Efficient removal of Pb(II) ions from aqueous solution by modified red mud

A comprehensive review on phytoremediation of fly ash and red mud: exploring environmental impacts and biotechnological innovations

Fly ash (FA) and red mud (RM) are industrial byproducts generated by thermal power plants and the aluminum industry, respectively. The huge generation of FA and RM is a significant global issue, and finding a safe and sustainable disposal method remains a challenge. These dumps contain harmful trace elements that have a significant impact on the environment and human health. It contributes to air, water, and soil pollution, disrupting the delicate balance of the ecosystems. It also introduces toxins into the food chain through biomagnification. Utilizing a vegetation cover can assist in addressing environmental health concerns associated with FA and RM dumps. Nevertheless, the presence of alkaline pH, toxic metals, the absence of soil microbes, and the pozzolanic properties of both FA and RM pose challenges to plant growth. Taking a comprehensive approach to the ecological restoration of these dumps through phytoremediation is crucial. This review examines the role of various factors in the ecological restoration of FA and RM dumps, specifically the use of naturally occurring plants. However, the issue of slow plant growth due to a lack of nutrients and microbial activities is being resolved through various advances, such as amendments in conjunction with organic matter, microbial inoculants, and the use of genetically modified plants. Research has demonstrated the benefits of using amendments to stimulate vegetation growth on FA and RM dumps. In this review, we explore various approaches to restoring FA and RM dumps and transforming them into productive sites that enhance the ecosystem services.

Waste-treating-waste: Effective heavy metals removal from electroplating wastewater by ladle slag

Ladle slag, a by-product of steelmaking, presents a valuable strategy for waste reduction and valorization in wastewater treatment. This work demonstrates the successful simultaneous removal of Al(III), B(III), Ba(II), Cr(III), Mg(II), Sr(II), Pb(II), and Zn(II), from electroplating wastewater by ladle slag. First, Cr(III) and Pb(II) removals were evaluated in single synthetic systems by analyzing the influence of pH, temperature, and ladle slag dosage. Competitive removal was observed in binary batch experiments of Cr(III) - Pb(II), achieving 88% and 96% removal, respectively, with fast kinetics following a pseudo-second-order model. The findings of XRD, SEM, EDX, and FTIR of the slag after removal helped to elucidate the synergic removal mechanism involving ladle slag dissolution, precipitation, ion exchange, and adsorption in a tight relationship with the solution pH. Lastly, ladle slag was tested in real electroplating wastewater with the aforementioned ions at concentrations ranging from <1 to 1700 mg/L. The removal was performed in two steps, the first attained the following efficiencies: 73% for Al(III), 88% for B(III), 98% for Ba(II), 80% for Cr(III), 82% for Mg(II), 99% for Pb(II), 88% for Sr(II), and 88% for Zn(II). Visual MINTEQ simulation was utilized to identify the different species of ions present during the removal process. Furthermore, the leaching tests indicated a minimal environmental risk of secondary pollution in its application. The results promote an effective and sustainable approach to wastewater treatment within the circular economy.

Assembly of core-shell Fe3O4 @CD-MOFs derived hollow magnetic microcubes for efficient extraction of hazardous substances: Plausible mechanisms for selective adsorption

Hazardous heavy metals and organic substances removal is of great significance for ensuring the safety of aquatic-ecosystem, yet the highly effective and selective extraction always remains challenging. To address this problem, magnetic hollow microcubes were fabricated through thermal carbonization of Fe3O4-COOH@ γ-CD-MOFs, and core-shell structured precursors were in-situ greenly constructed on a large scale via microwave-assisted self-assembly strategy. As noted, the development of secondary crystallization was utilized to achieve uniform dispersion of cores within MOFs frameworks and thus improved magnetic and adsorption ability of composites. Acquired magnetic Fe3O4 @HC not only can harvest excellent extraction of heavy metals (Cd, Pb, and Cu of 129.87, 151.05, and 106.98 mg·g-1) but also exhibit highly selective adsorption ability for cationic organics (separation efficiency higher than 95.0 %). Impressively, Fe3O4 @HC achieved outstanding adsorption (60-80 %) of Cd in realistic mussel cooking broth with no obvious loss in amino acid. Characterizations better offer mechanistic insight into the enhanced selectivity of positively charged pollutants can be attributed to synergistic effect of ions exchange and electrostatic interaction of abundant oxygen-containing functional groups. Our study provides a feasible route by rationally developing core-shell structured composites to promote the practical applications of sustainable water treatment and value-added utilization of processing by-products.

Characterization of phosphate modified red mud-based composite materials and study on heavy metal adsorption

In this paper, Bayer red mud (RM) and lotus leaf powder (LL) were used as the main materials, and KH2PO4 was added to modify the material. Under the condition of high-temperature carbonization, RMLL was prepared and phosphate modified red mud matrix composite (PRMLL) was prepared based on KH2PO4 modification, which can effectively remove Pb2+ from water. The optimum preparation and application conditions were determined through orthogonal experiment: dosage 0.1g, ratio 1:1, and temperature 600 °C. The effects of pH, dosage, and initial concentration on the adsorption of Pb2+ were studied. The pseudo-first-order, pseudo-second-order, and Elovich kinetic models were fitted to the experimental data. It was found that RMLL and PRMLL were more consistent with the pseudo-second-order kinetic model and chemisorption. Langmuir, Freundlich, Timkin, and Dubinin-Radushkevich isothermal adsorption models were used to fit the experimental data. It was found that RMLL and PRMLL were more consistent with Langmuir model. In addition, the maximum adsorption capacity of RMLL and PRMLL was 188.1 mg/g and 213.4 mg/g, respectively. It is larger than the adsorption capacity of their monomers. Therefore, the use of RMLL and PRMLL as the removal of Pb2+ from water is a potential application material.

