Enhancing Cd(II) sorption by red mud with heat treatment: Performance and mechanisms of sorption

A comprehensive investigation of the adsorption behaviour and mechanism of industrial waste sintering and bayer red muds for heavy metals

The issue of heavy metal pollution is a critical global concern that requires urgent solution. However, conventional heavy metal adsorbents are too costly to be applied in large-scale engineering. In this study, adsorption behavior and mechanism of sintering red mud (RM-A) and bayer red mud (RM-B) for heavy metals were investigated to address the disposal of red mud as industrial waste and remediation of heavy metal pollution. Batch adsorption experiments were conducted to explore the adsorption performances of RM-A and RM-B under various conditions. Characterization of RM-A and RM-B before and after adsorption by XRD, FTIR and SEM-EDX was applied to investigate the specific adsorption behavior and mechanism. Adsorption experiments of both RM-A and RM-B fitted pseudo-second-order kinetic model and Langmuir isotherm model, with estimated maximum adsorption capacity of 21.96 and 25.19 mg/g for Cd2+, 21.47 and 26.06 mg/g for Cu2+ and 55.47 and 59.65 mg/g for Pb2+, respectively. Precipitation transformation of calcite was the primary adsorption mechanism for RM-A, whereas ion exchange of cancrinite, surface coordination compounds of hematite and minor precipitation transformation of calcite accounted for the adsorption mechanism for RM-B. Overall, RM-A and RM-B exhibited best adsorption performance for Pb2+, with RM-B showing greater adsorption capacity attributed to its higher specific surface area. This study compared the adsorption properties of RM-A and RM-B for the first time and demonstrated that both red muds can be effectively applied to remove heavy metals, thereby contributing to the sustainable industrial waste management and resourceful reuse.

Machine learning-based prediction and experimental validation of heavy metal adsorption capacity of bentonite

As a natural adsorbent material, bentonite is widely used in the field of heavy metal adsorption. The heavy metal adsorption capacity of bentonite varies significantly in studies due to the differences in the properties of bentonite, solution, and heavy metal. To achieve accurate predictions of bentonite's heavy metal adsorption capacity, this study employed six machine learning (ML) regression algorithms to investigate the adsorption characteristics of bentonite. Finally, an eXtreme Gradient Boosting Regression (XGB) model with outstanding predictive performance was constructed. Explanation analysis of the XGB model further reveal the importance and influence manner of each input feature in predicting the heavy metal adsorption capacity of bentonite. The feature categories influencing heavy metal adsorption capacity were ranked in order of importance as adsorption conditions > bentonite properties > heavy metal properties. Furthermore, a web-based graphical user interface (GUI) software was developed, facilitating researchers and engineers to conveniently use the XGB model for predicting the heavy metal adsorption capacity of bentonite. This study provides new insights into the adsorption behaviors of bentonite for heavy metals, offering guidance and support for enhancing its application efficiency and addressing heavy metal pollution remediation.

Applying Red Mud in Cadmium Contamination Remediation: A Scoping Review

Red mud is an industrial solid waste rarely utilized and often disposed of in landfills, resulting in resource waste and environmental pollution. However, due to its high pH and abundance of iron and aluminum oxides and hydroxides, red mud has excellent adsorption properties which can effectively remove heavy metals through ion exchange, adsorption, and precipitation. Therefore, red mud is a valuable resource rather than a waste byproduct. In recent years, red mud has been increasingly studied for its potential in wastewater treatment and soil improvement. Red mud can effectively reduce the migration and impact of heavy metals in soils and water bodies. This paper reviews the research results from using red mud to mitigate cadmium pollution in water bodies and soils, discusses the environmental risks of red mud, and proposes key research directions for the future management of red mud in cadmium-contaminated environments.

Comprehensive Analysis of Geopolymer Materials: Properties, Environmental Impacts, and Applications

