Mobility of metal(loid)s in Pb/Zn tailings under different revegetation strategies

Heavy metals release in lead-zinc tailings: Effects of weathering and acid rain

Heavy metals (HMs) release from lead (Pb)-zinc (Zn) tailings poses significant environmental risks to surrounding areas. Furthermore, with the natural weathering and frequently happened acid rain events, the release of HMs could be elevated. This study conducted a series of laboratory column experiments with thermodynamics and hydrogeochemical analysis to investigate the environmental behavior of HMs release in Pb-Zn tailings under natural weathering conditions and acid rain events. Results showed that the weathering of calcite facilitates the release of Pb (17.9 mg/kg) and cadmium (Cd) (0.15 mg/kg), while acid rain promotes Zn release (10.5 mg/kg) from the Fe-Mn oxides, with no significant change for arsenic (As). Among the influencing factors during the column experiments, the oxidation-reduction potential (ORP) was identified as the primary indicator for the predictions of the HMs release behavior based upon the Random Forest model (R2 = 0.973 - 0.997). Correlation analysis revealed a strong relationship between coexistent ions and HM release patterns. Therefore, saturation index (SI) could effectively identify the influence range of each mineral phase on HM release. This study provides scientific evidence for effective management in carbonate-type tailings ponds.

Miscanthus sp. root exudate alters rhizosphere microbial community to drive soil aggregation for heavy metal immobilization

The heavy metals (HMs) spatial distribution in soil is intricately shaped by aggregation processes involving chemical reactions and biological activities, which modulate HMs toxicity, migration, and accumulation. Pioneer plants play a central role in preventing HMs at source, yet the precise mechanisms underlying their involvement in soil aggregation remain unclear. This study investigates HMs distribution within rhizosphere and bulk soil aggregates of Miscanthus sp. grown in tailings to elucidate the impact of root exudates (REs) and rhizosphere microbes. The results indicate that Miscanthus sp. enhance soil stability, increasing the proportion of macroaggregates by 4.06 %-9.78 %. HMs tend to concentrate in coarse-aggregates, particularly within rhizosphere environments, while diminishing in fine-aggregates. Under HMs stress, lipids and lipid-like molecules are the most abundant REs produced by Miscanthus sp., accounting for under up to 26.74 %. These REs form complex with HMs, promoting microaggregates formation. Charged components such as sugars and amino acids further contribute to soil aggregation. REs also regulates rhizosphere bacteria and fungi, with Acidobacteriota, Chloroflexi were the dominant bacterial phyla, while Ascomycota and Basidiomycota dominate the fungal community. The synergistic effect of REs and microorganisms impact soil organic matter and nutrient content, facilitating HMs nanoparticle heteroaggregation and macroaggregates formation. Consequently, soil structure and REs shape the distribution of HMs in soil aggregation. Pioneer plants mediate REs interaction with rhizosphere microbes, promoting the distribution of HMs into macroaggregates, leading to immobilization. This study sheds light on the role of pioneer plants in regulating soil HMs, offering valuable insights for soil remediation strategies.

Biogenic manganese oxides promote metal(loid) remediation by shaping microbial communities in biological aqua crust

Biological aqua crust (biogenic aqua crust-BAC) is a potentially sustainable solution for metal(loid) bioremediation in global water using solar energy. However, the key geochemical factors and underlying mechanisms shaping microbial communities in BAC remain poorly understood. The current study aimed at determining the in situ metal(loid) distribution and the key geochemical factors related to microbial community structure and metal(loid)-related genes in BAC of a representative Pb/Zn tailing pond. Here we showed that abundant metal(loid)s (e.g. Pb, As) were co-distributed with Mn/Fe-rich minerals (e.g. biogenic Mn oxide, FeOOH) in BAC. Biogenic Mn oxide (i.e. Mn) was the most dominant factor in shaping microbial community structure in BAC and source tailings. Along with the fact that keystone species (e.g. Burkholderiales, Haliscomenobacter) have the potential to promote Mn ion oxidization and particle agglomeration, as well as Mn is highly associated with metal(loid)-related genes, especially genes related to As redox (e.g. arsC, aoxA), and Cd transport (e.g. zipB), biogenic Mn oxides thus effectively enhance metal(loid) remediation by accelerating the formation of organo-mineral aggregates in biofilm-rich BAC system. Our study indicated that biogenic Mn oxides may play essential roles in facilitating in situ metal(loid) bioremediation in BAC of mine drainage.

