Proposal for management and alkalinity transformation of bauxite residue in China

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.

Bauxite residue (red mud) treatment: Current situation and promising solution

Bauxite residue, an industrial solid waste generated during alumina production, with over 80 % of bauxite residue worldwide being accumulated around alumina plants, which occupying a significant amount of land resources and posing a threat to the natural environment in the surrounding areas. This paper reviews recent advances in extracting valuable resources from bauxite residue, and its applications in building materials, environmental adsorbents, energy storage materials, and soil alkalinization. It also highlighted the main problem existing in these researches, which is the inability of the existing single processes to achieve the comprehensive utilization of various types of bauxite residue or maximize the utilization of bauxite residue components, resulting in a low comprehensive utilization rate and insignificant absorption effects of bauxite residue. To address these issues, we proposed a strategy of classifying and utilizing bauxite residue based on its components and establishing a multi-industry application system, involving sectors such as steel and building materials. This collaborative approach aims to handle various types of bauxite residue more effectively. Additionally, we suggest selecting suitable treatment methods based on the specific characteristics of bauxite residue and implementing measures to promote its comprehensive and large-scale utilization.

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.

Halophyte Elymus dahuricus colonization regulates microbial community succession by mediating saline-alkaline and biogenic organic matter in bauxite residue

Alkalinity regulation and nutrient accumulation are critical factors in the construction of plant and microbial communities and soil formation in bauxite residue, and are extremely important for sustainable vegetation restoration in bauxite residue disposal areas. However, the establishment and succession of microbial communities driven by plant colonization-mediated improvements in the physicochemical properties of bauxite residues remain poorly understood. Thus, in this study, we determined the saline-alkali properties and dissolved organic matter (DOM) components under plant growth conditions and explored the microbial community diversity and structure using Illumina high-throughput sequencing. The planting of Elymus dahuricus (E. dahuricus) in the bauxite residue resulted in a significant decrease in total alkalinity (TA), exchangeable Na, and electrical conductivity (EC) as well as the release of more tryptophan-like protein compounds and low-molecular-weight humic substances associated with biological activities into the bauxite residue substrate. Taxonomical analysis revealed an initial-stage bacterial and fungal community dominated by alkaline-tolerant Actinobacteriota, Firmicutes, and Ascomycota, and an increase in the relative abundances of the phyla Bacteroidota, Cyanobacteria, Chloroflexi, and Gemmatimonadota. The biological activities of phylum Actinobacteriota, Bacteroidota, and Gemmatimonadota were significantly associated with protein-like and UVA-like humic substances. As eutrophic bacteria, Proteobacteria participate in the transformation of humic substances and can not only utilize small molecules of organic matter and convert them into humic substances but also promote the gradual conversion of humic acids into simple molecular compounds. Our results suggest that plant roots secrete organic matter and microbial metabolites as the main biogenic organic matter that participates in the establishment and succession of the microbial community in bauxite residues. Root length affects bacterial and fungal diversity by mediating the production of protein-like substances.

Experimental Study on the Curing Mechanism of Red Mud-Based Stabilized Soil Co-Modified by Nano-SiO2 and Gypsum

In order to effectively utilize red mud and reduce its occupation of land resources, as well as its impact on the environment, experiments were conducted to develop stabilized soil materials using nano-SiO2 synergistically modified red mud and to investigate the curing mechanism of stabilized soil. The unconfined compressive strength, microscopic morphology, and curing mechanism of the red mud-based stabilized soil materials with different amounts of modified materials were investigated. The test results show that after 7 days of curing, the unconfined compressive strength of red mud-based stabilized soil meets the compressive strength requirement of road base material when nano-SiO2, gypsum, and cement are synergistically modified. In such cases, the soil structure has the lowest fracture rate and the best structural compactness when the amount of nano-SiO2 is 1%. It is found that the needle-like and columnar calcium alumina in the modified red mud-based stabilized soil increases, and the binding energy of hydration product ions in the modified material is improved. The chemical curing mechanism of modified red mud-based stabilized soil includes hydration reaction, pozzolanic reaction, promotion effect of nano-SiO2, and enhancement effect of gypsum. On this base, a model of the early start hydration process of red mud-based stabilized soil promoted by nano-SiO2 is established.

Element distribution of grading in red mud at different temperatures based on TIMA and EDS analysis

In this paper, element distribution patterns of red mud particles with different grading temperatures were explored based on TIMA and EDS, and alkali removal performance of different particle sizes under high temperature grading was compared. The results show that non-clay phases in the particles coagulate with the clay phases of different sodium contents during stacking process, thus forming a mixture phase containing clay phase and other impurities. The potential of grading utilization of red mud is displayed by process mineralogy studies. The elements and phases of different particle sizes of red mud cannot be effectively separated by grading at room temperature. Due to high-temperature grading, red mud is divided into three particle sizes, namely, a (above 100 μm), b (38-100 μm), and c (below 38 μm), with Na2O contents of 3.25%, 2.31%, and 8.13%, respectively, decreasing to 1.00%, 0.27%, and 2.99% after alkali removal. The different elements and phases of red mud can be effectively separated by high-temperature grading, which promotes the classification of different particle sizes and the comprehensive utilization of red mud.

