Effectiveness and mechanism for the simultaneous adsorption of Pb(II), Cd(II) and As(III) by animal-derived biochar/ferrihydrite composite

Harnessing corn straw biochar: A breakthrough in eco-friendly Cu(II) wastewater treatment

To investigate an energy-efficient, environmentally friendly, and highly efficient biochar for adsorbing Cu(II)-containing wastewater, corn straw hydrothermal char prepared at 240 °C for 2 h was used as a precursor. Silicon (Si)-Manganese (Mn) impregnation modification was then performed to produce the modified biochar (b-BC). The study found that Si and Mn were loaded onto the b-BC surface in the form of oxides. The distinct hierarchical Si membrane effectively stabilized Mn oxides and increased the specific surface area. Under different pH conditions, the effect of Mn rendered b-BC consistently negatively charged in the solution, facilitating electrostatic attraction with Cu(II). Fourier-transform infrared and X-ray photoelectron spectroscopy results revealed that b-BC's surface had numerous oxygen-containing functional groups, effectively binding with Cu(II). Adsorption experiments showed that, at an addition amount of 1.47 g/L and pH of 7, b-BC displayed a significant adsorption capacity for Cu(II) at 167.884 mg/g. Pseudo-second-order adsorption kinetics and Freundlich isotherm models better described the adsorption behavior of b-BC for Cu(II). The adsorption process was primarily dominated by multilayer chemical adsorption. Webber-Morris analysis indicated that the key adsorption process occurred during the membrane diffusion stage. At this stage, Cu(II) formed bonds with the b-BC surface in the forms of Cu-O, -COOCu, Cu(OH)2, and Si/Mn-O-Cu. Chelation emerged as the most significant mechanism for b-BC adsorbing Cu(II). Due to its environmentally friendly preparation method and high efficiency in adsorbing Cu(II) from water, b-BC can be considered as a feasible rich-carbon adsorbent in the field of waste treatment.

Study on the adsorption performance of carbon-magnetic modified sepiolite nanocomposite for Sb(V), Cd(II), Pb(II), and Zn(II): Optimal conditions, mechanisms, and practical applications in mining areas

A carbon-magnetic modified sepiolite nanocomposite (γ-Fe2O3/SiO2-Mg(OH)2@BC) was synthesized using a hydrothermal method, consisting of γ-Fe2O3, activated sludge biochar (BC), and alkali-modified sepiolite. Its ability to remove heavy metals such as Sb(V), Pb(II), Cd(II), and Zn(II) was investigated through adsorption experiments. Using response surface optimization, the optimal adsorption conditions were determined: adsorption time = 3.78 h, pH = 2.63, initial concentration = 15.78 mg/L, temperature = 35.14°C, and adsorbent dosage = 100.71 mg. Characterization results revealed that the main adsorption mechanisms included complexation, π-π interactions, and electrostatic attraction. Kinetic and isotherm model analyses indicated that the adsorption process of γ-Fe2O3/SiO2-Mg(OH)2@BC adhered to the pseudo-second-order kinetic model and the Freundlich isotherm model, primarily involving multilayer chemical adsorption. The application of this composite material in complex aquatic environments in antimony mining areas demonstrated promising practical results, as well as excellent regeneration performance. This study provides technical and theoretical support for the treatment of complex heavy metal wastewater in antimony mining areas and lays a foundation for the development of novel carbon-magnetic modified nanocomposite adsorbents.

Simultaneous removal of Cd(II) and Sb(V) by MnFe2O4-biochar composite: Performance and mechanisms

