Mineralization of organic matter during the immobilization of heavy metals in polluted soil treated with minerals

Tailings particle size effects on pollution and ecological remediation: A case study of an iron tailings reservoir

The particle size distribution in tailings notably influences their physical properties and behavior. Despite this, our understanding of how the distribution of tailings particle sizes impacts in situ pollution and ecological remediation in in-situ environment remains limited. In this study, an iron tailings reservoir was sampled along a particle flow path to compare the pollution characteristic and microbial communities across regions with different particle sizes. The results revealed a gradual reduction in tailings particle size along the flow direction. The predominant mineral composition shifts from minerals such as albite and quartz to layered minerals. Total nitrogen, total organic carbon, and total metal concentrations increased, whereas the acid-generating potential decreased. The region with the finest tailings particle size exhibited the highest microbial diversity, featuring metal-resistant microorganisms such as KD4-96, Micrococcaceae, and Acidimicrobiia. Significant discrepancies were observed in tailings pollution and ecological risks across different particle sizes. Consequently, it is necessary to assess tailings reservoirs pollution in the early stages of remediation before determining appropriate remediation methods. These findings underscore that tailings particle distribution is a critical factor in shaping geochemical characteristics. The responsive nature of the microbial community further validated these outcomes and offered novel insights into the ecological remediation of tailings.

Mobility of Zn and Cu in Bentonites: Implications for Environmental Remediation

The aim of this study was to evaluate the mobility of copper (Cu) and zinc (Zn) and their impact on the properties of bentonites and unfrozen water content. Limited research in this area necessitates further analysis to prevent the negative effects of metal interactions on bentonite effectiveness. Tests involved American (SWy-3, Stx-1b) and Slovak (BSvk) bentonite samples with Zn or Cu ion exchange. Sequential extraction was performed using the Community Bureau of Reference (BCR) method. Elemental content was analyzed via inductively coupled plasma optical emission spectrometry (ICP-OES). Unfrozen water content was measured using nuclear magnetic resonance (1H-NMR) and differential scanning calorimetry (DSC). Results showed a significant influence of the main cation (Zn or Cu) on ion mobility, with toxic metal concentrations increasing mobility and decreasing residual fractions. Mobile Zn fractions increased with larger particle diameters, lower clay content, and shorter interplanar spacing, while the opposite was observed for Cu. Zn likely accumulated in larger clay pores, while Cu was immobilized in the bentonite complex. The stability of Zn or Cu ions increased with higher clay content or specific surface area. Residual Zn or Cu fractions were highest in uncontaminated bentonites with higher unfrozen water content, suggesting the potential formation of concentrated solutions in sub-zero temperatures, posing a threat to the clay-water environment, especially in cold regions.

A review on the transformation of birnessite in the environment: Implication for the stabilization of heavy metals

Birnessite is ubiquitous in the natural environment where heavy metals are retained and easily transformed. The surface properties and structure of birnessite change with the changes in external environmental conditions, which also affects the fate of heavy metals. Clarifying the effect and mechanism of the birnessite phase transition process on heavy metals is the key to taking effective measures to prevent and control heavy metal pollution. Therefore, the four transformation pathways of birnessite are summarized first in this review. Second, the relationship between transformation pathways and environmental conditions is proposed. These relevant environmental conditions include abiotic (e.g., co-existing ions, pH, oxygen pressure, temperature, electric field, light, aging, pressure) and biotic factors (e.g., microorganisms, biomolecules). The phase transformation is achieved by the key intermediate of Mn(III) through interlayer-condensation, folding, neutralization-disproportionation, and dissolution-recrystallization mechanisms. The AOS (average oxidation state) of Mn and interlayer spacing are closely correlated with the phase transformation of birnessite. Last but not least, the mechanisms of heavy metals immobilization in the transformation process of birnessite are summed up. They involve isomorphous substitution, redox, complexation, hydration/dehydration, etc. The transformation of birnessite and its implication on heavy metals will be helpful for understanding and predicting the behavior of heavy metals and the crucial phase of manganese oxides/hydroxides in natural and engineered environments.

