Long-term performance of a UASB reactor treating acid mine drainage: effects of sulfate loading rate, hydraulic retention time, and COD/SO42− ratio

Sensitivity of Pathogenic Bacteria Strains to Treated Mine Water

Mine water as a result of meteoric and/or underground water's contact with tailings and underground workings could have an elevated content of metals associated with sulfate, often acidic, due to the bio-oxidation of sulfides. When entering aquatic ecosystems, the mine water can cause significant changes in the species' trophic levels, therefore a treatment is required to adjust the alkalinity and to remove the heavy metals and metalloids. The conventional mine water treatment removes metals, but in many cases it does not reduce the sulfate content. This paper aimed to predict the impact of conventionally treated mine water on the receiving river by assessing the genotoxic activity on an engineered Escherichia coli and by evaluating the toxic effects generated on two Gram-negative bacterial strains, Pseudomonas aeruginosa and Escherichia coli. Although the main chemical impact is the severe increases of calcium and sulfate concentrations, no significant genotoxic characteristics were detected on the Escherichia coli strain and on the cell-viability with a positive survival rate higher than 80%. Pseudomonas aeruginosa was more resistant than Escherichia coli in the presence of 1890 mg SO₄^2-/L. This paper reveals different sensitivities and adaptabilities of pathogenic bacteria to high concentrations of sulfates in mine waters.

Lactate as an effective electron donor in the sulfate reduction: impacts on the microbial diversity

The competition between sulfate-reducing bacteria and methane-producing archaea has a major influence on organic matter removal, as well as the success of sulfidogenic systems. This study investigated the performance of six batch sulfidogenic reactors in response to different COD/sulfate ratios (1.0 and 2.0) and electron donors (cheese whey, ethanol, and sodium lactate) by evaluating the biochemical mechanisms of sulfate reduction, organic matter oxidation, and microbial structure modification. A COD/sulfate ratio of 1.0 resulted in high sulfidogenic activity for all electron donors, thereby achieving a nearly 80% sulfate removal. Lactate provided high sulfate removal rates at COD/sulfate ratios of 1.0 (80%) and 2.0 (90%). A COD/sulfate ratio of 2.0 decreased the sulfate removal rates by 25 and 28% when ethanol and cheese whey were used as substrates. The sulfate-reducing bacteria populations increased using ethanol and lactate at a COD/sulfate ratio of 1.0. Particularly, Desulfovibrio, Clostridium, and Syntrophobacter were predominant. Influent composition and COD/sulfate ratio influenced the relative abundance of the microbial communities. Therefore, controlling these parameters may facilitate the wastewater treatment with high sulfate levels through bacterial activity.

Ferns and lycophytes in coal mining waste and tailing landfills

Mineral coal extraction in Santa Catarina State (Brazil) Carboniferous Basin has degraded the local ecosystem, restricting the use of its areas. One of the biggest environmental impacts in the mining areas is the uncontrolled disposal of waste and sterile mining with high concentrations of pyrite, which in the presence of air and water is oxidized promoting the formation of acid mine drainage (AMD). These contaminants can be leached into water resources, restrict the use of water and soil, and cause threats to fauna and flora. This study aimed to characterize these areas as to the content of Cd, Pb, Ni and Zn metals in the tailings and waste resulting from coal mining and to survey the species of ferns and lycophytes present. Wastes and tailing samples and specimens of ferns and lycophytes were collected in 23 landfills in six municipalities in the region and in four underlying areas used as controls. Chemical and physical analyses (pH in water and pH in KCl, Ca, Mg, P, K, Na, Mn, Fe, Al, clay and OM contents) were carried out and the total contents of heavy metals Cd, Pb, Ni and Zn were determined. Sampling of ferns and lycophytes was carried out by walking. The levels of heavy metals, Cd, Ni and Zn, were below the prevention concentrations established by CONAMA Resolution 420/2009. Pb levels were above prevention values in four landfills. Sixteen species of ferns and one lycophyte were found, with hemicryptophytes the most frequent and helophytes the most adapted to the environment. Of the species found, Pteridium esculentum (G. Forst.) Cockayne, Pityrogramma calomelanos (L.) Link and Telmatoblechnum serrulatum (Rich.) Perrie, DJ Ohlsen & Brownsey demonstrated resistance to degraded and contaminated environments with Pb, which may constitute an alternative for project monitoring and environmental recovery.

