Acid mine drainage formation and arsenic mobility under strongly acidic conditions: Importance of soluble phases, iron oxyhydroxides/oxides and nature of oxidation layer on pyrite

Removal Efficiency of Heavy Metals Such as Lead and Cadmium by Different Substrates in Constructed Wetlands

In order to find an efficient and economical wetland substrate to treat mine wastewater containing various heavy metals, and effectively realize the resource utilization of water treatment residuals, in this paper, the treatment efficiency of mine wastewater containing various heavy metals was investigated using unburned ceramsite prepared from water treatment residuals (UCWTR) and clay ceramsite. The continuous dynamic test results showed that the removal rate of Pb, Cd, Cu, Zn, and Fe can reach more than 98.5% after the UCWTR-based CWs runs for 56 days, and its concentration was 30.05%, 24.85%, 20.82%, 14.63%, and 7.91% lower than that of the clay ceramsite-based CWs, respectively. SEM, XPS, and FT-IR showed that the characteristic peaks of two ceramsites were basically similar. The ceramsite undergoes ion exchange, coordination complexation, and chelation reaction with Pb, Cd, Cu, Zn, and Fe under the action of the gel of internal groups -OH, C=O, Al-OH, Si-Fe-O and C-S-H. Compared with clay ceramsite, the ion exchange reaction and chelation reaction of -OH effect and the coordination reaction of C=O effect of carboxyl group in UCWTR were enhanced. In conclusion, using UCWTR as a CWs substrate can effectively enhance the adsorption capacity of heavy metals, providing a scientific basis for the application of UCWTR-based CWs in mine wastewater treatment.

Sulfide-induced repartition of chromium associated with schwertmannite in acid mine drainage: Impacts and mechanisms

The cycling of Fe/S is often related to the formation and fate of schwertmannite which is particularly suitable as a scavenger for heavy metals and metalloids in acid mine drainage (AMD). However, the interactions between reactivity of S(-II) and schwertmannite with structurally incorporated Cr(VI) remain elusive. This work evaluated dissolution experiments in combination with XRD, SEM, FTIR, TEM, and XPS characterization to provide detailed information regarding the transformation of schwertmannite induced by S(-II) following changes in pH, Cr loading, and S(-II) concentration. Our results found that the presence of sulfide significantly decreased the stabilization of schwertmannite under acidic conditions. The reactivity of the three schwertmannite samples depended on the contents of Cr(VI) that were structurally incorporated and followed the order Sch > 0.13Cr-Sch > 0.17Cr-Sch. High S(-II) concentrations and low Cr doping favored the release of Fe and SO₄^2- from schwertmannite. Attenuation of Cr mobility occurred via elevating the S(-II) concentrations and pH values resulting in Cr concentrations spanning ∼1.39 to ∼0.10 mg L^-1 and ∼1.58 to ∼0.12 mg L^-1 for 0.13Cr-Sch and 0.17Cr-Sch, respectively. Combining the results of characterization, goethite was the dominant end product constituted secondary phase together with sulfide minerals (FeS, FeS₂), iron oxides (Fe₃O₄, Fe₂O₃), and CrFe minerals on the bulk mineral surface. The substituted Cr significantly inhibited the reductive transformation of schwertmannite by sulfide and led to the formation of lepidocrocite. Thus, we concluded that a three-stage transformation mechanistic pathway governed partial conversion of schwertmannite to goethite. This finding provides new understanding of the biogeochemical processes of iron minerals affected by reducing substances that control the transport and fate of immobilizing contaminants in an AMD-polluted area.

Effective immobilization of geogenic As and Pb in excavated marine sedimentary material by magnesia under wet-dry cycle, freeze-thaw cycle, and anaerobic exposure scenarios

Massive amounts of marine sedimentary materials with geogenic heavy metal(loids) are excavated by the subsurface construction projects and then exposed to weathering conditions, which pose potential threats to the environment. In the present study, 2 % magnesia (MgO) was applied to immobilize geogenic arsenic (As) and lead (Pb) in excavated marine sedimentary material. To better evaluate the immobilization efficiency under different environmental scenarios, the untreated and amended solids were subjected to wet-dry cycles, freeze-thaw cycles, and anaerobic incubation until 49 days. The leaching behaviors of As and Pb were investigated and their size fractionations in the leachates were compared. The results indicate that most Pb exists in particulate and agglomerated colloidal fractions (0.1-5 μm) in the leaching suspensions, while most As is found in dissolved forms (<0.1 μm). It is therefore necessary to consider the element type and exposure scenarios during environmental risk evaluation, particularly using the batch test as a routine compliance testing procedure. In the control test without MgO addition, the wet-dry cycle resulted in the "self-induced" immobilization of As and Pb. The pH decreases to the neutral range and the formation of amorphous Fe-(oxyhydr)oxides following pyrite oxidation largely explained the decreased As and Pb leaching. In comparison, the freeze-thaw cycle and anaerobic incubation tended to enhance As and Pb leaching. Overall, MgO addition significantly reduced the leachability of As and Pb and displayed sustained immobilization performance under all studied scenarios. These findings could be largely attributed to solid particle aggregation induced by MgO addition, including the adsorption of As and Pb onto newly formed Fe-(oxyhydr)oxides and/or MgSi precipitates. This study offers a simple and effective strategy for the sustainable management of excavated marine sedimentary materials contaminated by geogenic As and Pb.

Utilization of Lead Slag as In Situ Iron Source for Arsenic Removal by Forming Iron Arsenate

In situ treatment of acidic arsenic-containing wastewater from the non-ferrous metal smelting industry has been a great challenge for cleaner production in smelters. Scorodite and iron arsenate have been proved to be good arsenic-fixing minerals; thus, we used lead slag as an iron source to remove arsenic from wastewater by forming iron arsenate and scorodite. As the main contaminant in wastewater, As(III) was oxidized to As(V) by H₂O₂, which was further mineralized to low-crystalline iron arsenate by Fe(III) and Fe(II) released by lead slag (in situ generated). The calcium ions released from the dissolved lead slag combined with sulfate to form well-crystallized gypsum, which co-precipitated with iron arsenate and provided attachment sites for iron arsenate. In addition, a silicate colloid was generated from dissolved silicate minerals wrapped around the As-bearing precipitate particles, which reduced the arsenic-leaching toxicity. A 99.95% removal efficiency of arsenic with initial concentration of 6500 mg/L was reached when the solid-liquid ratio was 1:10 and after 12 h of reaction at room temperature. Moreover, the leaching toxicity of As-bearing precipitate was 3.36 mg/L (As) and 2.93 mg/L (Pb), lower than the leaching threshold (5 mg/L). This work can promote the joint treatment of slag and wastewater in smelters, which is conducive to the long-term development of resource utilization and clean production.

A new organosilane passivation agent prepared at ambient temperatures to inhibit pyrite oxidation for acid mine drainage control

Acid mine drainage (AMD) is a significant environmental problem caused by the oxidation of pyrite and other metal sulfide ores. Organosilane passivation is an effective strategy to inhibit pyrite oxidation. However, synthetic organic silane passivation agents generally require temperatures of 50-80 °C, resulting in high energy consumption and high synthesis cost. In this study, a 3-aminopropyltrimethoxysilane -methyltrimethoxysilane (APS-MTMS) coatings was successfully prepared at ambient temperatures of 15-40 °C as a passivation agent to inhibit pyrite oxidation. Chemical leaching tests were used to study the inhibition performance of APS-MTMS for pyrite oxidation. The experimental results showed that the release of the total Fe from APS-MTMS-coated pyrite was 11.31 mg/L after chemical oxidation for 7 hours, and the passivation rate can reach 77.78%. The contact angle of the APS-MTMS-coated pyrite was significantly larger (140.4°) than that of the bare pyrite (58.8°), indicating that APS-MTMS prompted the formation of a superhydrophobic surface of pyrite, improving the oxidation resistance. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were applied to probe the interaction mechanism of APS-MTMS with pyrite. The results indicated that APS accelerated the Si-O-Si formation by amino protonation and enriched a crosslinked network of Si-O-Si and Fe-O-Si on the pyrite surface to prevent pyrite oxidation. This study provides a novel method for preparing organosilane passivation materials at ambient temperatures for AMD control.

