Adsorption Mechanism Of Heavy Metals Research Articles

Interaction of biochar with extracellular polymers of resistant bacteria restrains Pb(II) adsorption onto their composite: Macro and micro scale investigations

Pb(II) sequestration in extracellular polymers-biochar composites (EPS-BC) was explored using macroscopic models and microscopic technology. The results showed that the actual adsorption capacity of EPS-BC was 52.2% lower than the calculated capacity based on adsorption onto pure components due to the interaction of polysaccharide and amide group in extracellular polymers with biochar, which masked the reactive sites related to Pb(II) in EPS-BC. The bond of Pb–O (40.8%) and Pb–OOC (31.5%) mainly contributed to Pb(II) speciation on the EPS-BC surfaces. Furthermore, each Pb atom coordinated with 6O atoms in the first shell and with 0.5C atoms in the second shell, indicating that the carboxyl group in composite was complexed with Pb(II) as a monodentate inner-sphere structure. The findings provide an in-depth understanding of the adsorption mechanism of heavy metals by extracellular polymers coupled with biochar at molecular scale, guiding bioremediation with respect to heavy metal contamination.

Active sites regulation and adsorption performance of Al-pillared bentonite for the removal of lead from aqueous phase

Developing an environmentally friendly adsorbent with high adsorption capacity and economic benefits is crucial for the treatment of lead-contaminated wastewater. In this study, active clay (A-bent) and Al pillared acid-activated clay (A-AlPILC) were prepared using natural bentonite as the carrier via acidification and pillar-supported technology. The samples were characterized by X-ray diffraction (XRD), N2 adsorption-desorption experiments, Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray fluorescence spectrometer (XRF). The results showed that the amended clay exhibited an increase in specific surface area (SSA) from 39.37 m2/g to 175.28 m2/g and 245.42 m2/g compared to raw bentonite. The interlayer spacing and total pore volume rose respectively. The adsorption technique achieved an impressive removal rate of 90.6 % for Pb(II). Adsorption data were fitted by adsorption kinetics and adsorption isotherm model. Based on the response surface method (RSM), and analysis of variance (ANOVA) showed that the influence of these variables was significant. The simulation of lead ions showed that the acidic functional groups and ligands promote adsorption more than they inhibit it. Using the density function theory (DFT) calculation, the bentonite cycle model was established to demonstrate the interaction between bentonite and lead, with a binding energy of −0.369 eV. Similarly, the density and energy band structure of the state were computed to explore the mechanism of heavy metal adsorption by clay minerals.

Fabricating nepheline syenite-chitosan composite for heavy metals uptake: Mechanism insight via statistical physics modeling

Given the substantial costs associated with conventional wastewater treatment methods, researchers are increasingly exploring mineral compounds for their cost-effectiveness, widespread availability, and efficiency. In this study, nepheline syenite, an alkaline igneous rock, was selected among these compounds. The research investigates the single and binary adsorption of heavy metals (Ni(II) and Cd(II)) from synthetic and electroplating industrial effluents using a nepheline syenite-chitosan composite (NS-CS). The physical and chemical properties of the adsorbents were characterized using XRD, XRF, FT-IR, BET, and SEM analyses. The study examined the effects of solution acidity, contact time, initial concentration, and adsorbent dosage to determine the optimal conditions for the uptake of Ni(II) and Cd(II). The isothermal data were best described by the Langmuir isotherm model, with NS-CS showing adsorption capacities of up to 40.48 mg/g for Ni(II) and 41.79 mg/g for Cd(II). Additionally, the competitive adsorption of nickel and cadmium in the binary system was more accurately described by the modified Freundlich equation. Kinetic data were best fitted by the pseudo-second-order model. Thermodynamic analysis indicated that the adsorption of both ions was endothermic and nonspontaneous. Statistical physics modeling was employed to elucidate the adsorption mechanism of heavy metals on the NS-CS composite. The single-layer model with one energy level provided the most precise fit to the experimental data. Using this model, key factors were calculated, revealing that the adsorption energies for both Ni(II) and Cd(II) ions were below 20 kJ/mol, suggesting that the adsorption process is likely physical in nature.