The advancement of eco-friendly technology in the construction sector has been improving rapidly in the last few years. As a result, multiple building materials were developed, enhanced, and proposed as replacements for some traditional materials. One notable example presents geopolymer as a substitute for ordinary Portland concrete (OPC). The manufacturing process of (OPC) generates CO2 emissions and a high energy demand, both of which contribute to ozone depletion and global warming. The implementation of geopolymer concrete (GPC) technology in the construction sector provides a path to more sustainable growth and a cleaner environment. This is due to geopolymer concrete's ability to reduce environmental pollutants and reduce the construction industry's carbon footprint. This is achieved through its unique composition, which typically involves industrial byproducts like fly ash or slag. These materials, rich in silicon and aluminum, react with alkaline solutions to form a binding gel, bypassing the need for the high-energy clinker production required in OPC. The use of such byproducts not only reduces CO2 emissions but also contributes to waste minimization. Additionally, geopolymer offers extra advantages compared to OPC, including improved mechanical strength, enhanced durability, and good stability in acidic and alkaline settings. Such properties make GPC particularly suitable for a range of construction environments, from industrial applications to infrastructure projects exposed to harsh conditions. This paper comprehensively reviews the different characteristics of geopolymers, which include their composition, compressive strength, durability, and curing methods. Furthermore, the environmental impacts related to the manufacturing of geopolymer materials were evaluated through the life-cycle assessment method. The result demonstrated that geopolymer concrete maintains positive environmental impacts due to the fact that it produces fewer carbon dioxide CO2 emissions compared to OPC concrete during its manufacturing; however, geopolymer concrete had some minor negative environmental impacts, including abiotic depletion, human toxicity, freshwater ecotoxicity, terrestrial ecotoxicity, and acidification. These are important considerations for ongoing research aimed at further improving the sustainability of geopolymer concrete. Moreover, it was determined that silicate content, curing temperature, and the proportion of alkaline solution to binder are the major factors significantly influencing the compressive strength of geopolymer concrete. The advancement of geopolymer technology represents not just a stride toward more sustainable construction practices but also paves the way for innovative approaches in the field of building materials.

Effective Removal of Dyes from Wastewater by Osmanthus Fragrans Biomass Charcoal

The exploration of low-cost, high-performance adsorbents is a popular research issue. In this work, a straightforward method that combined hydrothermal with tube firing was used to produce Osmanthus fragrans biomass charcoal (OBC) from low-cost osmanthus for dye adsorption in water. The study examined the parameters of starting concentration, pH, and duration, which impacted the process of adsorption of different dyes by OBC. The analysis showed that the adsorption capacities of OBC for six dyes: malachite green (MG, C0 = 800 mg/L, pH = 7), Congo red (CR, C0 = 1000 mg/L, pH = 8), rhodamine B (RhB, C0 = 500 mg/L, pH = 6), methyl orange (MO, C0 = 1000 mg/L, pH = 7), methylene blue (MB, C0 = 700 mg/L, pH = 8), and crystalline violet (CV, C0 = 500 mg/L, pH = 7) were 6501.09, 2870.30, 554.93, 6277.72, 626.50, and 3539.34 mg/g, respectively. The pseudo-second-order model and the Langmuir isotherm model were compatible with the experimental findings, which suggested the dominance of ion exchange and chemisorption. The materials were characterized by using XRD, SEM, FTIR, BET, and XPS, and the results showed that OBC had an outstanding specific surface area (2063 m2·g-1), with potential adsorption mechanisms that included electrostatic mechanisms, hydrogen bonding, and π-π adsorption. The fact that the adsorption capacity did not drastically decrease after five cycles of adsorption and desorption suggests that OBC has the potential to be a dye adsorbent.

Synthesis of co-pyrolyzed biochar using red mud and peanut shell for removing phosphate from pickling wastewater: Performance and mechanism

Iron (Fe)/iron oxide-modified biochar has practicable adsorption capability for phosphorus (P), but it is expensive. In this study, we synthesized novel low-cost and eco-friendly adsorbents co-pyrolyzed biochars using Fe-rich red mud (RM) and peanut shell (PS) wastes via a one-step pyrolysis process for removing P from pickling wastewater. The preparation conditions (heating rate, pyrolysis temperature, and feedstock ratio) and P adsorption behaviors were systematically investigated. In addition, a series of characterization and approximate site energy distribution (ASED) analyses were conducted to understand the P adsorption mechanisms. The magnetic biochar (BR7P3) with m (RM):m (PS) of 7:3 prepared at 900°C and 10 °C/min had a high surface area (164.43 m2/g) and different abundant ions (including Fe3+, and Al3+). In addition, BR7P3 exhibited the best P removal capability (142.6 mg/g). The Fe2O3 from RM was successfully reduced to Fe0, which was easily oxidized as Fe3+ to precipitate with H2PO4-. The electrostatic effect, Fe-O-P bonding, and surface precipitation were the main mechanisms of P removal. ASED analyses revealed that high distribution frequency and solution temperature led to a high P adsorption rate of the adsorbent. Therefore, this study provides new insight into the waste-to-wealth strategy by transforming PS and RM into mineral-biomass biochar with excellent P adsorption capability and environmental adaptability.

