Thermodynamic Analysis of Nickel(II) and Zinc(II) Adsorption to Biochar

Immobilization mechanisms of Sr(II), Ni(II), and Cd(II) on glomalin-related soil protein in mangrove sediments at the microscopic scale

Glomalin-related soil protein (GRSP) is a significant component in the sequestration of heavy metal in soils, but its mechanisms for metal adsorption are poorly known. This study combined spectroscopic data with molecular docking simulations to reveal metal adsorption onto GRSP's surface functional groups at the molecular level. The EXAFS combined with FTIR and XPS analyses indicated that the adsorption of Cd(II), Sr(II), and Ni(II) by GRSP occurred mainly through the coordination of -OH and -COOH groups with the metal. The -COOH and -OH groups bound to the metal as electron donors and the electron density of the oxygen atom decreased, suggesting that electrostatic attraction might be involved in the adsorption process. Two-dimensional correlation spectroscopy revealed that preferential adsorption occurred on GRSP for the metal in sequential order of -COOH groups followed by -OH groups. The presence of the Ni-C shell in the Ni EXAFS spectrum suggested that Ni formed organometallic complexes with the GRSP surface. However, Sr-C and Cd-C were absent in the second shell of the Sr and Cd spectra, which was attributed to the adsorption of Sr and Cd ions with large hydration ion radius by GRSP to form outer-sphere complexes. Through molecular docking simulations, negatively charged residues such as ASP151 and ASP472 in GRSP were found to provide electrostatic attraction and ligand combination for the metal adsorption, which was consistent with the spectroscopic analyses. Overall, these findings provided new insights into the interaction mechanisms between GRSP and metals, which will help deepen our understanding of the ecological functions of GRSP in metal sequestration.

New Insights into Aggregation Behaviors of the UV-Irradiated Dissolved Biochars (DBioCs) in Aqueous Environments: Effects of Water Chemistries and Variation in the Hamaker Constant

The aggregation behavior of ubiquitous dissolved black carbon (DBC) largely affects the fate and transport of its own contaminants and the attached contaminants. However, the photoaging processes and resulting effects on its colloidal stability remain yet unknown. Herein, dissolved biochars (DBioCs) were extracted from common wheat straw biochar as a proxy for an anthropogenic DBC. The influences of UV radiation on their aggregation kinetics were systematically investigated under various water chemistries (pH, electrolytes, and protein). The environmental stability of the DBioCs before and after radiation was further verified in two natural water samples. Hamaker constants of pristine and photoaged DBioCs were derived according to Derjaguin-Landau-Verwey-Overbeek (DLVO) prediction, and its attenuation (3.19 ± 0.15 × 10-21 J to 1.55 ± 0.07 × 10-21 J after 7 days of radiation) was described with decay kinetic models. Pearson correlation analysis revealed that the surface properties and aggregation behaviors of DBioCs were significantly correlated with radiation time (p < 0.05), indicating its profound effects. Based on characterization and experimental results, we proposed a three-stage mechanism (contended by photodecarboxylation, photo-oxidation, and mineral exposure) that DBioCs might experience under UV radiation. These findings would provide an important reference for potential phototransformation processes and relevant behavioral changes that DBC may encounter.

Sustainable management of campus fallen leaves through low-temperature pyrolysis and application in Pb immobilization

Realizing campus sustainability requires the environmental-friendly and economical treatment of tremendous fallen leaves. Producing fallen leaf biochar at a low temperature is a candidate approach. In this study, six common types of fallen leaves on the campus were pyrolyzed at 300 °C. The obtained biochars were characterized and the adsorption mechanisms of lead (Pb) by the fallen leaf biochars were investigated. The adsorption capacity of leaf biochar for Pb was relatively high, up to 209 mg/g (Yulania denudata leaf biochar). Adsorption of Pb onto active sites was the rate-limiting step for most leaf biochars. But for Platanus leaf biochar, intraparticle diffusion of Pb2+ dominated owing to the lowest adsorption capacity. However, the highest exchangeable Pb fraction (27%) indicated its potential for removing aqueous Pb2+. Ginkgo and Prunus cerasifera leaf biochar immobilized Pb by surface complexation and precipitation as lead oxalate. Hence, they were suitable for soil heavy metal remediation. This study shed the light on the sustainable utilization of campus fallen leaves and the application of fallen leaf biochars in heavy metal remediation.

Application of machine learning in prediction of Pb2+ adsorption of biochar prepared by tube furnace and fluidized bed

Data mining by machine learning (ML) has recently come into application in heavy metals purification from wastewater, especially in exploring lead removal by biochar that prepared using tube furnace (TF-C) and fluidized bed (FB-C) pyrolysis methods. In this study, six ML models including Random Forest Regression (RFR), Gradient Boosting Regression (GBR), Support Vector Regression (SVR), Kernel Ridge Regression (KRR), Extreme Gradient Boosting (XGB), and Light Gradient Boosting Machine (LGBM) were employed to predict lead adsorption based on a dataset of 1012 adsorption experiments, comprising 422 TF-C groups from our experiments and 590 FB-C groups from literatures. The XGB model showed superior accuracy and predictive performance for adsorption, achieving R2 values for TF-C (0.992) and FB-C (0.981), respectively. Contrasting inferior results were observed in other models, including RF (0.962 and 0.961), GBR (0.987 and 0.975), SVR (0.839 and 0.763), KRR (0.817 and 0.881), and LGBM (0.975 and 0.868). Additionally, a hybrid dataset combining both biochars in Pb adsorption also indicated high accuracy (0.972) as obtained from XGB model. The investigation revealed that the influence of char characteristics and adsorption conditions on Pb adsorption differs between the two biochar. Specific char characteristics, particularly nitrogen content, significantly influence lead adsorption in both biochar. Interestingly, the influence of pyrolysis temperature (PT) on lead adsorption is found to be greater for TF-C than for FB-C. Consequently, careful consideration of PT is crucial when preparing TF-C biochar. These findings offer practical guidance for optimizing biochar preparation conditions during heavy metal removal from wastewater.

Highly efficient Ni(II) adsorption by industrial lignin-based biochar: a pivotal role of dissolved substances within biochar

In the context of carbon neutrality, promoting resource utilization of industrial alkali lignin addressing heavy metal pollution is crucial for China's pollution alleviation and carbon reduction. Microwave pyrolysis produced functionalized biochar from industrial alkali lignin for Ni(II) adsorption. LB400 achieved 343.15 mg g-1 saturated adsorption capacity in 30 min. Pseudo-second-order kinetic and Temkin isotherm models accurately described the adsorption, which was endothermic and spontaneous (ΔGϴ < 0, ΔHϴ > 0). Quantitative analysis revealed that both dissolved substances and carbon skeleton from biochar contributed to adsorption, with the former predominates (93.76%), including mineral precipitation NiCO3 (Qp) and adsorption of dissolved organic matter (QDOM). Surface complexation (Qc) and ion exchange (Qi) on the carbon skeleton accounted for 6.3%. Higher biochar preparation temperature reduced Ni(II) adsorption by dissolved substances. Overall, biochar which comes from the advantageous disposal of industrial lignin effectively removes Ni(II) contamination, encouraging ecologically sound treatment of heavy metal pollution and sustainable resource utilization.

