Efficient Removal of Trivalent Iron and Sulfate from Coal Mine Gushing Water Using Zero-Valent Iron Powder Combined with Hydrogen Peroxide

The high concentration of trivalent iron (Fe3+) and sulfate (SO4 2-) ions in coal mine gushing water is a major ecological hazard and difficult to treat industrially. In this paper, effective purification of coal mine gushing water was achieved by the two-step cyclic process using zero-valent iron (ZVI) powder and hydrogen peroxide. We investigated the effect of different doses of ZVI and hydrogen peroxide on the removal of Fe3+ and SO4 2-. Single-factor experiments indicated that as the dosage of ZVI increased, the removal of Fe3+ increased and then decreased, with the highest Fe3+ removal rate approaching 100% at the ZVI dosage of 100 mg/L, while the maximum removal rate of SO4 2- for sulphate was achieved at 400 ppm of hydrogen peroxide. The precipitate produced in the purification system was characterized and the results demonstrated that it was a typical secondary mineral, schwertmannite, and that it contained considerably more iron and sulfate than the precipitate formed by the natural sedimentation process in coal mine gushing water. Overall, after the two-step cycle process, Fe3+ and SO4 2- can be effectively removed via the formation of schwertmannite, and the ion concentrations all meet the discharge standards for Chinese mining industry wastewater.


Introduction
For a long time, Coal has occupied the main position in the energy consumption structure of China, and has played an important role in the economic construction and social development, and is the resource guarantee for the sustainable development strategy [1,2].Nevertheless, the long-term underground coal mining has damaged the hydrogeology and ecological environment of the coal mining area, resulting in water shortage in the coal mining area, which affects the economic construction and the life of the residents around the mine [3,4].According to the relevant data, each 1 ton of coal mined discharges 1~1.5 tons of wastewater and damages water resources up to 2.5 tons [5].The coal mine wastewater management constrains the development of the coal industry, and there is an urgent need to address the issue of water environment management in coal mining.
Coal mine wastewater is made up of three main components: coal mine gushing water, coal mine washing wastewater, and gangue field leaching wastewater [6].Coal mine gushing water mainly comes from the underground gushing and seeping into the tunnel during the mining process and is discharged into the groundwater, containing a large amount of suspended matter, high content of sulfate and trivalent iron ions, low pH value, and has a strong toxic effect on the water environment and ecosystem [7,8].During coal mining, the original reducing environment of the sulfide minerals (mainly sulfide iron ore) in the coal seam and surrounding rocks is destroyed, which brings it together with oxygen and water, resulting in a series of complex geochemical reactions and the generation of large quantities of acid under the microbiological influence [9].The strongly acidic water leaches out the harmful metal ions from the coal seam and surrounding rocks, thus forming acidic mine water.
For coal mine gushing water, especially in abandoned coal mines, it exists in huge volumes of water, many sources, large differences in salt ion composition and the difficulty of resource treatment [10].The common methods include membrane separation [11], flocculation-sedimentation [12], and microbial anaerobic reduction [13], but there are a series of governance defects, such as the limitations of the sewage site, the small volume of treated water, secondary pollution, and low resource utilization.Therefore, it is imperative to develop long-lasting, sustainable, and resourceful treatment technologies for coal mine gushing water.
In this work, ZVI powder and hydrogen peroxide were used to treat trivalent iron and sulfate in coal mine gushing water in a stepwise cycle, and the influence of the dosage of ZVI powder and hydrogen peroxide on the treatment effect of trivalent iron and sulfate was investigated.It can effectively recover metal resources from wastewater and the generated precipitates (schwertmannite) can be used as highly effective commercial adsorbents in the treatment of heavy metal effluent.This experiment provides an economic and feasible method for the treatment of coal mine gushing water by forming in-situ precipitates containing sulfate and trivalent iron to achieve effective purification.

Chemicals and materials
The coal mine gushing water samples used for this experiment were obtained from an abandoned coal mine factory in Leiyang, Hunan Province.The samples were analyzed and indicated that the total iron content was approximately 150 mg/L and the SO 4 2-content was approximately 720 mg/L.zero-valent iron powder (D = 50 μm), hydrogen peroxide (H 2 O 2 , 30%), barium chloride (BaCl 2 ), hydrochloric acid (HCl), and nitric acid (HNO 3 ) were purchased from Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China).All the chemical reagents were analytical grade and used without further purification.

