Acid Mine Drainage Remediation Research Articles

Adsorption properties of Pb(II) and Cd(II) in acid mine drainage by oyster shell loaded lignite composite in different morphologies

A new idea to alleviate environmental pollution is the development of low-cost adsorbents using natural minerals and fishery wastes to treat high concentrations of heavy metal pollutants in acid mine drainage (AMD). Adsorbent morphology, adsorptive and regenerative capacity, and application potential are limiting factors for their large-scale use. Oyster shells capable of releasing alkalinity were loaded on the surface of lignite to develop two composite adsorbents with different morphologies (powdery and globular) for the treatment of AMD containing Pb(II) and Cd(II). The results show that the ability of the adsorbent to treat AMD is closely related to its morphologies. The pseudo-second-order kinetic model and the Langmuir model are suitable to describe the adsorption process of OS-M(P), and the maximum adsorption saturation capacities of Pb(II) and Cd(II) are 332.6219 mg/g and 318.9854 mg/g, respectively. The pseudo-second-order kinetic model and the Freundlich model are suitable to describe the adsorption process of OS-M(G). A synergistic result of electrostatic adsorption, neutralization precipitation, ion exchange and complex reaction is achieved in the removal of Pb(II) and Cd(II) by two morphologies of adsorbents. The regeneration times (5 times) and recovery rate (75.75%) of OS-M(G) are higher than those of OS-M(P) (3 times) and recovery rate (20%). The ability of OS-M(G) to treat actual AMD wastewater is still better than that of OS-M(P). OS-M(G) can be used as a promising environmentally friendly adsorbent for the long-term remediation of AMD. This study provides a comprehensive picture of resource management and reuse opportunities for natural mineral and fishery wastes.

Bioelectrochemical treatment of acid mine drainage: Microbiome synergy influences sulfidogenesis and acetogenesis

Bioelectrochemical systems (BES) are emerging as potential technologies that can remediate acid mine drainage (AMD) by cathodic reduction of sulfates to metal sulfides. This study evaluated bioelectrochemical remediation of sulfate rich AMD at two applied cathode potentials; BES-1: −1.0 V and BES-2: −0.8 V. Sulfate reducing bacteria were selectively enriched to be used as biocatalyst in BES. Initially, lactate was fed as carbon source and switched to chemolithoautotrophy with only CO2-fed conditions. Both BESs were operated at 3±0.2 g/l of sulfate with synthetic AMD (SAMD) fed first, and gradually changed to 50% AMD from mining site with 50% SAMD. Sulfate reduction was relatively higher with BES-1: 82% than BES-2: 76% coupled with sulfidogenesis. Interestingly, acetogenesis (BES-1: 2.12±0.2 g/l, BES-2: 1.9±0.2 g/l) was also noticed with high reduction currents (BES-1&2: >-70 mA). Microbiome community analysis revealed the dominant presence of sulfate reducers, acetogens, syntrophic bacteria and Methanobacterium, probing microbial synergy aiding sulfate reduction. An added advantage was the iron-sulfide (FeS) particles formation on cathode, which might have contributed to increased reduction currents. This study reveals insights into microbial synergy for autotrophic sulfate reduction within mixed microbiome communities along with the impact of FeS particles as conducive facilitator for electron transfer in BES, thereby enhancing electrosynthetic acetate production.

In-situ utilization of As(III) enhances selective recovery of heavy metals from acid mine drainage by hydroxyl-enriched Fe-Mn binary oxide

As(III), a common pollutant in acid mine drainage (AMD), can provide potential active sites for the adsorption of heavy metals on adsorbents. In this study, Pb(II), As(III), and Cu(II) were sequentially recovered from AMD using hydroxyl-enriched Fe-Mn binary oxide (O-FMO) via the in-situ utilization strategy of As(III). The results showed that the prepared O-FMO demonstrated better adsorption performance under acidic conditions than FMO. O-FMO exhibited an outstanding selectivity (＞99.6%) for the removal of Pb(II) and As(III) via the generation of insoluble As-Pb complex in AMD and the immobilized Pb and As could be subsequently separated using respective acid and alkaline solutions. Furthermore, the recovered O-FMO containing residual As could promote the selective removal of Cu(II) (Selectivity ＞98.4 %) from the treated AMD. Mechanism analysis revealed that the oxidized As(V) on the O-FMO played a dominant role in promoting metals adsorption via electrostatic interaction and complexation. Pb(II) can directly form a generally stable complex with H3AsO4 (As(V)) under acidic conditions, thereby replacing the Cu(II) and Cd(II) which only form acid-soluble complexes with H2AsO4-1 (As(V)). Besides, metal electronegativity affects the traits and the formation sequence of As-metal complexes. Hence, the recovery of metal(loid)s (purity > 98.4 %) from AMD following the order: Pb(II), As(III), and Cu(II) could be achieved by reutilizing O-FMO. This work offers a unique approach for AMD remediation by utilizing pollutants to treat pollution.