The coexistence of cadmium (Cd) and antimony (Sb) in soils severely threatens environmental safety and human health. While biochar is widely used for soil remediation, its effectiveness in removing multiple metals, especially in the presence of anions, lacks dynamic quantification and mechanistic understanding. This study synthesized a MnFe2O4-biochar composite (MF-RBC) using the coprecipitation method, and explored its adsorption performance and mechanisms for coexisting Cd(II) and Sb(V). The maximum adsorption capacities of MF-RBC for Cd(II) and Sb(V) were 11.24 and 57.33 mg g-1, respectively, higher than those of pure RBC (6.27 mg g-1 for Cd(II), and 19.43 mg g-1 for Sb(V)) and MF (9.25 mg g-1 for Cd(II), and 48.01 mg g-1 for Sb(V). The enhanced performance is attributed to the large specific surface area of MF-RBC (318.86 m2 g-1), improved dispersion of MF particles by biochar, and the attachment of oxygen-containing groups. Additionally, Cd(II) exhibited a synergistic effect on Sb(V) removal, likely due to reduced negative charge repulsion between MF-RBC and Sb(V), and the formation of MF-Cd(II)-Sb(V) ternary complexes. MF-RBC also decreased the availability of Cd and Sb by transforming them into more stable forms. Microscopic and mechanistic analysis revealed that Cd(II) forms complexes with both the CO/CO groups of biochar and the MnO bonds of MnFe2O4, while Sb(V) is primarily complexed with FeO bonds of MnFe2O4 in the Cd&Sb coexistence system. These findings facilitate a better understanding of Cd(II) and Sb(V) behavior in natural environments and offer valuable insights for improving soil remediation strategies.

Insights into the role of dissolved organic matter derived from paddy soils with different parent materials in the environmental behavior of heavy metal adsorbed by ferrihydrite

The interaction between dissolved organic matter (DOM) and ferrihydrite (Fh) is a crucial process to control the environmental behavior of heavy metals (HMs) in soil environments, with DOM playing a particularly strong role in HMs fate. Since chemical properties of DOM vary based on different soil parent materials, the underlying impact of DOM-Fh associations on HMs binding remains unclear. This study systematically investigated the interactions between DOM from three soil parent materials (fluvial alluvium: FDOM, sand-shale: SDOM and granite: GDOM) and Fh, and meanwhile understand their effects on the environmental behavior of Cd and Pb under various environmental conditions. An increased Cd and Pb binding during DOM-Fh interactions was observed and attributed to the introduction of additional binding sites by the organic functional groups with a variety of metal affinities. Specifically, more aromatic carboxyl groups in FDOM and more aliphatic groups in SDOM strongly promoted the adsorption of Pb and Cd, respectively. Meanwhile, Higher pH and increased C/Fe also promoted HMs adsorption, particularly in the presence of DOM. Further characterization indicated that electrostatic attraction, ion exchange and surface complexation were primary mechanisms of HMs adsorption. These finding highlight the significant impact of DOM-Fh interactions, dependent on different soil parent materials, on the mobility and fate of HMs in soils, providing valuable insights into the role of DOM composition in influencing HMs contamination, which offer theoretical guidance for environmental management, especially in agricultural and contaminated soils.

Pb alleviates As mobilization during the biological reductive dissolution of Pb-As jarosite

The biological dissolution of jarosite accelerates As mobilization in acid mine drainage environments, which can be influenced by coexisting metals. However, few studies have focused on the effects of coexisting Pb on this biogeochemical process. Here, we investigated the behavior of As during the reductive dissolution of Pb-As jarosite (PbFe3(SO4, AsO4)2(OH)6) by a sulfate-reducing bacterium (SRB) of Desulfovibrio desulfuricans. Pb incorporation inhibited jarosite dissolution and As mobilization. One reason was that Pb incorporation caused lattice distortion of jarosite, decreasing its solubility. Another reason was that S2- produced via anaerobic respiration of D. desulfuricans was partially captured by Pb2+ to form PbS, which negatively affected the reduction of Fe3+ in jarosite to aqueous Fe2+, slowing down the reductive dissolution of Pb-As jarosite and As release. Furthermore, the dissolved As5+ was reduced and quickly precipitated by forming As2S3 or being absorbed on the formed amorphous secondary minerals, resulting in only trace amounts of As being retained in the aqueous environment. Transcriptomic analysis provided additional evidence for As reduction, sulfate reduction, and extracellular electron transfer mediated by SRB. These findings offered new sights for understanding the geochemical cycling of As in acid mine drainage, and provided a reference basis for the treatment of Pb and As co-contaminated environment.

