Recovering iron and sulfate in the form of mineral from acid mine drainage by a bacteria-driven cyclic biomineralization system

Chemical Mineralization of AMD into Schwertmannite Fixing Iron and Sulfate Ions by Structure and Adsorption: Paving the Way for Enhanced Mineralization Capacity

Abundant iron and sulfate resources are present in acid mine drainage. The synthesis of schwertmannite from AMD rich in iron and sulfate could achieve the dual objectives of resource recovery and wastewater purification. However, schwertmannite cannot emerge spontaneously due to the Gibbs free energy greater than 0. This results in the iron and sulfate in AMD only being able to use the energy generated by oxidation in the coupling reaction to promote the formation of minerals, but this only achieved partial mineralization, which limited the remediation of AMD through mineralization. In order to clarify the mechanism of iron and sulfate removal by the formation of schwertmannite in AMD, kinetic and thermodynamic parameters were crucial. This work used H2O2 oxidation of Fe2+ as a coupling reaction to promote the formation of schwertmannite from 64.4% of iron and 15.7% of sulfate in AMD, and determined that 99.7% of the iron and 89.9% of sulfate were immobilized in the schwertmannite structural, and only a small fraction was immobilized by the adsorption of schwertmannite, both of which were consistent with second-order kinetics models. The thermodynamic data suggested that reducing the concentration of excess sulfate ions or increasing the energy of the system may allow more iron and sulfate to be immobilized by forming schwertmannite. Experimental verification using the reaction of potassium bicarbonate with the acidity in solution to increase the energy in the system showed that the addition of potassium bicarbonate effectively promoted the formation of schwertmannite from Fe3+ and SO42-. It provided a theoretical and research basis for the direct synthesis of schwertmannite from Fe3+ and SO42- rich AMD for the removal of contaminants from water and the recovery of valuable resources.

Bacterial communities shift and influence in an acid mine drainage treatment using barium carbonate disperse alkaline substrate system

Chemical passive treatment systems used to remediate acid mine drainage has been evaluated based mainly on the reactivity of the chemical alkaline reagents, overlooking the activity of the microorganisms that proliferate in these artificial ecosystems. In this study, the bacterial communities of a unique passive treatment system known as BDAS (Barium carbonate Dispersed Alkaline Substrate) were investigated using 16S rRNA gene metagenomic sequencing combined with hydrochemical characterization of the AMD and phenotypic characterization of biogenic precipitates. According to the hydrochemical characterization, the water quality improved as the water progressed through the system, with a drastic increase in the pH (up to alkaline conditions) and total organic carbon, as well as the removal of main contaminants such as Ca²⁺, SO₄^2-, Fe³⁺, Al³⁺, and Mn²⁺. These environmental changes resulted in an increase in bacterial diversity (richness) after the inlet and in the shift of the bacterial communities from chemoautotrophs (e.g., Ferrovum and Acidiphilum) to chemoheterotrophs (e.g., Brevundimonas and Geobacter). Some of these taxa harbour potential to immobilize metals, aiding in the treatment of the water. One of the mechanisms involved in the immobilization of metals is microbially induced calcium carbonate precipitation, which seems to occur spontaneously in BDAS. The production of biofilm was also observed in most parts of the system, except in the inlet, helping with the removal of metals. However, in the long run, the build-up of biofilm and precipitation of metals could clog (i.e., biofouling) the pores of the matrix, reducing the treatment efficiency. Potential human pathogens (e.g. Legionella) were also detected in BDAS indicating the need for a treatment step at the end of the system to remove pathogenic microorganisms. These findings present a new perspective of the bacterial communities and their effects (both positively and negatively) in a chemical passive treatment system.

The interplay between microalgae and toxic metal(loid)s: mechanisms and implications in AMD phycoremediation coupled with Fe/Mn mineralization

Acid mine drainage (AMD) is low-pH with high concentration of sulfates and toxic metal(loid)s (e.g. As, Cd, Pb, Cu, Zn), thereby posing a global environmental problem. For decades, microalgae have been used to remediate metal(loid)s in AMD, as they have various adaptive mechanisms for tolerating extreme environmental stress. Their main phycoremediation mechanisms are biosorption, bioaccumulation, coupling with sulfate-reducing bacteria, alkalization, biotransformation, and Fe/Mn mineral formation. This review summarizes how microalgae cope with metal(loid) stress and their specific mechanisms of phycoremediation in AMD. Based on the universal physiological characteristics of microalgae and the properties of their secretions, several Fe/Mn mineralization mechanisms induced by photosynthesis, free radicals, microalgal-bacterial reciprocity, and algal organic matter are proposed. Notably, microalgae can also reduce Fe(III) and inhibit mineralization, which is environmentally unfavorable. Therefore, the comprehensive environmental effects of microalgal co-occurring and cyclical opposing processes must be carefully considered. Using chemical and biological perspectives, this review innovatively proposes several specific processes and mechanisms of Fe/Mn mineralization that are mediated by microalgae, providing a theoretical basis for the geochemistry of metal(loid)s and natural attenuation of pollutants in AMD.