Potential risk, leaching behavior and mechanism of heavy metals from mine tailings under acid rain

The leaching of heavy metals from abandoned mine tailings can pose a severe threat to surrounding areas, especially in the regions influenced by acid rain with high frequency. In this study, the potential risks of heavy metals in the tailings collected from a small-scale abandoned multi-metal mine was assessed, and their leaching behavior and mechanism were investigated by batch, semi-dynamic and in situ leaching experiments under simulated and natural rainfall conditions. The results suggested that Zn, Cu, Pb, and Cd in the tailings could cause high/very high risks. Both batch and semi-dynamic leaching tests consistently confirmed that the leaching of heavy metals (particularly Cd) could lead to serious pollution of the surrounding environment. The leaching rates of heavy metals were pH-dependent and related to their chemical speciations in the mine tailings. The leaching behavior of Cu and Cd was dominated by surface wash-off, Zn was controlled by diffusion initially and then surface wash-off, and the leaching mechanisms of Pb and As varied with the pH conditions. It was estimated that acid rain could greatly elevate the release fluxes of Zn (20.8%), Cu (36.7%), Pb (49.9%) and Cd (35.3%) in the study area. These findings could improve the understanding of the leaching behavior of heavy metals from mine tailings and assist in developing appropriate management strategies.

A critical review on the migration and transformation processes of heavy metal contamination in lead-zinc tailings of China

The health risks of lead-zinc (Pb-Zn) tailings from heavy metal (HMs) contamination have been gaining increasing public concern. The dispersal of HMs from tailings poses a substantial threat to ecosystems. Therefore, studying the mechanisms of migration and transformation of HMs in Pb-Zn tailings has significant ecological and environmental significance. Initially, this study encapsulated the distribution and contamination status of Pb-Zn tailings in China. Subsequently, we comprehensively scrutinized the mechanisms governing the migration and transformation of HMs in the Pb-Zn tailings from a geochemical perspective. This examination reveals the intricate interplay between various biotic and abiotic constituents, including environmental factors (EFs), characteristic minerals, organic flotation reagents (OFRs), and microorganisms within Pb-Zn tailings interact through a series of physical, chemical, and biological processes, leading to the formation of complexes, chelates, and aggregates involving HMs and OFRs. These interactions ultimately influence the migration and transformation of HMs. Finally, we provide an overview of contaminant migration prediction and ecological remediation in Pb-Zn tailings. In this systematic review, we identify several forthcoming research imperatives and methodologies. Specifically, understanding the dynamic mechanisms underlying the migration and transformation of HMs is challenging. These challenges encompass an exploration of the weathering processes of characteristic minerals and their interactions with HMs, the complex interplay between HMs and OFRs in Pb-Zn tailings, the effects of microbial community succession during the storage and remediation of Pb-Zn tailings, and the importance of utilizing process-based models in predicting the fate of HMs, and the potential for microbial remediation of tailings.

Free nitrous acid-assisted asymmetrical alternating current electrochemistry (FNA-AACE) for multi-heavy metals decontamination in waste activated sludge

Heavy metal contamination of waste activated sludge (WAS) is a key factor limiting the land application of sludge for nutrients recovery. This study proposes a novel free nitrous acid (FNA)-assisted asymmetrical alternating current electrochemistry (FNA-AACE) process to achieve high-efficiency decontamination of multi-heavy metals (Cd, Pb, and Fe) in WAS. The optimal operating conditions, the heavy metal removal performance of FNA-AACE, and the related mechanisms for maintaining the high performance were systematically investigated. During the FNA-AACE process, FNA treatment was optimal with an exposure time of 13 h at a pH of 2.9 and an FNA concentration of 0.6 mg/g TSS. Then the sludge was washed with EDTA in a recirculating leaching system under asymmetrical alternating current electrochemistry (AACE). The 6-h working and the following electrode cleaning were defined as a working circle of AACE. After three cycles of working-cleaning periods in AACE treatment, the cumulative removal efficiency of the toxic metals Cd and Pb reached over 97% and 93%, respectively, whilst that of Fe was greater than 65%. This surpasses most previously reported efficiencies and possesses a shorter treatment duration and sustainable EDTA circulation. The mechanism analysis suggested that FNA pretreatment provoked the migration of heavy metals for leaching enhancement, as well as reduced the demand for EDTA eluent concentration and increased conductivity, which can improve the AACE efficiency. Meanwhile, the AACE process absorbed the anionic chelates of heavy metals and reduced them to zero-valent particles on the electrode, regenerating the EDTA eluent and maintaining its high extraction efficiency for heavy metals. In addition, FNA-AACE could provide different electric field operation modes, allowing it to have flexibility for the real application processes. This proposed process is expected to be coupled with anaerobic digestion in wastewater treatment plants (WWTPs) for high efficiency of heavy metal decontamination, sludge reduction, and resource/energy recovery.

