Journal of Chemistry
Volume 2018 (2018), Article ID 1265168, 8 pages
https://doi.org/10.1155/2018/1265168

Research Article

Removal of High-Concentration Sulfate Ions from the Sodium Alkali FGD Wastewater Using Ettringite Precipitation Method: Factor Assessment, Feasibility, and Prospect

Ping Fang, Zi-jun Tang, Xiong-bo Chen, Jian-hang Huang, Zhi-xiong Tang, and Chao-ping Cen

South China Institute of Environmental Sciences, Ministry of Environmental Protection, Guangzhou 510655, China

Correspondence should be addressed to Chao-ping Cen; cenchaoping@scies.org

Received 6 December 2017; Revised 6 April 2018; Accepted 19 April 2018; Published 3 June 2018

Academic Editor: Pedro Avila Pérez

Copyright © 2018 Ping Fang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The feasibility of removal of sulfate ions from the sodium alkali FGD wastewater using the ettringite precipitation method was evaluated. Factors affecting the removal of sulfate ions, such as NaAlO₂ dosage, Ca(OH)₂ dosage, solution temperature, anions (Cl⁻, NO₃⁻ and F⁻), and heavy metal ions (Mg²⁺ and Mn²⁺), were investigated, and the optimal experimental conditions for the removal of sulfate ions were determined. Experimental results indicate that the ettringite precipitation method can effectively remove SO₄²⁻ with removal efficiency of more than 98%. All the investigated factors have influences on the removal of sulfate ions, and among them, the dosage of reagents, solution temperature, and fluoride ions have the strongest influence. In addition, the method can effectively synergistically remove F⁻ and heavy metal ions with removal efficiencies of more than 90% and 99%, respectively; meanwhile, Cl⁻ and NO₃⁻ also can be removed minimally by the method. The result of actual wastewater treatment shows that the method is feasible for treating high-concentration sulfate wastewater. The ettringite precipitation method has the potential to be a commercial application in the future.

1. Introduction

At present, the wet flue gas desulfurization (WFGD) is the most commonly used technology in the field of industrial flue gas desulfurization in China [1]. For power plant flue gas desulfurization, the main technology used in China is limestone-gypsum wet process. High desulfurization efficiency and gypsum recycling are the main advantages of this technology. However, some disadvantages, such as the complex system, higher investment cost, and requirement for a large area, have limited the application of this technology in the flue gas desulfurization of industrial boilers and furnaces in China. Therefore, the sodium alkali (NaOH) FGD technology with relatively simple process and high SO₂ removal efficiency (>95%) is widely used in the flue gas desulfurization of industrial boilers and furnaces. However, there are some shortcomings in this technology that need further improvement. One of the major problems is the disposal of complex and sulfate-rich wastewater.

The sulfate (SO₄²⁻) concentration in the sodium alkali (NaOH) FGD wastewater is generally more than 10,000 mg/L; the maximum can be greater than 20,000 mg/L. In addition, the wastewater also contains a large number of inorganic anions (NO₃⁻, Cl⁻, and F⁻) and heavy metal ions (Mg²⁺ and Mn²⁺). Though SO₄²⁻ is a common and nontoxic component of various types of water bodies, high concentrations of SO₄²⁻ in the water can cause a series of serious environmental problems, leading to water mineralization, metal corrosion, pipes and equipment scaling, toxic hydrogen sulfide release, and disruption in the balance of the natural sulfur cycle [2–5]. In addition, high concentrations of SO₄²⁻ (>600 mg/L) in the water can cause laxative effects in mammals [6]. Hence, to protect the environment, the SO₄²⁻ concentration in the industrial effluents is set ranging from 250 mg/L to 500 mg/L in many countries [6]; for example, the sulfate ion concentration limit values in the industrial recycling water and surface water are all set as 250 mg/L in China. Hence, in order to meet the discharge standard or achieve the recycling of desulfurization wastewater, it is necessary to remove the sulfate ion in the sodium alkali FGD wastewater.

Currently, several technologies, such as biological treatment [7, 8], membrane filtration [9], adsorption [10], ion exchange [11, 12], electrocoagulation [13], crystallization [14, 15], and chemical precipitation [16], have been developed to treat sulfate in water. However, these methods are not very suitable for the treatment of high concentration of SO₄²⁻ wastewater, except the chemical precipitation method. The chemical precipitation method mainly includes lime precipitation [6], barium chloride precipitation [17, 18], and ettringite (Ca₆Al₂(OH)₁₂(SO₄)₃·26H₂O) precipitation methods [19]. The limestone precipitation method is widely used in the field of water treatment; however, due to the relatively high solubility of gypsum [19], the SO₄²⁻ removal efficiency is low. The barium chloride precipitation method can reach high SO₄²⁻ removal efficiency, but a large number of corrosive chloride ions and toxic barium ions will be introduced into the water; besides, barium chloride is more expensive than lime, so the technology is rarely used in the field of flue gas desulfurization wastewater treatment.