Rapid conversion of alkaline bauxite residue through co-pyrolysis with waste biomass and its revegetation potential

The extreme alkalinity of bauxite residue (BR) leads to difficulty with its reuse. Alkaline leachate and dust generation during the stacking process can pollute surrounding soil, air and water. In this work, co-pyrolysis of bauxite residue and sawdust was applied to rapidly produce a soil-like matrix that met the conditions for plant growth as demonstrated by ryegrass pot experiments. The present study aimed to characterize the detailed changes in physicochemical, mineral weathering, and microbial communities of the pyrolyzed BR with different ratios of saw dust after plant colonization for 2 months. With increasing sawdust addition during co-pyrolysis, the pH of BR decreased from 11.21 to 8.16, the fraction of macro-aggregates 0.25-2 mm in the water-stable agglomerates increased by 29.3%, and the organic carbon concentration increased from 12.5 to 320 mg/kg, whilst facilitating the degree of humification, which were all beneficial to its revegetation performance. The backscattered electron-scanning electron microscope-energy-dispersive X-ray spectrometry (BSE-SEM-EDS) results confirmed the occurrence of sodalite and calcite weathering on aggregate surfaces, and X-ray photoelectron spectroscopy (XPS) results of surface Al and Si compounds identified that some weathering products were clay minerals such as kaolinite. Furthermore, bacterial community composition and structure shifted towards typical soil taxonomic groups. These results demonstrate soil development of treated BR at an early stage. The technique is a combination of alkalinity regulation and agglomerate construction, which accelerates soil formation of BR, thus proving highly promising for potential application as an artificial soil substitute.

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.

Solidification/stabilization of soil heavy metals by alkaline industrial wastes: A critical review

Solidification/stabilization technology is one of the most desirable technologies for the remediation of heavy metal contaminated soils due to its convenience and effectiveness. The annual production of alkaline industrial wastes in China is in the hundreds of millions of tons. Alkaline industrial wastes have the potential to replace conventional stabilizers because of their cost effectiveness and performance in stabilizing heavy metals in soils. This paper systematically summarizes the use of four alkaline industrial wastes (soda residue, steel slag, carbide slag, and red mud) for the solidification/stabilization of heavy metal contaminated soils and provides a comprehensive analysis of the three mechanisms of action (hydration, precipitation, and adsorption) and factors that influence the process. In addition, the environmental risks associated with the use of alkaline industrial wastes are highlighted. We found that soda residues, steel slag and carbide slag are appropriate for solidification/stabilization of Pb, Cd, Zn and Cu, while red mud is a potential passivation agent for the stabilization of As in soils. However, implementation of remediation methods using alkaline industrial wastes has been limited because the long-term effectiveness, synergistic effects, and usage in soils containing multiple heavy metals have not been thoroughly studied. This review provides the latest knowledge on the mechanisms, risks, and challenges of using alkaline industrial wastes for solidification/stabilization of heavy metal contaminated soils.

Dynamic Adsorption Characteristics of Cr(VI) in Red-Mud Leachate onto a Red Clay Anti-Seepage Layer

Red-mud leachate from tailings ponds contains Cr(VI), which can pollute groundwater via infiltration through anti-seepage layers. This paper investigates leachate from a red-mud tailings pond in southwest China and the red clay in the surrounding area to simulate the adsorption of Cr(VI) onto clay at different pHs, using geochemical equilibrium software (Visual MINTEQ). We also performed dynamic adsorption testing of Cr(VI) on a clay anti-seepage layer. The dynamic adsorption behaviors and patterns in the dynamic column were predicted using the Thomas and Yoon-Nelson models. Visual MINTEQ predicted that Cr(VI) adsorption in red-mud leachate onto clay was 69.91%, increasing gradually with pH, i.e., adsorption increased under alkaline conditions. Cr(VI) concentration in the effluent was measured using the permeability test through a flexible permeameter when the adsorption saturation time reached 146 days. At a low seepage rate, Cr(VI) adsorption onto the clay anti-seepage layer took longer. Saturation adsorption capacity, q₀, and adsorption rate constant, K_th, were determined using the Thomas model; the Yoon-Nelson model was used to determine when the effluent Cr(VI) concentration reached 50% of the initial concentration. The results provide parameters for the design and pollution prediction of the clay anti-seepage layer of red-mud tailings ponds.

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.

