Effects of increasing concentrations of unamended and gypsum modified bauxite residues on soil microbial community functions and structure - A mesocosm study

Interactions between organic matter and alkaline minerals in bauxite residue: implication for soil restoration

Ecological restoration of bauxite residue has received extensive attention, and organic matter plays a crucial role in the soil formation process of bauxite residue. However, the interaction between organic matter and alkaline minerals in bauxite residue is not well understood. In this work, molecular spectroscopic techniques combined with isothermal titration calorimetry (ITC) were employed to investigate the interactions between humic acid (HA) and four representative alkaline minerals in bauxite residue (calcite, garnet, sodalite, and cancrinite). The results show that the adsorption processes of HA onto calcite and garnet were primarily governed by monolayer surface adsorption and controlled by surface reactions, which were different for sodalite and cancrinite. Both garnet and cancrinite had strong binding affinities with fluorescent HA, while cancrinite only bound with a small fraction of HA. In contrast, the bindings of calcite and sodalite with fluorescent HA were weak. The ITC results indicate distinct thermodynamic properties of different alkaline minerals in the interaction with HA. The molar enthalpy of calcite was - 45.88 kJ/mol, which was much higher than those of garnet, sodalite, and cancrinite, suggesting that calcite exhibited a relatively uniform interaction mechanism with HA dominated by enthalpy change, while the others showed heterogeneous entropy-driven mechanisms. The findings contribute to a better understanding on the microscale connections between organic matter and alkaline minerals in bauxite residue.

Straw addition increases enzyme activities and microbial carbon metabolism activities in bauxite residue

Recovery of microbial functions is one of the critical processes in the nutrient cycling of bauxite residue for improving revegetation. Straw is considered to be effective to increase microbial diversity and drive the development of the microbial community, but its effect on microbial carbon metabolism has not been illustrated. The present study evaluated the effects of phosphogypsum (PG), straw (SF) and phosphogypsum plus straw (PGSF) on physicochemical properties, enzyme activities, and microbial carbon metabolism activities in bauxite residue. After 180 days incubation, PG, SF and PGSF treatment significantly reduced the residue pH from 10.85 to 8.64, 9.39 and 8.06, respectively. Compared to CK treatment, SF treatment significantly increased the content of total organic carbon (TOC) and organic carbon fractions (DOC, MBC, EOC, and POC). In addition, straw addition significantly increased glucosidase, cellulose, urease, and alkaline phosphatase by 7.2-9.1 times, 5.8-7.1 times, 11.1-12.5 times, and 1.1-2.2 times, respectively. The Biolog results showed that straw addition significantly increased microbial metabolic activity (AWCD) and diversity in bauxite residue. Redundancy analysis indicated total nitrogen (TN) and carbon fractions (POC, MBC and DOC) were the most important environmental factors affecting microbial metabolic activity and diversity in bauxite residue. These findings provided us with a biogeochemical perspective to reveal soil formation in bauxite residue and suggested that nutrient supplement and regulation of salinity-alkalinity benefit the establishment of microbial communities and functions in bauxite residue.

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

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

Evaluation of Biological Characteristics of Soil as Indicator for Sustainable Rehabilitation of a Post-Bauxite-Mining Land

This paper presents a study of the microbial abundance in post-bauxite-mining land soil from Zece Hotare, Bihor county, Romania. The soil samples were collected from 12 soil variants, in the year 2020, after 15 years of long-term restoration. Some chemical parameters and bacterial numbers of six groups of microorganisms were determined in the restored mining land, and these characteristics were compared with those of the soil from a beech forest situated in an adjacent area unaffected by bauxite exploitation. On the basis of the total number of microorganisms belonging to each group studied, the bacterial potential of the soil quality was assessed, calculating the bacterial soil quality index (BSQI), while the Shannon diversity index and the Jaccard distance were applied to show the level of bacterial diversity. The characteristics of the studied chemical and microbiological parameters determined in the beech adjacent area were very similar to those observed in the high-level plateau, low-level plateau, and Black locust areas, indicating similar soil conditions; therefore, the ecological reconstruction 15 years ago, had a very favorable impact on restoration in some affected areas.

