Current approaches for mitigating acid mine drainage

Prevention of acid rock drainage formation through pyrite inhibition by silica coating

Passive treatment of acid rock drainage (ARD) is a sustainable approach to control ARD, with sulfide inhibition by silica being a promising alternative. In a small-scale column leaching, a total of four cells loaded with pyritic waste rock (11 wt% S) from an operating Cu mine in Sweden were kept in a climatic chamber at a controlled temperature and humidity. The waste rock was leached for 11 weeks before treatment using alkaline silicate solution was applied, without pH buffer and adjuster. One cell was left untreated, whereas the others were treated with silicate solution as a source of dissolved silica, with and without H2O2 pre-oxidation. The pH in silica-treated cells generated leachate with circumneutral pH until the end of the leaching cycle, whereas sulfide oxidation accelerated in the absence of treatment. Leachate quality in all Si-treated cells improved, as evidenced by the suppressed release of sulfur and other metals (e.g., Al, Fe, Cu, Co, Mn, and Ni). Upon treatment with a longer contact time, silica (SiO2) layer developed on waste rock and inhibited pyrite. The layer remained stable upon extended exposure to air and water for up to 10 weeks after treatment. Despite forming a siliceous Fe-O phase, H2O2 pre-oxidation resulted in indirect oxidation of sulfides and other phases. With an excess of silicate solution and at alkaline pH, pyrite surfaces are devoid of coating and metal ions were mobilized. Finally, this study suggested that treatment of pyritic waste rock using silica can attenuate ARD formation and prevent metal leaching by pyrite inhibition and maintaining a circumneutral pH environment or both.

Critical Review of Field Studies of Chemical Surface Coatings to Mitigate Leaching from Mining Wastes

Source control of mine drainage and leachate from mining wastes is ideal, but there is limited use of control methods aside from removal of wastes to repositories or capping wastes in situ. In some locations, neither removal nor capping is possible; thus, there is renewed interest in understanding technologies for coating mining wastes. This paper presents the current state of knowledge from a critical review of the few field case studies where chemical coating technologies have been applied to mined surfaces (e.g. a highwall) or mining wastes. It also includes laboratory studies where the same technology was examined on the same type of waste material as the field studies. The chemicals used in field studies for coating mining wastes were magnesium oxide, potassium permanganate, silicate compounds, phosphate compounds, and a furfuryl alcohol resin sealant. Descriptions of the methods employed, and results obtained are provided. Limitations of the individual studies and of the technologies in general are discussed. Potential reasons why coating technologies have not been commonly used are presented, along with further research needs.

The comparison of 7.5 and 15% hydrogen peroxide as oxidizing agent in static tests of acid mine drainage potential in Indonesia

Static tests of acid mine drainage potential are an important part of mining water management. Net acid generation (NAG) test is widely used in Indonesian coal mines because of its convenience. This test uses H2O2 to oxidize sulfide minerals within rock samples to determine their net acid-forming capacity. This study aimed to determine the difference between H2O2 at concentrations of 7.5% (the standard in several Indonesian coal mines) and 15% (the standard of the Acid Rock Drainage Test Handbook and Indonesian National Standard) in categorizing rocks as potentially acid-forming and non-acid-forming and in terms of NAG solution characteristics. A total of 564 rock samples collected from two Indonesian coal mining sites were analyzed using pH, NAG, total sulfur, and acid-base-accounting tests. The results of the study showed that there was no significant difference in rock classification or the behavior of contaminants in the NAG solution between 7.5% and 15% H2O2. The characteristics of sulfide minerals in Indonesian coal mines were the main factors influencing the results of the NAG test and behavior of contaminants in the NAG solution. Therefore, H2O2 at a concentration of 7.5% can be used in Indonesian coal mines with relatively low total sulfur concentrations (<5%) and minerals in framboidal form.