Exploring adsorption dynamics of heavy metals onto varied commercial microplastic substrates: Isothermal models and kinetics analysis

The increasing presence of plastics in the environment has raised concerns about their potential impact, especially as carriers of heavy metals such as Cd, Ni, and Pb. However, the adsorption mechanism of heavy metals on microplastics remains poorly understood. In this study, we investigated the adsorption behavior of Cd, Ni, and Pb by polystyrene (PS) and polypropylene (PP) microplastics to better comprehend their interaction and potential environmental implications. Our results revealed that equilibrium adsorption of microplastics with different heavy metals was achieved within a 6-h contact time. The FTIR analysis findings, which suggest that physical interactions play a significant role in the adsorption of heavy metals onto microplastics, are further supported by the observed changes in surface morphology after adsorption. We explored the influence of solution pH, contact duration, and initial concentration on the adsorption capacity and found significant effects on the adsorption behavior. To model the adsorption process, we applied Langmuir and Freundlich adsorption isotherm models and observed that the Langmuir model better fit the experimental data. Furthermore, we compared the pseudo-first and pseudo-second-order kinetic models and found that the pseudo-second-order model provided a more accurate description of the adsorption kinetics. Notably, the adsorption percentages varied depending on the type of microplastic and experimental conditions. Overall, this study enhances our understanding of the adsorption mechanism of heavy metals on microplastics and provides valuable insights into their behavior in aquatic environments. These findings have implications for the development of effective strategies for mitigating pollution caused by microplastics and heavy metals in aquatic ecosystems.

Adsorption characteristics and applications of andesite in removing some pollutants from wastewater

Andesite was employed to effectively extract mercury(II) in an aqueous solution. After evaluating its characteristics, andesite was characterized by applying modern techniques such as BET and TGA methods. The study employed SEM and TEM measurements to analyze the variation in the surface shape and crystallinity of the metal due to adsorption. Using the EDX process, the chemical composition, weight, and atomic percentage of each element of andesite were determined. FTIR techniques were also used to confirm the TEM–EDX findings. Zeta potential was estimated. Cycles of regeneration and desorption have been examined. 99.03% was the highest uptake percentage. Adsorbent quantity (0.0025–0.05) g/L, contact time (5–60) min, pH (2–10), temperature (25–60) °C, and dose (0.0027, 0.0044, 0.0125, 0.0155, and 0.0399) mg/L all affect the amount of removal that increases with the increase in contact time, pH, dose, and temperature but drops as the metal ion concentration rises. The ideal values for contact time, pH, metal ion concentration, dose, and temperature were found to be, respectively, 30 min, 0.0155 mg/l, 0.02 g/l, and 40 °C. The calculation of thermodynamic parameters, including ΔH, ΔG, and ΔS, was imperative in establishing that the mechanism of heavy metal adsorption on andesite was endothermic, exhibiting a physical nature that escalated with temperature rise. The Freundlich adsorption equation's linear form is matched by the adsorption of mercury(II) on andesite; constant n was 1.85, 1.06, 1.1, and 1.1, whereas the Langmuir constant qm was found to be 1.85, 2.41, 3.54, and 2.28 mg/g at 25–60 °C. Furthermore, adsorption follows a pseudo-second-order rate constant of (3.08, 3.24, 3.24, and 13) g/mg/min under identical temperature conditions, as opposed to a first-order rate constant of 4, 3, 2.6, and 2. Hg2+, NH4+, Cl−, Br−, NO3−, SO42−, Na+, K+, H2S, and CH3SH were all extracted from wastewater by this application.

Prediction of adsorption of metal cations by clay minerals using machine learning

Adsorption of heavy metals by clay minerals occurs widely at the solid-liquid interface in natural environments, and in this paper, the phenomenon of adsorption of Cd2+, Cu2+, Pb2+, Zn2+, Ni2+ and Co2+ by montmorillonite, kaolinite and illite was simulated using machine learning. We firstly used six machine learning models including Random Forest(R), Extremely Forest(E), Gradient Boosting Decision Tree(G), Extreme Gradient Boosting(X), Light Gradient Boosting(LGB) and Category Boosting(CAT) to feature engineer the metal cations and the parameters of the minerals, and based on the feature engineering results, we determined the first order hydrolysis constant(log K), solubility product constant(SPC), and higher hydrolysis constant (HHC) as the descriptors of the metal cations, and site density(SD) and cation exchange capacity(CEC) as the descriptors of the clay minerals. After comparing the predictive effects of different data cleaning methods (pH50 method, Box method and pH50-Box method) and six model combinations, it was finally concluded that the best simulation results could be achieved by using the pH 50-Box method for data cleaning and Extreme Gradient Boosting for modelling (RMSE = 4.158 %, R2 = 0.977). Finally, model interpretation was carried out using Shapley explanation plot (SHAP) and partial dependence plot(PDP) to analyse the potential connection between each input variable and the output results. This study combines machine learning with geochemical analysis of the mechanism of heavy metal adsorption by clay minerals, which provides a different research perspective from the traditional surface complexation model.