Compared with the above methods, the ettringite precipitation method is considered as an effective method for treating high-concentration sulfate effluents. In this method, lime and aluminum salts are added into the wastewater to react with sulfate to form insoluble ettringite (pKsp = 111.6) [20], and then sulfate is effectively removed. The ettringite precipitation method has become a preferred method due to its high removal efficiency and cost-effectiveness, and many studies have used this method to treat industrial wastewaters, such as aluminum anodizing, textile industries, and mine water [19–21]. However, in existing researches the sulfate concentrations in simulated or actual wastewaters were usually lower; hence, anions and heavy metal ions were not considered in the removal of sulfate; meanwhile, there exists little information on the removal of SO₄²⁻ from the sodium alkali (NaOH) FGD wastewater using the method in the literatures.

The aims of this study are to evaluate the feasibility of removal of high-concentration SO₄²⁻ from the sodium alkali FGD wastewater by using the ettringite precipitation method and investigate the influence of different parameters on SO₄²⁻ removal, in particular, anions and heavy metal ions. In addition, the removal of SO₄²⁻ and other ions in actual sodium alkali FGD wastewater by using ettringite precipitation method was also studied. Finally, the optimal experimental conditions for the removal of high-concentration sulfate ions were determined.

2. Experimental Section

2.1. Materials

Na(SO₄)₂, NaAlO₂, Ca(OH)₂, NaF, NaCl, KNO₃, NaOH, MgSO₄, MnSO₄, and HNO₃ were of analytical grade and used directly without purification. NaAlO₂ and Ca(OH)₂ were used as aluminum and calcium sources in this study. All reagents were purchased from Guangzhou Chemical Reagent Factory. The sulfate-rich simulated wastewater used in this study was prepared by dissolving Na₂SO₄ in deionized water to get initial SO₄²⁻ concentration of 10,000 mg/L. The initial pH of the solution was adjusted using HNO₃ (1.0 mol/L) and NaOH (1.0 mol/L).

The actual sodium alkali (NaOH) FGD wastewater was obtained from a ceramic production enterprise located in Guang Dong Province, China. The composition of the ions in the wastewater was analyzed and the results are shown in Tables 1 and 2 (in Section 3.8).

Table 1: Results of anions removal.

SO₄²⁻
Cl⁻
F⁻
NO₃⁻
pH

Actual wastewater, mg/L
15,274.8
205.56
101.43
58.30
7.8

Purified wastewater, mg/L
172.61
199.71
7.95
54.94
13.1

Average removal efficiency, %
98.87
2.85
92.16
5.76
—

Table 2: Results of heavy metal ions removal.

Cr²⁺
Mg²⁺
Mn²⁺
Ni²⁺
Pb²⁺
Zn²⁺

Actual wastewater, mg/L
3.55
15.48
22.76
5.53
5.15
5.29

Purified wastewater, mg/L
—
0.0102
0.0019
—
—
—

Average removal efficiency, %
100
99.93
99.99
100
100
100

2.2. Analytical Methods

The concentrations of SO₄²⁻, NO₃⁻, F⁻, and Cl⁻ in the solution were analyzed with an ion chromatography system (Metrohm 883, Switzerland). The concentrations of heavy metal ions, such as Mg²⁺ and Mn²⁺, were measured using an inductively coupled plasma emission spectrometer (ICP-AES 710, Agilent Technologies). An MP511 pH detector (Shanghai Precision Instruments Co., Ltd.) was used to determine the pH of the solution.

2.3. Removal of Sulfate Ions

Experiments were performed on a six league electric blender (ZR4-6, China). The experimental steps of the ettringite precipitation method are as follows: (1) 1 L of solution sample was taken in a glass reactor, and a certain amount of Ca(OH)₂ and NaAlO₂ was added to the solution. (2) Then, the sample was stirred at a certain speed for a certain time. (3) Finally, the sample was taken and filtered under vacuum using a 0.45 μm microporous membrane filter. The filtrates were analyzed for SO₄²⁻ and other ions, and finally, the SO₄²⁻ and other ions removal efficiencies were calculated by (1). According to the above steps, batch experiments effecting different experimental conditions on SO₄²⁻ removal were implemented, including Ca(OH)₂ dosage (the molar ratios of Ca(OH)₂ to SO₄²⁻ of 1∼6 : 1), NaAlO₂ dosage (the molar ratios of NaAlO₂ to SO₄²⁻ of 0.7∼3 : 1), solution initial pH (3.0∼11.0), solution temperature (25∼80°C), reaction time (15∼120 min), stirring speed (100∼500 r/min), and the concentrations of Cl⁻ (500∼4000 mg/L), NO₃⁻ (100∼2000 mg/L), F⁻ (200∼1000 mg/L), Mg²⁺ (50∼1000 mg/L), and Mn²⁺ (50∼1000 mg/L):

