Microbial reductive transformation of iron-rich tailings in a column reactor and its environmental implications to arsenic reactive transport in mining tailings

Extracellular polymeric substances altered ferrihydrite (trans)formation and induced arsenic mobilization

The behavior of As is closely related to trans(formation) of ferrihydrite, which often coprecipitates with extracellular polymeric substances (EPS), forming EPS-mineral aggregates in natural environments. While the effect of EPS on ferrihydrite properity, mineralogy reductive transformation, and associated As fate in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this research, ferrihydrite-EPS aggregates were synthesized and batch experiments combined with spectroscopic, microscopic, and geochemical analyses were conducted to address these knowledge gaps. Results indicated that EPS blocked micropores in ferrihydrite, and altered mineral surface area and susceptibility. Although EPS enhanced Fe(III) reduction, it retarded ferrihydrite transformation to magnetite by inhibiting Fe atom exchange in systems with low SO42-. As a result, 16% of the ferrihydrite was converted into magnetite in the Fh-0.3 treatment, and no ferrihydrite transformation occurred in the Fh-EPS-0.3 treatment. In systems with high SO42-, however, EPS promoted mackinawite formation and increased As mobilization into the solution. Additionally, the coprecipitated EPS facilitated As(V) reduction to more mobilized As(III) and decreased conversion of As into the residual phase, enhancing the potential risk of As contamination. These findings advance our understanding on biogeochemistry of elements Fe, S, and As and are helpful for accurate prediction of As behavior.

Revealing the release and migration mechanism of heavy metals in typical carbonate tailings, East China

Refining the occurrence characteristics of tailings hazardous materials at source is of great importance for pollution management and ecological reclamation. However, the release and transport of heavy metals (HMs) from tailings under rainfall drenching in simulated real-world environments is less well portrayed, particularly highlighting the inherent neutralisation in tailings wastes under superimposed dynamic conditions. In this study, dynamic leaching columns simulating actual conditions were used to observe the release and transport of HMs from tailings under acid rainfall infiltration at spatial and temporal scales. The release rate of trace elements (e.g., As, Cr, Ni, Pb, Cd) is high. Neutralisation in the presence of carbonate rocks in the gangue reduces HMs release intensity from tailings with high heavy metal content, along with the precipitation of iron oxides and chromium-bearing minerals, etc. In addition, the vertical differentiation of HMs is more relevant to physical processes. In the absence of carbonate rocks in gangue, the lowest pH value is reached within 1.2 h after acid rain infiltrates the tailings. At the same time, Cu, Zn and Cd are released significantly from the minerals at the superficial level. The release of As(III) is mainly concentrated in the early and late stages of water-rock contact.

Dynamic coupling of ferrihydrite transformation and associated arsenic desorption/redistribution mediated by sulfate-reducing bacteria

Sulfate-reducing bacteria play an important role in the geochemistry of iron (oxyhydr)oxide and arsenic (As) in natural environments; however, the associated reaction processes are yet to be fully understood. In this study, batch experiments coupled with geochemical, spectroscopic, microscopic, and thermodynamic analyses were conducted to investigate the dynamic coupling of ferrihydrite transformation and the associated As desorption/redistribution mediated by Desulfovibrio vulgaris (D. vulgaris). The results indicated that D. vulgaris could induce ferrihydrite transformation via S2--driven and direct reduction processes. In the absence of SO42-, D. vulgaris directly reduced ferrihydrite, and As desorption and re-sorption occurred simultaneously during the partial transformation of ferrihydrite to magnetite. The increase in SO42- loading promoted the S2--driven reduction of ferrihydrite and accelerated the subsequent mineralogical transformation. In the low and medium SO42- treatments, ferrihydrite was completely transformed to a mixture of magnetite and mackinawite, which increased the fraction of As in the residual phase and stabilized As. In the high SO42- treatment, although the replacement of ferrihydrite by only mackinawite also increased the fraction of As in the residual phase, 22.1% of the total As was released into the solution due to the poor adsorption affinity of As to mackinawite and the conversion of As5+ to As3+. The mechanisms of ferrihydrite reduction, mineralogy transformation, and As mobilization and redistribution mediated by sulfate-reducing bacteria are closely related to the surrounding SO42- loadings. These results advance our understanding of the biogeochemical behavior of Fe, S, and As, and are helpful for the risk assessment and remediation of As contamination.