Experimental and theoretical-based study of heavy metal capture by modified silica-alumina-based materials during thermal conversion of coal at high temperature combustion

Heavy metals emitted from coal combustion can easily pose many threats. In this study, five modified additives based on kaolin were prepared for reducing the emission of heavy metals from high temperature coal combustion and characterized by XRD, FT-IR, XRF, BET, SEM and zeta potential. The heavy metal retention characteristics of the additives under high temperature (900–1300 °C) coal combustion were investigated by static and dynamic experiments, and the results showed that pseudo-boehmite-metakaolin (BMK) was the most effective additive, and the ability of heavy metal capture was Cr > Zn > Pb > Cd, and the dynamic experiments were better than the static experiments. The quantum chemical models of heavy metals (monomers, oxides and chlorides), kaolin and adsorbent-active substances were developed and optimized, and the adsorption energy was calculated. The morphological changes of the active substances were taken into account in the simulation, and new mathematical models for adsorption energy calculation were proposed, which effectively solved the extreme results of adsorption energy calculation and made a comprehensive evaluation of heavy metal adsorption for the whole studied temperature range system. The mechanism of heavy metal adsorption on the active substance was further investigated by the simulations. Heavy metal atoms can form covalent or ionic bonds with O atoms of the active substance, Cl atoms of heavy metal chlorides can be covalently bonded or non-bonded with Al atoms of the active substance, and O atoms of heavy metal oxides can combine with Al atoms of the active substance to form ionic bonds. This study sheds new light on the control of heavy metals in high-temperature coal combustion, the methods for the optimization of simulation and the adsorption mechanism of heavy metals on relevant reactive substances. The method has a promising potential for large-scale application due to its simple preparation, low cost and significant effect.

Effects of humic acids on the adsorption of Pb(II) ions onto biofilm-developed microplastics in aqueous ecosystems

Microplastics (MPs), as emerging contaminants can behave as carriers for heavy metals in the water environments. Although the adsorption performance of heavy metals on MPs has been widely investigated, the effects of humic acids (HA) on the adsorption have seldom been explored. The authors were compared the Pb(II) adsorption onto biofilm-developed polyvinyl chloride (Bio-PVC) MPs with Pb(II) adsorption onto virgin PVC MPs (V-PVC), and explored the relationship between surface characteristics and the adsorption properties in the coexistence of HA. Our results showed that due to a larger specific surface area and more oxygen containing groups, Bio-PVC had a larger adsorption capability with a value of 3.57 mg/g than original ones (1.85 mg/g) due to its huge specific surface area and more oxygen containing groups. Microbial community analysis showed that the predominate bacteria in biofilms as Proteobacteria, Acidobacteria, Cyanobacteria, Firmicutes, and Bacteroidetes. Notably, the Pb(II) adsorption onto the V-PVC surfaces was increased, but the adsorption capacities of Pb(II) on Bio-PVC were suppressed with increasing HA. With the co-existence of HA, the increasing complexation and electrostatic attraction had attributed to the increased Pb(II) adsorption ability on V-PVC. Except for its competitive ability, HA has a shield effect which decreases the sorption sites on Bio-PVC. Overall, our findings provide a better understanding of the HA effect on the adsorption mechanism of heavy metals onto MPs in aquatic ecosystems.

Removal of Cd2+ from wastewater to form a three-dimensional fiber network using Si-Mg doped industrial lignin-based carbon materials

In this study, SiMg doped industrial lignin-based carbon materials (SLCs) were prepared by water bath silicification and MgCl2 activation to remove Cd2+ from aqueous solutions. What’s more, the doping of SiMg jointly promoted the excellent physicochemical properties of the material, e.g., high specific surface area, good pore volume, and numerous oxygen-containing groups. The Cd2+ batch adsorption experiments proved that SLCs have good Cd2+ removal capacity within pH 3–7, and the adsorption model demonstrated the adsorption process as a physicochemically complex process. The maximum adsorption of Cd2+ in the SLC was 665.35 mg/g, and the contributing factors to the removal of Cd2+ were as follows: ion exchange (59.36 %) > Cd2+ precipitation (24.93 %) > oxygen-containing functional group complexation (14.79 %) > Cd2+–π interactions (0.92 %). In addition, the complexation of SiO, MgO, and Cd precipitates allowed the formation of a three-dimensional fiber mesh structure. The application of SLCs has the potential to eliminate Cd2+ pollution in water bodies, and its preparation is simple and environmentally friendly. Finally, this study provides a theoretical basis for an in-depth understanding of the mechanism of heavy metal adsorption by inorganic nonmetals in combination with metal oxides.

