Simultaneous leaching of arsenite, arsenate, selenite and selenate, and their migration in tunnel-excavated sedimentary rocks: II. Kinetic and reactive transport modeling

Fluoride leaching from tuff breccia and its removal by natural and commercial adsorbents

In Japan, the concentration of fluoride (F-) leached from rocks, such as tuff breccia, excavated in tunnel construction projects often exceeds the Japanese environmental standard of 0.8 mg/L. Because of this, proper disposal methods are necessary for managing F--bearing excavated rocks. One effective solution based on circular economy is the use of an adsorption layer system. This system can simultaneously prevent the migration of F- released from excavated rocks and allow the recycling of this construction waste material. To determine the most suitable material for the disposal of excavated F--bearing tuff breccia from a tunnel construction in Hokkaido, Japan, four types of natural geological materials (S-1, S-2, S-3, and S-4) obtained near the tunnel construction site, as well as three types of commercial adsorbents (calcium (Ca), magnesium (Mg), and CaMg adsorbents) were selected for evaluation. The batch adsorption test results showed that S-1 and S-4 had high adsorption capacities for F-, and the adsorption process followed the Langmuir isotherm. The adsorption of F- to the natural adsorbents was strongly influenced by the pH and the presence of bicarbonate ions (HCO3-), but unaffected by chloride (Cl-) and sulfate (SO42-). There was also a strong positive correlation between the abundance of amorphous aluminum (Al) and iron (Fe) extracted and the adsorption of F-, indicating the importance of ion exchange reactions associated with surface OH- in immobilizing F-. Meanwhile, the Mg-bearing adsorbent exhibited the highest adsorption affinity for F- among the commercial adsorbents. This was attributed to adsorption through electrostatic interactions and coprecipitation with magnesium hydroxide (Mg(OH)2) formed during the hydration of magnesium oxide (MgO). To effectively incorporate these adsorbents into the adsorption layer system, parameters such as permeability and residence time need to be determined in order to maximize the retention of F- through adsorption, ion exchange and coprecipitation reactions.

Soil selenium enrichment in the Loess Plateau of China: Geogenic evidence, spatial distribution, and it's influence factors

The selenium (Se)-deficient soil belt in China has gained widespread attention. During large-scale soil surveys in China, Se-rich soils within low-Se belts have been identified. However, the sources of Se in those soils and the controlling factors for their enrichment remain unclear. Here we summarize Se concentrations and spatial distributions in the Yuanzhou district of the Loess Plateau. We evaluated Se variations in soil profiles, Se migration into water bodies, and considered soil indicators, topographic characteristics and the influence of land-use types on soil Se concentrations. The average Se concentration in the topsoil of the Yuanzhou district was 0.164 μg/g. High-Se soils (>0.222 μg/g) were found in the western valley plain and the southern red bed hilly area, as well as sporadically in higher elevation forestland and grassland areas in the east. Enrichment of Se in the topsoil in the eastern and southern areas was primarily due to Se adsorption and accumulation by soil organic matter as well as enrichment in gypsum, berlinite, and clay minerals during soil formation. Widespread enrichment in the southern area was linked to high Se concentrations in red Tertiary sedimentary rocks. In the western area, enrichment of Se in topsoil was found on both sides of the Qingshui River at low elevations with gentle slopes, with river water being the primary carrier of Se enrichment. These findings provide valuable insights into the epigenetic geochemical behavior of soil Se in China's low-Se belt that accounts for development of Se-rich soils in the region.

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.

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.