Efficient treatment of palladium from wastewater by acrolein cross-linked chitosan hydrogels: Adsorption, kinetics, and mechanisms

Herein we present a study on the preparation and properties of a hydrogel adsorbent for treatment of wasted palladium souring from actial petrochemical industrial wastewater. Chitosan was used as the raw material and acrolein as the cross-linking agent for the hydrogel (A/CS). The adsorption behaviors of the hydrogel for Pd(II) ions were characterized and analyzed. The effect of pH, temperature, adsorption kinetics, and thermodynamics were investigated. Langmuir models were employed to describe the adsorption isotherms, while the pseudo-second-order equation was applied to describe the adsorption kinetics. The experimental results demonstrated that the adsorption was a monolayer chemical adsorption, and the adsorption capacity was found to reach 505.05 mg/g under optimal conditions. In addition, FT-IR and XPS analyses, combined with MS calculations confirmed that chelation and electrostatic attraction were dominated in the adsorption process. Overall, the development of this hydrogel adsorbent will provide a practical approach to the treatment of industrial wastewater containing palladium and have great potential for practical applications.

Evaluation of a pH- and time-dependent model for the sorption of heavy metal cations by poultry litter-derived biochar

Common isotherm and kinetic models cannot describe the pH-dependent sorption of heavy metal cations by biochar. In this paper, we evaluated a pH-dependent, equilibrium/kinetic model for describing the sorption of cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) by poultry litter-derived biochar (PLB). We performed sorption experiments across a range of solution pH, initial metal concentration, and reaction time. The sorption of all five metals increased with increasing pH. For Cd, Cu, and Pb, kinetics experiments demonstrated that sorption rates were greater at pH 6.5 than at pH 4.5. For each metal, all sorption data were described using single set of four adjustable parameters. Sorption edge and isotherm data were well described with R2 > 0.93 in all cases. Time-dependent sorption was well described (R2 ≥ 0.90) for all metals except Pb (R2 = 0.77). We then used the best-fit model parameters to calculate linear distribution coefficients (KD) and equilibration times as a function of pH and initial solution concentration. These calculations provide a more robust way of characterizing biochar affinity for metal cations than Freundlich distribution coefficients or Langmuir sorption capacity. Because this model can characterize metal cation sorption by biochar across a wider range of reaction conditions than traditional isotherm or kinetic models, it is better suited for estimating metal cation/biochar interactions in engineered or natural systems.

Qualitative and Quantitative Investigation of Biochar-Cu0 Composite for Nickel Adsorption

The current investigation deals with the treatment of water pollution that is caused by the leaching of nickel ions from the metallurgical industry and new-energy batteries. Therefore, an eco-friendly treatment of nickel through the use of a composite of cotton stalk biochar with nanozerovalent copper has been presented in this investigation signifying the impact of zerovalent copper in enhancing the adsorption capacity of biochar for nickel adsorption. Thermogravimetric analysis data showed the adsorbent to be significantly stable in the higher thermal range, whereas transmission electron microscopy analysis confirmed the particles to be 27 nm and also showed the cubic geometry of the particles. A much closer scanning electron microscopy analysis shows the morphology of particles to be cubic in shape. Batch adsorption indicated a positive influence of pH increase on adsorption due to the electrostatic attraction between positive nickel ions and post point of zero charge (pHPZC) negative surface of copper biochar composite (pH > 5.5). A high adsorption rate was observed in the first 60 min, whereas adsorption increased with the increase in temperature from 303 to 318 K. Kinetic modeling confirmed the pseudo-first-order to fit best to the data. The apparent activation energy (11.96 kJ mol-1) is indicative of the chemical nature of the process. The adsorption data fitted well to the Langmuir adsorption model. The negative values of apparent ΔG° and the positive values of apparent ΔH° indicate the spontaneity and endothermicity of the process, respectively, whereas the positive values of apparent ΔS° point toward increased randomness during the process. Postadsorption XPS suggests the adsorption of nickel on the surface of biochar composites in the form of Ni(OH)2 and NiO(OH).

Selective Adsorption Behavior and Mechanism for Cd(II) in Aqueous Solution with a Recoverable Magnetie-Surface Ion-Imprinted Polymer

A novel recoverable magnetic Cd(II) ion-imprinted polymer was synthesized on the surface of silica-coated Fe₃O₄ particles via the surface imprinting technique and chemical grafting method. The resulting polymer was used as a highly efficient adsorbent for the removal of Cd(II) ions from aqueous solutions. The adsorption experiments revealed that Fe₃O₄@SiO₂@IIP had a maximum adsorption capacity of up to 29.82 mg·g^-1 for Cd(II) at an optimal pH of 6, with the adsorption equilibrium achieved within 20 min. The adsorption process followed the pseudo-second-order kinetic model and the Langmuir isotherm adsorption model. Thermodynamic studies showed that the adsorption of Cd(II) on the imprinted polymer was spontaneous and entropy-increasing. Furthermore, the Fe₃O₄@SiO₂@IIP could rapidly achieve solid-liquid separation in the presence of an external magnetic field. More importantly, despite the poor affinity of the functional groups constructed on the polymer surface for Cd(II), we improved the specific selectivity of the imprinted adsorbent for Cd(II) through surface imprinting technology. The selective adsorption mechanism was verified by XPS and DFT theoretical calculations.

Aggregation behaviour of black carbon in aquatic solution: Effect of ionic strength and coexisting metals

Black Carbon (BC) is an important constituent of both aquatic and terrestrial environment, but also has several adverse effects on human health, aquatic life, and contributes to the global climate change. Thus, to understand the fate and transport of BC nanoparticles (NPs) in the environment, it's important to understand the colloidal stability or aggregation behaviour and factors affecting it, under various environmental conditions, including both aquatic and atmospheric. This study investigated the individual influence of ionic strengths, valence (Na+, Ca2+ and Mg2+), metals (Zn2+, Cu2+, Ni2+ and Cd2+), and organic substances (PO43- and Humic Acid: HA) on the effective diameter or hydrodynamic diameter and zeta potential of BC-NPs in aquatic systems. A dynamic light scattering (DLS) principle-based 90 Plus Particle Size Analyzer was used for measurements of BC particle size and zeta potential at varying ionic chemistry. The results showed that strong ionic strength promotes aggregation of BC-NPs till the repulsion forces become dominant due to more negative zeta potential. The Aggregation of BC-NPs was observed to be significantly dependent on the ionic valence, where divalent ions caused more aggregation than monovalent ions. Metal ions at higher concentration (around 1 mM) promoted the aggregation rate of BC-NPs, and Cu+2 dominated among all selected metals. Conversely, organic matter (PO43- and HA) tends to promote stabilisation of BC-NPs instead of aggregation. Though this study investigated individual effect of substances, influence of possible environmental combination of substances will help to get more clear idea.