The effective Ni(II) removal of red mud modified chitosan from aqueous solution

This study used red mud modified with chitosan (RM/CS) as a novel adsorbent to remove Ni(II) ions from an aqueous solution. The adsorbent was characterized by the techniques of the BET method, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analysis. According to the findings, the surface area of RM/CS is nearly doubled compared to CS, from 68.6 to 105.7 m².g^-1. The Ni(II) batch adsorption of RM/CS was performed as a function of pH value, contact time, and volume of adsorbent. Three isotherm adsorption models (Langmuir, Freundlich, and Sips) and three kinetic models (the pseudo-first-order, the pseudo-second-order, and the intra-diffusion models) were fitted with the experimental data to calculate the maximum adsorption capacity and to estimate the uptake in nature. The Langmuir monolayer adsorption capacity for Nickel (II) is 31.66 mg.g^-1 at a pH of 6.0, with an adsorption time of 180 min and a temperature of 323 K. The Ni(II) adsorption on RM/CS is the exothermic process and is controlled by the intra-diffusion model.

A review of comprehensive utilization of red mud

Red mud (RM) is a solid waste generated during the process of alumina production. RM has already posed a serious environmental threat with the development of the alumina refining industry. The comprehensive utilization of RM has attracted much attention due to its large-scale generation and harmful nature. This paper introduces the characteristics and state of RM and summarizes the relevant research on the comprehensive utilization of RM. The results show that comprehensive utilization of RM is mainly focused on the preparation of building materials, the extraction of valuable metals, catalyst synthesis and environmental protection. Besides, the article discusses the existing problems while utilizing RM. Prospects and suggestions for different utilization methods of RM are proposed.

Clean dealkalization technology from aluminum industry hazardous tailings-red mud by displacement with Mg-based agent

Red mud is a kind of strong alkaline hazardous slag discharged from aluminum metallurgy industry. In this study, the water immersion with high temperature and high pressure was developed for the selective dealkalization from red mud by adding Mg-based additives. The removal efficiency of alkali could reach 92% by using 12% MgCl₂ with 9 mL/g at 250 °C for 60 min. The MgCl₂ was the most effective leaching reagent to promote the decomposion of cancrinite lattice. The new minerals bearing Mg, i.e., chlorite (Mg₅Al₂Si₃O₁₀(OH)₈) and pyrope (Mg₃Al₂Si₃O₁₂) could be formed, which was in favor of transforming the structural alkali into the free alkali by the analysis and validation of XRD and SEM-EDS. The dealkalization process was mainly controlled by chemical reactions according to the analysis of unreacted shrinking core model (USCM) of leaching kinetics. The leaching kinetics equation of 1 - (1 - x)^1/3 = 32.2 × exp[4582.6 / T] × t was built and the apparent activation energy of 38.1 kJ/mol was obtained. This method may provide a new and cleaner way for the efficient dealkalization of red mud and a basis for the utilization of leaching residue as the soil amendment.

Preparation of Iron Carbon Composite Material by Extracting Iron from Bauxite Residue and Its Adsorption of Heavy Metal Cd(II)

An effective method of iron extraction from bauxite residue was explored, and iron was used to prepare iron carbon composite material, which have a good adsorption effect on the heavy metal cadmium. After acid washing, acid leaching, Fe(III) reduction and ferrous oxalate decomposition, FeSO₄·H₂O(RM) was successfully extracted from bauxite residue, and the iron loss was only 4.35%. FexOy-BC(RM) nanocomposite materials were prepared by loading FeSO₄·H₂O(RM) onto walnut shell biochar (BC) (a kind of agricultural and forestry waste) by an in situ reduction and oxidation method. The results showed that the adsorption effect of FexOy-BC(RM) on Cd(II) was better than that of commercial FexOy-BC. XPS, TEM, SEM characterization analysis showed that FexOy-BC(RM) immobilized Cd(II) by adsorption, complexation, etc.to achieve a highly efficient adsorption of heavy metal Cd(II) in wastewater.