Efficient dealkalization of red mud and recovery of valuable metals by a sulfur-oxidizing bacterium

Red mud (RM) is a highly alkaline polymetallic waste generated via the Bayer process during alumina production. It contains metals that are critical for a sustainable development of modern society. Due to a shortage of global resources of many metals, efficient large-scale processing of RM has been receiving increasing attention from both researchers and industry. This study investigated the solubilization of metals from RM, together with RM dealkalization, via sulfur (S0) oxidation catalyzed by the moderately thermophilic bacterium Sulfobacillus thermosulfidooxidans. Optimization of the bioleaching process was conducted in shake flasks and 5-L bioreactors, with varying S0:RM mass ratios and aeration rates. The ICP analysis was used to monitor the concentrations of dissolved elements from RM, and solid residues were analyzed for surface morphology, phase composition, and Na distribution using the SEM, XRD, and STXM techniques, respectively. The results show that highest metal recoveries (89% of Al, 84% of Ce, and 91% of Y) were achieved with the S0:RM mass ratio of 2:1 and aeration rate of 1 L/min. Additionally, effective dealkalization of RM was achieved under the above conditions, based on the high rates (>95%) of Na, K, and Ca dissolution. This study proves the feasibility of using bacterially catalyzed S0 oxidation to simultaneously dealkalize RM and efficiently extract valuable metals from the amassing industrial waste.

Migration of Alkaline Constituents and Restoration Evaluation in Bauxite Residue Disposal Areas

Bauxite residue is a highly alkaline waste from alumina refining, and is mainly disposed by stacking with high environmental risks. Here, the migration of alkaline constituents and the restoration evaluation with phosphogypsum were discussed by soil column experiments to investigate the alkaline regulation in bauxite residue disposal areas (BRDAs). The pH, free alkali, exchangeable sodium in the top layer (0-25 cm depth) covered with BR and phosphogypsum mixtures were reduced from 10.89 ± 0.02, 285.45 ± 21.15 mmol/kg, 385.63 ± 30.34 mg/kg to 9.00 ± 0.50, 12.50 ± 1.50 mmol/kg, 97.00 ± 10.50 mg/kg. For the sublayers, including depths of 35, 45, 55 cm, these values dropped to 9.86, 10.06, 10.03; 38.23, 86.12, 148.00 mmol/kg; 152.90, 246.00, 305.00 mg/kg, respectively. These results indicated alkaline indicators for phosphogypsum amended BR declined dramatically, and the parameters for sublayers were also decreased due to the migration of alkaline constituents. The physicochemical properties for amended BR could meet the conditions for plant growth. This research provided a reference for alkalinity regulation in BRDAs by phosphogypsum.

Spatial Characteristics and Risk Assessment of Heavy Metals in the Soil-Vegetation System of a Red Mud Slag Yard, SW China

The purpose of this study was to investigate the distribution pattern, pollution status and potential ecological risk of Cr, Co, Ni, Cu, As, Cd, Sb, and Pb in soils and dominant plants around an abandoned red mud (RM) slag yard in Southwestern China. Soils exhibited representative enrichment and combination characteristics of these metals compared to the background values, ascribed to the leaching of long-term acid rain on the RM dump. The soil was moderately to severely polluted with As and Sb. Cd also posed a moderate ecological risk. Asteraceae species predominated in the RM slag yard, followed by Coriaria sinica and Robinia pseudoacacia. No plants were identified as hyperaccumulators because of low bioconcentration values, whereas Cosmos bipinnata can act as a potential phytostabilizer of heavy metals based on the translocation factor. The results provided effective decision support for reducing heavy metal pollution by phytoremediation RM stacking fields.

Bioleaching Performance of Titanium from Bauxite Residue Under a Continuous Mode Using Penicillium Tricolor

The present study performed a continuous mode of bioleaching to investigate the leaching efficiency of Titanium (Ti) from bauxite residue using Penicillium Tricolor at between 4% and 12% pulp densities during a 120-day running. Obtained results of the current study showed that increased pulp density led to a decrease in biomass, dissolved oxygen, and amount of leaching Ti as well as an increase in pH value. Further, it was found that efficiency of bioleaching can be enhanced by increasing the rate of aeration, retention time, and concentration of carbon source. However, it was also evident that, at high pulp density, excessive agitation did not give an expected leaching efficiency but a collapse of biomass. In addition, results of the present study showed that the maximum leaching amount of Ti was 3202 mg/L with a corresponding leaching ratio of 50.35% during the whole bioleaching process. Moreover, it was noted that the biomass showed a significant negative correlation with the pH value and dissolved oxygen. However, the biomass showed a significant positive correlation with leaching amount of Ti and thus indicate that microbial metabolic activities are the uppermost factor affecting the continuous leaching performance.

Alkali Recovery of Bauxite Residue by Calcification

Bauxite residue (red mud) generated during alumina production is a highly alkaline solid waste. The red mud is mainly stored on land, but it can cause harm to the surrounding environment and human health. The transformation of red mud into soil is a feasible method for the large-scale disposal of red mud, but alkali removal is the key process that controls the transformation of red mud into soil. In this study, the calcification dealkalization of red mud with a small particle size was carried out below 100 °C. The results show that the sodium in red mud is predominately distributed in small particles, mainly because the lattice alkali and alkali present between the crystals are exposed to the surface of red mud particles by ball milling. The dealkalization process was controlled by the internal diffusion of the shrinking-core model (SCM), and the apparent activation energy was 23.55 kJ/mol. The dealkalization rate and the Na2O content of dealkalized red mud reached 92.44% and 0.61%, respectively. The dealkalization rate increased with increasing reaction time, reactant concentration, and leaching temperature, and this result was consistent with the results of the kinetic study. In addition, calcification enhances the flocculation of particles, so the filtration performance of red mud improved.

