochars from the pyrolysis of maize straw at 300, 400, and 500 °C (BC300, BC400, and BC500, respectively) and wheat straw at 400 °C (WBC400) were investigated. The influence of production temperature on the adsorption of Hg2+, Cd2+, and Pb2+ by maize straw biochar was investigated by the characterization of the biochars and by adsorption tests. The adsorption capacities of maize and wheat straw biochar were compared in an adsorption experiment. Biochar BC400 showed the best physical and chemical properties and had the largest number of surface functional groups. The pseudo-second-order kinetic model was more suitable for describing the adsorption behavior of metal ions to biochar. The Langmuir model better fit the experimental data. Biochar BC400 had a higher adsorption speed and a stronger adsorption capacity than WBC400. The sorption of Pb2+ and Hg2+ to maize straw biochar followed the mechanisms of surface precipitation of carbonates and phosphates and complexation with oxygenated functional groups and delocalized π electrons. The adsorption mechanism for Cd2+ was similar to those of Hg2+ and Pb2+, but precipitation mainly occurred through the formation of phosphate. In the multi-heavy-metal system, the adsorption of Cd2+ by BC400 was inhibited by Pb2+ and Hg2+. In summary, BC400 biochar was most suitable for the adsorption effect of heavy metals in aqueous solution.

Two-step modification (sodium dodecylbenzene sulfonate composites acid-base) of sepiolite (SDBS/ABsep) and its performance for remediation of Cd contaminated water and soil

To enhance the remediation capability of cadmium (Cd) polluted water and soil, our approach involved two-step modification of sepiolite (Sep) through acid-base compound treatment and sodium dodecylbenzene sulfonate (referring as SDBS/ABsep), and then the batch adsorption and soil culture experiments were conducted to investigate its immobilization potential and mechanisms of Cd. The findings revealed that the SDBS/ABsep had a rougher surface and higher porosity, and the maximum adsorption capacity of Cd²⁺ onto SDBS/ABsep was 241.39 mg g^-1, which was 5.32 times higher than that on Sep. It conformed to the pseudo-second-order kinetic and Redlich-Paterson isotherm models. SDBS/ABsep exhibited a high efficiency for immobilization-induced remediation of Cd polluted soils. Upon the addition of different concentrations of SDBS/ABsep, DTPA-Cd content decreased by 17.41-47.33% compared with the control groups, and the ratio of residual fraction-Cd increased from 4.67% in unamended soil to 14.05% in the presence of 4% SDBS/ABsep. SEM-EDS, TEM, FTIR, XRD, and XPS analyses indicated that the interaction mechanisms between SDBS/ABsep and Cd included the electrostatic force, precipitation, ion exchange, and complexation of sulfonic acid groups. Therefore, SDBS/ABsep can be used as a promising effective passivation agent for remediation of Cd contaminated soil and water.

Purification mechanism of city tail water by constructed wetland substrate with NaOH-modified corn straw biochar

The pollution of corn straw to the environment had attracted much attention. The preparation and alkali modification of corn straw biochar as the constructed wetland (CW) substrate was conducive to solving the environment pollution caused by straw and improving the purification effect of CW. The NaOH modification mechanism of corn straw biochar was analyzed by measuring the surface morphology, element content, specific surface area (SSA), pore volume, crystal structure, surface functional groups and CO₂ adsorption. Biochar prepared under relatively optimal NaOH-modified conditions was used as the vertical flow CW substrate to treat city tail water. The results showed that controlling the modification condition of NaOH (< 1.0 mol·L^-1, ≤ 24 h) was conducive to prevent the biochar structure destruction and C element reduction. The SSA and pore volume of NaOH (0.1 mol·L^-1) modified biochar are 360 m²·g^-1 and 0.109 cm³·g^-1, respectively. The biochar adsorption for CO₂ conformed to the Langmuir and Freundlich isothermal adsorption theoretical model (R² > 0.9). The maximum adsorption capacity of CO₂ by modified biochar with NaOH (0.1 mol·L^-1) was 64.516 cm³·g^-1 and increased by 10.3%. The city tail water treated by CW with plants showed that the removal rates of ammonia nitrogen, total nitrogen and nitrate nitrogen reached about 90%. The research results improved the utilization value of straw, realized straw carbon sequestration and promoted the progress of CW technology.

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.