Geochemical audit of a historical tailings storage facility in Japan: Acid mine drainage formation, zinc migration and mitigation strategies

Historical tailings storage facilities (TSFs) are either abandoned or sparsely rehabilitated promoting acid mine drainage (AMD) formation and heavy metal release. To sustainably manage these sites, a geochemical audit coupled with numerical simulation to predict AMD flow paths and heavy metal migration are valuable. In this study, a 40-year-old TSF in Hokkaido, Japan was investigated. Tailings in this historical TSF contain pyrite (FeS₂) while its copper (Cu) and zinc (Zn) contents were 1400-6440 mg/kg and 2800-22,300 mg/kg, respectively. Copper and Zn were also easily released in leaching tests because they are partitioned with the exchangeable phase (29% of Zn; 15% of Cu) and oxidizable fraction (25% of Zn; 33% of Cu). Kinetic modeling results attributed AMD formation to the interactions of pyrite and soluble phases in the tailings with oxygenated groundwater, which is supported by the sequential extraction and leaching results. Calibrations of groundwater/AMD flow and solute transport in the 2D reactive transport model were successfully done using hydraulic heads measured on-site and leaching results, respectively. The model forecasted the quality of AMD to deteriorate with time and AMD formation to continue for 1000 years. It also predicted ~24% AMD flux reduction, including lower Zn release with time when recharge reduction interventions are implemented on-site.

Redox Behavior of Secondary Solid Iron Species and the Corresponding Effects on Hydroxyl Radical Generation during the Pyrite Oxidation Process

During the pyrite oxidation process, aqueous ferrous/ferric ions (Fe²⁺/Fe³⁺), as well as surface-adsorbed Fe²⁺/Fe³⁺, have been widely recognized to dominate hydroxyl radical (^•OH) generation, while this study reveals that the secondary solid iron species also play non-negligible roles. Based on the different forms and the presence of sites, the secondary solid iron species were classified as Fe_coat (iron-containing coating on the pyrite surface) and Fe_dep (ex situ-deposited iron (oxyhydr)oxide that is not in contact with pyrite). Instead of participating in building a stubborn passivation layer on the pyrite surface, Fe_coat is easy to fall off from the pyrite surface as the oxidation of pyrite deepens, while large fractions of Fe_dep and Fe_coat are found to be extractable with nitrilotriacetic acid (NTA). Achieved by cyclically oxidizing pyrite within different NTA levels (0/0.1/10 mM), Fe_coat and Fe_dep were proved to have distinct redox behavior during the pyrite oxidation process. Amorphous Fe_dep, originated from the hydrolyzation of dissolved Fe³⁺, accelerates the nonradical decay of hydrogen peroxide (H₂O₂); as a result, the accumulation of Fe_dep always decreases the ^•OH production during the pyrite oxidation process. However, part of Fe_dep adsorbs on the pyrite surface through electrostatic attraction and converts into Fe_coat. The electron conduction between Fe_coat and pyrite was verified, which accelerates the oxidative dissolution of pyrite, produces reactive Fe(II), and therefore favors ^•OH generation. This study improves our understanding of the redox behavior of pyrite in complex media such as natural processes and practical engineering systems.

Photooxidation of Fe(II) to schwertmannite promotes As(III) oxidation and immobilization on pyrite under acidic conditions

Pollution of arsenic (As) in acid mine drainage (AMD) is a universal environmental problem. The weathering of pyrite (FeS₂) and other sulfide minerals leads to the generation of AMD and accelerates the leaching of As from sulfide minerals. Pyrite can undergo adsorption and redox reactions with As, affecting the existing form and biotoxicity of As. However, the interaction process between As and pyrite in AMD under sunlight radiation remains unclear. Here, we found that the oxidation and immobilization of arsenite (As(III)) on pyrite can be obviously promoted by the reactive oxygen species (ROS) in sunlit AMD, particularly by OH. The reactions between hole-electron pairs and water/oxygen adsorbed on excited pyrite resulted in the production of H₂O₂, OH and O₂^-, and OH was also generated through the photo-Fenton reaction of Fe²⁺/FeOH²⁺. Weakly crystalline schwertmannite formed from the oxidation of Fe²⁺ ions by OH contributed much to the adsorption and immobilization of As. In the mixed system of pyrite (0.75 g L^-1), Fe²⁺ (56.08 mg L^-1) and As(III) (1.0 mg L^-1) at initial pH 3.0, the decrease ratio of dissolved total As concentration was 1.6% under dark conditions, while it significantly increased to 69.0% under sunlight radiation. The existence of oxygen or increase in initial pH from 2.0 to 4.0 accelerated As(III) oxidation and immobilization due to the oxidation of more Fe²⁺ and production of more ROS. The present work shows that sunlight significantly affects the transformation and migration of As in AMD, and provides new insights into the environmental behaviors of As.

Reduction of acid mine drainage by passivation of pyrite surfaces: A review

Acid mine drainage (AMD), a source of considerable environmental pollution worldwide, has prompted the development of many strategies to alleviate its effects. Unfortunately, the methods available for remedial treatment of AMD and the damage it cause are generally costly, labor-intensive, and time-consuming. Furthermore, such treatments may result in secondary pollution. Alternatively, treating the AMD problem at its source through pyrite surface passivation has become an important topic for research because it has the potential to reduce or prevent the generation of AMD and associated pollution. This review summarizes various pyrite anti-corrosion technologies, including the formation of various passivating coatings (inorganic, organic and organosilane) and carrier-microencapsulation. Several effective long-term passivators are identified, although many of them currently have important deficiencies that limit their practical application. Combining the mechanisms of existing passivation agents or new artificial materials, while considering environmental conditions, costs, and long-term passivation performance, is a feasible direction for future research.

Removal of arsenic in acidic wastewater using Lead-Zinc smelting slag: From waste solid to As-stabilized mineral

High-arsenic wastewater has long been considered a major threat to ecological balance and human health because of its strong toxicity and high mobility. Herein, an environmentally friendly process was proposed for As removal and fixation in the form of As-stabilized mineral, using Lead-Zinc smelting (LZS) slag as the in situ Fe donor, neutralizer, and crystal seed. The slag was dissolved in the wastewater and released Fe and Ca ions, while simultaneously increasing the pH value of the solution to help scorodite synthesis. The dissolved Ca²⁺ ion preferentially reacted with SO₄^2- ion in the form of CaSO₄·2H₂O precipitate as in situ "seeds" for As precipitation. The dissolved Fe(II) and As(III) ions were oxidized to Fe(III) and As(V) ions by H₂O₂, and later reacted with each other to generated amorphous ferric arsenate on the surface of CaSO₄·2H₂O, and then evolved into scorodite crystals with high stability. With a Fe/As molar ratio of 2, a reaction temperature of 90 °C, and a reaction time of 12 h, 98.42% of As was effectively precipitated from the wastewater with an initial As concentration of 7530.00 mg/L. Moreover, the leached As concentration of the As-bearing precipitate in the TCLP test was 3.46 mg/L. The concentration of the residual As and heavy metals ions in the final filtrate was lower than local wastewater discharge standards, successfully realizing the treatment of smelting wastewater. In summary, a prospective process successfully shows a great potential for co-treatment of LZS wastewater and slag, which could advance the large-scale disposal of LZS plants.

Cu and As(V) Adsorption and Desorption on/from Different Soils and Bio-Adsorbents

This research is concerned with the adsorption and desorption of Cu and As(V) on/from different soils and by-products. Both contaminants may reach soils by the spreading of manure/slurries, wastewater, sewage sludge, or pesticides, and also due to pollution caused by mining and industrial activities. Different crop soils were sampled in A Limia (AL) and Sarria (S) (Galicia, NW Spain). Three low-cost by-products were selected to evaluate their bio-adsorbent potential: pine bark, oak ash, and mussel shell. The adsorption/desorption studies were carried out by means of batch-type experiments, adding increasing and individual concentrations of Cu and As(V). The fit of the adsorption data to the Langmuir, Freundlich, and Temkin models was assessed, with good results in some cases, but with high estimation errors in others. Cu retention was higher in soils with high organic matter and/or pH, reaching almost 100%, while the desorption was less than 15%. The As(V) adsorption percentage clearly decreased for higher As doses, especially in S soils, from 60−100% to 10−40%. The As(V) desorption was closely related to soil acidity, being higher for soils with higher pH values (S soils), in which up to 66% of the As(V) previously adsorbed can be desorbed. The three by-products showed high Cu adsorption, especially oak ash, which adsorbed all the Cu added in a rather irreversible manner. Oak ash also adsorbed a high amount of As(V) (>80%) in a rather non-reversible way, while mussel shell adsorbed between 7 and 33% of the added As(V), and pine bark adsorbed less than 12%, with both by-products reaching 35% desorption. Based on the adsorption and desorption data, oak ash performed as an excellent adsorbent for both Cu and As(V), a fact favored by its high pH and the presence of non-crystalline minerals and different oxides and carbonates. Overall, the results of this research can be relevant when designing strategies to prevent Cu and As(V) pollution affecting soils, waterbodies, and plants, and therefore have repercussions on public health and the environment.