Recovery of microbial community in strongly alkaline bauxite residues after amending biomass residue

The aim of this study was to characterize the effects of cornstalk biomass amendments on microbial communities in bauxite residues (BRs) by phylogenetic analysis. Improvements in soil geochemical, physical, and biological properties were assessed to identify the major factors controlling microbial community development in BRs. After one year of incubation, the salinity and structure of the amended BRs had gradually improved, with pH dropping from 11.39 to 9.89, the exchangeable sodium percentage (ESP) dropping from 86.3% to 35.2%, and the mean weight diameter (MWD) rising from 0.12 mm to 0.38 mm. Further analysis of community level physiological profiles (CLPP) showed that the microbial utilization of different carbohydrates had shifted significantly, in addition to increases in the diversity index H' (0.7-7.34), U (2.16-3.14), and the average well color development (0.059-1.08). Over the one-year outside incubation, the dominant fungal phyla in the BRs had shifted gradually from Ascomycota (85.64%) to Ascomycota (52.07%) and Basidiomycota (35.53%), while the dominant bacterial phyla had shifted from Actinobacteria (38.47%), Proteobacteria (21.39%), and Gemmatimonadetes (12.72%) to Actinobacteria (14.87%), Proteobacteria (23.53%), and Acidobacteria (14.37%). Despite these shifts, microbial diversity remained lower in the amended BRs than in the natural soil. Further redundancy analysis indicated that pH was the major factor driving shifts in the bacterial community, while aggregates were the major factor driving shifts in the fungal community. This study demonstrated that amendment with cornstalk biomass shifted the microbial community in the BRs from halophilic groups to acidogenic groups by improving the soil environmental conditions.

Analysis of Engineering Characteristics and Microscopic Mechanism of Red Mud–Bauxite Tailings Mud Foam Light Soil

In order to effectively utilize aluminum industrial waste—red mud and bauxite tailings mud—and reduce the adverse impact of waste on the environment and occupation of land resources, a red mud–bauxite tailings mud foam lightweight soil was developed based on the existing research results. Experiments were conducted to investigate the mechanical properties and microscopic characteristics of the developed materials with different proportions of red mud and bauxite tailings mud. Results show that with the increase in red mud content, the wet density and fluidity of the synthetic sample was increased. With 16% red mud content, the water stability coefficient of the synthetic sample reached its maximum of 0.826, as well as the unconfined compressive strength (UCS) of the sample cured for 28 d (1.056 MPa). SEM images reveal that some wastes of the sample without red mud were agglomerated, the peripheral hydration products were less wrapped, and when the amount of red mud was 16%, the hydration products tightly wrapped the waste particles and increased the structural compactness. The final concentration of alkali leaching of samples increased with the addition of red mud. The maximum concentration of alkali leaching was 384 mg/L for the group with the addition of red mud of 16%. Based on the obtained mechanical strength and alkali release analysis, the sample B24R16 was selected as the optimum among all tested groups. This study explored a way to reuse aluminum industrial waste, and the results are expected to be applied to roadbed and mining filling.

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.

Reuse of Red Mud and Bauxite Tailings Mud as Subgrade Materials from the Perspective of Mechanical Properties

In order to reuse red mud and bauxite tailings mud (two typical aluminum industrial wastes) to reduce the occupation of land resources and environmental damage, these two wastes were combined to develop subgrade materials for the first time. With different combinations, the effects of the amounts of red mud, tailings mud, and cementitious materials on the strength of tested subgrade materials were investigated. The mechanism of strength growth was analyzed by a micro-test. The test results showed that the material strength of three combinations met the requirements when the unconfined compression strength (UCS) of all combinations increased with age. The UCS of the A₁BC₂ combination (the mass ratio of red mud and tailings mud was 2:1, the mass ratio of cement and quicklime was 1:1, and the mass ratio of waste and cementitious materials was 1:0.2) was the best, with the UCS being 3.03 MPa in 7 days. Microscopic imaging showed that specimens with high red mud contents had compact structures without cracks. The strength of these materials is mainly due to hydration reactions and pozzolanic reactions; the cementitious products generated by the reactions solidify Na⁺ and inhibit the release of OH⁻, while the addition of tailings mud can reduce the content of Na₂O in the material, which makes the environmental compatibility of the A₃BC₂ combination the best (the mass ratio of red mud and tailings mud was 1:2, the mass ratio of cement and quicklime was 1:1, and the mass ratio of waste and cementitious materials was 1:0.2). Its pH value was 8.75. This experiment verifies the feasibility of the combined application of red mud and tailings mud in subgrade materials. To this end, a feasible scheme for the simultaneous consumption of these two kinds of aluminum industrial wastes has been proposed.

