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

Predicting Cd(II) adsorption capacity of biochar materials using typical machine learning models for effective remediation of aquatic environments

The screening and design of "green" biochar materials with high adsorption capacity play a pivotal role in promoting the sustainable treatment of Cd(II)-containing wastewater. In this study, six typical machine learning (ML) models, namely Linear Regression, Random Forest, Gradient Boosting Decision Tree, CatBoost, K-Nearest Neighbors, and Backpropagation Neural Network, were employed to accurately predict the adsorption capacity of Cd(II) onto biochars. A large dataset with 1051 data points was generated using 21 input variables obtained from batch adsorption experiments, including preparation conditions for biochar (2 features), physical properties of biochar (4 features), chemical composition of biochar (9 features), and adsorption experiment conditions (6 features). The rigorous evaluation and comparison of the ML models revealed that the CatBoost model exhibited the highest test R2 value (0.971) and the lowest RMSE (20.54 mg/g), significantly outperforming all other models. The feature importance analysis using Shapley Additive Explanations (SHAP) indicated that biochar chemical compositions had the greatest impact on model predictions of adsorption capacity (42.2 %), followed by adsorption conditions (37.57 %), biochar physical characteristics (12.38 %), and preparation conditions (7.85 %). The optimal experimental conditions optimized by partial dependence plots (PDP) are as follows: as high Cd(II) concentration as possible, C(%) of 33 %, N(%) of 0.3 %, adsorption time of 600 min, pyrolysis time of 50 min, biochar dosage of less than 2 g/L, O(%) of 42 %, biochar pH value of 11.2, and DBE of 1.15. This study unveils novel insights into the adsorption of Cd(II) and provides a comprehensive reference for the sustainable engineering of biochars in Cd(II) wastewater treatment.

Enhanced reduction of Cd uptake by wheat plants using iron and manganese oxides combined with citrate in Cd-contaminated weakly alkaline arable soils

Iron (Fe) and manganese (Mn) oxides can be used to remediate Cd-polluted soils due to their excellent performance in heavy metal adsorption. However, their remediation capability is rather limited, and a higher content of available Mn and Fe in soils can reduce Cd accumulation in wheat plants due to the competitive absorption effect. In this study, goethite and cryptomelane were first respectively used to immobilize Cd in Cd-polluted weakly alkaline soils, and sodium citrate was then added to increase the content of available Mn and Fe content for further reduction of wheat Cd absorption. In the first season, the content of soil-available Cd and Cd in wheat plants significantly decreased when cryptomelane, goethite and their mixture were used as the remediation agents. Cryptomelane showed a better remediation effect, which could be attributed to its higher adsorption performance. The grain Cd content could be decreased from 0.35 mg kg-1 to 0.25 mg kg-1 when the content of cryptomelane was controlled at 0.5%. In the second season, when sodium citrate at 20 mmol kg-1 was further added to the soils with 0.5% cryptomelane treatment in the first season, the content of soil available Cd was increased by 14.8%, and the available Mn content was increased by 19.5%, leading to a lower Cd content in wheat grains (0.16 mg kg-1) probably due to the competitive absorption. This work provides a new strategy for the remediation of slightly Cd-polluted arable soils with safe and high-quality production of wheat.

Simultaneous removal of nitrate, manganese, zinc, and bisphenol a by manganese redox cycling system: Performance and mechanism

The manganese(Mn) redox cycling system in this work was created by combining Mn(IV)-reducing bacteria MFG10 with Mn(II)-oxidizing bacteria HY129. The biomanganese oxides (BMO) generated by strain HY129 were transformed by strain MFG10 to Mn(II), finishing the Mn redox cycling, in which nitrate (NO3--N) was converted to nitrite, which was further reduced to nitrogen gas. The system could achieve 85.7 % and 98.8 % elimination efficiencies of Mn(ⅠⅠ) and NO3--N, respectively, at Mn(ⅠⅠ) = 20.0 mg/L, C/N = 2.0, pH = 6.5, and NO3--N = 16.0 mg/L. The removal of bisphenol A (BPA) and zinc (Zn(II)) at 36 h reached 91.7 % and 89.7 % under the optimal condition, respectively. Furthermore, the Mn redox cycling system can reinforce the metabolic activity and electron transfer activity of microorganisms. The findings showed that the adsorption by bioprecipitation throughout the Mn cycling was responsible for the elimination of Zn(II) and BPA.

A new strategy for utilization of gasification ash: Manganese oxides-modified activated carbon for efficient copper citrate removal

To address the challenges posed by solid waste generated from coal gasification ash, a pyrolysis self-activation method was employed to prepare activated carbon by gasification ash, followed by the modification with manganese oxide to enhance its adsorption performance. Subsequently, the removal efficiency and mechanism for copper citrate were investigated. The results demonstrated the successful preparation of manganese oxides modified gasification ash-derived activated carbon (GAC-MnOx), exhibiting a specific surface area of 158.3 m2/g and a pore volume of 0.1948 cm³/g. The kinetic process could be described by the pseudo-second-order kinetic model (R2 = 0.958). High removal efficiency and low concentration of dissolved Mn were observed within the pH range of 3-10, where the adsorption capacity of GAC-MnOx for copper citrate exhibited an inverse relationship with pH. Notably, the fitting results of the Langmuir model demonstrated that the maximum adsorption capacity of GAC-MnOx for copper citrate is determined to be 7.196 mg/g at pH 3. The adsorption capacity of GAC-MnOx was found to be significantly reduced to 0.26 mg/g as the pH decreased below 2, potentially attributed to the dissolution of Mn. The findings of the Dual-Mode model demonstrated that the copper citrate removal mechanism by GAC-MnOx involved both surface adsorption and precipitation processes as follows: the porous structure of activated carbon enables physical adsorption of copper citrate, the MnOx or oxygen-containing functional groups establish chemical bonds with copper citrate and subsequently precipitate onto the surface of the adsorbent. The physical adsorption remains predominant in the removal of copper citrate, despite a gradual decrease in its proportion with increasing pH and equilibrium concentrations. Moreover, the X-ray photoelectron spectroscopy results indicated that copper citrate might be oxidized by MnOx to release copper ions and be retained on the surface of the adsorbent, meaning the adsorption efficiency of Cu(II)-Cit by GAC was enhanced through MnOx oxidation. This study could provide a new strategy for the high-value resource utilization of gasification ash.

Interfacial interaction mechanism between Mn doped highly conjugated biochar and berberine hydrochloride

MnSO4-modified biochar (Mn-BC) was synthesized to remove berberine hydrochloride (BH) from wastewater by utilizing tea waste as raw material and MnSO4 as modifier. Brunel Emmett Taylor (BET) analysis reveals that the specific surface area (SSA) and average pore size (Dave) of Mn-BC are 1.4 and 7 times higher than those of pristine biochar apart, attributing to the dissociation effect can promote the dispersion of MnSO4 in the pores of the biochar. Meanwhile, the doping of Mn not only introduces additional oxygen-containing functional groups (OCFGs), but also modulates the π electron density. Furthermore, Response surface method (RSM) analysis reveals that Mn-BC dosage has the most significant effect on BH removal, followed by BH concentration and pH value. Kinetic and isothermal studies reveal that the BH adsorption process of Mn-BC was mainly dominated by chemical and monolayer adsorption. Meanwhile, density functional theory (DFT) calculations confirm the contribution of Mn doping to the conjugation effect in the adsorption system. Originally proposed Mn-BC is one potentially propitious material to eliminate BH from wastewater, meanwhile this also provides a newfangled conception over the sustainable utilization of tea waste resources.

Straw removal reduces Cd availability and rice Cd accumulation in Cd-contaminated paddy soil: Cd fraction, soil microorganism structure and porewater DOC and Cd

The impacts of straw removal on rice Cd absorption, behaviour of Cd and microbial community in rhizosphere soil were investigated in paddy fields over two consecutive seasons. The results of the experiments in two fields revealed that straw removal promoted the transformation of soil Cd from acid-extractable and oxidisable fraction to residual fraction and reduced soil DTPA-Cd content with the reduction in DOC and Cd ions in soil porewater, thereby decreasing Cd content in rice. Specifically, the Cd content in brown rice of early rice was below 0.2 mg·kg-1 when all rice straw and roots were removed in the slightly Cd-contaminated soils. The α-diversity of soil microbial communities was less influenced by continuous straw removal, β-diversity was altered and the relative abundances of Anaeromyxobacter, Methylocystis and Mycobacterium microbes were increased. Redundancy analysis and network analysis exhibited that soil pH predominantly influenced the microbial community. Path analysis revealed that the Cd content in brown rice could be directly influenced by the soil Total-Cd and DTPA-Cd, as well as soil pH and OM. Straw removal, including roots removal, is an economical and effective technique to reduce Cd accumulation in rice plants.