Review of Remediation Solutions for Acid Mine Drainage Using the Modified Hill Framework

This paper reviews the Acid Mine Drainage (AMD) remediation potential and operational costs of twelve existing AMD remediation methods against Class 0 and Class I AMD geochemical characteristics as defined in the Modified Hill Framework. Of the twelve remediation options reviewed in this study, eleven required additional process steps either for further treatment to achieve the discharge limits or for the safe management of hazardous waste by-products. Chemical desalination showed the greatest potential with high quality treated water and operational costs between USD 0.25 and USD 0.75 per cubic meter treated. The management of the toxic metal and sulphide by-products remains a key challenge that requires further research for sustainable mine water remediation. Further development of end-to-end methods suitable for Class 0 AMD with economical operational costs is recommended in order to effectively address the ongoing environmental challenges posed by AMD globally.

Sulfate and metal removal from acid mine drainage using sugarcane vinasse as electron donor: Performance and microbial community of the down-flow structured-bed bioreactor

The down flow structured bed bioreactor (DFSBR) was applied to treat synthetic acid mine drainage (AMD) to reduce sulfate, increase the pH and precipitate metals in solutions (Co, Cu, Fe, Mn, Ni and Zn) using vinasse as an electron donor for sulfate-reducing bacteria (SRB). DFSBR achieved sulfate removal efficiencies between 55 and 91%, removal of Co and Ni were obtained with efficiencies greater than 80%, while Fe, Zn, Cu and Mn were removed with average efficiencies of 70, 80, 73 and 60%, respectively. Sulfate reduction increased pH from moderately acidic to 6.7-7.5. Modelling data confirmed the experimental results and metal sulfide precipitation was the mainly responsible for metal removal. The main genera responsible for sulfate and metal reduction were Geobacter and Desulfovibrio while fermenters were Parabacteroides and Sulfurovum. Moreover, in syntrophism with SRB, they played an important role in the efficiency of metal and sulfate removal.

Over-sulfated soils and sediments treatment: A brief discussion on performance disparities of biological and non-biological methods throughout the literature

High sulfate concentrations in industrial effluents as well as solid materials (excavated soils, dredged sediments, etc.) are a major hindrance for circular economy outlooks. SO₄^2- acceptability standards are indeed increasingly restrictive, given the potential outcomes for public health and ecosystems. This literature review deals with the treatment pathways relying on precipitation, adsorption and microbial redox principles. Although satisfactory removal performances can be achieved with each of them, significant yield differences are displayed throughout the bibliography. The challenge here was to identify the parameters leading to this variability and to assess their impact. The precipitation pathway is based on the formation of two main minerals (ettringite and barite). It can lead to total sulfate removal but can also be limited by aqueous wastes chemistry. Stabilizer kinetics of formation and equilibrium are highly constrained by background properties such as pH, Eh, SO₄^2- saturation state and inhibiting metal occurrences. Regarding the adsorption route, sorbents' intrinsic features such as the q_max parameter govern removal yields. Concerning the microbial pathway, the chemical oxygen demand/SO₄^2- ratio and the hydraulic retention time, which are classically evoked as yield variation factors, appear here to be weakly influential. The effect of these parameters seems to be overridden by the influence of electron donors, which constitute a first order factor of variability. A second order variability can be read according to the nature of these electron donors. Approaches using simple monomers (ethanol lactates, etc.) perform better than those using predominantly ligneous organic matter.

Sulfate removal rate and metal recovery as settling precipitates in bioreactors: Influence of electron donors

Four down-flow structured bed bioreactors were operated targeting biological sulfate-reduction and metal recovery. Three different electron donors were tested: glycerol (R1), lactate (R2), sucrose (R3), and a blend of the previous three (R4) with an increasing copper influent load (5, 15, and 30 mg Cu²⁺.L^-1). Copper inhibited sulfate-reduction in R1 (15 mg Cu²⁺.L^-1) and R3 (5 mg Cu²⁺.L^-1), but the fermentative activity was not affected. R2 and R4 were not inhibited by the copper influent concentration. R2 provided the highest sulfate reduction rate (1767.3 ± 240.1 mg SO₄^2-.L.day^-1). Nonetheless, the accumulation of settling precipitates was 22 % higher in R4 than in R2, indicating the former yielded the highest metal recovery as settling precipitates (24.8 g FSS.L^-1, 25 % Fe²⁺, 5% Cu²⁺). 16S rRNA sequencing showed highest diversity of sulfate-reducing bacteria in R2. A predominance of sulfate-reducing and fermentative bacteria with more similarity was observed between microbial populations in R1 and R4, despite the difference in toxicity thresholds. Hence, the electron donor influenced not only the biological sulfate reduction, but also metal toxicity thresholds and metal recovery as settling precipitates.