Novel biological aqua crust enhances in situ metal(loid) bioremediation driven by phototrophic/diazotrophic biofilm

BACKGROUND

Understanding the ecological and environmental functions of phototrophic biofilms in the biological crust is crucial for improving metal(loid) (e.g. Cd, As) bioremediation in mining ecosystems. In this study, in combination with metal(loid) monitoring and metagenomic analysis, we systematically evaluated the effect of biofilm in a novel biological aqua crust (biogenic aqua crust-BAC) on in situ metal(loid) bioremediation of a representative Pb/Zn tailing pond.

RESULTS

We observed strong accumulation of potentially bioavailable metal(loid)s and visible phototrophic biofilms in the BAC. Furthermore, dominating taxa Leptolyngbyaceae (10.2-10.4%, Cyanobacteria) and Cytophagales (12.3-22.1%, Bacteroidota) were enriched in biofilm. Along with predominant heterotrophs (e.g. Cytophagales sp.) as well as diazotrophs (e.g. Hyphomonadaceae sp.), autotrophs/diazotrophs (e.g. Leptolyngbyaceae sp.) in phototrophic biofilm enriched the genes encoding extracellular peptidase (e.g. family S9, S1), CAZymes (e.g. CBM50, GT2) and biofilm formation (e.g. OmpR, CRP and LuxS), thus enhancing the capacity of nutrient accumulation and metal(loid) bioremediation in BAC system.

CONCLUSIONS

Our study demonstrated that a phototrophic/diazotrophic biofilm constitutes the structured communities containing specific autotrophs (e.g. Leptolyngbyaceae sp.) and heterotrophs (e.g. Cytophagales sp.), which effectively control metal(loid) and nutrient input using solar energy in aquatic environments. Elucidation of the mechanisms of biofilm formation coupled with metal(loid) immobilization in BAC expands the fundamental understanding of the geochemical fate of metal(loid)s, which may be harnessed to enhance in situ metal(loid) bioremediation in the aquatic ecosystem of the mining area. Video Abstract.

Heavy Metal Sources, Contamination and Risk Assessment in Legacy Pb/Zn Mining Tailings Area: Field Soil and Simulated Rainfall

This study investigated heavy metal(HM) soil pollution and evaluated the risk and sources at a legacy tailings pond's area in Meizhou, China. Result shows that HM accumulation in soil, particularly Cd, Pb, and Zn, were serious. Zn and Cd in tailing soil and all studied elements in field soil had a significant release potential. Four HM sources were identified by positive matrix factorization (PMF) model: cinder and vehicle emissions (11.3%), natural sources (16.3%), tailings pond and human activities (32.8%), tailings pond (39.7%). The soil was severely polluted with Cd, Pb, and Zn, which posed a high potential environmental risk near surrounding area. Column leaching tests showed that large quantities of HMs were released from the tailings soil during simulated rainfall with different pH. This study indicates that the study area has been severely polluted and continues to have a great risk of HM pollution under natural conditions.

Effect of different direct revegetation strategies on the mobility of heavy metals in artificial zinc smelting waste slag: Implications for phytoremediation

The establishment of vegetation cover is an important strategy to reduce wind and water erosion at metal smelting waste slag sites. However, the mobility of heavy metals in waste slag-vegetation-leachate systems after the application of revegetation strategies is still unclear. Large microcosm experiments were conducted for revegetation of waste slag for 98 d using combined amendments, i.e., phosphate rock and an organic waste coming from the anaerobic digestion of pig manure (named as biogas residue), and by single- and co-planted perennial ryegrass (Lolium perenne L.) and Trifolium repens (T. repens). The results showed that the application of biogas residue slightly increased the concentrations of Zn and Cd in the leachates; however, the establishment of plants could avoid the excessive leaching of heavy metals coming from the biogas residue. The bioavailability of Cu, Zn, and Cd slightly increased, but Pb bioavailability significantly decreased regardless of single- or co-planting patterns. Additionally, the bioavailability of Cu, Zn, and Cd in the waste slag revegetated with perennial ryegrass was lower than that in T. repens under the single-planting pattern. The change in the heavy metals bioavailability under different revegetation strategies was mainly due to the root-induced change in the pH and speciation of heavy metals in the waste slag. The application of biogas residue and phosphate rock tends to the immobilization of Pb. Heavy metals mainly accumulated in the underground parts of the two herbs, and the heavy metal contents in the underground parts of perennial ryegrass were higher than those in T. repens regardless of single- or co-planting patterns. The heavy metals accumulated in T. repens were lower than those in perennial ryegrass in the single-planting pattern. The bioaccumulation and transportation factors of the two herbs were extremely low. Thus, the two herbs are potential candidates for phytostabilization of zinc smelting waste slag sites.

