Sb/As immobilization and soil function improvement under the combined remediation strategy of modified biochar and Sb-oxidizing bacteria at a smelting site.

Antimony (Sb) and arsenic (As) lead to soil pollution and structural degradation at Sb smelting sites. However, most sites focus solely on Sb/As immobilization, neglecting the restoration of soil functionality. Here, we investigated the effectiveness of Fe/H2O2 modified biochar (Fe@H2O2-BC) and Sb-oxidizing bacteria (Bacillus sp. S3) in immobilizing Sb/As and enhancing soil functional resilience at an Sb smelting site. Over a twelve-month period, the leaching toxicity of As and Sb was reduced to 0.05 and 0.005 mg L-1 (GB3838-2002) respectively, with 1% (w/w) Fe@H2O2-BC and 2% (v/v) Bacillus sp. S3 solution. Compared to CK, the combination of Fe@H2O2-BC and Bacillus sp. S3 significantly reduced the bioavailable As/Sb by 98.00%/93.52%, whilst increasing residual As and reducible Sb fractions by 210.31% and 96.51%, respectively. The combined application generally improved soil aggregate structure, pore characteristics, and water-holding capacity. Fe@H2O2-BC served as a pH buffer and long-term reservoir of organic carbon, changing the availability of carbon substrates to bacteria. The inoculation of Bacillus sp. S3 facilitated the transformation of Sb(III)/As(III) to Sb(V)/As(V) and differentiated the composition and functional roles of bacterial communities in soils. The combination increased the abundance of soil saprotrophs by 164.20%, whilst improving the relative abundance of N- and S-cycling bacteria according to FUNGuild and FAPROTAX analysis. These results revealed that the integrated application was instrumental in As/Sb detoxification/immobilization and soil function restoration, which demonstrating a promising microbially-driven ecological restoration strategy at Sb smelting sites.

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.

Remediation potential of magnetic biochar in lead smelting sites: Insight from the complexation of dissolved organic matter with potentially toxic elements.

Magnetic biochar has been widely used in potentially toxic elements (PTEs) polluted soils due to its magnetic separation capability and synchronous immobilization for multiple metals. However, the contribution of magnetic biochar to soil dissolve organic material (SDOM) and its binding behavior with PTEs needs to be further clarified prior to its remediation application on lead smelting sites. In this study, multi-spectral techniques of excitation-emission matrix (EEM) fluorescence spectroscopy and two-dimensional FTIR correlation spectroscopy (2D-FTIR-COS) were used to explore the evolution characteristics of SDOM in the lead smelting site under the remediation of magnetic biochar, and to further analyze its affinity and binding behavior with Pb and As. Results showed that magnetic biochar significantly increased SDOM content and decreased Pb and As available content. EEM and parallel factor analysis (EEM-PARAFAC) and Self-Organizing map analysis showed that humus-like and aromatic DOM increased and microbial-derived SDOM decreased after magnetic biochar cultivation. Furthermore, 2D-FTIR-COS correlation spectroscopy analysis indicated that BDOM had a stronger binding affinity to Pb, while SDOM has a stronger binding affinity to As. The binding sequences of different DOMs to PTEs varied greatly, the carboxyl and amide groups of SDOM and BDOM showed a remarkable and rapid response. Our results enhance the insights of magnetic biochar on soil function and PTEs remediation potential, providing novel information for its environmental remediation application.

The assembly process and co-occurrence patterns of soil microbial communities at a lead smelting site.

The potential toxic elements of the site are diverse and complex, seriously threatening the land utilization potential and soil ecological function. Microbial community is critical to maintaining ecosystem function, their assembly processes and diversity play an essential role in predicting changes in soil ecological function. However, our understanding of the mechanisms that shape community composition and successional direction in complex polluted environments is very limited. In this study, to explore the mechanisms driving community assembly and symbiosis in different contaminated regional environments, the biological characteristics of bacterial and fungal communities in four different polluted areas of a typical lead smelting site were studied. Contamination by PTEs appears to increase microbial networks, as well as altering microbial community composition, with relative abundance of dominant phyla such as Actinomycetes and Acidobacteria decreasing, whilst Proteobacteria and Ascomycota increased, this indicated that communities may shift from K-strategy to r-strategy and become opportunistic. Dispersal limitation (DL, 42 %-86 %), drift (Dr, 8 %-37 %) and homogeneous selection (HoS, 1 %-31 %) proved to be the important community assembly process. The top ten bins controlling the contribution of different biological processes were identified, and the relative abundance of these bacterial and fungal taxa varied with CPI. Collectively, our results suggest that CPI and nutrient availability regulate soil bacterial and fungal community assembly processes. The results of this study provide potential guidance for community regulation in the process of ecological restoration and mitigating degraded soils at smelting sites.