Mine drainage: Remediation technology and resource recovery

Drainage from current and historic mining operations remains a persistent environmental problem. Numerous research and development efforts were made in 2019 with a goal to minimize the impact of mine drainage on the environment, while other research endeavors addressed the mine drainage issue from a different perspective, where mine drainage was considered a resource for water and valuable products, such as metals, sulfuric acid, and rare earth elements. Thus, this review has two main sections: (a) focusing on research efforts in mine drainage remediation technology, and (b) emphasizing advances in resource recovery from mine drainage. The first section covers traditional and emerging passive and active treatment technologies. The second section summarizes resource recovery efforts using various technologies, such as selective precipitation, membrane process, and biological systems. PRACTITIONER POINTS: Significant progress continued to be made in the management of mine drainage and related issues. Recent remediation technology advances in mine drainage were presented. Technologies focusing on resource recovery from mine drainage were reviewed.

Sulfate and metals removal from acid mine drainage in a horizontal anaerobic immobilized biomass (HAIB) reactor

The acid mine drainage (AMD) can causes negative impacts to the environment. Physico-chemical methods to treat AMD can have high operational costs. Through passive biological methods, such as anaerobic reactors, sulfate reduction, and recovery of metals are promoted. This study evaluated the performance of a horizontal anaerobic immobilized biomass (HAIB) reactor for the treatment of synthetic AMD using polyurethane foam as support material, and anaerobic sludge as inoculum. Ethanol was used as an electron donor for sulfate reduction, resulting in an influent chemical oxygen demand (COD) in the range of 500-1,500 mg/L and COD/sulfate ratio at 1. A gradual increase of sulfate and COD concentration was applied that resulted in COD removal efficiencies higher than 78%, and sulfate removal efficiencies of 80%. Higher sulfate and COD concentrations associated with higher hydraulic retention times (36 h) proved to be a better strategy for sulfate removal. The HAIB reactor was able to accommodate an increase in the SLR up to 2.25 g SO₄^2-/L d^-1 which achieved the greatest performance on the entire process. Moreover, the reactor proved a suitable alternative for reaching high levels of metal removal (86.95 for Zn, 98.79% for Fe, and 99.59% for Cu).

Advances in heavy metal removal by sulfate-reducing bacteria

Industrial development has led to generation of large volumes of wastewater containing heavy metals, which need to be removed before the wastewater is released into the environment. Chemical and electrochemical methods are traditionally applied to treat this type of wastewater. These conventional methods have several shortcomings, such as secondary pollution and cost. Bioprocesses are gradually gaining popularity because of their high selectivities, low costs, and reduced environmental pollution. Removal of heavy metals by sulfate-reducing bacteria (SRB) is an economical and effective alternative to conventional methods. The limitations of and advances in SRB activity have not been comprehensively reviewed. In this paper, recent advances from laboratory studies in heavy metal removal by SRB were reported. Firstly, the mechanism of heavy metal removal by SRB is introduced. Then, the factors affecting microbial activity and metal removal efficiency are elucidated and discussed in detail. In addition, recent advances in selection of an electron donor, enhancement of SRB activity, and improvement of SRB tolerance to heavy metals are reviewed. Furthermore, key points for future studies of the SRB process are proposed.

Simultaneous Biological and Chemical Removal of Sulfate and Fe(II)EDTA-NO in Anaerobic Conditions and Regulation of Sulfate Reduction Products

In the simultaneous flue gas desulfurization and denitrification by biological combined with chelating absorption technology, SO2 and NO are converted into sulfate and Fe(II)EDTA-NO which need to be reduced in biological reactor. Increasing the removal loads of sulfate and Fe(II)EDTA-NO and converting sulfate to elemental sulfur will benefit the application of this process. A moving-bed biofilm reactor was adopted for sulfate and Fe(II)EDTA-NO biological reduction. The removal efficiencies of the sulfate and Fe(II)EDTA-NO were 96% and 92% with the influent loads of 2.88 kg SO42−·m−3·d−1 and 0.48 kg NO·m−3·d−1. The sulfide produced by sulfate reduction could be reduced by increasing the concentrations of Fe(II)EDTA-NO and Fe(III)EDTA. The main reduction products of sulfate and Fe(II)EDTA-NO were elemental sulfur and N2. It was found that the dominant strain of sulfate reducing bacteria in the system was Desulfomicrobium. Pseudomonas, Sulfurovum and Arcobacter were involved in the reduction of Fe(II)EDTA-NO.