Prediction of acid mine drainage generation potential of various lithologies using static tests: Etili coal mine (NW Turkey) as a case study

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

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

The geochemistry and hydrology of coal waste rock dumps: A systematic global review

Coal has been a major global resource for at least the past 250 years. The major waste product of coal mining is waste rock, which is stored in dumps of various sizes. Although the adverse effects of coal waste rock dumps on ecosystems and human health are widely recognised, there is little information on their internal hydrological and geochemical processes in the peer-reviewed literature. Coal and conventional waste rock dumps share many similarities, but coal waste rock dumps differ in structure, organic matter content, and size, which can affect the timing and rate of aqueous chemical release. In this global systematic review, we identify limited links to climate setting and dump construction, and inconsistent reporting of sampling and monitoring approaches, as limitations to the generalisation of findings. Furthermore, sources of aqueous constituents of interest (COIs) are not routinely or adequately identified, which can lead to incorrect assumptions regarding COI availability and geochemical mobility. Water flow regimes within dumps are dominated by matrix and/or preferential flow, depending on dump texture; these flow mechanisms exert a primary control on patterns of aqueous COI release. The inability to successfully transfer COI release rates from laboratory or field scale trials to operational scale dumps is primarily due to limitations of testing methods and fundamental characteristics of scale. Prediction of future release rates is hampered by a lack of long-term studies that fully characterise geochemistry (e.g., source and COI production rates) as well as dump hydrology (e.g., water balance, water migration). Five critical elements to include in best practice investigations are climate setting, dump physical characteristics, geochemical processes, water regime, and environmental load over time, as aqueous release of COIs from coal waste rock dumps occurs over decades to centuries. Key considerations are identified for each of these elements to guide best practice.

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

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

Metal removal from acid mine lake using ultrasound-assisted modified fly ash at different frequencies

Acid mine drainage/lakes (AMD/AMLs) have a low pH with high concentrations of metals and sulfate and have been a major environmental problem in the Can Coal Basin, in northwestern Turkey. In this study, metal removal from Hayirtepe AML by using fly ash (FA) and modified fly ash (MFA) was investigated in batch experiments. The effects of various parameters, such as ultrasonic frequency, dose, contact time, pH, and temperature, were examined to determine the optimum conditions for metal removal from AML. This study also focused on the application of ultrasound-assisted modification by using a 20-kHz ultrasonic probe and a 40-kHz ultrasonic bath to increase the FA surface and improve its adsorption capacity for metal removal. FA modification at 20 kHz showed better results than that at 40 kHz because it produced rapid bubble implosion with acoustic cavitation. The FA and MFAs selectivity for metal removal was 98%-99% for Fe, 96%-99% for Al, 94%-97% for Zn, 90%-95% for Co, 88%-94% for Ni, 77%-92% for Cu, and 74%-92% for Mn according to the determined optimum parameters. Scanning electron microscopy coupled with the energy-dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffractometry of the solid residues (SRs) identified gypsum as a new mineral phase from sulfate removal from the AML. Inductively coupled plasma mass spectrometry and SEM/EDX analysis revealed that the metal content of the SRs increased. The adsorption process fitted the pseudo-second order kinetic model. Thermodynamic parameters showed that the process was exothermic and the randomness of the solid/solution interface increased during adsorption. Reuse experiments indicated that the MFAs were reused more effectively for metal removal from AML compared with the FA. This study showed that the use of MFAs with a high adsorption capacity and surface area is economic and efficient for metal removal from AML.

Evolution of Acid Mine Drainage from a Coal Waste Rock Pile Reclaimed with a Simple Soil Cover

Waste rock piles (WRPs) are commonly remediated with cover systems to limit water and oxygen influx and mitigate the impacts of acid mine drainage (AMD) on the environment. While numerous types of cover systems exist, simple, single-layer soil covers remain an attractive option due to their low cost and simplicity of installation. Since knowledge of their long-term performance in humid climates is limited, this study was undertaken to assess and predict a single-layer cover system at a WRP in Nova Scotia, Canada. A two-dimensional finite element model was developed to simulate variably saturated flow and solute transport at the WRP and surrounding area. Key parameters collected during five years of field monitoring, including moisture contents, groundwater levels and dissolved metal concentrations, were used to produce a well-calibrated and verified model. Early results confirm that the cover system has already decreased AMD into both groundwater (reduced water infiltration/seepage in the WRP) and surface water (eliminated contaminated surface water runoff). Long-term acidity depletion rates indicate that all sulphidic minerals within the pile will be oxidized within 34 years, but due to the slow leaching rates into water, it will take over 9000 years to deplete all acidity. Numerical simulations predict the evolution of groundwater and surface water quality over time until full acidity depletion. Current work involves kinetic tests on waste rock samples to more accurately access the annual generation and release of AMD.

Performance assessment of a single-layer moisture store-and-release cover system at a mine waste rock pile in a seasonally humid region (Nova Scotia, Canada)

Cover systems are commonly applied to mine waste rock piles (WRPs) to control acid mine drainage (AMD). Single-layer covers utilize the moisture "store-and-release" concept to first store and then release moisture back to the atmosphere via evapotranspiration. Although more commonly used in semi-arid and arid climates, store-and-release covers remain an attractive option in humid climates due to the low cost and relative simplicity of installation. However, knowledge of their performance in these climates is limited. The objective of this study was to assess the performance of moisture store-and-release covers at full-scale WRPs located in humid climates. This cover type was installed at a WRP in Nova Scotia, Canada, alongside state-of-the-art monitoring instrumentation. Field monitoring was conducted over 5 years to assess key components such as meteorological conditions, cover material water dynamics, net percolation, surface runoff, pore-gas, environmental receptor water quality, landform stability and vegetation. Water balances indicate small reductions in water influx to the waste rock (i.e., 34 to 28% of precipitation) with the diminished AMD release also apparent by small improvements in groundwater quality (increase in pH, decrease in sulfate/metals). Surface water quality analysis and field observations of vegetative/aquatic life demonstrate significant improvements in the surface water receptor. The WRP landform is stable and the vegetative cover is thriving. This study has shown that while a simple store-and-release cover may not be a highly effective barrier to water infiltration in humid climates, it can be used to (i) eliminate contaminated surface water runoff, (ii) minimize AMD impacts to surface water receptor(s), (iii) maintain a stable landform, and (iv) provide a sustainable vegetative canopy.

Evaluation of net acid generation pH as a single indicator for acid forming potential of rocks using geochemical properties

The main purpose of this research was to evaluate the geochemical properties of rocks for a single indicator of acid-forming potential. The indicators, such as net acid generation (NAG), NAG pH and total S, were applied to 312 rock samples of various geological characteristics. Additional indicators, such as a Modified NAG pH, paste pH and available acid neutralizing capacity (ANC), were applied to 22 selected samples. Among them, NAG pH was considered the most plausible single indicator in evaluating acid-forming potential, as it is simple to measure, widely applicable to various samples and can be used to estimate the NAG value. The acid-forming potential of 287 samples (92% of samples examined in this research) was classified as either non-acid forming (NAF) or potentially acid forming (PAF) by NAG pH, with an NAF criteria of <3.21 and PAF of >4.52. The NAG pH was also a good estimate of the risk of short-term acid release when combined with paste pH information. However, application of NAG pH to coal mine wastes, with high organic carbon contents, produced erroneous results due to the generation of organic acid during the NAG test. In this research, a Modified NAG pH was assessed as an alternative to NAG pH in such situations.