Remediation potential of mining, agro-industrial, and urban wastes against acid mine drainage

Acid mine drainage (AMD) poses serious consequences for human health and ecosystems. Novel strategies for its treatment involve the use of wastes. This paper evaluates the remediation potential of wastes from urban, mining and agro-industrial activities to address acidity and high concentrations of potentially toxic elements (PTE) in AMD. Samples of these waste products were spiked with an artificially prepared AMD, then pH, electrical conductivity (EC), and PTE concentrations in the leachates were measured. The artificial AMD obtained through oxidation of Aznalcóllar's tailing showed an ultra-acid character (pH - 2.89 ± 0.03) and extreme high electrical conductivity (EC - 3.76 ± 0.14 dS m^-1). Moreover, most PTE were above maximum regulatory levels in natural and irrigation waters. Wastes studied had a very high acid neutralising capacity, as well as a strong capacity to immobilise PTE. Inorganic wastes, together with vermicompost from pruning, reduced most PTE concentrations by over 95%, while organic wastes retained between 50 and 95%. Thus, a wide range of urban, mining, and agro-industrial wastes have a high potential to be used in the treatment of AMD. This study provides valuable input for the development of new eco-technologies based on the combination of wastes (eg. Technosols, permeable reactive barriers) to remediate degraded environments.

Straw Biochar at Different Pyrolysis Temperatures Passivates Pyrite by Promoting Electron Transfer from Biochar to Pyrite

To control acid mine drainage (AMD) at source, biochar, a new green and environmentally friendly passivator has been introduced to passivate pyrite. However, the raw material and pyrolysis temperature largely determine the physical and chemical properties of biochar, the causal relationship between biochar and pyrite and the underlying mechanism are still unknown. Here, biochar materials (rice-straw biochar (RSB) and sugarcane bagasse biochar (SBB)) at different pyrolysis temperatures (300–600 °C) were utilized for the passivation of pyrite. The results of our investigations revealed that the passivation ability of RSB was superior to that of SBB. The addition of RSB with higher pyrolysis temperatures could greatly enhance the passivation efficiency of pyrite. RSB-500 (produced at a pyrolysis temperature of 500 °C) achieved the best passivation effect on pyrite. RSB can form Fe-O bonds through C=O bonding with pyrite. Moreover, the addition of RSB created a reducing environment in the mixture system because of its strong electron-donation capacity (EDC) and altered the energy-band structure of pyrite, which promoted the transfer of electrons from biochar to pyrite. On the contrary, the addition of SBB did not result in the formation of Fe-O bonds with pyrite. In addition, the EDC of SBB was also lower than that of RSB and it had almost no effect on the band structure of pyrite. Hence it did not alter the direction of the electron migration. These findings shed light on the mechanism of biochar passivation of pyrite and provide a theoretical foundation for selecting suitable biochar materials for AMD prevention at source.

Resource Utilization of Acid Mine Drainage (AMD): A Review

Acid mine drainage (AMD) is a typical type of pollution originating from complex oxidation interactions that occur under ambient conditions in abandoned and active mines. AMD has high acidity and contains a high concentration of heavy metals and metalloids, posing a serious threat to ecological systems and human health. Over the years, great progress has been made in the prevention and treatment of AMD. Remediation approaches like chemical neutralization precipitation, ion exchange, membrane separation processes, and bioremediation have been extensively reported. Nevertheless, some limitations, such as low efficacy, excessive consumption of chemical reagents, and secondary contamination restrict the application of these technologies. The aim of this review was to provide updated information on the sustainable treatments that have been engaged in the published literature on the resource utilization of AMD. The recovery and reuse of valuable resources (e.g., clean water, sulfuric acid, and metal ions) from AMD can offset the cost of AMD remediation. Iron oxide particles recovered from AMD can be applied as adsorbents for the removal of pollutants from wastewater and for the fabrication of effective catalysts for heterogeneous Fenton reactions. The application of AMD in beneficiation fields, such as activating pyrite and chalcopyrite flotation, regulating pulp pH, and leaching copper-bearing waste rock, provides easy access to the innovative utilization of AMD. A review such as this will help researchers understand the progress in research, and identify the strengths and weaknesses of each treatment technology, which can help shape the direction of future research in this area.

