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

Migration and transformation behaviors of potentially toxic elements and the underlying mechanisms in bauxite residue: Insight from various revegetation strategies

Revegetation is a promising strategy for large-scale bauxite residue disposal and management, potentially influencing the geochemical stability of potentially toxic elements (PTEs) through rhizosphere processes. However, the geochemical behaviors of PTEs and the underlying mechanisms during bauxite residue revegetation remain unclear. This study examined the migration and transformation behaviors of PTEs and their underlying mechanisms in the bauxite residue-vegetation-leachate system under various revegetation strategies, including single and co-planting of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), over a 100-day microcosm experiment. The results showed significant decreases in pH, EC, Na, Al, and Cr levels in the leachate under various revegetation strategies, with slight increases in Cu, V, As, and Pb. Over time, the pH, EC, Na, Cr, Cu, V, Pb, and As levels in the leachate decreased, while those of Al, Fe, Mn, and Zn increased. The mean pH, EC, and concentrations of Na, Al, Fe, and Cr in the leachate of the revegetated treatments decreased by 6%-8%, 21%-33%, 2%-4%, 19%-27%, 7%-22%, and 15%-26%, respectively, while the mean concentrations of Mn, V, Zn, and As increased by 47%-134%, 26%-46%, 39%-47%, and 3%-10%, respectively, compared to the unamended treatment. Co-planting generally exhibited a greater impact on leachate components compared to single planting. Available contents of Al, Cr, and Pb decreased by 81%-83%, 57%-77%, and 55%-72%, respectively, while those of other PTEs increased in the revegetated bauxite residue. Co-planting significantly reduced the availability of PTEs compared to single planting. Except for Na and Mn, the bioaccumulation and transportation factors of PTEs in both vegetation species remained below 1 under various revegetation strategies. The migration and transformation behaviors of PTEs in the bauxite residue-vegetation-leachate system were mainly influenced by pH and nutrient levels. These findings provide new insights into the migration and transformation behaviors of PTEs during bauxite residue revegetation.

Application of positively charged red mud-based carriers for anaerobic ammonium oxidizing bacteria biofilm formation

Enrichment of anaerobic ammonium oxidizing bacteria (AnAOB) in the anaerobic ammonium oxidation (anammox) process remains a challenge in engineering applications. In this study, for faster enrichment of AnAOB, red mud-based carriers were positively charged with FeCl3 and microbial promoters. The experimental results showed that the average nitrogen removal rates of the red mud-based carriers (RMGA), FeCl3-modified red mud-based carriers (RMGA-Fe), and microbial promoter-modified red mud-based carriers (RMGA-MA) increased by 24.4%-175% compared with the commercially available K3 carriers. Combining the results of SEM and laser confocal microscope, electrostatic attraction played a leading role on loading AnAOB. The increase in the surface roughness and specific area of the carriers after the attachment and the presence of Fe2+, Ca2+, and Mg2+ plasma were necessary for the subsequent growth of AnAOB, while the leaching of Cr(VI) promoted the nitrogen removal effect of AnAOB.

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

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

Red mud with enhanced dealkalization performance by supercritical water technology for efficient SO2 capture

The total de-alkalization treatment of industrial solid waste red mud (RM) has been a worldwide challenge. Removing the insoluble structural alkali fraction from RM is the key to enhancing the sustainable utilization of RM resources. In this paper, supercritical water (SCW) and leaching agents were used for the first time to de-alkalize the Bayer RM and to remove sulfur dioxide (SO2) from flue gas with the de-alkalized RM slurry. The results showed that the optimum alkali removal and Fe leaching rates of RM-CaO-SW slurry were 97.90 ± 0.88% and 82.70 ± 0.95%, respectively. Results confirmed that the SCW technique accelerated the disruption of (Al-O) and (Si-O) bonds and the structural disintegration of aluminosilicate minerals, facilitating the conversion of insoluble structural alkalis to soluble chemical alkalis. The exchangeable Ca2+ displaced Na+ in the remaining insoluble base, producing soluble sodium salts or alkalis. CaO consumed SiO2, which was tightly bound to Fe2O3 in RM, and released Fe2O3, which promoted Fe leaching. RM-SCW showed the best desulfurization performance, which maintained 88.99 ± 0.0020% at 450 min, followed by RM-CaO-SW (450 min, 60.75 ± 6.00%) and RM (180 min, 88.52% ± 0.00068). The neutralization of alkaline components, the redox of metal oxides, and the liquid-phase catalytic oxidation of Fe contributed to the excellent desulfurization performance of the RM-SCW slurry. A promising approach shown in this study is beneficial to RM waste use, SO2 pollution control, and sustainable growth of the aluminum industry.