Zirconium-modified attapulgite was used for removing of Cr(vi) in aqueous solution

This work fabricated the zirconium-modified attapulgite (Zr@ATP) for removing Cr(vi) ions in aqueous solutions. According to X-ray diffraction, scanning electron microscopy, TEM, Fourier transform infrared, and X-ray photoelectron spectroscopy analyses, Zr was successfully grafted onto the attapulgite rod surface. Cr(vi) adsorption onto Zr@ATP surface fitted well with the Langmuir isotherm and pseudo-second-order kinetic models, which suggested that the adsorption is primarily chemisorption. When the pH of the aqueous solution is 3, Zr@ATP achieved the highest Cr(vi) absorption, of about 32.84 mg/g. Density functional theory studies revealed that the hydroxyl functional group introduced through the modification process supplies more active sites to form the hydrogen bond with CrO 4 2 − {\text{CrO}}_{4}^{2-} and HCrO 4 − {\text{HCrO}}_{4}^{-} .

Application of zeolites in permeable reactive barriers (PRBs) for in-situ groundwater remediation: A critical review

Permeable reactive barrier (PRB) is one of the most promising in-situ groundwater remediation technologies due to its low costs and wide immobilization suitability for multiple contaminants. Reactive medium is a key component of PRBs and their selection needs to consider removal effectiveness as well as permeability. Zeolites have been extensively reported as reactive media owing to their high adsorption capacity, diverse pore structure and high stability. Moreover, the application of zeolites can reduce the PRBs fouling and clogging compared to reductants like zero-valence iron (ZVI) due to no formation of secondary precipitates, such as iron monosulfide, in spite of their reactivity to remove organics. This study gives a detailed review of lab-scale applications of zeolites in PRBs in terms of sorption characteristics, mechanisms, column performance and desorption features, as well as their field-scale applications to point out their application tendency in PRBs for contaminated groundwater remediation. On this basis, future prospects and suggestions for using zeolites in PRBs for groundwater remediation were put forward. This study provides a comprehensive and critical review of the lab-scale and field-scale applications of zeolites in PRBs and is expected to guide the future design and applications of adsorbents-based PRBs for groundwater remediation.

Facile Construction of Iron/Nickel Phosphide Nanocrystals Anchored on N-B-Doped Carbon-Based Composites with Advanced Catalytic Capacity for 4-Nitrophenol and Methylene Blue

The search for a simple and effective method to remove organic dyes and color intermediates that threaten human safety from the water environment is urgent. Herein, we report a simple method for constructing iron/nickel phosphide nanocrystals anchored on N-B-doped carbon-based composites, using steam-exploded poplar (SEP) and graphene oxide (GO) as a carrier. The stability and catalytic activity of N-B-Ni_xFe_yP/SEP and GO were achieved by thermal conversion in a N₂ atmosphere and modifying the Fe/Ni ratio in gel precursors. N-B-Ni₇Fe₃P/SEP was employed for the catalytic hydrogenation of 4-nitrophenol (4-NP) and methylene blue (MB), using sodium borohydride in aqueous media at room temperature. This showed much better catalytic performances in terms of reaction rate constant (0.016 S⁻¹ and 0.041 S⁻¹, respectively) and the activity factor, K (1.6 S⁻¹·g⁻¹ and 8.2 S⁻¹·g⁻¹, respectively) compared to the GO carrier (0.0053 S⁻¹ and 0.035 S⁻¹ for 4-NP and MB, respectively). The strong interaction between the carrier’s morphology and structure, and the vertically grown bimetallic phosphide nanoclusters on its surface, enhances charge transfer, electron transfer kinetics at the interface and Ni-Fe phosphide dispersion on the nanoclusters, and prevents dissolution of the nanoparticles during catalysis, thereby improving stability and achieving catalysis durability. These findings provide a green and simple route to efficient catalyst preparation and provide guidance for the rational selection of catalyst carriers.

Biochars and activated carbons as adsorbents of inorganic and organic compounds from multicomponent systems - A review

Biochars are obtained by biomass pyrolysis, whereas activated carbon is a biochar that has undergone chemical or physical activation. Owing to the large surface area and easy surface modification both solids are widely applied as adsorbents. They are low-costs materials, they could be regenerated and their disposal is not troublesome. Adsorption of heavy metals, dyes, pharmaceuticals on the surface of biochars and activated carbons, from simple systems of adsorbate containing only one compound, are described extensively in the literature. The present paper provides an overview of reports on adsorption of inorganic and organic compounds onto these two types of adsorbents from the mixed adsorbate systems. The described adsorbate systems have been divided into those consisting of: two or more inorganic ions, two or more organic compounds and both of them (inorganic and organic ones). The research of this type is carried out much less frequently due to the more complicated description of interactions in the mixed adsorbate systems.

The Removal of Pb2+ from Aqueous Solution by Using Navel Orange Peel Biochar Supported Graphene Oxide: Characteristics, Response Surface Methodology, and Mechanism

The value-added utilization of waste resources to synthesize functional materials is important to achieve the environmentally sustainable development. In this paper, the biochar supported graphene oxide (BGO) materials were prepared by using navel orange peel and natural graphite. The optimal adsorption parameters were analyzed by response surface methodology under the conditions of solution pH, adsorbent dosage, and rotating speed. The adsorption isotherm and kinetic model fitting experiments were carried out according to the optimal adsorption parameters, and the mechanism of BGO adsorption of Pb2+ was explained using Scanning Electron Microscope (SEM-EDS), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). Compared with virgin biochar, the adsorption capacity of Pb2+ on biochar supported graphene oxide was significantly increased. The results of response surface methodology optimization design showed that the order of influence on adsorption of Pb2+ was solution pH > adsorbent dosage > rotating speed. The optimal conditions were as follows: solution pH was 4.97, rotating speed was 172.97 rpm, and adsorbent dosage was 0.086 g. In the adsorption−desorption experiment, the desorption efficiency ranged from 54.3 to 63.3%. The process of Pb2+ adsorption by BGO is spontaneous and endothermic, mainly through electrostatic interaction and surface complexation. It is a heterogeneous adsorption process with heterogeneous surface, including surface adsorption, external liquid film diffusion, and intra-particle diffusion.

Wastewater post-coagulation sludge recycled as a multifunctional adsorbent via pyrolysis enhanced in carbon dioxide (CO2)

Massive wastewater post-coagulation sludge (WPCS) generated from the tertiary treatment facilities has been regarded as an environmentally burdensome waste. Herein, to take advantage of the abundant amounts of Al/Fe (hydr)oxides, the WPCS was converted into functional char via pyrolysis under CO₂ and N₂ atmosphere. The higher organic matter content and porous structure of WPCS than drinking water treatment sludge made it a more suitable precursor for biochar and adsorbent production. CO₂ expedited the thermolysis of the organics in WPCS and the Fe (hydr)oxides in WPCS further decreased the temperature of CO₂-mediated reaction. Therefore, the corresponding products outcompeted the chars in N₂, achieving ∼37% higher specific surface area, stronger aromaticity and more amorphous Al and Fe contents of 201.19 ± 2.25 and 27.03 ± 0.56 mg g^-1, accompanied by more loss of surface functional groups like carboxyl and hydroxyl. Accordingly, WPCS chars under CO₂ showed superior performance for removing phosphate (15.58 ± 0.19 mg g^-1), along with the adsorption of heavy metal (37.17 ± 1.25 mg g^-1 of Pb (II)) and dye (14.45 ± 0.11 mg g^-1 of methylene blue). In sum, this study proposes a win-win strategy to convert coagulation sludges into resources and a new candidate for multifunctional adsorbent production.

