Bauxite residue neutralization with simultaneous mineral carbonation using atmospheric CO2

Highly efficient CO2 mineralization: Mechanisms and feasibility of utilizing electrolytic manganese residue as a feedstock and implementing ammonia recycling

Electrolytic manganese residue (EMR) and CO2 emissions from the electrolytic manganese metal (EMM) production process present significant challenges to achieving cleaner production within the industry. Given the high capacity for CO2 sequestration and the stability of the sequestered forms, CO2 mineralization methods utilizing minerals or industrial residues have garnered considerable research interest. The efficacy of such methods is fundamentally dependent on the properties of the materials employed. EMR, due to its calcium sulfate dihydrate (CaSO4·2H2O) content, possesses an intrinsic potential for CO2 solidification. In this study, we propose a novel method for CO2 mineralization utilizing EMR, coupled with NH3·H2O recycling. Experimental results indicated that under conditions of a reaction temperature of 55 °C and a pH of approximately 8, each ton of EMR can sequester 0.16 t of CO2, with equilibrium achieved within 10 min. The mineralization mechanism was elucidated using SEM, TG curves, and XRD analyses, which revealed that Ca2+ ions are initially leached from CaSO4·2H2O in the EMR, subsequently precipitating with CO32- ions to form CaCO3. This CaCO3 layer effectively covers the surface of CaSO4·2H2O, inhibiting further Ca2+ release and stabilizing the reaction equilibrium. Furthermore, the ammonia in the solution is regenerated into NH3·H2O, facilitating its reuse and preventing secondary pollution. The utilization of EMR for CO2 mineralization not only mitigates carbon emissions in the EMM production process but also promotes environmentally sustainable practices in the industry. This study highlights a promising pathway towards achieving carbon neutrality and cleaner production in electrolytic manganese production.

Ecological restoration affects the dynamic response of alkaline minerals dissolution in bauxite residue

Regulating alkalinity is the key process to eliminating environmental risk and implementing sustainable management of bauxite residue. Nevertheless, continuous release of free alkali from the solid phase (mainly sodalite and cancrinite) is a major challenge for long-term stability of alkalinity in amended bauxite residue. In order to understand the dissolution behavior of sodalite and cancrinite, their dissolution kinetics under simulated pH conditions of 8, 9 and 10 were investigated. Additionally, PHREEQC software and shrinking core model (SCM) were employed to analyze the release pattern of saline ions. The results revealed that the ratio of Na/Si and Na/Al values exhibited greater stability in sodalite than in cancrinite. The dissolution of elemental Na, Si, and Al in sodalite and cancrinite was matched with non-chemometric characteristics. The kinetic calculations by the shrinking core model (SCM) suggested that both sodalite and cancrinite exhibited slow dissolution kinetics, and their dissolution processes belong to internal diffusion control and external diffusion control, respectively. pH controlled the dissolution kinetic rates of sodalite and cancrinite mainly by changing their coupled dissolution-precipitation processes. More importantly, these findings can predict the change of alkaline components accurately, thus facilitating the implementation of efficient alkalinity regulation strategies for the ecological restoration of bauxite residue disposal areas.

Trends in research on characterization, treatment and valorization of hazardous red mud: A systematic review

Meta-analysis of red mud-related literature in English published from 1976 to 2022 and in Chinese from 1990 to 2022 was performed to support critical analysis and evaluation of the available literature based on the following aspects of red mud research: (a) characterization, (b) treatment for harmfulness minimization, (c) recovery of valuable metals, (d) environmental applications, and (e) uses as construction materials. It was found that (a) sinter red mud tended to contain more silica and calcium, and less iron, sodium and aluminium compared to Bayer red mud; (b) gypsum was the most frequently used agent for harmfulness reduction treatment of red mud, followed by flue gas/CO2; (c) the mean optimal pH for adsorption of major anionic pollutants was 8.42 ± 1.13 (arsenite), 3.73 ± 0.68 (arsenate), 3.50 ± 2.38 (phosphate), 4.43 ± 1.04 (fluoride) and 3.80 ± 1.54 (chromate); (d) wastewater treatment has attracted more attention compared to contaminated soils and waste gases; (e) recovery of iron and scandium has attracted more attention compared to other metals; (f) cement making has been the focus in construction uses. Most of the research findings were based on laboratory-scale experiments that focused on efficacy rather than efficiency. There was a lack of integrated approaches for research in red mud valorization.

