Suppressive effects of ferric-catecholate complexes on pyrite oxidation

Potential high-risk release sources of thallium and arsenic from surrounding rocks of a typical thallium and arsenic mining area in southwest China

Abundant naturally and anthropogenically exposed surrounding rocks (NESRs and AESRs) in mining areas may pose persistent threats as sources of potentially toxic elements (PTEs), but this has been historically overlooked, especially for thallium (Tl) and arsenic (As). Here, the release risks of Tl and As from both NESRs and AESRs in a typical TlAs sulfide mining area were investigated. In a single leaching process, AESRs released 10.4 % of total Tl (157 μg L-1) and 32.5 % of total As (4089 μg L-1), 2-3 orders of magnitude higher than NESRs. Prolonged multiple leaching tests revealed notable and long-term risks of release of Tl and As from AESRs, associated with oxidation and dissolution of iron/sulfur-bearing minerals. Substantial release of PTEs was linked to the transformation/degradation of the -OH functional group and extensive dissolution of secondary sulfate minerals in AESRs. Ultrafiltration and STEM-EDS indicate that 18.4 % of water-extracted As released from AESRs existed as natural nanoparticles consisting of iron/sulfur-bearing minerals. This study highlights the high risks of Tl and As release from anthropogenically exposed surrounding rocks and the importance of nanoparticles in PTE transport, and provides insights into the control of PTEs in mining areas.

A highly efficient process to enhance the bioleaching of spent lithium-ion batteries by bifunctional pyrite combined with elemental sulfur

Bioleaching technologies have been shown to be an environmentally friendly and economically beneficial tool for extracting metals from spent lithium-ion batteries (LIBs). However, conventional bioleaching methods have exhibited low efficiency in recovering metals from spent LIBs. Therefore, relied on the sustainability principle of using waste to treat waste, this study employed pyrite (FeS2) as an energy substance with reducing properties and investigated its effects in combination with elemental sulfur (S0) or FeSO4 on metals bioleaching from spent LIBs. Results demonstrated that the bioleaching efficiency was significantly higher in the leaching system constructed with FeS2 + S0, than in the FeS2 + FeSO4 or FeS2 system. When the pulp densities of FeS2, S0 and spent LIBs were 10 g L-1, 5 g L-1 and 10 g L-1, respectively, the leaching efficiency of Li, Ni, Co and Mn all reached 100%. Mechanistic analysis reveals that in the FeS2 + S0 system, the activity and acid-producing capabilities of iron-sulfur oxidizing bacteria were enhanced, promoting the generation of Fe (Ⅱ) and reducible sulfur compounds. Simultaneously, bio-acids were shown to disrupt the structure of the LIBs, thereby increasing the contact area between Fe (Ⅱ) and sulfur compounds containing high-valence metals. This effectively promoted the reduction of high-valence metals, thereby enhancing their leaching efficiency. Overall, the FeS2 + S0 bioleaching process constructed in this study, improved the leaching efficiency of LIBs while also effectively utilizing waste, providing technical support for the comprehensive and sustainable management of solid waste.

Performance and mechanisms of PropS-SH/Ce(dbp)3 coatings in the inhibition of pyrite oxidationtion for acid mine drainage control

Inhibition of tailings oxidation could availably control the generation of acid mine wastewater from its source. Organosilanes serving as a high-efficiency inhibitor of the oxidation of pyrite, bring some problems including safety hazards caused by large amounts of organic solvents, difficult high-temperature curing, poor long-term properties, and so on. In our work, the PropS-SH/Ce (dbp)₃ (PS/Ce (dbp)₃) passivator with excellent passivation performance and self-healing properties was prepared by choosing 3-mercaptopropyltrimethoxysilane (PropS-SH) and dibutyl phosphate (Ce (dbp)₃) as the main passivating agent and the repair agent, respectively. We reduced the ratio of ethanol to water by adjusting the pH of the organosilane condensation and also achieved room-temperature curing by extending the curing time. Electrochemical and chemical leaching experiments results showed that the most appropriate addition of Ce (dbp)₃ was 0.2 wt% for enhancing the passivation performance of the passivated coating. In a 6-month chemical leaching experiment, the PS/Ce (dbp)₃-0.2 passivation coating cured at room temperature showed a better passivation effect and maintained 90.55% and 78.54% of total Fe and SO₄^2- passivation efficiencies. The passivation and self-healing mechanisms were investigated by FT-IR, XPS, ²⁹Si NMR, and other characterization methods, which were as follows: silane formed a cross-linked mesh structure by Si-O-Si bonding, in which Ce (dbp)₃ was physically filled. And the Si-OH on the surface of the passivation film formed Fe-O-Si bonds with the hydroxyl groups on the surface of the pyrite, thus attaching to the surface of the pyrite and isolating the oxidation medium. When the passivation coating was locally damaged, the oxidation reaction caused a change in pH, which accelerated the dissolution of Ce (dbp)₃ in the passivation layer. Ce³⁺ underwent a valence change and formed a CeO₂ precipitate, while dbp^- could form a complex with Fe²⁺ on the pyrite surface, both of which worked together to repair the broken passivation coating and prevent the oxidation reaction.