Mechanistic insight into selective adsorption and easy regeneration of carboxyl-functionalized MOFs towards heavy metals

The development of heavy metal adsorbents with high selectivity has become a research hotspot due to the interference of coexisting ions (e.g., Na⁺, Ca²⁺) in the actual wastewater, but the more difficult regeneration caused by high adsorption selectivity severely limits its practical applications. Herein, a carboxyl adsorbent, MIL-121, demonstrated high adsorption selectivity for heavy metals at 10,000 mg/L of Na⁺ (removal > 99% for Cu²⁺) as well as unexpected easy regeneration (desorption > 99%) at low H⁺ concentration (10^-3.5-10^-3.0 M), which is hundreds of times lower than that of ever reported selective adsorbents (> 10^-1 M H⁺). X-ray photoelectron spectrometry (XPS), extended X-ray absorption fine structure (EXAFS) coupled with Density functional theory (DFT) simulation unveil that the -COOH groups in MIL-121 for heavy metals adsorption is specific inner-sphere coordination with higher binding energy (1.31 eV for Cu), and less energy required for regeneration (0.26 eV for H). Similar high selectivity and easy regeneration were also satisfied with other heavy metals (e.g., Pb²⁺, Ni²⁺), and removal of heavy metals remained > 99% in 10 consecutive adsorption-desorption cycles. For actual copper electroplating wastewater treatment, MIL-121 could produce ~ 3600 mL clean water/g sample, outperforming 300 mL that of the benchmark commercial adsorbent D-113. This study shows the potential of MIL-121 for heavy metal wastewater treatment and provides mechanistic insight for developing adsorbents with high selective adsorption and easy regeneration.

A critical review of biochar-based materials for the remediation of heavy metal contaminated environment: Applications and practical evaluations

The contamination of heavy metals (HMs) in the environment has aroused a global concern. The valid remediation of HM contaminated environment is a highly significant issue. As alternative to carbon materials, biochar has been vastly documented for the remediation of HM contaminated environment. However, there are some possible imperfections to meet the actual remediation tasks as the finite properties of raw biochar, and the remediation process is complex and unexpectedly. This review focuses on the progress made on environmental HM remediation by biochar-based materials within the past six years. The property analysis and key modifications of biochar are summarized inspired by their applicability or necessity for HM decontamination, and the environmental remediation as well as the implicated mechanisms are thoroughly elaborated from multiple pivotal sides. The evaluations of practical application associated with biochar amendment are also presented. Finally, some pertinent improvements and research directions are proposed. To our knowledge, this article is the first time to make a systematic summary on the reliability and practicability of biochar-based materials for environmental HM remediation, and critically pointed out the existing issues to facilitate the judicious design of biochar-based materials and understanding the research trends. It is also aims to provide reference for subsequent research and propel the practical applications.

Characterization of Waste Amidoxime Chelating Resin and Its Reutilization Performance in Adsorption of Pb(II), Cu(II), Cd(II) and Zn(II) Ions

The continuous expansion of the market demand and scale of commercial amidoxime chelating resins has caused large amounts of resin to be discarded around the world. In this study, the waste amidoxime chelating resin was reutilized as an adsorbent for the removal and recovery of Pb(II), Cu(II), Cd(II) and Zn(II) ions from aqueous solutions. The physical morphology and chemical composition of the waste amidoxime chelating resin (WAC-resin) from the factory was characterized by the elemental analyzer, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. The influence of the initial metal ions concentration, contact time, temperature and the solution pH on the adsorption performance of the metal ions was explored by batch experiments. It was shown that the optimal pH was 4. Kinetic studies revealed that adsorption process corresponded with the pseudo-second-order kinetic model and the adsorption isotherm was consistent with the Langmuir model. At room temperature, the adsorption capacities of WAC-resin for Pb2+, Cu2+, Zn2+ and Cd2+ reached 114.6, 93.4, 24.4 and 20.7 mg/g, respectively.

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.

Artificial intelligence (AI) applications in adsorption of heavy metals using modified biochar

The process of removal of heavy metals is important due to their toxic effects on living organisms and undesirable anthropogenic effects. Conventional methods possess many irreconcilable disadvantages pertaining to cost and efficiency. As a result, the usage of biochar, which is produced as a by-product of biomass pyrolysis, has gained sizable traction in recent times for the removal of heavy metals. This review elucidates some widely recognized harmful heavy metals and their removal using biochar. It also highlights and compares the variety of feedstock available for preparation of biochar, pyrolysis variables involved and efficiency of biochar. Various adsorption kinetics and isotherms are also discussed along with the process of desorption to recycle biochar for reuse as adsorbent. Furthermore, this review elucidates the advancements in remediation of heavy metals using biochar by emphasizing the importance and advantages in the usage of machine learning (ML) and artificial intelligence (AI) for the optimization of adsorption variables and biochar feedstock properties. The usage of AI and ML is cost and time-effective and allows an interdisciplinary approach to remove heavy metals by biochar.

Nutrient alterations following biochar application to a Cd-contaminated solution and soil

Biochars, when applied to contaminated solutions or soils, may sequester potentially toxic elements while releasing necessary plant nutrients. This purpose of this study focused on quantifying both phenomenon following wheat straw (Triticum aestivum L.) biochar application (0, 5, and 15% by wt) to a Cd containing solution and a Cd-contaminated paddy soil using 240-day laboratory batch experiments. Following both experiments, solid phases were analyzed for elemental associations using a combination of wet chemical sequential extractions and synchrotron-based X-ray absorption spectroscopy (XAS). When wheat straw biochar was applied at 15% to Cd containing solutions, Cd and Zn concentrations decreased to below detection in some instances, Ca and Mg concentrations increased by up to 290%, and solution pH increased as compared to the 5% biochar application rate. Similar responses were observed when biochar was added to the Cd-contaminated paddy soil, suggesting that this particular biochar has the ability to sequester potentially toxic elements while releasing necessary plant nutrients to the soil solution. When significant, positive correlations existed between nutrient release over time, while negative correlations were present between biochar application rate, potentially toxic element sorption and pH. The latter suggests that potentially toxic elements were sorbed by a combination of organic functional groups or mineral precipitation based on whether pH was above or below ~ 7. In support of this contention, the wet chemical sequential extraction procedure in conjunction with previously observed Cd or current Zn XAS showed that biochar application promoted the formation of layered double hydroxides, sorption to (oxy)hydroxides, and organically bound to biochar as Zn species. As a multifunctional material, biochar appears to play an important role in sequestering Cd while releasing essential plant nutrients. These findings suggest that biochar may be a 'win-win' for improving environmental quality in potentially toxic element contaminated agroecosystems.