A review of hydroxyapatite synthesis for heavy metal adsorption assisted by machine learning

Heavy metals (HMs) represent a persistent and significant threat to aquatic ecosystems. Hydroxyapatite (HAp) has emerged as a utilized material in the remediation of environmental HMs, owing to its exceptionally high porosity, expansive surface area, and the presence of three-dimensional ordered channels. An in-depth study of the synthesis strategy of HAp and its adsorption properties can help reduce the cost of remediating HMs in aquatic environments and alleviate the water shortage. In this paper, we reviewed 466 works of literature on the adsorption of heavy metals by HAp based on the Web of Science database between 2013 and 2023 that focused on the adsorption of heavy metals by HAp. We meticulously synthesized the findings related to the synthesis conditions-namely precipitation, hydrothermal, and calcination-as well as the characterization parameters and the adsorption capacity of HAp for heavy metals such as Pb2+, Cd2+, Cu2+, Zn2+, and Ni2+. Synthesizing advanced materials by reducing the number of experiments is essential to accelerate material development. Machine learning (ML) holds significant promise in material discovery and performance enhancement. We have consolidated the qualitative and quantitative relationships between HAp synthesis conditions, characterization parameters, and heavy metal adsorption capacity across previous studies, utilizing both the Statistical Package for Social Sciences (SPSS) and ML techniques. Building on the most recognized heavy metal adsorption mechanisms, we have evaluated the influence of characterization parameters on adsorption performance. We have outlined the optimal synthetic conditions for enhancing the adsorption of Pb2+, Cd2+, Cu2+, Zn2+, and Ni2+ through precipitation, hydrothermal, and calcination methods, offering a practical guide for the targeted synthesis of HAp tailored to specific heavy metal adsorption capacities.

Predicting the efficiency of arsenic immobilization in soils by biochar using machine learning

Arsenic (As) pollution in soils is a pervasive environmental issue. Biochar immobilization offers a promising solution for addressing soil As contamination. The efficiency of biochar in immobilizing As in soils primarily hinges on the characteristics of both the soil and the biochar. However, the influence of a specific property on As immobilization varies among different studies, and the development and application of arsenic passivation materials based on biochar often rely on empirical knowledge. To enhance immobilization efficiency and reduce labor and time costs, a machine learning (ML) model was employed to predict As immobilization efficiency before biochar application. In this study, we collected a dataset comprising 182 data points on As immobilization efficiency from 17 publications to construct three ML models. The results demonstrated that the random forest (RF) model outperformed gradient boost regression tree and support vector regression models in predictive performance. Relative importance analysis and partial dependence plots based on the RF model were conducted to identify the most crucial factors influencing As immobilization. These findings highlighted the significant roles of biochar application time and biochar pH in As immobilization efficiency in soils. Furthermore, the study revealed that Fe-modified biochar exhibited a substantial improvement in As immobilization. These insights can facilitate targeted biochar property design and optimization of biochar application conditions to enhance As immobilization efficiency.

Biochar derived from animal and plant facilitates synergistic transformation of heavy metals and phosphorus in sewage sludge composting

This study investigated the influence of plant-derived biochar (PB) and animal-derived biochar (AB) on behavior of heavy metals and phosphorus fractions during sewage sludge composting. PB was highly effective in reducing the bioavailability of Zn and Cu by 39% and 50%, respectively, while AB decreased the bioavailability of Pb (30%) and Cd (12%). Both biochar increased available phosphorus by over 38%. Acid extractable and bioavailable Pb in AB, and water-soluble, oxidizable and total Zn, acid extractable and oxidizable Cu in PB were positively correlated with moderately resistant organic phosphorus (MROP). Besides, in AB, Cd had strong and positive correlation with highly resistant organic phosphorus (HROP). This suggested biochar facilitated the formation of stable organometallic complexes through binding metal ions to phosphorus fractions, with notable differences based on biochar source. FT-IR showed biochar promoted humification, with PB enhancing carboxyl and polysaccharide formation, while AB encouraged quinone and aryl ether structures. These surface functional groups on the biochar likely contributed to heavy metals and phosphorus binding through chelation, adsorption, and electron shuttling.