where is the SO₄²⁻ or other ions removal efficiency and and are the initial and final SO₄²⁻ or other ions concentrations of solutions (mg/L), respectively.

3. Results and Discussion

3.1. Effect of NaAlO₂ Dosage and Ca(OH)₂ Dosage

Previous researches have shown that SO₄²⁻ removal was significantly affected by NaAlO₂ dosage and Ca(OH)₂ dosage [21, 22]. So the effects of NaAlO₂ dosage and Ca(OH)₂ dosage on SO₄²⁻ removal were investigated and the results are shown in Figures 1 and 2.

Figure 1: The effect of NaAlO₂ dosage. Vsolution = 1 L, T = 25°C,  = 200 r/min, t = 30 min, and pH = 7.0.

Figure 2: The effect of Ca(OH)₂ dosage. Vsolution = 1 L, T = 25°C,  = 200 r/min, t = 30 min, and pH = 7.0.

As depicted in Figure 1, the SO₄²⁻ removal was greatly affected by the NaAlO₂ dosage, but the effect of NaAlO₂ dosage on the removal of sulfate was different under different molar ratios of Ca(OH)₂ to SO₄²⁻ (Ca/S ratio). The SO₄²⁻ removal decreased with increasing NaAlO₂ dosage, when the Ca/S ratio was less than 3 : 1. However, it was found that the SO₄²⁻ removal increased with the increase of the molar ratios of NaAlO₂ to SO₄²⁻ (Al/S ratio) at first, then decreased rapidly with the increase of NaAlO₂ dosage when the Ca/S ratio was more than 4 : 1. The theoretical Al/S ratio found in Ca₆Al₂(SO₄)₃(OH)₁₂ is about 0.67; the NaAlO₂ dosage was added in excess (Al/S ratio ≥ 0.7) in the series of experiments. It was found that the amount of ettringite generated was reduced with an increase of NaAlO₂ dosage; meanwhile, the monosulfate (Ca₄Al₂(SO₄)(OH)₁₂) generation increased [21]. The Al/S ratio in the monosulfate is 2 : 1 higher than the ettringite (2 : 3), and the Ca/S ratio in the monosulfate is 4 : 1 higher than the ettringite (2 : 1). Therefore, sulfate ions are mainly removed in monosulfate form at the high NaAlO₂ dosage condition, resulting in large consumption of calcium and aluminum, and reduction of sulfate ion removal. The results have shown that overdosing of NaAlO₂ is not conducive to sulfate ions removal; the preferred Al/S ratio is 1 : 1.

The effect of Ca(OH)₂ dosage on SO₄²⁻ removal is shown in Figure 2. The results show that SO₄²⁻ removal increased with an increase of Ca(OH)₂ dosage at first, then decreased slowly with the Ca(OH)₂ dosage further increasing, when the Al/S ratio was less than 1.5 : 1. This is because Ca²⁺ concentration in the solution increased with an increase of Ca(OH)₂ dosage, which facilitated the SO₄²⁻ removal. However, the pH of the solution increased with an increase of Ca(OH)₂ dosage. The reaction of NaAlO₂ hydrolysis could be inhibited by hydroxide ions; meanwhile, the hydroxide ions could promote the formation of monosulfate, and finally, the SO₄²⁻ removal decreased with the increase of the solution pH. When the Al/S ratio was higher than 1.5 : 1, SO₄²⁻ removal increased with the increase of Ca(OH)₂ dosage. The main reason is that sulfate ions are mainly removed in the monosulfate form at the high NaAlO2 dosage condition. The molar ratio of Ca²⁺ to SO₄²⁻ in the monosulfate is twice as much as that in the ettringite, and the concentration of Ca²⁺ in the solution increased with the Ca(OH)₂ dosage increasing, leading to generate a large amount of ettringite. Thus, the SO₄²⁻ removal increased. However, it is deduced that SO₄²⁻ removal will decrease with further increase in the Ca(OH)₂ dosage, as a large number of hydroxide ions are not conducive to the formation of ettringite. So overdosing of Ca(OH)₂ is neither desirable nor cost-effective; the preferred Ca/S ratio is 4 : 1.