Effects of Environmental Factors on the Leaching and Immobilization Behavior of Arsenic from Mudstone by Laboratory and In Situ Column Experiments

Hydrothermally altered rocks generated from underground/tunnel projects often produce acidic leachate and release heavy metals and toxic metalloids, such as arsenic (As). The adsorption layer and immobilization methods using natural adsorbents or immobilizer as reasonable countermeasures have been proposed. In this study, two sets of column experiments were conducted, of which one was focused on the laboratory columns and other on the in situ columns, to evaluate the effects of column conditions on leaching of As from excavated rocks and on adsorption or immobilization behavior of As by a river sediment (RS) as a natural adsorbent or immobilizer. A bottom adsorption layer consisting of the RS was constructed under the excavated rock layer or a mixing layer of the excavated rock and river sediment was packed in the column. The results showed that no significant trends in the adsorption and immobilization of As by the RS were observed by comparing laboratory and in situ column experiments because the experimental conditions did not influence significant change in the leachate pH which affects As adsorption or immobilization. However, As leaching concentrations of the in situ experiments were higher than those of the laboratory column experiments. In addition, the lower pH, higher Eh and higher coexisting sulfate ions of the leachate were observed for the in situ columns, compared to the results of the laboratory columns. These results indicate that the leaching concentration of As became higher in the in situ columns, resulting in higher oxidation of sulfide minerals in the rock. This may be due to the differences in conditions, such as temperature and water content, which induce the differences in the rate of oxidation of minerals contained in the rock. On the other hand, since the leachate pH affecting As adsorption or immobilization was not influenced significantly, As adsorption or immobilization effect by the RS were effective for both laboratory and in situ column experiments. These results indicate that both in situ and laboratory column experiments are useful in evaluating leaching and adsorption of As by natural adsorbents, despite the fact that the water content which directly affects the rate of oxidation is sensitive to weathering conditions.

A novel arsenic immobilization strategy via a two-step process: Arsenic concentration from dilute solution using schwertmannite and immobilization in Ca-Fe-AsO4 compounds

Acid mine drainage (AMD) with toxic arsenic (As) is commonly generated from the tailings storage facilities (TSFs) of sulfide mines due to the presence of As-bearing sulfide minerals (e.g., arsenopyrite, realgar, orpiment, etc.). To suppress As contamination to the nearby environments, As immobilization by Ca-Fe-AsO₄ compounds is considered one of the most promising techniques; however, this technique is only applicable when As concentration is high enough (>1 g/L). To immobilize As from wastewater with low As concentration (~10 mg/L), this study investigated a two-step process consisting of concentration of dilute As solution by sorption/desorption using schwertmannite (Fe₈O₈(OH)_8-2x(SO₄)_x; where (1 ≤ x ≤ 1.75)) and formation of Ca-Fe-AsO₄ compounds. Arsenic sorption tests indicated that As(V) was well adsorbed onto schwertmannite at pH 3 (Q_max = 116.3 mg/g), but its sorption was limited at pH 13 (Q_max = 16.1 mg/g). A dilute As solution (~11.2 mg/L As) could be concentrated by sorption with large volume of dilute As solution at pH 3 followed by desorption with small volume of eluent of which pH was 13. The formation of Ca-Fe-AsO₄ compounds from As concentrate solution (2 g/L As(V)) was strongly affected by temperature and pH. At low temperature (25-50 °C), amorphous ferric arsenate was formed, while at high temperature (95 °C), yukonite (Ca₂Fe_3-5(AsO₄)₃(OH)_4-10·xH₂O; where x = 2-11) and johnbaumite (Ca₅(AsO₄)₃OH) were formed at pH 8 and 12, respectively. Among the synthesized products, johnbaumite showed strongest As retention ability even under acidic (pH < 2) and alkaline (pH > 9) conditions.

Exploring the role of Fe species from biochar-iron composites in the removal and long-term immobilization of SeO42- against competing oxyanions

Carbothermal reduction is a convenient and cost-effective method to produce biochar (BC) supported iron-based nano-particles (INP) for oxyanion contaminants removal. However, considering the possible desorption of the target oxyanion during change of the surrounding environment, the detailed removal mechanisms remain unclear and the long-term efficiency of different INPs cannot be predicted. In this study, different BC/Fe composites were synthesized by controlling the pyrolysis temperatures (500-800 °C). BC/Fe₃O₄ composite synthesized at 500 °C (BC/Fe₅₀₀) possessed the strongest surface acidity thus with the best SeO₄^2- removal performance, and BC/Fe⁰/Fe₃O₄ composite synthesized at 650 °C (BC/Fe₆₅₀) possessed the best reducing ability toward SeO₄^2-. Through the co-removal experiments (SeO₄^2- and common competing oxyanions co-existed) and the investigation of Se stability loaded on BC/Fe composites, the removal of SeO₄^2- by BC/Fe₅₀₀ through highly reversible adsorption could not achieve long-term immobilization of Se, making it an appropriate adsorbent for pre-treatment only, while the efficient reduction of SeO₄^2- to Se⁰ by BC/Fe₆₅₀ could largely improve its long-term stability. This study supplies a possible strategy for Se immobilization against common competing oxyanions.