Suppression of pyrite oxidation by co-depositing bio-inspired PropS-SH-tannic acid coatings for the source control acid mine drainage

In previous works, both tannic acid (TA) and organosilane-based passivators have been proven to possess good inhibition effects on pyrite oxidation, which could effectively prevent acid mine drainage (AMD) generation at the source. However, the hydrophilicity of TA passivation film and the complex coating process of organosilane-based passivators (high temperature conditions were required during the process carried out) may limit their further practical use. Therefore, to achieve the purpose of better coating treatment of pyrite under mild conditions, TA and γ-mercaptopropyltrimethoxysilane (PropS-SH) were introduced to synergistically passivate pyrite in this work. Electrochemistry tests and chemical leaching experiments both confirmed that PropS-SH-TA coated pyrite had better oxidation resistance than raw pyrite and single PropS-SH or TA coated pyrite. Additionally, the analyses of scanning electron microscopy (SEM) measurements and static water contact angle tests demonstrated that a scaly coating was formed on PropS-SH-TA coated pyrite surface, which may be the reason for the significant improvement of its surface hydrophobicity. Finally, the study on the film-forming mechanism of PropS-SH-TA composite passivator displayed that the benzoquinone derivatives formed by TA could copolymerize with PropS-SH through Michael addition or Schiff base reaction, which constructed a dense hydrophobic film on pyrite surface. The newly formed composite film could provide a better oxidation barrier for pyrite based on TA passivation film.

Conventional and recent advances in gravity separation technologies for coal cleaning: A systematic and critical review

"Affordable and clean energy" is enshrined in the UN Sustainable Development Goals (SDGs; #7) because of its importance in supporting the sustainable development of society. As an energy source, coal is widely used because it is abundant and its utilization for electricity and heat generation do not require complex infrastructures and technologies, which makes it ideal for the energy needs of low-income and developing countries. Coal is also essential in steel making (as coke) and cement production and will continue to be on high demand for the foreseeable future. However, coal is naturally found with impurities or gangue minerals like pyrite and quartz that could create by-products (e.g., ash) and various pollutants (e.g., CO₂, NO_X, SO_X). To reduce the environmental impacts of coal during combustion, coal cleaning-a kind of pre-combustion clean coal technology-is essential. Gravity separation, a technique that separates particles based on their differences in density, is widely used in coal cleaning due to the simplicity of its operation, low cost, and high efficiency. In this paper, recent studies (from 2011 to 2020) related to gravity separation for coal cleaning were systematically reviewed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A total of 1864 articles were screened after removing duplicates, and after a thorough evaluation 189 articles were reviewed and summarized. Among of conventional separation techniques, dense medium separator (DMS), particularly dense medium cyclone (DMC), is the most popular technologies studied, which could be attributed to the growing challenges of cleaning/processing fine coal-bearing materials. In recent years, most of works focused on the development of dry-type gravity technologies for coal cleaning. Finally, gravity separation challenges and future applications to address problems in environmental pollution and mitigation, waste recycling and reprocessing, circular economy, and mineral processing are discussed.

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.

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.