Assessment of the induced effect of selected iron hydroxysulfates biosynthesized using Acidithiobacillus ferrooxidans for biomineralization of acid mine drainage

Soluble iron and sulfate in acid mine drainage (AMD) can be greatly removed through the formation of minerals facilitated by seed crystals. However, the difference in the effects of jarosite and schwertmannite as endogenous seed crystals to induce AMD mineralization remains unclear. This paper intends to study the effect of Fe²⁺ oxidation and Fe³⁺ mineralization in the biosynthesis of minerals using different addition amounts and methods of jarosite or schwertmannite. The results showed that the addition amount and method of different seed crystals had no effect on the Fe²⁺ bio-oxidation but would change the Fe³⁺ mineralization efficiency. With the same amount of seed crystals added, jarosite exhibited a higher capacity to promote Fe³⁺ mineralization than schwertmannite. Adding seed crystals before the initiation of Fe²⁺ oxidation (0 h) could significantly promote Fe³⁺ mineralization efficiency. With the increase of seed crystals, jarosite could not only shorten the time required for mineral synthesis but also improve the final mineral yield, whereas schwertmannite could only shorten the time required for mineral synthesis. When Fe²⁺ was completely oxidized to Fe³⁺ (48 h), the supplementary of jarosite could still effectively improve Fe³⁺ mineralization efficiency, but the addition of schwertmannite no longer affected the final mineralization degree.

Algae, biochar and bacteria for acid mine drainage (AMD) remediation: A review

Acid mine drainage (AMD) is a global issue and causes harmful environmental impacts. AMD has high acidity and contains a high concentration of heavy metals and metalloids, making it toxic to plants, animals, and humans. Traditional treatments for AMD have been widely used for a long time. Nevertheless, some limitations, such as low efficacy and secondary contamination, have led them to be replaced by other methods such as bio-based AMD treatments. This study reviewed three bio-based treatment methods using algae, biochar, and bacteria that can be used separately and potentially in combination for effective and sustainable AMD treatment to identify the removal mechanisms and essential parameters affecting AMD treatment. All bio-based methods, when applied as a single process and in combination (e.g. algae-biochar and algae-bacteria), were identified as effective treatments for AMD. Also, all these bio-based methods were found to be affected by some parameters (e.g. pH, temperature, biomass concentration and initial metal concentration) when removing heavy metals from AMD. However, we did not identify any research focusing on the combination of algae-biochar-bacteria as a consortium for AMD treatment. Therefore, due to the excellent performance in AMD treatment of algae, biochar and bacteria and the potential synergism among them, this review provides new insight and discusses the feasibility of a combination of algae-biochar-bacteria for AMD treatment.

A novel approach for treating acid mine drainage by forming schwertmannite driven by a combination of biooxidation and electroreduction before lime neutralization

Acid mine drainage (AMD) contains abundant iron, sulfates, and various metal ions, and it causes environmental pollution. The traditional AMD lime neutralization forms a layer of iron hydroxide and gypsum on the surface of the lime particles, preventing continuous reaction and leading to excessive lime addition and neutralized sludge production. In this study, an approach for treating AMD using a cyclic process of biooxidation and electroreduction before lime neutralization was proposed, in which the Fe²⁺ in AMD was oxidized to Fe³⁺ and induced to form schwertmannite through Acidithiobacillus ferrooxidans. The remaining Fe³⁺ was reduced to Fe²⁺ using an electric field. After three biooxidation and two electroreduction cycles, 98.2% of Fe and 62.4% of SO₄^2- in AMD precipitated as schwertmannite (Fe₈O₈(OH)_5.16(SO₄)_1.37). The yield of schwertmannite reached 33.98 g/L_AMD, with a high specific surface area of 112.59 m²/g. The lime dosage and sludge yield of the treated AMD in the subsequent neutralization stage (pH = 7.00) decreased by 85.0% and 74.5%, respectively, than those of raw AMD. The pilot test results showed that the integrated treatment of biooxidation-electroreduction cyclic mineralization and lime neutralization has practical applications.

Modified chemical mineralization-alkali neutralization technology: Mineralization behavior at high iron concentrations and its application in sulfur acid spent pickling solution

Mineralization coupled with neutralization is a dual-function technology for disposing acidic iron-rich waters, which can recover the valuable iron in the form of secondary mineral and concurrently purify the wastewater. In this study, a modified technology for treating high Fe wastewater (sulfur acid spent pickling liquor, 62 g Fe/L) was proposed based on the specific investigation of the mineralization behaviors in Fe concentration range of 20-70 g/L. Results showed that high SO₄^2-/Fe²⁺ molar ratio (> 2.0) tended to trigger gelation phenomena at Fe concentrations above 30 g/L. Fe specie distribution suggested that the insufficient polymerization among Fe-OH complexes might be responsible for the gelation phenomena, since the strong Fe-SO₄ coordination almost completely suppressed the Fe_x(OH)_y^(3x-y)+ form (a general terms of Fe³⁺ hydrolysates and their polymers). Modified mineralization strategies were proposed, including pretreatment with dilution or BaCl₂/CaCl₂ precipitation, of which CaCl₂ pretreatment was a versatile and low-cost method. Following CaCl₂ pretreatment, chemical mineralization converted above 90% of iron into secondary mineral, which therefore drastically reduced the alkali consumption (from 164.2 g/L to 1.4 g/L) and sludge yield (from 328.1 g/L to 2.4 g/L) in subsequent neutralization treatment. The resultant mineral was identified as schwertmannite, and exhibited efficient adsorption capacity toward arsenite (364.2 mg/g). The modified chemical mineralization-alkaline neutralization is a cost-effective technology for the treatment of the acidic iron-rich waters. In practical applications, several regulating strategies should be further explored to improve the mineral purity, and the mineralization conditions must be optimized according to the Fe and SO₄^2- concentrations in wastewater.