Geochemical and petrological studies of high sulfur coal and overburden from Makum coalfield (Northeast India) towards understanding and mitigation of acid mine drainage

Opencast coal mining produces trash of soil and rock containing various minerals, that are usually dumped nearby the abandoned sites which causes severe environmental concern including the production of acid mine drainage (AMD) through oxidation pyrite minerals. The current study entailed assessing the potential production of AMD from an opencast coal mining region in Northeast part of India. In order to have a comprehensive overview of the AMD problem in Makum coalfield, the physico-chemical, geochemical, and petrological characteristics of the coal and overburden (OB) samples collected from the Makum coalfield (Northeast India) were thoroughly investigated. The maceral compositions reveal that coal features all three groups of macerals (liptinite, vitrinite, and inertinite), with a high concentration of liptinite indicating the coal of perhydrous, thereby rendering it more reactive. Pyrite (FeS2) oxidation kinetics were studied by conducting the aqueous leaching experiments of coal and (OB) samples to interpret the chemical weathering under controlled laboratory conditions of various temperature and time periods, and to replicate the actual mine site leaching. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was operated to detect the disposal of some precarious elements from coal and OB samples to the leachates during our controlled leaching experiment. The Rare earth element (REE) enrichment in the samples shows the anthropogenic incorporation of the REE in the coal and OB. These experiments reveal the change in conductivity, acid producing tendency, total dissolved solid(TDS), total Iron(Fe) and dissolved Sulfate(SO42−) ions on progress of the leaching experiments. Moreover, the discharge of FeS2 via atmospheric oxidation in laboratory condition undergoes a significant growth with the rise of temperature of the reaction systems in the environment and follows pseudo first order kinetics. A bio-remediative strategies is also reported in this paper to mitigate AMD water by employing size-segregated powdered limestone and water hyacinth plant in an indigenously developed site-specific prototype station. Apart from neutralisation of AMD water, this eco-friendly AMD remediation strategy demonstrates a reduction in PHEs concentrations in the treated AMD water.

Potassium-Bicarbonate-Induced Mineralized Acid Mine Drainage into Iron Hydroxyl Sulfate Minerals for Better Water Remediation and Resource Reuse

Iron hydroxyl sulfate minerals (IHSMs, including schwertmannite and jarosite) are a promising material for environmental applications with excellent adsorption of metal oxygen anions. The acid mine drainage (AMD) abundant in iron and sulfate ions holds potential for the production of valuable IHSMs, thereby achieving resource recycling whilst simultaneously mitigating water contamination, which is important for the sustainable remediation of AMD. Conventional mineralization, which promotes the generation of minerals from Fe3+ and SO42− through the energy provided by chemical or biological oxidation, can only partially mineralize iron in AMD containing substantial quantities of Fe2+. In this study, an improved method for mineralizing AMD containing iron of a different valence into IHSMs under the induction of KHCO3 was proposed. For AMD containing Fe2+, the combination of KHCO3 and H2O2 was used to promote the hydrolysis of iron (92.7%) and sulfate (11.1%) into IHK minerals, which resulted in a significant increase in iron removal of 75.2% and a minor increase in sulfate removal of 4.9%, compared with the formation of schwertmannite from oxidation solely involving H2O2. For the AMD containing Fe3+, the energy generated by the acid–base reaction in water could also directly promote the formation of IK minerals from 97.2% iron and 6.9% sulfate. XRD and FTIR analyses confirmed the identification of the IHK and IK minerals as IHSMs transitioning from schwertmannite to jarosite. SEM and elemental analyses indicated that the mineral exhibited denser aggregate spheres with the incorporation of KHCO3 in mineralization yet displayed enhanced mineralization abilities for the contaminant ions in AMD. Moreover, despite the SSA of the modified minerals being diminished (2.02, 1.83 and 1.83 m2/g for IH, IHK and IK, respectively), the presence of more sulfate in the mineral enhanced the adsorption capacity of Cr(VI). Furthermore, the water quality results also illustrated that the removal ratios of iron and sulfate in AMD notably increased with the involvement of KHCO3 in mineralization. In conclusion, the KHCO3-induced mineralization of iron-containing (either divalent or trivalent) AMD into IHSMs not only improved the mineralization ratios and contaminant removal ratios for better remediation of AMD but also obtained mineral resources with better adsorption of Cr(VI), thereby fostering the sustainable advancement of the remediation of AMD. Therefore, this innovative strategy employing KHCO3-assisted chemical mineralization to form IHSMs holds ample potential and promises to be an efficacious methodology for the sustainable remediation of iron-rich AMD.