Enhanced adsorption of Pb2+ by the oxygen-containing functional groups enriched activated carbon

Lead is one of the primary pollutants found in water and poses significant toxicity risks to humans; thus, it is necessary to investigate techniques for removing it economically and efficiently. In order to enhance the removal capacity of Pb2+, coconut shell-based activated carbon (AC) was modified with introducing oxygen-containing functional groups (OFGs) via nitric acid (HNO3) or hydrogen peroxide (H2O2) modification in this study. The characterization results show that after oxidation treatment, the content of OFGs increased, and the textural properties of the samples do not change significantly. This indicates that the modification conditions used in this study effectively introduced OFGs while avoiding the adverse effects on physical adsorption ability of AC caused by oxidation treatment. The Pb2+ adsorption capacities of the AC modified with 10 M HNO3 and 30 wt.% H2O2 were 4.26 and 3.64 times that of the pristine AC, respectively. The experimental data can be well fitted using the Langmuir isotherm model and the Elovich kinetic model, suggesting that the adsorption of Pb2+ on AC belongs to single-layer adsorption, and chemical adsorption dominates the adsorption process. In summary, the hydrothermal-assisted HNO3/H2O2-modified coconut shell-based AC shows great potential in efficiently removing Pb2+ from solutions, offering a solution for utilizing coconut shell waste.

Biotransformation of Pb and As from sewage sludge and food waste by black soldier fly larvae: Migration mechanism of bacterial community and metalloregulatory protein scales

The accumulation and transformation of lead (Pb) and arsenic (As) during the digestion of sewage sludge (SS) by black soldier fly larvae (BSFL) remain unclear. In this study, we used 16 s rRNA and metagenomic sequencing techniques to investigate the correlation between the microbial community, metalloregulatory proteins (MRPs), and Pb and As migration and transformation. During the 15-day test period, BSFL were able to absorb 34-48 % of Pb and 32-45 % of As into their body. Changes in bacterial community abundance, upregulation of MRPs, and redundancy analysis (RDA) results confirmed that ZntA, EfeO, CadC, ArsR, ArsB, ArsD, and ArsA play major roles in the adsorption and stabilization of Pb and As, which is mainly due to the high contribution rates of Lactobacillus (48-59 %) and Enterococcus (21-23 %). Owing to the redox reaction, the regulation of the MRPs, and the change in pH, the Pb and As in the BSFL residue were mainly the residual fraction (F4). The RDA results showed that Lactobacillus and L.koreensis could significantly (P < 0.01) reduce the reducible fraction (F2) and F4 of Pb, whereas Firmicutes and L.fermentum can significantly (P < 0.05) promote the transformation of As to F4, thus realizing the passivation Pb and As. This study contributes to the understanding of Pb and As in SS adsorbed by BSFL and provides important insights into the factors that arise during the BSFL-mediated migration of Pb and As.

Formulation Optimization and Performance Prediction of Red Mud Particle Adsorbents Based on Neural Networks

Red mud (RM), a bauxite residue, contains hazardous radioactive wastes and alkaline material and poses severe surface water and groundwater contamination risks, necessitating recycling. Pretreated RM can be used to make adsorbents for water treatment. However, its performance is affected by many factors, resulting in a nonlinear correlation and coupling relationship. This study aimed to identify the best formula for an RM adsorbent using a mathematical model that examines the relationship between 11 formulation types (e.g., pore-assisting agent, component modifier, and external binder) and 9 properties (e.g., specific surface area, wetting angle, and Zeta potential). This model was built using a back-propagation neural network (BP) based on single-factor experimental data and orthogonal experimental data. The model trained and predicted the established network structure to obtain the optimal adsorbent formula. The RM particle adsorbents had a pH of 10.16, specific surface area (BET) of 48.92 m2·g-1, pore volume of 2.10 cm3·g-1, compressive strength (ST) of 1.12 KPa, and 24 h immersion pulverization rate (ηm) of 3.72%. In the removal of total phosphorus in flotation tailings backwater, it exhibited a good adsorption capacity (Q) and total phosphorous removal rate (η) of 48.63 mg·g-1 and 95.13%, respectively.