Transformation of As and Cd associated with Fe-Mn-modified biochar during simultaneous remediation on the contaminated soil

Here, Fe- and Mn-modified biochar (BC-Fe-Mn) was applied to simultaneously stabilize As and Cd in the contaminated soil. The removal efficiencies for NaHCO3-extractable As and DTPA-extractable Cd by BC-Fe-Mn were 60.8% and 49.6%, respectively. The speciation analyses showed that the transformation to low-crystallinity Fe-bound (F3) As, Fe-Mn oxide-bound (OX) of Cd, and residual As and Cd was primarily attributed to stabilizing the two metal(loid)s. Moreover, the correlation analyses showed that the increase of As in F3 fraction was significantly and positively associated with the increase of OX fraction Mn (r = 0.64). Similarly, OX fraction Cd was increased notably with increasing OX fraction Fe (r = 0.91) and OX fraction Mn (r = 0.76). In addition, a novel dialysis experiment was performed to separate the reacted BC-Fe-Mn from the soil for intensively investigating the stabilization mechanisms for As and Cd by BC-Fe-Mn. The characteristic crystalline compounds of (Fe0.67Mn0.33)OOH and Fe2O3 on the surface of BC-Fe-Mn were revealed by SEM-EDS and XRD. And FTIR analyses showed that α-FeOOH, R-COOFe/Mn+, and O-H on BC-Fe-Mn potentially served as the reaction sites for As and Cd. A crystalline compound of MnAsO4 was found in the soil treated by BC-Fe-Mn in the dialysis experiment. Thus, our results are beneficial to deeper understand the mechanisms of simultaneous stabilization of As and Cd by BC-Fe-Mn in soil and support the application of the materials on a large scale.

Pinecone biochar for the Adsorption of chromium (VI) from wastewater: Kinetics, thermodynamics, and adsorbent regeneration

High concentration of chromium in aquatic environments is the trigger for researchers to remediate it from wastewater environments. However, conventional water treatment methods have not been satisfactory in removing chromium from water and wastewater over the last decade. Similarly, many adsorption studies have been focused on one aspect of the treatment, but this study dealt with all aspects of adsorption packages to come up with a concrete conclusion. Therefore, this study aimed to prepare pinecone biochar (PBC) via pyrolysis and apply it for Cr(VI) removal from wastewater. The PBC was characterized using FTIR, SEM-EDX, BET surface area, pHpzc, Raman analyses, TGA, and XRD techniques. Chromium adsorption was studied under the influence of PBC dose, solution pH, initial Cr(VI) concentration, and contact time. The characteristics of PBC are illustrated by FTIR spectroscopic functional groups, XRD non-crystallite structure, SEM rough surface morphology, and high BET surface area125 m2/g, pore volume, 0.07 cm3/g, and pore size 1.4 nm. On the other hand, the maximum Cr (VI) adsorption of 69% was found at the experimental condition of pH 2, adsorbent dosage 0.25 mg/50 mL, initial Cr concentration 100 mg/L, and contact time of 120 min. Similarly, the experimental data were well-fitted with the Langmuir adsorption isotherm at R2 0.96 and the pseudo-second-order kinetics model at R2 0.99. This implies the adsorption process is mainly attributed to monolayer orientation between the adsorbent and adsorbate. In the thermodynamics study of adsorption, ΔG was found to be negative implying the adsorption process was feasible and spontaneous whereas the positive values of ΔH and ΔS indicated the adsorption process was endothermic and increasing the degree of randomness, respectively. Finally, adsorbent regeneration and reusability were successful up to three cycles. In conclusion, biochar surface modification and reusability improvements are urgently required before being applied at the pilot scale.

Efficient co-stabilization of arsenic and cadmium in farmland soil by schwertmannite under long-term flooding-drying condition

Simultaneously stabilizing of arsenic (As) and cadmium (Cd) in co-contaminated soil presents substantial challenges due to their contrasting chemical properties. Schwertmannite (Sch) is recognized as a potent adsorbent for As pollution, with alkali modification showing promising results in the simultaneous immobilization of both As and Cd. This study systematically investigated the long-term stabilization efficacy of alkali-modified Sch in Cd-As co-contaminated farmland soil over a 200-day flooding-drying period. The results revealed that As showed significant mobility in flooded conditions, whereas Cd exhibited increased soil availability under drying phases. The addition of Sch did not affect the trends in soil pH and Eh fluctuations; nonetheless, it led to an augmentation in the levels of amorphous iron oxides and SO42- concentration in soil pore water. At a dosage of 0.5% Sch, there was a notable decrease in the mobility and soil availability of As and Cd under both flooding (34.5% and 53.6% at Day 50) and drying conditions (27.0% and 29.4% at Day 130), primarily promoting the transformation of labile metal(loid) fraction into amorphous iron oxide-bound forms. Throughout the flooding-drying treatment period, Sch maintained stable mineral morphology and mineralogical phase, highlighting its long-term stabilization effect. The findings of this study emphasize the promising application of Sch-based soil remediation agents in mitigating the challenges arising from As-Cd co-contamination. Further research is warranted to explore their application in real farmland settings and their impact on the uptake of toxic metal(loid)s by plants.

Ecological consequences of biochar and hydrochar amendments in soil: assessing environmental impacts and influences

Anthropogenic activities have caused irreversible consequences on our planet, including climate change and environmental pollution. Nevertheless, reducing greenhouse gas (GHG) emissions and capturing carbon can mitigate global warming. Biochar and hydrochar are increasingly used for soil remediation due to their stable adsorption qualities. As soil amendments, these materials improve soil quality and reduce water loss, prevent cracking and shrinkage, and interact with microbial communities, resulting in a promising treatment method for reducing gas emissions from the top layer of soil. However, during long-term studies, contradictory results were found, suggesting that higher biochar application rates led to higher soil CO2 effluxes, biodiversity loss, an increase in invasive species, and changes in nutrient cycling. Hydrochar, generated through hydrothermal carbonization, might be less stable when introduced into the soil, which could lead to heightened GHG emissions due to quicker carbon breakdown and increased microbial activity. On the other hand, biochar, created via pyrolysis, demonstrates stability and can beneficially impact GHG emissions. Biochar could be the preferred red option for carbon sequestration purposes, while hydrochar might be more advantageous for use as a gas adsorbent. This review paper highlights the ecological impact of long-term applications of biochar and hydrochar in soil. In general, using these materials as soil amendments helps establish a sustainable pool of organic carbon, decreasing atmospheric GHG concentration and mitigating the impacts of climate change.

Diclofenac sodium and paracetamol removal with ZnCl2 activated carbon produced from rice straw

This study explored the efficacy of activated carbon derived from rice straw and treated with ZnCl2 (ZnCl2-RS) for the removal of diclofenac sodium (DCF) and paracetamol (PCM) through an adsorption process. The investigation included examining the variations in removal efficiency at different pH levels and ZnCl2-RS doses. The characteristics of the ZnCl2-RS, prepared for the study, were determined through SEM and FTIR analyses, revealing a composition of 49.4% carbon and 8.3% zinc. At pH 5, the adsorption efficiency for DCF and PCM was enhanced, achieving removal rates of 92.2% for DCF and 89.1% for PCM with 0.2 g of ZnCl2-RS. The adsorption of DCF and PCM by ZnCl2-RS followed pseudo-second-order kinetic and adhered to the Langmuir isotherm model. The maximum adsorption capacities were calculated as 26.04 mg/g for DCF and 19.05 mg/g for PCM. In conclusion, the cost-effective production of activated carbon from agricultural waste like rice straw yielded a promising adsorbent material for efficiently removing pharmaceuticals such as diclofenac sodium and paracetamol. This approach not only contributes to waste reduction but also promotes the repurposing of agricultural waste materials.