Potential of major by-products from non-ferrous metal industries for CO2 emission reduction by mineral carbonation: a review

By-products from the non-ferrous industry are an environmental problem; however, their economic value is high if utilized elsewhere. For example, by-products that contain alkaline compounds can potentially sequestrate CO₂ through the mineral carbonation process. This review discusses the potential of these by-products for CO₂ reduction through mineral carbonation. The main by-products that are discussed are red mud from the alumina/aluminum industry and metallurgical slag from the copper, zinc, lead, and ferronickel industries. This review summarizes the CO₂ equivalent emissions generated by non-ferrous industries and various data about by-products from non-ferrous industries, such as their production quantities, mineralogy, and chemical composition. In terms of production quantities, by-products of non-ferrous industries are often more abundant than the main products (metals). In terms of mineralogy, by-products from the non-ferrous industry are silicate minerals. Nevertheless, non-ferrous industrial by-products have a relatively high content of alkaline compounds, which makes them potential feedstock for mineral carbonation. Theoretically, considering their maximum sequestration capacities (based on their oxide compositions and estimated masses), these by-products could be used in mineral carbonation to reduce CO₂ emissions. In addition, this review attempts to identify the difficulties encountered during the use of by-products from non-ferrous industries for mineral carbonation. This review estimated that the total CO₂ emissions from the non-ferrous industries could be reduced by up to 9-25%. This study will serve as an important reference, guiding future studies related to the mineral carbonation of by-products from non-ferrous industries.

Ca-driven stable regulatory of alkalinity within desilication products: Experimental, modeling, transformation mechanism and DFT study

The prevalent pH rebound phenomenon in the bauxite residue alkalinity regulation is primarily caused by the presence of alkaline minerals, including sodalite and cancrinite. Calcium ion is widely used to remove the free alkali for reducing the alkalinity of bauxite residue, but its underlying mechanism on alkaline minerals is still unclear. In this work, we investigated the action mechanism of calcium ion on sodalite and cancrinite by various microspectroscopic methods, and then employed spin-polarized density functional theory (DFT) calculations to reveal the reaction pathways of calcium ion substitution and migration in minerals. The calcium ion can effectively regulate the stability of alkaline minerals by inhibiting alkaline ions release, which respectively enters sodalite and cancrinite by displacing Na adsorbed inside the mineral lattice and on the mineral surface. The entered calcium ion acts as competitive protection against sodium during the neutralization process, thus inhibiting the proton-promoted dissolution of sodalite and cancrinite. Moreover, the amount of entry calcium ion controls their acid neutralization ability. DFT calculations revealed calcium ions readily replaced sodium on the internal channels of minerals rather than on the surface. These new findings contribute to the understanding of potential options to directly stabilize critical alkaline components in bauxite residue.

Summary of research progress on separation and extraction of valuable metals from Bayer red mud

Bayer red mud is a strong alkaline solid waste discharged during alumina production. Due to large emissions and strong alkalinity, red mud is now mostly dammed or buried, which not only occupies huge land but also contaminates the surrounding ecosystem, causing the risk of collapse and landslide. In addition to its overall utilization in building materials, agriculture, the environment, and the chemical industry, red mud also contains valuable metals such as sodium, aluminum, iron, titanium, and scandium and is considered to be an important secondary resource. In this paper, the physicochemical properties and hazards of red mud are first introduced, and then, the overall utilization of red mud is summarized. Then, the latest research progress on the separation and extraction of valuable metals from red mud is reviewed in detail and a new comprehensive utilization method is recommended and evaluated. This paper also provides suggestions for the future development direction of the comprehensive utilization technology of red mud.

