The high concentrations of Fe2+ and Mn2+ in acid mine drainage make it difficult and expensive to treat. It is urgent that we find a cheap and efficient adsorption material to treat Fe2+ and Mn2+. As a solid waste in mining areas, coal gangue occupies a large area and pollutes the surrounding environment during the stacking process. Developing a method of resource utilization is thus a research hotspot. In this study, we modified spontaneous combustion gangue using NaOH, NaCl, and HCl by chemically modifying the minerals. We determined the optimal conditions for treating Fe2+ and Mn2+ in acid mine drainage with spontaneous combustion gangue and modified coal gangue using the single factor test method. Based on results of the static test, two dynamic test columns, column No. 1 (spontaneous combustion gangue) and column No. 2 (NaOH modified spontaneous combustion gangue), were constructed, and the repair effects of acid mine drainage were compared and analyzed using dynamic experiments. The results show that overall, NaOH modified spontaneous combustion gangue is the most efficient at removing the Fe2+ and Mn2+ in acid mine drainage. The optimal conditions for NaOH modification are an NaOH concentration of 3 mol/L, a liquid to solid ratio of 2 L/kg, and a modification time of 8 h. The overall efficiency of column No. 2 at removing Fe2+ and Mn2+ from acid mine drainage is better than that of column No. 1. Among them, the average removal efficiency of Fe2+ and Mn2+ from acid mine drainage in column No. 2 were 97.73% and 44.82%, respectively. The above results show that NaOH modified spontaneous combustion gangue is a good adsorbent, which has application potential in wastewater remediation, as it can achieve the purpose of “treating dust with waste”.
Acid mine drainage is formed by the oxidation and weathering of sulfides. The pH value of acid mine drainage is low, and its Fe2+, Fe3+, Al3+, Mn2+, and SO42− contents are high [
Coal gangue is a solid waste produced in the process of coal mining and washing, and its average output accounts for about 10–15% of the output of raw coal [
Because the silicon to aluminum ratio of coal gangue is similar to that of zeolite, several researchers have made zeolite after the chemical modification of coal gangue, thereby improving the ability of the coal gangue to treat acid mine drainage. Bu et al. [
Based on the above considerations, this study focuses on the following aspects: (1) Using the chemical modification method, NaOH, NaCl, and HCl were used to modify spontaneous combustion coal gangue. The modified coal gangue was analyzed using SEM and XRD, and the influence of the different modification methods on the morphology and composition of the coal gangue was investigated. (2) The single factor test was used to explore the application conditions of the spontaneous combustion coal gangue as an adsorbent, to optimize the preparation conditions of the modified spontaneous coal gangue, and to obtain the best modification method. (3) Dynamic experiments were conducted to verify the long-term adsorption performance of NaOH modified spontaneous combustion gangue. Through the above research, a new method of converting coal gangue into an inexpensive, green adsorbent was developed, which can generate local materials for the treatment of acid mine drainage, thereby reducing the cost of wastewater treatment, effectively solving the problem of large amounts of waste coal gangue, and making it possible to reuse coal gangue.
Spontaneous combustion gangue (SCG), was collected from the Gaode mining area, Fuxin City, Liaoning Province, China. Its main chemical composition is presented in
Constituent | SiO2 | Al2O3 | Fe2O3 | MgO | CaO | Na2O | K2O | TiO2 | MnO | P2O5 | SO3 | CO2 | Loss on ignition |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
SCG | 60.16 | 16.56 | 6.26 | 3.63 | 3.4 | 1.84 | 3.09 | 0.71 | 0.16 | 0.19 | 0.73 | 2.50 | 0.79 |
The modified spontaneous combustion gangue (MSCG) samples were prepared using NaOH, NaCl, and HCl soaking modification. These samples were labeled as NaOH-MSCG, NaCl-MSCG, and HCl-MSCG. The SCG was placed in different modified solutions according to the liquid to solid ratio set in the single factor test, soaked in an oven at 100°C for several hours, taken out of the modified solution, washed with distilled water until neutral, and dried in an oven at 105°C to obtain MSCG.