Applications of engineered biochar in remediation of heavy metal(loid)s pollution from wastewater: Current perspectives toward sustainable development goals

Environmental pollution of heavy metal(loid)s (HMs) caused adverse impacts, has become one of the emerging concerns and challenges worldwide. Metal(loid)s can pose significant threats to living organisms even when present in trace levels within environmental matrices. Extended exposure to these substances can lead to adverse health consequences in humans. Removing HM-contaminated water and moving toward sustainable development goals (SDGs) is critical. In this mission, biochar has recently gained attention in the environmental sector as a green and alternative material for wastewater removal. This work provides a comprehensive analysis of the remediation of typical HMs by biochars, associated with an understanding of remediation mechanisms, and gives practical solutions for ecologically sustainable. Applying engineered biochar in various fields, especially with nanoscale biochar-aided wastewater treatment approaches, can eliminate hazardous metal(loid) contaminants, highlighting an environmentally friendly and low-cost method. Surface modification of engineered biochar with nanomaterials is a potential strategy that positively influences its sorption capacity to remove contaminants. The research findings highlighted the biochars' ability to adsorb HM ions based on increased specific surface area (SSA), heightened porosity, and forming inner-sphere complexes with oxygen-rich groups. Utilizing biochar modification emerged as a viable approach for addressing lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), and chromium (Cr) pollution in aqueous environments. Most biochars investigated demonstrated a removal efficiency >90 % (Cd, As, Hg) and can reach an impressive 99 % (Pb and Cr). Furthermore, biochar and advanced engineered applications are also considered alternative solutions based on the circular economy.

Current progress on manganese in constructed wetlands: Bibliometrics, effects on wastewater treatment, and plant uptake

Constructed wetlands (CWs) are a pollutant treatment design inspired by natural wetlands and are widely utilized for the removal of common pollutants. The research focus lies in the circulation of manganese (Mn) in the environment to enhance pollutant removal within CWs. This paper provides a comprehensive review of recent advancements in understanding the role and effects of Mn in chemical weapons, based on literature retrieval from 2002 to 2021. Ecological risk assessment and heavy metals within CWs emerge as current areas of research interest. Mn sources within CWs primarily include natural deposition, heavy metal wastewater, and intentional addition. The cycling between Mn(II) and Mn(IV) facilitates enhanced wastewater treatment within CWs. Moreover, employing a Mn matrix proves effective in reducing ammonia nitrogen wastewater, organic pollutants, as well as heavy metals such as Cd and Pb, thereby addressing complex pollution challenges practically. To comprehensively analyze influencing factors on the system's performance, both internal factors (biological species, design parameters, pH levels, etc.) and external factors (seasonal climate variations, precipitation patterns, ultraviolet radiation exposure, etc.) were discussed. Among these factors, microorganisms, pollutants, and temperature are the most important influencing factors, which emphasizes the importance of these factors for wetland operation. Lastly, this paper delves into plant absorption of Mn along with coping strategies employed by plants when faced with Mn poisoning or deficiency scenarios. When utilizing Mn for the regulation of constructed wetlands, it is crucial to consider the tolerance levels of associated plant species. Furthermore, the study predicts future research hotspots encompass high-efficiency catalysis techniques, matrix-filling approaches, and preparation of resource utilization methods involving Mn nanomaterials.

The role of an Sb-oxidizing bacterium in modulating antimony speciation and iron plaque formation to reduce the accumulation and toxicity of Sb in rice (Oryza sativa L.)

Microbial antimony (Sb) oxidation in the root rhizosphere and the formation of iron plaque (IP) on the root surface are considered as two separate strategies to mitigate Sb(III) phytotoxicity. Here, the effect of an Sb-oxidizing bacterium Bacillus sp. S3 on IP characteristics of rice exposed to Sb(III) and its alleviating effects on plant growth were investigated. The results revealed that Fe(II) supply promoted IP formation under Sb(III) stress. However, the formed IP facilitated rather than hindered the uptake of Sb by rice roots. In contrast, the combined application of Fe(II) and Bacillus sp. S3 effectively alleviated Sb(III) toxicity in rice, resulting in improved rice growth and photosynthesis, reduced oxidative stress levels, enhanced antioxidant systems, and restricted Sb uptake and translocation. Despite the ability of Bacillus sp. S3 to oxidize Fe(II), bacterial inoculation inhibited the formation of IP, resulting in a reduction in Sb absorption on IP and uptake into the roots. Additionally, the bacterial inoculum enhanced the transformation of Sb(III) to less toxic Sb(V) in the culture solution, further influencing the adsorption of Sb onto IP. These findings highlight the potential of combining microbial Sb oxidation and IP as an effective strategy for minimizing Sb toxicity in sustainable rice production systems.

Biomass chitosan/sodium alginate colorimetric imprinting hydrogels with integrated capture and visualization detection for cadmium(II)

Due to Cd(II) with highly toxic, persistent and bioaccumulative, the discharge of it into the environment brings serious pollution. Developing strategies that are efficient, low-cost, pollution-free and specific to removing Cd(II) from wastewater is therefore of great urgency and prime importance. A novel chitosan/sodium alginate ionic imprinting(IICA) hydrogels with specific adsorption capacity for Cd(II) was prepared through freeze-thaw and ion imprinting, and finally the colorimetric sensor (IICAS) was prepared via introducing Rhodamine B(RhB) and Victoria blue(VBB) by immersion to achieve visual detection of Cd(II). The IICA hydrogels with imprinted hole structure had higher adsorption capacity and better specific selectivity for Cd(II). As well as internal diffusion, coordination, ion exchange, and hydrogen bonding influenced the adsorption rate. Moreover, the IICAS exhibited good selective detection ability and linearity for Cd(II) with the fitted correlation coefficient (R2) = 0.98, limit of detection (LOD) = 35 nmol/L. Combined with the smartphone platform, portable and quantitative detection of Cd(II) can be achieved, Within the 0-100 mg/L range, R2 remained 0.94, and LOD was 75 nmol/L. This strategy of preparing a novel whole biomass IICAS integrating capture and visual detection provides a new insight into the construction of a promising candidate sensor for the removal and detection of Cd(II).

Birnessite-Type MnO2 Modified Sustainable Biomass Fiber toward Adsorption Removal Heavy Metal Ion from Actual River Aquatic Environment

In this work, a novel birnessite-type MnO2 modified corn husk sustainable biomass fiber (MnO2@CHF) adsorbent was fabricated for efficient cadmium (Cd) removal from aquatic environments. MnO2@CHF was designed from KMnO4 hydrothermally treated with corn husk fibers. Various characterization revealed that MnO2@CHF possessed the hierarchical structure nanosheets, large specific surface area, and multiple oxygen-containing functional groups. Batch adsorption experimental results indicated that the highest Cd (II) removal rate could be obtained at the optimal conditions of adsorbent amount of 0.200 g/L, adsorption time of 600 min, pH 6.00, and temperature of 40.0 °C. Adsorption isotherm and kinetics results showed that Cd (II) adsorption behavior on MnO2@CHF was a monolayer adsorption process and dominated by chemisorption and intraparticle diffusion. The optimum adsorption capacity (Langmuir model) of Cd (II) on MnO2@CHF was 23.0 mg/g, which was higher than those of other reported common biomass adsorbent materials. Further investigation indicated that the adsorption of Cd (II) on MnO2@CHF involved mainly ion exchange, surface complexation, redox reaction, and electrostatic attraction. Moreover, the maximum Cd (II) removal rate on MnO2@CHF from natural river samples (Xicheng Canal) could reach 59.2% during the first cycle test. This study showed that MnO2@CHF was an ideal candidate in Cd (II) practical application treatment, providing references for resource utilization of agricultural wastes for heavy metal removal.