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.

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.

A worksheet-based tool to implement reactive transport models in COMSOL Multiphysics

The increasing needs for modelling of reactive transport phenomena in different areas of environmental modelling have led to the development of many numerical codes. However, many of them suffer from a lack of flexibility, which hinders the adaptation of the codes to new problems. Moreover, in many cases, changes can be done by a very reduced group of people, and often by a single person, the main developer. Implementation platforms based on multiphysics modelling removes these barriers, although until now within that programming environments has been only possible the coupling of geochemical codes to transport equations using operator splitting techniques. This paper presents the EE4MGM tool, a MS Excel worksheet, provided in supplementary material, for the edition and complete implementation of reactive transport models in COMSOL. The tool automatically generates the code needed to solve the desired reactive transport problem by selecting only which species make up the geochemical system. This way, the numerical model will be completely adapted to the idealisation to be applied, being able to choose easily and effortlessly from a wide range of different levels of conceptual complexity. The organization of data input and the equation libraries obtained for the implementation in the multiphysics COMSOL environment are first described. Afterwards, two examples, in one and two-dimensional domains, to check the utility of the tool are presented.

Integrating biometallurgical recovery of metals with biogenic synthesis of nanoparticles

Industrial activities, such as mining, electroplating, cement production, and metallurgical operations, as well as manufacturing of plastics, fertilizers, pesticides, batteries, dyes or anticorrosive agents, can cause metal contamination in the surrounding environment. This is an acute problem due to the non-biodegradable nature of metal pollutants, their transformation into toxic and carcinogenic compounds, and bioaccumulation through the food chain. At the same time, platinum group metals and rare earth elements are of strong economic interest and their recovery is incentivized. Microbial interaction with metals or metals-bearing minerals can facilitate metals recovery in the form of nanoparticles. Metal nanoparticles are gaining increasing attention due to their unique characteristics and application as antimicrobial and antibiofilm agents, biocatalysts, in targeted drug delivery, for wastewater treatment, and in water electrolysis. Ideally, metal nanoparticles should be homogenous in shape and size, and not toxic to humans or the environment. Microbial synthesis of nanoparticles represents a safe, and environmentally friendly alternative to chemical and physical methods. In this review article, we mainly focus on metal and metal salts nanoparticles synthesized by various microorganisms, such as bacteria, fungi, microalgae, and yeasts, as well as their advantages in biomedical, health, and environmental applications.

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.

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.

Distribution of Selenium in the Soil–Plant–Groundwater System: Factors Controlling Its Bio-Accumulation

Selenium (Se) is an essential micronutrient for humans and animals, but both Se excess and deficiency can cause various health risks. Since Greece is among the European countries where people have very low Se-serum, the present study is focused on the Se distribution in cultivated and non-cultivated plants and relative soil coming from the Neogene basins of Greece (Assopos-Thiva and Attica), aiming to define potential Se-source/es and factors controlling Se bio-accumulation and enrichment in food. The dry weight Se values are relatively low (0.1–0.9 mg/kg) with the highest Se contents in garlic, beet and lettuce from the Assopos basin, where the translocation percentage [(mplant/msoil) × 100] for Se, P and S is much higher compared to non-cultivated Attica basin. There is a diversity between the Se source in soil and coastal groundwater which is used for irrigation in the cultivated Assopos–Thiva basin. The soil pH and oxidizing conditions (Eh) are considered the main driving force to make Se available for plant uptake. Potential sources for Se in Greece are Fe-Cu-Zn-sulphide ores and peat deposits in northern Greece, with a Se content ranging from decades to hundreds of mg/kg. Application of the leaching testing protocol is necessary to select the most appropriate proportion of additives to improve the Se deficiencies in agricultural soil.