Fe Crystalline Phases in Fe/C Composites Modulated the Selective Adsorption of Pb(II) from Industrial Wastewater with Cd(II): An Electronic-Scale Perspective

Fe oxide or Fe0-based materials display weak removal capacity for Pb(II), especially in the presence of Cd(II), and the electronic-scale mechanisms are not reported. In this study, Fe3C(220) modified black carbon (BC) [Fe3C(220)@BC] with high adsorption and selectivity for Pb(II) from industrial wastewater with Cd(II) was developed. The quantitative experiment suggested that Fe species accounted for 80.5-100 and 18.4-33.8% of Pb(II) and Cd(II) removal, respectively. Based on X-ray absorption near-edge structure analysis, 57.3% of adsorbed Pb2+ was reduced to Pb0; however, 61.6% of Cd2+ existed on Fe3C@BC. Density functional theory simulation unraveled that Cd(II) adsorption was attributed to the cation-π interaction with BC, whereas that of Pb(II) was ascribed to the stronger interactions with different Fe phases following the order: Fe3C(220) > Fe0(110) > Fe3O4(311). Crystal orbital bond index and Hamilton population analyses were innovatively applied in the adsorption system and displayed a unique discovery: the stronger Pb(II) adsorption on Fe phases was mediated by a combination of covalent and ionic bonding, whereas ionic bonding was mainly accounted for Cd(II) adsorption. These findings open a new chapter in understanding the functions of different Fe phases in mediating the fate and transport of heavy metals in both natural and engineered systems.

Simultaneous removal of Cd(II) and As(V) by ferrihydrite-biochar composite: Enhanced effects of As(V) on Cd(II) adsorption

The coexistence of cadmium (Cd(II)) and arsenate (As(V)) pollution has long been an environmental problem. Biochar, a porous carbonaceous material with tunable functionality, has been used for the remediation of contaminated soils. However, it is still challenging for the dynamic quantification and mechanistic understanding of the simultaneous sequestration of multi-metals in biochar-engineered environment, especially in the presence of anions. In this study, ferrihydrite was coprecipitated with biochar to investigate how ferrihydrite-biochar composite affects the fate of heavy metals, especially in the coexistence of Cd(II) and As(V). In the solution system containing both Cd(II) and As(V), the maximum adsorption capacities of ferrihydrite-biochar composite for Cd(II) and As(V) reached 82.03 µmol/g and 531.53 µmol/g, respectively, much higher than those of the pure biochar (26.90 µmol/g for Cd(II), and 40.24 µmol/g for As(V)) and ferrihydrite (42.26 µmol/g for Cd(II), and 248.25 µmol/g for As(V)). Cd(II) adsorption increased in the presence of As(V), possibly due to the changes in composite surface charge in the presence of As(V), and the increased dispersion of ferrihydrite by biochar. Further microscopic and mechanistic results showed that Cd(II) complexed with both biochar and ferrihydrite, while As(V) was mainly complexed by ferrihydrite in the Cd(II) and As(V) coexistence system. Ferrihydrite posed vital importance for the co-adsorption of Cd(II) and As(V). The different distribution patterns revealed by this study help to a deeper understanding of the behaviors of cations and anions in the natural environment.

Adsorption of Sb(III) and Pb(II) in wastewater by magnetic γ-Fe2O3-loaded sludge biochar: Performance and mechanisms

Magnetically modified carbon-based adsorbent (BC@γ-Fe2O3) was prepared through facile route using activated sludge biomass and evaluated for the simultaneous removal of Sb(III) and Pb(II). BC@γ-Fe2O3 exhibited outstanding Sb(III) and Pb(II) adsorption capacity when 200 mg of adsorbent was employed at pH 5.0 for 240 min, with the removal efficiency higher than 90%. The experiments demonstrated the excellent reusability and the potent anti-interference properties of the prepared absorbent. Freundlich and pseudo-second-order kinetic were prior to describe the adsorption process. The adsorption of Sb(III) and Pb(II) onto BC@γ-Fe2O3 was spontaneous and endothermic. BC@γ-Fe2O3 with high specific surface area revealed the exceptional competence to absorb Sb(III) and Pb(II) through pore filling, electrostatic adsorption and complexation. The adsorption mechanisms of Sb(III) and Pb(II) showed similarities with slight disparities. The removal of Sb(III) involved the Fe-O-Sb bond and π-π bond, while the adsorption of Pb(II) was closely related to ion exchange. Moreover, Sb(III) was oxidized to Sb(V) in a minor part during adsorption. The Fe-O-Cl active sites on BC allowed for the binding of γ-Fe2O3, guaranteeing the abundant adsorption sites and stability. BC@γ-Fe2O3 provides an efficient and green insight into the simultaneous removal of complex heavy metals with promising application in wastewater treatment.