Sodium hydroxide/magnesium chloride multistage activated sludge biochar: interfacial chemical behavior and Cd(II) adsorption performance

To enhance the adsorption performance of municipal sludge biochar on Cd(II), modified sludge biochar was prepared by sodium hydroxide/magnesium chloride (NaOH/MgCl2) graded activation, and the Cd(II) adsorption performance on sludge biochar (BC), NaOH-activated sludge biochar (NBC) and NaOH/MgCl2 activated sludge biochar (NBC-Mg) was investigated. The results showed that NaOH/MgCl2 graded activation upgraded the surface structure and enhanced the graphitization of sludge biochar. The adsorption experiments indicated that the adsorption kinetic and adsorption isotherm for Cd(II) were in accordance with the pseudo second-order kinetic and Langmuir model. The adsorption capacity of NBC-Mg (143.49 mg/g) for Cd(II) was higher than that of BC (50.40 mg/g) and NBC (85.20 mg/g). The mechanism of Cd(II) adsorption included ion exchange, complexation, cation-π interaction, and mineral precipitation. After five regeneration, the removal efficiency of Cd(II) by NBC-Mg remained above 90%. This work indicated that sludge biochar prepared by multistage activation could be an effective material for Cd-containing wastewater treatment.

Denitrification driven by additional ferrous (Fe2+) and manganous (Mn2+) and removal mechanism of tetracycline and cadmium (Cd2+) by biogenic Fe-Mn oxides

Zoogloea sp. MFQ7 achieved excellent denitrification of 91.71% at ferrous to manganous ratio (Fe/Mn) of 3:7, pH of 6.5, nitrate concentration of 25 mg L-1 and carbon to nitrogen ratio of 1.5. As the Fe/Mn ratio increasd, the efficiency of nitrate removal gradually decreased, indicating that strain MFQ7 had a higher affinity for Mn2+ than Fe2+. In situ generated biogenic Fe-Mn oxides (BFMO) contained many iron-manganese oxides (MnO2, Mn3O4, FeO(OH), Fe2O3, and Fe3O4) as well as reactive functional groups, which play an significant part in tetracycline (TC) and cadmium (Cd2+) adsorption. The adsorption of TC and Cd2+ by BFMO can better fit the pseudo-second-order and Langmuir models. In addition, multiple characterization results of before and after adsorption indicated that the removal mechanism of BFMO on TC and Cd2+ was probably surface complexation adsorption and redox reactions.

Adsorption mechanism of Cd2+ on microbial inoculant and its potential for remediation Cd-polluted farmland soils

The adsorption characteristics and mechanism of Cd2+ on microbial inoculant (MI) mainly composed of Bacillus subtilis, Bacillus thuringiensis and Bacillus amyloliquefaciens, and its potential for remediation Cd polluted soils through batch adsorption and soil incubation experiments. It was found that the Freundlich isotherm model and the pseudo-second-order kinetics were more in line with the adsorption processes of Cd2+. The maximum adsorption capacity predicted by Langmuir isotherm model suggested that of MI was 57.38 mg g-1. Scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) images exhibited the surface structure of MI was damaged to varying degrees after adsorption, and Cd element was distributed on the surface of MI through ion exchange. X-ray diffraction (XRD) results showed that CdCO3 was formed on the surface of MI. Moreover, the functional groups (-OH, C-H, and -NH) involved in the adsorption of Cd2+ through fourier transform infrared spectroscopy (FTIR). After applying MI to Cd-contaminated soil, it was found that soil pH, conductivity (EC) and soil organic matter (SOM) increased by 0.84 %-2.43 %, 31.6 %-241.48 %, and 8.11 %-24.1 %, respectively, when compared with the control treatments. The content of DTPA-Cd in the soils was significantly (P < 0.05) reduced by 15.48 %-29.68 % in contrast with CK, and the Cd speciation was transformed into a more stable residual fraction. The activities of urease, phosphatase and sucrose were increased by 3.5 %-45.18 %, 57.00 %-134.18 % and 52.51 %-70.52 %, respectively, compared with CK. Therefore, MI could be used as an ecofriendly and sustainable material for bioremediation of Cd-contaminated soils.

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

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

Quantification of the effect of biochar application on heavy metals in paddy systems: Impact, mechanisms and future prospects

Biochar (BC) has shown great potential in remediating heavy metal(loid)s (HMs) contamination in paddy fields. Variation in feedstock sources, pyrolysis temperatures, modification methods, and application rates of BC can result in great changes in its effects on HM bioavailability and bioaccumulation in soil-rice systems and remediation mechanisms. Meanwhile, there is a lack of application guidelines for BC with specific properties and application rates when targeting rice fields contaminated with certain HMs. To elucidate this topic, this review focuses on i) the effects of feedstock type, pyrolysis temperature, and modification method on the properties of BC; ii) the changes in bioavailability and bioaccumulation of HMs in soil-rice systems applying BC with different feedstocks, pyrolysis temperatures, modification methods, and application rates; and iii) exploration of potential remediation mechanisms for applying BC to reduce the mobility and bioaccumulation of HMs in rice field systems. In general, the application of Fe/Mn modified organic waste (OW) derived BC for mid-temperature pyrolysis is still a well-optimized choice for the remediation of HM contamination in rice fields. From the viewpoint of remediation efficiency, the application rate of BC should be appropriately increased to immobilize Cd, Pb, and Cu in rice paddies, while the application rate of BC for immobilizing As should be <2.0 % (w/w). The mechanism of remediation of HM-contaminated rice fields by applying BC is mainly the direct adsorption of HMs by BC in soil pore water and the mediation of soil microenvironmental changes. In addition, the application of Fe/Mn modified BC induced the formation of iron plaque (IP) on the root surface of rice, which reduced the uptake of HM by the plant. Finally, this paper describes the prospects and challenges for the extension of various BCs for the remediation of HM contamination in paddy fields and makes some suggestions for future development.

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

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

Regulation of rhizosphere microenvironment by rice husk ash for reducing the accumulation of cadmium and arsenic in rice

It is important to reduce Cd and As content in brown rice in contaminated paddy soils. We conducted research on the effects of rice husk ash (RHA) on the Cd and As in the rhizosphere microenvironment (soil, porewater, and iron plaque) and measured the Cd, As, and Si content in rice plants. The main elements in RHA were Si (29.64%) and O (69.17%), which had the maximum adsorption capacity for Cd was 42.49 mg/kg and for As was 18.62 mg/kg. Soil pH and available Si content increased, while soil available Cd and As decreased following application of 0.5%-2% RHA. RHA promote the transformation of Cd to insoluble fraction, while As was transformed from a poorly soluble form to a more active one. RHA reduced Cd content and increased Si content in porewater, and reduced As only at the later rice growth stages. RHA increased the amount of iron plaque, thereby decreasing the Cd content in iron plaque, while increased the As content in it. Cd and inorganic As content in brown rice were decreased, to 0.31 mg/kg and 0.18 mg/kg, respectively. The decrease of Cd in brown rice was due to the decrease of Cd mobility in soil, thereby reducing root accumulation, while the decrease of As in brown rice was affected by the transport from roots to stems. Therefore, RHA can be considered as a safe and efficient in-situ remediation amendment for Cd and As co-contaminated paddy soil.

Au doped metal organic frameworks as di-functional photocatalysts for clearing organics in wastewater

Along with the development of productive forces, the use of organic compounds including diversified dyes and multiple drugs has become more and more commonly, resulting in the accelerating water contamination. Herein in this paper, Au doped PCN 224 are designed as bi-functional wastewater treatment agents to absorb and decompose organics molecules efficiently under light irradiation. After inserted with Au, the PCN 224 nanoparticles, which is kind of porous, stable and photosensitive metal-organic framework, show enhanced photodegradeability. Because the Au inserted could inhibit the re-combination of electrons and holes by absorbing photo-electrons; decrease the nanoparticles' band gap, and finally produce much more free radicals. In the meanwhile, due to the lower binding energy between S and Au, the Au modified PCN 224 perform better in absorbing organic compounds consisted of S contained heterocyclic ring (such as methylene blue). This work provides new insights into the precious design of materials in clearing organic compounds.