Considering the SO₄²⁻ removal and cost-effectiveness, in the next series of experiments the molar ratios of Ca(OH)₂ to NaAlO₂ to SO₄²⁻ (Ca : Al : S) were constant at 4 : 1 : 1. In addition, based on the literatures and experimental results [19, 21], the following chemical equilibrium reactions can be used to describe the SO₄²⁻ removal reaction process:

3.2. Effect of the Solution Initial pH

It can be seen from Figure 3 that the removal of SO₄²⁻ was negligibly affected by the solution initial pH when the pH was ranging from 3.0 to 9.0 and the SO₄²⁻ removal efficiencies maintained at around 99% within the pH range. However, SO₄²⁻ removal efficiencies decreased slowly when the initial pH was more than 9.0; for example, the SO₄²⁻ removal efficiencies decreased from 98.69% to 94.19% in the pH range of 9.0∼11.0. There are two main reasons for this outcome. One of the reasons is that Ca(OH)₂ solubility decreased with the increase of solution pH, resulting in lower Ca²⁺ concentration in the solution and less ettringite production. Another reason is that the amount of CO₂ absorbed by the solution increased with increasing alkalinity of the solution, resulting in an increase of CO₃²⁻ concentration in the solution; thus CO₃²⁻ can react with ettringite to form hydrated carbonated calcium aluminate (3CaO·Al₂O₃·CaCO₃·11H₂O) [19, 23], leading to decrease in SO₄²⁻ removal efficiencies. Therefore, it is necessary to control the wastewater pH in practical engineering applications in the range of 5.0∼9.0 to achieve a higher SO₄²⁻ removal.

Figure 3: The effect of the solution initial pH. solution = 1 L, T = 25°C,  = 200 r/min, and t = 30 min.

3.3. Effect of Solution Temperature

Solution temperature is one of the important factors in SO₄²⁻ removal. As Figure 4 shows, SO₄²⁻ removal sharply decreased from 99.29% to 37.62%, when the solution temperature increased from 25 to 80°C. The main reason is that the solubility of Ca(OH)₂ increases with increasing temperature, and that the solution pH increases significantly with the increase of temperature. For example, when the reaction temperatures were 25 and 80°C, the solution pH after the reaction were 12.8 and 13.5, respectively. However, previous studies have shown that the optimal pH range for producing stability of ettringite is about 11∼12.5 [21, 24]. On the one hand, increasing the solution pH will inhibit the hydrolysis of sodium aluminate and further hinder the formation of Al(OH)₄⁻ and Al(OH)₆³⁻; on the other hand, increasing the pH will promote the decomposition of ettringite [21]. In addition, the solubility of ettringite increases with increasing temperature. Therefore, in order to achieve a higher SO₄²⁻ removal, it is necessary to reduce the wastewater temperature in practical engineering applications.

Figure 4: The effect of solution temperature. Vsolution = 1 L,  = 200 r/min, t = 30 min, and pH = 7.0.

3.4. Effect of Reaction Time

Figure 5 displays the effect of reaction time on SO₄²⁻ removal. The results show that the reaction time has a minimal effect on SO₄²⁻ removal. SO₄²⁻ removal efficiencies were 98.51%, 99.09%, 99.33%, 99.45%, 99.23%, and 99.48% when the reaction time were 15, 30, 45, 60, 90, and 120 min, respectively. SO₄²⁻ removal slightly increased with the increase of the reaction time. However, due to the low solubility of Ca(OH)₂, SO₄²⁻ removal was negligibly affected by the reaction time and remained almost constant at about 99%. The results show that the reaction of Ca²⁺, Al³⁺, and SO₄²⁻ is a rapid reaction under alkaline condition, being almost complete within 30 min. Taking into account the economic factor, area required, and SO₄²⁻ removal, the reaction time was selected as 30 min.

Figure 5: The effect of reaction time. Vsolution = 1 L,  = 200 r/min, T = 25°C, and pH = 7.0.

3.5. Effect of Stirring Speed

The influence of stirring speed on SO₄²⁻ removal was investigated and the results are shown in Figure 6. The results indicate that SO₄²⁻ removal was slightly affected by the stirring speed, and the SO₄²⁻ removal increased slowly from 98.62% to 99.85% with the increase of stirring speed from 100 to 500 r/min. In general, the increase of stirring speed helps promote the dissolution of the reagents by increasing the opportunities for contact and collision of ions and promote SO₄²⁻ removal. However, in this study the increase of stirring speed cannot significantly increase the SO₄²⁻ removal; the main reason is that the reaction of ettringite formation is a rapid reaction, and the appropriate stirring speed just can promote the completion of the reaction. Hence, it is not necessary to use high stirring speed to promote the reaction completion. Considering the cost and SO₄²⁻ removal, the stirring speed was selected as 200 r/min.

