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

*e 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 (Mg and Mn), 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 SO4with 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. *e result of actual wastewater treatment shows that the method is feasible for treating high-concentration sulfate wastewater. *e ettringite precipitation method has the potential to be a commercial application in the future.


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.
erefore, the sodium alkali (NaOH) FGD technology with relatively simple process and high SO 2 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.
e sulfate (SO 4 2− ) 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 (NO 3 − , Cl − , and F − ) and heavy metal ions (Mg 2+ and Mn 2+ ).ough SO 4 2− is a common and nontoxic component of various types of water bodies, high concentrations of SO 4 2− 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][3][4][5].In addition, high concentrations of SO 4 2− (>600 mg/L) in the water can cause laxative effects in mammals [6].Hence, to protect the environment, the SO 4 2− 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.
e limestone precipitation method is widely used in the field of water treatment; however, due to the relatively high solubility of gypsum [19], the SO 4 2− removal efficiency is low.e barium chloride precipitation method can reach high SO 4 2− 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.e 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][20][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 SO 4 2− from the sodium alkali (NaOH) FGD wastewater using the method in the literatures.
e aims of this study are to evaluate the feasibility of removal of high-concentration SO 4 2− from the sodium alkali FGD wastewater by using the ettringite precipitation method and investigate the influence of different parameters on SO 4 2− removal, in particular, anions and heavy metal ions.In addition, the removal of SO 4 2− 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− concentration of 10,000 mg/L.e initial pH of the solution was adjusted using HNO 3 (1.0 mol/L) and NaOH (1.0 mol/L).e actual sodium alkali (NaOH) FGD wastewater was obtained from a ceramic production enterprise located in Guang Dong Province, China.e composition of the ions in the wastewater was analyzed and the results are shown in Tables 1 and 2 (in Section 3.8).

Analytical Methods.
e concentrations of SO 4 2− , NO 3 − , F − , and Cl − in the solution were analyzed with an ion chromatography system (Metrohm 883, Switzerland).e concentrations of heavy metal ions, such as Mg 2+ and Mn 2+ , 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.

Removal of Sulfate Ions.
Experiments were performed on a six league electric blender (ZR4-6, China).e 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 NaAlO 2 was added to the solution.(2) en, 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.e filtrates were analyzed for SO 4 2− and other ions, and finally, the SO 4 2− and other ions removal efficiencies were calculated by (1).According to the above steps, batch experiments effecting different experimental conditions on SO 4 2− removal were implemented, including Ca(OH) 2 dosage (the molar ratios of Ca(OH) 2 to SO 4 2− of 1∼6 : 1), NaAlO 2 dosage (the molar ratios of NaAlO 2 to SO 4 2− 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), NO 3 − (100∼2000 mg/L), F − (200∼1000 mg/L), Mg 2+ (50∼1000 mg/L), and Mn 2+ (50∼1000 mg/L): where η is the SO 4 2− or other ions removal efficiency and C 0 and C t are the initial and final SO 4 2− or other ions concentrations of solutions (mg/L), respectively.removal were investigated and the results are shown in Figures 1 and 2.
As depicted in Figure 1, the SO 4 2− removal was greatly a ected by the NaAlO 2 dosage, but the e ect of NaAlO 2 dosage on the removal of sulfate was di erent under different molar ratios of Ca(OH) 2 to SO 4 2− (Ca/S ratio).e SO 4 2− removal decreased with increasing NaAlO 2 dosage, when the Ca/S ratio was less than 3 : 1.However, it was found that the SO 4 2− removal increased with the increase of the molar ratios of NaAlO 2 to SO 4 2− (Al/S ratio) at rst, then decreased rapidly with the increase of NaAlO 2 dosage when the Ca/S ratio was more than 4 : 1. e theoretical Al/S ratio found in Ca 6 Al 2 (SO 4 ) 3 (OH) 12 is about 0.67; the NaAlO 2 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 NaAlO 2 dosage; meanwhile, the monosulfate (Ca 4 Al 2 (SO 4 )(OH) 12 ) generation increased [21].e 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).erefore, sulfate ions are mainly removed in monosulfate form at the high NaAlO 2 dosage condition, resulting in large consumption of calcium and aluminum, and reduction of sulfate ion removal.e results have shown that overdosing of NaAlO 2 is not conducive to sulfate ions removal; the preferred Al/S ratio is 1 : 1. e e ect of Ca(OH) 2 dosage on SO 4 2− removal is shown in Figure 2. e results show that SO 4 2− removal increased with an increase of Ca(OH) 2 dosage at rst, then decreased slowly with the Ca(OH) 2 dosage further increasing, when the Al/S ratio was less than 1.5 : 1.
is is because Ca 2+ concentration in the solution increased with an increase of Ca(OH) 2 dosage, which facilitated the SO 4 2− removal.However, the pH of the solution increased with an increase of Ca(OH) 2 dosage.e reaction of NaAlO 2 hydrolysis could be inhibited by hydroxide ions; meanwhile, the hydroxide ions could promote the formation of monosulfate, andnally, the SO 4 2− removal decreased with the increase of the solution pH.When the Al/S ratio was higher than 1.5 :1, SO 4 2− removal increased with the increase of Ca(OH) 2 dosage.e main reason is that sulfate ions are mainly removed in the monosulfate form at the high NaAlO2 dosage condition.
e molar ratio of Ca 2+ to SO 4 2− in the monosulfate is twice as much as that in the ettringite, and the concentration of Ca 2+ in the solution increased with the Ca(OH) 2 dosage increasing, leading to generate a large amount of ettringite.us, the SO 4 2− removal increased.However, it is deduced that SO 4 2− 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-e ective; the preferred Ca/S ratio is 4 :1.
Considering the SO 4 2− removal and cost-e ectiveness, in the next series of experiments the molar ratios of Ca(OH) 2 to NaAlO 2 to SO 4 2− (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 SO 4 2− removal reaction process:

E ect of the Solution Initial pH.
It can be seen from Figure 3 that the removal of SO 4 2− was negligibly a ected by the solution initial pH when the pH was ranging from 3.0 to 9.0 and the SO 4 2− removal e ciencies maintained at around 99% within the pH range.However, SO 4 2− removal eciencies decreased slowly when the initial pH was more than 9.0; for example, the SO 4 2− removal e ciencies decreased from 98.69% to 94.19% in the pH range of 9.0∼11.0.ere 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 Ca 2+ concentration in the solution and less ettringite production.Another reason is that the amount of CO 2 absorbed by the solution increased with increasing alkalinity of the solution, resulting in an increase of CO 3 2− concentration in the solution; thus CO 3 2− can react with ettringite to form hydrated carbonated calcium aluminate [19,23], leading to decrease in SO 4 2− removal e ciencies.erefore, it is necessary to control the wastewater pH in practical engineering applications in the range of 5.0∼9.0 to achieve a higher SO 4 2− removal.

E ect of Solution
Temperature.Solution temperature is one of the important factors in SO 4 2− removal.As Figure 4 shows, SO 4 2− removal sharply decreased from 99.29% to 37.62%, when the solution temperature increased from 25 to 80 °C.e main reason is that the solubility of Ca(OH) 2 increases with increasing temperature, and that the solution pH increases signi cantly 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) 6 3− ; on the other hand, increasing the pH will promote the decomposition of ettringite [21].In addition, the solubility of ettringite increases with increasing temperature.erefore, in order to achieve a higher SO 4 2− removal, it is necessary to reduce the wastewater temperature in practical engineering applications.removal e ciencies 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.SO 4 2− removal slightly increased with the increase of the reaction time.However, due to the low solubility of Ca(OH) 2 , SO 4 2− removal was negligibly a ected by the reaction time and remained almost constant at about 99%.e results show that the reaction of Ca 2+ , Al 3+ , and SO 4

E ect of Reaction Time.
2− is a rapid reaction under alkaline condition, being almost complete within 30 min.Taking into account the economic factor, area required, and SO 4 2− removal, the reaction time was selected as 30 min.

E ect of Stirring Speed.
e in uence of stirring speed on SO 4 2− removal was investigated and the results are shown in Figure 6. e results indicate that SO 4 2− removal was slightly a ected by the stirring speed, and the SO 4 2− 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 SO 4 2− removal.However, in this study the increase of stirring speed cannot signi cantly increase the SO 4 2− 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 SO 4 2− removal, the stirring speed was selected as 200 r/min.

