Enhancement of nitrogen on core taxa recruitment by Penicillium oxalicum stimulated microbially-driven soil formation in bauxite residue.

Microbially-driven soil formation process is an emerging technology for the ecological rehabilitation of alkaline tailings. However, the dominant microorganisms and their specific roles in soil formation processes remain unknown. Herein, a 1-year field-scale experiment was applied to demonstrate the effect of nitrogen input on the structure and function of the microbiome in alkaline bauxite residue. Results showed that the contents of nutrient components were increased with Penicillium oxalicum (P. oxalicum) incorporation, as indicated by the increasing of carbon and nitrogen mineralization and enzyme metabolic efficiency. Specifically, the increasing enzyme metabolic efficiency was associated with nitrogen input, which shaped the microbial nutrient acquisition strategy. Subsequently, we evidenced that P. oxalicum played a significant role in shaping the assemblages of core bacterial taxa and influencing ecological functioning through intra- and cross-kingdom network analysis. Furthermore, a recruitment experiment indicated that nitrogen enhanced the enrichment of core microbiota (Nitrosomonas, Bacillus, Pseudomonas, and Saccharomyces) and may provide benefits to fungal community bio-diversity and microbial network stability. Collectively, these results demonstrated nitrogen-based coexistence patterns among P. oxalicum and microbiome and revealed P. oxalicum-mediated nutrient dynamics and ecophysiological adaptations in alkaline microhabitats. It will aid in promoting soil formation and ecological rehabilitation of bauxite residue. ENVIRONMENT IMPLICATION: Bauxite residue is a highly alkaline solid waste generated during the Bayer process for producing alumina. Attempting to transform bauxite residue into a stable soil-like substrate using low-cost microbial resources is a highly promising engineering. However, the dominant microorganisms and their specific roles in soil formation processes remain unknown. In this study, we evidenced the nitrogen-based coexistence patterns among Penicillium oxalicum and microbiome and revealed Penicillium oxalicum-mediated nutrient dynamics and ecophysiological adaptations in alkaline microhabitats. This study can improve the understanding of core microbes' assemblies that affect the microbiome physiological traits in soil formation processes.

Pollution characteristics and quantitative source apportionment of heavy metals within a zinc smelting site by GIS-based PMF and APCS-MLR models.

The abandoned smelters present a substantial pollution threat to the nearby soil and groundwater. In this study, 63 surface soil samples were collected from a zinc smelter to quantitatively describe the pollution characteristics, ecological risks, and source apportionment of heavy metal(loid)s (HMs). The results revealed that the average contents of Zn, Cd, Pb, As, and Hg were 0.4, 12.2, 3.3, 5.3, and 12.7 times higher than the risk screening values of the construction sites, respectively. Notably, the smelter was accumulated heavily with Cd and Hg, and the contribution of Cd (0.38) and Hg (0.53) to ecological risk was 91.58%. ZZ3 and ZZ7 were the most polluted workshops, accounting for 25.7% and 35.0% of the pollution load and ecological risk, respectively. The influence of soil parent materials on pollution was minor compared to various workshops within the smelter. Combined with PMF, APCS-MLR and GIS analysis, four sources of HMs were identified: P1(25.5%) and A3(18.4%) were atmospheric deposition from the electric defogging workshop and surface runoff from the smelter; P2(32.7%) and A2(20.9%) were surface runoff of As-Pb foul acid; P3(14.5%) and A4(49.8%) were atmospheric deposition from the leach slag drying workshop; P4(27.3%) and A1(10.8%) were the smelting process of zinc products. This paper described the distribution characteristics and specific sources of HMs in different process workshops, providing a new perspective for the precise remediation of the smelter by determining the priority control factors.

Driving factors for distribution and transformation of heavy metals speciation in a zinc smelting site.