Magnetization of Bauxite Residue to Enhance the Removal Efficiency Towards Heavy Metals

Bauxite residues are a mass of industrial wastes derived from aluminum metallurgy. This work provided a simple pyrolysis method to magnetize the bauxite residue to serve as a magnetic adsorbent towards heavy metals removal. The X-ray diffraction patterns and Mossbauer spectrum results confirmed the partial reduction of iron species with an obvious enhancement in magnetization. The magnetized bauxite residue exhibited excellent removal efficiencies for Cu²⁺, Cd²⁺ and Pb²⁺ with maximum adsorption capacities of 219.0 mg g^-1, 275.4 mg g^-1, and 100.4 mg g^-1, which could be quickly separated through a magnet. The adsorption equilibrium data were fitted to the Langmuir isotherm model, while the adsorption kinetics followed a pseudo-first-order model. According to the characterization results, chemical precipitation and sorption was the major mechanism for the removal of Cu²⁺, Pb²⁺, and Cd²⁺. Thus, the magnetized bauxite residue exhibited promising applications for heavy metals removal in wastewater.

Immobilization of lead, copper, cadmium, nickel, and zinc in sediment by red mud: adsorption characteristics, mechanism, and effect of dosage on immobilization efficiency

The objective of this work was to determine the effect of dosage on the immobilization of lead (Pb), copper (Cu), cadmium (Cd), nickel (Ni), and zinc (Zn) in sediment by red mud (RM). To achieve this aim, the adsorption characteristics and mechanism of Pb, Cu, Cd, Ni, and Zn from aqueous solution on RM were studied at first, and then the influence of the RM dosage on the fractionation and leaching potential of Pb, Cu, Cd, Ni, and Zn in sediment was investigated. The results showed that RM possessed high adsorption capacities for Pb(II), Cu(II), Cd(II), Ni(II), and Zn(II) in aqueous solution. The maximum monolayer Pb(II), Cu(II), Cd(II), Ni(II), and Zn(II) adsorption capacities for RM derived from the Langmuir isotherm model were found to be 296, 39.2, 70.2, 46.0, and 50.7 mg/g, respectively. The addition of RM into sediment could effectively reduce the toxicity characteristic leaching procedure (TCLP)-leachable concentrations of Pb, Cu, Cd, Ni, and Zn in the sediment. The added RM could effectively immobilize the mobile (exchangeable, reducible, and oxidizable fractions) Pb in sediment by the conversion of the exchangeable and reducible fractions into the residual fraction, and it could effectively immobilize the mobile Cu, Cd, Ni, and Zn in sediment by the conversion of the exchangeable fraction into the residual fraction. The quantities of mobile Pb, Cu, Cd, and Ni immobilized by RM had a good linear relationship with the added RM. The above results suggest that RM is a promising amendment for the immobilization of mobile Pb, Cu, Cd, Ni, and Zn in sediment, and the linear relationship between the RM dosage and the quantities of immobilized Pb, Cu, Cd, and Ni by RM can be employed to determine the RM dosage required for the immobilization of mobile Pb, Cu, Cd, and Ni in sediment.

Effects of Phosphate, Red Mud, and Biochar on As, Cd, and Cu Immobilization and Enzymatic Activity in a Co-Contaminated Soil

Arsenic (As), cadmium (Cd), and copper (Cu) are the primary inorganic pollutants commonly found in contaminated soils. The simultaneous stabilization of the three elements is a preferred approach for mixture-contaminated soils which has received extensive research attention. However, few studies have focused on the immobilization efficiency of a single amendment on the three elements. In this study, phosphate, red mud, and biochar were used to remediate As (237.8 mg kg−1), Cd (28.72 mg kg−1), and Cu (366.5 mg kg−1) co-contaminated soil using a 180-day incubation study. The BCR (European Community Bureau of Reference) extraction method, NH4H2PO4–extractable As, and diethylenetriamine penta-acetic acid (DTPA)–extractable Cd and Cu were analyzed at different time intervals. The results indicated that the application of red mud and biochar significantly reduced soil DTPA–Cd and Cu concentrations during the incubation, while the decrease in soil NH4H2PO4–As was much less than that of soil DTPA–Cd and Cu. After 180 days of incubation, the concentrations of NH4H2PO4–As in red mud and biochar treatments decreased by 2.15~7.89% and 3.01~9.63%, respectively. Unlike red mud and biochar, phosphate significantly reduced the concentration of soil DTPA–Cd and Cu, but failed to lower that of As. The BCR extraction method confirmed that red mud and biochar addition increased the reducible fraction of As due to the surface complexes of As with Fe oxide. Canonical correspondence analysis (CCA) demonstrated that soil pH in addition to available As, Cd, and Cu concentrations were the primary factors in driving the changes in soil enzymatic activity. Soil pH showed positive correlation with soil urease and catalase activities, while negative correlation was observed between soil-available As, Cd, and Cu, and soil enzyme activities. This study revealed that it is difficult to simultaneously and significantly reduce the bioavailabilities of soil As, Cd, and Cu using one amendment. Further research on modifying these amendments or applying combined amendments will be conducted, in order to develop an efficient method for simultaneously immobilizing As, Cd, and Cu.

