The aim of the study was to describe the sorption interactions between potentially toxic metals (Cd, Co, Cu, Pb) and materials from an underground coal gasification (UCG) experimental zone. These interactions seem to be significant in terms of the impact of in situ UCG on the groundwater environment. Sorption parameters were determined for two different sample types: subbituminous coal mined from the coal-bed and then subjected to gasification and coal char from the cavity formed by the UCG process. Laboratory-scale tests were carried out using deionized water and aqueous solutions of metals with increasing concentrations. The Freundlich isotherm model was applied to describe sorption phenomena due to nonlinear mass distribution of adsorbed metal ions as a function of equilibrium concentration and assuming physical interactions only. In addition, the efficiency of the tested sorbents for metal removal was calculated. In the case of subbituminous coal, the percent removal ranged from a minimum of 3.6–9.8% (for cobalt) to a maximum of 43.4–79.8% (for lead). Char removed metals more efficiently (min. 26.6–94.8% for cadmium; max. 98.5–99.9% for lead). Furthermore, the sorbates can be ranked according to the metal ion binding efficiency to sorbents in the following order: Co < Cd < Cu < Pb. The sorption characteristics of materials obtained from the post-UCG cavity may be used to evaluate the retardation parameters of inorganic pollutant migration in the environment around a georeactor.

Underground coal gasification (UCG) can be considered as one of the clean coal technologies. During the process, the gas of industrial value is produced, which can be used to produce heat and electricity, liquid fuels or can replace natural gas in chemistry. However, UCG does carry some environmental risks, mainly related to potential negative impacts on surface and groundwater. Wastewater and sludge from UCG contain significant amounts of aliphatic and aromatic hydrocarbons, phenols, ammonia, cyanides and hazardous metals such as arsenic. This complicated matrix containing high concentrations of hazardous pollutants is similar to wastewater from the coke industry and, similarly to them, requires complex mechanical, chemical and biological treatment. The focus of the review is to explain how the wetlands systems, described as one of bioremediation methods, work and whether these systems are suitable for removing organic and inorganic contaminants from heavily contaminated industrial wastewater, of which underground coal gasification wastewater is a particularly challenging example. Wetlands appear to be suitable systems for the treatment of UCG wastewater and can provide the benefits of nature-based solutions. This review explains the principles of constructed wetlands (CWs) and provides examples of industrial wastewater treated by various wetland systems along with their operating principles. In addition, the physicochemical characteristics of the wastewater from different coal gasifications under various conditions, obtained from UCG’s own experiments, are presented.

This paper presents a series of surface experiments simulating underground coal gasification (UCG). The main goal of the experiments was to investigate the influence of the gasification medium and the coal rank on the gasification process. Four multi-day trials were carried out using a laboratory gasification facility designed for the large-scale experimental simulations of UCG and located in the Experimental Mine “Barbara”, located at Mikołów, Poland. Two Polish bituminous coals were investigated: coal sourced from “Piast-Ziemowit” mine and coal sourced from “Wesoła” mine. Each of the two coals was gasified in two separate experiments using oxygen-enriched air (OEA) and pure oxygen as the respective gasifying agents. Gasification with oxygen resulted in significantly higher gas quality and higher process efficiency than gasification with OEA. Higher concentrations of hydrogen (23.2% and 25.5%) and carbon monoxide (31.8% and 33.4%) were obtained when oxygen was used as a gasifying reagent, while lower concentrations were obtained in the case of gasification with OEA (7.1% and 9.5% of hydrogen; 6.4% and 19.7% of carbon monoxide). Average gas calorific values were 7.96 MJ/Nm³ and 9.14 MJ/Nm³ for the oxygen experiments, compared to 2.25 MJ/Nm³ and 3.44 MJ/Nm³ for the OEA experiments (“Piast-Ziemowit” coal and “Wesoła” coal, respectively). The higher coalification degree of “Wesoła” coal (82.01% of carbon) compared to the “Piast-Ziemowit” coal (68.62% of carbon) definitely improves the gas quality and energy efficiency of the process. The rate of water condensate production was higher for the oxygen gasification process (5.01 kg/h and 3.63 kg/h) compared to the OEA gasification process (4.18 kg/h and 2.63 kg/h, respectively), regardless of the type of gasified coal. Additionally, the textural characteristics (porosity development) of the chars remaining after coal gasification experiments were analyzed. A noticeable development of pores larger than 0.7 nm was only observed for the less coalified “Piast-Ziemowit” coal when analyzed under the more reactive atmosphere of oxygen.