Interactive effects of biochar and mussel shell activated concoctions on immobilization of nickel and their amelioration on the growth of rapeseed in contaminated aged soil

Mussel shell (MS) and biochar (BC) are commonly used for the remediation of metal contaminated soil. However, less research has been focused to examine the efficacy of their combinations to reduce metal toxicity in crop plants. This study was therefore conducted to investigate the effects of BC, MS and their activated concoctions on the soil properties, enzyme activities and nickel (Ni) immobilization in aged Ni contaminated soil. Moreover, the growth, photosynthetic pigments and anti-oxidative machnery of Brassica napus plants has also been investigated in order to determine amendments efficiency in reducing soil Ni toxicity for plants. The results showed that the application of Ni adversely affected soil health and trigged stress responses by inducing oxidative stress in B. napus. However, the incorporation of amendments reduced the bioavailability of Ni, and the concoctions of BC and MS showed promising results in the immobilization of Ni. Among various combinations of BC and MS, treatment with BC + MS (3:1) significantly reduced Ni uptake, decreased reactive oxygen species (ROS) and enhanced antioxidant defense of B. napus plants. Results showed that amendment's combinations stimulated the transcriptional levels of ROS scavenging enzymes and suppressed the expression level of Ni transporters. The morphological and physical characterization techniques (i.e. SEM, BET, EDS, FTIR and X-ray diffraction analyses) showed that amendment's combinations had relatively higher Ni adsorption capacity, indicating that BC and MS concoctions are efficient immobilizing agents for minimizing Ni availability, preventing oxidative toxicity and promoting growth and biomass production in rapeseed plants under metal stress conditions.

Nickel in soil and water: Sources, biogeochemistry, and remediation using biochar

Nickel (Ni) is a potentially toxic element that contaminates soil and water, threatens food and water security, and hinders sustainable development globally. Biochar has emerged as a promising novel material for remediating Ni-contaminated environments. However, the potential for pristine and functionalized biochars to immobilize/adsorb Ni in soil and water, and the mechanisms involved have not been systematically reviewed. Here, we critically review the different dimensions of Ni contamination and remediation in soil and water, including its occurrence and biogeochemical behavior under different environmental conditions and ecotoxicological hazards, and its remediation using biochar. Biochar is effective in immobilizing Ni in soil and water via ion exchange, electrostatic attraction, surface complexation, (co)precipitation, physical adsorption, and reduction due to the biogeochemistry of Ni and the interaction of Ni with surface functional groups and organic/inorganic compounds contained in biochar. The efficiency for Ni removal is consistently greater with functionalized than pristine biochars. Physical (e.g., ball milling) and chemical (e.g., alkali/acidic treatment) activation achieve higher surface area, porosity, and active surface groups on biochar that enhance Ni immobilization. This review highlights possible risks and challenges of biochar application in Ni remediation, suggests future research directions, and discusses implications for environmental agencies and decision-makers.

Highly selective removal and recovery of Ni(II) from aqueous solution using magnetic ion-imprinted chitosan nanoparticles

Nickel (Ni) is one of the most common heavy metals. In this study, nano-sized magnetic ion-imprinted polymers (MIIPs) were synthesized using chitosan as the functional monomer, and used for selective adsorption and recovery of Ni(II) from solutions. The results showed MIIPs possessed high sorption selectivity for Ni(II), and the change in pH (5.0-9.0) exerted insignificant influence on the ion adsorption, allowing almost complete elution and recovery of adsorbed Ni(II) ions by using 0.5% EDTA-Na solution. Moreover, the sorption capacity of the recycled MIIPs decreased by only about 10% after 15 adsorption-desorption cycles. The time required for establishing the adsorption equilibrium was less than 1 h. The sorption process was predominant and endothermic, and could be well described by both Langmuir isotherm model and pseudo-second-order kinetic model. Therefore, the synthesized MIIPs was a suitable adsorbent for highly selective, fast and efficient removal and recovery of low-concentration Ni(II) ions from wastewaters.

Influence of dissolved black carbon on the aggregation and deposition of polystyrene nanoplastics: Comparison with dissolved humic acid

Dissolved black carbon (DBC), widely found in soil and water environments is likely to affect the transport of nanoplastics in aquatic environments. The aggregation and deposition behaviors of fresh and aged polystyrene nanoplastics (PSs) with and without DBC in NaCl solution were investigated by time-resolved dynamic light scattering (DLS) and quartz crystal microbalance with dissipation monitoring equipment (QCM-D) techniques. The results suggest that DBC can screen the surface charges of PSs by interacting with PSs through hydrogen bonding, hydrophobic interactions and π-π interactions, although they were negatively charged. DBC promoted the aggregation of PSs under relatively low ionic strengths, and it minimally affected the stability of PSs under high ionic strength. Deposition experiments showed that both DBC in salt solution and DBC adsorption on silica surface facilitated the deposition of fresh PSs while HA inhibited both deposition processes. After aging, PSs were more stable, and the effects of DBC and HA were weakened. This study investigated the influence mechanism of DBC on the aggregation and deposition behaviors, which provides new insights into the stability and transport of PSs in complex aquatic environments.

Selective adsorption of Pb2+ and Cu2+ on amino-modified attapulgite: Kinetic, thermal dynamic and DFT studies

Amino-modified attapulgite (M-ATP) was prepared to remove Pb²⁺ and Cu²⁺ from the aqueous solution. Fourier transform infrared spectroscopy (FT-IR) spectrums and X-ray powder diffraction (XRD) patterns revealed that a new Si-O-Si bond formed after modification. The result indicates that the graft reaction of ATP occurred at Si-O (2 0 0) tetrahedron crystal face. No matter whether in a single or binary heavy metal ion system, the adsorption experiments displayed that the equilibrium adsorption capacity of M-ATP towards Pb²⁺ was much higher than Cu²⁺, which indicated M-ATP more readily adsorbs the Pb²⁺. The selective adsorption mechanism of Pb²⁺ and Cu²⁺ on modified attapulgite was studied by density functional theory (DFT). The E_ads of Pb (- 2.01 eV) adsorbed on M-ATP is lower than Cu (- 1.79 eV) through the DFT calculation of adsorption energy (E_ads), which indicate that the Pb²⁺ adsorbed on M-ATP is more stable than Cu²⁺. Both adsorption experiments and theoretical calculations revealed that due to the stability of Pb²⁺ adsorption on M-ATP, Pb²⁺ is more readily adsorbed by M-ATP, and it is difficult for Cu²⁺ to exchange Pb²⁺ from M-ATP.