Sulfate-accelerated photochemical oxidation of arsenopyrite in acidic systems under oxic conditions: Formation and function of schwertmannite

The weathering of arsenopyrite is closely related to the generation of acid mine drainage (AMD) and arsenic (As) pollution. Solar radiation can accelerate arsenopyrite oxidation, but little is known about the further effect of SO₄^2- on the photochemical process. Here, the photooxidation of arsenopyrite was investigated in the presence of SO₄^2- in simulated AMD environments, and the effects of SO₄^2- concentration, pH and dissolved oxygen on arsenopyrite oxidation were studied as well. SO₄^2- could accelerate the photooxidation of arsenopyrite and As(III) through complexation between nascent schwertmannite and As(III). Fe(II) released from arsenopyrite was oxidized to form schwertmannite in the presence of SO₄^2-, and the photooxidation of arsenopyrite occurred through the ligand-to-metal charge-transfer process in schwertmannite-As(III) complex along with the formation of reactive oxygen species in the presence of O₂. The photooxidation rate of arsenopyrite first rose and then fell with increasing SO₄^2- concentration. In the pH range of 2.0-4.0, the photooxidation rate of arsenopyrite progressively increased in the presence of SO₄^2-. This study reveals how SO₄^2- promotes the photooxidation of arsenopyrite and As release in the AMD environment, and improves the understanding of the transformation and migration of As in mining areas.

Highly efficient poly(6-acryloylamino-N-hydroxyhexanamide) resin for adsorption of heavy metal ions

Heavy metal wastewater pollution has become an ecological challenge worldwide. This study reports the development of a novel poly (6-acryloylamino-N-hydroxyhexanamide) (PAHHA) resin for effective adsorption of heavy metal ions, including Cu²⁺, Pb²⁺ and Ni²⁺. The chelating resin was synthesized by the grafting reaction between 6-amino-N-hydroxyhexanamide and polyacrylic resin, thus containing the hydroxamate and acylamino groups. The batch adsorption experiments revealed that the PAHHA resin exhibited an excellent adsorption performance for Cu²⁺, Pb²⁺ and Ni²⁺. The maximum adsorption capacities of Cu²⁺, Pb²⁺ and Ni²⁺ were determined to be 238.59, 232.48 and 115.77 mg·g^-1, respectively. Based on the adsorption kinetics, the pseudo-second-order kinetic model was noted to fit well for all metal ions. The metal ion concentration as a function of the equilibrium adsorption capacity fitted well with the Langmuir isotherm, thus indicating the single layer adsorption process. The adsorption mechanism was investigated by using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS) and adsorption isotherms. It was revealed that the PAHHA resin possessed multiple active sites, including -CONHOH, -CONH- and -COOH, which could strongly adsorb the metal ions. Specifically, the -CONHOH group displayed a high affinity by forming a stable five-membered ring with heavy metal ions. Overall, the developed resin exhibits advantages such as simple synthesis, inexpensive raw material and good recyclability, along with high adsorption ability, thus providing a new approach for efficiently treating wastewater contaminated with heavy metal ions.

Geo-Accumulation Index of Manganese in Soils Due to Flooding in Boac and Mogpog Rivers, Marinduque, Philippines with Mining Disaster Exposure

This paper presents the effects of flooding on the accumulation of manganese (Mn) in soils within proximity of the Boac and Mogpog rivers in Marinduque of The Philippines. Marinduque, an island province in the Philippines, experienced two catastrophic tailings storage facility (TSF) failures in the 1990s that released sulfide-rich tailings into the two major rivers. The Philippines experiences 21–23 typhoons every year, 11 of which pass thru Marinduque that causing inundation of floodplain areas in the province. A flood hazard map developed using LiDAR DEM was utilized for the Boac and Mogpog rivers for an accurate representation of flooding events. A portable X-ray fluorescence spectrometer (pXRF) and a Hannah multi-parameter device were used for the on-site analyses of Mn concentration and water physico-chemical properties, respectively. Spatial grid mapping with zonal statistics was employed for a comprehensive analysis of all the data collected and processed. Correlation analysis was carried out on Mn concentrations in soil and surface water, electrical conductivity (EC), total dissolved solids (TDS), pH, temperature, curve number (CN), and flood heights. The curve number indicates the runoff response characteristic of the Mogpog-Boac River basin. The results show that 40% of the total floodplain area of Boac and Mogpog were subjected to high hazards with flood heights above 1.5 m. The Mn content of soils had a statistically significant moderate positive correlation with flood height (r = 0.458) and a moderate negative correlation with pH (r = −0.438). This condition suggested that more extensive flooding promotes Mn contamination of floodplain soils in the two rivers, the source of which includes the mobilization of Mn-bearing silt, sediments, and mine drainage from the abandoned mine pits and TSFs. There is also a strong negative correlation between pH and Mn concentrations in surface water, a relationship attributed to the solubilization of Mn-bearing precipitates based on geochemical modeling results. Using Muller’s geo-accumulation index, 77.5% of the total floodplain of the two rivers was identified as “moderately contaminated” with an average Mn soil content of 3.4% by weight (34,000 mg/kg). The Mn contamination map of floodplain soils in the Mogpog and Boac rivers described in this study could guide relevant regional, national, and local government agencies in planning appropriate intervention, mitigation, remediation, and rehabilitation strategies to limit human exposure to highly contaminated areas.

Trace element adsorption from acid mine drainage and mine residues on nanometric hydroxyapatite

Mining Ag, Cu, Pb, and Zn sulfides by flotation produces great volume of residues, which oxidized through time and release acid solutions. Leachates from tailing heaps are a concern due to the risk of surface water pollution. Hydroxyapatite nanoparticles may remove trace elements from acid leachate collected from an oxidized tailing heap (pH ranged 1.69 ± 0.3 to 2.23 ± 0.16; [SO₄^2-] = 58 ± 0.67 to 60.69 ± 0.39 mmol). Based on the batch experiments under standard conditions, the average removal efficiency was 96%, 92%, 86%, and 67% for Cd, Pb, Zn, and Cu, respectively. The Zn adsorption was modeled by the Freundlich equation, but Cd, Cu, and Pb isotherms do not fit to Freundlich nor Lagmuir equations. Adsorption and other mechanisms occur during trace elements removal by hydroxyapatite. In the polymetallic system, trace elements saturate the specific surface of hydroxyapatite in the following order Zn, Cd, Cu, and Pb. The pH values must be higher than 7.5 to adsorb trace elements. The dose of 3.8% of hydroxyapatite to acid mine drainage removed efficiently > 80% of the soluble Fe, Cu, Mn, Zn, Cd, Ni, and Pb: 4020.0, 37.3, 34.8, 432.0, 4.4, 0.7, and 0.11 mg L^-1 from leachate A and 3357.1, 46.6, 27.8, 569.0, 4.7, 0.6, and 1.7 from leachate B, respectively. The application of 0.7% of hydroxyapatite decreased the extractable Pb in unoxidized tailing heaps from 272 to 100 mg kg^-1. It is likely to use hydroxyapatite to control trace element mobility from mine residues to surrounding soils and surface water.

Vertical-flow constructed wetland based on pyrite intensification: Mixotrophic denitrification performance and mechanism

Deep nitrogen removal from low-carbon wastewater is a pressing water treatment challenge as of yet. Eight sets of vertical-flow constructed wetland (VFCW) intensified by pyrite were designed and applied to treat with low C/N ratio wastewater in this research. The results showed that the addition of pyrite (100% added) significantly promoted TN removal with an efficiency higher than 27.05% under low C/N ratio conditions, indicating that mixotrophic denitrification was achieved in VFCW. Microbial analysis showed that the community structure and diversity of microorganisms were changed significantly, and the growth of autotrophic (Thiobacillus) and heterotrophic bacteria (Thauera) concomitantly enhanced. It is recommended that the addition amount of pyrite is 75% of the wetland volume, meantime, mixing evenly with 25% high porosity substrate (such as activated carbon, volcanic stone, etc.), which could enhance the effective adhesion of microorganisms and their contact area with pyrite, ultimately improve the denitrification capacity of the VFCW.