Improved adsorption properties of tetracycline on KOH/KMnO4 modified biochar derived from wheat straw

Modification of pristine biochars has received increasing attentions due to the significant potential in enhancing adsorption performance. In this work, the co-modification of KOH and KMnO₄ on biochar (K-Mn-BC) was performed, with the effect of KOH/KMnO₄ modification on biochar properties and their adsorption toward tetracycline (TC) being extensively explored. Results showed that KOH/KMnO₄ modification can significantly regulate biochars to form hierarchical structure. The obtained K-Mn-BC was characterized with a high specific surface area (1524.6 m² g^-1) and total pore volume (0.85 cm³ g^-1). In addition, the K-Mn-BC exhibited a high adsorption capacity of 584.19 mg g^-1 toward TC at 318 K, and pseudo-second-order (R²:0.993～0.998) and Langmuir (R²: 0.834～0.874) models can fit well with the adsorption behavior. Moreover, the obtained K-Mn-BC can efficiently adsorb TC within a wide pH range (3.0-10.0), and were not affected by the co-existing ions. The possible mechanisms for the high adsorption capacity were ascribed to the pore filling and π-π interaction, following by hydrogen bonding and metal complexation. The obtained K-Mn-BC is a suitable adsorbent for TC removal from water due to the hierarchical structure, high adsorption capacity, and stable adsorption effect.

Biochar-based fertilizers and their applications in plant growth promotion and protection

Soil is an integral part of the ecosystem because it serves as a habitat for various microorganisms and lays the foundation for supporting plant growth and development. Therefore, factors such as increased anthropogenic activities hand by hand with other natural processes that harm the ecosystem may eventually lead to a decline in soil quality and fertility, hindering the growth of plants and soil microbial communities. Given the current global scenario of increasing human intervention, it is essential to find effective measures and reliable technologies to restore soil quality. Biochar is an emerging soil ameliorant employed for soil health restoration and is primarily generated through the anoxygenic pyrolysis of biomass. The biochar application in soil remediation may be beneficial due to biochar's unique physicochemical properties, including high carbon and metal fixation abilities. In addition, biochar possesses abilities to reduce the plant's environmental stress injuries. This review briefly overviewed the ingredients and mechanism of biochar productions. We then emphatically reviewed the advances in biochar applications in soil bioremediation, soil microflora growth stimulation, and the alleviation of various biotic and abiotic stresses in plants.

Adsorptive behavior of thallium using Fe3O4-kaolin composite synthesized by a room temperature ferrite process

Thallium (Tl) contaminants pose serious threats to the ecological environment and human health due to its acute/chronic poisoning on the health of most organisms even at low concentrations. To find a rapid and efficient technology in removing Tl from waters thus becomes a crucial issue. A magnetic Fe₃O₄-kaolin composite (denoted by FKC) with high specific surface area (133.7 m²/g) was successfully synthesized via a simple and low-cost technique for Tl(I) removing from various water media. The HRTEM images confirmed the existence of lattice fingers Fe₃O₄ and displayed that a large number of Fe₃O₄ nanoparticles dispersed on the surface of kaolin sheets. Compared with kaolin or Fe₃O₄ alone, FKC enhanced obviously the adsorption rate and capacity of Tl(I) over a wide pH range (4.5-9.0). The maximum adsorption capacity of FKC for Tl(I) was 19,347 mg/kg (calculated by Langmuir model), which was almost one hundred times and two times higher than those of kaolin and Fe₃O₄, respectively. Importantly, FKC was observed to have a great potential in removing Tl(I) from surface water, groundwater, and tap water in more alkaline conditions. By applying the external magnetic field, FKC could be recovered efficiently (99%) and rapidly (20 s). Moreover, Tl L₃-edge XANES spectra revealed that Tl(I) was adsorbed on the FKC and would not be converted to more toxic Tl(III). The cations (CaCl₂, NaCl, and KCl) and the ionic strength with concentrations of 0.001-1.0 mol/L showed a great influence on the adsorption of Tl(I) by FKC, implying that this adsorption was dominated by outer-sphere surface complexation at investigated pH values. The information provided is essential for designing a rapid and effective scavenger for removing Tl in various natural waters.