Preliminary construction of a microecological evaluation model for uranium-contaminated soil

With the extensive development of nuclear energy, soil uranium contamination has become an increasingly prominent problem. The development of evaluation systems for various uranium contamination levels and soil microhabitats is critical. In this study, the effects of uranium contamination on the carbon source metabolic capacity and microbial community structure of soil microbial communities were investigated using Biolog microplate technology and high-throughput sequencing, and the responses of soil biochemical properties to uranium were also analyzed. Then, ten key biological indicators as reliable input variables, including arylsulfatase, biomass nitrogen, metabolic entropy, microbial entropy, Simpson, Shannon, McIntosh, Nocardioides, Lysobacter, and Mycoleptodisus, were screened by random forest (RF), Boruta, and grey relational analysis (GRA). The optimal uranium-contaminated soil microbiological evaluation model was obtained by comparing the performance of three evaluation methods: partial least squares regression (PLS), support vector regression (SVR), and improved particle algorithm (IPSO-SVR). Consequently, partial least squares regression (PLS) has a higher R2 (0.932) and a lower RMSE value (0.214) compared to the other. This research provides a new evaluation method to describe the relationship between soil ecological effects and biological indicators under nuclear contamination.

Role of genes encoding microbial carbohydrate-active enzymes in the accumulation and dynamics of organic carbon in subtropical forest soils

Microbial anabolism and catabolism regulate the accumulation and dynamics of soil organic carbon (SOC). However, very little attention has been paid to the role of microbial functional traits in the accumulation and dynamics of SOC in forest soils. In this study, nine forest soils were selected at three altitudes (600 m, 1200 m, and 1500 m) and three soil depths (0-15 cm, 15-30 cm, and 30-45 cm) located in Jiugong Mountain. Vertical traits of functional genes encoding microbial carbohydrate-active enzymes (CAZymes) were observed using metagenomic sequencing. Soil amino sugars were used as biomarkers to indicate microbial residue carbon (MRC). The results showed that GH1 (β-glucosidase: 147.49 TPM) and GH3 (β-glucosidase: 109.09 TPM) were the dominant genes for plant residue decomposition, and their abundance increased with soil depth and peaked in the deep soil at 600 m (GH1: 147.89 TPM; GH3: 109.59 TPM). The highest abundance of CAZymes for fungal and bacterial residue decomposition were GH18 (chitinase: 30.81 TPM) and GH23 (lysozyme: 58.02 TPM), respectively. The abundance of GH18 increased with soil depth, while GH23 showed the opposite trend. Moreover, MRC accumulation was significantly positively correlated with CAZymes involved in the degradation of hemicellulose (r = 0.577, p = 0.002). Compared with the soil before incubation, MRC in the topsoil at the low and middle altitudes after incubation increased by 4 % and 8 %, respectively, while MRC in the soils at 1500 m tended to decrease (p > 0.05). The mineralization capacity of SOC at 1500 m was significantly higher than that at 1200 m and 600 m (p < 0.05). Our results suggested that microbial function for degrading plant residue components, especially hemicellulose and lignin, contributed greatly to SOC accumulation and dynamics. These results were vital for understanding the roles of microbial functional traits in C cycling in forest.

Immobilization of zinc and cadmium by biochar-based sulfidated nanoscale zero-valent iron in a co-contaminated soil: Performance, mechanism, and microbial response