Industrial waste-based adsorbents as a new trend for removal of water-borne emerging contaminants

Emerging contaminants in wastewater are one of the growing concerns because of their adverse effects on human health and ecosystems. Adsorption technology offers superior performance due to its cost-effectiveness, stability, recyclability, and reliability in maintaining environmental and health standards for toxic pollutants. Despite extensive research on the use of traditional adsorbents to remove emerging contaminants, their expensiveness, lack of selectivity, and complexity of regeneration remain some of the challenges. Industrial wastes viz. blast furnace slag, red mud, and copper slag can be used to develop efficacious adsorbents for the treatment of emerging contaminants in water. Advantages of the use of such industrial wastes include resource utilization, availability, cost-effectiveness, and waste management. Nevertheless, little is known so far about their application, removal efficacy, adsorption mechanisms, and limitations in the treatment of emerging contaminants. A holistic understanding of the application of such unique industrial waste-derived adsorbents in removing emerging contaminants from water is need of the hour to transform this technology from bench-scale to pilot and large-scale applications. This review investigates different water treatment techniques associated with industrial waste-based adsorbents derived from blast furnace slag, red mud, and copper slag. Besides, this review provides important insights into the growing trends of utilizing such novel types of adsorbents to remove emerging contaminants from water with an emphasis on removal efficacy, controlling measures, adsorption mechanisms, advantages, and limitations. The present timely review brings the current state of knowledge into a single reference which could be a strong platform for future research in understanding the latest advancements, decision making, and financial management related to the treatment of wastewater using industrial waste-based adsorbents.

Weathered Coal-Immobilized Microbial Materials as a Highly Efficient Adsorbent for the Removal of Lead

Most research on immobilized microorganisms employs biomass charcoal as a carrier, but limited studies explore coal-based resources for microbial immobilization. Herein, lead-resistant functional strains were immobilized using weathered coal as a carrier, resulting in the development of a weathered coal-immobilized microbial material (JK-BW) exhibiting high efficiency in lead removal from solutions. A quadratic polynomial model for the adsorption capacity and adsorption rate of JK-BW on Pb2+ was developed using the Box-Behnken method to determine the optimal adsorption conditions. The Pb2+ adsorption mechanism of JK-BW was studied through batch adsorption and desorption experiments along with SEM-EDS, BET, FT-IR, and XPS analyses. Findings indicated that optimal conditions were identified at 306 K temperature, 0.36 g/L adsorbent dosage, and 300 mg/L initial solution concentration, achieving a peak adsorption performance of 338.9 mg/g (308 K) for the immobilized material, surpassing free cell adsorption by 3.8 times. Even after four cycles of repeated use, the material maintained its high adsorption capacity. Pb2+ adsorption by JK-BW involved monolayer chemisorption with ion exchange, complexation, precipitation, physical adsorption, and microbial intracellular phagocytosis. Ion exchange accounted for 22-42% and complexation accounted for 39-57% of the total adsorption mechanisms, notably involving exchanges with K, Ca, Na, and Mg ions as well as complexation with -OH, -COOH, CO-OH, -COOH, CO-, NH2, and the β-ring of pyridine for Pb2+ adsorption.

Application of common industrial solid waste in water treatment: a review

Industrial solid waste has a wide range of impacts, and it is directly or indirectly related to land, atmosphere, water, and other resources. Industrial solid waste has a large amount of production, complex and diverse components and contains a variety of harmful substances. However, as industrial by-products, it also has a lot of available value. Industrial solid waste has been continuously studied in water treatment due to its special composition and porous and loose structure. It is known that there are few reviews of various industrial solid wastes in the field of wastewater treatment, and most of them only discuss single industrial solid waste. This paper aims to sort out the different studies on various solid wastes such as fly ash, red mud, wastewater sludge, blast furnace slag and steel slag in dyeing, heavy metal, and phosphorus-containing wastewater. Based on the modification of industrial solid waste and the preparation of composite materials, adsorbents, coagulants, catalysts, filtration membranes, geological polymers, and other materials with high adsorption properties for pollutants in wastewater were formed; the prospect and development of these materials in the field of wastewater were discussed, which provides some ideas for the mutual balance of environment and society. Meanwhile, some limitations of solid waste applications for wastewater treatment have been put forward, such as a lack of further researches about environment-friendly modification methods, application costs, the heavy metal leaching, and toxicity assessment of industrial solid waste.

Assessing the potential of red mud and dehydrated mineral mud mixtures as soil matrix for revegetation

The disposal of red mud (RM) and dehydrated mineral mud (DM) presents a significant challenge for the global alumina industry. This study proposes a novel disposal method for RM and DM, which uses mixtures of RM and DM as a soil matrix for revegetation in the mining area. RM mixed with DM effectively alleviated its salinity and alkalinity. X-ray diffraction analysis indicated that reduction of salinity and alkalinity may be due to the release of chemical alkali from sodalite and cancrinite. Applications of ferric chloride (FeCl3), gypsum, and organic fertilizer (OF) improved the physicochemical properties of the RM-DM mixtures. FeCl3 significantly reduced available Cd, As, Cr, and Pb content in the RM-DM, while OF significantly increased the cation exchange capacity, microbial carbon and nitrogen, and aggregate stability (p < 0.05). Micro-computed tomography and nuclear magnetic resonance analysis showed that amendment with OF and FeCl3 increased the porosity, pore diameter, and hydraulic conductivity in the RM-DM mixture. The RM-DM mixtures had low leaching of toxic elements, indicating low environmental risk. Ryegrass grew well in the RM-DM mixture at a ratio of 1:3. OF and FeCl3 significantly increased the ryegrass biomass (p < 0.05). These results suggested that RM-DM amended with OF and FeCl3 has a potential application in the revegetation of areas after bauxite mining.