Exploring adsorption capacity and mechanisms involved in cadmium removal from aqueous solutions by biochar derived from euhalophyte

Biochar has shown potential as a sorbent for reducing Cd levels in water. Euhalophytes, which thrive in saline-alkali soils containing high concentrations of metal ions and anions, present an intriguing opportunity for producing biochar with inherent metal adsorption properties. This study focused on biochar derived from the euhalophyte Salicornia europaea and aimed to investigate its Cd adsorption capacity through adsorption kinetics and isotherm experiments. The results demonstrated that S. europaea biochar exhibited a high specific surface area, substantial base cation content, and a low negative surface charge, making it a highly effective adsorbent for Cd. The adsorption data fit well with the Langmuir isotherm model, revealing a maximum adsorption capacity of 108.54 mg g-1 at 25 °C. The adsorption process involved both surface adsorption and intraparticle diffusion. The Cd adsorption mechanism on the biochar encompassed precipitation, ion exchange, functional group complexation, and cation-π interactions. Notably, the precipitation of Cd2+ with CO32- in the biochar played a dominant role, accounting for 73.7% of the overall removal mechanism. These findings underscore the potential of euhalophytes such as S. europaea as a promising solution for remediating Cd contamination in aquatic environments.

Competitive sorption and desorption of cadmium, lead, and zinc onto peat, compost, and biochar

Soil and water contamination by potentially toxic metals (PTMs) has exerted adverse environmental impacts, which justifies studies of promising remediation alternatives. This article investigated the competitive sorption of cadmium (Cd), lead (Pb), and zinc (Zn) onto peat, compost, and biochar derived from the organic fraction of municipal solid waste (OFMSW), but its main innovation was the post-sorption assessment. The effects of contact time on competition between contaminants were systematically analyzed by batch experiments and the effectiveness of the sorption process was evaluated in desorption tests (H2O, HCl, NaOH, and NaCl) and sequential extraction. Kinetic data were well-fitted to pseudo-first-order (PFO) and pseudo-second-order (PSO) models and the intra-particle diffusion model revealed the existence of multiple linear regions, indicating the sorption process was controlled by a multi-step mechanism. The sorption capacities followed a biochar > compost > peat order, with biochar retaining more than 99% of Cd, Pb, and Zn in all samples. The general order of desorption percentage was peat > compost > biochar, with a below 0.60% biochar release, suggesting the importance of chemical processes. HCl solution (more acid pH) showed the highest release of previously sorbed contaminants and, therefore, can be employed for the reuse of sorbents (sorption/desorption cycles). The only exception was Pb desorption on biochar, with maximum release in NaOH solution. A negative Pearson correlation with F1 (acid-soluble/exchangeable fraction) for Cd and Zn and a positive one with the other steps were reported. Pb exhibited an opposite behavior, showing the highest sorption performances and the lowest desorption rates for all sorbents, justified by positive correlations with F4 (residual fraction) and negative ones with desorption. The findings suggest the evaluated sorbents, especially compost and biochar, can be effective materials in the simultaneous sorption of Cd, Pb, and Zn in wastewater, as well as an amendment for PTMs immobilization in contaminated soils.