Physiological tolerance of black locust (Robinia pseudoacacia L.) and changes of rhizospheric bacterial communities in response to Cd and Pb in the contaminated soil

Woody plants possess great potential for phytoremediation of heavy metal-contaminated soil. A pot trial was conducted to study growth, physiological response, and Cd and Pb uptake and distribution in black locust (Robinia pseudoacacia L.), as well as the rhizosphere bacterial communities in Cd and Pb co-contaminated soil. The results showed that R. pseudoacacia L. had strong physiological regulation ability in response to Cd and Pb stress in contaminated soil. The total chlorophyll, malondialdehyde (MDA), soluble protein, and sulfhydryl contents, as well as antioxidant enzymes (superoxide dismutase, peroxidase, catalase) activities in R. pseudoacacia L. leaves under the 40 mg·kg-1 Cd and 1000 mg·kg-1 Pb co-contaminated soil were slightly altered. Cd uptake in R. pseudoacacia L. roots and stems increased, while the Pb content in the shoots of R. pseudoacacia L. under the combined Cd and Pb treatments decreased in relative to that in the single Pb treatments. The bacterial α-diversity indices (e.g., Sobs, Shannon, Simpson, Ace, and Chao) of R. pseudoacacia L. rhizosphere soil under Cd and Pb stress were changed slightly relative to the CK treatment. However, Cd and Pb stress could significantly (p < 0.05) alter the rhizosphere soil microbial communities. According to heat map and LEfSe (Linear discriminant analysis Effect Size) analysis, Bacillus, Sphingomonas, Terrabacter, Roseiflexaceae, Paenibacillus, and Myxococcaceae at the genus level were notably (p < 0.05) accumulated in the Cd- and/or Pb-contaminated soil. Furthermore, the MDA content was notably (p < 0.05) negatively correlated with the relative abundances of Isosphaeraceae, Gaiellales, and Gemmatimonas. The total biomass of R. pseudoacacia L. was positively (p < 0.05) correlated with the relative abundances of Xanthobacteraceae and Vicinamibacreraceae. Network analysis showed that Cd and Pb combined stress might enhance the modularization of bacterial networks in the R. pseudoacacia L. rhizosphere soil. Thus, the assembly of the soil bacterial communities in R. pseudoacacia L. rhizosphere may improve the tolerance of plants in response to Cd and/or Pb stress.

Adsorption characteristics and mechanisms of Cd(II) from wastewater by modified chicken manure biochar

Due to the threat to food supply and human health posed by cadmium-contaminated wastewater, a highly effective adsorbent is under necessary development to remove cadmium from wastewater. In this study, four new types of modified biochars with different modifier concentrations were prepared from chicken manure using K2FeO4 as a modifier, and the modified biochar KFBC1 with the best adsorption effect was obtained through optimal experiments. Various characterization analyses have shown that KFBC1 has a rough surface structure, abundant pore structure, and a large number of functional groups. Additionally, iron oxides are introduced on the surface of the biochar, which provided a favorable condition for the adsorption of Cd(II) in wastewater. The adsorption performance of Cd(II) on the biochar before and after modification was investigated through batch adsorption experiments. The adsorption kinetic model of KFBC1 to Cd(II) in solution was in accordance with the quasi-secondary kinetic model, and the adsorption isothermal model was in accordance with the Langmuir model, with a maximum adsorption capacity of 330.06 mg/g, which was 5.15 fold of pristine BC. Meanwhile, the adsorption rate of Cd(II) by KFBC1 was positively correlated with dosage and pH. Pore adsorption, ion exchange, surface precipitation, interaction with -π electrons, and complexation of oxygen-containing functional groups on the surface were considered as important mechanisms for the removal of Cd(II) by KFBC1. According to the results, KFBC1 is a novel and efficient adsorbent that can be used as a treatment agent for cadmium-contaminated wastewater.

Bacteria-loaded biochar for the immobilization of cadmium in an alkaline-polluted soil

The combination of biochar and bacteria is a promising strategy for the remediation of Cd-polluted soils. However, the synergistic mechanisms of biochar and bacteria for Cd immobilization remain unclear. In this study, the experiments were conducted to evaluate the effects of the combination of biochar and Pseudomonas sp. AN-B15, on Cd immobilization, soil enzyme activity, and soil microbiome. The results showed that biochar could directly reduce the motility of Cd through adsorption and formation of CdCO3 precipitates, thereby protecting bacteria from Cd toxicity in the solution. In addition, bacterial growth further induces the formation of CdCO3 and CdS and enhances Cd adsorption by bacterial cells, resulting in a higher Cd removal rate. Thus, bacterial inoculation significantly enhances Cd removal in the presence of biochar in the solution. Moreover, soil incubation experiments showed that bacteria-loaded biochar significantly reduced soil exchangeable Cd in comparison with other treatments by impacting soil microbiome. In particular, bacteria-loaded biochar increased the relative abundance of Bacillus, Lysobacter, and Pontibacter, causing an increase in pH, urease, and arylsulfatase, thereby passivating soil exchangeable Cd and improving soil environmental quality in the natural alkaline Cd-contaminated soil. Overall, this study provides a systematic understanding of the synergistic mechanisms of biochar and bacteria for Cd immobilization in soil and new insights into the selection of functional strain for the efficient remediation of the contaminated environments by bacterial biochar composite.

The Physicochemical Characteristics and Heavy Metal Retention Capability of Black Liquor Lignin-Based Biochars

Due to its high carbon content, lignin, particularly for lignin-containing solid waste, is considered an excellent raw material for the preparation of carbon materials like biochar. To produce high-quality lignin-based biochar (LGBCs), lignin extracted from black liquor was employed to prepare biochar at various pyrolysis temperatures (300~600 °C). The physicochemical properties of LGBCs were assessed using scanning electron microscopy, N2 adsorption/desorption, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction. Furthermore, the adsorption capability and potential mechanism of LGBCs in removing Cd(II) were investigated as well. The results indicate that LGBCs produced at higher pyrolysis temperatures exhibit rougher surfaces and more developed pore structures, which facilitate the exposure of numerous active adsorption sites. The adsorption of Cd(II) by LGBCs generally follows the order of LG-300C < LG-400C < LG-500C < LG-600C. According to the Langmuir adsorption isotherm model, the theoretical maximum adsorption capacity of LG-600C for Cd(II) is calculated to be 18.54 mg/g. Adsorption mechanism analysis reveals that the complexation interaction, dependent on the surface functional groups, plays a crucial role in the adsorption of Cd(II) by LGBCs prepared at higher pyrolysis temperatures. This study demonstrates that, by controlling the pyrolysis temperature during biochar preparation, high-quality lignin-based biochar can be readily obtained.

Use of thermally activated Fenton sludge for Cd removal in zinc smelter wastewater: Mechanism and feasibility of Cd removal

Fenton sludge is a byproduct of the Fenton process that contains large amounts of Fe and Ca. Because of the secondary contamination generated during the disposal of this byproduct, ecofriendly treatment methods are needed. In this study, we used Fenton sludge to remove the Cd discharged from a zinc smelter factory, using thermal activation to enhance the Cd adsorption capacity. Among the various temperatures considered (300-900 °C), the Fenton sludge that was thermally activated at 900 °C (TA-FS-900) adsorbed the highest amount of Cd because of its high specific surface area and high Fe content. Cd was adsorbed onto TA-FS-900 via complexation with C-OH, C-COOH, FeO-, and FeOH and cation exchange with Ca2+. The maximum adsorption of TA-FS-900 was 260.2 mg/g, indicating that TA-FS-900 is an efficient adsorbent, comparable to those reported in the literature. The initial Cd concentration in the zinc smelter wastewater discharged was 105.7 mg/L, 98.4% of which was removed by applying TA-FS-900, suggesting the applicability of TA-FS-900 for real wastewater containing high concentrations of various cations and anions. The leaching of heavy metals from TA-FS-900 was within the EPA standard limits. We concluded that the environmental impact of Fenton sludge disposal can be reduced, and the use of Fenton sludge can add value to the treatment of industrial wastewater in terms of the circular economy and environment.