Unravelling the effects of complexation of transition metal ions on the hydroxylation of catechol over the whole pH region

Catechol pollutants (CATPs) serving as chelating agents could coordinate with many metal ions to form various CATPs-metal complexes. Little information is available on the effects of complexation of metal ions on CATPs degradation. This work presents a systematical study of •OH-mediated degradation of catechol and catechol-metal complexes over the whole pH range in advanced oxidation processes (AOPs). Results show that the pH-dependent complexation of metal ions (Zn²⁺, Cu²⁺, Ti⁴⁺ and Fe³⁺) promotes the deprotonation of catechol under neutral and even acidic conditions. The radical adduct formation (RAF) reactions are both thermodynamically and kinetically favorable for all dissociation and complexation species, and OH/O^- group-containing C positions are more vulnerable to •OH attack. The kinetic results show that the complexation of the four metal ions offers a wide pH range of effectiveness for catechol degradation. At pH 7, the apparent rate constant (k_app) values for different systems follow the order of catechol+Ti⁴⁺ ≈ catechol+Zn²⁺ > catechol+Cu²⁺ > catechol+Fe³⁺ > catechol. The mechanistic and kinetic results would greatly improve our understanding of the degradation of CATPs-metal and other organics-metal complexes in AOPs. The toxicity assessment indicates that the •OH-based AOPs have the ability for decreasing the toxicity and increasing the biodegradability during the processes of catechol degradation.

Inhibition of organosilane/ATP@HQ self-healing passivator for pyrite oxidation

Organosilane, with functional organic groups attached to inorganic silicon atoms, exhibits excellent passivation performance for pyrite. However, a considerable number of micro-cracks will gradually appear on the surface of passivation film under long-term corrosion of oxidizing medium, resulting in a significant decrease of passivation effect. To improve the stability and long-term performance of organosilane coating, a novel passivator (PT-ATP@HQ) with self-healing function was prepared to inhibit the oxidation of pyrite. We chose 3-mercaptopropyltrimethoxysilane (Prop-SH) and tetraethoxysilane (TEOS) as the host coating (PT), and attapulgite clay (ATP) loaded with 8-hydroxyquinoline (8-HQ) was used to endow the coating with better passivation and self-healing performance. The electrochemical and chemical leaching results showed that the addition of ATP@HQ greatly improved the passivation performance of PT coating. The passivation efficiencies of total Fe and SO₄^2- reached to 88.1% and 79.2%, respectively. We also found that the protective capability of the scratched PT-ATP@HQ coating can be recovered automatically through 8-HQ release from ATP. The passivation and self-healing mechanisms were investigated by FT-IR, XPS, ²⁹Si NMR, and other characterization methods, which were as follows: firstly, the organosilanes hydrolyzed to form highly active silanol groups, then dehydration condensation reaction occurred between silanol molecules and ATP@HQ to obtain cross-linked network structure connected by Si-O-Si bonds. After that, Si-OH groups reacted with the hydroxyl groups of pyrite to form Fe-O-Si bonds, thereby an inert and dense passivation film attached to the surface of pyrite. Once the passivation film is locally damaged, 8-HQ will automatically release to repair the cracks.

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.

Performance Evaluation of Fe-Al Bimetallic Particles for the Removal of Potentially Toxic Elements from Combined Acid Mine Drainage-Effluents from Refractory Gold Ore Processing

Acid mine drainage (AMD) is a serious environmental issue associated with mining due to its acidic pH and potentially toxic elements (PTE) content. This study investigated the performance of the Fe-Al bimetallic particles for the treatment of combined AMD-gold processing effluents. Batch experiments were conducted in order to eliminate potentially toxic elements (including Hg, As, Cu, Pb, Ni, Zn, and Mn) from a simulated waste solution at various bimetal dosages (5, 10, and 20 g/L) and time intervals (0 to 90 min). The findings show that metal ions with greater electrode potentials than Fe and Al have higher affinities for electrons released from the bimetal. Therefore, a high removal (> 95%) was obtained for Hg, As, Cu, and Pb using 20 g/L bimetal in 90 min. Higher uptakes of Hg, As, Cu, and Pb than Ni, Zn, and Mn also suggest that electrochemical reduction and adsorption by Fe-Al (oxy) hydroxides as the primary and secondary removal mechanisms, respectively. The total Al3+ dissolution in the experiments with a higher bimetal content (10 and 20 g/L) were insignificant, while a high release of Fe ions was recorded for various bimetal dosages. Although the secondary Fe pollution can be considered as a drawback of using the Fe-Al bimetal, this issue can be tackled by a simple neutralization and Fe precipitation process. A rapid increase in the solution pH (initial pH 2 to >5 in 90 min) was also observed, which means that bimetallic particles can act as a neutralizing agent in AMD treatment system and promote the precipitation of the dissolved metals. The presence of chloride ions in the system may cause akaganeite formation, which has shown a high removal capacity for PTE. Moreover, nitrate ions may affect the process by competing for the released electrons from the bimetal owing to their higher electrode potential than the metals. Finally, the Fe-Al bimetallic material showed promising results for AMD remediation by electrochemical reduction of PTE content, as well as acid-neutralization/metal precipitation.