Temperature regulated adsorption and desorption of heavy metals to A-MIL-121: Mechanisms and the role of exchangeable protons

Adsorption is a viable technology to remove trace heavy metals from wastewater, but regeneration of adsorbents in an economic and environmentally friendly manner often represents a limiting factor of its application. Compared with traditional strong acid desorption, developing a chemical-free method is of great significance to both economic and the environmental welfare. Herein, we synthesized a novel thermoresponsive absorbent, A-MIL-121, which could effectively remove trace Cu(II) (> 95 %) from a high-salinity ([Na⁺]/[Cu²⁺] = 20000) water at normal temperature. At elevated temperature, A-MIL-121 could quickly and efficiently desorb Cu(II), with over 90% desorption rate at 80°C within 3 h. Fourier transform infrared spectroscopy (FTIR) analysis revealed that two types of -COOH groups existed in the material. One was in free form and acted as the sites for Cu(II) adsorption; the other was in dimer connected by two H-bonds, which cleaved at elevated temperature. As a result, massive exchangeable protons were released to the solution, which caused the desorption of Cu(II). Similar temperature dependent adsorption-desorption behavior was also found to other heavy metals, such as Cd²⁺, Pb²⁺, Ni²⁺. No significant capacity loss was observed after 10 successive adsorption-desorption cycles. Finally, Column experiments using a real copper electroplating wastewater showed that a total of ~ 1650 mL of clean water was generated before breakthrough (Cu²⁺ < 0.5 mg/L), while less than 45 mL of 80°C water was used for regeneration. This study indicates the potential of A-MIL-121 as a novel green adsorbent to address trace heavy metals in wastewater.

Green synthesis of reusable super-paramagnetic diatomite for aqueous nickel (II) removal

Adsorption is an effective method for treating wastewater containing nickel due to its minimal equipment requirements and flexible operation. Therefore, an environmental friendly, inexpensive, efficient and recyclable adsorbent is needed. In this work, a reusable dual-functional super-paramagnetic adsorbent was prepared by combining APTES (3-Aminopropyltriethoxysilane) and EDTA (ethylenediaminetetraacetic acid disodium) with magnetic diatomite for the removal of Ni²⁺. It is named diatomite/CoFe₂O₄@APTES-EDTA (DECFASEs). The synthetic material was characterized and studied by XRD (X-ray Powder Diffractometer), FTIR (Fourier Transform Infrared Spectrometer), SEM (Scanning Electron Microscope), TEM (Transmission Electron Microscope), EDS (Energy Dispersive Spectrometer), VSM (Vibrating-Sample Magnetometer), BET (Brunauer-Emmett-Teller) method, Zeta potential analyzer and XPS (X-ray Photoelectron Spectroscopy), respectively. The performance of adsorption Ni²⁺ by DECFASEs was studied on effect of pH, reaction time and initial concentrations. The adsorption and desorption capacity and recyclability of the adsorbent material were estimated. A adsorption kinetic data had a significant correlation with the pseudo second-order kinetic and also adsorption isotherm data corresponded well with Freundlich adsorption isotherm. The maximum adsorption capacity of the adsorbent material was 19.22 mg/g. The Ni²⁺ adsorption capacity of DECFASEs decreased slightly from 9.11 to 8.25 mg/g after 4 recycles. The XPS results of DECFASEs before and after Ni²⁺ uptake showed N and O participated in the complexation of Ni²⁺ in the adsorption process, which verified the chemical interaction between Ni²⁺ and DECFASEs. Modified-diatomite is a promising adsorbent for aqueous Ni²⁺ removal.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Effects of aging and weathering on immobilization of trace metals/metalloids in soils amended with biochar

Biochar is an effective amendment for trace metal/metalloid (TMs) immobilization in soils. The capacity of biochar to immobilize TMs in soil can be positively or negatively altered due to the changes in the surface and structural chemistry of biochar after soil application. Biochar surfaces are oxidized in soils and induce structural changes through physical and biochemical weathering processes. These changes in the biochar surface and structural chemistry generally increase its ability to immobilize TMs, although the generation of dissolved black carbon during weathering may increase TM mobility. Moreover, biochar modification can improve its capacity to immobilize TMs in soils. Over the short-term, engineered/modified biochar exhibited increased TM immobilization capacity compared with unmodified biochar. In the long-term, no large distinctions in such capacities were seen between modified and unmodified biochars due to weathering. In addition, artificial weathering at laboratories also revealed increased TM immobilization in soils. Continued collection of mechanistic evidence will help evaluate the effect of natural and artificial weathering, and biochar modification on the long-term TM immobilization capacity of biochar with respect to feedstock and synthesis conditions in contaminated soils.

Study on the Adsorption of CuFe2O4-Loaded Corncob Biochar for Pb(II)

A series of the magnetic CuFe2O4-loaded corncob biochar (CuFe2O4@CCBC) materials was obtained by combining the two-step impregnation of the corncob biochar with the pyrolysis of oxalate. CuFe2O4@CCBC and the pristine corncob biochar (CCBC) were characterized using XRD, SEM, VSM, BET, as well as pHZPC measurements. The results revealed that CuFe2O4 had a face-centered cubic crystalline phase and was homogeneously coated on the surface of CCBC. The as-prepared CuFe2O4@CCBC(5%) demonstrated a specific surface area of 74.98 m2·g-1, saturation magnetization of 5.75 emu·g-1 and pHZPC of 7.0. The adsorption dynamics and thermodynamic behavior of Pb(II) on CuFe2O4@CCBC and CCBC were investigated. The findings indicated that the pseudo-second kinetic and Langmuir equations suitably fitted the Pb(II) adsorption by CuFe2O4@CCBC or CCBC. At 30 °C and pH = 5.0, CuFe2O4@CCBC(5%) displayed an excellent performance in terms of the process rate and adsorption capacity towards Pb(II), for which the theoretical rate constant (k2) and maximum adsorption capacity (qm) were 7.68 × 10-3 g·mg-1··min-1 and 132.10 mg·g-1 separately, which were obviously higher than those of CCBC (4.38 × 10-3 g·mg-1·min-1 and 15.66 mg·g-1). The thermodynamic analyses exhibited that the adsorption reaction of the materials was endothermic and entropy-driven. The XPS and FTIR results revealed that the removal mechanism could be mainly attributed to the replacement of Pb2+ for H+ in Fe/Cu-OH and -COOH to form the inner surface complexes. Overall, the magnetic CuFe2O4-loaded biochar presents a high potential for use as an eco-friendly adsorbent to eliminate the heavy metals from the wastewater streams.

Effect of Fe-Mn-Ce modified biochar composite on microbial diversity and properties of arsenic-contaminated paddy soils

In this study, we investigated the mechanism of decrease in arsenic (As) bioavailability after addition of biochar (BC) supplemented with iron (Fe)- manganese (Mn)- cerium (Ce) oxide (FMCBC) to As-contaminated paddy soil. We explored the effects of these composites on the oxidation, reduction, microbial community, and soil enzyme activity of As-contaminated paddy soil. Results showed that FMCBCs improve soil pH, significantly improve the redox capacity of soil, and reduce bioavailable forms of As. FMCBCs can convert As from a specifically or non-specifically bound form into amorphous hydrous oxide bound- and crystalline hydrous oxide bound form. The application of FMCBCs increased soil enzyme activity (urease, catalase, alkaline phosphatase, and peroxidase), and greatly influenced the relative abundance of certain microorganisms (Proteobacteria, Acidobacteria, and Gemmatimonadetes), which improved soil enzyme heavy metal tolerance and prevented their denaturation. Thus, FMCBCs can not only change the form and distribution of As in soil but also create an environment suitable for microbial growth, consequently affecting the geochemical cycling of As in soil.