Heavy metal pollution at an abandoned smelter pose a significant risk to environmental health. However, remediation strategies are constrained by inadequate knowledge of the polymetallic distribution, speciation patterns, and transformation factors at these sites. This study investigates the influence of soil minerals, heavy metal occurrence forms, and environmental factors on heavy metal migration behaviors and speciation transformations. X-ray diffraction analysis revealed that the minerals associated with heavy metals are mainly hematite, franklinite, sphalerite, and galena. Sequential extraction results suggest that lead and zinc are primarily present in the organic-sulfide fractions (F4) and residual form (F5) in the soil, accounting for over 70% of the total heavy metal content. Zinc displayed greater instability in carbonate-bound (16%) and exchangeable (2%) forms. The migration and diffusion patterns of heavy metals in the subsurface environment were visualized through the simulation of labile state heavy metals, demonstrating high congruence with groundwater pollution distribution patterns. The key environmental factors influencing heavy metal stable states (F4 and F5) were assessed by integrating random forest models and redundancy analysis. Primary factors facilitating Pb transformation into stable states were available phosphorus, clay content, depth, and soil organic matter. For Zn, the principal drivers were Mn oxides, soil organic matter, clay content, and inorganic sulfur ions. These findings enhance understanding of the distribution and transformation of heavy metal speciation and can provide valuable insights into controlling heavy metal pollution at non-ferrous smelting sites.

The formation of multi-metal(loid)s contaminated groundwater at smelting site: Critical role of natural colloids.

The occurrence and migration of colloids at smelting sites are crucial for the formation of multi-metal(loid)s pollution in groundwater. In this study, the behavior of natural colloids (1 nm-0.45 µm) at an abandoned smelting site was investigated by analyzing groundwater samples filtered through progressively decreasing pore sizes. Smelting activities in this site had negatively impacted the groundwater quality, leading to elevated concentrations of zinc (Zn), lead (Pb), arsenic (As), and cadmium (Cd). The results showed that heavy metal(loid)-bearing colloids were ubiquitous in the groundwater with the larger colloidal fractions (∼75 -450 nm) containing higher abundances of pollutants. It was also observed that the predominant colloids consisted of Zn-Al layered double hydroxide (LDH), sphalerite, kaolinite, and hematite. By employing multiple analytical techniques, including leaching experiments, soil colloid characterization, and Pb stable isotope measurements, the origin of groundwater colloids was successfully traced to the topsoil colloids. Most notably, our findings highlighted the increased risk of heavy metal(loid)s migration from polluted soils into adjacent sites through the groundwater because of colloid-mediated transport of contaminants. This field-scale investigation provides valuable insights into the geochemical processes governing heavy metal(loid) behavior as well as offering pollution remediation strategies specifically tailored for contaminated groundwater.

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.

[Research Progress on Solidification and MICP Remediation of Soils in Heavy Metal Contaminated Site].

Heavy metal pollution in soils of smelting sites is an important environmental problem to be solved urgently. Solidification technology has become one of the mainstream technologies for heavy metal remediation in contaminated sites owing to its shorter remediation time, low cost, and high treatment efficiency. On the basis of summarizing the latest research progress on the remediation of heavy metal pollution in sites by solidification in the past 10 years, this study focused on the mechanisms of solidification technology and analyzed the advantages and disadvantages of different mechanisms (mechanism of inorganic materials, mechanism of organic materials, mechanism of mechanical ball milling, and mechanism of microbial-induced carbonate mineralization (MICP)) and their scope of application. Then, according to the research focus and development trend presented by CiteSpace, the application prospects and limiting factors of MICP technology for the solidification and remediation of heavy metal pollution in sites were summarized from three aspects:the application of MICP in multi-metal remediation, the application of MICP composites in contaminated sites, and the influencing factors of MICP technology application. Finally, the prospects and challenges in solidification technology were put forward in order to provide reference for the future development.

Using Fe/H2O2-modified biochar to realize field-scale Sb/As stabilization and soil structure improvement in an Sb smelting site.