Dealkalization and Leaching Behavior of Fe, Al, Ca, and Si of Red Mud by Waste Acid from Titanium White Production

Dealkalization is the necessary step for the multipurpose use of red mud (RM), and acid leaching is a productive method to realize the dealkalization of RM. Most researches focus on recovering metals from the highly alkaline waste by pure acid leaching or stabilization by dealkalization. In this study, according to the strong alkalinity of RM and strong acidity of the waste acid from titanium dioxide production, the waste acid was used for the dealkalization of RM. The effects of leaching temperature, reaction time, the concentration of waste acid, liquid-solid ratio (L/S), and stirring rate on the dealkalization of RM were investigated, and the main metal ions in the dealkalization solution were analyzed. The results show that the leaching ratio of sodium can reach 92.3591% when the leaching temperature is 30 °C, the reaction time is 10 min, the concentration of waste acid is 0.6238 mol/L, the L/S is 4:1, and the stirring rate is 300 rpm. The residual alkali content in the treated RM is 0.2674%, which is a reduction to less than 1%. The phase analysis results show that the sodalite and cancrinite in RM are dissolved, decomposed, and transformed after acid leaching. Therefore, RM meets the requirements of building materials after dealkalization, which provides further development as building material products.

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.

Co-Disposal of Coal Gangue and Red Mud for Prevention of Acid Mine Drainage Generation from Self-Heating Gangue Dumps

The seepage and diffusion of acid mine drainage (AMD) generated from self-heating coal gangue tailings caused acid pollution to the surrounding soil and groundwater. Red mud derived from the alumina smelting process has a high alkali content. To explore the feasibility of co-disposal of coal gangue and red mud for prevention of AMD, coal gangue and red mud were sampled from Yangquan (Shanxi Province, China), and dynamic leaching tests were carried out through the automatic temperature-controlled leaching system under the conditions of different temperatures, mass ratios, and storage methods. Our findings indicated that the heating temperature had a significant effect on the release characteristics of acidic pollutants derived from coal gangue, and that the fastest rate of acid production corresponding to temperature was 150 °C. The co-disposal dynamic leaching tests indicated that red mud not only significantly alleviated the release of AMD but also that it had a long-term effect on the treatment of acid pollution. The mass ratio and stacking method were selected to be 12:1 (coal gangue: red mud) and one layer was alternated (coal gangue covered with red mud), respectively, to ensure that the acid-base pollution indices of leachate reached the WHO drinking-water quality for long-term discharge. The results of this study provided a theoretical basis and data support for the industrial field application of solid waste co-treatment.

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.

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.

Removal of Cr (Ⅲ) from aqueous solution by using bauxite residue (red mud): Identification of active components and column tests

Bauxite residue is the by-product of the aluminium industry with an annual output of more than 200 million metric tons in China. Its treatment is still a big problem because more than 96% of that is stockpiled on land causing environmental pollution and threatening the human health. This study used bauxite residue to remove Cr (Ⅲ) from aqueous solution and analyzed the removal mechanism. The removal time was dependent on the initial concentrations of Cr (Ⅲ) and different active components acted on different reaction period. Reaction time increased from <5 min to >2 h with an increase of Cr (Ⅲ) concentration from 5 to 100 and 170 mg/L. The existing forms of adsorbed-Cr were iron oxide-bound Cr (40.80%-87.85%), sulfide-bound Cr (4.04%-20.28%) and residue (6.60%-33.72%). All the components started to react as soon as bauxite residue was added. Cr did not precipitate in the presence of high alkalinity bauxite residue due to the slow release of alkalinity maintaining pH < 6, thus producing Cr(OH)²⁺, Cr₂(OH)₂⁴⁺ and Cr₃(OH)₄⁵⁺ by hydrolysis without precipitation. Fe₂O₃ and Al-containing components were the main active phases for Cr (Ⅲ) removal, with the reaction time lasting more than 2 h and producing Ca₆Al₄Cr₂O₁₅, AlCr₂, (Si, Al)₂O₄, Fe(Cr, Al)₂O₄, FeCr₂Si₃O₁₂, MgCr_0·1Fe_1·9O₄ and MgCr_0·4Fe_1·6O₄. Finally, bauxite residue was granulated and used for column tests. Cr (Ⅲ) wastewater (1 and 50 mg/L) was treated and the effluent can meet the first level of the Shanghai standard (0.1 mg/L) defined by Integrated Wastewater Discharge Standard (DB 31/199-2009).