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.

Dissolution Test Protocol for Estimating Water Quality Changes in Minerals Processing Plants Operating With Closed Water Circulation

To save freshwater resources and comply with environmental regulations, minerals processing operations are transitioning to partially or fully closed water circulation. However, the accumulation of electrolytes and the addition of reagents lead to changes in water composition and may compromise flotation performance and plant maintenance. As a consequence, costly modifications are often required to cope with these challenges. Therefore, knowledge about water quality variation owing to closed water circulation and its potential effect on the flotation performance is crucial. The experimental methodology presented in this paper targeted three main objectives: (1) predicting the tendency of the accumulation of elements and compounds into the process water during comminution, flotation, and storage in tailings facilities; (2) establishing a relationship between laboratory results and plant historical water quality data; and (3) predicting the potential effect of recycling water on flotation performance. The results obtained with Boliden Kevitsa ore showed a good correlation between the water matrix of the actual process water on-site and that obtained in the ore dissolution tests done in the laboratory. The final water composition came close to the process water in terms of major elements and some of the minor elements. Additionally, the work presented in this paper demonstrated that a dissolution loop allowed us to predict the potential impact of the recycling water on the ore flotability. This methodology could serve as an aid for predicting water quality matrix variation and designing closed water circulation systems at existing and new plants.

Reducing geogenic arsenic leaching from excavated sedimentary soil using zero-valent iron amendment followed by dry magnetic separation: A case study

Although the deep-layer sedimentary soils excavated from construction sites contain low level of geogenic arsenic (As), remediation is necessary when the As leachability exceeds the environmental standard (10 μg/L) in Japan. In this study, the zero-valent iron (ZVI) amendment followed by dry magnetic separation (ZVI-DMS) was implemented for the treatment of a geogenic As-contaminated alkaline sedimentary soil (pH 8.9; 7.5 mg/kg of total As; 0.33 mg/kg of water-extractable As). This technology involves pH adjustment (adding H₂SO₄), ZVI addition, water content reduction (adding water adsorbent CaSO₄·0.5H₂O), and dry magnetic separation. The short-term and long-term As leachability before and after treatment was compared using sequential water leaching tests (SWLT). The results illustrated that As could be removed from the bulk soil through the magnetic separation of As-ZVI complexes, although the amount was limited (about 2% of total As). Moreover, immobilization played a dominant role in suppressing As leaching. The H₂SO₄ addition decreased pH to a circumneutral range and thereby suppress As release. The CaSO₄·0.5H₂O addition also contributed to the pH decrease and reduced As leachability. Besides, CaSO₄·0.5H₂O-dissolution released Ca²⁺ that favored As adsorption, and enhanced dissolved organic carbon (DOC) coagulation that decelerated As dissolution. SWLT results indicated that As leachability from remediated soil satisfied the environmental standard (10 μg/L) in both short-term and long-term perspective. However, the secular stability of treated soil deserves more attention due to the easy re-release of As caused by As-bearing framboidal pyrite oxidation. Additionally, during ZVI-DMS process, there is a need to scientifically decide the dosage of ZVI to avoid excessive addition. Our results demonstrated that ZVI-DMS technology could be a promising remediation strategy for geogenic As contaminated sedimentary soils/rocks.

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.