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

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

Quantification adsorption mechanisms of arsenic by goethite-modified biochar in aqueous solution

In this study, rice straw biochar (BC), goethite (GT), and goethite-modified biochar (GBC) were prepared and their differences in adsorption characteristics and mechanisms of arsenic were explored to provide theoretical and data reference for future design of modified biochar, aiming to address adsorption mechanism weakness and improve the efficiency of arsenic removal in water. Various characterization methods were employed to evaluate the influence of pH, adsorption kinetics, isotherms, and chemical analyses of the materials. At temperatures of 283 K, 298 K, and 313 K, the maximum actual adsorption capacity followed the order GBC > GT > BC, while at 313 K, the maximum Langmuir adsorption capacity of GBC reached 149.63 mg/g which was 95.92 times that of BC and 6.27 times of GT. Due to precipitation and complexation mechanisms, GBC exhibited more superior arsenic adsorption capacities than BC and GT, contributing to total adsorption ranging from 88.9% to 94.2%. BC was dominated by complexation and ion exchange mechanisms in arsenic adsorption, with contribution proportions of 71.8%-77.6% and 19.1%-21.9%, respectively. In GT, the precipitation mechanism played a significant role in total adsorption, contributing from 78.0% to 84.7%. Although GBC has significant potential for removing arsenic from aqueous solutions, the findings suggest that its ion exchange capacity needs improvement.

Adsorption of antibiotic, heavy metal and antibiotic plasmid by a wet-state silicon-rich biochar/ferrihydrite composite to inhibit antibiotic resistance gene proliferation/transformation

Decreasing bioaccessible antibiotics, heavy metals, and antibiotic resistance genes (ARGs) in soil by adsorption is an attractive, but unrealized, approach for ARG risk reduction. This approach has the potential to reduce the (co)selection pressure from antibiotics and heavy metals on bacteria and ARG horizontal gene transformation to pathogens. Here, a wet-state silicon-rich biochar/ferrihydrite composite (SiC-Fe(W)) synthesized by loading ferrihydrite onto rice straw-derived biochar was examined for i) adsorption of oxytetracycline and Cu²⁺ to reduce (co)selection pressure and ii) adsorption of extracellular antibiotic resistance plasmid pBR322 (containing tetA and bla_TEM-1) to inhibit ARG transformation. SiC-Fe(W) gained the adsorption priority of biochar (for Cu²⁺) and wet-state ferrihydrite (for oxytetracycline and pBR322) and showed adsorptive enhancement (for Cu²⁺ and oxytetracycline) from a more wrinkled and exposed surface from biochar silica-dispersed ferrihydrite and a more negatively charged biochar, and the adsorption capacity for SiC-Fe(W) was 17-135 times that of soil. Correspondingly, 10 g/kg SiC-Fe(W) amendment increased the soil adsorption coefficient K_d by 31%-1417% and reduced the selection pressure from dissolved oxytetracycline, co-selection pressure from dissolved Cu²⁺, and transformation frequency of pBR322 (assessed with Escherichia coli). The development of Fe-O-Si bonds on silicon-rich biochar in alkaline enhanced ferrihydrite stability and adsorption capacity (for oxytetracycline), presenting a new potential strategy of biochar/ferrihydrite composite synthesis for adsorptive inhibition of ARG proliferation and transformation in ARG pollution control.

One-pot synthesis of siliceous ferrihydrite - coated halloysite nanorods in alkaline medium: Structure, properties and cadmium adsorption performance

The application of amorphous ferrihydrite (Fh) for Cd(II) removal is restricted by its unstable and easily transformable nature. Although doping with silicates stabilized ferrihydrite, its product siliceous ferrihydrite (SiFh) again suffered from the disadvantage of spontaneous agglomeration. Herein, ferrihydrite was hybridized with halloysite nanotubes (HNTs) to prepare a novel siliceous ferrihydrite - coated halloysite nanorods (SiFh@HNTs) in alkaline medium, to break through the current barriers. The characterization results showed that SiFh@HNTs could simultaneously overcome the defects of easy phase transformation of ferrihydrite and easy aggregation of SiFh nanoparticles (NPs). Meanwhile, the optimal SiFh@HNT₄₀ with halloysite content of 40 % formed a well-developed mesoporous structure and exhibited the desired surface properties: a high specific surface area of 303.4 m²/g, an isoelectric point as low as pH_iep = 4.5, and rich functional Fe - OH groups. The formation mechanism of such excellent sturcture-properties of SiFh@HNT₄₀ were mainly attributed to two factors: the generation of smaller (∼5 nm) SiFh NPs induced by the integration of halloysite-derived SiO₄^4- into ferrihydrite, and the dispersion of SiFh NPs on clay nanotubes. Furthermore, the adsorption capacity of SiFh@HNT₄₀ for Cd(II) was up to 137.8 mg/g at 30 °C and pH 6, which was much higher than that of aggregated ferrihydrite (11.2 mg/g), halloysite (18.8 mg/g) and goethite (49.4 mg/g). The adsorption thermodynamics study revealed the adsorption of Cd(II) on SiFh@HNT₄₀ was clearly chemisorption with a (ΔH_ads)_q of 43.3 kJ/mol. Characterization results of XPS and FTIR confirmed that the rich Fe - OH groups on SiFh@HNT₄₀ was the main adsorption sites, and Cd(II) was specifically adsorbed by inner-sphere surface complexation. In addition, SiFh@HNT₄₀ had application potential in the mixed-metal wastewaters treatment. Cyclic regeneration experiments showed that SiFh@HNT₄₀ had good regeneration performance and could be reused many times.