Biopolymer-based fly ash-activated zeolite for the removal of chromium from acid mine drainage

Acid mine drainage (AMD), acid rock drainage, and fly ash from coal burning in power plants both posses widespread environmental and economic problems for many countries. Various biopolymers like chitosan have been used as a low-cost adsorbent for removal of toxic metals from AMD. The present investigation involves the microwave synthesis of low-cost chitosan biopolymer-based fly ash zeolite (CS@FAZ) adsorbent for the removal of chromium (Cr) from the acidic environment. The compositional and surface morphological characterization were conducted using FTIR, XRD, and SEM-EDS analysis. The Synthesized adsorbents were used for the removal of Cr (VI) ion under different experimental conditions, including initial concentration (15 to 60 mg L-1), contact time (0 to 180 minutes), dose (0.25 to 4 g L-1), solution pH (2 to 6). The optimum removal was 67.48% at room temperature 27℃, pH 3, concentration 45 mg L-1 and 0.1 g L-1 dose of CS@FZ adsorbent. The field evaluation parameters such as ionic strength and regeneration capacity show the adsorbent’s potential applicability in the remediation of AMD. The kinetics models and adsorption isotherm were determined to be pseudo-second order (R2 > 0.8952) validated with 27.78 mg g-1maximum adsorption capacity and fitted by Freundlichisotherm (R2 > 0.9264). The present low-cost adsorbent can be used for large scale to eliminate the Cr (VI) from the acid mine drainage.

A review: The formation, prevention, and remediation of acid mine drainage.

In abandoned open-pit coal mines, surface water and groundwater form acidic waters with high concentrations of metal ions due to chemical interactions with ores such as pyrite, and the formation of acid mine drainage (AMD) is one of the major sources of pollution of world concern. For this reason, this paper reviews the formation mechanisms and influencing factors of AMD. It also describes the prediction, prevention, and remediation techniques for AMD, identifying key research gaps. It also discusses the current challenges and shortcomings faced globally in the management of AMD. The formation of AMD is mainly caused by the oxidation of pyrite in mines, but it is mainly influenced by history, climate, topography, and hydrogeology, making the formation mechanism of AMD extremely complex. Currently, the remediation technologies for AMD mainly include active treatment and passive treatment, which can effectively neutralize acidic wastewater. However, the prediction technology for AMD is blank, and the source treatment technology such as passivation and microencapsulation only stays in the experimental stage. This leads to the high cost of treatment technologies at this stage and the inability to identify potential risks in mines. Overall, this review provides remediation tools for AMD from predicting root causes to treatment. Geophysical technology is an effective method for predicting the motion path and pollution surface of AMD in the future, and resource recovery for AMD is a key point that must be paid attention to in the future. Finally, integrated treatment technologies that deserve further exploration need to be emphasized.

