Modification of red mud by acid treatment and its application for CO removal

Red mud-based catalysts for the catalytic removal of typical air pollutants: A review

Red mud, as a solid waste produced during the alumina production, can cause severe eco-environmental pollution and health risks to human. Therefore, the resourcing of this type of solid waste is an effective way for the sustainable development. This paper reviews the recent progress on red mud-based catalysts for the removal of typical air pollutants, such as the catalytic reduction of nitrogen oxides (NO_x) by NH₃ (NH₃-SCR) and the catalytic oxidation of CO and volatile organic compounds (VOCs). The factors influencing the catalytic performance and the structure-activity relationship have been discussed. Future prospects and directions for the development of such catalysts are also proposed. This review would benefit for the high value-added utilizations of red mud in mitigating atmospheric pollutions.

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.

Activation of bisulfite by LaFeO3 loaded on red mud for degradation of organic dye

In this study, red mud (RM) was used as a support for LaFeO₃ to prepare LaFeO₃-RM via the ultrasonic-assisted sol-gel method for the removal of methylene blue (MB) assisted with bisulfite (BS) in the aqueous solution. Characterization by scanning electron microscopy and the Brunauer-Emmett-Teller method indicated that LaFeO₃-RM exhibited a large surface area and porous structure with a higher pore volume (i.e. 10 times) compared with the bulk LaFeO₃. The XRD, XPS and FTIR results revealed that the support of porous RM not only dispersed LaFeO₃ particles but also increased Fe oxidation capability, oxygen-containing functional groups and chemically adsorbed oxygen (from 44.3% to 90.3%) of LaFeO₃-RM, which improved the catalytic performance in structure and chemical composition. MB was removed through the synergistic effect of adsorption and catalysis, with MB molecules first absorbed on the surface and then degraded. The removal efficiency was 88.19% in the LaFeO₃-RM/BS system under neutral conditions but only 27.09% in the LaFeO₃/BS system. The pseudo-first-order kinetic constant of LaFeO₃-RM was six times higher than that of LaFeO₃. Fe(III) in LaFeO₃-RM played a key role in the activation of BS to produce SO 4 ⋅ - by the redox cycle of Fe(III)/Fe(II). Dissolved oxygen was an essential factor for the generation of SO 4 ⋅ - . This work provides both a new approach for using porous industrial waste to improve the catalytic performance of LaFeO₃ and guidance for resource utilization of RM in wastewater treatment.

Conversion of alkaline characteristics of bauxite residue by mechanical activated pretreatment: Implications for its dealkalization

Following the strict environmental policies of various countries, the strong alkalinity of bauxite residue (BR) has become a worldwide problem limiting the sustainable development of the global alumina industry. Continuous conversion of solid-phase alkalinity to free alkali is a major challenge for BR dealkalization to reduce its environmental impact. This work aimed to investigate the effect of mechanical grinding pretreatment on the transformation mechanisms of alkaline solids to free alkali at the BR interface under acids leaching, by monitoring the morphology, phase, and speciation transformations of Al and Si using primarily cross-section scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) elemental mapping, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The results indicated that particle grinding wrapped some of the alkaline minerals inside the particles to inhibit its release process. The leaching kinetics revealed the order of the buffering effect of minerals against acids leaching is firstly dissolved by minerals containing Na and Ca via the ion-exchange process, followed by Si and Al through the hydrolysis of the desilicated products. The mineral dissemination characteristics and surface compositions further confirmed the undissolved minerals block the interface reaction between embedded alkaline solids and acids to result in the difficult reaction dissolution of alkaline minerals, which is induced by ball milling. This novel approach provides new insight into the efficient dealkalization of BR on a large scale in the industry.

Binary reaction behaviors of red mud based cementitious material：Hydration characteristics and Na+ utilization