Transport of arsenic loaded by ferric humate colloid in saturated porous media

The transport behavior of arsenic (As(V)) loaded by ferric humate (HA-Fe) colloid, denoted as HA-Fe/As(V), moving in a saturated quartz sand column, was tested in the laboratory under varying pH values, ionic strengths, and HA and Fe(III) content. The time-fractional advection-dispersion equation (fADE) model was then employed to analyze the observed migration of HA-Fe/As(V). Results showed that the stability of the HA-Fe colloid exhibited an upward trend with an increasing pH and HA content. An increasing HA content led to a decrease in the particle size of the HA-Fe colloid. However, the effect of Fe(III) concentration on colloidal particle size exhibited the opposite phenomenon. The ability of the HA-Fe colloid to load As(V) gradually increased with the increase of the Fe(III) concentration. During the co-transport of the HA-Fe/As(V) colloid, transport of As(V) was promoted with increasing pH, increasing HA and Fe(III) content, and decreasing ionic strength in the saturated porous medium. The transport behavior of As(V) can be well fitted by the fADE model. The model analysis revealed that sub-diffusion of As(V) was weakened in the HA-Fe/As(V) colloid with high HA content. Sub-diffusion of As(V) in the low pH colloid was stronger than that of the high-pH colloid, and the molecular diffusion and mechanical dispersion were more weakened in the high-pH colloid than that of the low-pH colloid. When observing varying ionic strengths, As(V) exhibited stronger sub-diffusion in the HA-Fe/As(V) colloid with a higher ionic strength. As for the Fe(III) content, transport of As(V) was mainly affected by sub-diffusion in the HA-Fe/As(V) colloid with a low Fe(III) content. These findings provided direct and necessary insights into the effects of the HA-Fe colloid on the migration of As(V) throughout saturated porous media under different hydrochemical conditions found in natural environments.

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.

The role of the freshwater oligochaete Limnodrilus hoffmeisteri in the distribution of Se in a water/sediment microcosm

Selenite(IV) and selenate(VI) are the major species of Se in the seleniferous aquatic ecosystem. The redistribution of Se in the water/sediment microcosm by bioturbation remains largely unknown. In this study, the redistribution of Se in the water/sediment microcosm by the benthic oligochaete Limnodrilus hoffmeisteri was assessed. The worms were exposed to 2-40 μg/g dry weight of Se(IV) or Se(VI) in the sediment (diet) for 2 months. The changes in the Se levels in different compartments of the microcosm (sediment, overlying water, and worms) were quantified after 2 weeks and 2 months. The subcellular distribution of Se in the worms were also evaluated. Finally, the volatilization of Se from the two Se sources was estimated. The results showed that Se concentration in the overlying water and Se bioaccumulation in the worms were increased with Se levels in the sediments. Approximately 1.6-9.8% of Se was volatilized in the absence of the worms and was intensified in the presence of the worms (2.1-25.7%). The subcellular distribution witnessed high levels of Se in the cell debris (>60%). Se(IV) and Se(VI) differ in their bioaccumulation, redistribution and the effects on the growth of the worms. Our results suggest that the bioturbation by benthos play an essential role in the redistribution of Se in the water/sediment microcosm.

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.

Acid mine drainage sources and hydrogeochemistry at the Yatani mine, Yamagata, Japan: A geochemical and isotopic study

This paper describes the geochemistry of groundwater and its flow system in the closed Yatani mine in southern Yamagata Prefecture, Japan. The mine is located in a sulfide deposit containing pyrite and has been generating acid mine drainage (AMD). The study was intended to elucidate the formation of AMD and its flow patterns using geological, hydrological, geochemical, and isotopic techniques. The results indicate that AMD is formed by the interaction of groundwater with sulfide minerals, sand slime, and tailings back-filled into excavated mine areas. Groundwater recharge areas were identified on the mountain slope at an elevation of ~900 m. The formation of AMD in the drifts and shaft was more extensive than that in the deeper drainage levels. Principal component analysis was applied to the hydrogeochemical data to identify the causes of AMD formation. The first, second, and third principal components reveal that the increased ion concentrations in mine drainage are a result of water-mineral reactions in excavated mine areas, the contribution of groundwater in deep reductive environments, and isotopic fractionation during precipitation, respectively. A promising method of reducing AMD formation is to prevent contact between dissolved oxygen and sulfide minerals by increasing the drainage level or by filling the shallow underground excavated area with cementitious materials.