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.

Flotation Separation of Chalcopyrite and Molybdenite Assisted by Microencapsulation Using Ferrous and Phosphate Ions: Part II. Flotation

Porphyry-type deposits are the major sources of copper and molybdenum, and flotation has been adopted to recover them separately. The conventional reagents used for depressing copper minerals, such as NaHS, Na2S, and Nokes reagent, have the potential to emit toxic H2S gas when pulp pH was not properly controlled. Thus, in this study the applicability of microencapsulation (ME) using ferrous and phosphate ions as an alternative process to depress the floatability of chalcopyrite was investigated. During ME treatment, the use of high concentrations of ferrous and phosphate ions together with air introduction increased the amount of FePO4 coating formed on the chalcopyrite surface, which was proportional to the degree of depression of its floatability. Although ME treatment also reduced the floatability of molybdenite, ~92% Mo could be recovered by utilizing emulsified kerosene. Flotation of chalcopyrite/molybdenite mixture confirmed that the separation efficiency was greatly improved from 10.9% to 66.8% by employing ME treatment as a conditioning process for Cu-Mo flotation separation.

Reutilization of gangue wastes in underground backfilling mining: Overburden aquifer protection

Gangue backfilling mining (GBM) can effectively alleviate the gangue accumulation pollution and the overburden aquifer destruction. To efficiently evaluate the reutilization of gangue wastes by GBM and its advantage in overburden aquifer protection, non-Darcy hydraulic properties and deformation behaviors of granular gangues were studied through laboratorial, theoretical, and in-situ aspects. A series of compression and seepage tests on granular gangues under the variable original grain size grade (GSG) and stress rate were conducted. Laboratorial testing results convince that, hydraulic properties (porosity and permeability) of the granular gangue decline with the increasing original GSG and decreasing stress rate. The crushing ratio of the sample increases with the increase of original GSG and the decrease of stress rate. The fractal dimension reveals more obvious increases in the samples with the higher original GSGs and lower stress rates. The Kruger model (a classical theoretical model) was employed to predict the permeability evolution based on the porosity. However, the invalid pores in rocks would result in the model's underestimation. To this end, an improved model was established to predict the permeability evolution by the fractal dimension, and the improved Kruger model has better accuracy than the original one. Finally, according to the laboratorial testing and theoretical predicted results, friendly-environmental strategies for overburden aquifer protection were proposed. The effectiveness of these strategies was successfully verified by an in-situ application. It is concluded that the high filling stress, low gangue original GSG, and low filling stress rate in GBM can effectively reduce the risk of overburden aquifer destruction.

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.

The Use of Mining Waste Materials for the Treatment of Acid and Alkaline Mine Wastewater

The mining of metal ores generates both liquid and solid wastes, which are increasingly important to manage. In this paper, an attempt was made to use waste rocks produced in the mining of zinc and lead to neutralizing acid mine drainage and alkaline flotation wastewater. Waste rock is a quartz-feldspar rock of hydrothermal origin. It is composed of, besides quartz and potassium feldspar (orthoclase), phyllosilicates (chlorite and mica), and sulfides (chiefly pyrite). To determine its physicochemical parameters and their variability, acid mine water and flotation wastewater were monitored for 12 months. Acid mine drainage (AMD) is characterized by a low pH (~3), high zinc concentration (~750 mg·L−1), and high sulfate content (~6800 mg·L−1). On the other hand, the determinations made for flotation wastewater showed, among others, a pH of approximately 12 and ca. 780 mg·L−1 of sulfates. AMD and flotation wastewater neutralization by the waste rock was shown to be possible and efficient. However, in both cases, the final solution contained elevated concentrations of metals and sulfates. Premixing AMD with alkaline flotation wastewater in the first step and then neutralizing the obtained mixture with the waste rock was considered the best solution. The produced solution had a circumneutral pH. However, the obtained solution does not meet the legislative requirements but could be further treated by, for example, passive treatment systems. It is noteworthy that the proposed approach is low cost and does not require any chemical reagents.