Effect of synthetic and natural polymers on reducing bauxite residue dust pollution

Dust emission from the bauxite residue drying areas is of great concern for its serious environmental and health impacts. The application of polymer stabilizers is a promising approach to mitigate such issues. However, limited studies have been done on their application on red sand and the investigation of the effect of polymers on penetration resistance, which is a key mechanical property closely related to the dust control performance. Stabilizers reduce the dust emission potential by forming a crust on the surface of treated material. This paper investigates the relationship of crust properties and dust control performance by applying synthetic (polyacrylamide) and natural (xanthan gum and guar gum) polymers. Results illustrated that the water retention property of treated sand is greatly improved after the application of stabilizers. The thickness of the formed crust decreases with stabilizer concentration. Results from wind tunnel simulation illustrated that the polymer stabilizer offers nearly 8 times longer protective period than water when used as a stabilizing agent. Cationic PAM performed the best, and cationic guar gum outperformed anionic xanthan gum. The penetration resistance is proved in this study to be a primary index for predicting dust control performance of polymer stabilizers.

Attenuation of Cr/Pb in bauxite leachates by bentonite–polymer composite geosynthetic clay liners

Three commercially available bentonite–polymer composite geosynthetic clay liners (BPC GCLs) were selected for hydraulic conductivity testing, respectively permeated by two types of bauxite leachates with high alkalinity (pH > 12) and high ionic strength (620.3 mM). The influence of BPC GCLs on the attenuation behavior of Cr/Pb in the bauxite leachates was analyzed. The BPC GCLs with a low hydraulic conductivity (k < 10⁻¹⁰ m s⁻¹) retard the migration of Cr and Pb and the Cr had a higher mobility than Pb in the BPC GCLs. Scanning electron microscope (SEM) microstructure analysis showed that the migration and attenuation behavior of Cr/Pb mainly depended on the chemical properties of the leachates, polymer content and the microstructure of the polymer. Higher attenuation of heavy metals was obtained with bauxite leachates having higher ionic strength. Sufficient polymer content is needed to ensure BPC GCLs have adequately low hydraulic conductivity to suppress attenuation of heavy metals. The gelatinous structure associated with hydrated linear or crosslinked polymer diminishes when the polymer in a BPC is in contact with bauxite leachates. Compromising the hydrogel structure promotes polymer elution and leaves pore space open, resulting in attenuation of heavy metals.

Three commercially available bentonite–polymer composite geosynthetic clay liners (BPC GCLs) were selected for hydraulic conductivity testing, respectively permeated by two types of bauxite leachates with high alkalinity (pH > 12) and high ionic strength (620.3 mM).

Ecotoxicological risk assessment of revegetated bauxite residue: Implications for future rehabilitation programmes

Around 3 billion tonnes of bauxite residue (BR), the by-product of alumina extraction, have been produced and stockpiled worldwide, representing a potential risk for the environment due to the high alkalinity and the presence of relatively high concentrations of trace elements. Phytoremediation (or simply revegetation) is regarded as the most promising in situ remediation option to mitigate the environmental risk that might arise from the land-disposal of BR. Rehabilitation strategies (including the incorporation of amendments such as gypsum and organic matter) have been employed to address the main limitations to plant establishment and growth on BR, typically the high alkalinity, salinity and sodicity. However, the potential for trace element uptake and phytotoxicity have been largely unreported in revegetated BRs. In order to assess the ecotoxicological risk, samples of previously revegetated BR were collected from the field, characterized in the laboratory, and used to conduct ex-situ plant bioassays (Phytotoxkit™ and the RHIZOtest). Without rehabilitation, fresh BR severely inhibits seed germinationand root/shoot development in test species Lepidium sativum, Sinapis alba and Sorghum saccharatum. Plant uptake for Al, As, Cr, V was assessed with RHIZOtest bioassay trials with Lolium perenne and demonstrated that plants exposed to fresh BR take up and translocated trace elements to their shoots at concentrations (As = 4.13 mg/kg dm; Cr = 3.29 mg/kg dm; V = 85.66 mg/kg dm) exceeding phytotoxic levels (vanadium) or maximum levels specified for animal feed (arsenic), showing visible stress symptoms in the seedlings. Conversely, revegetated BR show improved chemical properties, allow seed germination, and permits seedling growth with no evidence of trace element phytotoxicity. However, Na can be taken up at concentrations that could elicit phytotoxicity and impair the success of revegetation. For future rehabilitation programmes, direct revegetation on BR after the incorporation of amendments such as gypsum and organic matter is recommended.

Influence of Na2CO3 and K2CO3 Addition on Iron Grain Growth during Carbothermic Reduction of Red Mud

Red mud is a by-product of alumina production from bauxite ore by the Bayer method, which contains considerable amounts of valuable components such as iron, aluminum, titanium, and scandium. In this study, an approach was applied to extract iron, i.e., carbothermic reduction roasting of red mud with sodium and potassium carbonates followed by magnetic separation. The thermodynamic analysis of iron and iron-free components’ behavior during carbothermic reduction was carried out by HSC Chemistry 9.98 (Outotec, Pori, Finland) and FactSage 7.1 (Thermfact, Montreal, Canada; GTT-Technologies, Herzogenrath, Germany) software. The effects of the alkaline carbonates’ addition, as well as duration and temperature of roasting on the iron metallization degree, iron grains’ size, and magnetic separation process were investigated experimentally. The best conditions for the reduction roasting were found to be as follows: 22.01% of K2CO3 addition, 1250 °C, and 180 min of duration. As a generalization of the obtained data, the mechanism of alkaline carbonates’ influence on iron grain growth was proposed.