Investigating Biochar-Derived Dissolved Organic Carbon (DOC) Components Extracted Using a Sequential Extraction Protocol

Biochar-derived dissolved organic carbon (DOC), as the most important component of biochar, can be released on farmland, improving fertility and playing a role in soil amendment and remediation. The complexity of molecular structures and diversity of DOC compounds have influenced these functions to some extent. A sequential extract protocol consisting of water (25 °C), hot water (80 °C), and NaOH solution (0.05 M) was used to fully extract DOC compounds and gain a thorough understanding of the possible DOC components released from biochar. Rape straw (RS), apple tree branches (ATB), and pine sawdust (PS) were pyrolyzed at 300, 500, and 700 °C, respectively, to make nine distinct biochars. A TOC analyser, ultraviolet-visible spectroscopy (UV–vis), and excitation–emission fluorescence (EEM) spectrophotometer were used in conjunction with parallel factor analysis (PARAFAC) to determine the distribution of DOC content, the diversity of aromaticity, molecular weight characteristics and components of biochar-derived DOC. The results show that the relative distribution of water-extractable fractions ranged from 3.21 to 35.57%, with a low-aromaticity and extremely hydrophilic fulvic-acid-like compounds being found in the highest amounts (C2 and C3). The smallest amount of hot water-extractable components was produced from the release of small-molecule aliphatic compounds adsorbed on biochar and susceptible to migration loss once in a soil solution. More than half of the biochar-derived DOC was released in a NaOH solution, which primarily consisted of humic-acid-like compounds (C1), with higher molecular weights, more aromaticity, and lower bioavailability, according to the distribution of DOC in various extractants. In addition, the pyrolysis temperature and biomass type had a significant impact on the DOC properties released by biochar. As a result, the findings of this study showed that using a sequential extract protocol of water, hot water, and NaOH solution in combination with spectroscopic methods could successfully reveal the diversity of biochar-derived components, which could lead to new insights for the accurate assessment of potential environmental impacts and new directions for biochar applications.

Efficient with low-cost removal and adsorption mechanisms of norfloxacin, ciprofloxacin and ofloxacin on modified thermal kaolin: experimental and theoretical studies

Quinolone antibiotics (QNs) can be potential hazard to environment and human. Combination of experimental and theoretical studies was used to analyze the adsorption properties of norfloxacin, ciprofloxacin and ofloxacin on modified thermally activated kaolin (KL). Main factors (calcination temperature, dose, pH, cations and regeneration) affecting the adsorption were discussed. Adsorption processes fit the pseudo-second order kinetic and Langmuir model well. The adsorption removal of norfloxacin, ciprofloxacin and ofloxacin can reach 88.53%, 89.43% and 91.46%, respectively. Cations inhibited adsorption, and AlS-KLB can maintain 80% efficiency in five cycles under optimal conditions. Simulations showed that the materials had good adsorption capacity for QNs, and the "①" of KL had the best capacity. Simulations explain the adsorption mechanism: F, H, O atoms of QNs are covalently bonded to O atoms from KL, Al₂O₃ and Al (OH)₃, C atoms from amorphous carbon and H atoms from C-H and Al (OH)₃. The Al atoms of Al₂O₃ and Al, Si atoms of KL are ionically bonded to F, H, O atoms of QNs. This study shed new light on the removal of QNs by providing low-cost and efficient modified KL and elucidating the adsorption mechanism in conjunction with DFT simulations.

Biogeochemical Fe(II) generators as a new strategy for limiting Cd uptake by rice and its implication for agricultural sustainability

This work has developed a new strategy of biogeochemical Fe(II) generators for activating microbial Fe(II) generation to immobilize Cd in soils through protons scavenging and coprecipitation. A new biochar modified magnetite (FeBC15) has been fabricated through a top-down method, with which microbial respiration can be stimulated in paddy soil. The FeBC15 exhibits a higher adsorption capacity for Cd than pristine magnetite (1.7 times). The results show that the available Cd can be reduced by 14.4% after adding FeBC15 compared to the control. More importantly, FeBC15 particles promote the conversion of MgCl₂ - Cd to stable crystalline Fe/Al bound Cd under the incubation period. The enhanced pH and Fe(II) leads to a comparably lower Cd availability in soils than in pristine soils, which are supported by the enhanced relative abundance of Geobacter and Clostridium with the FeBC15 treatment (i.e. up to 7.44-7.68 × 10⁹ copies/g soil). The Diffusive Gradients in Thin-films (DGT) study indicates that FeBC15 can lower the replenish capacity of soils (i.e. K_dL values of 0.2-3.6 mL/g) to soil pore waters and limit root absorption. Pot experiments demonstrate that this strategy can alleviate the rice Cd content by 38.4% (< 0.2 mg/kg). This work paves a new pathway for reducing Cd uptake in rice, enabling sustainable remediation of paddy soil.