Neutralization and Improvement of Bauxite Residue by Saline-Alkali Tolerant Bacteria

The high salt-alkalinity of bauxite residue (BR) hinders plant growth and revegetation of bauxite residue disposal areas (BRDA), which cause serious potential environmental and ecological risks. Bioneutralization is a promising method for improving the properties of BR and plant colonization. In the present study, a strong saline-alkali tolerant bacteria (ZH-1) was isolated from aged BR and identified as Bacillus sp. The medium of ZH-1 was optimized by orthogonal tests, and ZH-1 could decrease the medium pH from 11.8 to 6.01 (agitated culture) and 6.48 (static culture) by secretion of citric acid, oxalic acid and tartaric acid. With the inoculation of ZH-1, the pH of BR decreased from 11.6 to 8.76, and the water-soluble salt in BR increased by 68.11%. ZH-1 also changed the aggregate size distribution of BR, the mechanical-stable aggregates and water-stable aggregates increased by 18.76% and 10.83%, respectively. At the same time, the stability of the aggregates obviously increased and the destruction rate decreased from 94.37% to 73.46%. In addition, the microbial biomass carbon increased from 425 to 2794 mg/kg with the inoculation of ZH-1. Bacterial community analysis revealed that Clostridia, Bacilli, Gammaproteobacteria, Betaproteobacteria and Alphaproteobacteria were the main classes in the naturalized BR, and the inoculation of ZH-1 increased the diversity of bacteria in the BR. Overall, ZH-1 has great potential for neutralization and improvement the properties of BR and may be greatly beneficial for the revegetation of BRDA.

The utilization of alkaline wastes in passive carbon capture and sequestration: Promises, challenges and environmental aspects

Alkaline wastes have been the focus of many studies as they act as CO₂ sinks and have the potential to offset emissions from mining and steelmaking industries. Passive carbonation of alkaline wastes mimics natural silicate weathering and provides a promising alternative pathway for CO₂ capture and storage as carbonates, requiring marginal human intervention when compared to ex-situ carbonation. This review summarizes the extant research that has investigated the passive carbonation of alkaline wastes, namely ironmaking and steelmaking slag, mine tailings and demolition wastes, over the past two decades. Here we report different factors that affect passive carbonation to address challenges that this process faces and to identify possible solutions. We identify avenues for future research such as investigating how passive carbonation affects the surrounding environment through interaction with the biosphere and the hydrosphere. Future research should also consider economic analyses to provide investors with an in-depth understanding of passive carbonation techniques. Based on the reviewed materials, we conclude that passive carbonation can be an important contributor to climate change mitigation strategies, and its potential can be intensified by applying simple waste management practices.

Opportunities and challenges in CO2 utilization

CO₂ utilizations are essential to curbing the greenhouse gas effect and managing the environmental pollutant in an energy-efficient and economically-sound manner. This paper seeks to critically analyze these technologies in the context of each other and highlight the most important utilization avenues available thus far. This review will introduce and analyze each major pathway, and discuss the overall applicability, potential extent, and major limitations of each of these pathways to utilizing CO₂. This will include the analysis of some previously underreported utilization avenues, including CO₂ utilization in industrial filtration and the processing of raw industrial materials such as iron and alumina. The core theme of this paper is to seek to treat CO₂ as a commodity instead of a liability.

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.

Green conversion of hazardous red mud into diagnostic X-ray shielding tiles

Red mud is a solid hazardous alumina industrial waste, which is rich in iron, titanium, aluminum, silicon, calcium, etc. The red mud contains 30-60% of hematite, which is suitable for shielding high energy X- and gamma rays. So, the iron rich red mud was converted into diagnostic X-ray shielding tiles through ceramic route by adding a certain weight percentage of BaSO₄ and binders (kaolin clay or sodium hexametaphosphate) with it. The kaolin clay tile possess sufficient impact strength (failure point is 852 mm for 19 mm steel ball) and flexural strength of ~25 N/mm², which is suitable for wall applications. The 10.3 mm and 14.7 mm thick red mud:BaSO₄:kaolin clay tile possess the attenuation equivalent to 2 mm and 2.3 mm lead at 125 kVp and 140 kVp, respectively. No heavy elements were found to leach out except chromium and arsenic from the sintered tiles. However, the leaching of Cr (0.6 ppm) and As (0.015 ppm) was found to be well below the permissible limit. These tiles can be used in the X-ray diagnosis, CT scanner, bone densitometry, and cath labs instead of toxic lead sheet and thereby to protect the operating personnel, public, and environment from radiation hazards.