Experimental Procedure
The two-step cycle was as follows: in a 250 mL conical flask, 15 mg of ZVI powder (150 mg/L) was added to 100 mL of water sample, this process was the reduction process and performed in a shaker (180 rpm; 30 °C) for 2 h.After the reduction, the residual iron powder was filtered and subsequently oxidized by adding H 2 O 2 (400 ppm), the oxidation process was carried out in the same environment as the reduction process.After oxidation, the precipitate was collected by filtration and the supernatant is poured back into the conical flask.The above steps were repeated several times to achieve efficient treatment of Fe 3+ and SO 4 2-.After each cycle of the reduction-oxidation process, 1 mL of the system was taken and passed through a 0.45 μm aqueous filter to determine the Fe 3+ and SO 4 2-concentrations.

Analytical Methods
Total iron ions were observed by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Spectro Blue II, SPECTRO, Germany).The concentration of sulfate was determined by the barium sulphate turbidimetric method at 420 nm using a microplate reader (Epoch, BioTek, USA).The morphology of the precipitate was explored by scanning electron microscopy (FE-SEM, TESCAN MIRA3 LMH/LMU, Czech).The crystalline of the precipitate was observed by X-ray diffraction (XRD, XRD-6000, SHIMADZU, Japan) using a Cu Kα radiation source.X-ray photoelectron spectrometer (XPS, EscaLab Xi+, ThermoFischer, USA) was used to analyze the chemical compositions and the element valence state.

Effect of dosage on the iron ion removal
As shown in Fig. 1A, the total iron concentration changed under the condition of gradient ZVI powder dosage, when the hydrogen peroxide concentration was 400 ppm.With the increase of ZVI powder concentration, the total iron removal rate in the system gradually increased, and at the ZVI powder dosage of 150 mg/L, the complete removal of iron ions could be achieved within 11 days, which was nearly 2 times higher than the removal efficiency of the control group.As shown in Fig. 1B, under different dosage conditions of ZVI powder and hydrogen peroxide, all the iron ions in coal mine gushing water were almost completely removed by the fifth day.As hydrogen peroxide reacted with ZVI powder and divalent iron ions in Fenton reaction, trivalent iron was formed and precipitated hydrolytically, thus significantly reducing the concentration of iron ions.The optimal conditions for iron removal were 150 mg/L iron powder concentration, while the variation of hydrogen peroxide concentration had little effect on the removal of iron ions.As shown in Fig. 2B, when the ZVI powder was dosed at 150 mg/L, the removal of sulfate increased first and then decreased with the hydrogen peroxide concentration, with the highest removal of sulfate at 40 μL of hydrogen peroxide, which could reach 96.1%.Therefore, the optimum conditions for sulfate removal were 150 mg/L ZVI powder concentration and 40 μL hydrogen peroxide addition.

Effect of dosage on the sulfate removal
The effect of different ZVI powder and hydrogen peroxide dosage on the sulfate removal is shown in Fig. 2. As shown in Fig. 2A, when the ZVI powder dosage was higher than 150 mg/L, the removal rate was decreased, and the optimal dosage was 150 mg/L.In addition, the sulfate concentration in the control group was also decreased, which was mainly attributed to the hydrolysis of iron ions in coal mine gushing water by the reaction with hydrogen peroxide.

Precipitate characterization
The morphological structure of the precipitates produced by the cyclic reduction-oxidation process was analyzed by SEM.As shown in Fig. 3, the sediment precipitated during the oxidation of the water sample consisted of uniformly small spherical particles covered with needle-like ridges and present in an aggregated state, which is presumed to be a typical sea urchin-like schwertmannite.As shown in Fig. 3A, the precipitate particle size in the experimental group with 300 mg/L ZVI powder had an average diameter of about 270 nm, while in the experimental group with 150 mg/L ZVI powder (Fig. 3B), the precipitate particles had an average diameter of about 500 nm.The lower iron to sulfur ratios facilitates the formation of more stable schwertmannite.The larger size particles of schwertmannite produced in the 150 mg/L ZVI powder system were more effective in precipitating iron and sulfate ions away from the water sample as a precipitate.It also confirms the former experimental results that the addition of 150 mg/L ZVI powder was most effective in removing Fe 3+ and SO The high-resolution Fe 2p and S 2p XPS spectra were compared for the precipitates of the two-step cyclic process and the naturally sedimentation, the results are shown in Fig. 4. The Fe 2p3/2 was divided into two peaks of trivalent iron with binding energies at 709.6 and 713.3 eV, respectively, while the S 2p was divided into 2p3/2 and 2p1/2 of sulfate with binding energies at 168.1 and 169.3 eV, respectively.Notably, the precipitates obtained by the two-step cyclic process had significantly higher intensity of iron and sulfur elements than the naturally settled precipitate.This indicates that the schwertmannite surface produced after the cyclic reduction-oxidation reaction has an increased content of both Fe and S than the surface of the naturally settled precipitate, but no valence change has occurred.