Bayer red mud (RM) occupies a large amount of land, which excessive Na⁺ seriously damages groundwater resources. In this research, a RM based cementitious material (RMC) composed of RM and ordinary Portland cement (OPC) was developed. It is interesting to find that a binary reaction consisting of cement hydration and geopolymer reaction in RMC. The mechanical and Na⁺ consolidation rate of RMC were improved by the synergistic effect of binary reaction. The results indicated that the compressive strength of RMC is the highest and reaches 32.5R OPC when the mass ratio of CaO/(SiO₂+Al₂O₃) is 1.37, and the Na⁺ leaching concentration is environmentally acceptable. The 7 days compressive strength of RM-based cementitious material No.2 (RMC2) can reach 93.80% of that of 28 days. As the predominant hydration products, cement hydration product (Ca₅(SiO₄)₂(OH)₂) and geopolymer (CaAl₂Si₂O₈·2H₂O and Na₃Al₃Si₃O₁₂·2H₂O) were principally responsible for the strength development of RMC2 at 7 days. The optimal densification microstructure and [SiO₄] polymerization structure was presented in RMC2. The supreme Na⁺ consolidation rate was 99.23% in RMC2 due to the cooperation of physical fixation and [Si(Al)O₄] charge balance principle. This paper provides a fresh theoretical guidance for the utilization of RM and its Na⁺ in building materials.

Applications of red mud as an environmental remediation material: A review

Red mud is an alkaline by-product produced by alumina plants. The accumulation of red mud is becoming an increasingly serious problem with the growth of the aluminum industry. Various waste treatment methods utilizing red mud as an environmental remediation material have been developed. Red mud environmental remediation materials (RM-ERMs) are environmental remediation materials prepared by activating red mud, synergistically using red mud and other ingredients, or by extracting effective components from red mud. There are three general categories of use for RM-ERMs: for waste water purification, waste gas purification and soil remediation. As well as providing an opportunity to improve the environment through purification technologies, the highly alkaline red mud is consumed in the production of RM-ERMs. The use of RM-ERMs has been shown to be a promising strategy for the simultaneous treatment of various wastes. In this paper, the developregeneration characteristics of various red mud granularent status of RM-ERMs is described, the physical and chemical properties of red mud are introduced, and the active mechanism of RM-ERMs on target pollutants in waste water, waste gas and soil is summarized. Moreover, a discussion on the current existing problems of RM-ERMs provides important tips and suggestions for future research on RM-ERMs.

Dealkalization processes of bauxite residue: A comprehensive review

Bauxite residue is a kind of strong alkaline waste produced in the production of alumina. Its long-term storage poses a potential threat to the environment. With the tightening of environment policies in various countries, the strong alkalinity of bauxite residue has become a bottleneck restricting the sustainable development of aluminum industry all over the world. This review covers the composition characteristics of bauxite residue, and describes the Bayer process in detail, where emphasis is put on the formation of alkaline substances in bauxite residue and its release process in long-term storage. This review focuses on several typical processes for the management of bauxite residue alkalinity in recent decades around the world. The phase transformation mechanisms, merits and limitations, as well as application status are discussed. The potential application values of these typical methods are evaluated based on process characteristics. The large amount and varied characteristics of bauxite residue determine that it is unrealistic to solve the dealkalization problem of all bauxite residue with one method. It is recommended that the appropriate dealkalization process of bauxite residue should be selected according to the characteristics of bauxite residue and regional resources, as well as the planning of subsequent application.

Red-mud based porous nanocatalysts for valorisation of municipal solid waste

Red mud samples were used to catalyse in-situ co-pyrolysis of pinewood and low-density polyethylene for the production of high-quality bio-oil. The sodium cation in the crude red-mud was exchanged with barium and calcium cations and further tested to explore their role in oil upgrading. The relationship between red-mud catalytic activity and its constituents was explored using synthetic sodalite. The red-mud catalysts exhibited a considerable aromatisation capacity compared to the thermal co-pyrolysis, as the selectivity towards monocyclic aromatic hydrocarbons increased from 12.7 to 19.6%, respectively. Long-chain molecules cracking was more significant in synthetic sodalite associated with their acidic active sites. The addition of barium and calcium cations to the red-mud largely improved oxygen elimination as a result of the enhanced catalyst basicity. In contrast, the aromatisation ability of red-mud significantly impeded by the large cation size (Ba²⁺ and Ca²⁺) due to the limited diffusion of pyrolysis vapours to the active sites. Ba-exchanged red-mud catalysts reduced the content of carboxylic acids in the bio-oil to 1.8 % while maintained a high yield of the organic fraction (34 %). Ca-exchanged red-mud catalysts produced the lowest fraction of oxygenated compounds (35.1 %); however, the organic phase yield was as low as 23.6 %. The modified red-mud catalysts reduced the fraction of oxygenated compounds from 69.9-35.1% during the biomass-plastic co-pyrolysis.