Co-transport of arsenic and micro/nano-plastics in saturated soil

Contaminants can co-exist and migrate together in the environment, causing complex (and sometimes unexpected) transport dynamics which challenge the efficient remediation of individual contaminants. The co-transport dynamics, however, remained obscure for some contaminants, such as arsenic and micro/nano-plastics (MNPs). To fill this knowledge gap, this study explored the co-transport dynamics of arsenic and MNP particles in saturated soil by combining laboratory experiments and stochastic model analysis. Isothermal adsorption and sand column transport experiments showed that the adsorption of arsenic by MNP particles followed the Freundlich model, with a maximum adsorption of 2.425 mg/g for the MNP particles with a diameter of 100 nm. In the presence of MNP particles, the efflux concentration of arsenic ions declined due to adsorption, where the decline rate decreased with the increasing MNP size and increased with the increasing adsorption capacity. Experimental results also showed that the 100 nm nano-plastic particles prohibited arsenic transport in saturated sand columns, while the 5 μm microplastics enhanced arsenic transport due to electrostatic adsorption and media pore plugging. A tempered time fractional advective-dispersion equation was then proposed to quantify the observed breakthrough curves of arsenic. The results showed that this model can reliably capture the co-transport behavior of arsenic with MNPs in the saturated soil with all coefficients of determination over 0.97, and particularly, the small MNP particles facilitated anomalous transport of arsenic. This study therefore improved the understanding and quantification of the co-transport of arsenic and MNPs in soil.

Shewanella oneidensis MR-1 and oxalic acid mediated vanadium reduction and redistribution in vanadium-containing tailings

The microbially- and chemically-mediated redox process is critical in controlling the fate of vanadium (V) in tailing environment. Although the microbial reduction of V has been widely studied, the coupled biotic reduction mediated by beneficiation reagents and the underlying mechanism remain unclear. Herein, the reduction and redistribution of V in V-containing tailings and Fe/Mn oxide aggregates mediated by Shewanella oneidensis MR-1 and oxalic acid were explored. The dissolution of Fe-(hydr)oxides by oxalic acid promoted the microbe-mediated V release from solid-phase. After 48-day of reaction, the dissolved V concentrations in the bio-oxalic acid treatment reached maximum values of 1.72 ± 0.36 mg L^-1 and 0.42 ± 0.15 mg L^-1 in the tailing system and the aggregate system, respectively, significantly higher than those in control (0.63 ± 0.14 mg L^-1 and 0.08 ± 0.02 mg L^-1). As the electron donor, oxalic acid enhanced the electron transfer process of S. oneidensis MR-1 for V(V) reduction. The mineralogical characterization of final products indicates that S. oneidensis MR-1 and oxalic acid promoted solid-state conversion from V₂O₅ to NaV₆O₁₅. Collectively, this study demonstrates that microbe-mediated V release and redistribution in solid-phase were promoted by oxalic acid, suggesting that the role of organic agents for the V biogeochemical cycle in natural systems deserves greater attention.

Effect of phosphate on ferrihydrite transformation and the associated arsenic behavior mediated by sulfate-reducing bacterium

Although PO₄^3- is commonly found in association with iron (oxyhydr)oxide, the effect of PO₄^3- on ferrihydrite reduction, mineralogical transformation, and associated As behavior in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this study, batch experiments, together with geochemical, mineralogical, and biological analyses, were conducted to elucidate these processes. The results showed that SRB can reduce ferrihydrite via direct and indirect processes, and PO₄^3- promoted ferrihydrite reduction by supporting SRB growth at low and medium PO₄^3- loadings. However, at high loadings, PO₄^3- stabilized the ferrihydrite. PO₄^3- shifted the transformation of ferrihydrite from magnetite and mackinawite to vivianite, which scavenges As effectively by incorporating As into its particle. In systems with 0.5 mM SO₄^2-, PO₄^3- exerted a weak effect on As mobilization. However, in systems with 10 mM SO₄^2-, substantial amounts of As were released into the solution, and PO₄^3- impacted As behavior strongly. Low PO₄^3- loadings increased the mobilization of As because of the competitive adsorption of PO₄^3- on mackinawite. Medium and high PO₄^3- loadings were beneficial for As immobilization because of the substitution of mackinawite by vivianite. These findings have important implications for understanding the biogeochemistry of iron (oxyhydr)oxide and As behavior in SRB-containing sediments.

Water security threats and challenges following the rupture of large tailings dams

The rupture of mine-tailings dams can severely contaminate rivers, because released tailings can interact with water for years keeping contaminant concentrations high. The general purpose of this study was to examine the rupture of B1 tailings dam in Ferro-Carvão stream (municipality of Brumadinho, state of Minas Gerais, Brazil), which occurred in 25 January 2019 and contaminated the main water course (Paraopeba River) with 2.8 Mm³ of metal-rich tailings. The specific purpose was to assess the percentage of non-conforming concentrations following the event, considering the Normative Deliberation COPAM/CERH-MG no. 1. The results showed non-conforming aluminum, iron, manganese, lead, phosphorus and turbidity concentrations, clearly above pre-rupture averages, especially in the rainy period. The catastrophe triggered the suspension of Paraopeba River as drinking water source to the Metropolitan Region of Belo Horizonte (BHMR; 6 million people). Since then, the supply to the BHMR became an everyday challenge to water management authorities, because the Paraopeba source represented a 30% share. Mitigation measures are therefore urgently needed. As complementary objective to this study, we aimed to verify the possibility to restore drinking water supply through conventional treatment. The treatability of Paraopeba River water was assessed by the Raw Water Quality Index considering the rainy and dry periods in separate. The results suggested the possibility to lift up the suspension in the dry period, improving the regional water security. Considering the huge dataset on which this study is standing, our results are generalizable to similar events with sparser information.