Environmental and agronomic assessment of soil conditioners produced from bauxite residue and oil palm wastes

Soil conditioner is class of products used to enhance physics, physicochemical or soil biological activities, being able to recover disturbed or nutritional unbalanced soils. The formulation of a soil conditioner composed by bauxite residue (BR), and organic oil palm wastes, as raw materials, was recently proposed as an innovative strategy for the Brazilian acid soils amendment. Here we show the results of soil conditioner amended soil leaching tests and agronomical performance. The soil conditioners were formulated by BR mixed with decomposed POC (palm oil compost) and non-decomposed POMW (palm oil mill waste) oil palm wastes, in the proportion of 25% BR + 75% POC (T1) and 50% BR + 50% POMW (T2), in addition to the treatment with 100% POMW without BR (T3) and limestone at a dose calculated to raise soil pH to 6.0 (T4). Except for T4, all conditioners were applied to the soil at doses of 40, 80, and 120 t ha-1 for leaching tests. The experimental plots were composed of polyvinyl chloride columns, filled with 5 kg of soil, with bottles adapted with hoses at the bottom to facilitate drainage of the leachate. After leaching tests, the respective columns were used as pots for the cultivation of Brachiaria grass, stage with addition of a control composed by undisturbed soil (T5). The pH of the leachates had changes, but the use of BR associated with POMW was similar to the use of limestone. Of the 65 chemical elements evaluated, only nine were identified in the leachate, being most of them considered as plant nutrients. As for soil pH, limestone was slightly higher (6.6) than treatments that had BR (5.5). Brachiaria grass cultivated in the soil amended with conditioners showed similar results of limestone treated soil for the parameters of plant development and showed fertility improvement.

Molecular insights into the chemodiversity of dissolved organic matter and its interactions with the microbial community in eco-engineered bauxite residue

Dissolved organic matter (DOM) plays an important role in the biogeochemical function development of bauxite residue. Nevertheless, the DOM composition at the molecular level and its interaction with microbial community during soil formation of bauxite residue driven by eco-engineering strategies are still relatively unknown. In the present study, the DOM composition at the molecular level and its interactions with the microbial community in amended and revegetated bauxite residue were explored. The results showed that the amendment applications and revegetation enhanced the accumulation of unsaturated molecules with high values of double bond equivalent (DBE) and nominal oxidation of carbon (NOSC) and aromatic compounds with high values of modified aromaticity index (AI_mod) as well as the reduction of average weighted molecular mass of DOM molecules. Significant correlations between DOM molecules and the microbial community and Fe/Al oxides were found. DOM molecules were decomposed by the microbial community and sequestered onto Fe/Al oxides, which were the main driving factors that changed DOM chemodiversity in the amended and revegetated bauxite residue. These findings are beneficial for understanding the biogeochemical behaviours of DOM and providing a critical basis for the development of eco-engineering strategies towards soil formation and the sustainable revegetation of bauxite residue.

Bauxite residue valorization - Soil conditioners production through composting with palm oil mill residual biomass