Mechanism for iron and sulfur ion removal
The above results reveal that after the cyclic reduction and oxidation process, the resulting precipitate has a typical schwertmannite morphology and characteristic diffraction peaks, which are hydroxy iron sulfate aggregates formed from a large amount of Fe 3+ and SO 4 2-after the two-step cyclic process.This process mainly involves a redox reaction between hydrogen peroxide and divalent iron, resulting in the formation of large amounts of Fe 3+ and the production of hydroxyl radicals, while high concentrations of sulfate lead to the formation of large amounts of hydroxy iron sulfate, as well as the adsorption of the iron and sulfate ions from the coal mine gushing water on the particle surface to form iron sulfate.It has been demonstrated that at the pH of 2.4 to 3.0, trivalent iron hydrolysis products interact with sulfate to form hydroxy iron sulfate, particularly schwertmannite.The mechanism for the two-step cyclic process is illustrated in Scheme 1 and the reaction equations as shown below: Scheme 1.The two-step cyclic process for iron and sulfur ion removal mechanism.

Conclusion
The two-step cyclic process was adopted for the deep purification of wastewater for iron ions and sulfate in coal mine gushing water, and realized the resource utilization.The results indicated that when 150 mg/L of ZVI powder and 40 μL of hydrogen peroxide were added, the removal of iron ions and sulfate reached 100% and 96.1% respectively on 5 and 11 days of the reaction.The total iron ion concentration in the treated coal mine gushing water was much less than the discharge standard of < 10 mg/L stipulated in China's "Comprehensive Sewage Discharge Standard: GB8978-1996", while the sulfate concentration reached the discharge standard of < 250 mg/L.Characterization revealed that iron ions and sulfate in coal mine gushing water were separated from coal mine gushing water by precipitation formation (schwertmannite) and accelerated the adsorption and crystallization of iron ions and sulfate.The two-step cyclic process yields schwertmannite that can be used as environmental heavy metal adsorbents, enabling the resourceful recycling of ions in coal mine gushing water.Overall, it provides an economically viable method for the treatment of coal mine gushing water to achieve effective purification.

Figure 1 .
Figure 1.Effect of ZVI powder and hydrogen peroxide dosage on total iron concentration.

Figure 2 .
Figure 2. Effect of ZVI powder and hydrogen peroxide dosage on sulfate concentration.

Figure 3 .
Figure 3. SEM images of the precipitates.(A) the precipitate of the experimental group with 300 mg/L ZVI powder and (B) the precipitate of the experimental group with 150 mg/L ZVI powder.

Figure 4 .
Figure 4. Fe 2p XPS spectra of (a) the naturally settled precipitate and (b) the two-step cyclic process of precipitate, S 2p XPS spectra of (c) the naturally settled precipitate and (d) the two-step cyclic process of precipitate.

Fig. 5 Figure 5 .
Fig.5illustrates the XRD patterns of the precipitates in the 100 mg/L and 150 mg/L ZVI powder processed the coal mine gushing water.Te precipitate in the 100 mg/L ZVI powder processed coal mine gushing water is schwertmannite (PDF#47-1175), where the peak at 18.2°, 26.3°, and 35.2° correspond to the (210), (310), and (212) crystal plane of schwertmannite.Notably, there are also appearances of ferrohexahydrite (FeSO 4 • 6H 2 O, PDF#15-0393) in the precipitate produced by the processed with 150 mg/L ZVI powder.The rise in the dosage of ZVI powder increased the divalent iron content of the precipitate.The iron and sulfate ions are grown in the form of ferrous sulfate in the particles of schwertmannite and form crystal.The removal of sulfate from the coal mine gushing water was improved, which is consistent with the results of the removal ions described above.