Selective Parameters and Bioleaching Kinetics for Leaching Vanadium from Red Mud Using Aspergillus niger and Penicillium tricolor

In the present study, using Aspergillus niger and Penicillium tricolor, the influence of the selected parameters, including sucrose concentration, inoculation size of spores, pulp density, and pre-culture time, on the bioleaching efficiency (biomass, organic acids production, and vanadium extraction, respectively) of red mud were studied. The bioleaching kinetics under optimal conditions were also explored. Sucrose concentration showed a positive linear effect on bioleaching efficiency below 143.44 and 141.82 g/L using A. niger and P. tricolor, respectively. However, a higher concentration was unfavorable for vanadium extraction. The inoculation size of spores showed an insignificant effect on both biomass and vanadium extraction if it exceeded the lowest coded levels (0.5 × 107/mL). Red mud pulp density showed a negative effect on the bioleaching efficiency of A. niger but a positive effect on organic acids production and vanadium extraction of P. tricolor. A pre-culture was indispensable for A. niger but not for P. tricolor due to the fact of its isolation from the red mud examined in this study. The kinetics analysis showed that the leaching rate of vanadium followed a two-domain behavior: initially, a rapid leaching period of approximately 10–15 days and, subsequently, a slow leaching period. Considering the change of the particles’ appearance as well as in the elemental composition of the bioleached red mud, it is speculated that the rate of leaching agents through the silicon minerals was the rate-limiting step of dissolution kinetics under the fungal bioleaching process.

Effect of amendments on the leaching behavior of alkaline anions and metal ions in bauxite residue

A column leaching experiment was used to investigate the efficacy of amendments on their ability to remove alkaline anions and metal ions from bauxite residue leachates. Treatments included, simulated acid rain (AR), phosphogypsum + vermicompost (PVC), phosphogypsum + vermicompost + simulated acid rain (PVA), and biosolids + microorganisms (BSM) together with controls (CK). Results indicated that amendment could effectively reduce the leachate pH and EC values, neutralize OH^-, CO₃^2-, HCO₃^-, and water soluble alkali, and suppress arsenic (As) content. Correlation analysis revealed significant linear correlations with pH and concentrations of OH^-, CO₃^2-, HCO₃^-, water-soluble alkali, and metal ions. BSM treatment showed optimum results with neutralizing anions (OH^-, CO₃^2-, and HCO₃^-), water soluble alkali, and removal of metal ions (Al, As, B, Mo, V, and Na), which was attributed to neutralization from the generation of small molecular organic acids and organic matter during microbial metabolism. BSM treatment reduced alkaline anions and metal ions based on neutralization reactions in bauxite residue leachate, which reduced the potential pollution effects from leachates on the soil surrounding bauxite residue disposal areas.

Characterization of Trichoderma asperellum RM-28 for its sodic/saline-alkali tolerance and plant growth promoting activities to alleviate toxicity of red mud

Red mud (RM) is a highly alkaline, saline and sodic solid by-product released by alumina industries, which pose an economical and environmental problem and establishment of vegetation on these sites is a big challenge. In the present study, a fungus RM-28 exhibiting high tolerance to alkaline (pH 12), saline/sodic (NaCl 4%) was isolated from RM flooded rhizosphere soil of bermudagrass and tested its ability to reduce RM toxicity and promote the growth of sorghum-sudangrass seedlings. This fungus also exhibited high tolerance to heavy metal(loid)s (HMs) and desirable plant growth-promoting traits. This fungus was identified as Trichoderma asperellum based on its internal transcribed spacer (ITS) of rDNA and translation elongation factor-1α (TEF 1α) gene analysis. This fungus was effective in reducing the pH and solubilizing tricalcium phosphate under high alkaline and saline conditions in vitro. Further, RM-28 inoculation significantly lowered the pH and EC of the red mud from 11.8 to 8.2 and 2.23 to 1.42, respectively. Inoculation of RM-28 significantly improved the growth, chlorophyll content and reduced the oxidative stress of sorghum-sudangrass seedlings grown in red mud leachate. These observations suggest that T. asperellum RM-28 serves as potential source for the establishment of vegetation on red mud/red mud contaminated soils.

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.