Simultaneous removal of multiple metal(loid)s and neutralization of acid mine drainage using 3D-printed bauxite-containing geopolymers

The mining industry is one of the largest sources of environmental concern globally. Herein we report for the first time the application of highly porous 3D-printed sorbents containing high amounts (50 wt%) of red mud, a hazardous waste derived from the alumina industry, for the remediation of acid mine drainage (AMD). The sorption capacity of the inorganic polymers was initially evaluated for the simultaneous removal of five metal(loid) elements, namely Cu(II), Ni(II), Zn(II), Cd(II) and As(V) in synthetic wastewater. The effect of the initial concentration, pH and contact time were assessed, reaching removal efficiencies between 64% and 98%, at pH 4 and initial concentration of 50 mg L−1 of each cation, after 24 h of contact time. The 3D-printed lattices were then used for the remediation of the real AMD water samples, and the role of adsorption and acidic neutralization was investigated. Lattices were also successfully regenerated and reused up to five cycles without compromising their performance. This work paves the way for the use of an industrial waste derived from the production of alumina as raw material for the management of the hazardous AMD.

Hydrogeochemical evolution of groundwater impacted by acid mine drainage (AMD) from polymetallic mining areas (South China)

Mining activities have long-term impacts on the groundwater of surrounding areas and deserve in-depth analysis and study. Herein, the geochemical mechanisms of acid mine drainage (AMD)-affected groundwaters were examined, and groundwater quality was assessed through water quality indices. 15 water samples from 7 domestic and 4 groundwater monitoring wells were tested for physical and chemical parameters in 2022, and multivariate statistical analysis was carried out with monitoring data from 21 domestic wells in 2010. The groundwater chemical composition varied from a predominantly Ca-HCO3 type in 2010 to a Ca-SO4 type in 2022. The isotopic values of δ18O and δD indicate that groundwater has not been significantly affected by evaporation. Changes in groundwater sulfate and total dissolved solids (TDS) levels over the twelve-year period confirmed the AMD infiltration impact on groundwater quality. The groundwater chemical properties changed more slowly than those of surface waters affected by AMD based on a cumulative increase in sulfate concentration of 29.94 mg/L. Changes in groundwater quality were investigated, namely, the spatiotemporal distribution of potentially toxic elements (PTEs), including Fe, Mn, Cd, Pb, and As. Mn concentrations in upstream groundwater areas near the mine decreased by 61.8% between 2010 and 2022. Conversely, groundwater in midstream areas had Mn concentrations of 2.25 mg/L and arsenic concentrations of 11.8 μg/L, both exceeding the WHO, 2022 standard. According to multivariate statistical analysis, Mn, Cd, and Pb originated from polymetallic minerals, whereas As was likely derived from the reduction of Fe/Mn hydroxyl oxides. AMD remediation improved contaminated upstream groundwater quality over 12 years, with a 36.8% improvement in WQI values. PTE distribution determined water quality changes; therefore, PTE contamination should be treated in mid- and downstream regions while contaminated groundwater should be treated upstream.

Global trends and future prospects of acid mine drainage research.

The uncontrolled release of acid mine drainage (AMD) results in the ongoing deterioration of groundwater and surface water, along with harmful impacts on aquatic ecosystems and surrounding habitats. This study employed a bibliometric analysis to examine research activities and trends related to AMD from 1991 to 2021. The analysis demonstrated a consistent growth in AMD research over the years, with a notable surge in the number of publications starting from 2014. Applied Geochemistry and Science of the Total Environment emerged as the top two extensively published journals in the field of AMD research. The USA held a prominent position, achieving the highest h-index (96) and central value (0.36) among 111 countries/territories, with China and Spain following closely behind. The author keyword analysis provides an overview of the main focuses in AMD research. Furthermore, the co-citation reference analysis reveals four primary domains of AMD research. Moreover, the prevention and remediation of AMD, including source prevention and migration control, as well as the hazards posed by heavy metals/metalloids and the mechanisms and techniques employed for their removal, are discussed in detail.

Remediation of acid mine drainage and immobilization of rare earth elements: Comparison between natural and residual alkaline materials