Passive multi-unit field-pilot for acid mine drainage remediation: Performance and environmental assessment of post-treatment solid waste

This study evaluated the performance of a passive multi-unit field-pilot operating for 16 months to treat acid mine drainage (AMD) from a coal mine in Colombia Andean Paramo. The multi-unit field-pilot involved a combination of a pre-treatment unit (550 L) filled with dispersed alkaline substrate (DAS), and six passive biochemical reactors (PBRs; 220 L) under two configurations: open (PBRs-A) and closed (PBRs-B) to the atmosphere. The AMD quality was 1200 ± 91 mg L^-1 Fe, 38.0 ± 1.3 mg L^-1 Mn, 8.5 ± 1.6 mg L^-1 Zn, and 3200 ± 183.8 mg L^-1 SO₄^2-, at pH 2.8. The input and output effluents were monitored to establish AMD remediation. Physicochemical stability of the post-treatment solids, including metals (Fe²⁺, Zn²⁺, and Mn²⁺) and sulfates for environmental contamination from reactive mixture post-treatment, was also assessed. The passive multi-unit field-pilot achieved a total removal of 74% SO₄^2-, 63% Fe²⁺, and 48% Mn²⁺ with the line of PBRs-A, and 91% SO₄^2-, 80% Fe²⁺, and 66% Mn²⁺ with the line of PBRs-B, as well as 99% removal for Zn²⁺ without significant differences (p < 0.05) between the two lines. The study of the physicochemical stability of the post-treatment solids showed they can produce acidic leachates that could release large quantities of Fe and Mn, if they are disposed in oxidizing conditions; contact with water or any other leaching solutions must be avoided. Therefore, these post-treatment solids cannot be disposed of in a municipal landfill. The differences in configuration between PBRs, open or closed to the atmosphere, induced changes in the performance of the passive multi-unit field-pilot during AMD remediation.

A novel method based on membrane distillation for treating acid mine drainage: Recovery of water and utilization of iron

Rapid and highly efficient treatment of acid mine drainage (AMD) is still challenging due to the low pH and high metal concentrations in it. This research focuses on a novel treatment method of AMD using direct contact membrane distillation (DCMD) and photocatalysis to recover water and utilize iron. In the DCMD process without pretreatment, the flux decreased by 93.38%. If pretreated by adding sodium oxalate, scale formation potential was effectively mitigated due to the removal of calcium and complexing of iron. For the treatment of the pretreated AMD (PAMD), 60% of water was recovered in the DCMD process with the flux decrease of 22%. The concentrate obtained from the DCMD process demonstrated high photocatalytic activity in the methylene blue (MB) degradation in an aqueous solution. In addition, the Fe (III)-oxalate complexes in the concentrate were reduced to insoluble Fe (II)-oxalate with visible light irradiation, which could be separated by sedimentation and used as a Fenton catalyst. Hence, this novel method exhibits great advantages on effectively inhibiting DCMD membrane fouling during AMD treatment, producing high-quality distillate with low conductivity, and realizing near zero-discharge of AMD.

Continuous and efficient immobilization of heavy metals by phosphate-mineralized bacterial consortium

Traditional sewage treatment technology cannot remove heavy metals, which needs to be improved urgently. Lysinibacillus with the function of bio-mineralization was screened and loaded on granular sludge to form a phosphate-mineralized bacterial consortium, which demonstrated the ability of self-regulating pH and automatic solid-liquid separation. Heavy metals could be fixed on the bacterial consortium to produce stable and harmless phosphate minerals. The highest removal efficiency of Pb(Ⅱ), Cd(Ⅱ), and Ni(Ⅱ) were 97.9%, 70%, and 40%, respectively. Organic matter and other metal ions in actual polluted water had little effect on the Pb(Ⅱ) removal efficiency. Mechanism analysis was conducted through 3D-EEM, XRD, SEM-EDS, XPS, FTIR, and high-throughput sequencing analyses. The bacterial consortium was a multi-species coexistence system, but Lysinibacillus played a major role in removing Pb(Ⅱ). C-O and O-H bonds of tyrosine and phosphorous organics were broken by enzyme catalysis and the metal-oxygen bond (Pb-O) was formed. Mineral crystals in the reactor accumulated, transforming from the initial phase non-crystalline structure to the metaphase Pb₃(PO₄)₂ and eventually to the Pb₅(PO₄)₃OH. This research obtained a promising technique for immobilizing Pb(Ⅱ) or other hazardous metals continuously and efficiently.