Changes in distribution and microstructure of bauxite residue aggregates following amendments addition

Bauxite residue is a highly alkaline byproduct which is routinely discarded at residue disposal areas. Improving soil formation process to revegetate the special degraded lands is a promising strategy for sustainable management of the refining industry. A laboratory incubation experiment was used to evaluate the effects of gypsum and vermicompost on stable aggregate formation of bauxite residue. Aggregate size distribution was quantified by fractal theory, whilst residue microstructure was determined by scanning electron microscopy and synchrotron-based X-ray micro-computed tomography. Amendments addition increased the content of macro-aggregates (>250 μm) and enhanced aggregate stability of bauxite residue. Following gypsum and vermicompost addition, fractal dimension decreased from 2.84 to 2.77, which indicated a more homogeneous distribution of aggregate particles. Images from scanning electron microscopy and three-dimensional microstructure demonstrated that amendments stimulate the formation of improved structure in residue aggregates. Pore parameters including porosity, pore throat surface area, path length, and path tortuosity increased under amendment additions. Changes in aggregate size distribution and microstructure of bauxite residue indicated that additions of gypsum and vermicompost were beneficial to physical condition of bauxite residue which may enhance the ease of vegetation.

Phosphogypsum stabilization of bauxite residue: Conversion of its alkaline characteristics

Reduction of the high alkalinity of bauxite residue is a key problem to solve to make it suitable for plant growth and comprehensive utilization. In this study, phosphogypsum, a waste product from the phosphate fertilizer industry, was used to drive the alkaline transformation of the bauxite residue. Under optimal water washing conditions (liquid/solid ratio of 2 mL/g, 30°C, 24 hr), the impact of quantity added, reaction time and reaction mechanism during phosphogypsum application were investigated. Phosphogypsum addition effectively lowered pH levels and reduced the soluble alkalinity by 92.2%. It was found that the concentration of soluble Na and Ca ions in the supernatant increased gradually, whilst the exchangeable Na⁺ and Ca²⁺ in solid phase changed 112 mg/kg and 259 mg/kg, respectively. Ca²⁺ became the dominant element in the solid phase (phosphogypsum addition of 2%, liquid/solid ratio of 2 mL/g, 30°C, 12 hr). X-ray diffraction data indicated that cancrinite and hydrogarnet were the primary alkaline minerals. SEM images suggested that phosphogypsum could promote the formation of stable macro-aggregates, whilst the content of Ca²⁺ increased from 5.6% to 18.2% and Na reduced from 6.8% to 2.4%. Treatment with phosphogypsum could significantly promote the transformation of alkalinity cations by neutralization, precipitation and replacement reactions. This research provided a feasible method to promote soil formation of bauxite residue by phosphogypsum amendment.

Growth of Rhodes grass and leaching of ions from seawater neutralized bauxite residues after amendment with gypsum and organic wastes

A 32-week leaching column study was carried out in the greenhouse to investigate the effects of incorporation of gypsum, cattle manure, biosolids, gypsum plus cattle manure and gypsum plus biosolids into the topsoil layer (0-10 cm) on growth of Rhodes grass, and on root distribution and chemical and microbial properties in the topsoil and subsoil (10-30 cm) layers of seawater neutralized bauxite residues. The columns were leached for a period of 8 weeks prior to sowing Rhodes grass and during that time the bulk of the salts accumulated during seawater neutralization were leached out. The main cation leached was Na⁺ and the main balancing anions were Cl^- and SO₄^2-. During this period the pH of leachates rose from 7 to 8 up to 9-10. At the end of the study, exchangeable Na and pH were lowered in the surface horizon by all treatments with a combination of gypsum plus organic amendments having the greatest effect. The latter treatments also caused a significant decrease in pH, extractable Al and exchangeable Na in the subsoil. Rhodes grass dry matter production followed the order Control < gypsum < cattle manure = gypsum plus cattle manure < biosolids = gypsum plus biosolids. Growth of roots into the subsoil layer was inhibited in the Control and gypsum treatments but when organic amendments were applied, 50% or more of root dry matter was recovered in the subsoil layer. It was concluded that incorporating a combination of gypsum and organic matter into the surface soil is an effective strategy for revegetation of bauxite residue.

Bioleaching of Major, Rare Earth, and Radioactive Elements from Red Mud by using Indigenous Chemoheterotrophic Bacterium Acetobacter sp

The aim was to study the bioleaching performance of chemoheterotrophic bacterium involved in leaching of major, rare earth, and radioactive elements from red mud (RM), and to explore the underlying mechanism. An acid-producing bacterium, identified as Acetobacter sp., was isolated from RM impoundment and used in the bioleaching experiments under one-step, two-step and spent medium process at up to 10% pulp density. The results showed that the leaching ratios of Al, Lu, Y, Sc, and Th were 55%, 53%, 61%, 52%, and 53% respectively under one-step process at 2% pulp density. Under both one- and two-step processes at 2% pulp density, the radioactivity of bioleached RM can meet the relevant regulation in China. The total amount of organic acids excreted by Acetobacter sp. increased with an increase of RM pulp density. After bioleaching, contents of hematite and gibbsite decreased but perovskite increased in RM. Micromorphology analysis indicated that the cells of Acetobacter sp. adhered to RM particles and formed large-size aggregates, and a new crystal of weddellite emerged. In view of the shorter lag phase and smaller biomass comparing to fungi even under direct contact with RM, bacterium Acetobacter sp. is supposed to apply to in situ heap or dump bioleaching of RM.