Effect of potentially toxic elements on soil multifunctionality at a lead smelting site.

Contaminated soil at smelting sites affects land utilization and environmental regulation, resulting in soil degradation. However, the extent to which potentially toxic elements (PTEs) contribute to site soil degradation and the relationship between soil multifunctionality and microbial diversity in the process remains poorly understood. In this study, we investigated changes in soil multifunctionality and the correlation between soil multifunctionality and microbial diversity under the influence of PTEs. The change in microbial community diversity was closely related to changes in soil multifunctionality caused by PTEs. Microbial diversity, not richness, drives the delivery of ecosystem services in smelting site PTEs-stressed environments. Structural equation modeling identified that soil contamination, microbial taxonomic profile and microbial functional profile could explain 70% of the variance in soil multifunctionality. Furthermore, our findings demonstrate that PTEs limit soil multifunctionality by affecting soil microbial communities and functionality, whilst the positive effect of microorganisms on soil multifunctionality was mainly driven by the fungal diversity and biomass. Finally, specific fungal genera closely related to soil multifunctionality were identified, with saprophytic fungi being particularly important for maintaining multiple soil functions. The results of the study provide potential guidance for the remediation, pollution control practices and mitigation of degraded soils at smelting sites.

Combination of Biochemical, Molecular, and Synchrotron-Radiation-Based Techniques to Study the Effects of Silicon in Tomato (Solanum Lycopersicum L.).

The work focused on the analysis of two cultivars of tomato (Solanum lycopersicum L.), Aragon and Gladis, under two different treatments of silicon, Low, 2 L of 0.1 mM CaSiO3, and High, 0.5 mM CaSiO3, weekly, for 8 weeks, under stress-free conditions. We subsequently analyzed the morphology, chemical composition, and elemental distribution using synchrotron-based µ-XRF techniques, physiological, and molecular aspects of the response of the two cultivars. The scope of the study was to highlight any significant response of the plants to the Si treatments, in comparison with any response to Si of plants under stress. The results demonstrated that the response was mainly cultivar-dependent, also at the level of mitochondrial-dependent oxidative stress, and that it did not differ from the two conditions of treatments. With Si deposited mainly in the cell walls of the cells of fruits, leaves, and roots, the treatments did not elicit many significant changes from the point of view of the total elemental content, the physiological parameters that measured the oxidative stress, and the transcriptomic analyses focalized on genes related to the response to Si. We observed a priming effect of the treatment on the most responsive cultivar, Aragon, in respect to future stress, while in Gladis the Si treatment did not significantly change the measured parameters.

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.

Novel Plant Growth-Promoting Bacteria Isolated from Bauxite Residue: The Application for Revegetation.

Microbial inoculation with appropriate inorganic-organic amendments is a promising strategy for ecological rehabilitation at bauxite residue disposal areas. Nevertheless, research on screening suitable plant growth-promoting bacteria with tolerance to highly sodic-alkalinity is very limited in the literature. In this study, novel plant growth-promoting bacteria isolated from bauxite residue were used to investigate their potential for revegetation. Under high saline-alkalinity stress, inoculation of Z18 and Z28 increased the activity of antioxidative enzymes, whilst improving chlorophyll and carotenoid contents in ryegrass. Inoculation of the selected strains greatly reduced damage to organelles in ryegrass as observed by transmission electron microscopy. Based on 90-day soil incubation, inoculated strains improved physicochemical properties of bauxite residue and improved plant growth. These findings suggest that Z18 and Z28 may be selected as potential strains for vegetation establishment, aiding microbial remediation at bauxite disposal areas.

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.

Spatial distribution of toxic metal(loid)s at an abandoned zinc smelting site, Southern China.

