Self-hydrolyzing Submerged Flow Technology for Acid Coal Mine Drainage Treatment: Effective Removal of Iron Ions

In this study, mine wastewater from Leiyang, Hunan Province was taken as the research object to investigate the effect of using the method of self-hydrolytic submerged sedimentation to treat acid coal mine drainage. It was found in this study that the removal rate of sulfate ions and iron ions from the acidic coal mine drainage stock solution at pH=2.7 was very limited by self-hydrolytic sedimentation experiments relying only on pure vertical sedimentation flow treatment, but the hydrolytic treatment could greatly improve the solution environment and promote the formation of secondary minerals of sulfate ions and iron ions in the acidic coal mine drainage, which greatly improved the removal rate of iron ions. Both suspended and precipitated materials produced in the self-hydrolysis sedimentation experiments were hydroxylated iron sulfate secondary minerals- Schwertmannite [Fe8O8(OH)8-2X(SO4)X (1 ≤ x ≤ 175)], the formation of which is inextricably linked to the low pH environment and high concentration of sulfate and iron ions in coal mine wastewater. Self-hydrophoretic submerged flow can remove iron ions from acidic coal mine drainage at low cost and high efficiency, and the total iron removal rate of this system can reach 90.93% after 24 h of treatment, which has a favorable comprehensive utilization value.


Introduction
Coal resources occupy a large proportion of China's mineral resources and influence the status of domestic energy production and consumption in the international arena [1].However, due to the overutilization of coal resources and large-scale mining, the ecological environment of coal mining areas is facing a severe test, the most serious problem is the discharge of wastewater and solid pollutants from acidic coal mines [2].
As a large number of sulfide minerals such as pyrite and magnetite, which can be stable in a reducing environment, exist in symbiosis in coal mines, they are exposed to oxygen after mining, and subsequently, a large amount of acid coal mine drainage is generated under the combined action of oxygen, water, and microorganisms [3].Acidic coal mine wastewater has low pH, high content of sulphate ions and iron ions, and contains many types of sulfides, as well as a small number of heavy metals, and the flow of mine drainage changes seasonally, and the values of various pollution indicators change continuously with the flow, which can cause great damage to the nearby rivers and ecological environment [4,5].Therefore, it is imperative to treat acidic coal mine drainage to ensure the safety of human life and ecosystems.
At present, the main methods for treating acidic coal mine wastewater are chemical precipitation [6], physical adsorption [7], and biological methods [8].Traditional refined wastewater treatment methods and crude lime precipitation tanks have some limitations for targeted prevention and control of this type of wastewater economically and technically, such as consuming more reagents, complex equipment, requiring skilled mechanics to be on duty for a long time, and overall low economic and technical feasibility [9].Therefore, in response to the problem of severe exceedance of heavy metal element Fe concentration in acidic coal mine drainage in Leiyang City, Hunan Province, this experiment uses simple physicochemical treatment methods and technologies for self-flowing vertical submerged treatment to remove pollutants while achieving self-sustaining, low-cost and efficient operation of the treatment system, which can solve the problems of high operating costs and heavy treatment burden of traditional water treatment stations.

Materials
In this experiment, the coal mine drainage used is from a mine in Leiyang, Hunan Province, which mainly contains a large amount of iron ions and sulfate ions.

Self-hydrolysis submerged sedimentation experiment
Three sets of reaction columns (60 cm high, 4 cm inner diameter) were set up, filled with river stones at a height of 40 cm.1500 mL of raw coal mine wastewater (pH=2.7)and diluted coal mine wastewater (pH=4 and 5) were pumped to the top of the columns, and the mine wastewater was filtered by river stones in the columns and flowed out from the bottom of the reaction columns to the collection vessel, and then a peristaltic pump (flow rate 250 mL/min) to pump the mine wastewater from the collection vessel to the top of the column to complete the cycle.Samples were taken every 1 hour from the collection vessel below each of the three sets of reaction columns for changes in the concentration of iron and sulfate ions therein.The solid fractions in the collection vessels below were collected at the end of the self-hydrolysis submerged sedimentation experiment.

Analysis and Characterization
Sulphate ion concentration and iron ion and heavy metal contaminant concentrations were detected by Inductive Coupled Plasma Emission Spectrometer (ICP).The collected precipitated solids were analyzed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Xray diffractometer (XRD).

Variation of sulfate ion concentration and iron ion concentration during self-hydrolysis
As shown in Fig. 1(a), the concentration of sulphate ions in the acidic coal mine drainage water at pH=2.7 remained almost unchanged after vertical submerged sedimentation treatment, while the concentration of sulfate ions started to decrease after dilution to pH=4 and 5 and then treated by vertical submerged sedimentation.And it was observed during the experiment that both groups of coal mine drainage after hydrolysis treatment produced yellow suspensions and precipitates, which may be secondary minerals produced by the combination of sulfate ions and iron ions and need further verification.
The total iron ion concentration in the acidic coal mine drainage at pH=2.7 did not change much after the vertical submerged treatment, and the removal rate was only 8.26%, as shown in Fig. 1(b).On the contrary, when it was diluted to pH=4 and 5, and then self-hydrolysis reaction occurred during vertical submerged flow, the removal rate of total iron ions could be greatly increased, reaching 86.11% and 90.93%, respectively.This indicates that the iron ions in acidic coal mine drainage can be effectively removed by self-hydrolysis vertical submerged treatment.