Roasted modified lead-zinc tailings using alkali as activator and its mitigation of Cd contaminated: Characteristics and mechanisms

To comprehensively reuse lead-zinc tailings, leaching residue (LR) of solid by-products was produced after the recovery of valuable metals. This study provided a "waste-ecology" strategy by a simple, inexpensive method of roasting prepared highly active silicon modified tailing (HAST) to eliminate the environment risk of LR, and investigates performance and mechanism of HAST as sorbents and passivators. The results indicated that HAST possesses high pH, abundant mineral content, microporous structure and high stability. The adsorption kinetic experiment revealed that chemisorption is the main reaction and the Q_m of Cd via Langmuir model is 72.75 mg/g. As further demonstrated by X-ray diffraction (XRD), energy dispersive X-ray (EDX), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis, the Cd was adsorbed onto the HAST surface successfully, with the main interaction mechanisms involving ion exchange, complexation, precipitation and electrostatic interaction. Besides, the soil incubation experiment results showed that HAST had positive effects on exchange fractions (Cd) converting to stable fractions in soil, which modifies Cd migration and transformation, HAST added into soil decreased the DTPA-Cd by 4.7%-8.1%, 5.9-9.8% and 9.1%-13.4%, respectively, in different stages, as compared with the control. Therefore, this study provides a novel strategy to address LR recycling, and the relevant, wastewater and soil treatment, which has high practicability for industrial applications.

Effects of Mn(II) addition on Cd(II) removal by hydrated manganese dioxide

In this study, the redox homogeneous precipitation method was applied to synthesize hydrated manganese dioxide (HMO) and to study the removal performance of Cd(II) from wastewater. Moreover, a small amount of Mn(II) could still combine with HMO without causing the desorption of Cd(II). A novel discovery was that the synergistic effect of KMnO₄ and Mn(II) could directly regenerate MnO₂ to deeply remove Cd(II), realizing the recycling of MnO₂. The influence of Mn(II) addition on the adsorption behavior of Cd(II) was discussed in terms of pH, Mn(II) addition mode, initial concentration of Mn(II) or Cd(II), and contact time. The adsorption of Cd(II) on HMO could be better in line with the Langmuir model, while that of Mn(II) accorded with the Freundlich model. The pseudo-second-order kinetic fitting results indicated that the removal of Cd(II) and Mn(II) on HMO belonged to chemisorption. SEM, XRD, FTIR, and XPS analysis demonstrated that Cd(II) was trapped by forming an inner-sphere complex on HMO, and the added Mn(II) and KMnO₄ could regenerate MnO₂ through oxidation-reduction reaction, wrapping outside of Cd(II) for a purpose of deep removal of Cd(II). HIGHLIGHTS: 1)A small amount of Mn(II) can bind to HMO without causing Cd(II) desorption 2)A large amount of Mn(II) can cause partial desorption of Cd(II) 3)Large amount of Mn(II) will partially bind to the surface of HMO or Cd(II) complexes 4)Deeply remove Cd(II) without eluting and regenerating HMO.

Waste Material via Geopolymerization for Heavy-Duty Application: A Review

Due to the extraordinary properties for heavy-duty applications, there has been a great deal of interest in the utilization of waste material via geopolymerization technology. There are various advantages offered by this geopolymer-based material, such as excellent stability, exceptional impermeability, self-refluxing ability, resistant thermal energy from explosive detonation, and excellent mechanical performance. An overview of the work with the details of key factors affecting the heavy-duty performance of geopolymer-based material such as type of binder, alkali agent dosage, mixing design, and curing condition are reviewed in this paper. Interestingly, the review exhibited that different types of waste material containing a large number of chemical elements had an impact on mechanical performance in military, civil engineering, and road application. Finally, this work suggests some future research directions for the the remarkable of waste material through geopolymerization to be employed in heavy-duty application.