Effect of land-use change along altitudinal gradients on soil micronutrients in the mountain ecosystem of Indian (Eastern) Himalaya

Management of soil micronutrients for better crop production needs a sound understanding of their status and causes of variability. This is more relevant for acid soils of the mountain ecosystem of Eastern Himalaya (Northeast India). We assessed the status, and the effect of land uses along altitudinal gradients (14 to 4090 masl) on soil properties and micronutrient concentrations (DTPA extractable Fe, Mn, Cu, and Zn) across the region. Soils varied widely in micronutrient concentrations: Fe from 0.665 to 257.1 mg kg⁻¹ while Mn, Cu, and Zn from traces to 93.4, 17.1, and 34.2 mg kg⁻¹, respectively. On conversion of evergreen forests (EF) to upland agriculture (Shifting—SC and Settled—SA) and plantation (PH), Mn, Cu, and Zn concentrations decreased significantly from 30.5, 1.74, and 2.13 mg kg⁻¹ to 6.44–17.8, 0.68–0.81, and 1.06–1.42 mg kg⁻¹, respectively. Grassland (GL) and lowland paddy (LP) had comparable Fe, Mn, and Cu concentrations (except Zn). Degradation of EF to scrubland (SL) recorded the lowest Mn (5.91 mg kg⁻¹), Cu (0.59 mg kg⁻¹), and Zn (0.68 mg kg⁻¹) concentrations. Fe concentration was however increased in degraded SL (+ 73%) over EF (48.7 mg kg⁻¹). The distribution of micronutrients among the land uses was inconsistent and followed the order: (i) Fe: SL > PH > LP > EF > GL > SC > SA, (ii) Mn: EF > GL > LP > PH > SC > SA > SL; (iii) Cu: EF > GL > LP > SC > SA = PH > SL; and (iv) Zn: GL > EF > LP > SC > SA > PH > SL. Four micronutrients responded differently and followed a non-linear, 6th—order polynomial trend along the altitudinal gradients (< 500 to 4100 masl). Peak concentrations of Fe, Mn, and Cu were recorded at 1001–2000 m while Zn was recorded at > 4000 masl. The variability (54–64%) in soil micronutrients was mainly controlled by three key soil properties: acidity, clay, and organic carbon contents. Thus, altitude-specific land-use management holds significance in the distribution of available soil micronutrients in hilly ecosystems.

Effect of growth conditions on cell wall composition and cadmium adsorption in Chlorella vulgaris: A new approach to biosorption research

Biosorption of toxic metals in microalgae is a process relying on the presence of cell wall reactive groups acting as binding sites. This work studied the effect of culture conditions on the outer cell wall composition of C. vulgaris and cadmium biosorption. The experiments were conducted in continuous culture under light and nitrogen limitation at two growth rates (0.4 and 0.2 d^-1). Functional groups were profiled using ATR-FTIR spectrometry, and total cadmium biosorption was assayed. Significant differences in composition were attested the most salient being the absence of carboxyl groups in the light deprived states and a larger number of carbohydrates and amino groups in the nitrogen deprived cultures, particularly amino groups from deacetylated D-glucosamine polysaccharides. Higher biosorption was obtained with the nitrogen-restricted biomass, reaching a maximum of 11.9 mg_Cd/g_biomass, as compared to a minimum of 8.0 mg_Cd/g_biomass achieved in the light-restricted states. The increased biosorption exhibited by nitrogen-restricted strains was attributed to the deacetylated amino groups that have enhanced cation affinity. This work has shown that the characteristics of the outer cell wall can be engineered by culture conditions to improve biosorption, providing a new approach that opens up new research frontiers for the biosorption of hazardous metals.