Bauxite residue (BR) is a by-product of Bayer process, which is applied for alumina production. Due to its inherent alkalinity and sodicity, the use of BR is globally limited to 23% of the 150 million tons (Mt) produced annually. Maximizing alternative and large-scale uses of BR is a game changer to promote the sustainability of the aluminum production chain. As a strategy for BR valorization, a soil conditioner composed of BR and palm oil residual biomass was proposed. Here we evaluate the BR (25%, 50% and 75%) batch composting with raw palm oil mill waste (POMW) and palm oil compost (POC). The pH, EC, total N and organic carbon, C:N ratio, water holding capacity (WHC), cation exchange capacity (CEC), granulometry and elemental composition were determined after 90 days of composting. Changes in temperature, pH and EC curves were observed during composting of soil conditioners for 90 days. Composting reduced the alkalinity and sodicity of BR, increasing CEC, moisture, organic carbon and total nitrogen. The formulation containing 25% of BR and 75% of POC showed WHC ≥ 60% and CEC ≥ 200 mmol_c·kg^-1, meeting the Brazilian legislation for production and commercialization of soil conditioners. This strategy could potentially consume 7.6% of all BR produced annually in the largest Brazilian alumina refinery. Concentrations of potentially toxic elements were far below the allowable levels in all formulations. Major and minor plant nutrients were present and the composting aggregated small particles in BR. Composting of BR is a new alternative for the valorization of mining tailings, allowing the development of an environmentally friendly and zero-waste product, which can be applied on a large scale in agriculture to improve soil fertility.

Dynamic Variations of Soil-Formation Indicators in Bauxite Residue Driven by the Integration of Waste Solids and Microorganisms

Soil-formation process is critical to ecological rehabilitation on bauxite residue disposal areas. In this study, a soil column experiment was taken to assess the dynamic variations of soil-formation indicators in bauxite residue driven by the integration of waste solids and microorganisms. Results showed that the combination of waste solids and microorganisms significantly decreased the alkalinity, accumulated organic carbon content, and improved aggregate stability of bauxite residue. Compared with waste solids treatments, the addition of acid-producing microorganisms enhanced the changes of soil-formation indicators. The integration of waste solids and microorganisms increased the content of aliphatic carbon, presenting low thermal stability in the residues. The integration of waste solids and microorganisms provides a potentially effective method for soil formation and ecological remediation on bauxite residue disposal areas.

Improvements on physical conditions of bauxite residue following application of organic materials

Soil formation and ecological rehabilitation is the most promising strategy to eliminate environmental risks of bauxite residue disposal areas. Its poor physical structure is nevertheless a major limitation to plant growth. Organic materials were demonstrated as effective ameliorants to improve the physical conditions of bauxite residue. In this study, three different organic materials including straw (5% W/W), humic acid (5% W/W), and humic acid-acrylamide polymer (0.2% and 0.4%, W/W) were selected to evaluate their effects on physical conditions of bauxite residue pretreated by phosphogypsum following a 120-day incubation experiment. The proportion of 2-1 mm macro-aggregates, mean weight diameter (MWD) and geometric mean diameter (GWD) increased following organic materials addition, which indicated that organic materials could enhance aggregate stability. Compared with straw, and humic acid, humic acid-acrylamide polymer application had improved effects on the formation of water-stable aggregates in the residues. Furthermore, organic materials increased the total porosity, total pore volume and average pore diameter, and reduced the micropore content according to nitrogen gas adsorption (NA) and mercury intrusion porosimetry (MIP) analysis, whilst enhancing water retention of the residues based on water characteristic curves. Compared with traditional organic wastes, humic acid-acrylamide polymer could be regarded as a candidate according to the comprehensive consideration of the additive amount and the effects on physical conditions of bauxite residue. These findings could provide a novel application to both Ca-contained acid solid waste and high-molecular polymers on ecological rehabilitation at disposal areas.

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.

An Eco-Friendly Acid Leaching Strategy for Dealkalization of Red Mud by Controlling Phase Transformation

The alkaline components in red mud represent one of the crucial factors restricting its application, especially for the construction and building industry. The phase state of alkaline components has a significant influence on the dealkalization of red mud. In this work, an environmentally friendly acid leaching strategy is proposed by controlling the phase transformation of red mud during active roasting pretreatment. With a moderate roasting temperature, the alkaline component is prevented from converting into insoluble phases. After acid leaching with a low concentration of 0.1 M, a high dealkalization rate of 92.8% is obtained. Besides, the leachate is neutral (pH = 7) and the valuable metals in red mud are well preserved, manifesting a high selectivity and efficiency of diluted acid leaching. The calcination experiment further confirms the practicability of the strategy in the construction field, where the cementitious minerals can be formed in large quantities. Compared with the traditional acid leaching routes, the diluted acid leaching strategy in this work is acid saving with low valuable element consumption. Meanwhile, the secondary pollution issue can be alleviated. Hence, the findings in this work provide a feasible approach for the separation and recovery of alkali and resource utilization of red mud.