Functional biochar fabricated from red mud and walnut shell for phosphorus wastewater treatment: Role of minerals

A novel functional biochar (BC) was prepared from industrial waste red mud (RM) and low-cost walnut shell by one facile-step pyrolysis method to adsorb phosphorus (P) in wastewater. The preparation conditions for RM-BC were optimized using Response Surface Methodology. The adsorption characteristics of P were investigated in batch mode experiments, while a variety of techniques were used to characterize RM-BC composites. The impact of key minerals (hematite, quartz, and calcite) in RM on the P removal efficiency of the RM-BC composite was studied. The results showed that RM-BC composite produced at 320 °C for 58 min, with a 1:1 mass ratio of walnut shell and RM, had a maximum P sorption capacity of 15.48 mg g^-1, which was more than double that of the raw BC. The removal of P from water was found to be facilitated significantly by hematite, which forms Fe-O-P bonds, undergoes surface precipitation, and exchanges ligands. This research provides evidence for the effectiveness of RM-BC in treating P in water, laying the foundation for future scaling-up trials.

Effective recovery of Ti as anatase nanoparticles from waste red mud via a coupled leaching and boiling route

Red mud (RM) a solid waste generated by the bauxite smelting industry, is a rich source of metal resources, especially Ti, and its recycling can bring significant environmental and economic benefits. In this study, precious metal Ti was efficiently recovered from red mud using a coupled acid leaching and boiling route for the effective separation of low-value metals. The red mud which contained mainly 10.69% Si, 12.1% Al, 15.2% Ca, 10.99% Fe, and 4.37% Ti, was recovered in five steps. First, a nitric acid solution was used to leach the metals in multiple stages, resulting in an acidic leach solution with high concentrations of Fe, Al, Ti, and Ca ions 2.7 g/L, 4.7 g/L, 5.43 g/L, and 1.8 g/L, respectively. Then, a small amount of sucrose was added as a catalyst to recover Ti from the leach solution under hydrothermal conditions, resulting in the targeted recovery of 98.6% of Ti in the form of high-purity anatase while Fe, Al, and Ca remained in the solution. Next, the Fe in solution was separated as hematite products at a temperature of 110°C and a reaction time of 4 h. Similarly, the Al in the solution was separated and precipitated as boehmite by heating it at 260°C for a reaction time of 20 h. Finally, the remaining Ca in solution was recovered by simple pH regulation. Economic accounting assessment showed that the method yields $101.06 for 1 t of red mud treated, excluding labor costs. This study provides a novel approach to recover precious metals from metal wastes through the whole process resource recovery of solid waste red mud.

A strategy-purifying wastewater with waste materials: Zn2+ modified waste red mud as recoverable adsorbents with an enhanced removal capacity of congo red

The strategy, called purifying wastewater with waste materials (PWWM), can simultaneously improve the secondary utilization of industrial waste materials and in turn, reduce environmental pollution. However, the PWWM strategy has still not been extensively used because of its low purification efficiency of organic pollutants and extremely difficult secondary utilization process. Herein, we use zinc aluminum silicate (ZAS) to modify waste granular red mud (GRM) to form a recoverable adsorbent, called ZAS/GRM adsorbent. The ZAS has been found to exhibit exceptional adsorption performance with the ability to firmly anchor onto the surface of GRM, in which heavy metal ions can effectively solidify and reduce their outflow. Furthermore, many voids have been tactfully designed in the ZAS/GRM adsorbents by using a water vapor project, which provide more active sites for congo red (CR) organic dye, thereby remarkably improving the removal efficiency of CR. From our purification of CR, we find that the CR adsorption capacity of the ZAS/GRM adsorbent is 3.509 mg g^-1, which is four times higher than pure GRM (0.820 mg g^-1). This study demonstrates our PWWM strategy is highly effective and can inspire more research on waste reuse.

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

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

Two recyclable and complementary adsorbents of coal-based and bio-based humic acids: High efficient adsorption and immobilization remediation for Pb(II) contaminated water and soil

Humic acid can effectively bind heavy metals and is a promising remediation agent for heavy metals-contaminated water and soil. Many successful applications of humic acid have been reported, but rarely studied the specific process and mechanism of heavy metal removal by humic acids from water and soil, especially the simultaneous application of coal-based and bio-based humic acids. In this work, two kinds of coal-based and bio-based humic acid materials (CHA and BHA) from weathered coal and rice husk were industrially produced and studied their Pb(II) adsorption and immobilization characteristics and mechanisms in water and soil. The batch adsorption experiments obtained the Pb(II) adsorption by CHA and BHA both were spontaneous and endothermic monolayer chemisorption and controlled by three rate-limiting steps (bulk, film, and pore) in the adsorption process. CHA and BHA had highly efficient Pb(II) adsorption capacities, obtained their maximum adsorption capacity was 201 and 188 mg g^-1, respectively. In addition to the two main adsorption mechanisms of ion exchange and surface complexation, electrostatic interaction, precipitation reaction, and π-π interaction were also involved. Soil culture experiments showed that CHA and BHA both exhibited a highly efficient immobilization effect on Pb(II)-contaminated soil, and CHA and BHA had a better synergistic promotion effect. Compared with the CK soil, the content of DTPA-Pb(II) decreased by 10.2-13.2% and the content of RES-Pb(II) increased by 14-22% in soils treated with different humic acids. Ion exchange, complexation, precipitation, and electrostatic attraction promote the transformation of unstable Pb(II) to stable Pb(II), which was of great significance for the immobilization of Pb(II) in soil. Overall, CHA and BHA have the potential to be used as green, efficient, and promising adsorbents to remove and immobilize Pb(II) from wastewater and soil.