Manganese-doped hydroxyapatite as an effective adsorbent for the removal of Pb(II) and Cd(II)

The water polluted by lead(Pb(II)) and cadmium(Cd(II)) seriously endangers ecological safety and needs to be solved urgently. Because of the relatively low adsorption rate of pure hydroxyapatite for heavy metals, a series of manganese-doped hydroxyapatites (MnHAPs) were prepared by using manganese, a common impurity in hydroxyapatite, as a doping element to improve the adsorption performance. The structural and functional groups of the materials with different Mn/(Ca +Mn) molar ratios (0%, 5%, 10%, 20%, and 30%) were investigated by scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET), X-Ray diffraction (XRD), Raman spectrometer and Fourier transform infrared spectroscopy (FTIR) characterization. The presence of manganese influenced the formation and growth of hydroxyapatite crystals, resulting in lattice distortion and a large number of lattice defects in materials. Among them, manganese-doped hydroxyapatite with a Mn/(Ca +Mn) molar ratio of 10% (MnHAP-10) could most effectively remove Pb(II) and Cd(II), with the adsorption capacity of 1806.09 mg g^-1 for Pb(II) at pH = 5 and 176.88 mg g^-1 for Cd(II) at pH = 5.5. Then the adsorption behavior and mechanism were further researched systemically. It was concluded that the immobilization of Pb(II) by MnHAP-10 was mainly through dissolution precipitation and ion exchange, while Cd(II) was adsorbed by ion exchange and electrostatic interaction. In conclusion, MnHAP-10 has the potential to be applied as an effective adsorbent for the removal of Pb(II) and Cd(II) pollution.

Optimization of Cd (II) removal from aqueous solution by natural hydroxyapatite/bentonite composite using response surface methodology

Toxic cadmium (Cd) was removed from water using eggshell-based hydroxyapatite (HAp) grafted bentonite (HAp/bentonite) composite through a straightforward chemical synthesis route. The as-prepared adsorbents were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller analysis (BET). Optimization of the initial adsorbate concentration, adsorbent dosage, pH, and contact time—all of which affect the adsorption process—was performed using the central composite design (CCD) of the response surface methodology (RSM). 99.3 percent adsorptive removal efficiency was observed at an initial concentration of 61.58 mg/L of Cd (II), with an adsorbent dosage of 1.58 g, a solution pH of 5.88, and a contact time of 49.63 min. The analysis of variance (ANOVA) was performed, and the multiple correlation coefficient (R2) was found to be 0.9915 which confirms the significance of the predicted model. The Langmuir isotherm model best represented the adsorption isotherm data, which also predicted a maximum sorption capacity of 125.47 mg/g. The kinetic data were best described by the pseudo-second order model.