Geological and geochemical characterizations of sediments in six borehole cores from the arsenic-contaminated aquifer of the Mekong Delta, Vietnam

The Mekong Delta, situated between Cambodia and Vietnam, is one of the most productive aquifer systems in the region. In recent years, however, several studies have shown that groundwater in several areas of the delta is highly contaminated with arsenic (As). Although more than 80% of the total area of the Mekong Delta is situated in Vietnam, most of the studies have been conducted on the Cambodian-side of the delta. In this study, borehole core samples were collected around the Tien and Hau Rivers, the two main branches of the Mekong River as it enters Vietnam. We present a raw data collection of the chemical and mineralogical composition of distinct lithological features from six borehole core samples drilled up to a depth of 40 m. The data also include the pH, Eh, EC, As, Si, Al, DOC, dissolved heavy metals (Fe and Mn) and major coexisting ions of leachates obtained by leaching the 34 selected sediment samples in deionized water. The information provided in this paper would be useful as a baseline for reactive transport or geochemical modeling to understand and predict As migration in naturally contaminated aquifers under various conditions. For more insights, the reader is referred to our paper entitled “The solid-phase partitioning of arsenic in unconsolidated sediments of the Mekong Delta, Vietnam and its modes of release under various conditions” Huyen et al., 2019.

Zea mays L. Grain: Increase in Nutraceutical and Antioxidant Properties Due to Se Fortification in Low and High Water Regimes

This work aimed to investigate the effect of selenium (Se) and irrigation on the grain yield, on the forms of Se, phenols, and carotenes, and on some antioxidant activities of maize ( Zea mays L.) grains. To reach this goal, a 2 year experiment was undertaken. Maize was fertigated with sodium selenite at the rate of 200 g of Se ha^-1 and grown under two water regimes. While the irrigation did not show a clear effect on the selected parameters, Se fertigation increased the contents of inorganic and organic Se forms, xanthophyll, and salicylic acid. Furthermore, while Se fertigation decreased the hydroxycinnamic acid content, generally higher antioxidant activities were found in Se-treated grains than in the control. These findings suggest that Se fertigation increases most of the nutraceutical values of maize grains, which therefore might improve human and livestock health and could increase the maize grain shelf life and its byproducts.

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.

Suppressive effects of ferric-catecholate complexes on pyrite oxidation

Pyrite, a common gangue mineral in complex sulfide ores and coals, is rapidly oxidized in water by ferric ions and dissolved oxygen to form a very acidic and heavy metal-laden leachate called acid mine drainage (AMD). Carrier-microencapsulation (CME) using Ti⁴⁺, Si⁴⁺, and Al³⁺ was reported as a promising new approach to prevent pyrite oxidation by forming a passivating barrier on the pyrite surface. In CME, the presence of Fe³⁺-catecholate complexes is unavoidable but their effects on pyrite oxidation remain unclear. In this study, the effects of Fe³⁺-catecholate complexes on pyrite oxidation were investigated. Formations of mono-, bis-, and tris-catecholate complexes of Fe³⁺ were verified by UV-Vis spectrophotometry and their speciation with pH was consistent with thermodynamic considerations. Linear sweep voltammetry was conducted to evaluate the redox properties of Fe³⁺-catecholate complexes, and the results indicate that ligands in the three complexes were sequentially oxidized until Fe³⁺ is released. Coating formation on pyrite was confirmed after treatment with mono- and bis-catecholate complexes. Results of SEM-EDX and ATR-FTIR indicate that the coating is composed primarily of iron oxyhydroxide phases. The results of leaching experiments showed that pyrite oxidation was suppressed by Fe³⁺-catecholate complexes via two mechanisms: (1) electron donating effects of the complexes, and (2) formation of a protective coating on pyrite. The results provide not only a better understanding of the effects of Fe³⁺-catecholate complexes on pyrite oxidation but also some possible applications of Fe³⁺-based CME such as the suppression of pyrite oxidation to prevent AMD formation and depression of pyrite floatability in mineral processing.