Synthesis of amino-functionalized biochar/spinel ferrite magnetic composites for low-cost and efficient elimination of Ni(II) from wastewater

Novel amino-modified rice bran biochar/MgFeAlO₄ (RB@MgFeAlO₄-NH₂) magnetic composites were synthesized via a simple one-step solvothermal approach and applied for removing toxic Ni(II) from wastewater. The elimination process and sorption performance of Ni(II) on RB@MgFeAlO₄-NH₂ were analyzed by combining batch experiments and spectral techniques. The sorption isotherms and kinetic data indicated that Ni(II) sorption on RB@MgFeAlO₄-NH₂ was monolayer and rapid. The experimental results confirmed that the obtained RB@MgFeAlO₄-NH₂ magnetic composite had high sorption capacity for Ni(II). The maximum sorption capacity of Ni(II) on RB@MgFeAlO₄-NH₂ was 201.62 mg g^-1. The researches based on the sorption mechanism showed that the ion exchange mechanism accounted for 76.51% of Ni(II) sorption. In addition, the amino, carboxyl and hydroxyl functional groups were also involved in the complexation with Ni(II). In view of its multiple advantages of environmental friendliness, low cost, easy magnetic separation and high sorption capacity, RB@MgFeAlO₄-NH₂ will be an excellent adsorbent for low-cost and efficient elimination of Ni(II) from aqueous solutions.

Nanostructured chitosan/molecular sieve-4A an emergent material for the synergistic adsorption of radioactive major pollutants cesium and strontium

A fresh adsorbent nanostructured chitosan/molecular sieve 4A hybrid (NSC@MS-4A) was fabricated for the rapid adsorption of strontium (Sr²⁺) and cesium (Cs⁺) ions from aqueous solutions. The as-obtained NSC@MS-4A were thoroughly characterized by XRD, FE-SEM, EDS, BET, XPS and FT-IR. The physio-chemical properties and structural aspects revealed that NSC@MS-4A acquires fine surface area (72 m²/g), porous structure as well as compatible functional groups (-P-O-P and -C-O-C) for the admission of Cs⁺ and Sr²⁺ ions. The batch adsorption studies concluded that prepared adsorbent displayed a maximum adsorption of 92-94 % within 40 min. Fast adsorption of Cs⁺ and Sr²⁺ was achieved at neutral pH (6-7), ambient temperature (25-30 °C) and slow agitation speed (50-60 rpm) which could propose vast benefits such as little power utilization and uncomplicated operation. Among six types of adsorption isotherms, Freundlich isotherm showed the best fit with R²>0.997. Pseudo-second order made a better agreement as compare to other kinetic models. The thermodynamic coefficients suggested the passage of Cs⁺ and Sr²⁺ ions through the liquid solid boundary is exothermic and spontaneous. The NSC@MS-4A displayed excellent regenerability properties over five repetitive adsorption/desorption cycles, which specified that as-obtained NSC@MS-4A is a sustainable as well as efficient adsorbent for practical decontamination of radioactive liquid waste.

Comparative study on Pb2+ removal from aqueous solutions using biochars derived from cow manure and its vermicompost

Waste emissions have increased the amount of water and soil contaminated with heavy metals such as Pb. To broaden the methods for the recycling and environmental usage of cow manure (CM) and its vermicompost (CV), CM, CV, and their derived biochars produced by the pyrolysis of CM or CV at 350 and 700 °C were used as adsorbents for Pb²⁺ removal in this batch adsorption experiment to reveal their different Pb²⁺ removal efficiencies and the underlying mechanisms. The batch experiment results revealed that all adsorbents rapidly removed Pb²⁺ within 30 min. A pH between 2.0 and 6.0 positively affected Pb²⁺ removal by CM and its biochar, whereas that by CV and its biochar was only positively affected by pH between 2.0 and 3.0. CV-derived biochar was more effective in the removal of Pb²⁺ than the other absorbents, with the maximum adsorption capacities (Qm) fitted from the Langmuir model reaching approximately 230.0 mg·g^-1 and the desorption rate (DR) being approximately 0.00-0.02%. Material physiochemical characterization, including X-ray diffraction analysis, showed that high pH, high ash content, rich mineral content, and high mineral contents might have been the main reasons for more effective removal of Pb²⁺ from aqueous solutions by CV-derived biochar. Fourier-transform infrared analysis indicated that surface functional groups such as -OH, CO, -COO-, and C-O; original and newly produced carbonate; and phosphate in CV also led to more effective Pb²⁺ removal efficiency from aqueous solution via surface functional group binding. Thus, pyrolyzing CVs may be used to produce biochar as a cost-effective adsorbent for heavy metal remediation in soil and water in the future.

The application of machine learning methods for prediction of metal sorption onto biochars

The adsorption of six heavy metals (lead, cadmium, nickel, arsenic, copper, and zinc) on 44 biochars were modeled using artificial neural network (ANN) and random forest (RF) based on 353 dataset of adsorption experiments from literatures. The regression models were trained and optimized to predict the adsorption capacity according to biochar characteristics, metal sources, environmental conditions (e.g. temperature and pH), and the initial concentration ratio of metals to biochars. The RF model showed better accuracy and predictive performance for adsorption efficiency (R² = 0.973) than ANN model (R² = 0.948). The biochar characteristics were most significant for adsorption efficiency, in which the contribution of cation exchange capacity (CEC) and pH_H2O of biochars accounted for 66% in the biochar characteristics. However, surface area of the biochars provided only 2% of adsorption efficiency. Meanwhile, the models developed by RF had better generalization ability than ANN model. The accurate predicted ability of developed models could significantly reduce experiment workload such as predicting the removal efficiency of biochars for target metal according to biochar characteristics, so as to select more efficient biochar without increasing experimental times. The relative importance of variables could provide a right direction for better treatments of heavy metals in the real water and wastewater.

Fabrication and environmental applications of multifunctional mixed metal-biochar composites (MMBC) from red mud and lignin wastes

This study fabricated a new and multifunctional mixed metal-biochar composites (MMBC) using the mixture of two abundant industrial wastes, red mud (RM) and lignin, via pyrolysis under N₂ atmosphere, and its ability to treat wastewater containing various contaminants was comprehensively evaluated. A porous structure (BET surface area = 100.8 m² g^-1) was created and metallic Fe was formed in the MMBC owing to reduction of Fe oxides present in RM by lignin decomposition products during pyrolysis at 700 °C, which was closely associated with the transformation of liquid to gaseous pyrogenic products. The potential application of the MMBC was investigated for the removal of heavy metals (Pb(II) and Ni(II)), oxyanions (As(V) and Cr(VI)), dye (methylene blue), and pharmaceutical/personal care products (para-nitrophenol and pCBA). The aluminosilicate mineral, metallic Fe, and porous carbon matrix derived from the incorporation of RM and lignin contributed to the multifunctionality (i.e., adsorption, chemical reduction, and catalytic reaction) of the MMBC. Thus, engineered biochar composites synthesized from selected industrial wastes can be a potential candidate for environmental applications.