Variation of green liquor dregs from different pulp and paper mills for use in mine waste remediation

The geotechnical, chemical, and mineralogical properties of green liquor dregs (GLDs) generated as byproducts from five paper mills were investigated to assess their buffering and heavy metal immobilization capacities and their roles as water and oxygen barriers. One type of GLD was further studied to test the effects of the retrieval process and the storage, drying, and hydration of GLD. The high water retention capacity of the GLDs is valuable for limiting O₂ diffusion. Laboratory results showed that the GLDs had hydraulic conductivities of 3.7 × 10^-9-4.6 × 10^-8 m/s and varied regularly in plasticity. The chemical and mineralogical compositions of the GLDs varied greatly, reflecting the raw material used to produce paper and the process used to retrieve GLDs. Although they had high total heavy metal contents, none of the leached elements from the GLDs (L/S 10 cm³/kg) exceeded the European Union's limits for landfills of non-hazardous waste. The GLDs exhibited high buffering capacities. In a supplementary test, the buffering capacities varied (0.0041-0.0114 M H⁺/g GLD) over 72 d after acid was added to the GLD. Changing the filtration process did not greatly affect the GLDs' properties but mainly affected the hydraulic conductivity, total heavy metal contents and sulfur content. Analyzing the storage of GLDs is necessary in the mining industry because remediation measures require large amounts of material over short periods. The buffering capacity of the dried GLD decreased slightly. The effect of dewatering caused by the mixing of 2% Na-lignosulfate with GLD (w/w) was low.

Prevention of sulfide oxidation in waste rock by the addition of lime kiln dust

During the operation of a mine, waste rock is often deposited in heaps and usually left under ambient conditions allowing sulfides to oxidize. To focus on waste rock management for preventing acid rock drainage (ARD) formation rather than ARD treatment could avoid its generation and reduce lime consumption, costs, and sludge treatment. Leachates from 10 L laboratory test cells containing sulfide-rich (> 60% pyrite) waste rock with and without the addition of lime kiln dust (LKD) (5 wt.%) were compared to each other to evaluate the LKD's ability to maintain near neutral pH and reduce the sulfide oxidation. Leaching of solely waste rock generated an acidic leachate (pH < 1.3) with high concentrations of As (21 mg/L), Cu (20 mg/L), Fe (18 g/L), Mn (45 mg/L), Pb (856 μg/L), Sb (967 μg/L), S (17 g/L), and Zn (23 mg/L). Conversely, the addition of 5 wt.% LKD generated and maintained a near neutral pH along with decreasing of metal and metalloid concentrations by more than 99.9%. Decreased concentrations were most pronounced for As, Cu, Pb, and Zn while S was relatively high (100 mg/L) but decreasing throughout the time of leaching. The results from sequential extraction combined with element release, geochemical calculations, and Raman analysis suggest that S concentrations decreased due to decreasing sulfide oxidation rate, which led to gypsum dissolution. The result from this study shows that a limited amount of LKD, corresponding to 4% of the net neutralizing potential of the waste rock, can prevent the acceleration of sulfide oxidation and subsequent release of sulfate, metals, and metalloids but the quantity and long-term stability of secondary minerals formed needs to be evaluated and understood before this method can be applied at a larger scale.

Walnut Shell Powder Can Limit Acid Mine Drainage Formation by Shaping the Bacterial Community Structure

The formation of acid mine drainage (AMD), which results from the oxidation of sulfur minerals by air and water, can be accelerated by acidophilic and chemolithotrophic bacteria such as Acidithiobacillus ferrooxidans. Our previous study revealed that walnut shell powder and its phenolic component inhibit the growth of A. ferrooxidans. However, their inhibitory effect on AMD formation in the environment needs verification. We established a bioleaching system to test whether walnut shell powder and its phenolic component can limit AMD formation. Our results showed that lignin and cellulose isolated from walnut shell decreased metal ion concentrations through absorption, whereas the phenolic component increased pH by downregulating the expression of Fe²⁺-oxidizing genes and rus operon genes of A. ferrooxidans. Only walnut shell powder showed an excellent ability to curb AMD by binding metal ions and increasing the pH value. On probing deeper into the alteration of the bacterial community structure in the bioleaching system, we found that the bacterial community became more diverse-the amount of A. ferrooxidans decreased and that of some non-acidophilic bacteria increased. The bacterial community in samples treated with walnut shell powder or its phenolic component had low abundance in the pathways of metabolism and energy production, as determined by phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt). In other words, preponderant microbes, mainly A. ferrooxidans, lacked energy to grow well in the treated samples. Our findings provide a practical applicability of walnut shell powder to reduce leaching from a complex environmental community.