Correlation Between Fe/S/As Speciation Transformation and Depth Distribution of Acidithiobacillus ferrooxidans and Acidiphilium acidophilum in Simulated Acidic Water Column

It is well known that speciation transformations of As(III) vs. As(V) in acid mine drainage (AMD) are mainly driven by microbially mediated redox reactions of Fe and S. However, these processes are rarely investigated. In this study, columns containing mine water were inoculated with two typical acidophilic Fe/S-oxidizing/reducing bacteria [the chemoautotrophic Acidithiobacillus (At.) ferrooxidans and the heterotrophic Acidiphilium (Aph.) acidophilum], and three typical energy substrates (Fe²⁺, S⁰, and glucose) and two concentrations of As(III) (2.0 and 4.5 mM) were added. The correlation between Fe/S/As speciation transformation and bacterial depth distribution at three different depths, i.e., 15, 55, and 105 cm from the top of the columns, was comparatively investigated. The results show that the cell growth at the top and in the middle of the columns was much more significantly inhibited by the additions of As(III) than at the bottom, where the cell growth was promoted even on days 24–44. At. ferrooxidans dominated over Aph. acidophilum in most samples collected from the three depths, but the elevated proportions of Aph. acidophilum were observed in the top and bottom column samples when 4.5 mM As(III) was added. Fe²⁺ bio-oxidation and Fe³⁺ reduction coupled to As(III) oxidation occurred for all three column depths. At the column top surfaces, jarosites were formed, and the addition of As(III) could lead to the formation of the amorphous FeAsO₄⋅2H₂O. Furthermore, the higher As(III) concentration could inhibit Fe²⁺ bio-oxidation and the formation of FeAsO₄⋅2H₂O and jarosites. S oxidation coupled to Fe³⁺ reduction occurred at the bottom of the columns, with the formations of FeAsO₄⋅2H₂O precipitate and S intermediates. The formed FeAsO₄⋅2H₂O and jarosites at the top and bottom of the columns could adsorb to and coprecipitate with As(III) and As(V), resulting in the transfer of As from solution to solid phases, thus further affecting As speciation transformation. The distribution difference of Fe/S energy substrates could apparently affect Fe/S/As speciation transformation and bacterial depth distribution between the top and bottom of the water columns. These findings are valuable for elucidating As fate and toxicity mediated by microbially driven Fe/S redox in AMD environments.

Acid mine drainage potential of waste rocks in a gold mine (Thailand): application of a weathering cell test and multivariate statistical analysis

In the process of gold mining, large amounts of broken waste rocks are produced and left at the surface under atmospheric conditions, which may generate acid mine drainage (AMD). This study aimed to predict the AMD generation potential and determine the concentrations of potentially toxic metals at three dump sites for a gold mine in Thailand. The AMD generation potentials of waste rock samples collected from the oxide, transition and sulfide dump sites was determined using the weathering cell test. The kinetic test had a 7-d cycle and was run for ~ 21 cycles; the effluent pH, conductivity, redox potential and levels of sulfate, and major and trace metals (i.e., As, Co, Cu, Fe, Mn, Pb and Zn) present in each cycle were measured. Some samples generated significant amounts of AMD, especially the massive sulfide samples from the transition and sulfide dump sites. The effluent water pH in the oxide and sulfide dump sites was neutral to slightly alkaline (pH ~ 6-9), while it was acidic to neutral (pH ~ 3-7) in the transition dump site. The transition dump site samples generated significantly higher acidity and sulfate levels than those from the oxide and sulfide dump sites. Furthermore, some waste rock samples, including the massive sulfide from the transition dump site, released relatively high amounts of heavy metals; in addition, sulfate reached levels (9.48 mg kg^-1 of waste rock) high enough to pose a risk to ecosystems. The long-term acid generation suggested that some waste rock samples from sulfide dump site and transition dump site will continue to generate acid for long periods. Based on data from the weathering cell test and multivariate statistical analysis, the transition dump site potentially generates a lower pH leachate than other waste rock dumps.

Effect of diurnal temperature range on bioleaching of sulfide ore by an artificial microbial consortium

Temperature is considered to be one of the main factors affecting bioleaching, but few studies have assessed the effects of diurnal temperature range (DTR) on the bioleaching process. This study investigates the effects of different bioleaching temperatures (30 and 40 °C) and DTR on the bioleaching of metal sulfide ores by microbial communities. The results showed that DTR had an obvious inhibitory effect on the bioleaching efficiency of the artificial microbial community, although this effect was mainly concentrated in the early and middle stages (0-18 days) of exposure, gradually decreasing until almost disappearing in the late stage (18-24 days). Extracellular polymeric substance (EPS) analysis showed that DTR did not change the composition of the EPS matrix (humic acid-like substances, polysaccharides and protein-like substances), but had a significant effect on the generative behavior of EPS, inhibiting the secretion of EPS during the early and middle stages of the bioleaching process. However, the continual increase in EPS secretion in the bioleaching system gradually reduced the adverse effects of DTR on mineral dissolution. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Scanning electron microscopy- energy dispersive spectrometry (SEM-EDS) analysis of the bioleached residue showed that DTR had no obvious effect on the mineralogical characteristics of sulfide ore. Therefore, in industrial sulfide ore bioleaching applications, in order to accelerate the artificial microbial community start-up process, temperature control measures should be increased in the bioleaching process to reduce the adverse effects of DTR on mineral dissolution.

Co-treatment of acid mine drainage and municipal wastewater effluents: Emphasis on the fate and partitioning of chemical contaminants

The co-management of different wastewater matrices can lead to synergistic effects in terms of pollutants removal. Here, the co-treatment of real municipal wastewater (MWW) and acid mine drainage (AMD) is comprehensively examined. Under the identified optimum co-treatment condition, i.e., 15 min contact time, 1:7 AMD to MWW liquid-to-liquid ratio, and ambient temperature and pH, the metal content of AMD (e.g., Al, Fe, Mn, Zn) was grossly (~95%) reduced along with sulphate (~92%), while MWW's phosphate content was practically removed (≥99%). The PHREEQC geochemical model predicted the formation of (oxy)-hydroxides, (oxy)-hydro-sulphates, metals hydroxides, and other mineral phases in the produced sludge, which were confirmed using state-of-the-art analytical techniques such as FE-SEM-EDS and XRD. The key mechanisms governing pollutants removal include dilution, precipitation, co-precipitation, adsorption, and crystallization. Beneficiation and valorisation of the produced sludge and co-treated effluent could promote resource recovery paradigms in wastewater management. Overall, the co-treatment of AMD and MWW appear to be feasible, yet not practical due to the excessive volume of MWW that is required to attain the desired treatment quality. Future research could focus on chemical addition for the control of the pH and the use of (photo)-Fenton for enhancing treatment efficiency.

Enrichment of psychrophilic and acidophilic sulfate-reducing bacterial consortia - a solution toward acid mine drainage treatment in cold regions

Failure of sulfate-reducing bacteria (SRB)-mediated treatment of acid mine drainage (AMD) in cold regions due to inhibition of bacteria by acidic pH and low temperature can be overcome by enriching psychrophilic and acidophilic microbial consortia from local metal-rich sediments. In this study, we enriched microbial consortia from Arctic mine sediments at varying pH (3-7) and temperatures (15-37 °C) under anaerobic conditions with repeated sub-culturing in three successive stages, and analyzed the microbial community using 16S rRNA gene sequencing. The enriched SRB genera resulted in high sulfate reduction (85-88%), and significant metal removal (49-99.9%) during the initial stages (stage 1 and 2). Subsequently, sub-culturing the inoculum at pH 3-4.5 resulted in lower sulfate reduction (9-34%) due to the inhibition of SRB by accumulated acetic acid (0.3-9 mM). The microbial metabolic interactions for successful sulfate and metal removal involved initial glycerol co-fermentation to acetic acid at acidic pH (by Desulfosporosinus, Desulfotomaculum, Desulfurospora, and fermentative bacteria including Cellulomonas and Anaerovorax), followed by acetic acid oxidation to CO₂ and H₂ (by Desulfitobacterium) at neutral pH, and subsequent H₂ utilization (by Desulfosporosinus). The results, including the structural and functional properties of enriched microbial consortia, can inform the development of effective biological treatment strategies for AMD in cold regions.