Getting insight into the effect of CuO on red mud for the selective catalytic reduction of NO by NH3

A series of copper-modified red mud catalysts (CuO/PRM) with different copper oxide contents were synthesized by wet impregnation method and investigated for selective catalytic reduction of NO by NH₃ (NH₃-SCR). The catalytic results demonstrated that the red mud catalyst with 7 wt% CuO content exhibited the excellent catalytic performance as well as resistance to water and sulfur poisoning. The red mud support and copper-containing catalysts were characterized by XRF, XRD, N₂ adsorption-desorption, HRTEM, EDS mapping, XPS, H₂-TPR, NH₃-TPD and in situ DRIFT. The obtained results revealed that well dispersed copper oxide originating from 1 to 7 wt% CuO contents was more facile for the redox equilibrium of Cu²⁺ + Fe²⁺ ↔ Cu⁺ + Fe³⁺ shifting to right to form Cu⁺ and surface oxygen species than crystalline CuO generating from high CuO loading (9 wt% CuO), which was beneficial to the enhancement of reducibility and the formation of Lewis acid sites on the catalyst surface. All these factors made significant contributions to the improvement of NH₃-SCR activities for CuO/PRM catalysts. Moreover, in situ DRIFT analysis combined with DFT calculated results confirmed that the finely dispersed copper species not only enhanced the NH₃ activation but also promoted the NO_x desorption, which synergistically facilitated the NH₃-SCR process via the Eley-Rideal mechanism.

Parent, alkylated, oxygenated and nitro polycyclic aromatic hydrocarbons from raw coal chunks and clean coal combustion: Emission factors, source profiles, and health risks

Residential coals are still inevitable using in developing areas in China. Clean coal briquettes, normally using alkaline substance such as lime or red mud (RM) as the additive, were helpful in pollution emission reduction even without changes of stoves. Studies of atmospheric polycyclic aromatic hydrocarbons (PAHs) emission characteristics from RM clear coal combustion were limited. In this study, emission factors (EFs), sources profiles, and health risks of polycyclic aromatic hydrocarbons (PAHs) in PM_2.5 were investigated for raw coal chunks and clean coal (with red mud) through combustion experiments. EFs of total PAHs were found to be 160.1 ± 100.9 mg·kg^-1 and 19.4 ± 6.1 mg·kg^-1 for bituminous and anthracite raw coal chunks (B-C and A-C), respectively. EFs values were highest for parent PAHs (p-PAHs), followed by oxygenated PAHs (o-PAHs), alkylated PAHs (a-PAHs), and nitro PAHs (n-PAHs). EFs of p-PAHs account for 80% and 52% of total PAHs emissions for B-C and A-C, respectively, while those for o-PAHs are 22.9% and 44.9%, demonstrating residential coal combustion as a significant primary source for p-PAHs and o-PAHs. Clean coals were developed through cold-press technology with red mud (RM) as additive, and clean coals with RM contents of 10% are referred to as B-10% (bituminous) and A-10% (anthracite). Compared to raw coals chunks, EFs were reduced from 128.1, 2.5, 29.3 mg·kg^-1 and 161.8 μg·kg^-1 to 83.5, 1.3, 16.4 mg·kg^-1 and 102.2 μg·kg^-1 by B-10%, and from 10.1, 0.6, 8.7 mg·kg^-1 and 20.6 μg·kg^-1 to 11.9, 0.2, 2.4 mg·kg^-1 and 15.3 μg·kg^-1 by A-10% for p-PAHs, o-PAHs, a-PAHs and n-PAHs, respectively. Diagnostic ratios of 5-Nitroacenaphthene / Acenaphthene (0.02-0.05 for coal, 0.0002 for biomass) can be used to separate residential coal and biomass burning in source analysis. When B-C was replaced by B-10%, both noncancer (0.58 to 0.33 for male, 1.65 to 0.95 for female in hazard quotient) and cancer risks (5.68 × 10^-4 to 2.73 × 10^-4 for male, 2.63 × 10^-3 to 1.27 × 10^-3 for female) can be reduced. o-PAHs should be paid more attention because of its high cancer risks caused by 6H-Benzo(C,D)Pyrene-6-One (1.74 × 10^-5 for male, 8.07 × 10^-5 for female), which are even more than the total risks caused by n-PAHs (3.59 × 10^-7 for male, 1.66 × 10^-6 for female). Results from this study highlighted the environment and health effects of PAHs originated from residential coal combustion, and proposed an effective way by using clean coal to alleviate the associated negative impacts.