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

Acid mine drainage (AMD) formation and toxic arsenic (As) pollution are serious environmental problems encountered worldwide. In this study, we investigated the crucial roles played by common secondary mineral phases formed during the natural weathering of pyrite-bearing wastes-soluble salts (melanterite, FeSO₄·7H₂O) and metal oxides (hematite, Fe₂O₃)-on AMD formation and As mobility under acidic conditions (pH 1.5-4) prevalent in historic tailings storage facilities, pyrite-bearing rock dumps and AMD-contaminated soils and sediments. Our results using a pyrite-rich natural geological material containing arsenopyrite (FeAsS) showed that melanterite and hematite both directly-by supplying H⁺ and/or oxidants (Fe³⁺)-and indirectly-via changes in the nature of oxidation layer formed on pyrite-influenced pyrite oxidation dynamics. Based on SEM-EDS, DRIFT spectroscopy and XPS results, the oxidation layer on pyrite was mainly composed of ferric arsenate and K-Jarosite when melanterite was abundant with/without hematite but changed to Fe-oxyhydroxide/oxide and scorodite when melanterite was low and hematite was present. This study also observed the formation of a mechanically 'strong' coating on pyrite that suppressed the mineral's oxidation. Finally, As mobility under acidic conditions was limited by its precipitation as ferric arsenate, scorodite, or a Fe/Al arsenate phase, including its strong adsorption to Fe-oxyhydroxides/oxides.

Mine drainage: Remediation technology and resource recovery

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

A Review of Recent Advances in Depression Techniques for Flotation Separation of Cu–Mo Sulfides in Porphyry Copper Deposits

Porphyry copper deposits (PCDs) are some of the most important sources of copper (Cu) and molybdenum (Mo). Typically, the separation and recovery of chalcopyrite (CuFeS2) and molybdenite (MoS2), the major Cu and Mo minerals, respectively, in PCDs are achieved by two-step flotation involving (1) bulk flotation to separate Cu–Mo concentrates and tailings (e.g., pyrite, silicate, and aluminosilicate minerals) and (2) Cu–Mo flotation to separate chalcopyrite and molybdenite. In Cu–Mo flotation, chalcopyrite is depressed using Cu depressants, such as NaHS, Na2S, Nokes reagent (P2S5 + NaOH), and NaCN, meaning that it is recovered as tailings, while molybdenite is floated and recovered as froth product. Although conventionally used depressants are effective in the separation of Cu and Mo, they have the potential to emit toxic and deadly gases such as H2S and HCN when operating conditions are not properly controlled. To address these problems caused by the use of conventional depressants, many studies aimed to develop alternative methods of depressing either chalcopyrite or molybdenite. In this review, recent advances in chalcopyrite and molybdenite depressions for Cu–Mo flotation separation are reviewed, including alternative organic and inorganic depressants for Cu or Mo, as well as oxidation-treatment technologies, such as ozone (O3), plasma, hydrogen peroxide (H2O2), and electrolysis, which create hydrophilic coatings on the mineral surface.

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.

Electrochemical Oxidation of Metal-Catechol Complexes as a New Synthesis Route to the High-Quality Ternary Photoelectrodes: A Case Study of Fe2TiO5 Photoanodes

A new electrochemical, solution-based synthesis method to prepare uniform multinary oxide photoelectrodes was developed. This method involves solubilizing multiple metal ions as metal-catechol complexes in a pH condition where they are otherwise insoluble. When some of the catechol ligands are electrochemically oxidized, the remaining metal complexes become insoluble and are deposited as metal-catechol films on the working electrode. The resulting films are then annealed to form crystalline multinary oxide electrodes. Because catechol can serve as a complexing agent for a variety of metal ions, the newly developed method can be used to prepare a variety of multinary oxide films. In the present study, we used this method to prepare n-type Fe₂TiO₅ photoanodes and investigated their photoelectrochemical properties for use in a photoelectrochemical water-splitting cell. We also performed a computational investigation with two goals. The first goal was to investigate small electron polaron formation in Fe₂TiO₅. Charge transport in most oxide photoelectrodes involves small polaron hopping, but small polaron formation in Fe₂TiO₅ has not been examined prior to this work. The second goal was to investigate the effect of substitutional Sn doping at the Fe site on the electronic band structure and the carrier concentration of Fe₂TiO₅. The combined experimental and theoretical results presented in this study greatly improve our understanding of Fe₂TiO₅ for use as a photoanode.