Potential of Adsorption of Diverse Environmental Contaminants onto Microplastics

Microplastics are regarded as vectors of hazardous contaminants due to their ability to adsorb xenobiotic chemicals. This has led to increased interest in the risk of previously neglected microplastic contaminants in the aquatic environment. Here, we assessed the possibility of transferring chemical contaminants to microplastics by evaluating the adsorption performance of (in)organic pollutants on various types of microplastics (polystyrene, PS; polyethylene terephthalate, PET; high-density polyethylene, HDPE; and low-density polyethylene, LDPE;). Considering the toxicity and polarity of each pollutant, dyes (BB9 and RR120) and heavy metals (Cd(II), Pb(II), As(III), and As(V)) were selected for the adsorption experiments. Dye was found to be adsorbed through physical adsorption. The adsorption capacity of microplastics for RR120 and BB9 was the highest for HDPE-1 and LDPE-1, respectively. Additionally, the smaller the size of the microplastics, the higher the adsorbed amounts. The main adsorption mechanism of heavy metals was found to be through physical and chemical adsorption. And adsorption mechanism of dye depends on physical adsorption. Thus, the adsorption of microplastic contaminants was affected more by the condition than by the type of microplastics.

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

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

A combined DFT, Monte Carlo, and MD simulations of adsorption study of heavy metals on the carbon graphite (111) surface

This work investigated the adsorption mechanism of heavy metals: silver (Ag), mercury (Hg), cadmium (Cd), palladium (Pd), and zinc (Zn) on the carbon graphite (111) surface based on density functional theory (DFT), Metropolis Monte Carlo (MMC), and the molecular dynamics (MD) simulation methods. The obtained results from the adsorption of these species showed that the process is spontaneous and exothermic in nature. The maximum adsorption capacities were obtained in neutral to low acid medium, and the interaction between Hg-carbon graphite was more favored than other systems. These findings showed that carbon graphite was more efficient in the removal of the studied metals. Moreover, this study better explains the adsorption mechanism of heavy metals onto carbon graphite and gives a theoretical basis for the wider application of graphite adsorbent in the removal of heavy metals. This paper provides theoretical support for heavy metals removal in various mediums and also provides some new ideas for the secondary utilization of quantum chemical descriptors and molecular dynamics simulation methods.

A review on preparation, surface enhancement and adsorption mechanism of biochar‐supported nano zero‐valent iron adsorbent for hazardous heavy metals

AbstractHeavy metal pollution of water is a global concern, which adversely affects human health because of its resistance to biodegradation and thus its transmission in the food chain via bioaccumulation. Nano zerovalent iron (nZVI) is very effective for the removal of heavy metals and is cost effective in terms of production. However, the main problems of nZVI are agglomeration and ease of oxidation. Several stabilization materials have been implemented to limit the aggregation of nZVI, such as silica, activated carbon and biochar. In comparison, as a support material, biochar possesses a large surface area, high stability and strong adsorption capacity, as well as being obtainable from various types of materials. Thus, this work aims to establish the opportunities available on the use of biochar‐supported nZVI in utilizing its ability to stabilize and immobilize the nZVI. This review also reports the preparation, modification and surface enhancement of biochar, nZVI and biochar‐nZVI for practical use as adsorbents. This review shows that modifications of the nZVI surface can help in their stabilization and reduction of aggregation. Additionally, this review is able to increase one's understanding of heavy metal sorption behavior by biochar‐supported nZVI as it is the important as heavy metal sorption is driven based on biochar‐nZVI type and heavy metal species which involve numerous mechanisms, including physical binding, complexation, ion exchange, surface precipitation and electrostatic interactions. Furthermore, this research reviews the adsorption parameters, including the crucial adsorption mechanism of heavy metals onto biochar‐nZVI; the reusability of the biochar‐nZVI is also discussed in this work. © 2022 Society of Chemical Industry (SCI).