Variation on leaching behavior of caustic compounds in bauxite residue during dealkalization process

Bauxite residue, a byproduct of alumina manufacture, is a serious environmental pollutant due to its high leaching contents of metals and caustic compounds. Four typical anions of CO₃^2-, HCO₃^-, Al(OH)₄^- and OH^- (represented caustic compounds) and metal ions (As, B, Mo and V) were selected to assess their leaching behavior under dealkalization process with different conditions including liquid/solid ratio (L/S ratio), temperature and leaching time. The results revealed that washing process could remove the soluble composition in bauxite residue effectively. The leaching concentrations of typical anions in bauxite residue decreased as follows: c(CO₃^2-) > c(HCO₃^-) > c[Al(OH)₄^-] > c(OH^-). L/S ratio had a more significant effect on leaching behavior of OH^-, whilst the leaching concentration of Al(OH)₄^- varied larger underleaching temperature and time treatment. Under the optimal leaching, the total alkaline, soluble Na concentrations, exchangeable Ca concentrations were 79.52, 68.93, and 136.0 mmol/L, respectively, whilst the soluble and exchangeable content of As, B, Mo and V in bauxite residue changed slightly. However, it should be noted that water leaching has released metal ions such as As, B, Mo and V in bauxite residue to the surrounding environment. The semiquantitative analysis of XRD revealed that water leaching increased the content of gismondine from 2.4% to 6.4%. The SEM images demonstrated the dissolution of caustic compounds on bauxite residue surface. The correlation analysis indicated that CO₃^2- and HCO₃^- could effectively reflect the alkalinity of bauxite residue, and may be regarded as critical dealkalization indicators to evaluate alkalinity removal in bauxite residue.

Synergistic effect of Cu loading on Fe sites of fly ash for enhanced catalytic reduction of nitrophenol

A novel Cu catalyst was developed using water-washed coal fly ash (WFA) as a support material for catalytic reduction of p-nitrophenol (p-NP) in the presence of NaBH₄. Cu/WFA showed ~ × 10⁵ times higher estimated rate constant k_obs-p-NP/C_Cu (L min^-1 g_Cu^-1) compared with Cu/SiO₂, Cu/Al₂O₃, and other Cu catalysts previously reported. Surprisingly, we obtained a significant lower price value (Price'/K) (0.027-0.068 USD/L min^-1) for Cu/WFA in comparison with other Cu catalysts and precious metallic catalysts (Pd, Au, Ag, and Pt). Various surface analyses and additional experiments using Fe/SiO₂, Cu/Fe₂O₃/SiO₂, and Cu/HCl-treated WFA demonstrated that Cu(0) nanoparticles were well loaded on the surface of WFA, where Fe elements were abundant, resulting in a dramatic enhancement of the Cu/WFA catalytic activity. Particularly, X-ray photoelectron spectroscopy revealed the abundance of Cu(0)/Fe(III) and Cu(0)/Fe(II) in the WFA surface. This indicates that Cu(0) was the main driving force for the activation of H_ad molecule, and that the reduction of Fe(III) to Fe(II) by NaBH₄ can accelerate the reduction of Cu(II) to Cu(0). Recycling and phytotoxicity tests showed that Cu/WFA can be applied as a reusable catalyst with low environmental impact, revealing the remarkable potential of non-precious metal/WFA catalyst in the field of environmental remediation.

Influence of phosphorus content on properties and performance of NiW nanocatalyst supported on activated red mud in atmospheric diesel hydrodesulfurization

The influence of phosphorus promoter addition on structural properties and hydrodesulfurization activity of NiW nanocatalysts supported on activated red mud, called NiW/P-ARM, was studied at atmospheric pressure. Before red mud usage, it was treated by Pratt and Christoverson technique and then NiW/P-ARM nanocatalysts with different phosphorus loadings were prepared by impregnation technique. The prepared samples were characterized by XRF, XRD, FESEM, BET, FTIR and H₂-TPR analyses. Base on the obtained results, XRF analysis indicated that activation process remarkably decreased Ca and Na content in the red mud. XRD results indicated the high dispersion of the tungstate, nickel and active phase (NiWO₄) species on the surface of activated red mud support. FESEM analysis illustrated that all particles of NiW/P-ARM were less than 100 nm. TPR measurements revealed that phosphorus-promoted nanocatalysts had higher reduction potentials than nanocatalyst without promoter. The catalytic ability of synthesized nanocatalysts was investigated in a fixed bed reactor at atmospheric pressure and 380 °C using Iso diesel and light diesel as the reaction feeds. It was found that NiW/P-ARM with 1.5 wt.% prompter showed higher activity for hydrodesulfurization process and further addition of P promoter decreased the catalytic activity that may be due to the decrease in surface area or agglomeration particles.