Figure 6: The effect of stirring speed. Vsolution = 1 L, t = 30 min, T = 25°C, and pH = 7.0.

3.6. Effect of Coexisted Anions

The flue gas usually contains chloride, fluoride, nitrogen oxides, and other components; they can be absorbed by the washing liquid. Therefore, certain concentrations of chloride, fluoride, and nitrate ions will be present in the desulfurization wastewater. In this study, the effects of coexisted anions such as Cl⁻, NO₃⁻, and F⁻ on SO₄²⁻ removal have been investigated, and the results are shown in Figures 7–9.

Figure 7: The effect of Cl⁻ concentration. Vsolution = 1 L, n (Ca : Al : S) = 4 : 1 : 1,  = 200 r/min, t = 30 min, T = 25°C, and pH = 7.0.

Figure 8: The effect of NO₃⁻ concentration. Vsolution = 1 L, n (Ca : Al : S) = 4 : 1 : 1,  = 200 r/min, t = 30 min, T = 25°C, and pH = 7.0.

Figure 9: The effect of F⁻ concentration. Vsolution = 1 L, n (Ca : Al : S) = 4 : 1 : 1,  = 200 r/min, t = 30 min, T = 25°C, and pH = 7.0.

As Figure 7 illustrates, SO₄²⁻ removal slowly decreased from 98.52% to 96.77% when the Cl⁻ concentration increased from 500 mg/L to 4000 mg/L. The results show that SO₄²⁻ removal is negligibly affected by the low Cl⁻ concentration (less than 2000 mg/L), but there is a certain negative impact on the SO₄²⁻ removal when the Cl⁻ concentration is high. It was reported that Cl⁻ can react with Ca²⁺ and Al³⁺ to form the Ca₄Al₂Cl₂(OH)₁₂ [25], though the solubility products of Ca₄Al₂Cl₂(OH)₁₂ (10−27.10) are much less than the solubility products of Ca₆Al₂(SO₄)₃(OH)₁₂ (10−111.6) [20, 25]; however, the high Cl⁻ concentration causes competitive reactions of Cl⁻ and SO₄²⁻ for the Ca²⁺ and Al³⁺, resulting in a decrease of SO₄²⁻ removal. Overall, chloride ions have a minimal effect on the removal of sulfate ions. Compared with the high SO₄²⁻ removal, Cl⁻ removal is low. When Cl⁻ concentration varied from 500 mg/L to 4000 mg/L, Cl⁻ removal almost remained stable at the range of 2% to 5%. Hence, in the case of coexistence of SO₄²⁻ and Cl⁻, SO₄²⁻ will be first effectively removed by using this method.

As demonstrated in Figure 8, the SO₄²⁻ removal was minimally affected when the NO₃⁻ concentration was less than 500 mg/L and remained stable at about 98%. However, when the NO₃⁻ concentration increased from 500 mg/L to 2000 mg/L, the removal of SO₄²⁻ decreased from 97.76% to 90.38%; meanwhile, the removal of NO₃⁻ almost remained stable at the range of 15% to 18% when the NO₃⁻ concentration increased from 500 mg/L to 2000 mg/L. The main reason is that high NO₃⁻ concentration causes severe competitive reactions of NO₃⁻ and SO₄²⁻ for the Ca²⁺ and Al³⁺. NO₃⁻ can react with Ca²⁺ and Al³⁺ to form Ca₄Al₂(NO₃)₂(OH)₁₂ [26]; hence, NO₃⁻ has a negative impact on SO₄²⁻ removal under high NO₃⁻ concentration.

It can be seen from Figure 9 that the presence of F⁻ in the solution has a significant inhibitory effect on SO₄²⁻ removal. SO₄²⁻ removal decreased rapidly from 98.49% to 61.39% when the F⁻ concentration increased from 200 mg/L to 1000 mg/L; meanwhile, the removal of F⁻ increased slowly from 92.24% to 96.34%. It is speculated that F⁻ could react with Ca²⁺ to form insoluble CaF₂, so the competitive reactions of Ca²⁺ with F⁻ and SO₄²⁻ existed in the solution and the amount of Ca²⁺ was insufficient in the solution due to the low solubility of Ca(OH)₂; hence, SO₄²⁻ removal decreased with the increase in the F⁻ concentration.