Effect of Carbide Slag on Removal of Na+/K+ from Red Mud Based on Water Leaching

Red mud (RM) is a hazardous solid waste discharged from the alumina production process. The stock of RM is very large, and it has strong alkalinity and certain radioactivity, which makes it have a very serious adverse effect on the environment. Many scholars have carried out extensive experimental investigations on the minimization, hazard-free treatment, and reutilization of RM, and encouraging results have been obtained. However, reutilization of RM has been restricted mainly due to its complex composition and strong alkalinity. In this study, carbide slag, a byproduct of acetylene production, was utilized to remove alkalis (Na⁺ and K⁺) from RM by calcium ion replacement. The effects of the temperature, liquid-to-solid ratio, carbide slag dose, and leaching time on dealkalization of RM by carbide slag were studied. The leaching mechanism of sodium was investigated and analyzed using inductively coupled plasma-atomic emission spectrometry, X-ray diffraction, and scanning electron microscopy with energy-dispersive spectrometry. Under the optimal conditions, the residual Na₂O and K₂O amount in the RM after dealkalization using the carbide slag diminished to less than 0.93 and 0.45 wt %. More than 78.80% of Na₂O and 58.84% of K₂O could be dissolved under the optimal conditions. The cancrinite structure in the initial RM was destroyed, and soluble sodium salts formed in the suspension can be easily replaced by carbide slag reducing Na⁺. The dealkalization process of RM by using carbide slag was controlled by chemical reaction of shrinking core model, where the apparent activation energy was 4.92 kJ/mol.

Neutralization of bauxite residue with high calcium content in abating pH rebound by using ferrous sulfate

The high alkalinity of bauxite residue and its sustained release impose major limitation on its reuse and ecological disposal. It has been confirmed from sustained rehabilitation that gypsum can effectively reduce the alkalinity of bauxite residue by continuously releasing Ca2+ to react with carbonate and hydroxide. However, the combined bauxite residue with high calcium content exhibits stubborn alkalinity for most alkaline reduction methods employing cations to consume carbonate. In this study, we have aimed to address this knowledge gap by investigating the dose-response relationship in the alkaline reduction induced by ferrous sulfate (FS) neutralization. The pH, exchangeable sodium percentage (ESP), and CO32-/HCO3- of bauxite residue decreased from 10.6, 44.1%, and 42.7/24.5 mg/kg to 8.1, 27.7%, and 0.7/18.0 mg/kg, respectively. Approximately 20-55 days were required for the neutralization reaction to reach equilibrium. The FS induced an increase in free iron oxide (Fed) and amorphous iron oxide (Feo), and partial dissolution of alkaline minerals including calcite, cancrinite, and kaolinite in bauxite residue. Further, addition of FS also affected the kinetic dissolution process of bauxite residue; the acid neutralization capacity of bauxite residue to pH 7 decreased from 0.21 mol H+/kg solid to 0.02 mol H+/kg solid. The results showed FS to be a potential candidate for improving the characteristics of the combined bauxite residue, and guide the FS application for the disposal of the combined bauxite residue.

Oyster Shell Powder, Zeolite and Red Mud as Binders for Immobilising Toxic Metals in Fine Granular Contaminated Soils (from Industrial Zones in South Korea)

Low-cost absorbent materials have elicited the attention of researchers as binders for the stabilisation/solidification technique. As, there is a no comprehensive study, the authors of this paper investigated the performance of Oyster shell powder (OS), zeolite (Z), and red mud (RM) in stabilising heavy metals in three types of heavy metal-contaminated soils by using toxicity characteristic leaching procedure (TCLP). Samples were collected from surroundings of an abandoned metal mine site and from military service zone. Furthermore, a Pb-contaminated soil was artificially prepared to evaluate each binder (100× regulatory level for Pb). OS bound approximately 82% of Pb and 78% of Cu in real cases scenario. While Z was highly effective in stabilizing Pb in highly polluted artificial soil (>50% of Pb) at lower dosages than OS and RM, it was not effective in stabilising those metals in the soils obtained from the contaminated sites. RM did not perform consistently stabilising toxic metals in soils from contaminated sites, but it demonstrated a remarkable Pb-immobilisation under dosages over than 5% in the artificial soil. Further, authors observed that OS removal efficiency reached up to 94% after 10 days. The results suggest that OS is the best low-cost adsorbent material to stabilize soils contaminated with toxic metals considered in the study.