Because most acid mine drainage pollution has high Fe2+ and Mn2+ contents, we used the Fe2+, Mn2+, and SO42− concentrations as the acid mine drainage quality indicators measured in Rumbel City, Inner Mongolia Autonomous Region, China. We simulated the acid mine drainage in the laboratory. In the field samples, the acid mine drainage contained 947–1118 mg/L of SO42−, 52.85–86.31 mg/L of Fe2+, and 13.42–28.51 mg/L of Mn2+, and it had a pH of 4.84–5.36. In order to ensure that the simulated acid mine drainage was realistic, we ensured that the simulated acid mine drainage contained 1000 mg/L of SO42−, 65 mg/L of Fe2+, and 20 mg/L of Mn2+ and had a pH of 5. All the chemicals used in the experiments were of analytical reagent grade.
The SCG was weighed and placed in a conical flask containing 50 mL of simulated acid mine drainage at 25°C. The flask was shaken at 100 r/min for 30 min, and then the supernatant was extracted and passed through a 0.45 μm filter membrane. Finally, the residual concentrations of Fe2+ and Mn2+ were measured. The effects of the particle size of the SCG (75–96, 96–120, 120–180, 180–425, and 425–1,700 μm), the dosing amount (1, 2, 3, 4, and 5 g/50 mL), and the reaction time (30, 60, 90, 120, 150, 180, and 240 min) on the adsorption of Fe2+ and Mn2+ from the acid mine drainage by the SCG were investigated using the static single factor test method [
The MSCG was prepared according to the optimal particle size of the SCG determined from the experiment described above, and the experiment to investigate the adsorption of Fe2+ and Mn2+ from acid mine drainage by MSCG was carried out using the previously determined optimal dosage amount and reaction time. The effects of the NaOH concentration (1, 2, 3, and 4 mol/L), the liquid-solid ratio (2, 4, 6, and 8 L/kg), and the modification time (1, 2, 3, 4, 6, 8, 10, and 12 h) on the adsorption of Fe2+ and Mn2+ from acid mine drainage by NaOH-MSCG were investigated using the static single factor test method [
Based on the optimum modification conditions of the NaOH-MSCG, NaCl-MSCG, and HCl-MSCG, the efficiencies of the three types of modified spontaneous combustion gangue at removing different concentrations of Fe2+ and Mn2+ were determined and compared with that of SCG to determine the best modification method for the removal of Fe2+ and Mn2+.
As shown in
A peristaltic pump was used to pass the simulated acid mine drainage through the column from top to bottom. The flowmeter in the influent path was adjusted so that the flow rate in each dynamic column was maintained at approximately 12 mL/h. The dynamic column experiment was operated at room temperature for 30 days. Samples of incoming and outgoing water were collected regularly at 8:00 am every morning. The residual concentrations of Fe2+ and Mn2+ in the effluent were determined after being passed through a 0.45 μm filter membrane. The removal efficiencies
where
The Fe2+ content was determined using o-phenanthroline spectrophotometry (HJ/T 345–2007), and the Mn2+ content was determined using potassium periodate spectrophotometry (GB 11906-89). The SCG and MSCG were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD) using a Quanta 250 field emission scanning electron microscope and an XRD-6100 X-ray diffractometer, respectively.
As shown in
As shown in the XRD diffraction pattern (
The effects of the particle size, dosage, and adsorption time of the efficiency of SCG at removing Fe2+ and Mn2+ in acid mine drainage were investigated using single factor experiments. The results are shown in
As shown in
According to
The optimal conditions for SCG to absorb Fe2+ and Mn2+ in acid mine drainage are particle sizes of 120–180 μm, a dosing amount of 4 g/50 mL, and a reaction time of 150 min.