Acid mine drainage (AMD) is a well-known source of toxic trace metals in freshwaters. Traditional passive treatment systems rely on AMD neutralization with limestone and removal of most common toxic transition metals such as Cu and Zn with little attention to rare earth elements (REE). Alkaline waste materials now receive increasing attention as low cost AMD treatment alternatives in the circular economy. This study was set up to identify the efficiency of alkaline waste materials remediating AMD and scavenging REE in addition to other toxic trace elements. An AMD sample was collected from a lixiviate coming from pyrite heaps in the Iberian Pyrite Belt (pH = 1.8, 30 μM ∑REY). The sample was treated with either blast furnace slag (BFS) generated during smelting of iron ore in a blast furnace or biomass ashes (BA) derived from combustion of biomass, thereby using analytical grade CaCO3, and NaOH as reference products. The batch alkalinization experiments were conducted by adding each alkaline material at an amount to obtain an equal pH to ≈6.5. The required amounts of the products were NaOH < CaCO3<BFS < BA in line with their acid neutralizing capacities. The largest removal of sulfate from water was obtained in the CaCO3 treatment suggesting gypsum precipitation which was lower with BA and BFS and virtually absent with NaOH, these trends were confirmed by SEM-EDX and XRD. Both BFS and BA removed more Fe than CaCO3 and NaOH. The REE elements were well removed by all treatments (>99%) and the remaining REE concentrations in the solutions were clearly lower than values for Cu and Zn. The Zn and Cu removals were not consistently high enough (except with NaOH) to meet environmental limits in the discharge waters. The largest efficiency for REE removals was obtained with CaCO3. Indirect evidence here suggests that gypsum is a better host for the trivalent REE than Fe(III) minerals in the precipitates. The ionic radii of trivalent REE are more similar to Ca2+ than to Fe3+, explaining the better potential of gypsum as REE host. This study showed also the potential of BFS as alkaline agent for the remediation of AMD in terms of its higher alkalinity generation potential as compared to BA, thus making BA less promising than BFS.

Sustainable use of wastes as reactive material in permeable reactive barrier for remediation of acid mine drainage: Batch and continuous studies

The aim of this work was to evaluate the feasibility of the use of different industrial and agricultural wastes as reactive materials in Permeable Reactive Barriers (PRB) for Acid Mine Drainage (AMD) remediation. Sugar foam (SF), paper mill sludge (PMS), drinking water sludge (DWS) and olive mill waste (OMW) were evaluated in terms of pH neutralization and metal removal from AMD. Laboratory batch tests and continuous pilot scale up-flow columns containing 82% of Volcanic Slag (VS), as porous fill material, and 18% w/w of one of the industrial and agricultural wastes previously indicated, were tested.From the batch tests it was observed that the reactive material presenting the best results were the SF and the PMS. The results obtained in all the PRB were accurately described by a pseudo-first order model, presenting coefficient of determination higher than 0.96 in all the cases.During the continuous operation of the PRB, the porosity and hydraulic retention time (HRT) of most of the up-flow columns strongly decreased due to chemical precipitation and biofilm growth. The SF presented a significant number of fine particles that were washed out by the liquid flow, generating an effluent with very high total suspended solid concentration. Despite SF was the material with the highest alkalinity potential, the reduction of the HRT limited its neutralization and metal removal capacity. PMS and DWS presented the best pollutant removal yields in the continuous operation of the PRB, ranging from 55 to 99% and 55–95% (except in the case of the Mn), respectively. These results allowed the metal removal from the AMD. Additionally, these wastes presented very good biological sulphate reduction.Based on these results, the use of PMS and DWS as reactive material in PRB would allow to simultaneously valorise the industrial waste, which is very interesting within the circular economy framework, and to remove metals from the AMD by means of a low-cost and environmentally sustainable procedure.

Optical In Situ Monitoring of Acid Mine Drainage Remediation: Laboratory and Model Investigation

AbstractA permeable reactive barrier (PRB) can be deployed to remediate acid mine drainage. The performance of a PRB material under different boundary conditions (pH, flow velocity, and sulfate concentration) was investigated in a series of column experiments applying in‐situ optical sensing methods for pH and oxygen detection. The reactive material consisted of organic components (compost, wood, and coconut shell) mixed with calcium carbonate and fine gravel. The input concentrations were around 1000 mg/L for iron and 3000 mg/L for sulfate, and the pH value was 6.2. The remediation efficiency of iron was 14.6% and of sulfate 15.2%, but was expected to scale up when moving to a field‐site PRB with greater thickness. The iron and sulfate removal was influenced by decreasing the flow velocity and increasing the sulfate input concentration and the pH value. In an experiment with low pH boundary conditions (pH = 2.2), acidity was neutralized in the PRB by calcium carbonate during an experiment duration of 47 days. The modeling program MIN3P was used to create a simulation of the laboratory experiments. This helps to design parameters, for example, the residence time in the PRB, which is necessary for close to 100% remediation efficiency. This study shows the application of optical oxygen and pH monitoring in PRBs. In this context, they can be used to monitor the stability of a PRB for the remediation of acid mine drainage (AMD).