Antimony (Sb) and arsenic (As) released from the Sb smelting activities pose a major environmental risk and ecological degradation in Sb smelting sites. Here the effects of Fe/H2O2 modified biochar (Fe@H2O2-BC) on the synchronous stabilization of Sb/As and the improvement of soil structure in a typical Sb smelting site in Southern China based on a 1-year field experiment were studied. Application of ≥1 % (w/w) Fe@H2O2-BC could stably decrease the leaching concentrations of Sb and As of the polluted soils to Environmental quality standards for surface water Chinese Level III (GB3838-2002). Compared to the untreated soils, the stabilization efficiency of soil Sb and As treated by Fe@H2O2-BC reached 90.7 % ~ 95.7 % and 89.6 % ~ 90.8 %, respectively. The residue fractions of Sb/As in the soils increased obviously, and the bio-availability of Sb/As decreased by 65.0-95.6 % and 91.1-96.0 %, respectively. Moreover, Fe@H2O2-BC addition elevated soil organic carbon content, increased soil porosity, and improved water retention capacity, indicating the positive effects on soil structure and functions. Advanced mineral identification and characterization systems showed that Sb/As usually occurred in Fe-bearing minerals and stabilized by surface complexation and co-precipitation. The findings demonstrated that 1 % (w/w) Fe@H2O2-BC was appropriate to Sb/As stabilization and soil function recovery following field conditions, which provided potential application for ecological restoration in Sb smelting sites.

Groundwater heavy metal(loid)s risk prediction based on topsoil contamination and aquifer vulnerability at a zinc smelting site.

Groundwater pollution is a recurrent problem in abandoned non-ferrous metal smelting sites, and its severity is influenced by topsoil contamination, hydrogeological characteristics, and hydrogeochemical conditions. In such unique areas, traditional methods for evaluating groundwater pollution risk are biased, as the long production history of these sites have led to highly polluted and heterogeneous soil and groundwater. Herein, based on a typical lead-zinc smelting site, As, Pb, Zn, Cd, Mn, and Ni were found to be the predominant heavy metal (loid)s in groundwater, with respective exceedance rates of 44.4%, 50.0%, 72.2%, 88.9%, 88.9%, and 61.1%. Combined with the groundwater pollution characteristics, the representative hydrogeochemical factors were screened out to optimize the following aquifer vulnerability evaluation using the AHP-DRASTICH method. A comprehensive evaluation model (DI-NCPI) for groundwater pollution risk was established by combining the DRASTICH index (DI) obtained after optimization and the Nemerow comprehensive contamination index (NCPI) of topsoil. The fit between DI-NCPI and groundwater heavy metal (loid) pollution index reached 0.956, which laterally confirms that the model has some reference value. In terms of distribution, the high-risk and very high-risk zones were mainly concentrated in the zinc smelting system, located in the southeastern and central-western parts of the site. These areas have relatively high levels of topsoil contamination and aquifer vulnerability and require focused attention in site remediation. This research highlights the importance of combining topsoil contamination and aquifer vulnerability to evaluate groundwater pollution risk in smelting areas. It provides a more targeted reference for groundwater remediation strategies in abandoned smelting sites, as well as severely polluted industrial areas.

Geochemical fractionation and potential release behaviour of heavy metals in lead‒zinc smelting soils.

The lack of understanding of heavy metal speciation and solubility control mechanisms in smelting soils limits the effective pollution control. In this study smelting soils were investigated by an advanced mineralogical analysis (AMICS), leaching tests and thermodynamic modelling. The aims were to identify the partitioning and release behaviour of Pb, Zn, Cd and As. The integration of multiple techniques was necessary and displayed coherent results. In addition to the residual fraction, Pb and Zn were predominantly associated with reducible fractions, and As primarily existed as the crystalline iron oxide-bound fractions. AMICS quantitative analysis further confirmed that Fe oxyhydroxides were the common dominant phase for As, Cd, Pb and Zn. In addition, a metal arsenate (paulmooreite) was an important mineral host for Pb and As. The pH-stat leaching indicted that the release of Pb, Zn and Cd increased towards low pH values while release of As increased towards high pH values. The separate leaching schemes were associated with the geochemical behaviour under the control of minerals and were confirmed by thermodynamic modelling. PHREEQC calculations suggested that the formation of arsenate minerals (schultenite, mimetite and koritnigite) and the binding to Fe oxyhydroxides synchronously controlled the release of Pb, Zn, Cd and As. Our results emphasized the governing role of Fe oxyhydroxides and secondary insoluble minerals in natural attenuation of heavy metals, which provides a novelty strategy for the stabilization of multi-metals in 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.