BIOSORPTION CHARACTERISTICS OF HG(II) INTO BIOFILM

Biosorption is an alternative technology developed to overcome pollution, including heavy metal contamination in aquatic ecosystems. The accuracy of choosing a biosorbent strongly influences the success of biosorption. The organisms that are widely developed as biosorbents are microbes. Microbes live in ecosystems by forming biofilms. Therefore, knowledge related to the mechanism of heavy metal adsorption, such as Hg(II) by biofilms, will contribute to the development ofbiofilm-based biosorption. This study aims to analyze the biosorption characteristics of Hg(II) bybiofilms taken from aquatic ecosystems. The methods used are kinetic adsorption analysis, isotherm adsorption analysis, and FTIR spectra analysis of biofilms. This study indicates that the adsorption process of Hg(II) into the biofilm is a physicochemical process. The adsorption sites that bind Hg(II) are negatively charged sites resulting from the ionization of functional groups such as carboxyl groups present in polymeric biofilms. According to this study, biofilm is an excellent biosorbent to adsorb pollutants such as Hg(II) from aquatic ecosystems.

Surfactant-induced adsorption of Pb(II) on the cracked structure of microplastics

Surfactant molecules can change the hydrophobic nature of microplastic surfaces, thereby affecting the adsorption of heavy metals in the environment onto the microplastics. It is essential to explore the role of crack structure of microplastics in the adsorption of heavy metals, especially in the presence of surfactants. In this study, polyethylene (PE) and polypropylene (PP) were evaluated for Pb(II) adsorption and desorption mechanism in the presence of two surfactants: cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS). The experimental results were analyzed using kinetics and the isothermal model fitting and spectrogram (FTIR, XPS). This study showed that the application of surfactants could greatly enhance the Pb(II) adsorption capacities of PE and PP by promoting Pb(II) into the fissures. The Pb(II), S, and N contents did not significantly decrease at different depths in the presence of surfactants, and the Pb(II) content without surfactants decreased with an increasing depth. The adsorption behavior was consistent with the Bangham channel diffusion model and the DR model, suggesting that the adsorption process was related to the pore structure of the microplastics. Furthermore, the release of Pb(II) from desorption using high concentration of surfactant solution was less than that of low concentration, it was difficult to release heavy metals primarily because of the crack structure of the microplastics, especially when more surfactant molecules entered the pores. This work contributes to a better understanding of the adsorption mechanism of heavy metals on microplastics in the presence of surfactants, which will better control the ecological risks of microplastics.

Hierarchically porous biochar templated by in situ formed ZnO for rapid Pb2+ and Cd2+ adsorption in wastewater: Experiment and molecular dynamics study

3D hierarchical porous biochar (HPBC) was synthesized by a thermally removable template without post-activation. Zn(NO3)2 decomposition produced gases and ZnO in situ to activate and expand the three-dimensional micro-and mesopores. Compared with pristine biochar (BC), the specific surface area and pore volume of HPBC were increased by 223 and 75 times, respectively. The abundant pore structure of HPBC significantly enhanced the diffusion rate of heavy metals. For example, compared to BC, the time required for HPBC to adsorb Pb2+ reach adsorption equilibrium was reduced by 87.5% (40 min vs 5min). Such an adsorption performance of HPBC was also insensitive to different background ions (K+, Na+, Ca2+, and Mg2+) with a much higher concentration than that of heavy metals. When applied to treat desulfurization wastewater from power plants, HPBC yielded 100% removal of Pb2+ and Cd2+, much higher than that by using commercial activated carbon (28%). Molecular dynamics simulation revealed different locations preferred by the adsorption of Pb2+ (micropores) and Cd2+ (mesopores) in the hierarchical pore structures. The adsorption of Pb2+ and Cd2+ on HPBC was mainly achieved by diffusion, oxygen functional group complexation, and precipitation. These results provided better knowledge to understand the microscopic adsorption mechanisms of heavy metals in hierarchical pores and a facile yet robust strategy to design such structures in biochar for efficient wastewater treatment.