Remediation of Acid Mine Drainage in the Haizhou Open-Pit Mine through Coal-Gangue-Loaded SRB Experiments

To address the pollution problem of acid mine drainage (AMD) characterized by high concentrations of Fe2+, Mn2+, and SO42−, a combination of coal gangue (CG) and sulfate-reducing bacteria (SRB) was employed. The effects of coal-gangue dosage, SRB inoculation concentration, and temperature on AMD treatment with coal-gangue-loaded SRB were determined through single-factor experiments and response surface methodology (RSM) experiments. By considering the principles of adsorption isotherms, adsorption kinetics, and reduction kinetics, the removal mechanisms of SO42−, Fe2+, and Mn2+ in AMD using coal gangue-loaded SRB in the the Haizhou open-pit mine was revealed. The results showed that the overall effectiveness of the four types of coal-gangue-loaded SRB in repairing AMD was as follows: 3# CG-loaded SRB &gt; 2# CG-loaded SRB &gt; 1# CG-loaded SRB &gt; 4# CG-loaded SRB, with coal-gangue-loaded SRB in the the Haizhou open-pit mine showing the best performance. According to the RSM test, the optimum conditions for repairing AMD with coal-gangue-loaded SRB in the open-pit mine were a coal-gangue dosage of 52 g, SRB inoculation concentration of 11.7%, and temperature of 33.4 °C. The order of factors affecting the removal of SO42− and Fe2+ from AMD by SRB loaded on coal gangue was SRB inoculation concentration &gt; temperature &gt; coal-gangue dosage. For Mn2+, the order of influence was temperature &gt; SRB inoculation concentration &gt; coal-gangue dosage. In the process of repairing Fe2+ with coal-gangue-loaded SRB in the the Haizhou open-pit mine, the biological activity metabolism of SRB played a leading role, while the adsorption isotherm of Mn2+ followed the Freundlich model. The adsorption kinetics of coal-gangue-loaded SRB in the the Haizhou open-pit mine for Fe2+ and Mn2+ in AMD conformed to Lagergren’s second-order kinetic model, while the reduction kinetics of SO42− conformed to a first-order reaction model.

Sulfide Immobilization: Investigating the Impact of a Protective Sulfide-Bearing Layer Formed by Siderite (FeCO3)

Sulfate-rich wastewater poses ecological hazards to freshwater ecosystems, and sulfate is highly regulated in many Minnesota lakes. Biological sulfate reduction results in the reduction of sulfate to sulfide, and this process is used to remediate acid mine drainage. Theoretically, the aqueous sulfide can be immobilized into a solid-phase material and removed from the aqueous system. This study focuses on sulfide immobilization using iron-bearing waste minerals. Specifically, the extent of reaction of siderite (FeCO3), an abundant ferrous mineral in some mining wastes, with sulfide was studied. Mildly acidic batch reactors containing powdered siderite were consecutively injected with a sodium sulfide solution. Solid reaction products were identified and characterized using powder X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Mackinawite (FeS) appeared to be the most abundant product, with greigite (Fe3S4) also detected. Results reveal that the immobilization capacity of sulfide by siderite is limited by the concentration of the Fe2+(aq) presented in the system immediately before the initial sulfide exposure as the Fe2+(aq) levels are not replenished after sulfidation. These results improve our understanding of the sulfidation of siderite and provide insight to improve the viability of using siderite-containing mining waste rock in a sulfate remediation technology.