Insights into variations on dissolved organic matter of bauxite residue during soil-formation processes following 2-year column simulation

Bauxite residue, an industrial alkaline solid waste, has a low organic carbon content which hinders plant growth. Dissolved organic matter (DOM) drives many biogeochemical processes including carbon storage and soil formation in soils. Input of exogenous organic materials may provide organic carbon and accelerate soil formation processes in bauxite residue. However, the potential effects of ameliorants on the quantity and quality of DOM in bauxite residue are still poorly understood. Here, the integration of ultraviolet-visible (UV-Vis) spectra, fluorescence spectra, and parallel factor (PARAFAC) analysis were used to investigate the vertical characteristics of DOM in bauxite residue treated by PV (the combined addition of 2% phosphogypsum and 4% vermicompost, w/w) and BS (6% w/w including 4% bagasse and 2% bran) with 2-year column experiments. The content of DOM in untreated residues ranged from 0.064 to 0.096 g/kg, whilst higher contents of DOM were observed in PV (0.13 g/kg) and BS (0.26 g/kg) treatment. Meanwhile, with the increase of residue depth, the aromaticity and hydrophobic components of DOM in residue decreased, which indicated that the degree of humification of the treated residues in the upper layer was higher than that in the lower layer. Compared with BR, BS and PV treatment accumulated the related content of fulvic acid-like substance from 36.14% to 71.33% and 74.86%, respectively. The incorporation of vermicompost and biosolids increased the content of humic-like substances, whilst decreasing the content of protein-like substances in the surface layer, which may be due to the enrichment of the microbial community. During soil formation processes, the application of organic amendments reduced both salinity and alkalinity, enhanced microbial community diversity, and changed the quantity and quality of DOM in bauxite residue. These findings improve our understanding of the dynamics of DOM and response of DOM to soil formation processes in bauxite residue.

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

In the work, we employed a hydrothermal method for modification of red mud using colloidal silica and sodium hydroxide under mild conditions, and applied it into adsorbing Pb(II) ions in aqueous solutions. In the modification, zeolite structure was formed. The adsorption experiments found that the adsorption capacity of the modified red mud for Pb(II) ions was significantly improved, almost 10 times as much as that of the original red mud. Both the pseudo-first-order and pseudo-second-order kinetic equation can describe the adsorption process, indicating it a more complicated interaction. Langmuir and Dubinin-Radushkevich models well fit the adsorption isotherm, indicating that the modified red mud mainly removes lead ions from aqueous solution by monolayer physical adsorption. According to the fitting results, the saturated adsorption capacity of Pb (II) by the modified red mud is 551.11 mg/g, confirming its high efficiency adsorption performance. XRD, FTIR, XPS and SEM-EDS all detected the formation of PbCO₃ and Pb₃(CO₃)₂(OH)₂. It was speculated that the adsorption mechanism should be attributed to the joint contribution of ion exchange and precipitation. The excellent performance of the modified red mud on Pb(II) ions adsorption makes it a promising candidate for the treatment of wastewater contaminated by heavy metal ions.

Physical and thermal insulation properties of the composites based on seawater-neutralised bauxite residue

The objective of this study is to evaluate the possibility of using bauxite residue (BR), which is a highly hazardous waste, as a filler in thermoset polymer matrices for applications in insulating construction materials to minimise heat loss in buildings. Unsaturated polyester resin (UPR) blended with the seawater-neutralised BR in a ratio of 0-60 vol.% was transformed into a solid via a crosslinking process at room temperature. The pristine polyester and BR-UPR composites were characterised to ascertain their structural, physical and thermal properties. The results indicate that BR is a good filler that can be used with UPR to develop thermal insulation/construction materials. The UPR composite containing 40 vol.% BR is considered to be the optimum mixture with respect to the tested physical and thermal properties because it utilises a large amount of filler and shows promising thermal insulation characteristics with a thermal conductivity of 0.096 W/(m∙K), a thermal diffusivity of 0.161 mm²/s, a 24-h water retention of 0.15 % and a bulk density of 1484 kg/m³. Furthermore, the thermal stability of the prepared composites is positively influenced by the incorporation of BR. The overall weight loss upon heating to 900 °C decreased from 99.85 % (pristine polyester) to 46.68 % (60 vol.% BR composite).