Pb(II) bioremediation using fresh algal-bacterial aerobic granular sludge and its underlying mechanisms highlighting the role of extracellular polymeric substances

Lead (Pb) discharged from rural industries poses a significant threat to the environment and human health. Algal-bacterial aerobic granular sludge (A-B AGS) is a promising alternative for sewage treatment with high efficiency and good settleability. In this study, Pb(II) biosorption using fresh A-B AGS was investigated for the first time. The important role of extracellular polymeric substances (EPS) was revealed with the involved mechanisms being clarified. The desorbents for Pb recovery from Pb-loaded A-B AGS were also screened. Results showed that A-B AGS has an excellent maximum Pb adsorption capacity of 72.4 mg·g^-1 at pH 6.0. EPS plays an important role in keeping microbial activity, Pb bonding, and providing metal ions (Ca, Na and Mg) for Pb ion exchanges. Electrostatic interaction, ion exchange, and bonding to functional groups may occur orderly in the Pb biosorption process and the formation of pyromorphite (Pb₅(PO₄)₃Cl) contributes to Pb biosorption. About 66 % of the adsorbed Pb was accumulated in the A-B AGS microbial cells. Na₂EDTA (0.05 M) can recover 60.3 % of the loaded Pb with the highest microbial activity of granules being remained. All the findings will provide the theoretical basis for the large-scale application of A-B AGS to bioremediate Pb(II)-containing wastewater.

Strategies for microbial bioremediation of environmental pollutants from industrial wastewater: A sustainable approach

Heavy metals are hazardous and bring about critical exposure risks to humans and animals, even at low concentrations. An assortment of approaches has been attempted to remove the water contaminants and keep up with water quality, for that microbial bioremediation is a promising way to mitigate these pollutants from the contaminated water. The flexibility of microorganisms to eliminate a toxic pollutant creates bioremediation an innovation that can be applied in various water and soil conditions. This review insight into the sources, occurrence of toxic heavy metals, and their hazardous human exposure risk. In this review, significant attention to microbial bioremediation for pollutant mitigation from various ecological lattices has been addressed. Mechanism of microbial bioremediation in the aspect of factors affecting, the role of microbes and interaction between the microbes and pollutants are the focal topics of this review. In addition, emerging strategies and technologies developed in the field of genetically engineered micro-organism and micro-organism-aided nanotechnology has shown up as powerful bioremediation tool with critical possibilities to eliminate water pollutants.

A high-performance fluorescent hybrid material for fluorometric detection and removal of toxic Pb(ii) ions from aqueous media: performance and challenges

Lead(ii) is an extremely toxic heavy metal ion that causes various health problems that are difficult to recover from in many developing countries of the world. Fluorescence-based nanosensors have amazing characteristics such as high sensitivity/selectivity, portability, low detection limit, rapid on-site usability, low cost and capability for removal of heavy metal ions. In this paper, a new fluorescent hybrid material based on silica gel (Bodipy-Si) was developed via a click reaction between alkyne-terminal silica gel and azido-terminal Bodipy. The solid support surface was characterized by various techniques such as SEM, FT-IR, etc. The adsorption and fluorometric properties of the fluorescent nanoparticles were also examined using atomic absorption and fluorescence spectroscopies, and in the presence of metal ions, respectively. The results indicated that the prepared hybrid-fluorescent nanoparticles can be used in the removal and detection of toxic Pb(ii) ions. The limit of detection (LOD) was determined from the fluorescence data as 1.55 × 10-7 M and the maximum adsorption capacity was examined by AAS. The complexometric interactions between Pb(ii) and Bodipy-Si affect the adsorptions of the Pb(ii) metal ion at various concentrations.

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.

Application of Mesopore-Activated Red Mud for Phosphorus Adsorption

In this paper, the mesopore-activated red mud (M-ARM) was prepared by treating red mud (RM) with acid under an ultrasonic batch and through heat treatment at 750 C. The surface area and adsorption average pore width of M-ARM were calculated and obtained values of 13.408 m2 g-1 and 25.160 nm, respectively. Therefore, the maximum adsorption capacity of M-ARM for phosphorus was $200.85\pm 1.66\text{mg}{\text{g}}^{-1}$ at 318 K and $\text{pH}=5$ . At a low initial concentration (75 mg L-1), the phosphorus removal capacity by M-ARM material was up to $97.09\pm 0.15\%$ at 313 K. With the temperature scales varying from 298 to 313 K, the values of Gibbs free energy change ( $\Delta G°$ ) were negative and also vary from -37.47 to -36.68. The phosphorus adsorption process in an aqueous solution is spontaneous, and this adsorption process was exothermic with enthalpy change $∆{\text{H}}^{\text{o}}=-14.36$ . From the results of investigations and calculations of thermodynamic values, kinetics, and adsorption capacity of materials, we can confirm that the materials in this study had a low-cost and potential material for applications to treat phosphorus-contaminated water. In addition, the adsorption kinetics of this material for phosphorus were also studied and discussed.