The research has shown that different types of anions in the solution had different effects on the removal of sulfate ions. The ability of three anions affected the SO₄²⁻ removal is F⁻ > NO₃⁻ > Cl⁻. It suggests that if the wastewater contains high concentrations of F⁻ and SO₄²⁻, F⁻ must first be removed in order to achieve high SO₄²⁻ removal.

3.7. Effect of Coexisted Heavy Metal Ions

A variety of heavy metal ions such as Pb²⁺, Ni²⁺, Mg²⁺, and Mn²⁺ usually exist in the desulfurization absorption solution. The flue gas is a major source of heavy metal ions; in addition to circulating water, the reagents also typically contain a certain amount of heavy metal ions. As the flue gas desulfurization absorption solution is usually alkaline in the sodium alkali (NaOH) FGD process, some types of heavy metal ions such as Pb²⁺ and Ni²⁺ easily react with OH⁻ to form hydroxide precipitates; therefore, the concentrations of these heavy metal ions in the solution are relatively low. It was found that Mn²⁺ and Mg²⁺ concentrations in the desulfurization wastewater were usually high. Hence, the study focused on the effect of Mn²⁺ and Mg²⁺ on SO₄²⁻ removal, and the results are displayed in Figures 10 and 11.

Figure 10: The effect of Mn²⁺ concentration. Vsolution = 1 L, n (Ca : Al : S) = 4 : 1 : 1,  = 200 r/min, t = 30 min, T = 25°C, and pH = 7.0.

Figure 11: The effect of Mg²⁺ concentration. Vsolution = 1 L, n (Ca : Al : S) = 4 : 1 : 1,  = 200 r/min, t = 30 min, T = 25°C, and pH = 7.0.

Data shown in Figure 10 indicate that manganese ions have no effect on SO₄²⁻ removal, when using the ettringite precipitation method. The removal of SO₄²⁻ and Mn²⁺ almost remained stable at 99% and 100%, respectively, when Mn²⁺ concentration increased from 50 mg/L to 1000 mg/L. This is because the solution is strongly alkaline due to the addition of excess calcium hydroxide; Mn²⁺ can easily react with OH⁻ to form insoluble manganese hydroxide (pKsp = 13.40) under strong alkaline conditions. Hence, Mn²⁺ preferentially converts to Mn(OH)₂ under the experimental conditions and has no effect on SO₄²⁻ removal.

As shown in Figure 11, magnesium ions have a certain impact on the removal of sulfate ions. The removal of SO₄²⁻ decreased from 98.78% to 93.67%, when the Mg²⁺ concentration increased from 50 mg/L to 1000 mg/L; meanwhile, the Mg²⁺ removal was almost maintained at between 88.68% and 90.36%. Previous researches have revealed that Mg²⁺ can react with OH⁻ to convert to Mg(OH)₄²⁻ which preferentially reacts with Al(OH)₄⁻ and SO₄²⁻ to form hydrotalcite-type compound (Mg₆Al₂SO₄(OH)₁₆·nH₂O) rather than ettringite (Ca₆Al₂(OH)₁₂(SO₄)₃·26H₂O) [19, 27]. Comparing the two chemical formulas, it can be found that the Al/S ratio is 2 : 1 in hydrotalcite-type compound, which is three times of that in ettringite, and the molar ratio of OH⁻ to SO₄²⁻ in hydrotalcite-type compound is four times of that in ettringite. It can be derived from the results that hydrotalcite-type compound consumes more Al³⁺ and OH⁻ compared with ettringite, resulting in the decrease of Al³⁺ and OH⁻ concentrations in the solution, and finally, resulting in a decline of SO₄²⁻ removal. Thus, in the coexistence of Mg²⁺ and Ca²⁺, Mg²⁺ can compete with Ca²⁺ for Al³⁺ to form hydrotalcite-type compound, and the higher the concentration of magnesium ions, the stronger the inhibitory effect of magnesium ion on the SO₄²⁻ removal.

3.8. Feasibility and Application Prospect of the Method

The removal of sulfate ions and other ions in actual flue gas desulfurization wastewater by using the ettringite precipitation method was evaluated. The composition of wastewater and the results of purification various ions are shown in Tables 1 and 2. The results indicate that the ettringite precipitation method had high removal efficiencies for SO₄²⁻, F⁻, and heavy metal ions, with average removal efficiencies of more than 98%, 90%, and 99%, respectively. In addition, Cl⁻ and NO₃⁻ could also be removed minimally by the method.