Efficient removal of Cd(II) by phosphate-modified biochars derived from apple tree branches: Processes, mechanisms, and application

Phosphate (P)-modified biochar is a good material for cadmium (Cd) immobilization, and the pore-forming effect of potassium ions (K⁺) can favor the P loading on biochar. However, few studies have been done specifically on Cd(II) removal by composites of potassium phosphates with biochar, and the removal potential and mechanisms are not clear. Herein, apple tree branches, a major agricultural waste suitable for the development of porous materials, were pyrolyzed individually or together with KH₂PO₄, K₂HPO₄·3H₂O, or K₃PO₄·3H₂O to obtain biochars to remove Cd(II), denoted as pristine BC, BC-1, BC-2, and BC-3, respectively. The results showed that the orthophosphates containing more K⁺ enlarged the specific surface area, total pore volume and phosphorus loading of biochar. Co-pyrolysis of apple tree branches and P promoted the thermochemical transformation of P species. Only weak signal of orthophosphate was observed in the pristine BC, while the presence of orthophosphate, pyrophosphate and metaphosphate were detected in BC-1, and BC-2 and BC-3 showed the presence of orthophosphate and pyrophosphate. The maximum Cd(II) adsorption capacities of pristine BC, BC-1, BC-2 and BC-3 were 10.4, 88.5, 95.8, and 116 mg·g^-1, respectively. Orthophosphate modification enhanced the Cd(II) adsorption capacity due to the formation of Cd-P-precipitates, namely Cd₅(PO₄)₃Cl, Cd₅(PO₄)₃OH, Cd₃(PO₄)₂, Cd₂P₂O₇, and Cd(PO₃)₂. Furthermore, higher cation exchange efficiencies between Cd(II) and K⁺ in P-modified biochars also contributed to their high Cd(II) adsorption capacity. Cd(II) removal by BC-3 from artificially polluted water bodies showed more than 99.98% removal rates. Application of BC-3 also reduced the diethylene triamine pentaacetic acid-extracted Cd(II) in soil by 69.1%. The co-pyrolysis of apple tree branches and potassium phosphates shows great prospect in Cd(II) wastewater/soil treatment and provide a promising solution for agricultural waste utilization and carbon sequestration.

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 Pyrolysis Temperature and Chemical Modification on the Adsorption of Cd and As(V) by Biochar Derived from Pteris vittata

Phytoremediation can be applied successfully to solve the serious worldwide issue of arsenic (As) and cadmium (Cd) pollution. However, the treatment of biomass containing toxic elements after remediation is a challenge. In this study, we investigated the effective use of biomass resources by converting the As hyperaccumulator P. vittata into biochar to adsorb toxic elements. Plant biomass containing As was calcined at 600, 800, and 1200 °C, and its surface structure and adsorption performances for As(V) and Cd were evaluated. Pyrolysis at 1200 °C increased the specific surface area of the biochar, but it did not significantly affect its adsorption capacity for toxic elements. The calcined biochar had very high adsorption capacities of 90% and 95% for As(V) and Cd, respectively, adsorbing 6000 mmol/g-biochar for As(V) and 4000 mmol/g-biochar for Cd. The As(V) adsorption rate was improved by FeCl₃ treatment. However, the adsorption capacity for Cd was not significantly affected by the NaOH treatment. In conclusion, it was found that after phytoremediation using P. vittata biomass, it can be effectively used as an environmental purification material by conversion to biochar. Furthermore, chemical modification with FeCl₃ improves the biochar’s adsorption performance.