Formation of hydroperoxo (-OOH) species on the surface of self-doped Bi2.15WO6: reactivity towards As(iii) oxidation

Bi2+xWO6 is a cost-effective and environmentally friendly photocatalyst that shows high reactivity in the oxidation of various contaminants under visible light. However, under alkaline conditions, the reactive oxidative species in the Bi2+xWO6 system are still not clear yet. In this study, it is observed that the oxidation rates of As(iii) increase with increasing pH values in the Bi2.15WO6 system. Photoluminescence and the Mott-Schottky analyses confirm that OH- promotes the separation and transfer of photogenerated electron-hole pairs over Bi2.15WO6, thus facilitating the oxidation of As(iii). Electron spin resonance spectra analysis and quenching experiments rule out contributions of •OH, O2˙-, 1O2 and superoxo species to As(iii) oxidation and indicate that surface -OOH and/or H2O2 are indeed the predominant species under alkaline conditions. The improved production of H2O2 by H-donors such as glucose and phenol, as well as the UV-vis diffuse reflectance and Raman analyses, further confirms the formation of surface -OOH on Bi2.15WO6 under alkaline conditions. In the dark, the significant higher oxidation rate of As(iii) by H2O2-Bi2.15WO6 than that by H2O2 alone reveals that surface -OOH, instead of H2O2, plays an important role in As(iii) oxidation. This study enriches our understanding of the diversity of reactive oxygen species (ROS) in the Bi2.15WO6 system and gives new insight into the mechanism involved in the oxidation of As(iii) under alkaline conditions.

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.

Phosphate recovery with granular acid-activated neutralized red mud: Fixed-bed column performance and breakthrough curve modelling

Granular acid-activated neutralized red mud (AaN-RM) has been successfully prepared with good chemical stability and physical strength. However, its potential for industrial application remains unknown. Therefore, the performance of granular AaN-RM for phosphate recovery in a fixed-bed column was investigated. The results demonstrated that the phosphate adsorption performance of granular AaN-RM in a fixed-bed column was affected by various operational parameters, such as the bed depth, flow rate, initial solution pH and initial phosphate concentration. With the optimal empty-bed contact time (EBCT) of 24.27 min, the number of processed bed volumes and the phosphate adsorption capacity reached 496.95 and 84.80 mg/g, respectively. Then, the saturated fixed-bed column could be effectively regenerated with a 0.5 mol/L HCl solution. The desorption efficiency remained as high as 83.45% with a low weight loss of 3.57% in the fifth regeneration cycle. In addition, breakthrough curve modelling showed that a 5-9-1 feed-forward artificial neural network (ANN) could be effectively applied for the optimization of the fixed-bed adsorption system; the coefficient of determination (R²) and the root mean square error (RMSE) evaluated on the validation-testing data were 0.9987 and 0.0183, respectively. Therefore, granular AaN-RM fixed-bed adsorption exhibits promising potential for phosphate removal and recovery from polluted water.

The dynamic development of bacterial community following long-term weathering of bauxite residue

Bauxite residue is the industrial waste generated from alumina production and commonly deposited in impoundments. These sites are bare of vegetation due to the extreme high salinity and alkalinity, as well as lack of nutrients. However, long term weathering processes could improve residue properties to support the plant establishment. Here we investigate the development of bacterial communities and the geochemical drivers in bauxite residue, using Illumina high-throughput sequencing technology. Long term weathering reduced the pH in bauxite residue and increased its nutrients content. The bacterial community also significantly developed during long term weathering processes. Taxonomic analysis revealed that natural weathering processes encouraged the populations of Proteobacteria, Chloroflexi, Acidobacteria and Planctomycetes, whereas reducing the populations of Firmicutes and Actinobacteria. Redundancy analysis (RDA) indicated that total organic carbon (TOC) was the dominant factors affecting microbial structure. The results have demonstrated that natural weathering processes improved the soil development on the abandoned bauxite residue disposal areas, which also increased our understanding of the correlation between microbial variation and residue properties during natural weathering processes in Bauxite residue disposal areas.

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).