Cleanup of arsenic, cadmium, and lead in the soil from a smelting site using N,N-bis(carboxymethyl)-L-glutamic acid combined with ascorbic acid: A lab-scale experiment

Chemical washing has been carried out to remediate soil contaminated with heavy metals. In this study, the appropriate washing conditions for N,N-bis(carboxymethyl)-L-glutamic acid (GLDA) combined with ascorbic acid were determined to remove As, Cd, and Pb in the soil from the smelting site. The mechanism of heavy metal removal by the washing agent was also clarified. The results showed that heavy metals in the soil from the smelting site can be effectively removed. The removal percentages of As, Cd, and Pb in the soil from the smelting site were found to be 34.49%, 63.26%, and 62.93%, respectively, under optimal conditions (GLDA and ascorbic acid concentration ratio of 5:20, pH of 3, washing for 60 min, and the liquid-to-solid ratio of 10). GLDA combined with ascorbic acid efficiently removes As, Cd, and Pb from the soil through synergistic proton obstruction, chelation, and reduction. GLDA can chelate with iron and aluminum oxides while directly chelate with Cd and Pb. Ascorbic acid can reduce both Fe(III) to Fe(II) and As(III) to As⁰. The dissolution of As was promoted by indirectly preempting the binding sites of iron and aluminum in the soil while those of Cd and Pb were improved by directly interrupting the binding sites. This study suggested that GLDA combined with ascorbic acid is an effective cleanup technology to remove As, Cd, and Pb simultaneously from contaminated smelting site soils.

Solar irradiation induced oxidation and adsorption of arsenite on natural pyrite

The migration and bioavailability of toxic elemental arsenic (As) are influenced by the adsorption and redox processes of sulfide minerals in waters around mining areas. Pyrite is the most abundant sulfide mineral in the Earth's crust and exhibits certain photochemical activity. However, the adsorption and redox behaviors of arsenite (As(III)) on pyrite surface under solar irradiation remain unclear. Here, the interaction between As(III) and natural pyrite was investigated under light irradiation. The results indicated that solar irradiation promotes As(III) oxidation and adsorption on pyrite surface due to reactive oxygen species (ROS) intermediates. The reactions between H₂O/O₂ and hole-electron pairs (h_vb⁺-e_cb^-) on excited pyrite and the oxidation of Fe²⁺ released from pyrite by dissolved O₂ contributed much to the generation of OH^•, O₂^•- and H₂O₂ under light irradiation. ROS production and As(III) oxidation were accelerated by dissolved O₂. An increase in pH within 5.0 to 9.0 decreased the concentration of OH^• but increased that of H₂O₂ and the amount of oxidized As(III). In weakly acidic and neutral environments, OH^• was mainly responsible for As(III) oxidation, while H₂O₂ contributed much to As(III) oxidation in weakly alkaline environments. Partial arsenate (As(V)) was adsorbed on pyrite and newly formed ferrihydrite. The present work enriches the understanding of As migration and transformation in the waters around mining areas, and provides a potential method for As(III) removal by using pyrite under solar irradiation.

Synergistic oxidation of dissolved As(III) and arsenopyrite in the presence of oxygen: Formation and function of reactive oxygen species

As an important source of arsenic (As) pollution in mine drainage, arsenopyrite undergoes redox and adsorption reactions with dissolved As, which further affects the fate of As in natural waters. This study investigated the interactions between dissolved As(III) and arsenopyrite and the factors influencing the geochemical behavior of As, including initial As(III) concentration, dissolved oxygen and pH. The hydrogen peroxide (H₂O₂) and hydroxyl radical (OH^•) generated from the interaction between Fe(II) on arsenopyrite surface and oxygen were found to facilitate the rapid oxidation of As(III), and the production of As(V) in the reaction system increased with increasing initial As(III) concentration. An increase of pH from 3.0 to 7.0 led to a gradual decrease in the oxidation rate of As(III). At pH 3.0, the presence of As(III) accelerated the oxidation rate of arsenopyrite; while at pH 5.0 and 7.0, As(III) inhibited the oxidative dissolution of arsenopyrite. This work reveals the potential environmental process of the interaction between dissolved As(III) and arsenopyrite, and provides important implications for the prevention and control of As(III) pollution in mine drainage.

Role of secondary minerals in the acid generating potential of weathered mine tailings: Crystal-chemistry characterization and closed mine site management involvement

Mine tailings exposed to water and oxygen generate acid mine drainage (AMD) when the neutralizing minerals are insufficient to buffer the acid produced by sulfide oxidation. Mineral reactivity, such as sulfide oxidation and carbonate dissolution, leads to several changes within mine tailings in terms of their physical, mineralogical, and geochemical properties, which may lead to the release of metal(oid)s (e.g., As, Cu, Zn, Fe, S) into the environment. Fresh and oxidized tailings were sampled at two vertical profiles in a tailings storage facility (TSF). The TSF contains tailings from gold ore processing at a mine that has been closed for more than 25 years. Oxidized tailings have formed by in-situ oxidation of fresh tailings over more than 20 years. The collected samples were analyzed for: i) chemical composition by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence (XRF), and total S/C; and ii) mineralogical composition by X-ray diffraction (XRD), Mineral Liberation Analyzer (MLA), Mossbauer spectroscopy, and Fe L-edge X-ray absorption near-edge spectroscopy (XANES). Mineralogically, the fresh tailings included more than 22 wt% carbonates and more than 10 wt% sulfides. In contrast, the oxidized tailings were composed mainly of secondary minerals such as iron oxy-hydroxides and gypsum. Geochemically, the fresh tailings exhibited a circumneutral behavior during weathering cell experiments and contaminants such as As were negligibly released (<0.3 mg/L). The latter is explained by formation of secondary iron oxy-hydroxides, which are known for the capacity to uptake several contaminants from the leachate. Long term oxidation of fresh tailings will lead to highly oxidized tailings similar to those collected in situ. The oxidized tailings exhibited an acidic behavior despite sulfide depletion due to latent acidity. The geochemical behavior was strongly controlled by the reactivity of secondary minerals (e.g., dissolution of gypsum and iron oxy-hydroxides). Quantitatively, the oxidized tailings released 163 mg/kg Fe, around 12,000 mg/kg S, and around 6 mg/kg Zn.

Editorial for Special Issue “Novel and Emerging Strategies for Sustainable Mine Tailings and Acid Mine Drainage Management”

Climate change is one of the most pressing problems facing humanity this century [...]

Enhanced pyrite passivation by carrier-microencapsulation using Fe-catechol and Ti-catechol complexes

Acid mine drainage (AMD) formation is mainly caused by the oxidation of pyrite. Carrier-microencapsulation (CME) using metal-catecholate complexes has been proposed to passivate sulfide minerals by forming surface-protective coatings on their surfaces. Among the various metal-catecholate complexes, Ti-catecholate formed stable coatings having superior acid-resistance, but a thick enough passivating film required considerable time (ca. 14 days) to grow. Meanwhile, Fe-catecholates can form Fe-oxyhydroxide coatings within 2 days, however, they are less stable than Ti-based coating. To address these drawbacks of using a single metal-complex, this study investigated the concurrent use of Fe-catechol and Ti-catechol complexes for accelerating the formation of stable passivating coating on pyrite. Compared with a single metal-complex system, the coating formation was significantly accelerated in mixed system. Linear sweep voltammetry showed the simultaneous decomposition of [Fe(cat)]⁺ and [Ti(cat)₃]^2- as the main reason for improved coating formation. Electrochemical properties of coatings formed by single and mixed complex systems, confirmed by electrochemical impedance spectroscopy and cyclic voltammetry, indicated the coating formed in the mixed system had higher resistance and more electrochemically inert than the other cases. The simultaneous use of Fe-catechol and Ti-catechol complexes enhanced pyrite passivation by accelerating metal-complex decomposition and forming more stable coating composed of Fe₂TiO₅.