Removal of oxytetracycline from wastewater by biochar modified with biosynthesized iron oxide nanoparticles and carbon nanotubes: Modification performance and adsorption mechanism

The pollution problem of oxytetracycline (OTC) from wastewater becomes more serious, so an efficient, economical, and green adsorption material is urgently explored. In this study, the multilayer porous biochar (OBC) was prepared by coupling carbon nanotubes with iron oxide nanoparticles synthesized by Aquabacterium sp. XL4 to modify corncobs under medium temperature (600 °C) conditions. The adsorption capacity of OBC could reach 72.59 mg g-1 after preparation and operation parameters were optimized. In addition, various adsorption models suggested that OTC removal resulted from the combined effect of chemisorption, multilayer interaction, and disordered diffusion. Meanwhile, the OBC was fully characterized and exhibited a large specific surface area (237.51 m2 g-1), abundant functional groups, stable crystal structure, high graphitization, and mild magnetic properties (0.8 emu g-1). The OTC removal mechanisms mainly included electrostatic interactions, ligand exchange, π-π bonding reactions, hydrogen bonds, and complexation. pH and coexistence substance experiments revealed that the OBC possesses a wide pH adaptation range and excellent anti-interference ability. Finally, the safety and reusability of OBC were confirmed by repeated experiments. In summary, OBC as a biosynthetic material shows considerable potential for application in the field of purifying new pollution from wastewater.

Mitigation of Cd(II) contamination in aqueous solution and soil by multifunctional hydroxyapatite/sludge biochar composite

Biochar with well-developed pore structure is an ideal carrier for easily agglomerated hydroxyapatite (HAP). Hence, a novel multifunctional hydroxyapatite/sludge biochar composite (HAP@BC) was synthesized by chemical precipitation method and used for mitigating Cd(II) contamination form aqueous solution/soil. Compared to sludge biochar (BC), HAP@BC exhibited rougher and more porous surface. Meanwhile, the HAP was dispersed on the sludge biochar surface, which reduced the agglomeration of HAP. The adsorption performance of HAP@BC on Cd(II) was better than that of BC under the influence of different single-factor batch adsorption experiments. Moreover, the Cd(II) adsorption behavior by BC and HAP@BC was uniform monolayer adsorption, and this reaction process was endothermic and spontaneous. The Cd(II) maximum adsorption capacities of BC and HAP@BC were 79.96 and 190.72 mg/g at 298 K, respectively. Moreover, the Cd(II) adsorption mechanism on BC and HAP@BC included complexation, ion exchange, dissolution-precipitation and Cd(II)-π interaction. According to the semi-quantitative analysis, ion exchange was the main mechanism for Cd(II) removal by HAP@BC. Notably, HAP played a role in the Cd(II) removal by dissolution-precipitation and ion exchange. This result suggested that there was a synergistic effect between HAP and sludge biochar for the Cd(II) removal. HAP@BC reduced the leaching toxicity of Cd(II) in soil better than BC, indicating that the HAP@BC was able to mitigate Cd(II) contamination in soil more effectively. This work demonstrated that sludge biochar was an ideal carrier for dispersed HAP and provided an effective HAP/biochar composite for the mitigation of Cd(II) contamination in aqueous solution/soil.

Sulfoaluminate cement-modified straw biochar as a soil amendment to inhibit Pb-Cd mobility in the soil-romaine lettuce system

Biochar is widely used to remediate soil polluted by potentially toxic elements (PTEs), while the effect of a new type of biochar, obtained from modified cement material, on the mobility of Pb and Cd in the soil-plant system is still unknown. In this study, soils doped with sulfoaluminate cement modified biochar (SBC) were characterized using a series of approaches including FTIR, XRD, and XPS, and combined with pot experiments to explore its synergistic effects on the speciation transformation, accumulation, and mobility of both Pb and Cd in a soil-romaine lettuce system in heavily contaminated soils containing 500 mg·kg-1-Pb and 3 mg·kg-1-Cd. The results showed that SBC effectively immobilized Pb and Cd in the soil and that this was achieved through cation exchange, complexation, and gel encapsulation. Moreover, SBC also changed the soil physicochemical properties and indirectly affected the speciation transformation of Pb and Cd. FTIR and XRD analyses revealed that the groups such as -OH, -COOH, SO42-, and SiO32-introduced by SBC stimulated the conversion from the soluble to the residual state of Pb. XPS analysis indicated that, the deviation of the C-O-C, C-OOH, and O-CO peak and the increased in area suggested that organic groups in the SBC were engaged in the immobilization mechanism of Pb and Cd. The transformation of residual Cd in other extractable fractions might be due to either enhanced soil reducibility or competitive adsorption with Pb. In 5% SBC soil, Pb was reduced by 27.69% and 64.84%, and Cd was reduced by 20.45% and 35.87% for shoots and roots of romaine lettuce, respectively. SBC showed a significantly positive correlation with SOM, while SOM showed a highly significantly negative correlation with both Pb and Cd in the roots. In summary, SBC can be strongly recommended as a green amendment to remediate Pb-Cd contaminated soil and to inhibit the mobility to plant.

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.

Synergistic impacts of ferromanganese oxide biochar and optimized water management on reducing Cd accumulation in rice

Ferromanganese oxide biochar composite (FMBC) is an efficient remediation material for cadmium -contaminated soils. However, the effect of FMBC under varied water managements on the remediation of Cd-polluted soil is unclear. In this study, we conducted both incubation and field experiments to investigate the combined effects of corn-stover-derived biochar modified with ferromanganese on the immobilization and uptake of Cd by rice under continuous aerobic (A), aerobic-flooded (AF), and flooded-aerobic (FA) water management regimes. The results showed that loading iron-manganese significantly increased the maximum sorption capacity (Qm) of Cd on FMBC (50.46 mg g^-1) due to increased surface area, as compared to the pristine biochar (BC, 31.36 mg g^-1). The results revealed that soil Eh and pH were significantly affected by FMBC and it's synergistic application with different water regimes, thus causing significant differences in the concentrations of DTPA-extractable Cd under different treatments. The lowest DTPA-extractable Cd content (0.28-0.46 mg^-1) was observed in the treatment with FMBC (2.5 %) combined FA water amendment, which reduced the content of available Cd in soil by 2.63-28.4 %. Moreover, the treatments with FMBC-FA resulted the proportion of residual Cd increased by 22.2 % compared to the control. Variations in the content and fraction of Cd had a significant influence on its accumulation in the rice grains. The FMBC-FA treatments reduced the Cd concentration in roots, shoots and grains by 37.97 %, 33.98 %, and 53.66 %, respectively, when compared with the control. Predominantly because of the reduction in Cd biological toxicity and the improved soil nutrient content, the combined application increased the biomass and yield of rice to some extent. Taken together, the combination of the Fe-Mn modified biochar and flooded-aerobic water management may potentially be applied in Cd-polluted soil to mitigate the impacts of Cd on rice production.

Comprehensive study on the hydrochar for adsorption of Cd(II): preparation, characterization, and mechanisms

Hydrothermal carbonization process via converting invasive plants into functional materials may provide a novel strategy to comprehensively control and utilized the exotic invasive plants. In this study, Eupatorium adenophorum was utilized to fabricate the hydrochar via hydrothermal carbonization process, which was further applied to remove Cd(II). The results showed that the hydrochar was a mesoporous material with abundant O-containing functional groups (OFPs) on the surface. The adsorption isotherms were fitted by both the Langmuir and Freundlich models, and the maximum adsorption amount achieved 24.53 mg/g. The adsorption dynamics were governed by surface adsorption and film diffusion. pH and ionic strength can exert a strong influence on the adsorption efficiency. The mechanisms on the adsorption of Cd(II) on the hydrochar concluded the pore-filling effects, electrostatic interactions, ion exchange, precipitation, coordination with π electrons, and surface complexation with the OFPs, such as hydroxyl, carboxylic, phenol, acetyl, and ester groups. Thus, hydrothermal carbonization process may provide a promising technique to fabricate the hydrocar for the treatment of Cd(II), which may facilitate comprehensive control of invasive plants and boost to the carbon neutrality.

Sustainable green conversion of coal gangue waste into cost-effective porous multimetallic silicate adsorbent enables superefficient removal of Cd(II) and dye

Converting solid wastes into new materials for wastewater decontamination is a feasible "one stone, three birds" strategy to achieve sustainable value-added utilization of resources and minimize waste emissions, but significant challenges remain. In response to this, we proposed an efficient "mineral gene reconstruction" method to synchronously transform coal gangue (CG) into a green porous silicate adsorbent without using any harmful chemicals (i.e., surfactants, organic solvents). The one of the synthesized adsorbents with a high specific surface area (582.28 m²/g) and multimetallic active centres shows outstanding adsorption performance (adsorption capacities: 168.92 mg/g for Cd(II), 234.19 mg/g for methylene blue (MB); removal rate: 99.04% for Cd(II) and 99.9% for MB). The adsorbent can also reach a high removal rate of 99.05%∼99.46% and 89.23%∼99.32% for MB and Cd(II) in real water samples (i.e., Yangtze River, Yellow River, seawater and tap water), respectively. After 5 adsorption-desorption cycles, the adsorption efficiency remained above 90%. The adsorbents mainly adsorbed Cd(II) by electrostatic attraction, surface complexation and partial ion exchange and MB by electrostatic and hydrogen bonding interactions. This study provides a sustainable and promising platform for developing a new-generation cost-efficient adsorbent from waste for clean water production.