Cadmium adsorption performance and mechanism from aqueous solution using red mud modified with amorphous MnO2

In this study, red mud modified by manganese dioxide(MRM) was utilized as an adsorbent to effectively remove Cd²⁺ from aqueous solution. The characteristics were analysed by SEM-EDS, XRD, BET, FTIR and XPS. Different factors that affected the Cd²⁺ removal on MRM, such as dosage, initial pH, initial Cd²⁺ concentration, were investigated using batch adsorption experiments. Simultaneously, the adsorption kinetics, adsorption isotherms and adsorption thermodynamics of Cd²⁺ were also investigated using adsorption experiments data. The characterization results showed that MRM had a rougher, larger specific surface area and pore volume (38.91 m² g^-1, 0.02 cm³ g^-1) than RM (10.22 m² g^-1, 0.73 cm³ g^-1). The adsorption experiments found that the equilibrium adsorption capacity of MRM for Cd²⁺ was significantly increased to 46.36 mg g^-1, which was almost three times that of RM. According to the fitting results, the pseudo-second-order kinetic model described the adsorption process better than the pseudo-first-order kinetic model. The Langmuir model fitted the adsorption isotherms well, indicating that the adsorption process was unimolecular layer adsorption and the maximum capacity was 103.59 mg g^-1. The thermodynamic parameters indicated that the adsorption process was heat-trapping and spontaneous. Finally, combined XPS and FTIR studies, it was speculated that the adsorption mechanisms should be electrostatic attachment, specific adsorption (i.e., Cd-O or hydroxyl binding) and ion exchange. Therefore, manganese dioxide modified red mud can be an effective and economical alternative to the removal of Cd²⁺ in the wastewater treatment process.

Efficient removal of Cd2+ from aqueous solution with a novel composite of silicon supported nano iron/aluminum/magnesium (hydr)oxides prepared from biotite

Taking low cost silicate minerals to develop efficient Cd²⁺ adsorption materials was favorable to the comprehensive utilization of minerals and remediation of environmental pollution. In this study, a composite of silicon supported nano iron/aluminum/magnesium (hydr)oxides was prepared with biotite by combining acid leaching and base precipitation process, which was used to remove Cd²⁺. Cd²⁺ adsorption behaviors were in accordance of pseudo-second order kinetic model and Langmuir model, and the obtained maximal Cd²⁺ adsorption capacity was 78.37 mg/g. Increasing pH and temperature could accelerate the removal of Cd²⁺. The activation energy was calculated as 66.05 kJ/mol, meaning that Cd²⁺ removal process was mainly depended on chemical adsorption. XRD and SEM results showed that this composite was a micro-nano structure of layered silica supported nano iron/aluminum/magnesium (hydr)oxides. Cd²⁺ removal mechanisms were consisted of surface complexation and ion exchange between Cd²⁺ and other metal ions, and the ion exchange interaction played the major role. These results indicated that a novel efficient utilization way for silicate minerals was developed.

Pb(II) adsorption mechanism and capability from aqueous solution using red mud modified by chitosan

Red mud modified by chitosan (RM/CS) was utilized as an adsorbent to effectively remove Pb(II) from aqueous solution. The surface area of RM/CS was found to significantly increase by more than 50% compared to that of original red mud. Different factors that affected the Pb(II) removal on this material, such as initial Pb(II) concentration, pH, and contact time, were investigated. The pseudo-first-order, pseudo-second-order, and intra-diffusion models were used to fit the experimental data to investigate the Pb(II)'s removal kinetics. The Pb(II) removal followed the intra-diffusion model. Additionally, the non-zero C value obtained from this model indicates that the removal was controlled by many different mechanisms. We also found that the interaction of Pb(II) and carbonate group on the material's surface played a primary role once the adsorption equilibrium was reached. Finally, the maximum adsorptive capacity was found to be about 209 mg/g. This obtained value is higher than those obtained for some other materials. Therefore, the present RM/CS should be a potential material for removing Pb(II) from aqueous solution.

Effect of a Passivator Synthesized by Wastes of Iron Tailings and Biomass on the Leachability of Cd/Pb and Safety of Pak Choi (Brassica chinensis L.) in Contaminated Soil