The results show that the removal of high-concentration sulfate ions from the sodium alkali FGD wastewater is feasible by using the ettringite precipitation method. The SO₄²⁻ concentration in the purified wastewater met the requirements for reuse of water which is 250 mg/L in China; meanwhile, F⁻ and heavy metal ions were effectively removed. As displayed in Table 1, the pH of the purified wastewater was 13.1; thus, the water could be reused to decrease the consumption of water and alkali in the flue gas treatment system and to reduce the operating costs. In addition, the solid sediment produced by wastewater treatment can be used as a raw material for ceramics and other building materials production. Therefore, considering the cost-effectiveness, pollutant removal efficiencies, and resource reuse, the ettringite precipitation method has the potential to be a commercial application in the field of removal of high-concentration sulfate ions from the industrial wastewater in the future.

4. Conclusions

In this paper, the ettringite precipitation method was used to remove the sulfate ions, and the influences of experimental parameters on SO₄²⁻ removal were investigated. Based on the results of the experiments, the following conclusions can be obtained:

(1)The ettringite precipitation method can effectively synergistically remove SO₄²⁻, F⁻, and heavy metal ions with removal efficiencies of more than 98%, 90%, and 99%, respectively. In addition, Cl⁻ and NO₃⁻ also can be removed minimally by the method.(2)NaAlO₂ dosage, Ca(OH)₂ dosage, solution initial pH, solution temperature, reaction time, stirring speed, anions (Cl⁻, NO₃⁻ and F⁻), and heavy metal ions all have effects on the Cl⁻ removal. Finally, the optimal experimental conditions (Al/S ratio, 1 : 1; Ca/S ratio, 4 : 1; solution temperature, 25°C; reaction time, 30 min; and stirring speed, 200 r/min) were determined.(3)The ettringite precipitation method is feasible for treating high-concentration sulfate wastewater and has the potential to be a commercial application in the high-concentration sulfate wastewater treatment field in the future.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the National Key R&D Program of China (2017YFC0210704 and 2017YFC0210803), National Natural Science Foundation of China (NSFC-51778264), Project of Science and Technology Program of Guangdong Province (2015A020220008, 2015B020215008, 2016B020241002, and 2017B020237002), Youth Top-Notch Talent Special Support Program of Guangdong Province (2016TQ03Z576), and Pearl River S&T Nova Program of Guangzhou (201610010150).

References

P. Fang, Z. J. Tang, X. B. Chen et al., “Split, partial oxidation and mixed absorption: a novel process for synergistic removal of multiple pollutants from simulated flue gas,” Industrial & Engineering Chemistry Research, vol. 56, no. 17, pp. 5116–5126, 2017.

W. Cao, Z. Dang, X. Q. Zhou et al., “Removal of sulphate from aqueous solution using modified rice straw: preparation, characterization and adsorption performance,” Carbohydrate Polymers, vol. 85, no. 3, pp. 571–577, 2011.

I. Pikaar, K. R. Sharma, S. Hu, W. Gernjak, J. Keller, and Z. Yuan, “Reducing sewer corrosion through integrated urban water management,” Science, vol. 345, no. 6198, pp. 812–814, 2014.

H. Sun, B. Shi, F. Yang, and D. Wang, “Effects of sulfate on heavy metal release from iron corrosion scales in drinking water distribution system,” Water Research, vol. 114, pp. 69–77, 2017.

D. S. Baldwin and A. Mitchell, “Impact of sulfate pollution on anaerobic biogeochemical cycles in a wetland sediment,” Water Research, vol. 46, no. 4, pp. 965–974, 2012.

A. M. Silva, R. M. F. Lima, and V. A. Leao, “Mine water treatment with limestone for sulfate removal,” Journal of Hazardous Materials, vol. 221-222, pp. 45–55, 2012.

T. Najib, M. Solgi, A. Farazmand, S. M. Heydarian, and B. Nasernejad, “Optimization of sulfate removal by sulfate reducing bacteria using response surface methodology and heavy metal removal in a sulfidogenic UASB reactor,” Journal of Environmental Chemical Engineering, vol. 5, no. 4, pp. 3256–3265, 2017.

M. Gopi Kiran, K. Pakshirajan, and G. Das, “Heavy metal removal from multicomponent system by sulfate reducing bacteria: mechanism and cell surface characterization,” Journal of Hazardous Materials, vol. 324, pp. 62–70, 2017.

H. Lu, W. Zou, P. Chai, J. Wang, and L. Bazinet, “Feasibility of antibiotic and sulfate ions separation from wastewater using electrodialysis with ultra-filtration membrane,” Journal of Cleaner Production, vol. 112, pp. 3097–3105, 2016.