Arsenic, selenium, boron, lead, cadmium, copper, and zinc in naturally contaminated rocks: A review of their sources, modes of enrichment, mechanisms of release, and mitigation strategies

Massive and ambitious underground space development projects are being undertaken by many countries around the world to decongest megacities, improve the urban landscapes, upgrade outdated transportation networks, and expand modern railway and road systems. A number of these projects, however, reported that substantial portions of the excavated debris are oftentimes naturally contaminated with hazardous elements, which are readily released in substantial amounts once exposed to the environment. These contaminated excavation debris/spoils/mucks, loosely referred to as "naturally contaminated rocks", contain various hazardous and toxic inorganic elements like arsenic (As), selenium (Se), boron (B), and heavy metals like lead (Pb), cadmium (Cd), copper (Cu), and zinc (Zn). If left untreated, these naturally contaminated rocks could pose very serious problems not only to the surrounding ecosystem but also to people living around the construction and disposal sites. Several incidents of soil and ground/surface water contamination, for example, have been documented due to the false assumption that excavated materials are non-hazardous because they only contain background levels of environmentally regulated elements. Naturally contaminated rocks are hazardous wastes, but they still remain largely unregulated. In fact, standard leaching tests for their evaluation and classification are not yet established. In this review, we summarized all available studies in the literature about the factors and processes crucial in the enrichment, release, and migration of the most commonly encountered hazardous and toxic elements in naturally contaminated geological materials. Although our focus is on naturally contaminated rocks, analogue systems like contaminated soils, sediments, and other hazardous wastes that have been more widely studied will also be discussed. Classification schemes and leaching tests to properly identify and regulate excavated rocks that may potentially pose environmental problems will be examined. Finally, management and mitigation strategies to limit the negative effects of these hazardous wastes are introduced.

Simultaneous suppression of acid mine drainage formation and arsenic release by Carrier-microencapsulation using aluminum-catecholate complexes

Pyrite (FeS₂), the most common sulfide mineral in nature, plays an important role in the formation of acid mine drainage (AMD), one of the most serious environmental problems after the closure of mines and mineral processing operations. Likewise, arsenopyrite (FeAsS) is an important sulfide mineral because its dissolution releases toxic arsenic (As) into the environment. To mitigate the serious environmental problems caused by pyrite and arsenopyrite, this study investigated carrier-microencapsulation (CME) using Al-catecholate complexes, a technique that selectively forms protective coatings on the surfaces of sulfide minerals, by electrochemical techniques and batch leaching experiments coupled with surface sensitive characterization techniques. Cyclic voltammetry (CV) of Al-catecholate complexes (mono-, bis-, tris-catecholate) suggest that these three species could be oxidatively decomposed in this order: [Al(cat)₃]^3-→[Al(cat)₂]^-→[Al(cat)]⁺→Al³⁺, and these reactions were irreversible. Among these three species, [Al(cat)]⁺ was the most effective in suppressing pyrite and arsenopyrite oxidations because it requires less steps for complete decomposition than the other two complexes. Analyses of CME treated minerals by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that they were covered with Al-oxyhydroxide (γ-AlO(OH)), which became more extensive at higher [Al(cat)]⁺ concentrations. In addition, this coating was stable even at relatively high applied potentials that simulated surface oxidizing conditions. Based on these results, a detailed mechanism of Al-based CME is proposed: (1) adsorption of [Al(cat)]⁺ on the surface of mineral, (2) oxidative decomposition of [Al(cat)]⁺ and release of "free" Al³⁺, and (3) precipitation and formation of Al-oxyhydroxide coating.

Selenium contamination, consequences and remediation techniques in water and soils: A review

Selenium (Se) contamination in surface and ground water in numerous river basins has become a critical problem worldwide in recent years. The exposure to Se, either direct consumption of Se or indirectly may be fatal to the human health because of its toxicity. The review begins with an introduction of Se chemistry, distribution and health threats, which are essential to the remediation techniques. Then, the review provides the recent and common removal techniques for Se, including reduction techniques, phytoremediation, bioremediation, coagulation-flocculation, electrocoagulation (EC), electrochemical methods, adsorption, coprecipitation, electrokinetics, membrance technology, and chemical precipitation. Removal techniques concentrate on the advantages, drawbacks and the recent achievements of each technique. The review also takes an overall consideration of experimental conditions, comparison criteria and economic aspects.