Cadmium (Cd) and lead (Pb) carry a high heavy-metal-toxic risk for both animals and plants in soil. In this study, iron-based biochar (T-BC) was prepared by co-pyrolysis using wastes of iron tailings and biomass with urea as the functioning agents. Field-emission scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and toxicity-characteristic leaching procedure (TCLP) methods were employed to analyze the physicochemical characteristics of T-BC. Additionally, a pot trial was conducted to examine the effects of T-BC on the physiological characteristics of pak choi (Brassica campestris L.), the availability of heavy metals, and enzyme activities in the soils. The results show that toxic metals have been volatilized by the roasting process and immobilized within T-BC via the formation of stable metal-compounds during the co-pyrolysis process, which satisfies the requirements of a soil passivator. Incubation experiments showed that the DTPA-extractable Cd and Pb in contaminated soils decreased with an increasing amendment rate. Moreover, in the pot experiments, by adding 1% (w/w) T-BC into soils, the soils benefited from its large adsorption, complex precipitation, and immobilization capacity. Approximately 36% Cd and 29% Pb concentrations of edible parts in pak choi were reduced. The amendment proved promising for the stabilization of Cd and Pb in contaminated soils, while providing a strategy for solving the residual waste of tailings and biomass.

Toward broader applications of iron ore waste in pollution control: Adsorption of norfloxacin

Norfloxacin, a kind of antibiotic frequently detected in environments, represents a group of non-persistent organic pollutants with latent risks to the ecosystem. Iron ore waste, generated and accumulated in large quantities from the iron/steel industry, was evaluated as a potential sorbent for norfloxacin removal. Kinetics analysis showed that the adsorption process reached equilibrium at 72 h, and the adsorption process could be best defined by the pseudo-second-order kinetics with the primary mechanism of norfloxacin adsorption suggested to be cation exchange. Further, adsorption of norfloxacin to iron ore waste was shown to be facilitated by the pH range of 2-10, low cation concentration, and low temperature, which are characteristic of natural surface waters, suggesting the potential of practical applications in aquatic environments. These findings provide new insight into the potentials of beneficial reuse for iron ore waste in the adsorptive removal of environmental pollutants.

Simultaneous adsorption of Cr(VI) and phenol by biochar-based iron oxide composites in water: Performance, kinetics and mechanism

The pollution of heavy metals and organic compounds has received increased attention in recent years. In the current study, a novel biochar-based iron oxide composite (FeYBC) was successfully synthesized using pomelo peel and ferric chloride solution through one-step process at moderate temperature. Results clearly demonstrate that FeYBC exhibited more efficient removal of Cr(VI) and/or phenol compared with the pristine biochar, and the maximum adsorption amounts of Cr(VI) and phenol by FeYBC could reach 24.37 and 39.32 mg g^-1, respectively. A series of characterization data suggests that several iron oxides such as Fe₂O₃, Fe⁰, FeOOH and Fe₃O₄ were formed on the FeYBC surface as well as oxygen-containing groups. Thermodynamics study indicates that Cr(VI) and phenol adsorption by FeYBC were endothermic and exothermic processes, respectively. Langmuir adsorption isotherm and pseudo-second order models could better explain the Cr(VI) and phenol adsorption behaviors over FeYBC. The Cr(VI) adsorption might be primarily achieved through the ion exchange and surface complexation and reduction, whereas the π-π interaction and electron donor-acceptor complex mainly contributed to phenol adsorption. The findings indicate that the biochar-based iron oxide composites material was an efficient adsorbent for the remediation of industrial effluents containing Cr(VI) and phenol.

Red mud for the efficient adsorption of U(VI) from aqueous solution: Influence of calcination on performance and mechanism

Iron-rich red mud is a potent radioactive drainage treatment material. However, the calcite in red mud attenuates its U adsorption capacity by restricting U adsorption onto adsorbent; it captures U as a dissociative complex in aqueous systems. This study produced macroporous iron and carbon combined calcined red mud (ICRM) and carbon calcined red mud (CRM) through calcination in the range of 500-800 °C. XRD results revealed that both series generated advantageous magnetite and calcite were fully decomposed. SEM and batch experiments highlighted ICRM calcined at 600 °C has more stable and favorable performance. The components of post-adsorption ICRM remained active, as demonstrated by FT-IR results. Additionally, ICRM@600 displayed superior U adsorption capacity (59.45 mg/g) than did all red mud adsorbents from our previous research. Zeta-potential results revealed ICRM has positive potential charges in acidic conditions, indicating it adsorbs U(VI) ions via electrostatic attraction. The main adsorption mechanisms of ICRM are surface electrostatic attraction, physical adsorption by porous structure, and chemical adsorption by active Al and Fe components. In application, ICRM@600 obtained a 82.20% U adsorption ratio in uranium mine pit drainage. Overall, this study offers theoretical guidances to radioactive drainage management and red mud reuse.