Temporal fluctuations in water contamination from abandoned pyrite Wieściszowice mine (Western Sudetes, Poland)

Production of acid mine drainage may occur during mining operations and may continue for many years after closure. In some cases, especially when host rock is capable of reacting with acidic drainage, metal concentrations may decrease over time and distance. Seasonal variations in water flow rate also play an important role in metals concentration both in surface and groundwater. The present study evaluates the contamination of an abandoned pyrite mining area (Wieściszowice, SW Poland) and the temporal variation of the water contamination in selected locations of 2 sampling campaigns (2000 and 2015). The mining surrounding Rudawy Janowickie Mountains range is well known for the rich mining history and is considered as one of the oldest in Europe. The Wieściszowice pyrite mine was exploited for several hundreds of years and processed Fe and Cu sulfides, and sulfur. This mine was closed in 1925 because of the high competition of pyrites from Spain. Results show clearly that water samples collected in the mining area are mainly Ca-SO₄ and acid/high metal, while spring water and surface water samples, representing the local geochemical background, are Ca-HCO₃ and neutral-low metal. The analysis of data shows an improvement in water quality from 2000 to 2015 as well as a decreasing of water risk assessment for human use. This improvement can be related to the fact that 2015 was a very dry year, with over 60% less flow than in 2000, leading to less water contamination.

Study of the micromorphology and health risks of arsenic in copper smelting slag tailings for safe resource utilization

Slag tailings are produced by "cooling-grinding-ball milling-flotation" and other processes of slag, while slag is produced by the flash smelting of the original ore. The utilization and environmental hazards of arsenic in slag tailings have become a focus of attention. This study on slag tailings reveals the presence of arsenic in copper smelting tailings from the mineralogy and leaching perspectives, and the noncarcinogenic and carcinogenic risks of arsenic to the human body were assessed by using the USEPA health risk model. The surface particles of the slag tailings were unevenly dispersed, and the mineral crystals were relatively complete. A small amount of secondary minerals had grown on the mineral surface. Most of the fine particles adhered to the surface of the main mineral to form inclusions. The mineral composition of the slag tailings was dominated by maghemite (Fe₃O₄) and fayalite (Fe₂SiO₄), and the arsenic-bearing minerals were unevenly distributed, where As (Ⅴ) fine particles were embedded in maghemite, amorphous phase and fayalite. There was a large amount of residual arsenic in the slag tailing particles, and the leaching content of arsenic in the toxicity leaching procedure was always lower than the limit of 5 mg/L. The health risk to the exposed population was evaluated by the USEPA health risk model. Since the exposed population in the industrial land is mainly adults, it is determined that the tailings will not cause harm to children's health. In this evaluation, the exposure duration (length of service of the workers) of 30 years, exposure frequency of 314 d/y and body weight of 60 kg (average weight of the workers) were taken as the parameters of three exposure pathways: hand-oral ingestion, respiratory system inhalation and skin contact. Therefore, longer activity time of the workers in the tailing workshop corresponds to a higher HI (hazard index). Although the arsenic in the slag tailings had a certain degree of bioavailability, it was not sufficient to adversely affect human health.

Simultaneous enhancement of treatment performance and energy recovery using pyrite as anodic filling material in constructed wetland coupled with microbial fuel cells

Constructed wetland coupled with microbial fuel cells (CW-MFCs) are a promising technology for sustainable wastewater treatment. However, the performance of CW-MFCs has long been constrained by the limited size of its anode. In this study, we developed an alternative CW-MFC configuration that uses inexpensive natural conductive pyrite as an anodic filling material (PyAno) to extend the electroactive scope of the anode. As a result, the PyAno configuration significantly facilitated the removal of chemical oxygen demand, ammonium nitrogen, total nitrogen, and total phosphorus. Meanwhile, the PyAno increased the maximum power density by 52.7% as compared to that of the quartz sand control. Further, a typical exoelectrogen Geobacter was found enriched in the anodic zone of PyAno, indicating that the electroactive scope was extended by conductive pyrite. In addition, a substantial electron donating potential was observed for the anodic filling material of PyAno, which explained the higher electricity output. Meanwhile, a higher dissimilatory iron reducing potential was observed for the anodic sediment of PyAno, demonstrating the integrity of an iron redox cycling in the system and its promotive effect for the wastewater treatment. Together, these results implied that the PyAno CW-MFCs can be a competitive technology to enhance wastewater treatment and energy recovery simultaneously.

Phytoextraction of Lead Using a Hedge Plant [Alternanthera bettzickiana (Regel) G. Nicholson]: Physiological and Biochemical Alterations through Bioresource Management

Phytoremediation is a cost-effective and environmentally friendly approach that can be used for the remediation of metals in polluted soil. This study used a hedge plant–calico (Alternanthera bettzickiana (Regel) G. Nicholson) to determine the role of citric acid in lead (Pb) phytoremediation by exposing it to different concentrations of Pb (0, 200, 500, and 1000 mg kg−1) as well as in a combination with citric acid concentration (0, 250, 500 µM). The analysis of variance was applied on results for significant effects of the independent variables on the dependent variables using SPSS (ver10). According to the results, maximum Pb concentration was measured in the upper parts of the plant. An increase in dry weight biomass, plant growth parameters, and photosynthetic contents was observed with the increase of Pb application (200 mg kg−1) in soil while a reduced growth was experienced at higher Pb concentration (1000 mg kg−1). The antioxidant enzymatic activities like superoxide dismutase (SOD) and peroxidase (POD) were enhanced under lower Pb concentration (200, 500 mg kg−1), whereas the reduction occurred at greater metal concentration Pb (1000 mg kg−1). There was a usual reduction in electrolyte leakage (EL) at lower Pb concentration (200, 500 mg kg−1), whereas EL increased at maximum Pb concentration (1000 mg kg−1). We concluded that this hedge plant, A. Bettzickiana, has the greater ability to remediate polluted soils aided with citric acid application.

Synthesis and characterization of coal fly ash and palm oil fuel ash modified artisanal and small-scale gold mine (ASGM) tailings based geopolymer using sugar mill lime sludge as Ca-based activator

The continuous accumulation of artisanal and small-scale gold mining (ASGM) tailings in the Philippines without adequate storage and disposal facility could lead to human health and environmental disasters in the long run. In this study, ASGM tailings was simultaneously stabilized and repurposed as construction material via geopolymerization using coal fly ash, palm oil fuel ash and a powder-based alkali activator. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) identified iron sulfides in the tailings containing arsenic (As), cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn), which could be released via weathering. The average unconfined compressive strengths (UCS) of tailings-based geopolymers at 14 days curing were 7.58 MPa and 7.7 MPa with fly ash and palm oil fuel ash, respectively. The tailings-based geopolymers with palm oil fuel ash had higher UCS most likely due to CASH reaction product formation that improved strength formation. The toxicity characteristic leaching procedure (TCLP) results showed very low leachabilities of As, Pb and Fe in the geopolymer materials suggesting ASGM tailings was effectively encapsulated within the geopolymer matrix. Overall, the geopolymerization of ASGM tailings is a viable and promising solution to simultaneously stabilize mining and industrial wastes and repurpose them into construction materials.

Suppression of arsenopyrite oxidation by microencapsulation using ferric-catecholate complexes and phosphate

Mineral processing, pyro- and hydrometallurgical processes of auriferous sulfide ores and porphyry copper deposits (PCDs) generate arsenopyrite-rich wastes. These wastes are disposed of into the tailings storage facilities (TSF) in which toxic arsenic (As) is leached out and acid mine drainage (AMD) is generated due to the oxidation of arsenopyrite (FeAsS). To suppress arsenopyrite oxidation, this study investigated the passivation of arsenopyrite by forming ferric phosphate (FePO₄) coating on its surface using ferric-catecholate complexes and phosphate simultaneously. Ferric iron (Fe³⁺) and catechol form three types of complexes (mono-, bis-, and triscatecholate complexes) depending on the pH, but mono-catecholate complex (i.e.,[Fe(cat)]⁺) became unstable in the presence of phosphate because the chemical affinity of Fe³⁺-PO₄^3- is most probably stronger than that of Fe³⁺-catechol in [Fe(cat)]⁺. When two or more catechol molecules were coordinated with Fe³⁺ (i.e., [Fe(cat)₂]^- and [Fe(cat)₃]^3-), however, these complexes were stable irrespective of the presence of phosphate. The treatment of arsenopyrite with [Fe(cat)₂]^- and phosphate could suppress its oxidation due to the formation of FePO₄ coating, evidenced by SEM-EDX and XPS analyses. The mechanism of FePO₄ coating formation by [Fe(cat)₂]^- and phosphate was confirmed by linear sweep voltammetry (LSV): (1) [Fe(cat)₂]^- was oxidatively decomposed and (2) the resultant product (i.e., [Fe(cat)]⁺) reacts with phosphate, resulting in the formation of FePO₄.