Amino-modified chitosan/gold tailings composite for selective and highly efficient removal of lead and cadmium from wastewater

In this work, a novel amino-modified chitosan/tailings composite (CS-PEI-nGT) was successfully synthesized from gold tailings particle treated by ball milling (nGT), chitosan (CS) and polyethyleneimine (PEI) as raw materials, for Lead (Pb(Ⅱ)) and Cadmium (Cd(Ⅱ)) removal from aqueous solutions. The CS-PEI-nGT was characterized by using FTIR, XRD, SEM, BET, TGA and XPS techniques. The results showed that CS-PEI-nGT had maximum adsorption capacity of 192.78 mg·g^-1 and 99.46 mg·g^-1 for Pb(Ⅱ) and Cd(Ⅱ) respectively at pH 5. The adsorption kinetics was described well by pseudo-second-order kinetic adsorption model, and suggested that chemisorption as the rate-controlling step for adsorption of Pb(Ⅱ) and Cd(Ⅱ). The isotherm data was accurately explained by Langmuir model with higher correlation coefficient (R²) of 0.9911 and 0.9642 for Pb(Ⅱ) and Cd(Ⅱ) respectively. In addition, CS-PEI-nGT retained its selective adsorption capacity for Pb(Ⅱ) and Cd(Ⅱ), compared to other metals such as Zn(Ⅱ), Mn(Ⅱ), Mg(Ⅱ) and Al(Ⅲ). The mechanism of the adsorption was investigated and the results revealed that amino (-NH₂), silicon oxide groups (Si-O) and hydroxyl (-OH) functional groups on composite surface were accountable for metals adsorption, suggesting surface complexation, electrostatic interactions and ion exchange. Our work presents a promising strategy for tailings recycling and highly efficient removal of toxic metals ions from wastewater.

Synthesis and Characterization of Colistin-Functionalized Silica Materials for Rapid Capture of Bacteria in Water

In this study, a new colistin-functionalized silica gel material (SiO₂@NH₂@COOH@CST) was synthesized after carboxylation on the surface of amino-modified silica. The main factors affecting the adsorptive properties of the material, such as the types of linkers, the linking methods, the reaction buffers and the particle sizes of carriers, were systematically investigated. The SiO₂@NH₂@COOH@CST was characterized by means of electron microscopy, Fourier-transform infrared spectroscopy, zeta potential measurements, etc. We demonstrated that the sorbent showed good adsorption of Gram-negative bacteria. The adsorption efficiency of E. coli on SiO₂@NH₂@COOH@CST was 5.2 × 10¹¹ CFU/g, which was 3.5 times higher than that on SiO₂@NH₂@COOH, suggesting that electrostatic interactions between SiO₂@NH₂@COOH@CST and E. coli played a key role. The adsorption was quick, and was reached in 5 min. Both pseudo-first-order and pseudo-second-order kinetic models fit well with the dynamic adsorption process of SiO₂@NH₂@COOH@CST, indicating that physical adsorption and chemisorption might occur simultaneously during the adsorption process. SiO₂@NH₂@COOH@CST was successfully applied for the rapid capture of bacteria from water. The synthesized material could be used as a potential means of bacterial isolation and detection.

A novel utilization of raw sepiolite: preparation of magnetic adsorbent directly based on sol-gel for adsorption of Pb(II)

The constraints of industrial separation technology for low grade sepiolite greatly limit the development and utilization of these potential resources. In this work, a novel sepiolite adsorbent loaded with copper ferrite was prepared by sol-gel method to remove Pb(II) from wastewater. The effects of various factors on Pb(II) removal ratio were investigated. The maximum adsorption capacities at 293, 313, and 333 K were 1285.32, 1325.45, and 1390.54 mg/g, respectively. The adsorption of Pb(II) by magnetic sepiolite was a spontaneous endothermic process. Besides, the adsorption process followed Langmuir isothermal adsorption model and pseudo-second-order kinetic model. The main adsorption mechanism of Pb(II) removal was electrostatic attraction, ion exchange, and surface complexation. The improvement of Pb(II) absorption indicated that the efficient removal of Pb(II) can be realized by phosphate groups introduced in the preparation process and the carbonate groups contained in gangue minerals.

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.

Dissolution behavior of representative elements from red mud (RM) by leaching with titanium white waste acid (TWWA)

Titanium white waste acid (TWWA) was used to dissolve the representative elements from red mud (RM) to achieve the goal of "treating waste with waste." The leaching parameters on the leaching efficiency of Na, Sc, and Al were investigated, in which the analysis of XRD and SEM-EDS on RM and leaching residue was performed. The leaching kinetics of Na, Sc, and Al was studied with unreacted shrinking core model (USCM). The results show that the dealkalization efficiency was close to 100%, and the leaching efficiency of Sc and Al was 82% and 75%, respectively. Cancrinite was dissolved from RM, and then the elements such as Na, Al, and Ca reacted with H₂SO₄ of TWWA. Na existed in the leaching liquor in the form of ions. Ca reacted with sulfuric acid to form anhydrite, which existed in the leaching residue. The particles of RM became smaller and dispersed with each other by acid leaching. The leaching apparent activation energy of Na, Sc, and Al was 4.947 kJ/mol, 6.361 kJ/mol, and 31.666 kJ/mol, respectively. The leaching kinetic equation of Na, Sc, and Al was 1 - (1 - a)^2/3 = 0.084·exp[- 595.05/T]·t by external diffusion, 1 - 2a/3 - (1 - a)^2/3 = 0.021·exp[- 765.16/T]·t by internal diffusion, and ln(1 - a)/3 + (1 - a)^-2/3 - 1 = 67.12·exp[- 3808.8/T]·t by joint action, respectively.