Investigation of seawater mineral promoted pyrolysis at low temperature for improving the adsorption capabilities of biochar

Naturally abundant seawater mineral was employed to engineer banana pseudostem and bamboo biochars through pyrolysis at different low temperatures for improving their adsorption capabilities for methylene blue (MB) and tetracycline (TC). The adsorption capabilities were greatly enhanced as the biochars were pyrolyzed at 300 °C with 50/1 (mL/g) dosage of seawater to biomass. For instance, the engineered banana pseudostem biochar exhibited 8.00 and 6.54 times higher adsorption capabilities than the corresponding pristine biochar for MB (447.79 mg/g vs 55.96 mg/g) and TC (100.59 mg/g vs 16.75 mg/g) at 25 °C, respectively. The characterization results indicated that a large number of carboxylates, lactone acid salts, and alkoxides were generated on the engineered biochar and a high cation exchange capacity was gained. The adsorption of MB was mainly attributed to cation exchange complying with hydrogen bonding and electrostatic interaction, whereas the adsorption of TC was realized by hydrogen bonding and complexation.

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

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

Contributions of Various Cd(II) Adsorption Mechanisms by Phragmites australis-Activated Carbon Modified with Mannitol

Due to its high toxicity, persistence, and bioaccumulation in the food chain, controlling cadmium (Cd) pollution in wastewater is urgent. Activated carbon is a popular material for removing Cd. To improve the Cd(II) adsorption efficiency by increasing the number of oxygen-containing functional groups, Phragmites australis-activated carbon (PAAC) was modified with mannitol at a low temperature (150 °C). The textural and chemical characteristics of PAAC and modified PAAC (M-PAAC) were analyzed by surface area analysis, elemental analysis, Boehm's titration, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Batch adsorption experiments were conducted to investigate the influence of Cd(II) concentration, contact time, ionic strength, and pH on Cd(II) adsorption. The main adsorption mechanisms of Cd(II) on activated carbon were quantitatively calculated. The results showed that mannitol modification slightly decreased the S BET (5.30% of PAAC) and increased the content of carboxyl, lactone, and phenolic groups (total increase of 43.96% with PAAC), which enhanced the adsorption capacity of PAAC by 58.59%. The adsorption isotherms of PAAC and M-PAAC were described well using the Temkin model, while the intraparticle diffusion model fitted the Cd(II) adsorption kinetics best. Precipitation with minerals was a crucial factor for Cd(II) adsorption on activated carbon (50.40% for PAAC and 40.41% for M-PAAC). Meanwhile, the Cd(II) adsorption by M-PAAC was also dominated by complexation with oxygen-containing functional groups (33.60%). This research provides a method for recovering wetland plant biomass to prepare activated carbon and efficiently treat Cd-containing wastewater.

Magnetically Recyclable Loofah Biochar by KMnO4 Modification for Adsorption of Cu(II) from Aqueous Solutions

Novel KMnO₄-modified loofah biochar loaded with nano-Fe₂O₃ (FMLB) was successfully synthesized for Cu(II) adsorption. Nitrogen adsorption method, scanning electron microscope, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometer, and other characterization measurements were used to evaluate the physical and chemical properties of FMLB and nano-Fe₂O₃-loaded biochar (FLB). The results show that the adsorption behavior of Cu(II) can be best fitted by the Langmuir isotherm model and the pseudo-second-order (PSO) kinetic model, indicating that the surface of FMLB was composed of homogeneous adsorption, and chemical adsorption dominated the adsorption process under optimal reaction conditions. The adsorption capacity of FMLB is improved by 42.86% compared to FLB, and it remained over 75% after four cycles. The inner-sphere complexes with manganese oxide (MnO _x ) and oxygen-containing functional groups, as well as electrostatic interaction, physical adsorption, and ion exchange, play important roles in Cu(II) adsorption. The saturation magnetization of FMLB was 10.41 emu/g, ensuring that it can be easily separated from aqueous solutions. Therefore, magnetically recyclable biochar modified by KMnO₄ is a feasible method for Cu(II) adsorption.

Pristine and engineered biochar for the removal of contaminants co-existing in several types of industrial wastewaters: A critical review