Toxic metal(loid) (TM) soil pollution at large-scale non-ferrous metal smelting contaminated sites is of great concern in China, but there are no detailed reports relating to them. A comprehensive study was conducted to determine contamination characteristics and horizontal and vertical spatial distribution patterns of soils at an abandoned zinc smelting site in Southern China. The spatial distribution of TMs revealed that soil environmental quality was seriously threatened, with Cd, Zn, As, Pb and Hg being the main contaminants present. The distribution of all TMs showed strong spatial heterogeneity and were expressed as a "patchy aggregation" pattern due to strong anthropogenic and production activities. Vertical migration of TMs indicated that the pollutants were mainly concentrated in the fill layers. Different contaminants had various migration depths, with migration occurring as: Cd > Hg > As > Zn > Pb> Cu> Mn> Sb. Analysis of their spatial variability showed that As, Pb, Cd and Hg had strong regional spatial variability. This research provides a new approach to comprehensively analyze TM pollution characteristics of non-ferrous smelting sites. It provides valuable information for guiding post-remediation strategies at abandoned non-ferrous metal smelting sites.

Characteristics of Metabolites by Seed-Specific Inhibition of FAD2 in Brassica napus L.

Fatty acid desaturase-2 (FAD2) is a key enzyme in the production of polyunsaturated fatty acids in plants. RNAi technology can reduce the expression of FAD2 genes in Brassica napus seeds and acquire transgenic B. napus plants with a high oleic acid content, but the effect of seed-specific inhibition of FAD2 expression on B. napus seed metabolites is not clear. Here we use widely targeted metabolomics to investigate the metabolites of normal-oleic-acid rapeseed (OA) and high-oleic-acid rapeseed (HOA) seeds, resulting in a total of 726 metabolites being detected. Among them, 24 differential metabolites were significantly downregulated and 88 differential metabolites were significantly upregulated in HOA rapeseed. In further lipid profile experiments, more lipids in B. napus seeds were accurately quantified. The contents of glycolipids and phospholipids that contain C18:1 increased significantly and C18:2 decreased because FAD2 expression was inhibited. The changes in the expression of key genes in related pathways were also consistent with the changes in metabolites. The insertion site of the ihpRNA plant expression vector was reconfirmed through genomewide resequencing, and the transgenic event did not change the sequence of FAD2 genes. There was no significant difference in the germination rate and germination potential between OA and HOA rapeseed seeds because the seed-specific ihpRNA plant expression vector did not affect other stages of plant growth. This work provides a theoretical and practical guidance for subsequent molecular breeding of high OA B. napus.

Evaluation of aggregate formation, stability and pore characteristics of bauxite residue following polymer materials addition.

Bauxite residue has poor physical conditions, which hinders plant growth and causes potential environmental risks. Polymer materials have broad potential applications for holding water and improving soil aggregation. However, no attempt has been made to assess the effects of polymers on physical structure of bauxite residue. The purpose of this study was to evaluate the effects of polyacrylamide (BP), humic acid (BH), starch-acrylamide polymer (BSA) and humic acid-acrylamide polymer (BHA) on aggregate formation, stability, and pore characteristics in bauxite residue by 60-day pot experiment. Results demonstrated that 0.2% polymer addition increased the proportion of >0.25 mm mechanical-stable aggregates. Under wet sieving, BP and BHA treatments increased the values of mean weight diameter (WMWD) from 0.36 mm to 0.67 mm and 0.68 mm, respectively, which may result in the increase of the percentage of organic functional groups including OCO and CC. Laser diffraction analysis and the visualized 3D surface map revealed that >0.25 mm residue aggregate was more difficult to disintegrate following BHA treatment during 180-min hydraulic circulation. BP and BH treatments elevated <0.5 µm pore size volumes, whilst BHA treatment increased >5 µm pore size volumes and improved the porosity of bauxite residue. Polymer applications indicated that compared with polyacrylamide or humic acid, humic acid-acrylamide polymer could be regarded as an effective ameliorant due to its positive effects on both aggregate stability and pore characteristics. These findings were helpful for understanding the application potential of natural-synthetic polymers on physical conditions of bauxite residue prior to ecological reconstruction on the disposal areas.

Appropriate human intervention stimulates the development of microbial communities and soil formation at a long-term weathered bauxite residue disposal area.