Selenium and arsenic removal from water using amine sorbent, competitive adsorption and regeneration

Selenium (Se) and arsenic (As) are toxic contaminants in surface water and drinking water. The human body needs little quantity of Se, but too high dose is not allowed. Metal oxides such as iron oxides were used for adsorption or co-precipitation removal of As from water. However, the regeneration and stability problems of metals oxides sorbents are unsatisfactory , and there is not enough adsorbent for Se removal from water also. We developed the acrylic amine fiber (AAF) for adsorption reomval of Se and As from water and systematically studied the influenced factors. Batch experiments were conducted for investigating the adsorption edges, while column filtration tests were employed for dynamic application edges. At neutral pH, the Langmuir isotherm fittings gave the maximum adsorption capacities of As(V), As(III), Se(VI) and Se(IV) are 270.3, 40.5, 256.4, and 158.7 mg/g, respectively. Effects of co-existing inorganic anions on As(V) and Se(VI) adsorption using AAF gave the order of PO₄^3- > SO₄^2- > NO₃^- > SiO₃^2-, while different organic acids obey the order of citric acid > oxalic acid > formic acid. Fourier transform infrared analysis showed the PO₄^3- and SO₄^2- competition mechanisms are electrostatic repulsions, while the competition of organic acids derived from acid-base reaction between the carboxyl group and the amino group. Column filtration and regeneration results showed that the spent AAF can be regenerated using 0.5 mol/L HCl solution and reused with no much decrease of adsorption capacity.

Impacts of acid mine drainage on karst aquifers: Evidence from hydrogeochemistry, stable sulfur and oxygen isotopes

The pollution of karst aquifers by acid mine drainage (AMD) waters is increasing. Major and minor ions (Ca²⁺, Mg²⁺, HCO₃^-, SO₄^2-, F^-, and Fe), stable sulfur and oxygen isotopes of dissolved sulfates (δ³⁴S_SO4 and δ¹⁸O_SO4) and oxygen isotope of water (δ¹⁸O_H2O), were analyzed in rainwater, surface water, groundwater, and AMD water sampled from the Babu subterranean stream watershed, in Southwest China. The principal aim of this study was to explore the impact of AMD waters on the evolution of karst aquifers. Based on hydrogeochemistry and stable isotopes (δ¹⁸O_H2O, δ¹⁸O_SO4 and δ³⁴S_SO4): (1) the chemistry of AMD waters was primarily controlled by pyrite oxidation, karst conduit water by AMD waters and mixing with calcite and dolomite dissolution, and spring water by atmospheric precipitation and carbonate dissolution; (2) contamination of the karst conduit water was mainly attributed to the input of AMD waters, resulting in a shift of δ³⁴S_SO4 towards more negative values (from 3.4‰ to -13.2‰); (3) the quality of karst conduit water changed from suitable to unsuitable for irrigation and drinking, particularly due to the increase in total Fe, SO₄^2-, and F^- concentrations, reflecting the cumulative effect of AMD waters derived from tailings dumps; this influence was enhanced during rainstorm/drought and anthropogenic activities; and (4) the flow of contaminated groundwater through the conduit promoted the dissolution of carbonates, especially during the dry season due to the greater proportion of AMD in the groundwater. This released more CO₂ to the atmosphere. We believe that analysis of stable isotopes (δ¹⁸O_H2O, δ¹⁸O_SO4, and δ³⁴S_SO4), combined with hydrogeochemistry, is effective for exploring the impact of AMD on karst aquifers. Therefore, reasonable treatment methods should be taken to reduce the negative impacts of tailings dumps on karst aquifers.

Inhibition of pyrite oxidation using PropS-SH/sepiolite composite coatings for the source control of acid mine drainage

Pyrite, as one of the most abundant sulfide minerals, can be easily oxidized to generate acid mine drainage (AMD). In the present study, a new composite passivator named PropS-SH/sepiolite (PSPT) using γ-mercaptopropyltrimethoxysilane (PropS-SH) as the main passivator and natural sepiolite particles as filler was fabricated and used to suppress the oxidation of pyrite. Electrochemical tests and chemical leaching experiments were carried out to evaluate the passivation performance of PSPT coatings with different amount of sepiolite particles on pyrite oxidation. The results showed that the addition of appropriate sepiolite could significantly improve the inhibition ability of PropS-SH against pyrite oxidation. However, excessive addition of sepiolite particles weakened the inhibition ability of the PSPT coatings owing to aggregations of sepiolite. Additionally, the coating mechanism of PSPT on pyrite was also proposed based on the characterization of FTIR, XPS, and ²⁹SiNMR measurements, which indicated that sepiolite particles could be embedded in PropS-SH network through oxygen bridges, thus improving the stability of the composite coatings.

Influence of Extremophiles on the Generation of Acid Mine Drainage at the Abandoned Pan de Azúcar Mine (Argentina)

The risk of generation of acid drainages in the tailings of the Pan de Azúcar mine that closed its activities more than three decades ago, was evaluated through biooxidation studies using iron- and sulfur-oxidizing extremophilic leaching consortia. Most of tailings showed a high potential for generating acid drainage, in agreement with the results from net acid generation (NAG) assays. In addition, molecular analysis of the microbial consortia obtained by enrichment of the samples, demonstrated that native leaching microorganisms are ubiquitous in the area and they seemed to be more efficient in the biooxidation of the tailings than the collection microorganisms. The acid drainages detected at the site and those formed by oxidation of the tailings, produced a significant ecotoxicological effect demonstrated by a bioassay. These drainages, even at high dilutions, could seriously affect a nearby Ramsar site (Laguna de Pozuelos) that is connected to the Pan de Azúcar mine through a hydrological route (Cincel River).

Genomic Analysis of a Newly Isolated Acidithiobacillus ferridurans JAGS Strain Reveals Its Adaptation to Acid Mine Drainage

Acidithiobacillus ferridurans JAGS is a newly isolated acidophile from an acid mine drainage (AMD). The genome of isolate JAGS was sequenced and compared with eight other published genomes of Acidithiobacillus. The pairwise mutation distance (Mash) and average nucleotide identity (ANI) revealed that isolate JAGS had a close evolutionary relationship with A. ferridurans JCM18981, but whole-genome alignment showed that it had higher similarity in genomic structure with A. ferrooxidans species. Pan-genome analysis revealed that nine genomes were comprised of 4601 protein coding sequences, of which 43% were core genes (1982) and 23% were unique genes (1064). A. ferridurans species had more unique genes (205–246) than A. ferrooxidans species (21–234). Functional gene categorizations showed that A. ferridurans strains had a higher portion of genes involved in energy production and conversion while A. ferrooxidans had more for inorganic ion transport and metabolism. A high abundance of kdp, mer and ars genes, as well as mobile genetic elements, was found in isolate JAGS, which might contribute to its resistance to harsh environments. These findings expand our understanding of the evolutionary adaptation of Acidithiobacillus and indicate that A. ferridurans JAGS is a promising candidate for biomining and AMD biotreatment applications.

Mineralogical and Geochemical Characterization of Gold Mining Tailings and Their Potential to Generate Acid Mine Drainage (Minas Gerais, Brazil)

For more than 30 years, sulfide gold ores were treated in metallurgic plants located in Nova Lima, Minas Gerais, Brazil, and accumulated in the Cocoruto tailings dam. Both flotation and leaching tailings from a deactivated circuit, as well as roasted and leaching tailings from an ongoing plant, were studied for their acid mine drainage potential and elements’ mobility. Detailed characterization of both tailings types indicates the presence of fine-grain size material hosting substantial amounts of sulfides that exhibit distinct geochemical and mineralogical characteristics. The samples from the ongoing plant show high grades of Fe in the form of oxides, cyanide, and sulfates. Differently, samples from the old circuit shave higher average concentrations of Al (0.88%), Ca (2.4%), Mg (0.96%), and Mn (0.17%), present as silicates and carbonates. These samples also show relics of preserved sulfides, such as pyrite and pyrrhotite. Concentrations of Zn, Cu, Au, and As are higher in the tailings of the ongoing circuit, while Cr and Hg stand out in the tailings of the deactivated circuit. Although the obtained results show that the sulfide wastes do not tend to generate acid mine drainage, leaching tests indicate the possibility of mobilization of toxic elements, namely As and Mn in the old circuit, and Sb, As, Fe, Ni, and Se in the tailings of the plant that still works. This work highlights the need for proper management and control of tailing dams even in alkaline drainage environments such as the one of the Cocoruto dam. Furthermore, strong knowledge of the tailings’ dynamics in terms of geochemistry and mineralogy would be pivotal to support long-term decisions on wastes management and disposal.