Biochar has been widely studied as an adsorbent for the removal of contaminants from wastewater due to its unique characteristics, such as having a large surface area, well-distributed pores and high abundance of surface functional groups. Critical review of the literature was performed to understand the state of research in utilizing biochars for industrial wastewater remediation with emphasis on pollutants that co-exist in wastewater from several industrial activities, such as textile, pharmaceutical and mining industries. Such pollutants include organic (such as synthetic dyes, phenolic compounds) and inorganic contaminants (such as cadmium, lead). Multiple correspondence analyses suggest that through batch equilibrium, columns or constructed wetlands, researchers have used mechanistic modelling of isotherms, kinetics, and thermodynamics to evaluate contaminant removal in either synthetic or real industrial wastewaters. The removal of organic and inorganic contaminants in wastewater by biochar follows several mechanisms: precipitation, surface complexation, ion exchange, cation-π interaction, and electrostatic attraction. Biochar production and modifications promote good adsorption capacity for those pollutants because biochar properties stemming from production were linked to specific adsorption mechanisms, such as hydrophobic and electrostatic interactions. For instance, adsorption capacity of malachite green ranged from 30.2 to 4066.9 mg g^-1 depending on feedstock type, pyrolysis temperature, and chemical modifications. Pyrolyzing biomass at above 500 °C might improve biochar quality to target co-existing pollutants. Treating biochars with acids can also improve pollutant removal, except that the contribution of precipitation is reduced for potentially toxic elements. Studies on artificial intelligence and machine learning are still in their infancy in wastewater remediation with biochars. Meanwhile, a framework for integrating artificial intelligence and machine learning into biochar wastewater remediation systems is proposed. The reutilization and disposal of spent biochar and the contaminant release from spent biochar are important areas that need to be further studied.

The Use of Gigantochloa Bamboo-Derived Biochar for the Removal of Methylene Blue from Aqueous Solution

In this study, locally grown bamboo (Gigantochloa spp.) was used as feedstock for pyrolysis production of biochar under various pyrolysis temperatures (400–800°C). The resultant biochars were tested for their performance in adsorptive removal of the methylene blue (MB) dye. The scope of the adsorption experiment includes the effects of adsorbent dosage, solution pH, initial adsorbate concentration, and contact time. The adsorption data confirmed that pyrolysis temperature has a significant effect on adsorptive performance, whereas biochar pyrolysed at 500°C (BC500) has the highest adsorptive performance with the maximum adsorption capacity (derived from the Langmuir model) being 86.6 mg g-1. Basic characterisations (SEM, EDX, XRD, FTIR, and BET) were carried out for BC500 where FTIR and SEM confirmed the adsorption of MB onto the biochar, while the BET data showed the reduction of the BET surface area, total pore volume, and pore diameter after the adsorption process.

Competitive sorption of Cd, Cr, Cu, Ni, Pb and Zn from stormwater runoff by five low-cost sorbents; Effects of co-contaminants, humic acid, salinity and pH

For a comprehensive estimation of metals removal by sorbents in stormwater systems, it is essential to evaluate the impacts of co-contaminants. However, most studies consider only metals (single or multiple), which may overestimate performance. This study employed a batch method to investigate the performance of five low-cost sorbents - coconut coir fiber (CCF), blast furnace slag (BFS), waste tire crumb rubber (WTCR), biochar (BC), and iron coated biochar (FeBC) - for simultaneous removal of Cd, Cr, Cu, Ni, Pb and Zn from simulated stormwater (SSW) containing other contaminants (nutrients and polycyclic aromatic hydrocarbons). BFS and CCF demonstrated the highest sorption capacity of all metals (> 95% removal) in all systems (single and multi-contaminant). However, the presence of other contaminants in solution reduced metals removal for other sorbents, as follows (highest to lowest removal): single-metal > multi-metal > multi-contaminant solutions, and removal efficiency ranking among metals was generally Cr~Cu~Pb > Ni > Cd > Zn. Humic acid (HA) negatively affected the metal sorption, likely due to the formation of soluble HA-metal complexes; NaCl concentration did not impact removal, but alkaline pH improved removal. These findings indicate that sorbents need to be tested under realistic stormwater solution chemistry including co-contaminants to appropriately characterize performance prior to implementation.

Unraveling the molecular mechanisms of Cd sorption onto MnOx-loaded biochar produced from the Mn-hyperaccumulator Phytolacca americana