Biomass inherent metal interfere carbothermal reduction modification of biochar for Cd immobilization

Metal salt laden are frequently used to enhance the heavy metal adsorption capacity of biochar. The present study indicates that CaS loading biochar can be modified from the carbothermal reduction reaction between CaSO₃ (modification agent) and carbon matrix. The CaS transformation ratio as indicated by XPS spectra was significantly improved by the CaSO₃ loading content. The coprecipitation reaction induced by the CaS in biochar can significantly enhance the adsorption capacity of heavy metals (Cd). And, the Cd adsorption capacity can be enhanced up to >100 mg/g and increases with increasing CaS ratio in the biochar. In addition, the adsorption process was rapid and could be balanced within several minutes (~ 5 min). Furthermore, the interaction reaction between the modification agent and the inherent metal in the biomass was examined in the biochar pyrolysis preparation process. Interestingly, MgCl₂ inherent metal salt can combine with the original CaSO₃ to produce a new mineral, resulting in a decrease in CaS. However, KCl, a more thermally stable biomass-derived metal salt, exhibited a weak combination ability with the modification agent. Accordingly, this type of secondary reaction reduces the Cd adsorption capacity owing to the decrease in the number of adsorption sites (CaS).

Preparation of manganese-oxides-coated magnetic microcrystalline cellulose via KMnO4 modification: Improving the counts of the acid groups and adsorption efficiency for Pb(II)

Herein, the manganese-oxides-coated magnetic microcrystalline cellulose (MnOx@Fe₃O₄@MCC) was prepared by coprecipitation and subsequently modified with KMnO₄ solution at room temperature, which was in turn applied for the removal of Pb(II) from wastewater. The adsorption properties of Pb(II) on MnOx@Fe₃O₄@MCC were investigated. The kinetics and isothermal data of Pb(II) were described well by the Pseudo-second-order model and the Langmuir isotherm model, respectively. At pH = 5, 318 K, the Langmuir maximum Pb(II) adsorption capacity of MnOx@Fe₃O₄@MCC was 446.43 mg/g, which is higher than many documented bio-based adsorbents. The results of Fourier transform infra-red and X-ray photoelectron spectroscopy indicated that the adsorption mechanisms for Pb(II) mainly involved surface complexation, ion exchange, electrostatic interaction and precipitation. Interestingly, the increased amount of carboxyl group on the surface of microcrystalline cellulose modified by KMnO₄ was one of the important reasons for the high Pb(II) adsorption performance of MnOx@Fe₃O₄@MCC. Furthermore, MnOx@Fe₃O₄@MCC exhibited excellent activity (70.6 %) after five consecutive regeneration cycles, indicating its high stability and reusability. Endorsing to the cost-effectiveness, environmentally friendliness, and reusable nature, MnOx@Fe₃O₄@MCC can be counted as a great alternative contender for the remediation of Pb(II) from industrial wastewater.

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

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

Polyethylene and poly (butyleneadipate-co-terephthalate)-based biodegradable microplastics modulate the bioavailability and speciation of Cd and As in soil: Insights into transformation mechanisms

Microplastics (MPs) that enter the soil can alter the physicochemical and biochemical properties of soil and affect speciation of heavy metals (HMs), thereby perturbing the bioavailability of HMs. However, the mechanisms underlying these effects are not understood. Therefore, we investigated the effects of MPs from poly (butyleneadipate-co-terephthalate)-based biodegradable mulch (BM) and polyethylene mulch (PM) in Cd- or As-contaminated soil on soil properties and speciation of HMs. MPs were characterised using Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The addition of MPs reduced the bioavailability of HMs in soil and promoted the transformation of HMs into inert fractions. The mechanisms underlying the reduction of the bioavailability of HMs in soils could be as follows: (1) the entry of MPs into the soil changed its properties, which reduced the bioavailability of HMs; (2) FTIR and XPS analyses revealed that the hydroxyl and carboxyl groups and benzene ring present on the surface of aged MPs stabilized complexes (As(V)-O) with As(V) may have directly reduced the bioavailability of As(V) in soil; (3) aged BM exposed more amounts and types of reactive functional groups and was more effective in stabilising soil HMs than PM. Overall, this study provides new insights regarding the complexation mechanisms of soil HMs by MPs from different plastic mulch sources.

In situ preparation of a multifunctional adsorbent by optimizing the Fe2+/Fe3+/Mn2+/HA ratio for simultaneous and efficient removal of Cd(II), Pb(II), Cu(II), Zn(II), As(III), Sb(III), As(V) and Sb(V) from aqueous environment: Behaviors and mechanisms

Multiple potentially toxic elements (PTEs) often coexist in practical wastewater environment, which poses serious risks to the ecological environment and human health. However, few of the reported adsorbents are capable of simultaneously and effectively removing multiple PTEs from wastewater due to the unique properties of each element. In this work, a multifunctional adsorbent FMHs was developed by optimizing Fe²⁺/Fe³⁺/Mn²⁺/HA ratio, and applied to remove Cd(II), Pb(II), Cu(II), Zn(II), As(III), Sb(III), As(V) and Sb(V) from aqueous solution. Results revealed that the adsorption data obeyed the Elovich, Sips and Redlich-Peterson models in the mono-component system, and the maximum adsorption capacity of FMHs was superior to most adsorbents reported in the literatures. In addition, FMHs retained considerable removal capacity after four cycles, and maintained excellent adsorption performance under the interference of different environmental factors (including pH, ionic strength, co-existing ions and humic acid). In the multi-component system, FMHs also presented high adsorption capacity for all the selected PTEs, especially for Sb(III/V) and Pb(II). Characterization results confirmed that various removal mechanisms, such as precipitation, surface complexation, ion exchange, electrostatic attraction and redox, were responsible for the capture of PTEs by FMHs.

Biochar Derived from Chinese Herb Medicine Residues for Rhodamine B Dye Adsorption

In this study, one well-known CHM residue (Atropa belladonna L., ABL) was used to prepare biochar capable of adsorbing rhodamine B (RhB) with an ultrahigh surface area for the first time. Three micropore-rich ABL biochars including ABL@ZnCl₂ (1866 m²/g), ABL@H₃PO₄ (1488 m²/g), and ABL@KOH (590 m²/g) were obtained using the one-step carbonization method with activation agents (ZnCl₂, H₃PO₄, and KOH) via chemical activation and carbonization at 500 °C, and their adsorption performance for RhB was systematically studied with adsorption kinetics, isotherms, and thermodynamics. Through pore diffusion, π-π interaction, and hydrogen bonding, ABL biochar had excellent adsorption performance for RhB. Moreover, when C ₀ was 200 mg/L, biochar dosage was 1 g/L, and the contact time was 120 min; the maximum RhB adsorption capacity and removal efficiency on ABL@ZnCl₂ and ABL@H₃PO₄ were 190.63 mg/g, 95% and 184.70 mg/g, 92%, respectively, indicating that it was feasible to prepare biochar from the ABL residue for RhB adsorption. The theoretical maximum adsorption capacities of ABL@ZnCl₂ and ABL@H₃PO₄ for RhB were 263.19 mg/g and 309.11 mg/g at 25 °C, respectively. Furthermore, the prepared biochar showed good economic applicability, with pay back of USD 972/t (ABL@ZnCl₂) and USD 987/t (ABL@H₃PO₄), respectively. More importantly, even after five cycles, ABL@H₃PO₄ biochar still showed great RhB removal efficiency, suggesting that it had a good application prospect and provided a new method for the resource utilization of traditional CHM residues. Additionally, pore diffusion, π-π interactions, and hydrogen bonding all play roles in the physical adsorption of RhB on ABL biochar. π-π interactions dominated in the early stage of RhB adsorption on ABL@H₃PO₄, while pore diffusion played a crucial role in the whole adsorption process on both adsorbents.