Biological aqua crust mitigates metal(loid) pollution and the underlying immobilization mechanisms

Biocrust-mediated in situ bioremediation could be an alternative strategy to mitigate metal(loid) pollution in aquatic habitats. To better understand the roles of biocrusts in regulating the fate of metal(loid)s, we examined the morphology, composition and structure of biological aqua crusts (BAC) developed in the mine drainage of a representative Pb/Zn tailing pond, and tested their effectiveness for immobilizing typical metal(loid)s. Unlike terrestrial biocrusts, BAC results from an assembly of compounds produced by the strong microbial activity and mineral compounds present in the aquatic environment. The BAC exhibited a unique flexible, spongy and porous structure with a specific surface area of 12-22 m² g^-1, and was able to effectively concentrate various metal(loid)s (e.g. Cd, 0.26-0.60 g kg^-1; Pb, 0.52-0.66 g kg^-1; As, 10.4-24.3 g kg^-1). The concentrations of metal(loid)s (e.g. Cd and As) in the BAC were even three to seven times higher than those in the source tailings, and more than 98% of immobilized metal(loid)s were present as the highly stable non-EDTA-exchangeable fraction. Adsorption on the well distributed micro-particles of the clay minerals (e.g. kaolinite) and the organic matters (2.0-2.7 wt.%) were found to be the major mechanisms for BAC to bind metal cations, whereas adsorption and coprecipitation on Fe/Mn oxide (e.g. FeOOH), was proposed to be the dominant pathway for accumulating metal(loid)s, especially As. The decrease in aqueous concentrations of the metal(loid)s along the drainage could be attributed in part to the scavenging effects of the BAC. These findings therefore provide new insights into the possible and efficient strategy for metal(loid) removal from water bodies, and highlighted the important role of BAC as a nature-based solution to benefit the bioremediation of mining area.

Bioleaching for detoxification of waste flotation tailings: Relationship between EPS substances and bioleaching behavior

The production of large volumes of waste flotation tailings results in environmental pollution and presents a major ecological and environmental risk. This study investigates bioleaching of waste flotation tailings using Acidithiobacillus ferrooxidans. The experiments were performed with 5.00% solid concentration, pH 2.0 with 100 mL medium for 25 d in the lab. The pH, OPR, metal concentration, dissolved organic matter (DOM) in leachate and extracellular polymeric substances (EPS) were recorded. Bioleaching tailing materials were finally characterized. Results showed that microorganisms, acclimating with mine tailings, effectively accelerated the bioleaching process, achieving maximum Zn and Fe extraction efficiencies of 95.45% and 83.98%, respectively, after 25 days. Compared with raw mine tailings, bioleaching could reduce 96.36% and 95.84% leachable Zn and Pb, and Pb presented a low risk (4.13%), while Zn, Cu, and Cr posed no risk (0.34%, 0.64%, and 0%). Toxicity and environmental risk analysis revealed bioleaching process significantly reduced the environmental risk associated with mine tailings. EPS analysis indicated that the loosely-bound EPS (LB-EPS) and tightly-bound EPS (TB-EPS) fractions contained different organic substances, which played different roles in the bioleaching process. Pearson correlation analysis revealed that EPS was highly correlated with bioleaching behavior (p < 0.05), and EPS was the main factor affecting the bioleaching process, promoting bioleaching in the LB-EPS and TB-EPS fractions.

Soilless revegetation: An efficient means of improving physicochemical properties and reshaping microbial communities of high-salty gold mine tailings

Soilless revegetation is a cost-effective and eco-friendly method for the ecological restoration of gold mine tailings. However, due to gold mine tailings are high-salty, alkaline and low-nutrient, little research has been done on soilless revegetation of gold mine tailings. The aim of study was to apply soilless revegetation to gold mine tailings, and investigate the changes of physicochemical properties and microbial communities of tailings after soilless revegetation. Six selected herbaceous plants (Melilotus officinalis, Xanthium sibiricum, Festuca elata, Zoysia japonica, Amaranthus tricolor L., Artemisia desertorum) grew well on the bare tailings, and their heights reached as high as 16.28 cm after 90 days. After soilless revegetation, tailings salinity dramatically dropped from 547.15 to 129.24 μS cm^-1, and pH went down from 8.68 to 7.59 at most. The content of available phosphorus (AP), available nitrogen (AN) and organic matter (OM) in tailings gradually improved, especially the content of AP and OM increased 53.36% and 52.58%, respectively. Furthermore, microbial metabolic activity and diversity in tailings obviously increased 70.33-264.70% and 1.64-13.97% respectively. The relative abundance of potential plant growth-promoting bacteria increased 1.40-3.05%, while the relative abundance of opportunistic pathogens and halophilic bacteria decreased 10.58-17.03% and 2.98-6.52% respectively. Such variations of microbial communities were beneficial for tailings restoration. This study provided insight into soilless revegetation and its impact on tailings microorganisms, which could be a new strategy for ecological restoration of gold mine tailings.