Engineered biochar represents a promising material for green remediation practices. In this paper, we present an innovative approach to produce MnO_x-loaded biochars by pyrolyzing the biomass of a Mn-hyperaccumulator species (Phytolacca americana). Batch sorption and stirred-flow kinetic experiments were combined with spectroscopic techniques to elucidate the mechanisms behind the Cd sorption onto those biochars, named here as PABCs. The incorporation of MnO_x into the PABCs increased their surface densities of oxygen-containing functional groups. The average Mn leaching (< 9%) from PABCs was lower than that measured for the non-pyrolyzed biomass of P. americana (30-43%). PABCs pyrolyzed at 500 °C had Cd sorption capacities as high as 212-337 mg/g, which achieved by far the best performance reported for biochar materials. The stirred-flow experiments showed that MnO_x loading was instrumental in increasing both the Cd sorption onto PABCs as well as its irreversibility. Extended X-ray absorption fine structure spectroscopy revealed that the Cd immobilization occurred mainly through its association with organic matter (Cd-OM) and, to a lesser extent, with carbonate (CdCO₃) and MnO_x (Cd-MnO_x). In short, MnO_x-loaded biochar prepared from the biomass of a Mn-hyperaccumulator species proved to be an effective, sustainable, and eco-friendly material for remediating Cd-contaminated waters.

Efficient removal of Cd (II) from aqueous solution by chitosan modified kiwi branch biochar

Production of cost-efficient composite materials from low-cost modified biochar for the removal of Cd (II) from wastewater is much needed to meet the growing needs of industrial wastewater treatments. A novel chitosan-modified kiwi branch biochar (CHKB) was fabricated as low-cost modified biochar for the removal of Cd (II) from aqueous solution. Batch adsorption and characterization experiments indicated that the modification of kiwi biochar (KB) by chitosan remarkably improved its adsorption performance. The results revealed that the adsorption isotherms can be best described by a Langmuir model and that a pseudo-second-order model fits the Cd (II) adsorption kinetics well, which indicates that it is a monolayer process controlled by chemisorption. CHKB exhibited a Langmuir maximum adsorption capacity of Cd (II) (126.58 mg g^-1), whereas that of KB was only 4.26 mg g^-1. The adsorption ability of CHKB was improved by increasing the surface area and an abundance of surface functional groups (-OH, -NH, CO, etc.). The cation exchange, electrostatic interaction, surface complexation, and precipitation were the main mechanisms in the sorption of Cd (II) on CHKB. Excellent adsorption performance, low cost, and environmental-friendliness made CHKB a fantastic adsorbent for the removal of Cd (II) in wastewater.

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

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

Manganese oxide-modified biochar: production, characterization and applications for the removal of pollutants from aqueous environments - a review

The development of manganese (Mn) oxides (MnOx) modified biochar (MnOBC) for the removal of pollutants from water has received significant attention. However, a comprehensive review focusing on the use of MnOBC for the removal of organic and inorganic pollutants from water is missing. Therefore, the preparation and characterization of MnOBC, and its capacity for the removal of inorganic (e.g., toxic elements) and organic (e.g., antibiotics and dyes) from water have been discussed in relation to feedstock properties, pyrolysis temperature, modification ratio, and environmental conditions here. The removal mechanisms of pollutants by MnOBC and the fate of the sorbed pollutants onto MnOBC have been reviewed. The impregnation of biochar with MnOx improved its surface morphology, functional group modification, and elemental composition, and thus increased its sorption capacity. This review establishes a comprehensive understanding of synthesizing and using MnOBC as an effective biosorbent for remediation of contaminated aqueous environments.

Optimizing magnetic functionalization conditions for efficient preparation of magnetic biochar and adsorption of Pb(II) from aqueous solution

Recoverable magnetic biochar has great potential for treating wastewater contaminants such as Pb(II). However, whether magnetic modification could enhance metal adsorption efficiency is currently contradictory in the literature mainly due to the differences in selecting various magnetic functionalization conditions. Considering this gap in knowledge, the effects of magnetic functionalization method (impregnation and precipitation), concentration of precursor iron solution (0.01-1 M), and pyrolysis temperature (300-700 °C) on the characteristics and Pb(II) adsorption capacity of biochar were systematically investigated in this paper. Results indicated that Fe₃O₄ was the main product for magnetic biochars synthesized using the impregnation (denoted as FWFe(3)) and precipitation methods (denoted as FWFe(2)). Magnetic functionalization resulted in remarkably increased pH and more negative zeta potential for FWFe(2) samples, whereas FWFe(3) samples showed the opposite trends. The adsorption of Pb(II) on different biochars fitted the pseudo-second order model and the Langmuir model. The maximum adsorption capacity was 817.64 mg/g for FWFe(2)1M700C (precipitation by 1 M of Fe(II)/Fe(III), pyrolysis at 700 °C), outperforming FWFe(3) and pristine biochar samples by around 5-13 times. Mechanism study indicated that the adsorption mainly involved electrostatic attraction, ion exchange, co-precipitation, and complexation. Pb(II) adsorption capacity was strongly dependent on the alkali pH of biochar. However, this efficiency was less affected by biochar surface area and its morphology. The higher pH of FWFe(2) samples not only led to an increased surface charge for stronger electrostatic attraction and ion exchange but also favored the formation of co-precipitates. By contrast, FWFe(3) samples showed a decreased adsorption capacity for Pb(II) with increased concentration of embedded iron. Overall, magnetic biochar, prepared using precipitation followed by high-temperature pyrolysis (such as, FWFe(2)1M700C), can be a promising adsorbent for Pb(II) adsorption from wastewater.