Multi-technological integration in a smelting site: Visualizing pollution characteristics and migration pattern.

Heavy metal(loid)s pollution of industrial legacies has become a severe environmental issue worldwide. Linking soil pollution to groundwater contaminant plumes would make invisible pollution features visible across the site, but related studies are lacking and require the convergence of multiple technologies. This study uniformly managed the soil and groundwater data in a 3D visualization model to pellucidly assess the spatial distribution of critical contaminants beyond simple drilling information. The distribution of Pb, Zn, As, and Cd in soil-groundwater system has a strong correlation to historical production, substance type, soil property, and groundwater flow direction. Over 2600 measurements of High-density electrical resistivity tomography (ERT) data were used to guarantee the exactness of soil structures. Hydraulic conductivity showed a strongest correlation (R2 = 0.86), yielding a calibrated model to reveal the anisotropic and contaminant transport in the region, with the consequent minimize the drilling tests. This study provides a template for the description of a verifiable scenario of hydrogeological conditions and pollution characteristics at smelting sites, coupled with traditional exploration and non-invasive techniques. The findings highlight the significance of visualizing the internal state of the soil-groundwater system under consideration, thus providing a basis for targeted control measures against site contamination.

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.

Long-term weathering difference in soil-like indicators of bauxite residue mediates the multifunctionality driven by microbial communities.

Long-term weathering enhances the stability of ecosystem services and alters the microbiome, however, its influences on the relationship between microbial diversity and multifunctionality are still poorly understood. Hereby, 156 samples (0-20 cm) from five artificially divided functional zones including central bauxite residue zone (BR), the zone near residential area (RA), the zone near dry farming area (DR), the zone near natural forest area (NF), and the zone near grassland and forest area (GF) were collected in a typical disposal area to determine the heterogeneity and development of biotic and abiotic properties of bauxite residue. Residues in BR and RA exhibited higher values of pH, EC, heavy metals, and exchangeable sodium percentage compared to those in NF and GF. Our results showed a positive correlation between multifunctionality and soil-like quality during long-term weathering. Microbial diversity and microbial network complexity responded positively to multifunctionality within the microbial community, which was parallel with ecosystem functioning. Long-term weathering promoted oligotrophs-dominated bacterial assemblages (mostly Acidobacteria and Chloroflexi) and suppressed copiotrophs (including Proteobacteria and Bacteroidota), while the response of fungal communities was lower. Rare taxa from bacterial oligotrophs were particularly important at the current stage for maintaining ecosystem services and ensuring microbial network complexity. Our results underscore the significance of microbial ecophysiological strategies in response to changes in multifunctionality during long-term weathering, and highlight the necessity of conserving and augmenting the abundance of rare taxa to ensure the stable provision of ecosystem functions in bauxite residue disposal areas.

Root architectures differentiate the composition of organic carbon in bauxite residue during natural vegetation.