Mn-Doped material synthesized from red mud and rice husk ash as a highly active catalyst for the oxidation of carbon monoxide and p-xylene

Red muad and rice husk ash were treated without neutralization by acid to produce a support material (RR).

Sphingosine 1-Phosphate Receptors and Metabolic Enzymes as Druggable Targets for Brain Diseases

The central nervous system is characterized by a high content of sphingolipids and by a high diversity in terms of different structures. Stage- and cell-specific sphingolipid metabolism and expression are crucial for brain development and maintenance toward adult age. On the other hand, deep dysregulation of sphingolipid metabolism, leading to altered sphingolipid pattern, is associated with the majority of neurological and neurodegenerative diseases, even those totally lacking a common etiological background. Thus, sphingolipid metabolism has always been regarded as a promising pharmacological target for the treatment of brain disorders. However, any therapeutic hypothesis applied to complex amphipathic sphingolipids, components of cellular membranes, has so far failed probably because of the high regional complexity and specificity of the different biological roles of these structures. Simpler sphingosine-based lipids, including ceramide and sphingosine 1-phosphate, are important regulators of brain homeostasis, and, thanks to the relative simplicity of their metabolic network, they seem a feasible druggable target for the treatment of brain diseases. The enzymes involved in the control of the levels of bioactive sphingoids, as well as the receptors engaged by these molecules, have increasingly allured pharmacologists and clinicians, and eventually fingolimod, a functional antagonist of sphingosine 1-phosphate receptors with immunomodulatory properties, was approved for the therapy of relapsing-remitting multiple sclerosis. Considering the importance of neuroinflammation in many other brain diseases, we would expect an extension of the use of such analogs for the treatment of other ailments in the future. Nevertheless, many aspects other than neuroinflammation are regulated by bioactive sphingoids in healthy brain and dysregulated in brain disease. In this review, we are addressing the multifaceted possibility to address the metabolism and biology of bioactive sphingosine 1-phosphate as novel targets for the development of therapeutic paradigms and the discovery of new drugs.

High-surface-area activated red mud for efficient removal of methylene blue from wastewater

Red mud was activated by a digestion–precipitation method, resulting in a mesostructure with high surface area, and the activated red mud was further used as the adsorbent for methylene blue removal. The physicochemical properties of the resultant samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry analysis, and nitrogen sorption techniques. Batch studies were measured to investigate the influence factors including adsorbent dosage, contact time, pH, and initial concentration. It was revealed that the activated red mud was highly efficient for removal of methylene blue. Adsorption experiments were found to be better achieved in faintly acidic and alkaline conditions, where the adsorption capacity of activated red mud and activated red mud-200 reached 232 and 274 mg/g at pH 7.0, respectively. Langmuir, Freundlich, Temkin isotherms, and pseudo-second-order kinetic model fitted the experimental data well, demonstrating an electrostatic interaction mechanism.

Assessment of the use of red mud as a catalyst for photodegradation of bisphenol A in wastewater treatment

This study aimed to investigate the use of red mud (RM) - a byproduct of aluminum production, as a photocatalyst, which was characterized physical-chemically and used in the photodegradation of the target compound bisphenol A (BPA). Chemical processing was performed in the RM (acid treatment, chemical reduction and calcination) to verify the most active catalyst. From the results obtained, a complete degradation kinetics of BPA was carried out using a synthetic matrix (BPA in deionized water) and a real matrix (BPA in wastewater) using natural RM/calcined and TiO₂ for comparison. The results indicated the potential use of the RM/calcined, which was able to degrade between 88 and 100% of the pollutant in a synthetic sample. Tests on a real effluent sample resulted in degradation rates that ranged from 59 to 100% with chemical oxygen demand reductions of up to 23% using natural RM/calcined in comparison to TiO₂. The blank system (irradiation of the solution without the use of a photocatalyst) and the natural RM/calcined one, resulted in reductions of the toxicity in the effluent sample (measured by EC₂₀ using the marine bacteria Vibrio fischeri) of about 12 times, whereas the same treatment using TiO₂ resulted in a toxicity reduction of only seven times. Within these results, the RM/calcined showed potential to be used in wastewater treatment in polishing processes.