Environmental Decision Making on Acid Mine Drainage Issues in South Africa: An Argument for the Precautionary Principle

This paper examines the issue of acid mine drainage in South Africa and environmental decision making processes that could be taken to mitigate the problem in the context of both conventional risk assessment and the precautionary principle. It is argued that conventional risk assessment protects the status quo and hence cannot be entirely relied upon as an effective tool to resolve environmental problems in the context of South Africa, a developing country with complex environmental health concerns. The complexity of the environmental issues is discussed from historical and political perspectives. An argument is subsequently made that the precautionary principle is an alternative tool, and its adoption can be used to empower local communities. This work, therefore, adds to new knowledge by problematising conventional risk assessment and proposing the framing of the acid mine drainage issues in a complex and contextual scenario of a developing country-South Africa.

Recent Developments for Remediating Acidic Mine Waters Using Sulfidogenic Bacteria

Acidic mine drainage (AMD) is regarded as a pollutant and considered as potential source of valuable metals. With diminishing metal resources and ever-increasing demand on industry, recovering AMD metals is a sustainable initiative, despite facing major challenges. AMD refers to effluents draining from abandoned mines and mine wastes usually highly acidic that contain a variety of dissolved metals (Fe, Mn, Cu, Ni, and Zn) in much greater concentration than what is found in natural water bodies. There are numerous remediation treatments including chemical (lime treatment) or biological methods (aerobic wetlands and compost bioreactors) used for metal precipitation and removal from AMD. However, controlled biomineralization and selective recovering of metals using sulfidogenic bacteria are advantageous, reducing costs and environmental risks of sludge disposal. The increased understanding of the microbiology of acid-tolerant sulfidogenic bacteria will lead to the development of novel approaches to AMD treatment. We present and discuss several important recent approaches using low sulfidogenic bioreactors to both remediate and selectively recover metal sulfides from AMD. This work also highlights the efficiency and drawbacks of these types of treatments for metal recovery and points to future research for enhancing the use of novel acidophilic and acid-tolerant sulfidogenic microorganisms in AMD treatment.

Use of Walnut Shell Powder to Inhibit Expression of Fe(2+)-Oxidizing Genes of Acidithiobacillus Ferrooxidans

Acidithiobacillus ferrooxidans is a Gram-negative bacterium that obtains energy by oxidizing Fe(2+) or reduced sulfur compounds. This bacterium contributes to the formation of acid mine drainage (AMD). This study determined whether walnut shell powder inhibits the growth of A. ferrooxidans. First, the effects of walnut shell powder on Fe(2+) oxidization and H⁺ production were evaluated. Second, the chemical constituents of walnut shell were isolated to determine the active ingredient(s). Third, the expression of Fe(2+)-oxidizing genes and rus operon genes was investigated using real-time polymerase chain reaction. Finally, growth curves were plotted, and a bioleaching experiment was performed to confirm the active ingredient(s) in walnut shells. The results indicated that both walnut shell powder and the phenolic fraction exert high inhibitory effects on Fe(2+) oxidation and H⁺ production by A. ferrooxidans cultured in standard 9K medium. The phenolic components exert their inhibitory effects by down-regulating the expression of Fe(2+)-oxidizing genes and rus operon genes, which significantly decreased the growth of A. ferrooxidans. This study revealed walnut shell powder to be a promising substance for controlling AMD.