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

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

Modification of naturally abundant resources for remediation of potentially toxic elements: A review

Water and soil contamination due to potentially toxic elements (PTEs) represents a critical threat to the global ecosystem and human health. Naturally abundant resources have significant advantages as adsorbent materials for environmental remediation over manufactured materials such as nanostructured materials and activated carbons. These advantages include cost-effectiveness, eco-friendliness, sustainability, and nontoxicity. In this review, we firstly compare the characteristics of representative adsorbent materials including bentonite, zeolite, biochar, biomass, and effective modification methods that are frequently used to enhance their adsorption capacity and kinetics. Following this, the adsorption pathways and sites are outlined at an atomic level, and an in-depth understanding of the structure-property relationships are provided based on surface functional groups. Finally, the challenges and perspectives of some emerging naturally abundant resources such as lignite are examined. Although both unamended and modified naturally abundant resources face challenges associated with their adsorption performance, cost performance, energy consumption, and secondary pollution, these can be tackled by using advanced techniques such as tailored modification, formulated mixing and reorganization of these materials. Recent studies on adsorbent materials provide a strong foundation for the remediation of PTEs in soil and water. We speculate that the pursuit of effective modification strategies will generate remediation processes of PTEs better suited to a wider variety of practical application conditions.

Identifying adsorption sites for Cd(II) and organic dyes on modified straw materials

Turning agricultural waste into effective remediation materials is a highly promising approach for reducing in-field crop burning and promoting affordable wastewater treatment. This comparative study aims to identify active adsorption sites for methylene blue (MB), crystal violet (CV), and cadmium (Cd) as model pollutants on wheat straw materials modified by a thermal partial-oxidation process. The optimal modification temperature was found to be 160-180 °C for MB and CV adsorption, which is much lower than that of Cd(II) at 220-240 °C. A strong linear correlation exits between total surface group concentrations and Cd(II) uptake, indicating that both acidic and basic functional groups are favourable adsorption sites of Cd(II). By contrast, basic groups generated at higher modification temperatures might have adverse effects on MB and CV adsorption. These results provided mechanistic insights and predictive approach into reuse of agricultural waste for environmental remediation.

Characterization of cadmium accumulation in the cell walls of leaves in a low-cadmium rice line and strengthening by foliar silicon application

Cadmium (Cd) remobilization in leaves is affected by whether Cd is stored in nonlabile subcellular compartments, which might be regulated by silicon (Si) application. However, the underlying mechanism is still far from being completely understood. In this research, the Cd distribution pattern in leaves and a Cd-binding characterization in the cell wall of the low-Cd rice line YaHui2816 were investigated through one hydroponic experiment with 10 μM Cd in solutions. Foliar Si application was further adopted to explore its influence on the Cd accumulation in the cell walls of leaves in YaHui2816. Most of the Cd (69.4%) was distributed in the cell walls of YaHui2816 leaves, whereas the isolated cell walls of leaves from YaHui2816 exhibited a lower capacity for Cd chemisorption than the contrasting line C268A, which was resulted from its fewer relative peak areas of functional groups in the cell wall, such as carboxyl CO and OH stretching. Foliar Si application significantly increased the Cd concentration in leaves and various cell wall fractions (pectin, hemicellulose 1 and residue) by 191% and 137-160%, respectively. RNA-seq analysis revealed that foliar Si application depressed the expression of the metal transporters OsZIP7 and OsZIP8, up-regulated the expression of genes participating in the glutathione metabolism and the cellulose synthesis. Overall, the influence of foliar Si application on Cd-accumulation in the cell wall of leaves in a low-Cd rice line was demonstrated in this research, which inspires further avenues to ensure the food safety of rice grains.

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.