Molecular simulation of adsorption properties of thiol-functionalized titanium dioxide (TiO2) nanostructure for heavy metal ions removal from aqueous solution

We investigated the adsorption of Cd ion as a primary contaminant heavy metal in drinking water on titanium dioxide (TiO2) and thiol-functionalized TiO2 (thiol-TiO2). The dispersion corrected density functional theory (DFT-D2) calculation is used for structural relaxation and second-order MP2 (second-order Møller-Plesset) calculations for interaction energy (Eint) estimation. The results reveal that the Cd bound very strongly (Eint: −4.566 eV) to the TiO2 and strongly to the thiol-TiO2 (Eint: −1.882 eV). Our findings show that the H2O molecule is adsorbed very strongly on the TiO2 surface (Eint−4.535 eV) while it is attached weakly to the thiol-TiO2 (Eint−0.076 eV). The DFT-based molecular dynamic (MD) simulations at ambient conditions reveal that water molecules are first adsorbed on the nano-TiO2 surface, and then the Cd ion is co-adsorbed on the surface. Meanwhile, for thiol-nanoTiO2, a new chemical bond was formed between the Cd ion and S atom of the thiol group at 1 ps. The analysis of the AIM (Atom-In-Molecule) theory demonstrated that the types of interactions are typical for the electrostatics accompanied with partial covalent bonding between Cd ion and both O and S atoms. Our molecular simulations offer a well-founded understanding of the adsorption mechanisms of heavy metals on TiO2 and thiol-TiO2 at ambient conditions.

Sawdust for the Removal of Heavy Metals from Water: A Review.

The threat of the accumulation of heavy metals in wastewater is increasing, due to their abilities to inflict damage to human health, especially in the past decade. The world’s environmental agencies are trying to issue several regulations that allow the management and control of random disposals of heavy metals. Scientific studies have heavily focused on finding suitable materials and techniques for the purification of wastewaters, but most solutions have been rejected due to cost-related issues. Several potential materials for this objective have been found and have been compared to determine the most suitable material for the purification process. Sawdust, among all the materials investigated, shows high potential and very promising results. Sawdust has been shown to have a good structure suitable for water purification processes. Parameters affecting the adsorption mechanism of heavy metals into sawdust have been studied and it has been shown that pH, contact time and several other parameters could play a major role in improving the adsorption process. The adsorption was found to follow the Langmuir or Freundlich isotherm and a pseudo second-order kinetic model, meaning that the type of adsorption was a chemisorption. Sawdust has major advantages to be considered and is one of the most promising materials to solve the wastewater problem.

The adsorption mechanism of heavy metals from coal combustion by modified kaolin: Experimental and theoretical studies

Experimental and theoretical studies are combined to analyze the adsorption properties of modified kaolin for heavy metal (Pb, Cd, Zn and Cr) from coal combustion. The results indicate that the retention effect of kaolin for Pb, Cd, Zn and Cr has been significantly enhanced after intercalation-exfoliation combined with acid/alkali modification, which is mainly attributed to more active sites for adsorption, richer porosity and more effective in retarding coking of coal ash. The higher oxygen concentration is positive to the enrichment of heavy metals at 900–1200 ℃, while the coking of coal ash and the thermal conversion of additives become the main factors affecting the absorption at 1200–1300 ℃. The acid/alkali modification effectively promotes the inductive effect of electron transfer between modified kaolin and heavy metals to form stable chemical adsorption. The electron transfer induction of modified kaolin for Pb, Cd is higher than Zn, Cr at 900–1000 ℃, while the adsorption activity of mullite and cristobalite for Zn, Cr is stronger than Pb, Cd at 1200–1300 ℃. In addition, Pb, Cd and Zn are more readily adsorbed as oxides by additives at 900–1300 °C. The results shed new light on strengthening the adsorption activity of kaolin to Pb, Cd, Zn and Cr in high temperature.

Adsorption of Cd2+ and Pb2+ by biofuel ash-based geopolymer synthesized by one-step hydrothermal method

A hydrothermal alkali modification method was used to produce geopolymer (BFA-GP) from biofuel ash (BFA). The product was characterized by scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared reflection (FTIR), and X-ray photoelectron spectroscopy (XPS). Significant amount of geopolymer and gismondine was produced by this modification. The surface area increased from 20.41 m2 g−1 to 56.63 m2 g−1. The adsorption capacity of BFA-GP can reach 29.92 and 137.49 mg g−1 for Cd2+ and Pb2+, respectively, in a pure solution at 298 K, which is about triple of that by the original BFA. Competitive adsorption of Cd2+ and Pb2+ showed that the binding affinity is Pb2+> Cd2+. Chemical sorption including electrostatic attraction, chelate reaction, and ion-exchange is the dominant mechanism of heavy metal adsorption on BFA-GP. And the adsorption thermodynamics indicates that adsorption reaction of heavy metal ions by BFA-GP is spontaneous and endothermic. This modification provided a cost-effective and environmentally friendly way to change BFA to adsorbent for heavy metals with promising application prospects.