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Μining waste, processing by-products and mine water discharges pose a serious threat to the environment as in many cases they contain high concentrations of toxic substances. However, they may also be valuable resources. The main target of the current review is the comparative study of the occurrence of rare earth elements (REE) in mining waste and mine water discharges produced from the exploitation of coal, bauxite, phosphate rock and other ore deposits. Coal combustion ashes, bauxite residue and phosphogypsum present high percentages of critical REEs (up to 41% of the total REE content) with ΣREY content ranging from 77 to 1957.7 ppm. The total REE concentrations in mine discharges from different coal and ore mining areas around the globe are also characterised by a high range of concentrations from 0.25 to 9.8 ppm and from 1.6 to 24.8 ppm, respectively. Acid mine discharges and their associated natural and treatment precipitates seem to be also promising sources of REE if their extraction is coupled with the simultaneous removal of toxic pollutants.

The main objective of the present study is the investigation of the adsorption efficiency of raw and heat-treated attapulgite clay for removing Pb and Cu from aqueous solutions. The removal of each metal was studied separately with the use of one-substance solutions. The effect of certain factors, including solution pH and ionic strength, contact time, adsorbent concentration, temperature of treatment of the adsorbent, and initial metal concentration, was studied. In alkaline conditions, pH > 8.0, precipitation of Pb(OH)2 and Cu(OH)2 takes place, whereas at pH range 4.0–8.0 removal of metals is probably due to adsorption processes. Metal removal increases by 20% for Pb and by 80% for Cu with the increase of attapulgite content from 2 g·L−1 to 15 g·L−1. The removal of metals decreases with increasing solution ionic strength due to competition of Na with Pb and Cu for the available sites. Significant changes in the adsorption capacity of the used attapulgite clay occur after calcination in temperatures >550 °C due to destruction of the crystal lattice of the material and nano-porosity change. Finally, Pb adsorption is described well by both Langmuir and Freundlich isotherm models. According to the Langmuir model, the maximum adsorption capacity for Pb is 30 mg·g−1 and 4.41 mg·g−1 for Cu. The Freundlich model fitted better the experimental data of Cu.

The development of three-dimensional geological models has proven to be critical for conceptualizing complex subsurface environments. This is crucial for mining areas due to their various hazards and unstable conditions. Furthermore, three-dimensional (3D) models can be the initial step for the development of numerical models in order to support critical decisions and sustainable mining planning. This paper illustrates the results and the development phases of a 3D geological model within the boundaries of the Kardia lignite deposit in western Macedonia, Greece. It also highlights the usefulness of a Geographic Information System (GIS) methodology in the subsurface geological and hydrogeological analysis regarding the Underground Coal Gasification (UCG) methodology. In addition, the work focuses on the integrated geospatial framework that was developed to support the Coal-to-Liquids Supply Chain (CLSC) integration in unfavorable geological settings. A 3D subsurface geological model of the study area was developed to identify a suitable area for in situ coal conversion and UCG considering criteria related to specific coal thickness and depth. In this context, the suggested integrated geomodelling workflow can positively contribute to the implementation of conventional and innovative mining, saving time and reducing the cost to improve the quality of information needed to support decisions related to UCG implementation.