The two-step neutralization ferrite-formation process for sustainable acid mine drainage treatment: Removal of copper, zinc and arsenic, and the influence of coexisting ions on ferritization

Acid mine drainage (AMD) or acid rock drainage (ARD), the most notorious environmental problem in many mines and underground construction sites, is generally managed using lime neutralization. This approach is effective but unsustainable in the long term, so we introduced the two-step neutralization ferrite-formation process in our previous works as an alternative. However, several important issues related to this new approach-the partitioning of hazardous elements during treatment, stability of generated sludges, and influence of coexisting ions-remains unclear. In this study, real AMD containing zinc (Zn), copper (Cu) and arsenic (As) was treated using a laboratory-type continuous ferrite process flow setup. Partitioning of hazardous elements in the two sludges was elucidated by X-ray fluorescence spectroscopy (XRF) and X-ray absorption spectroscopy (XAS) while the stability of sludges was determined by standard leaching experiments. The bulk of Cu and As species (both As(III) and As(V) based on XANES spectra) were partitioned in the first sludge while ~64% of Zn was associated with the ferrite sludge. In terms of stability, both sludges were relatively inert and released only minute amounts of Zn, Cu and As, all of which were below the Japanese environmental standards. The roles played by two of the most ubiquitous coexisting ions in AMD on ferritization-dissolved silica (Si) and aluminum ion (Al³⁺)-were also elucidated using 10 synthetic AMDs. Between the two, dissolved Si exhibited stronger adverse effects on ferritization than Al³⁺. At dissolved Si above 4 mg/L, Si-O-Fe surface complex formation on amorphous Fe-precipitates or Fe-oxide precursor minerals became extensive, which protected these phases from the dissolution-transformation reactions required to form strongly magnetic magnesioferrite and magnetite. These results suggest that the flexibility and applicability of this new AMD treatment approach could be improved by controlling the dissolved Si concentration prior to the ferrite formation step.

Modeling of the groundwater flow system in excavated areas of an abandoned mine

This study evaluated the assumption that back-filled excavated areas of old mine workings can be modeled as porous media, where groundwater flow is governed by Darcy's law. The Yatani mine, located in Yamagata Prefecture, Japan, was selected for this study because several mining methods were used during its operation and detailed drawings of the excavated areas of the mine are available. The model was calibrated using combinations of hydraulic conductivities (k), with the best-matched case being selected by comparing calculated and measured AMD fluxes. Modeled AMD fluxes along the drainage tunnel (-2 L level) were consistent with measured data when the excavated areas were considered to be porous media with a specific hydraulic conductivity, and the presence of faults and permeability were taken into account. The model also successfully predicted the increasing trend of AMD flux from the shaft to adit mouth. In the numerical model, the back-filled excavated areas were assumed to behave as porous media, which was shown to be a valid assumption in this mine. The model demonstrated that back-filling the excavated areas and drainage tunnel with low permeability materials could reduce the flux of Zn in AMD by up to 61%.

Carrier-microencapsulation of arsenopyrite using Al-catecholate complex: nature of oxidation products, effects on anodic and cathodic reactions, and coating stability under simulated weathering conditions