Performance and mechanisms for Cd(II) and As(III) simultaneous adsorption by goethite-loaded montmorillonite in aqueous solution and soil

A series of goethite-modified montmorillonite (GMt) materials was synthesized for the amelioration of cationic cadmium (Cd) and anionic arsenic (As) complex contaminants in soil and water bodies. The results showed that goethite (Gt) was successfully loaded onto the surface of montmorillonite (Mt), which possessed more functional groups (such as Fe-O, and Fe-OH) and a larger specific surface area. GMt-0.5 (Mt loaded with Gt at a ratio of 0.5:1) showed the highest adsorption capacity for Cd(II) and As(III) with the maximum of 50.61 mg/g and 57.58 mg/g, respectively. The removal rate of Cd(II) was highly pH dependent, while the removal rate of As(III) showed little dependence on pH. The goethite on montmorillonite might contribute to the formation of surface complexes with As(III) and oxidation of As(III) to As(V). In the binary system, both, synergistic and competitive adsorption existed simultaneously. Importantly, in the binary system, the removal of As(III) was more favorable because of the electrostatic interaction, formation of a ternary complex, and co-precipitation. In addition, the amendment of GMt-0.5 significantly reduced the availability of Cd and As in the soil. This study suggests that GMt-0.5 is a promising candidate for the simultaneous immobilization of metal (loid)s in both, aqueous solution and mine soil.

Potential Mechanism of Long-Term Immobilization of Pb/Cd by Layered Double Hydroxide Doped Chicken-Manure Biochar

Layered double hydroxide (LDH)-doped chicken-manure biochar (CMB) with long-term stability was synthesized to immobilize Pb/Cd. MgAl-Cl-LDH-doped CMB (MHs) showed prominent long-term oxidation resistance and the least biodegradation sensitivity. Efficient Pb/Cd adsorption was observed on MHs, and the maximum adsorption capacities of Pb(II)/Cd(II) reached 1.95 mmol/g and 0.65 mmol/g, respectively. Precipitation and isomorphous substitution were identified as the key adsorption mechanisms, which formed highly stable Pb/Cd species (PbAl-CO₃-LDH, Pb₃(OH)₂CO₃, CdAl-Cl-LDH and CdCO₃). Pb(II) and Cd(II) precipitated with CO₃^2- in MHs; meanwhile, Mg(II) and Ca(II) in LDH layers were substituted by Pb(II) and Cd(II) respectively. Therefore, MHs had the potential for long-term stability of Pb/Cd. Moreover, complexation and electrostatic adsorption also contributed to the Pb/Cd immobilization to a certain extent. When 5% MHs (w/w) was applied to Pb/Cd contaminated smelting site soils, the soil pH increased from 5.9 to 7.3. After applying MHs for 25 d, the content of bioavailable Pb(II) and Cd(II) decreased by 98.8% and 85.2%, respectively, and the content of soluble Pb and Cd dropped by 99.5% and 96.7%. This study paves the way for designing a novel LDH doped CMB as efficient Pb/Cd immobilizers for smelting site soils.

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

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

Adsorption of chromium, copper, lead and mercury ions from aqueous solution using bio and nano adsorbents: A review of recent trends in the application of AC, BC, nZVI and MXene

Recent trends in adsorption of Chromium (Cr), Copper (Cu), Lead (Pb) and Mercury (Hg) in wastewater using (i) carbonaceous materials including activated carbon (AC) and biochar (BC), and (ii) nanomaterials including nano zero-valent iron (nZVI) and MXenes have been discussed in this paper. It has been found that adsorption capacity depends largely on the adsorbent modification technique, initial pH of wastewater, dosage of adsorbent, contact time and initial concentration of the pollutants. The pH value ranges for maximum removal of Cr, Cu, Pb and Hg have been reported as 2-4, 5-6, 5-8 and 3-8, respectively. Up to 99% removal of metals has been reported using AC, BC, nZVI and MXene. The mechanism involves the reduction and chemical adsorption of metals. AC and BC have a higher surface area (up to 5000 m²/g) compared to nZVI (up to 500 m²/g) and MXene (up to 67.66 m²/g). However, the higher reactivity and regeneration capacity of nZVI and MXene make them suitable adsorbents. From a practical point of view the application of adsorbents for real effluents, cost analysis, regeneration capability and reuse of heavy metals are some aspects that need attention in future studies. The removal efficiencies of AC and BC are comparable to the nZVI and MXene. The cost analysis may be an attractive aspect to decide the future application of these adsorbents at large scale.