Simultaneous alleviation of Cd availability in contaminated soil and accumulation in rice (Oryza sativa L.) by Fe-Mn oxide-modified biochar

Fe-Mn oxide-modified biochar (BC-FM) was used to remediate Cd-contaminated soil and mitigate Cd accumulation in rice. The roles of Fe and Mn in soil Cd immobilization and in controlling Cd uptake by rice were investigated via X-ray photoelectron spectroscopy (XPS) characterization and chemical analysis. Fe and Mn loaded on BC-FM increased the removal efficiencies of CaCl₂ extractable Cd in soil and Cd in pore water compared to those in only biochar (BC)-treated soil, with maximum removal rates at 67.9 % and 77.8 %, respectively. The XPS results indicated that the redox reactions of the Fe-Mn oxides on BC-FM surface affected Cd immobilization in the soil. The Fe (II/III) components on BC-FM were primarily converted to Fe₃O₄ in the soil system, which may form stable complexes with Cd²⁺ (Fe-O-Cd) during the entire rice growth period, and Cd may be bound to MnO or Mn₂O₃ in the form of CdMn₂O₄. The excellent adsorption performance of BC-FM enhanced by Fe-Mn oxides reduced the available Cd in the soil and stimulated Fe and Mn transport in rice, thereby inhibiting Cd accumulation in the aerial parts of rice. Cd concentrations in brown rice under BC-FM treatments reached the national safety standard (0.2 mg/kg, GB2762-2017). And BC-FM significantly increased the biomass of brown rice with a maximum rate of 26.8 %. These findings suggest that BC-FM could be used as an efficient material for Cd-contaminated soil remediation, and Fe-Mn plays important role in immobilizing Cd in soil and reducing Cd transport in rice.

Valorization of date palm leaves for adsorptive remediation of 2,4-dichlorophenoxyacetic acid herbicide polluted agricultural runoff

Alarming rates of water contamination by toxic herbicides have prompted the need and attention for easy, efficient, and affordable treatment options with a touch of circular economy aspects. This study valorized date palm leaf (DPL) wastes into a valuable adsorbent for remediating agricultural wastewater polluted with 2,4-Dichlorophenoxyacetic acid (2,4-DPA) herbicide. The DPL precursor was modified with H₂SO₄ treatment and both biomass samples were characterized by various analytical techniques. Acid treatment modified the morphology, thermal, and textural properties of the final product (TDPL) while maintaining the structure and surface chemistry intact. Simulated wastewaters containing 2,4-DPA were subsequently treated using TDPL as an adsorbent. Optimum adsorption conditions of pH 2, dosage 0.95 g/L, shaking speed 200 rpm, time 120 min, and temperature 30 °C showed a good herbicide removal efficiency in the range of 55.1-72.6% for different initial feed concentrations (50-250 mg/L). Experimental kinetic data were better represented by the pseudo-second-order model, while the Freundlich isotherm was reliable in describing the equilibrium behavior of the adsorption system. Further, the thermodynamic analysis revealed that the adsorption occurred spontaneously, favorably, and exothermically. Plausible sorption mechanism involved electrostatic interactions, weak van der Waals forces, hydrogen bonds, and π-π interactions between the participating phases. Conspicuously, TDPL application to real-world situations of treating actual herbicide-polluted agricultural runoff resulted in a 69.4% remediation efficiency. Thus, the study demonstrated the valorization of date palm leaves into a valuable and industry-ready adsorbent that can sequester toxic 2,4-DPA herbicide contaminant from aqueous streams.

Efficient removal of Sb(III) from water using β-FeOOH-modified biochar:Synthesis, performance and mechanism

Since the toxicity of Sb(III) is 10 times as high as that of Sb(V) in the environment, it is urgent to find a way to cut down Sb(III). β-FeOOH-modified biochar (β-FeOOH/BC) was prepared and used to remove Sb(III). The characterization results suggested that oxygen-containing functional groups formed on β-FeOOH/BC, which increased the Sb(III) removal efficiency. Even under complex water matrix conditions, the outstanding adsorption performance of β-FeOOH/BC for Sb(III) was obtained. The adsorption reaction rapidly reached a high removal efficiency within 5 min and approached adsorption equilibrium in about 6 h. The adsorption process was fitted to pseudo-second-order kinetics. Amount of maximum adsorption was 202.53 mg g^-1 at 308 K according to Langmuir model. β-FeOOH/BC removed Sb(III) mainly through pore-filling complexation, cation-π and coordination exchange. The CO sites and persistent free radicals (PFRs) acted as electron acceptors, facilitating the electron transfer. In brief, β-FeOOH/BC adsorbent material could adsorb and oxidize Sb(III), which showed excellent prospects for reducing the risk of Sb(III).

Enhanced nitrate and cadmium removal performance at low carbon to nitrogen ratio through immobilized redox mediator granules and functional strains in a bioreactor

The coexistence of multiple pollutants and lack of carbon sources are challenges for the biological treatment of wastewater. To achieve simultaneous removal of nitrate (NO₃^--N) and cadmium (Cd²⁺) at low carbon to nitrogen (C/N) ratios, 2-hydroxy-1,4-naphthoquinone (HNQ) was selected from three redox mediators as an accelerator for denitrification of heterotrophic strain Pseudomonas stutzeri sp. GF2 and autotrophic strain Zoogloea sp. FY6. Then, halloysite nanotubes immobilized with 2-hydroxy-1,4-naphthoquinone (HNTs-HNQ) were prepared and a bioreactor was constructed with immobilized redox mediator granules (IRMG) as the carrier, which was immobilized with HNTs-HNQ and inoculated with the two strains. The immobilized HNQ and the inoculated strains jointly improved the removal ability of NO₃^--N and Cd²⁺ and the removal efficiency of NO₃^--N (25.0 mg L^-1) and Cd²⁺ (5.0 mg L^-1) were 92.81% and 93.94% at C/N = 1.5 and hydraulic retention time (HRT) = 4 h. The Cd²⁺ was removed by adsorption of iron oxides (FeO(OH) and Fe₃O₄) and IRMG. The electron transport system activity (ETSA) of bacteria was improved and the composition of dissolved organic matter in the effluent was not affected by HNQ. The HNQ promoted the production of FeO(OH) and up-regulated the proportion of Zoogloea (54.75% in the microbial community), indicating that Zoogloea sp. FY6 was dominant in the microbial community. In addition, HNQ influenced the metabolic pathways and improved the relative abundance of some genes involved in nitrogen metabolism and the iron redox cycle.

Mechanism of microbiologically induced calcite precipitation for cadmium mineralization

Microbiologically induced calcite precipitation (MICP) technology shows potential for remediating heavy metal pollution; however, the underlying mechanism of heavy metal mineralization is not well-understood, limiting the application of this technology. In this study, we targeted Cd contamination (using 15:1, 25:1, and 50:1 Ca²⁺/Cd²⁺ molar ratios) and showed that the ureolytic bacteria Sporosarcina ureilytica ML-2 removed >99.7 % Cd²⁺ with a maximum fixation capacity of 75.61 mg-Cd/g-CaCO₃ and maximum precipitation production capacity of 135.99 mg-CaCO₃/mg-cells. Quantitative PCR analysis showed that Cd²⁺ inhibited the expression of urease genes (ureC, ureE, ureF, and ureG) by 70 % in the ML-2 strain. Additionally, the pseudo-first-order kinetics model (R² = 0.9886), intraparticle diffusion model (R² = 0.9972), and Temkin isotherm model (R² = 0.9828) described the immobilization process of Cd²⁺ by bio calcite in MICP-Cd system. The three Cd²⁺ mineralization products generated by MICP were attributed to surface precipitation (Cd²⁺ → Cd(OH)₂), direct binding with the CO₃^2-/substitution calcium site of calcite (Cd²⁺ → CdCO₃, otavite), and calcite lattice vacancy anchors (Cd²⁺ → (Ca_xCd_1-x)CO₃). Our findings improve the understanding of the mechanisms by which MICP can achieve in situ stabilization of heavy metals.