S. Q. Hong, F. S. Cannon, P. Hou, T. Byrne, and C. Nieto-Delgado, “Adsorptive removal of sulfate from acid mine drainage by polypyrrole modified activated carbons: effects of polypyrrole deposition protocols and activated carbon source,” Chemosphere, vol. 184, pp. 429–437, 2017.

O. Calinescu, N. M. Marin, D. Ionita et al., “Selective removal of sulfate ion from different drinking waters,” Environmental Nanotechnology, Monitoring and Management, vol. 6, pp. 164–168, 2016.

N. Arahman, S. Mulyati, M. RahmahLubis, R. Takagi, and H. Matsuyama, “Removal profile of sulfate ion from mix ion solution with different type and configuration of anion exchange membrane in electrodialysis,” Journal of Water Process Engineering, vol. 20, pp. 173–179, 2017.

M. A. Mamelkina, S. Cotillas, E. Lacasa et al., “Removal of sulfate from mining waters by electrocoagulation,” Separation and Purification Technology, vol. 182, pp. 87–93, 2017.

S. Tait, W. P. Clarke, J. Keller, and D. J. Batstone, “Removal of sulfate from high-strength wastewater by crystallization,” Water Research, vol. 43, no. 3, pp. 762–772, 2009.

M. D. G. Luna, D. P. M. Rance, L. M. Bellotindos, and M. C. Lu, “Removal of sulfate by fluidized bed crystallization process,” Journal of Environmental Chemical Engineering, vol. 5, no. 3, pp. 2431–2439, 2017.

C. T. Benatti, C. R. G. Tavares, and E. Lenzi, “Sulfate removal from waste chemicals by precipitation,” Journal of Environmental Management, vol. 90, no. 1, pp. 504–511, 2009.

H. Runtti, T. Luukkonen, M. Niskanen et al., “Sulphate removal over barium-modified blast-furnace-slag geopolymer,” Journal of Hazardous Materials, vol. 317, pp. 373–384, 2016.

D. N. Kartic, B.C. H. A. Narayana, and M. Arivazhagan, “Removal of high concentration of sulfate from pigment industry effluent by chemical precipitation using barium chloride: RSM and ANN modeling approach,” Journal of Environmental Management, vol. 206, pp. 69–76, 2018.

W. X. Dou, Z. Zhou, L. M. Jiang et al., “Sulfate removal from wastewater using ettringite precipitation: magnesium ion inhibition and process optimization,” Journal of Environmental Management, vol. 196, pp. 518–526, 2017.

E. T. Tolonen, T. Hu, J. Ramo, and U. Lassi, “The removal of sulphate from mine water by precipitation as ettringite and the utilisation of the precipitate as a sorbent for arsenate removal,” Journal of Environmental Management, vol. 181, pp. 856–862, 2016.

D. Almasri, K. A. Mahmoud, and A. Abdel-Wahab, “Two-stage sulfate removal from reject brine in inland desalination with zero-liquid discharge,” Desalination, vol. 362, pp. 52–58, 2015.

I. Kabdaslı, A. Bilgin, and O. Tünay, “Sulphate control by ettringite precipitation in textile industry wastewaters,” Environmental Technology, vol. 37, no. 4, pp. 446–451, 2016.

H. Z. Ma and M. Deng, “Effect of alkali on ettringite crystallization and solubility,” Journal of Nanjing University of Technology, vol. 29, pp. 37–40, 2007, in Chinese.

M. Chrysochoou and D. Dermatas, “Evaluation of ettringite and hydrocalumite formation for heavy metal immobilization: literature review and experimental study,” Journal of Hazardous Materials, vol. 136, no. 1, pp. 20–33, 2006.

A. Abdel-Wahab, B. Batchelor, and J. Schwantes, “An equilibrium model for chloride removal from recycled cooling water using the ultra-high lime with aluminum process,” Water Environment Research, vol. 77, no. 7, pp. 3059–3065, 2005.

J. P. Rapin, G. Renaudin, E. Elkaim, and M. Francois, “Structural transition of Friedel’s salt 3CaO·Al₂O₃·CaCl₂·10H₂O studied by synchrotron powder diffraction,” Cement and Concrete Research, vol. 32, no. 4, pp. 513–519, 2002.

R. Shi, P. Yang, Y. Yin, X. Dong, and J. Li, “Fabrication of porous microspheres and network arrays of Zn-Al hydrotalcite-like compounds on Al substrate via facile hydrothermal method,” Ceramics International, vol. 40, no. 5, pp. 6855–6863, 2014.