Removal of cadmium and lead from aqueous solutions by thermal activated electrolytic manganese residues

Electrolytic manganese residues (EMR) is produced from the electrolysis manganese industry. In this study, the thermal activated EMRs (T-EMR) were used to adsorb cadmium and lead from aqueous solution. X-ray diffractometer (XRD), scanning electron microscope-Energy Dispersive Spectrometer (SEM-EDS), X-ray photoelectron spectroscopy (XPS) were adopted to characterize EMR before and after the modification, and the performance and adsorption mechanisms of T-EMR for cadmium and lead were determined. Results show that the pH has a strong influence on the adsorption of cadmium and lead and the maximum adsorption capacity can be achieved at pH 6. The adsorption of Cd(II) can be better fitted by the Lagergren pseudo-first-order dynamic model, while that of Pb(II) fits the pseudo-second-order kinetic model better. The Freundlich isotherm model fits the adsorption of two metals better than Langmuir model. The thermodynamic results demonstrate that the adsorption of Cd(II) or Pb(II) on T-EMR is endothermic and spontaneous. As the nitric acid with pH 0.5 was used, nearly all of the adsorbed Cd(II) and 75% Pb(II) can be desorbed from the loaded T-EMR. It is concluded that the adsorption of Cd(II) and Pb(II) on T-EMR is in virtue of electrostatic attraction, ion-exchange and surface precipitation. The heavy metals are mainly adsorbed on ferric and manganese oxides and silicate minerals in T-EMR by electrostatic attraction. In addition, cadmium and lead also can be adsorbed via the ion exchange reaction. Moreover, some Pb(II) are adsorbed by forming lead sulfate. Thus, T-EMR may be an environmentally-friendly, effective adsorbent for the removal of heavy metals from aqueous solution.

Effect of lignosulfonate on the adsorption performance of hematite for Cd(II)

Lignin is a precursor of humus in soil and sediment. Lignin can be separated from vascular plants in the form of lignosulfonate via pulping processes. On the other hand, composites of iron oxide and organic matter can adsorb heavy metals, and thus influence the migration of these heavy metals in the environment. In this paper, a hematite/lignosulfonate composite (HLS) was prepared via coprecipitation to compare the adsorption performance of hematite (α-Fe₂O₃) toward Cd(II) before and after the incorporation of lignosulfonate (LS). The HLS is found to exhibit a weakly crystalline structure and possess a large number of nanoscale particles. Specific surface area of HLS (291.97 m²/g) is about 11 times that of α-Fe₂O₃, and the pore volume of HLS (0.22 cm³/g) is twice that of α-Fe₂O₃. The adsorption of Cd(II) is well illustrated by the pseudo-second-order adsorption kinetics and the initial adsorption rate (h) of HLS is 13.83 times that of α-Fe₂O₃. The maximum adsorption capacities are significantly improved from 4.89-6.35 mg/g (α-Fe₂O₃) to 39.03-53.65 mg/g (HLS). A greater affinity and more favorable association between Cd(II) and HLS is observed via fitting models. The incorporation of LS provides HLS with significantly better adsorption properties toward Cd(II) than α-Fe₂O₃, as is further confirmed by FT-IR and XPS characterization. Fe-O-O-H and Fe-O-H structures as well as more hydroxyl groups are observed, which promote the adsorption performance since the process are mainly influenced by complexation via coordination bonds.

A semi-industrial experiment of suspension magnetization roasting technology for separation of iron minerals from red mud

Red mud is a type of solid waste derived from the alumina extraction process. It can be considered as a secondary resource for recovering iron values because of its high content of ferric oxide. In this study, an innovative technology called suspension magnetization roasting (SMR) was applied to treat red mud to recycle iron. Based on the lab-scale experimental basis, we adopted the single factor method to perform the semi-industrial scale experiments. Under the optimum conditions, the recovery and grade of iron in the iron concentrate were 95.22 % and 55.54 %, respectively. Chemical phase analysis, vibrating sample magnetometer, and XRD combined with Mössbauer spectroscopy were employed to assess the characteristics of red mud and roasted products. Occupancy of Fe content in magnetite was raised to 89.91 % in SMR products from 0.75 % in the red mud; saturation magnetization was enhanced from 0.40 A·m²/kg to 32.44 A·m²/kg; and the hematite and goethite phase were transformed into Fe₃O₄ (A), Fe₃O₄ (B) and γ-Fe₂O₃ phase. In addition, transmission electron microscopy analysis revealed that both magnetite and maghemite were found in the roasted product. This study demonstrates that SMR is a promising technology for the recovery of iron from red mud.