Experimental Investigation on Red Mud from the Bayer Process for Cemented Paste Backfill

Red mud is a by-product of alumina production, and its disposal can have severe environmental consequences. This study experimentally investigates the feasibility of using red mud from the Bayer process for cemented paste backfill (CPB). Different binders and activators were used to improve the mechanical properties, water resistance, and environmental behaviors of red mud-based CPB. In addition, water immersion tests were introduced, for the first time, to evaluate the water resistance of CPB. Furthermore, the environmental behaviors of red mud-based CPB were investigated by conducting leaching experiments. The results showed that the red-mud specimens had an unconfined compressive strength (UCS) of less than 0.2 MPa and disintegrated after being immersed in water. Different binders significantly improved the mechanical properties of red mud-based CPB. In addition, the specimens with different binders showed excellent water resistance, and the softening coefficient of CPB with different binders could exceed 0.7 after being cured for 28 days. The binders exhibited a substantial inhibitory effect on the leaching of hazardous substances in red mud under the solidification and stabilization effects. The leaching concentration of hexavalent chromium, selenium, fluoride, arsenic, lead, and vanadium was reduced by more than 70%. Therefore, this study provides an effective method for the environmental-friendly and large-scale utilization of red mud from the Bayer process.

Quadruple analyte responsive platform: Point-of-care testing and multi-coding logic computation based on metal ions recognition and selective response

While it is recognized that instrumentation techniques can provide precise and sensitive solutions to heavy metal ion monitoring, it remains challenging to transform laboratory testing into a convenient, on-site, and quantitative sensing platform for point-of-care testing (POCT) in a resource-constrained setting. To address these limitations, an affordable and user-friendly colorimetric POCT sensing system is proposed here for selectively monitoring four metal ions (Fe³⁺, Co²⁺, Pb²⁺, and Cd²⁺) based on the sulfur quantum dots (S dots). Quadruple distinct visual signals (green, brown, precipitation, and bright yellow) are presented on the fabricated paper-based analytical devices (PADs) when mixing S dots and metal ions. The high-quality photographs of the PADs are captured by a scanner, while a smartphone App converts visual signals to HSV values. The quantitative analysis relies on the digital colorimetric reading, and the limits of detection are 0.59, 0.47, 0.82, and 0.53 μM for Fe³⁺, Co²⁺, Cd²⁺, and Pb²⁺, respectively. This metal ions-responsive platform is engineered as a smart strategy for multiple logic operations (YES, NOT, AND, INHIBIT, and NOR) by integrating multi-responsive blocks into the S dots with encoded patterns, which improves the computing capability. Accordingly, this strategy demonstrates its potential for on-site environmental testing and sophisticated molecular computation.

Remediation of Pb-contaminated soil using modified bauxite refinery residue

This study examines amendment of Pb-contaminated soil with modified bauxite refinery residue (MBRR) to decrease soil Pb mobility and bioaccessibility. Amendment experiments were conducted using four soils contaminated with Pb from various sources, including smelting, shooting-range activities and Pb-based paint waste. Lead L₃-edge X-ray absorption spectroscopy (XAS) indicated that Pb speciation in these soils was a mixture of Pb sorbed to Fe (hydr)oxide and clay minerals, along with Pb bound to organic matter. Amendment with MBRR decreased water-soluble Pb and/or Toxicity Characteristic Leachate Procedure (TCLP) Pb concentrations. Lead L₃-edge XAS and X-ray diffraction (XRD) indicated that Pb retention by MBRR occurred via sorption to Fe- and Al-(hydr)oxides at low Pb loadings, in addition to formation of hydrocerussite (Pb₃(CO₃)₂(OH)₂) at high loadings. Soil amendment with MBRR had relatively little effect on gastric-phase Pb bioaccessibility; as quantified via the Solubility/Bioavailability Research Consortium, SBRC, in vitro assay. In contrast, amendment with MBRR caused substantial decreases in relative intestinal-phase Pb bioaccessibility (Rel-SBRC-I) due to increased Pb sorption by MBRR's Fe- and Al-hydr(oxide) minerals as simulated GI tract conditions shifted from the gastric- to the intestinal-phase. These decreases in Rel-SBRC-I point to the potential efficacy of using amendment with MBRR to decrease soil Pb bioavailability.