SEM Characterization
After the self-hydrolysis submerged sedimentation experiment, the morphology of the collected solids was observed using scanning electron microscopy analysis.As shown in Fig. 2, both the suspension and the solid fraction in the precipitate can be observed as spherical particles with a fluffy structure on the surface, but there is a clear difference in size and microscopic morphology between the two types of particles.

Fig.2 SEM images of solid matter in suspensions (a-c) and precipitates (d-f).
The solid particles in the suspension were in the early stage of formation (Fig. 2 a-c), and their particles were small (about μm) with a relatively small and uneven distribution of fluffy structures on the surface.The solid particles after natural settling (Fig. 2 d-f), on the other hand, are significantly larger in size (about 5 μm), and the fuzz-like structures on their surfaces are more pronounced and more uniformly and densely distributed because the crystals have undergone sufficient growth after a longer reaction time.And this can increase the specific surface area of such solids, which facilitates their contact with metal ions in solution and increases the ability to adsorb other ions and substances.According to previous studies [10], this uniform and dense sea urchin-like structure is typical of Schwertmannite commonly found in acid mine drainage, and this secondary hydroxyferrate sulfate mineral is rich in iron/sulfur elements.This also suggests that the removal of sulfate and iron ions from acid mine drainage can be effectively promoted by improving the ambient acidity followed by self-hydrolysis vertical submerged treatment.

XPS and XRD analysis
The solid composition of the suspension or precipitate resembling Schwertmannite was further analyzed using X-ray photoelectron spectroscopy.The results are shown in Fig. 3.The solid material in the suspension contains mainly three elements, Fe-, S, and O (Fig. 3 a-c), Fe contains two valence states, Fe2+ and Fe3+, the fine spectrum of S belongs to the typical SO42-signal, and O corresponds to states such as Fe-O and -OH. he forms of these elements are very similar to those of the secondary mineral Schwertmannite.Fig. 3 XPS Spectra of solids in the suspensions (a-c) and precipitates (d-f).The XPS spectra of the solid components in the precipitate (Fig. 3 e-f) are almost identical to those in the suspension, both containing mainly Fe, S, and O, and the morphologies of the three elements also remain consistent.This indicates that although the morphologies of the solids in suspension and precipitate are more obviously different, the constituent elements do not change.This is most likely due to the formation of this secondary iron hydroxy sulfate mineral during the self-hydrolysis of sulfate ions and iron ions, which grows over time and eventually precipitates after the particles reach a certain size.Fig. 4 XRD pattern of solids in t precipitate.Fig. 4 shows the XRD pattern of the precipitate in the experiment, and it can be found that the XRD diffraction peaks of the substance match well compared with those of the standard card (JCPDS No. 47-1775), and the main crystalline phase of the precipitate is Schwertmannite (Fe 16 O 16 (SO 4 ) 3 (OH) 10 -10H 2 O), in which the heterogeneous phase of FeOOH appears at 31.15° and 42.7°.The orientation analysis of the different crystalline planes of the material phase shows that the (212) crystalline planes of the Schiff mineral corresponding to 35.12° are higher and the orientation growth is more obvious, and it can be found that the intensity of the diffraction peaks is weaker and the half-peak width is larger, which indicates that the disorder of the mineral crystallization is higher.In summary, it is proved that the precipitated solid material is indeed Schiff's mineral with low crystallinity, small grain size and faint heterogeneous phase generation.It also indicates that the sulfate ions and iron ions in the acidic coal mine drainage do form Schwertmannite during the self-hydrolysis process, and this is the main way they are removed.

Conclusion
In this study, it was found that the removal rate of sulfate ions and iron ions in the acidic coal mine drainage stock solution at pH=2.7 was very limited by self-hydrolytic sedimentation experiments relying only on pure vertical sedimentation flow treatment, but the simple dilution hydrolysis in acidic coal mine drainage could greatly promote the formation of secondary minerals of sulfate ions and iron ions in acidic coal mine drainage, which greatly improved the removal rate of iron ions.Both suspended and precipitated materials during the self-hydrolysis submerged sedimentation experiments were Schwertmannite [Fe 8 O 8 (OH) 8 -2X (SO 4 ) X (1 ≤ x ≤ 175)], an ochre-colored iron hydroxyl sulfate secondary mineral, whose formation is inextricably linked to the low pH environment of mine wastewater and high concentrations of sulfate ions and iron ions.The main reason for the formation of Schwertmannite in this experiment is the complexation of Fe 3+ with SO 4 2-to form complexes, and the ability of SO 4  2-to participate in the coordination position of hydroxyl groups in the polymerization of mineral particles, inhibiting the formation of iron hydroxyl compounds and leading to the appearance of Schwertmannite.The self-hydrolysis submerged flow can remove iron ions from acidic coal mine drainage at low cost and high efficiency, and the total iron removal rate can reach 90.93% after 24 h of treatment by this system, which has good comprehensive utilization value.

Fig. 1
Fig.1 Variation of sulfate ion (a) and iron ion (b)concentration at different pH conditions