Interactive effects of rice straw biochar and γ-Al2O3 on immobilization of Zn

Biochar system technology has been proved as a sustainable remediation method for metal contaminated soils. However, little attention has been paid to the interaction between biochar and oxide minerals and their influence on metal immobilization in soils. In this study, batch-type Zn sorption experiments were conducted using the mixture of γ-Al₂O₃ and rice straw biochar as a model binary geosorbent systems. In addition, advanced spectroscopic technics such as EXAFS, FTIR and XRD were performed to reveal the mechanism. EXAFS spectroscopy revealed that 62% of Zn existed as Zn-Al layered double hydroxide (LDH) on γ-Al₂O₃ at pH 7.5 (for 2 mM Zn loading) within 24 h, which was 19% in the mixture. The Zn in biochar samples mainly existed as Zn-OM (53%-76%) and Zn₂SiO₄ (21%-47%), while the proportion of Zn₂SiO₄ (0-6%) was negligible compared with Zn-Al silicate (26-48%) in the mixtures. The overall findings confirmed that Al released from γ-Al₂O₃ was sorbed in parallel with Zn on biochar to form Zn-Al silicate, rather than Zn-Al LDH on the γ-Al₂O₃ surface. These results unveiled the dynamic interactions between amended biochar and soil oxide minerals which can significantly affect the immobilization pathways of metals in contaminated soils.

Highly efficient nickel (II) removal by sewage sludge biochar supported α-Fe2O3 and α-FeOOH: Sorption characteristics and mechanisms

A novel approach was employed to load α-Fe₂O₃ and α-FeOOH onto sewage sludge biochar (SBC) with the purpose of efficient nickel (Ni) removal. A high Ni(II) adsorption capacity of 35.50 mg·g^-1 in 100 ppm Ni(II) solution with 10 mg modified sewage sludge biochar (MSBC) was achieved. The adsorption kinetic and isotherm were fitted well by the pseudo-second-order model and the Langmuir model, respectively. The optimal pH was found around a neutral pH of 7. The adsorption mechanisms of Ni(II) onto MSBC were described as the synergistic effects of electrostatic attraction, ion exchange, inner-sphere complexation and co-precipitation. The initial rapid adsorption phenomenon could be attributed to electrostatic attraction and ion exchange, and then inner-sphere complexation and co-precipitation acted as a crucial role in the following step. The remarkable performance of MSBC provides an effective waste utilization approach to simultaneous sewage sludge recycle and Ni removal from aqueous solution.

Effect of production temperature on lead removal mechanisms by rice straw biochars

Production temperature significantly affects biochar properties and consequently the removal mechanisms of heavy metals. In this study, rice straw biochars were produced at 300, 500 and 700 °C (RSB300, RSB500 and RSB700). The influence of production temperature on the adsorption characteristics and removal mechanisms of lead on this set of rice straw biochars were investigated by batch adsorption tests, micro-structural analyses and sequential metal extractions. Biochars produced at higher temperatures had significantly higher pH values and surface areas, resulting in higher metal removal capacities and faster uptake kinetics. Precipitation was a key mechanism for lead removal from solution for all biochars: lead oxalate was precipitated on RSB300, and hydrocerussite was precipitated on RSB500 and RSB700. The immobilized lead fraction on the biochars could be divided into exchangeable, acid soluble and non-available fractions. RSB300 had 11.34% of the total immobilized Pb attributed to the exchangeable fraction, whereas for RSB500 and RSB700, it was <1%. Immobilized Pb on RSB500 and RSB700 was almost exclusively attributable to the acid soluble and non-available fractions (>99%). Based on our results, RSB500 and RSB700 are likely much more appropriate for soil remediation of Pb as compared with RSB300.

Synthesis of MgO-coated corncob biochar and its application in lead stabilization in a soil washing residue

In this study, a magnesium oxide (MgO) coated corncob biochar (MCB) was synthesized by pyrolyzing MgCl₂ pretreated corncob, for a better performance in lead immobilization in a contaminated soil compared with corncob biochar (CB). The properties and microstructures of CB and MCB were investigated. It was observed that MgO particles ranging from 1 to 2 μm were well coated on MCB, and the MgO content in MCB was calculated at 29.90% in w/w. The surface area of the biochar was significantly enhanced from 0.07 to 26.56 m²/g after the MgO coating. The MgO coating also significantly facilitated the lead removal percentage from 23% to 74% in aqueous solution by biochar. CB failed to immobilize lead in a soil washing residue and could not reduce its environmental risks in a laboratory incubation study. In contrast, MCB was applied to the soil and resulted in a significant reduction in TCLP leached lead from 10.63 to 5.24 mg/L (reduced by 50.71%). The comparison between MCB and other amendments suggests that the biochar component of MCB adsorbed lead onto its surface through cation-π interaction and increased surface adsorption due to higher surface area, and then the MgO coated on MCB's surface further enhanced the adsorption through precipitation. The synergistic roles of biochar-mineral composites make them a promising candidate for soil remediation.

Mechanisms of the Removal of U(VI) from Aqueous Solution Using Biochar: A Combined Spectroscopic and Modeling Approach

Biochar has been touted as a promising sorbent for the removal of inorganic contaminants, such as uranium (U), from water. However, the molecular-scale mechanisms of aqueous U(VI) species adsorption to biochar remain poorly understood. In this study, two approaches, grounded in equilibrium thermodynamics, were employed to investigate the U(VI) adsorption mechanisms: (1) batch U(VI) adsorption experiments coupled to surface complexation modeling (SCM) and (2) isothermal titration calorimetry (ITC), supported by synchrotron-based X-ray absorption spectroscopy (XAS) analyses. The biochars tested have considerable proton buffering capacity and most strongly adsorb U(VI) between approximately pH 4 and 6. FT-IR and XPS studies, along with XAS analyses, show that U(VI) adsorption occurs primarily at the proton-active carboxyl (-COOH) and phenolic hydroxyl (-OH) functional groups on the biochar surface. The SCM approach is able to predict U(VI) adsorption behavior across a wide range of pH and at varying initial U(VI) and biochar concentrations, and U adsorption is strongly influenced by aqueous U(VI) speciation. Supporting ITC measurements indicate that the calculated enthalpies of protonation reactions of the studied biochar, as well as the adsorption of U(VI), are consistent with anionic oxygen ligands and are indicative of both inner- and outer-sphere complexation. Our results provide new insights into the modes of U(VI) adsorption by biochar and more generally improve our understanding of its potential to remove radionuclides from contaminated waters.