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.

CO2 Utilization via Direct Aqueous Carbonation of Synthesized Concrete Fines under Atmospheric Pressure

Mineral carbonation using alkaline wastes is an attractive approach to CO₂ utilization. Owing to the difference between waste CO₂ and feedstock CO₂, developing CO₂ utilization technologies without CO₂ purification and pressurization is a promising concept. This study investigated a potential method for CO₂ utilization via direct aqueous carbonation of synthesized concrete fines under atmospheric pressure and low CO₂ concentration. The carbonation reaction with different solid-liquid ratios and different concentrations of introduced CO₂ was examined in detail. Under basic conditions, a CO₂ uptake of 0.19 g-CO₂/g-concrete fines demonstrated that direct aqueous carbonation of concrete fines under atmospheric pressure and low CO₂ concentration is effective. The CaCO₃ concentration, degree of carbonation, and reaction mechanism were clarified. Furthermore, characterization of the carbonated products was used to evaluate ways of utilizing the carbonated products.

Alkalinity neutralization and structure upgrade of bauxite residue waste via synergistic pyrolysis with biomass

Bauxite residues, a large volume solid waste, are in urgent need of effective disposal and management. Especially, strategies to alleviate the high alkalinity of bauxite residue remain a big challenge. Here, we developed a synergistic pyrolysis to neutralize the alkalinity of bauxite residue and upgrade the structure of biomass simultaneously. By cooperating the catalytic feature from bauxite residue, rice straw, a cellulose-enriched biomass, could prefer to produce acidic components under a hypothermal pyrolysis temperature (below 250 °C) and partial oxygen-contained atmosphere as evidenced by the synchronous TGA-FTIR analysis. In return, these in-situ produced acidic components neutralized the bauxite residue profoundly (pH decreased from 11.5 to 7.2) to obtain a neutral product with long-term water leaching stability. Also, a higher pyrolysis temperature led to neutral biochar-based products with well-defined carbonization characteristics. Thus, the biomass-driven pyrolysis strategy provides a potential to dispose the alkalinity issue of bauxite residue and further opportunities for the sustainable reuse and continuing management of bauxite residue.

The removal of phosphate by thermally treated red mud from water: The effect of surface chemistry on phosphate immobilization

This study investigated thermal treatment of red mud (RM) and its effect on phase composition, surface property, and sorption capacity exemplified by phosphate. Dehydration (∼600 °C), decomposition of carbonate minerals (700 °C-800 °C), and silicate/aluminate formation (900 °C-1000 °C) occurred upon thermal treatment of RM. Grain growth and vitrification that rendered initial morphology changes and decreased the specific surface area of RM from 26.5 to 4.1 m²/g when treated from 600 to 1000 °C, respectively. Surface acidity, i.e., intrinsic acidity constant and surface acidity density, decreased as well after thermal treatment at 600 °C due to burnouts of organics then increased upon further elevated-temperature treatment because of phase transformation. Thermal activation enhanced phosphate adsorption density (μmol/m²). Multilayer sorption aided by leached metal ions was responsible for phosphate immobilization.

Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions

The main purpose of this study was to characterize the mineral and chemical composition of typical red muds in China. Changes in the physicochemical and microstructural properties of red muds collected from the Shanxi and Shandong provinces were investigated after they were immersed in an alkaline NaOH or an acidic HCl solution for 7, 28, and 120 days. The results showed that red mud has a high cation exchange capacity and active physicochemical properties, which can be closely related to its extremely high alkalinity and complex microstructure. The neutralization of red mud with the HCl solution results in the release of Na+ from the red mud particles into the leachate and can effectively decrease the pH value of the filtrate. The neutralization process can result in a significant decrease in the liquid limit, plastic limit and plasticity index, whereas the opposite was observed for the different parameters after the addition of the NaOH solution. In this sense, acid neutralization can significantly improve the cementation property of the red mud. This result will increase the water permeability of the acid-treated soil layer and improve the growth ability of plants. The specific surface area of red mud immersed in the NaOH solution decreased, whereas the specific surface area of red mud immersed in the HCl solution increased. This study contributes to our understanding of red mud properties after the red mud has been subjected to acidic and alkaline treatments, and the results can provide insights into the safe disposal of red mud.