Mining and smelting of mineral resources causes excessive accumulation of potentially toxic metals (PTMs) in surrounding soils. Here, biochar-based sulfidated nanoscale zero-valent iron (SNZVI/BC) was designed via a one-step liquid phase reduction method to immobilize cadmium (Cd) and zinc (Zn) in a copolluted arable soil. A 60 d soil incubation experiment revealed that Cd and Zn immobilization efficiency by 6 % SNZVI/BC (25.2-26.2 %) was higher than those by individual SNZVI (13.9-18.0 %) or biochar (14.0-19.3 %) based on the changes in diethylene triamine pentaacetic acid (DTPA)-extractable PTM concentrations in soils, exhibiting a synergistic effect. Cd2+ or Zn2+ replaced isomorphously Fe2+ in amorphous ferrous sulfide, as revealed by XRD, XPS, and high-resolution TEM-EDS, forming metal sulfide precipitates and thus immobilizing PTMs. PTM immobilization was further enhanced by adsorption by biochar and oxidation products (Fe2O3 and Fe3O4) of SNZVI via precipitation and surface complexation. SNZVI/BC also increased the concentration of dissolved organic carbon and soil pH, thus stimulating the abundances of beneficial bacteria, i.e., Bacilli, Clostridia, and Desulfuromonadia. These functional bacteria further facilitated microbial Fe(III) reduction, production of ammonium and available potassium, and immobilization of PTMs in soils. The predicted function of the soil microbial community was improved after supplementation with SNZVI/BC. Overall, SNZVI/BC could be a promising functional material that not only immobilized PTMs but also enhanced available nutrients in cocontaminated soils.

Oxidative compensation mechanism of Fe-S synergetic inhibition of Cd activity in paddy field during flooding and drainage

It is known that the transformation of Fe and S forms in soil affects the migration and activity of Cd, but the coordinated regulation of Cd activity by Fe and S under different redox conditions is still unclear. Here, Diffusive gradients in thin films (DGT), an in-situ monitoring technique, is used to explore the difference of the regulation of Cd activity in paddy fields with ferrihydrite (FH) and ferrihydrite coprecipitated by sulfate (FH-S) under the flooding and drainage conditions. The addition of FH-S and FH significantly reduced the activity of Cd (Dissolved, Exchanged, and CDGT-Cd). Compared with pure FH, the adsorption extent of Cd in FH was enhanced by increasing concentrations of SO42- (i.e., S/Fe ratio), which is attributed to the decrease in the crystallinity of FH by sulfate. During soil flooding, the addition of FH-S promoted the production of metal sulfide (CdS and FeS/FeS2). The activity of Cd increased after drainage, while the FH-S treatment groups delayed the release of Cd. After 30 days of drainage, the concentration of Cd in FH-S treatment groups decreased by 28.9-44.1 % compared with the control group. The fresh FeS/FeS2 is not the main adsorbent for fixing Cd, and due to the existence of oxidation compensation mechanism, the preferential oxidation of FeS/FeS2 delays the release of Cd in the drainage stage. Our study shed new light on the mechanism of Fe-S synergistic regulation of Cd and remediation of Cd-contaminated soils.

Effect of calcination temperatures on the performance of rectorite for cadmium immobilization in soil: Freeze-thaw, plant growth, and microbial diversity

The immobilization of cadmium (Cd(II)) in soil using calcined rectorite (REC) was investigated in this research. The results of immobilization show that a small amount of REC calcined at 700 °C (REC-700 °C) could effectively immobilize 90% of Cd(II) in soil, while the immobilization efficiency of REC only reached 42%. Moreover, the immobilization efficiency of REC calcined at 300 °C and 500 °C (REC-300 °C and REC-500 °C) were lower than REC. To investigate the mechanism, the materials before and after immobilization were fully analyzed by Fourier transform infrared spectroscopy (FT-IR), powdery X-ray diffraction analysis (XRD), and scanning electron microscopy (SEM). The results indicate that the structure of REC has been changed after calcination at different temperatures and Cd(II) was successfully immobilized on materials. Losing free water, structural water and OH groups respectively, the layer spacing of REC-300 °C and REC-500 °C was shrunk. However, the crystal structure of REC was destroyed after calcination at 700 °C, resulting in the generation of new phases. According to the XRD result, more cadmium hydroxide (Cd(OH)₂) were produced on REC-700 °C, indicating that more OH groups were formed during immobilization. Furthermore, Tessier test demonstrates that Cd(II) in soil changed from exchangeable state and water soluble state to carbonate bound state and iron manganese oxide bound state during immobilization. The result of microbial community indicates that REC-700 °C can restore the microbial composition of Cd(II)-contaminated soil. The effects of pH, freeze-thaw, REC dosage, and initial heavy metal concentration were also evaluated to provide a theoretical basis for the subsequent application of the material in the remediation of contaminated soil.