Bauxite residue discharged to disposal areas, which could generate environmental pollution issues. Long-term natural restoration may improve the physicochemical properties of the residues, in turn supporting vegetation establishment, and effectively managing pollution. Nevertheless, the effects of short-term human intervention on soil formation in the weathered disposal areas are still relatively unknown. Thus, residue samples with different depths from different regions including no vegetation, sparse vegetation, complete vegetation coverage, and complete vegetation coverage following sewage sludge treatment were selected to analyze microbial community using Illumina high-throughput sequencing technology and evaluate soil formation process. Long-term weathering changed pH, the fraction of water-stable aggregates and nutrient concentrations, whilst promoting Proteobacteria, Chloroflexi, Acidobacteria and Planctomycete populations. Sewage sludge addition enhanced aggregate stability and significantly changed microbial community diversity. Sewage sludge application enriched the relative abundances of Proteobacteria and Bacteroidetes, whilst decreasing the relative abundance of Acidobacteria, which may be due to variation in environmental factors. Canonical correspondence analysis revealed that pH and EC were the main factors affecting microbial structure, followed by organic carbon content and aggregate stability. The results enhance the understanding of soil formation in bauxite residue and reveal the potential benefit of human intervention in ecological reconstruction at disposal areas.

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 CO32-, 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(CO32-) > 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 CO32- 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.

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.

Effect of phosphogypsum and poultry manure on aggregate-associated alkaline characteristics in bauxite residue.

Bauxite residue is a highly alkaline solid waste with poor physical structure which ultimately limits plant growth. Ecological reconstruction is an effective strategy to improve its environmental management, although soil formation process still requires further investigation. Here, an incubation experiment was used to investigate the effects of phosphogypsum and poultry manure, on aggregate size distribution and aggregate-associated exchangeable bases of bauxite residue. Phosphogypsum and poultry manure additions significantly increased the proportion of 2-1 mm residue aggregates and enhanced mean weight diameter (MWD) of residues in the 0-20 cm and 20-40 cm layers, although little effect was evident in the 40-60 cm layer. Phosphogypsum addition reduced pH and EC values to approximately 8.5 and 200 mS/cm in different size aggregates at 0-20 cm. Exchangeable Ca2+ concentration was improved, especially in 0.25-0.05 mm and <0.05 mm aggregates, following amendment additions. The relative contents of katoite and cancrinite in >0.25 mm aggregate fractions were relatively higher, which was consistent with changes in pH. Phosphogypsum and poultry manure changed the microstructure and surrounding pores of residue aggregates, whilst the concentration of Ca on microaggregate surfaces was higher than that on macroaggregates. These findings reveal that application of phosphogypsum and poultry manure directly alter the distribution of exchangeable bases and alkaline indicators within residue aggregates, resulting in aggregate size distribution and microstructure variations.

Microbial driven iron reduction affects arsenic transformation and transportation in soil-rice system.

The microbe-driven iron cycle plays an important role in speciation transformation and migration of arsenic (As) in soil-rice systems. In this study, pot experiments were used to investigate the effect of bacterial iron (Fe) reduction processes in soils on As speciation and migration, as well as on As uptake in soil-rice system. During the rice growth period, pH and electrical conductivity (EC) in soil solutions initially increased and then decreased, with the ranges of 7.4-8.8 and 116.3-820 mS cm-1, respectively. The concentrations of Fe, total As and As(III) showed an increasing trend in the rhizosphere and non-rhizosphere soil solutions with the increasing time. Fe concentrations were significantly positively correlated with total As and As(III) concentrations (***p < 0.001) in the soil solutions. The abundances of the arsenate reductase gene (arsC) and the As(III) S-adenosylmethionine methyltransferase gene (arsM) in rhizosphere soils were higher than those in non-rhizosphere soils, while the abundance of the Fe-reducing bacteria (Geo) showed an opposite trend. Moreover, it showed that the Geo abundance was significantly positively correlated with that of the arsC (***p < 0.001) and arsM (**p < 0.01) genes, respectively. The abundances of Geo, arsC and arsM genes were significantly positively correlated with the concentrations of Fe, total As and As(III) in the soil solutions (*p < 0.05). Moreover, the abundances of arsC and arsM genes were significantly negatively correlated with total As and As(III) in rice grains (*P < 0.05). These results showed that the interaction of bacterial Fe reduction process and radial oxygen loss from roots promoted the reduction and methylation of As, and then decreased As uptake by rice, which provided a theoretical basis for alleviating As pollution in paddy soils.

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-, CO32-, HCO3-, and water soluble alkali, and suppress arsenic (As) content. Correlation analysis revealed significant linear correlations with pH and concentrations of OH-, CO32-, HCO3-, water-soluble alkali, and metal ions. BSM treatment showed optimum results with neutralizing anions (OH-, CO32-, 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.

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.