Evaluation of Hydrotalcites Produced in Seawater for Remediation of Metalliferous Acid Mine Drainage and Submarine Placement

Uncontrolled release of acid mine drainage (AMD) causes widespread detrimental impacts on the receiving environment. Thus, effective treatment to neutralise AMD effluent pH and capture a suite of metals is required. In-situ hydrotalcite (HTC) precipitation is an emerging technology for AMD remediation. HTC has an inherent capacity to accommodate a range of cations and anions during in situ formation, offering a method of broad-spectrum contaminant removal. This study explored the feasibility of using seawater as an Mg source and synthetic AMD in HTC formation. The HTC was formed from a stoichiometric combination of synthetic AMD and seawater. While three initial stoichiometric M2+:M3+ ratios of 2:1, 3:1, and 4:1 were investigated, only HTC with an M2+:M3+ ratio of 2:1 was generated, as confirmed by both mineralogical and geochemical analyses. Importantly, the HTC was demonstrated to effectively remove a suite of metals present in AMD such as Cu, Zn, Al, and Mn with removal rates of between 99.97 to 99.99%. The HTC precipitate contained ≈6.6% Cu and 4.1% Zn, and thus shows the potential, if required, for future metal recovery. Since submarine placement is often used in metal mining and processing operations proximal to the coast, the stability of the HTC precipitate in seawater was also investigated. Importantly, only 0.2% of the Cu and 1.1% of the Zn within the HTC were subsequently leaching in decreasing increments into seawater over 30 days with decreasing increments after the initial seven days. This indicates robust element retention and confirms the potential of HTC for AMD remediation with direct submarine placement.

Efficient remediation of acid mine drainage through sustainable and economical biochar-CaO composite derived from solid waste

Efficient remediation of acid mine drainage through sustainable and economical biochar-CaO composite derived from solid waste

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+, SO42−, 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.

Revealing the role of calcium alginate-biochar composite for simultaneous removing SO42− and Fe3+ in AMD: Adsorption mechanisms and application effects

The remediation of acid mine drainage (AMD) is particularly challenging because it contains a large amount of Fe3+ and a high concentration of SO42−. To reduce the pollution caused by SO42− and Fe3+ in AMD and realize the recycling of solid waste, this study used distillers grains as raw materials to prepare biochar at different pyrolysis temperatures. Calcium alginate-biochar composite (CA-MB) was further synthesized via the entrapment method and used to simultaneously remove SO42− and Fe3+ from AMD. The effects of different influencing factors on the sorption process of SO42− and Fe3+ were studied through batch adsorption experiments. The adsorption behaviors and mechanisms of SO42− and Fe3+ were investigated with different adsorption models and characterizations. The results showed that the adsorption process of CA-MDB600 on SO42− and Fe3+ could be well described by Elovich and Langmuir-Freundlich models. It was further proved by the site energy analysis that the adsorption mechanisms of SO42− onto CA-MDB600 were mainly surface precipitation and electrostatic attraction, while that of Fe3+ removal was attributed to ion exchange, precipitation, and complexation. The applications of CA-MDB600 in actual AMD proved its good application potential. This study indicates that CA-MDB600 could be applied as a promising eco-friendly adsorbent for the remediation of AMD.

Metal Pollution and Mining in the Iberian Pyrite Belt: New Remediation Technologies to Improve the Ecosystem Services of the River Basins

The highly metal-contaminated Odiel-Tinto River basin, located in the Iberian Pyrite Belt (IPB), has been the focus of many environmental studies as a natural lab for biodiversity and environmentally catastrophic scenarios and as a reference site for mining places with similar conditions. This study demonstrates the feasibility and effectiveness of two different technologies to recover ecosystems affected by acid mine drainage (AMD) in the area of IPB. The current study compiles results of two newest technologies for AMD remediation: passive remediation (dispersed alkaline substrate—DAS) and a new disruptive technology (Adiabatic Sonic Evaporation and Crystallization—ASE&amp;C) that purifies the contaminated water, obtaining two by-products (high-quality water and metal conglomerates) that improve the general quality of the ecosystem including biodiversity by eliminating more than 90% of the contaminants from AMD and mining waters. The removal of contaminants, enhancement of AMD treatment efficiency, and offset operating costs were compared and analyzed for the different uses of the decontaminated effluents, including an old tailing pond failure, the Aznalcóllar mining spill. The efficiency of the removal of elements from the contaminated water is significant using both technologies, although the passive DAS does not still reach the international benchmark for some compounds (such as Fe, sulfates, and Mn); whereas ASE&amp;C obtains distilled water fulfilling all the international benchmarks with conductivity values lower than 120 µS cm−1 or metal concentrations lower than µg/L. Both technologies are eco-friendly and cost-effective as a result of the generation of valuable by-products such as fresh water and metal conglomerates as potentially commercial products while remediating aquatic ecosystems impacted by mining activities.