Fabrication and environmental applications of multifunctional mixed metal-biochar composites (MMBC) from red mud and lignin wastes

This study fabricated a new and multifunctional mixed metal-biochar composites (MMBC) using the mixture of two abundant industrial wastes, red mud (RM) and lignin, via pyrolysis under N₂ atmosphere, and its ability to treat wastewater containing various contaminants was comprehensively evaluated. A porous structure (BET surface area = 100.8 m² g^-1) was created and metallic Fe was formed in the MMBC owing to reduction of Fe oxides present in RM by lignin decomposition products during pyrolysis at 700 °C, which was closely associated with the transformation of liquid to gaseous pyrogenic products. The potential application of the MMBC was investigated for the removal of heavy metals (Pb(II) and Ni(II)), oxyanions (As(V) and Cr(VI)), dye (methylene blue), and pharmaceutical/personal care products (para-nitrophenol and pCBA). The aluminosilicate mineral, metallic Fe, and porous carbon matrix derived from the incorporation of RM and lignin contributed to the multifunctionality (i.e., adsorption, chemical reduction, and catalytic reaction) of the MMBC. Thus, engineered biochar composites synthesized from selected industrial wastes can be a potential candidate for environmental applications.

Geochemical Characteristics and Toxic Elements in Alumina Refining Wastes and Leachates from Management Facilities

A nationwide investigation was carried out to evaluate the geochemical characteristics and environmental impacts of red mud and leachates from the major alumina plants in China. The chemical and mineralogical compositions of red mud were investigated, and major, minor, and trace elements in the leachates were analyzed. The mineral and chemical compositions of red mud vary over refining processes (i.e., Bayer, sintering, and combined methods) and parental bauxites. The main minerals in the red mud are quartz, calcite, dolomite, hematite, hibschite, sodalite, anhydrite, cancrinite, and gibbsite. The major chemical compositions of red mud are Al, Fe, Si, Ca, Ti, and hydroxides. The associated red mud leachate is hyperalkaline (pH > 12), which can be toxic to aquatic life. The concentrations of Al, Cl^-, F^-, Na, NO₃^2-, and SO₄^2- in the leachate exceed the recommended groundwater quality standard of China by up to 6637 times. These ions are likely to increase the salinization of the soil and groundwater. The minor elements in red mud leachate include As, B, Ba, Cr, Cu, Fe, Ni, Mn, Mo, Ti, V, and Zn, and the trace elements in red mud leachate include Ag, Be, Cd, Co, Hg, Li, Pb, Sb, Se, Sr, and Tl. Some of these elements have the concentration up to 272 times higher than those of the groundwater quality standard and are toxic to the environment and human health. Therefore, scientific guidance is needed for red mud management, especially for the design of the containment system of the facilities.

The potential for constructed wetland mechanisms to treat alkaline bauxite residue leachate: carbonation and precipitate characterisation

Leachates emanating from bauxite residue disposal areas are alkaline and require neutralisation prior to discharge. The use of passive technologies such as constructed wetlands has received increasing interest as possible treatments for alkaline leachates, including bauxite residues. Mechanisms proposed for wetland effectiveness have included calcite precipitation but it is not clear if such a pathway is feasible in the relatively low Ca residue leachates. Carbonation of Ca-spiked residue leachate treatments was conducted to observe rates of pH decrease and precipitate formation. For all treatments, carbonation effectively decreased pH to ca. 10.5 which remained stable following aeration. Decreases in Al content of 83-93% were also observed. Precipitates retrieved from carbonation experiments and from a constructed wetland trial were characterised using XRD, SEM, XPS and EDX. Calcium carbonates formed in Ca-spiked treatments and dawsonite precipitation occur in the absence of Ca. Rinsing of precipitates removes surface calcium indicating soluble forms adsorbed on precipitates. The results demonstrate that carbonation of bauxite residue leachate is an important component of passive treatments and neutralisation.