Magnetic sporopollenin supported polyaniline developed for removal of lead ions from wastewater: Kinetic, isotherm and thermodynamic studies

This study evaluated the synthesis of novel binary functionaladsorbent based on sporopollenin, magnetic nanoparticles, and polyaniline to produce MSP-PANI. The MSP-PANI was applied to enhance uptake of lead ions (Pb²⁺) from wastewater samples. The functionalities, surface morphology, magnetic properties, and elemental composition of the newly synthesized nanocomposite were investigated using Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), vibration sample magnetometer (VSM), and energy-dispersive X-ray spectroscopy (EDX), respectively. The experimental condition for the adsorption process was MSP/PANI ratio 1:1, pH ∼6, adsorbent dosage 40 mg, and contact time 90 min at room temperature. Under the proposed condition, lead ions removal were obtained as 83%, 88% and 95% for MSPE, PANI, and MSP/PANI, respectively. Based on the experimental and predicted data, the adsorption was corresponded to the psudo-second-order (R² = 0.999) kinetics model, and the adsorption equilibrium corresponded to the Langmuir model (R² = 0.996). Langmuir isotherm showed the maximum adsorption capacity of MSP-PANI for lead ions was 163 mg/g and followed the monolayer pattern. Hence, thermodynamic model under Van't Hoff equation suggested that the adsorption mechanism was physio-sorption with endothermic nature. Therefore, this research can help the researchers to use magnetic nanoparticles for lead removal in highly polluted areas.

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.

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.

Removal of toxic heavy metals using genetically engineered microbes: Molecular tools, risk assessment and management strategies

The direct release of industrial effluent into the water and other anthropogenic activities causes water pollution. Heavy metal ions are the primary contaminant in the industrial effluents which are exceptionally toxic at low concentrations, terribly disturb the endurance equilibrium of activities in the eco-system and be remarkably hazardous to human health. Different conventional treatment methodologies were utilized for the removal of toxic pollutants from the contaminated water which has several drawbacks such as cost-ineffective and lower efficiency. Recently, genetically modified micro-organisms (GMMs) stand-out for the removal of toxic heavy metals are viewed as an economically plausible and environmentally safe technique. GMMs are microorganisms whose genetic material has been changed utilizing genetic engineering techniques that exhibit enhanced removal efficiency in comparison with the other treatment methodologies. The present review comments the GMMs such as bacteria, algae and fungi and their potential for the removal of toxic heavy metals. This review provides current aspects of different advanced molecular tools which have been used to manipulate micro-organisms through genetic expression for the breakdown of metal compounds in polluted areas. The strategies, major limitations and challenges for genetic engineering of micro-organisms have been reviewed. The current review investigates the approaches working on utilizing genetically modified micro-organisms and effective removal techniques.

Effective removal of Pb(II) ions using piperazine-modified magnetic graphene oxide nanocomposite; optimization by response surface methodology

In this research, the piperazine-modified magnetic graphene oxide (Pip@MGO) nanocomposite was synthesized and utilized as a nano-adsorbent for the removal of Pb(II) ions from environmental water and wastewater samples. The physicochemical properties of Pip@MGO nanocomposite was characterized by X-ray diffraction analysis (XRD), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Energy-dispersive X-ray spectroscopy (EDAX), Thermo-gravimetric analysis (TGA), Vibrating Sample Magnetometery (VSM) and Fourier-transform infrared spectroscopy (FT-IR) analysis. In this method, the batch removal process were designed by response surface methodology (RSM) based on a central composite design (CCD) model. The results indicated that the highest efficiency of Pb(II) removal was obtained from the quadratic model under optimum conditions of prominent parameters (initial pH 6.0, adsorbent dosage 7 mg, initial concentration of lead 15 mg L⁻¹ and contact time 27.5 min). Adsorption data showed that lead ions uptake on Pip@MGO nanocomposite followed the Langmuir isotherm model equation and pseudo-second order kinetic model. High adsorption capacity (558.2 mg g⁻¹) and easy magnetic separation capability showed that the synthesized Pip@MGO nanocomposite has great potential for the removal of Pb(II) ions from contaminated wastewaters.

Pyrolysis temperature and feedstock affected Cr(VI) removal capacity of sulfidated zerovalent iron: Importance of surface area and electrical conductivity

Herein, feedstock (pinewood, rice straw, and dairy manure) and pyrolysis temperature (300, 500, and 700 °C) were selected as the influencing factors of properties of biochar (BC) to identify the contribution of biochar's matrix on Cr(VI) removal by BC-supported sulfidated zero-valent iron (S-ZVI/BC). Results showed that higher temperature was more conducible to improve the electrochemical properties and specific surface areas of composites. Raman spectra of S-ZVI supported by pinewood-derived BC (S-ZVI/PBC) showed the I_D/I_G ratio increased from 0.639 to 0.975 for the composites prepared at 300-700 °C, indicating the increased structural defects and resulting in the greatest Cr(VI) removal (35.81 mg g^-1) and reduction (30.21 mg g^-1) amounts of S-ZVI/PBC700. Besides, S-ZVI/PBC exhibited greater electrochemical reactivity and surface area than S-ZVI harbored by BC from dairy manure and rice straw. Additionally, Pearson correlation analysis revealed that Cr(VI) removal was significantly positively correlated to surface area (R² = 0.90) and negatively correlated to Tafel corrosive potential (R² = 0.88). Both desorption experiment and XPS spectra of spent sorbents showed that reduction predominated the detoxifying mechanism of Cr(VI) followed by adsorption (due to corrosively-generated iron oxides and BC) and precipitation (Cr₂S₃). This suggested that biochar with greater specific surface area and electrical conductivity is more favorable to immobilize S-ZVI with respect to Cr(VI) removal.

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.