Mining activities often generate large amounts of sulfide-rich wastes containing arsenopyrite (FeAsS), which when dissolved releases toxic arsenic (As) and generates acid mine drainage (AMD) that are both disastrous to the environment. To suppress arsenopyrite dissolution, a technique that selectively coats sulfide minerals with a protective layer of Al-oxyhydroxide called Al-based carrier-microencapsulation (CME) was developed. Although a previous study of the authors showed that Al-based CME could significantly limit arsenopyrite dissolution, nature of the coating formed on arsenopyrite, including its electrochemical properties, is still not well understood. Moreover, stability of the coating once exposed to weathering conditions remains unclear. Better understanding of these important issues would greatly improve Al-based CME especially in its application to real mine wastes. In this study, nature of the coating formed by Al-based CME was investigated using SEM-EDX, DRIFTS and XPS while the electrochemical properties of the coating were evaluated by cyclic voltammetry and chronoamperometry. Meanwhile, stability of the coating was elucidated using consecutive batch leaching experiments and weathering cell tests. SEM-EDX, DRIFTS and XPS results indicate that the protective coating formed on arsenopyrite by Al-based CME was mainly composed of bayerite (α-Al(OH)₃), gibbsite (γ-Al(OH)₃), and boehmite (γ-AlO(OH)). These Al-based coatings, which have insulating properties, made arsenopyrite less electrochemically active. The coatings also limited the extent of both the anodic and cathodic half-cell reactions of arsenopyrite oxidation that suppressed As release and acid generation. Weathering cell tests indicated that the oxidation of CME-treated arsenopyrite was effectively limited until about 15 days but after this, it started to gradually progress with time due to the increasing acidity of the system where Al-based coatings became unstable. Nonetheless, CME-treated arsenopyrite was less oxidized based on the released amounts of Fe, As and S suppressed by 80, 60 and 70%, respectively, compared with the one treated with control.

Improved pyrolysis behavior of ammonium polyphosphate-melamine-expandable (APP-MEL-EG) intumescent fire retardant coating system using ceria and dolomite as additives for I-beam steel application

This study describes the effects of ceria (CeO₂) and dolomite [CaMg(CO₃)₂] additives on the pyrolysis behavior and fire resistive property of conventional intumescent flame retardant (IFR) coating system for I-beam steel substrate called ammonium polyphosphate-melamine-expandable graphite (APP-MEL-EG) system. The fire resistance of various formulations was evaluated using the standard vertical Bunsen burner fire test. Thermogravimetric analysis (TGA) was used to understand the degradation of coating formulations. Observations by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) demonstrated that significant amounts of additives favored the formation of homogeneous compacted char structures, which were predominantly composed of carbon (C), phosphorus (P) and oxygen (O). These three main components of the char were also found to be in various binding combinations with other lighter elements like nitrogen (N) and hydrogen (H) as illustrated by the attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy results. X-ray photoelectron spectroscopy (XPS) further suggest that polyethylene([(CH₂–C₂H₂–CH₂)ⁿ⁻]) free radicals were abundant on the char surface for the two best formulations and the binding energy of this radical promoted the formation of aromatic carbon chains that enhanced the char's thermal stability. This means that the selection of appropriate additives and combinations of flame-retardant ingredients could significantly change the morphology of the char layer and improve its thermal stability during fire exposure.

Environmental Sustainability of Open-Pit Coal Mining Practices at Baganuur, Mongolia

In this paper, we studied one of the largest coal mines in Mongolia, the Baganuur Coal Mine, in terms of environmental sustainability related to mining practices, with a focus on discharged water and waste sediments. The present quality and potential for future pollution were assessed. Based on World Health Organization and Mongolian guidelines, groundwater pumped from the mining operations could be used for drinking and domestic purposes. In addition, based on the Na absorption ratio, groundwater samples from GW-2 and GW-3 could be used as agriculture water supplies with salinity reduction, or used to grow halophytes as a measure for desertification control and pasture production. All waste soil samples appeared to have a desertification potential. Dust particles smaller than 150 μm comprised more than 80% of soil samples, which had arsenic levels higher than the Mongolian soil pollution standards. In addition, soil collected between coal seams (S-5) showed high sulphur content based on X-ray fluorescence (XRF) and scanning electron microscopy–energy dispersive X-ray (SEM-EDX) spectroscopy analyses, strong potential for producing acid mine drainage in the analysis of pH of net acid generation and net acid production potential, and potential for leaching of metals, such as Co. Therefore, the Baganuur Coal Mine requires soil pollution control measures to mitigate the risks of dust and desertification. In this perspective, mine groundwater could be used to reduce environmental stresses by supporting pasture crops such as halophytes on waste disposal sites, thereby preventing dust issues and desertification. Continuous efforts, including monitoring and enacting environmental management measures, are needed from both the mining company and the government to ensure sustainable mine development.