In the SCG modification experiments, the influence of the modification solution concentrations, the liquid to solid ratio of the modification solutions and the SCG, and the modification time on the efficiency of the MSCG at removing Fe2+ and Mn2+ from acid mine drainage were studied. The results are shown in
As shown in
Increasing the modification time had little effect on the efficiency of the removal of Fe2+, but it effectively improved the efficiency of the removal of Mn2+. Therefore, the optimal conditions for NaOH modification are an NaOH concentration of 3 mol/L, a liquid to solid ratio of 2 L/kg, and a modification time of 8 h.
As shown in
As shown in
As shown in
Based on the results of the single factor experiments, two dynamic experimental columns were constructed for the dynamic test: column No. 1 was loaded with SCG, and column No. 2 was loaded with NaOH-MSCG. The dynamic removal of Fe2+ and Mn2+ from acid mine drainage by NaOH-MSCG was analyzed. The results are shown in
According to
The removal of the Mn2+ in acid mine drainage in the two dynamic experimental columns showed a general downward trend, with average Mn2+ removal efficiency of column No. 1 and column No. 2 were 21.64% and 44.82%, respectively. As the reaction time increased, the removal efficiency of Mn2+ in column No. 2 showed a downward trend, from an initial removal efficiency of 99.99% to 17.28%, indicating that there were many Mn2+ adsorption sites because of the initial amount of NaOH-MSCG. As the reaction progressed, the adsorption sites gradually became occupied, and the amount of Mn2+ adsorbed by the NaOH-MSCG gradually decreased as the gangue became saturated. The Mn2+ removal efficiency of column No.1 increased from 45.25% to 66.58% and then decreased to −17.27%. Zhang et al. [
Based on our experiments, we determined that NaOH-MSCG has a better dynamic Fe2+ and Mn2+ removal efficiency than SCG.
The accumulation of spontaneous combustion gangue poses large safety risks to the daily production and environmental protection of mining areas. In the process of acid mine drainage treatment, efficient and cheap adsorption materials are always scarce resources. Therefore, this paper uses spontaneous combustion gangue from the Gaode mining area as the raw material, and through NaOH, NaCl, and HCl chemical modification, we conducted a comparative study of the effects of modified coal gangue on acid mine wastewater treatment. The following conclusions were drawn based on the results of this study. The effectiveness of spontaneous combustion gangue at removing Fe2+ is better than its effectiveness at removing Mn2+, and the optimal conditions for spontaneous combustion gangue to absorb Fe2+ and Mn2+ include a particle size of 120–180 μm, a dosing amount of 4 g/50 mL, and a reaction time of 150 min. Compared with spontaneous combustion gangue and modified coal gangue, NaOH modification significantly improves the efficiency of the spontaneous combustion gangue at removing Fe2+ and Mn2+ from acid mine drainage. The optimal modification conditions are an NaOH concentration of 3 mol/L, a liquid to solid ratio of 2 L/kg, and a modification time of 8 h. Based on the results of the single factor test, the dynamic adsorption performance of the NaOH modified gangue was verified using column experiments. The results show that the dynamic treatment of Fe2+ and Mn2+ removal from acid mine drainage is as follows: NaOH modified gangue in column No. 2 > unmodified gangue in column No. 1. The average removal efficiencies of Fe2+ and Mn2+ by the NaOH modified gangue in column No. 2 were 97.73% and 44.82%, respectively. Using spontaneous combustion gangue to prepare an adsorbent has the advantages of numerous raw materials and low cost. It not only achieves recycling and utilization of coal gangue, but it also solves the pollution problem posed by nearby acid mine drainage. The results of our study provide new ideas for addressing solid waste and wastewater issues in mining areas, promoting clean production in mining areas, and fulfilling the research purpose of “treating dust with waste”.
This study introduces a new application of spontaneous combustion gangue, optimizes the preparation conditions of modified coal gangue, verifies the feasibility of the application of NaOH modified spontaneous coal gangue to wastewater remediation, provides a research basis for the secondary utilization of coal gangue in Fuxin, and is conducive to sustainable development in mining areas.