An Investigation into the Growth of Lolium perenne L. and Soil Properties Following Soil Amendment with Phosphorus-Saturated Bauxite Residue

Reuse options for bauxite residue include treatment of phosphorus (P)-enriched wastewaters where the P-saturated media offers fertiliser potential. However, few studies have assessed the impact on soil properties. Two types of spent P-saturated bauxite residue were applied to soil and compared to conventional superphosphate fertiliser as well as a control soil. Soil physico-chemical properties, worm Eisenia fetida L. choice tests, and Lolium perenne L. growth and elemental uptake were examined. Comparable biomass and plant content for L. perenne in the P-saturated bauxite residue treatments and those receiving superphosphate, indicated no phytotoxic effects. E. fetida L. showed a significant preference for the control soil (58 %± 2.1%) over the amended soils, indicating some form of salt stress. Overall, P-saturated bauxite residue was comparable to the superphosphate fertiliser in terms of the plant performance and soil properties, indicating the potential recycling of P from wastewaters using bauxite residue as a low-cost adsorbent.

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

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

Contrasted microbial community colonization of a bauxite residue deposit marked by a complex geochemical context

Bauxite residue is the alkaline byproduct generated during alumina extraction and is commonly landfilled in open-air deposits. The growth in global alumina production have raised environmental concerns about these deposits since no large-scale reuses exist to date. Microbial-driven techniques including bioremediation and critical metal bio-recovery are now considered sustainable and cost-effective methods to revalorize bauxite residues. However, the establishment of microbial communities and their active role in these strategies are still poorly understood. We thus determined the geochemical composition of different bauxite residues produced in southern France and explored the development of bacterial and fungal communities using Illumina high-throughput sequencing. Physicochemical parameters were influenced differently by the deposit age and the bauxite origin. Taxonomical analysis revealed an early-stage microbial community dominated by haloalkaliphilic microorganisms and strongly influenced by chemical gradients. Microbial richness, diversity and network complexity increased significantly with the deposit age, reaching an equilibrium community composition similar to typical soils after decades of natural weathering. Our results suggested that salinity, pH, and toxic metals affected the bacterial community structure, while fungal community composition showed no clear correlations with chemical variations.

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

The high alkalinity of bauxite residue and its sustained release impose major limitation on its reuse and ecological disposal. It has been confirmed from sustained rehabilitation that gypsum can effectively reduce the alkalinity of bauxite residue by continuously releasing Ca²⁺ to react with carbonate and hydroxide. However, the combined bauxite residue with high calcium content exhibits stubborn alkalinity for most alkaline reduction methods employing cations to consume carbonate. In this study, we have aimed to address this knowledge gap by investigating the dose-response relationship in the alkaline reduction induced by ferrous sulfate (FS) neutralization. The pH, exchangeable sodium percentage (ESP), and CO₃^2-/HCO₃^- of bauxite residue decreased from 10.6, 44.1%, and 42.7/24.5 mg/kg to 8.1, 27.7%, and 0.7/18.0 mg/kg, respectively. Approximately 20-55 days were required for the neutralization reaction to reach equilibrium. The FS induced an increase in free iron oxide (Fe_d) and amorphous iron oxide (Fe_o), and partial dissolution of alkaline minerals including calcite, cancrinite, and kaolinite in bauxite residue. Further, addition of FS also affected the kinetic dissolution process of bauxite residue; the acid neutralization capacity of bauxite residue to pH 7 decreased from 0.21 mol H⁺/kg solid to 0.02 mol H⁺/kg solid. The results showed FS to be a potential candidate for improving the characteristics of the combined bauxite residue, and guide the FS application for the disposal of the combined bauxite residue.