Red Mud as an Efficient, Stable, and Cost-Free Catalyst for COx-Free Hydrogen Production from Ammonia

Red mud, one of the mostly produced industrial wastes, was converted into a catalyst with exceptionally high and stable performance for hydrogen production from ammonia. Results showed that iron species produced after reduction of the HCl digested red mud were converted into ε-Fe₂N during the induction period of ammonia decomposition reaction at 700 °C. The catalytic performance measurements indicated that the modified red mud catalyst provides a record high hydrogen production rate for a non-noble metal catalyst at this temperature. For instance, stable hydrogen production rates were measured as 72 and 196 mmol H₂ min⁻¹ g_cat⁻¹ for the corresponding space velocities of 72 000 and 240 000 cm³ NH₃ h⁻¹ g_cat⁻¹, respectively, at 700 °C. These results offer opportunities to utilize one of the key hazardous industrial wastes as an eco-friendly, efficient, stable, and almost cost-free catalyst for CO_x-free hydrogen production from ammonia decomposition.

Proposal for management and alkalinity transformation of bauxite residue in China

Bauxite residue is a hazardous solid waste produced during the production of alumina. Its high alkalinity is a potential threat to the environment which may disrupt the surrounding ecological balance of its disposal areas. China is one of the major global producers of alumina and bauxite residue, but differences in alkalinity and associated chemistry exist between residues from China and those from other countries. A detailed understanding of the chemistry of bauxite residue remains the key to improving its management, both in terms of minimizing environmental impacts and reducing its alkaline properties. The nature of bauxite residue and the chemistry required for its transformation are still poorly understood. This review focuses on various transformation processes generated from the Bayer process, sintering process, and combined Bayer-sintering process in China. Problems associated with transformation mechanisms, technical methods, and relative merits of these technologies are reviewed, while current knowledge gaps and research priorities are recommended. Future research should focus on transformation chemistry and its associated mechanisms and for the development of a clear and economic process to reduce alkalinity and soda in bauxite residue.

Effects of thermal treatments on the characterisation and utilisation of red mud with sawdust additive

Extremely large amounts of red mud (bauxite residue) are generated globally every year from alumina refining industries, which are being disposed of on engineered landfills. The objective of this study is to investigate the effects of thermal treatments on red mud for development of utilisation strategies. Thermal treatments of red mud samples and their characterisations were investigated under inert (N2) and oxidative (air) conditions with and without sawdust addition at 200-600°C. After calcination, the resulting samples were analysed using thermogravimetric-infrared spectroscopy (TG-IR) for functional group transformations, thermogravimetric analysis (TGA) for thermal loss profiles and X-ray diffraction (XRD) for mineral transformations. The characterisation results showed that in N2 environment, boehmite in red mud was transferred to transition alumina at around 400°C while losing water from structural components. The addition of sawdust for incubation and calcination of red mud in air increased the surface area, whereas that in nitrogen atmosphere lead to reduction of hematite to magnetite at around 500°C. The incorporated carbon materials played a major role in increasing the surface area especially for pore size less than 2.5 nm. This treated red mud with altered mineral composition and improved properties for binding contaminants can be used for environmental remediation and in the process of metal recovery such as iron.

A review of the characterization and revegetation of bauxite residues (Red mud)

Bauxite residue (Red mud) is produced in alumina plants by the Bayer process in which Al-containing minerals are dissolved in hot NaOH. The global residue inventory reached an estimated 3.5 billion tons in 2014, increasing by approximately 120 million tons per annum. The appropriate management of bauxite residue is becoming a global environmental concern following increased awareness of the need for environmental protection. Establishment of a vegetation cover is the most promising way forward for the management of bauxite residue, although its physical and chemical properties can limit plant growth due to high alkalinity and salinity, low hydraulic conductivity, trace element toxicity (Al and Fe), and deficiencies in organic matter and nutrition concentrations. This paper discusses the various revegetation and rehabilitation strategies. Studies of the rehabilitation of bauxite residues have mainly focused on two approaches, amelioration of the surface layer and screening of tolerant plants and soil microorganisms. Amendment with gypsum can reduce the high alkalinity and salinity, promote soil aggregation, and increase the hydraulic conductivity of bauxite residues. Organic matter can provide a source of plant nutrients, form stable complexes with metal cations, promote hydraulic conductivity, stabilize soil structure, and provide an energy source for soil organisms. Tolerant plants and microorganisms such as halophytes and alkaliphilic microbes show the greatest potential to ameliorate bauxite residues. However, during restoration or as a result of natural vegetation establishment, soil formation becomes a critical issue and an improved understanding of the various pedogenic processes are required, and future direction should focus on this area.