Hematite-catalysed scorodite formation as a novel arsenic immobilisation strategy under ambient conditions

Scorodite is an important mineral not only for arsenic (As) removal from industrial wastewaters but also in the mobility and final fate of As in waste rocks, contaminated soils and sediments, and mine tailings. Because of the mineral's high As-loading capacity and stability, numerous studies have been done to understand its formation. Unfortunately, most of these studies were limited to elevated temperatures (>70 °C), so the processes involved in scorodite formation under ambient conditions remain unclear. This study provides evidence of the catalytic effects of hematite on the formation of scorodite at 25 °C in a pyrite-rich natural geologic material. Scorodite peaks were detected in the XRD patterns of the leaching residues with and without hematite, but those in the former were stronger and more pronounced than the latter. These results suggest that the formation of scorodite was catalysed by hematite, a generalisation that is further supported by strong characteristic IR absorption bands of scorodite at 819 cm^-1 (As-O bending vibration), 785 and 725 cm^-1 (As-O stretching vibrations), and 2990 cm^-1 (OH-vibration) as well as the distinct XPS binding energies of Fe(III)-As (709.7 eV), As(V)-O (44.8, 44.31 and 43.7 eV), O^2- (530.5 eV) and coordinated water (531.3 eV) in scorodite. This phenomenon could be attributed to three possible mechanisms: (1) more rapid precipitation promoted by the "seeding" effect of hematite particles, (2) additional supply of Fe³⁺ from hematite dissolution under acidic conditions, and (3) enhanced oxidations of Fe²⁺ to Fe³⁺ and As(III) to As(V) on the surface of hematite.

The solid-phase partitioning of arsenic in unconsolidated sediments of the Mekong Delta, Vietnam and its modes of release under various conditions

Arsenic (As) contamination of the groundwater in the Mekong Delta is a serious problem affecting millions of people who rely on this important resource for drinking and agriculture. In this study, borehole cores up to a depth of 40 m were collected in the Vietnamese-side of the delta, and the solid-phase partitioning of As with depth was investigated to understand the factors and processes controlling the release of this toxic element under oxic, acidic and reducing conditions. The results showed that in most of the sediments, substantial amounts of As are partitioned with exchangeable phases that are easily released into solution. Two borehole cores obtained between the Hau and Tien Rivers also had significantly high As partitioned with organic/sulfide phases and one of these cores had abundant As-bearing pyrite in 1-m thick peat layers. Leaching experiments in deionized (DI) water coupled with principal component analysis suggest that As release was controlled by sorption-desorption reactions with clays/phyllosilicates (i.e., kaolinite, muscovite and clinochlore), proton-promoted dissolution of iron-oxyhydroxides, and oxidation of pyrite/organic matter. The mobility of As was further promoted under acidic conditions in the presence of chloride (Cl^-), which suggests that seasonal drying/flooding episodes generating acid sulfate soils, as well as salt water intrusion due to excessive groundwater abstraction may exacerbate this problem in the future.

A review of recent strategies for acid mine drainage prevention and mine tailings recycling

Acid mine/rock drainage (AMD/ARD), effluents with low pH and high concentrations of hazardous and toxic elements generated when sulfide-rich wastes are exposed to the environment, is considered as a serious environmental problem encountered by the mining and mineral processing industries around the world. Remediation options like neutralization, adsorption, ion exchange, membrane technology, biological mediation, and electrochemical approach have been developed to reduce the negative environmental impacts of AMD on ecological systems and human health. However, these techniques require the continuous supply of chemicals and energy, expensive maintenance and labor cost, and long-term monitoring of affected ecosystems until AMD generation stops. Unfortunately, the formation of AMD could persist for hundreds or even thousands of years, so these approaches are both costly and unsustainable. Recently, two alternative strategies for the management of AMD and mine tailings are gaining much attention: (1) prevention techniques, and (2) mine waste recycling. In this review, recent advances in AMD prevention techniques like oxygen barriers, utilization of bactericides, co-disposal and blending, and passivation of sulfide minerals are discussed. In addition, recycling of mine tailings as construction and geopolymer materials to reduce the amounts of wastes for disposal are introduced.