Understanding plant root architectures induced changes in organic carbon accumulation and conversion is critical to predicting carbon cycling and screening appropriate plant species for ecological restoration on bauxite residue disposal areas. According to the ecological investigation of a weathered bauxite residue disposal area, three plants with different root architectures including Artemisia lavandulaefolia (A. lavandulaefolia), moss, and Zanthoxylum simulans (Z. simulans) were selected to investigate the rhizosphere effects on the composition and structure of organic carbon in bauxite residue. The physic-chemical properties, the contents and structure of different organic carbon fractions, and microbial communities of bauxite residue from rhizosphere and non-rhizosphere were analyzed. Plant growth decreased the saline-alkalinity, increased the contents of total organic carbon, particulate organic carbon and dissolved organic carbon, whilst enhancing the enzymatic activities of bauxite residue. Meanwhile, the rhizosphere effects had significant effects on the accumulation and stabilization of organic carbon in bauxite residue. A. lavandulaefolia had the strongest rhizosphere effects on the composition and structure of total organic carbon and dissolved organic carbon, whilst moss was more effective on the accumulation of particulate organic carbon in bauxite residue. Plant growth and root architecture changed the abundance of specific functional microorganisms and the complexity of microbial co-occurrence networks, thus elevating organic carbon levels in bauxite residue. During natural vegetation encroachment, rhizosphere exciting effects of the salt-tolerated plants could change the composition and structure of organic carbon fractions due to the comprehensive effectiveness of the improvement of physic-chemical properties and microbial communities. The findings improve our understanding of the responses of sequestration and stabilization of organic carbon pools to ecological restoration on bauxite residue disposal areas.

Anthropogenic processes drive heterogeneous distributions of toxic elements in shallow groundwater around a smelting site.

Smelting activities have a far-reaching influence on the quality of soil and groundwater, while most studies have neglected the information on the pollution characteristics of groundwater. The hydrochemical parameters of shallow groundwater and the spatial distributions of toxic elements were investigated in this study. Correlations analysis and groundwater evolution revealed that the major ions were primarily determined by silicate weathering and calcite dissolution process, and anthropogenic processes had a significant effect on groundwater hydrochemistry. Almost 79%, 71%, 57%, 89%, 100%, and 78.6% of samples exceeded the standards of Cd, Zn, Pb, As, SO42-, and NO3-, and their distribution is closely related to the production process. Analysis of soil geochemistry indicated that the relatively mobile forms of toxic elements strongly influence the origin and concentration in shallow groundwater. Besides, rainfall with high magnitude would lead to a decrease of toxic elements in shallow groundwater, whereas the area once stacked waste residue was the opposite. It is recommended to strengthen risk management of the limited mobility fraction while devising a plan for waste residue treatment in accordance with the local pollution conditions. The research on controlling the mechanism of toxic elements in shallow groundwater, along with sustainable development in the study area and other smelting zones may benefit from this study.

The effect of extracellular polymeric substances (EPS) of iron-oxidizing bacteria (Ochrobactrum EEELCW01) on mineral transformation and arsenic (As) fate.

Extracellular polymeric substances (EPS) are an important medium for communication and material exchange between iron-oxidizing bacteria and the external environment and could induce the iron (oxyhydr) oxides production which reduced arsenic (As) availability. The main component of EPS secreted by iron-oxidizing bacteria (Ochrobactrum EEELCW01) was composed of polysaccharides (150.76-165.33 mg/g DW) followed by considerably smaller amounts of proteins (12.98-16.12 mg/g DW). Low concentrations of As (100 or 500 µmol/L) promoted the amount of EPS secretion. FTIR results showed that EPS was composed of polysaccharides, proteins, and a miniscule amount of nucleic acids. The functional groups including -COOH, -OH, -NH, -C=O, and -C-O played an important role in the adsorption of As. XPS results showed that As was bound to EPS in the form of As3+. With increasing As concentration, the proportion of As3+ adsorbed on EPS increased. Ferrihydrite with a weak crystalline state was only produced in the system at 6 hr during the mineralization process of Ochrobactrum sp. At day 8, the minerals were composed of goethite, galena, and siderite. With the increasing mineralization time, the main mineral phases were transformed from weakly crystalline hydrous iron ore into higher crystallinity siderite (FeCO3) or goethite (α-FeOOH), and the specific surface area and active sites of minerals were reduced. It can be seen from the distribution of As elements that As is preferentially adsorbed on the edges of iron minerals. This study is potential to understand the biomineralization mechanism of iron-oxidizing bacteria and As remediation in the environment.

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.