High-surface-area activated red mud supported Co3O4 catalysts for efficient catalytic oxidation of CO

Red mud is activated and employed as the support of Co₃O₄ catalysts, exhibiting high catalytic activity for low-temperature CO oxidation.

Property and performance of red mud-based catalysts for the complete oxidation of volatile organic compounds

Red mud (RM) was assessed as a catalyst for the complete oxidation of volatile organic compounds (VOCs). The catalytic activity of RM was influenced by an acid treatment and the calcination temperature. Acid-treated RM (HRM) catalysts with a platinum loading (Pt/HRM) were prepared using a conventional impregnation method. Platinum catalysts supported on γ-Al2O3 (Pt/Al) were prepared for comparison. The physicochemical properties of the RM, HRM and Pt/HRM catalysts were characterized by BET analysis, ICP-AES, H2-TPD, XRD, FTIR, SEM, and FE-TEM. The acid treatment increased the BET surface area of the RM significantly, resulting in an increase in catalytic activity. Increasing the calcination temperature from 400°C to 600°C caused a decrease in its catalytic activity. Increasing the platinum loading on HRM(400) from 0.1 wt.% to 1 wt.% led to an increase in the toluene conversion, which was attributed to the better redox properties. The catalytic activities of the Pt/HRM(400) catalysts were superior to those of the Pt/Al catalysts. Benzene, toluene, o-xylene, and hexane were oxidized completely over the 1 wt.% Pt/HRM(400) catalyst at reaction temperatures less than 280°C. The presence of water vapor in the feed had a negative effect on the activity of the 1 wt.% Pt/HRM(400) catalyst.

A novel conversion of the groundwater treatment sludge to magnetic particles for the adsorption of methylene blue

Iron sludge, produced from filtration and backwash of groundwater treatment plant, has long been considered as a waste for landfill. In this study, iron sludge was reused to synthesize Fe3O4 magnetic particles (MPs) by using a novel solvothermal process. Iron sludge contained abundant amounts of silicon, iron, and aluminum and did not exhibit magnetic properties. After treatment for 4h, the amorphous Fe in iron sludge was transformed into magnetite Fe3O4, which could be easily separated from aqueous solution with a magnet. The prepared particles demonstrated the intrinsic properties of soft magnetic materials and could aggregate into a size of 1 μm. MPs treated for 10h exhibited excellent magnetic properties and a saturation magnetization value of 9 emu/g. The obtained particles presented the optimal adsorption of methylene blue under mild conditions, and the maximum adsorption capacity was 99.4 mg/g, which was higher than that of granular active carbon. The simple solvothermal method can be used to prepare Fe3O4 MPs from iron sludge, and the products could be applied to treatment of dyeing wastewater.

A dechlorination pathway for synthesis of horn shaped carbon nanotubes and its adsorption properties for CO2, CH4, CO and N2

Using metallic copper as reductant and tetrachloroethylene as carbon precursor, a simple, low temperature solvothermal method for the synthesis of horn shaped carbon nanotubes is reported. The detail study of reaction parameters such as temperature, time, carbon precursor amount, type and catalyst proportion has been carried out to optimize the conditions wherein that the copper metal (10 g) mediated reduction of tetrachloroethylene (25 mL) at 200°C for 5h resulted in the horn shaped carbon nanotubes with high yield and structural selectivity. The adsorption properties of horn shaped carbon nanotubes were investigated for carbon dioxide, methane, carbon monoxide and nitrogen as adsorbate by volumetric measurements up to 850 mm Hg. The prepared horn shaped carbon nanotubes showed good adsorption capacity for CO(2) (45 cm(3)/g) and CO (17 cm(3)/g), at 303 K and 850 mm Hg pressure, with high equilibrium selectivity (73.3 for CO(2) and 110.7 for CO at 318 K) and capacity selectivity (9.1 for CO(2) and 3.1 for CO at 850 mm Hg and 318 K) over nitrogen which provides the tool for the separation of CO(2) from its mixture with nitrogen observed in flue gas of thermal power plants and boilers, as well as with CO such as syngas.