Co-Disposal of Coal Gangue and Red Mud for Prevention of Acid Mine Drainage Generation from Self-Heating Gangue Dumps

The seepage and diffusion of acid mine drainage (AMD) generated from self-heating coal gangue tailings caused acid pollution to the surrounding soil and groundwater. Red mud derived from the alumina smelting process has a high alkali content. To explore the feasibility of co-disposal of coal gangue and red mud for prevention of AMD, coal gangue and red mud were sampled from Yangquan (Shanxi Province, China), and dynamic leaching tests were carried out through the automatic temperature-controlled leaching system under the conditions of different temperatures, mass ratios, and storage methods. Our findings indicated that the heating temperature had a significant effect on the release characteristics of acidic pollutants derived from coal gangue, and that the fastest rate of acid production corresponding to temperature was 150 °C. The co-disposal dynamic leaching tests indicated that red mud not only significantly alleviated the release of AMD but also that it had a long-term effect on the treatment of acid pollution. The mass ratio and stacking method were selected to be 12:1 (coal gangue: red mud) and one layer was alternated (coal gangue covered with red mud), respectively, to ensure that the acid-base pollution indices of leachate reached the WHO drinking-water quality for long-term discharge. The results of this study provided a theoretical basis and data support for the industrial field application of solid waste co-treatment.

Solid-phase partitioning and release-retention mechanisms of copper, lead, zinc and arsenic in soils impacted by artisanal and small-scale gold mining (ASGM) activities

Artisanal and small-scale gold mining (ASGM) operations are major contributors to the Philippines' annual gold (Au) output (at least 60%). Unfortunately, these ASGM activities lacked adequate tailings management strategies, so contamination of the environment is prevalent. In this study, soil contamination with copper (Cu), lead (Pb), zinc (Zn) and arsenic (As) due to ASGM activities in Nabunturan, Davao de Oro, Philippines was investigated. The results showed that ASGM-impacted soils had Cu, Pb, Zn and As up to 3.6, 83, 73 and 68 times higher than background levels, respectively and were classified as 'extremely' polluted (CD = 30-228; PLI = 5.5-34.8). Minerals typically found in porphyry copper-gold ores like pyrite, chalcopyrite, malachite, galena, sphalerite and goethite were identified by XRD and SEM-EDS analyses. Furthermore, sequential extraction results indicate substantial Cu (up to 90%), Pb (up to 50%), Zn (up to 65%) and As (up to 48%) partitioned with strongly adsorbed, weak acid soluble, reducible and oxidisable fractions, which are considered as 'geochemically mobile' phases in the environment. Although very high Pb and Zn were found in ASGM-impacted soils, they were relatively immobile under oxidising conditions around pH 8.5 because of their retention via adsorption to hydrous ferric oxides (HFOs), montmorillonite and kaolinite. In contrast, Cu and As release from the historic ASGM site samples exceeded the environmental limits for Class A and Class C effluents, which could be attributed to the removal of calcite and dolomite by weathering. The enhanced desorption of As at around pH 8.5 also likely contributed to its release from these soils.

Chemical Treatment of Highly Toxic Acid Mine Drainage at A Gold Mining Site in Southwestern Siberia, Russia

The critical environmental situation in the region of southwestern Siberia (Komsomolsk settlement, Kemerovo region) is the result of the intentional displacement of mine tailings with high sulfide concentrations. During storage, ponds of acidic water with incredibly high arsenic (up to 4 g/L) and metals formed on the tailings. The application of chemical methods to treat these extremely toxic waters is implemented: milk of lime Ca(OH)2, sodium sulfide Na2S, and sodium hydroxide NaOH. Field experiments were carried out by sequential adding pre-weighed reagents to the solutions with control of the physicochemical parameters and element concentrations for each solution/reagent ratio. In the experiment with Ca(OH)2, the pH increased to neutral values most slowly, which is contrary to the results from the experiment with NaOH. When neutralizing solutions with NaOH, arsenic-containing phases are formed most actively, arsenate chalcophyllite Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O, a hydrated iron arsenate scorodite, kaatialaite FeAs3O9·8H2O and Mg(H2AsO4)2. A common specificity of the neutralization processes is the rapid precipitation of Fe hydroxides and gypsum, then the reverse release of pollutants under alkaline conditions. The chemistry of the processes is described using thermodynamic modeling. The main species of arsenic in the solutions are iron-arsenate complexes; at the end of the experiments with Ca(OH)2, Na2S, and NaOH, the main species of arsenic is CaAsO4−, the most toxic acid H3AsO3 and AsO43−, respectively. It is recommended that full-scale experiments should use NaOH in the first stages and then Ca(OH)2 for the subsequent neutralization.

Effects of Backfilling Excavated Underground Space on Reducing Acid Mine Drainage in an Abandoned Mine

Three-dimensional groundwater flow around an abandoned mine was simulated to evaluate the effects of backfilling the excavated underground space of the mine on reducing the acid mine drainage (AMD). The conceptual model of the groundwater flow consists of not only variable geological formations but also vertical shafts, horizontal drifts, and the other excavated underground space. The steady-state groundwater flow in both days with high and little rainfall was calculated to calibrate the model. The calculated groundwater levels and flow rate of the AMD agreed with the measured ones by calibrating the hydraulic conductivity of the host rock, which was sensitive to groundwater flow in the mine. This validated model was applied to predict the flow rate of the AMD when backfilling the excavated underground space. The results showed that the flow rate of the AMD decreased by 5% to 30%. This indicates that backfilling the excavated space is one of the effective methods to reduce AMD of abandoned mines.

Mine Waste Rock: Insights for Sustainable Hydrogeochemical Management

Mismanagement of mine waste rock can mobilize acidity, metal (loid)s, and other contaminants, and thereby negatively affect downstream environments. Hence, strategic long-term planning is required to prevent and mitigate deleterious environmental impacts. Technical frameworks to support waste-rock management have existed for decades and typically combine static and kinetic testing, field-scale experiments, and sometimes reactive-transport models. Yet, the design and implementation of robust long-term solutions remains challenging to date, due to site-specificity in the generated waste rock and local weathering conditions, physicochemical heterogeneity in large-scale systems, and the intricate coupling between chemical kinetics and mass- and heat-transfer processes. This work reviews recent advances in our understanding of the hydrogeochemical behavior of mine waste rock, including improved laboratory testing procedures, innovative analytical techniques, multi-scale field investigations, and reactive-transport modeling. Remaining knowledge-gaps pertaining to the processes involved in mine waste weathering and their parameterization are identified. Practical and sustainable waste-rock management decisions can to a large extent be informed by evidence-based simplification of complex waste-rock systems and through targeted quantification of a limited number of physicochemical parameters. Future research on the key (bio)geochemical processes and transport dynamics in waste-rock piles is essential to further optimize management and minimize potential negative environmental impacts.

Arsenic (III) Removal from a High-Concentration Arsenic (III) Solution by Forming Ferric Arsenite on Red Mud Surface

Arsenic (As) is considered one of the most serious inorganic pollutants, and the wastewater produced in some smelters contains a high concentration of arsenic. In this paper, we purified the high-concentration arsenic solution with red mud and Fe3+ synergistically. In this system, arsenite anions reacted with Fe(III) ions to form ferric arsenite, which attached on the surface of red mud particles. The generated red mud/Fe1−x(As)x(OH)3 showed a better sedimentation performance than the pure ferric arsenite, which is beneficial to the separation of arsenic from the solution. The red mud not only served as the carrier, but also as the alkaline agent and adsorbent for arsenic treatment. The effects of red mud dosage, dosing order, pH, and molar ratio of Fe/As on arsenic removal were investigated. The efficiency of arsenic removal increased from a pH of 2 to 6 and reached equilibrium at a pH of 7. At the Fe/As molar ratio of 3, the removal efficiency of arsenic ions with an initial concentration of 500 mg/L reached 98%. In addition, the crystal structure, chemical composition, and morphological properties of red mud and arsenic removal residues (red mud/Fe1−x(As)x(OH)3) were characterized by XRD, XPS, X-ray fluorescence (XRF), SEM-EDS, and Raman spectroscopy to study the mechanism of arsenic removal. The results indicated that most of the arsenic was removed from the solution by forming Fe1−x(As)x(OH)3 precipitates on the red mud surface, while the remaining arsenic was adsorbed by the red mud and ferric hydroxide.