Immobilization and redistribution process of As(V) during As(V)-bearing ferrihydrite reduction by Geobacter sulfurreducens under the influence of TiO2 nanoparticles

The redistribution process of arsenate (As(V)) and the variation in As(V) content in different locations must be clarified to ensure low mobility of As(V) during microbial ferrihydrite reduction. In this study, we investigated As(V) immobilization and redistribution processes when ferrihydrite was incubated with Geobacter sulfurreducens in the presence of titanium dioxide (TiO₂) nanoparticles. Our study results showed that, As(V) in the aqueous phase and ferrihydrite were redistributed on light minerals (goethite), heavy minerals (ferrihydrite and magnetite), and extracellular polymeric substances (EPS) induced by G. sulfurreducens during ferrihydrite reduction. Interestingly, we found that As(V) in the form of arsenate ion (AsO₄^3-) was adsorbed by the functional groups of the EPS, while the formed Fe^II₃(As^VO₄)₂ was wrapped in the network structure of EPS. Moreover, the addition of TiO₂ nanoparticles did not promote but delayed the entire ferrihydrite reduction, As(V) immobilization and redistribution processes. Furthermore, changes in the aqueous arsenic and iron concentrations are closely related to the formation time of secondary minerals. Our study findings provide new insights into the As(V) immobilization process mediated by G. sulfurreducens under anaerobic conditions.

Fate of oxalic-acid-intervened arsenic during Fe(II)-induced transformation of As(V)-bearing jarosite

Jarosite is a metastable Fe(III)-oxyhydroxysulfate mineral that can act as an excellent scavenger for arsenic (As) in acid sulfate soils (ASSs) and in areas polluted by acid mine drainage (AMD). The Fe(II)-induced transformation of jarosite can influence the As mobility in reducing soil and sediment systems. Although organic acids are prevalent in these environments, their influence on the behavior of As during the Fe(II)-induced transformation of jarosite is yet to be fully understood. In this study, we investigated the effects of oxalic acid on the partitioning of As into dissolved, adsorbed, poorly crystalline, and residual phases during the Fe(II)-induced transformation of As(V)-bearing jarosite at pH 5.5 and 1 mM Fe(II) concentration. The results demonstrated that jarosite frequently transformed to lepidocrocite in treatments without oxalic acid or with low oxalic acid (0.1 mM), and As was typically redistributed in the surface-bound exchangeable and residual phases. While a high concentration of oxalic acid (1 mM) retarded the transformation of jarosite and produced goethite as the primary end product, it also changed the Fe(II)-induced transformation pathway and drove most As into the residual phase (approximately 92%). The results indicated that oxalic acid exerts a significant influence on the partitioning and speciation of As during the above-mentioned transformation. X-ray photo electron spectroscopy analysis of the reaction products also revealed that As(V) may be still the dominant redox species. Overall, this study provides critical information for understanding the fate of As during the transformation of secondary minerals under complex influencing factors, thereby assisting in more accurately predicting the geochemical cycling of As in natural systems.

A critical review on environmental implications, recycling strategies, and ecological remediation for mine tailings

Mine tailings, generated from the extraction, processing, and utilization of mineral resources, have resulted in serious acid mine drainage (AMD) pollution. Recently, scholars are paying more attention to two alternative strategies for resource recovery and ecological reclamation of mine tailings that help to improve the current tailing management, and meanwhile reduce the negative environmental outcomes. This review suggests that the principles of geochemical evolution may provide new perspective for the future in-depth studies regarding the pollution control and risk management. Recent advances in three recycling approaches of tailing resources, termed metal recovery, agricultural fertilizer, and building materials, are further described. These recycling strategies are significantly conducive to decrease the mine tailing stocks for problematic disposal. In this regard, the future recycling approaches should be industrially applicable and technically feasible to achieve the sustainable mining operation. Finally, the current state of tailing phytoremediation technologies is also discussed, while identification and selection of the ideal plants, which is perceived to be the excellent candidates of tailing reclamation, should be the focus of future studies. Based on the findings and perspectives of this review, the present study can act as an important reference for the academic participants involved in this promising field.