E ect of Coexisted
Anions.e ue gas usually contains chloride, uoride, nitrogen oxides, and other components; they can be absorbed by the washing liquid.erefore, certain concentrations of chloride, uoride, and nitrate ions will be present in the desulfurization wastewater.In this study, the e ects of coexisted anions such as Cl − , NO 3 − , and F − on SO 4 2− removal have been investigated, and the results are shown in Figures 7-9.
As Figure 7 illustrates, SO 4 2− removal slowly decreased from 98.52% to 96.77% when the Cl − concentration increased from 500 mg/L to 4000 mg/L.e results show that SO 4 2− removal is negligibly a ected by the low Cl − concentration (less than 2000 mg/L), but there is a certain negative impact on the SO 4 2− removal when the Cl − concentration is high.It was reported that Cl − can react with Ca 2+ and Al 3+ to form the Ca 4 Al 2 Cl 2 (OH) 12 [25], though the solubility products of Ca 4 Al 2 Cl 2 (OH) 12 (10 −27.10 ) are much less than the solubility products of Ca 6 Al 2 (SO 4 ) 3 (OH) 12 (10 −111.6 ) [20,25]; however, the high Cl − concentration causes competitive reactions of Cl − and SO 4 2− for the Ca 2+ and Al 3+ , resulting in a decrease of SO 4 2− removal.Overall, chloride ions have a minimal e ect on the removal of sulfate ions.Compared with the high SO 4 2− 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 SO 4 2− and Cl − , SO 4 2− will be rst e ectively removed by using this method.
As demonstrated in Figure 8, the SO 4 2− removal was minimally a ected when the NO 3 − concentration was less than 500 mg/L and remained stable at about 98%.However, when the NO 3 − concentration increased from 500 mg/L to 2000 mg/L, the removal of SO 4 2− decreased from 97.76% to 90.38%; meanwhile, the removal of NO 3 − almost remained stable at the range of 15% to 18% when the NO 3 − concentration increased from 500 mg/L to 2000 mg/L.e main reason is that high NO 3 − concentration causes severe competitive reactions of NO 3 − and SO 4 2− for the Ca 2+ and Al 3+ .NO 3 − can react with Ca 2+ and Al 3+ to form Ca 4 Al 2 (NO 3 ) 2 (OH) 12 [26]; hence, NO 3 − has a negative impact on SO 4 2− removal under high NO 3 − concentration.It can be seen from Figure 9 that the presence of F − in the solution has a signi cant inhibitory e ect on SO 4 2− removal.SO 4 2− 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 Ca 2+ to form insoluble CaF 2 , so the competitive reactions of Ca 2+ with F − and SO 4 2− existed in the solution and the amount of Ca 2+ was insu cient in the solution due to the low solubility of Ca(OH) 2 ; hence, SO 4 2− removal decreased with the increase in the F − concentration.
e research has shown that di erent types of anions in the solution had di erent e ects on the removal of sulfate ions.e ability of three anions a ected the SO 4 2− removal is F − > NO 3 − > Cl − .It suggests that if the wastewater contains high concentrations of F − and SO 4 2− , F − must rst be removed in order to achieve high SO 4 2− removal.

E ect of Coexisted Heavy Metal Ions.
A variety of heavy metal ions such as Pb 2+ , Ni 2+ , Mg 2+ , and Mn 2+ usually exist in the desulfurization absorption solution.e ue 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 ue gas desulfurization absorption solution is usually alkaline in the sodium alkali (NaOH) FGD process, some types of heavy metal ions such as Pb 2+  and Ni 2+ 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 Mn 2+ and Mg 2+ concentrations in the desulfurization wastewater were usually high.Hence, the study focused on the e ect of Mn 2+ and Mg 2+ on SO 4 2− removal, and the results are displayed in Figures 10 and 11.
Data shown in Figure 10 indicate that manganese ions have no e ect on SO 4 2− removal, when using the ettringite precipitation method.
e removal of SO 4 2− and Mn 2+

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Journal of Chemistry almost remained stable at 99% and 100%, respectively, when Mn 2+ concentration increased from 50 mg/L to 1000 mg/L. is is because the solution is strongly alkaline due to the addition of excess calcium hydroxide; Mn 2+ can easily react with OH − to form insoluble manganese hydroxide (pKsp � 13.40) under strong alkaline conditions.Hence, Mn 2+ preferentially converts to Mn(OH) 2 under the experimental conditions and has no effect on SO 4 2− removal.As shown in Figure 11, magnesium ions have a certain impact on the removal of sulfate ions.e removal of SO  [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 SO 4 2− in hydrotalcitetype compound is four times of that in ettringite.It can be derived from the results that hydrotalcite-type compound consumes more Al 3+ and OH − compared with ettringite, resulting in the decrease of Al 3+ and OH − concentrations in the solution, and finally, resulting in a decline of SO 4 2− removal.us, in the coexistence of Mg 2+ and Ca 2+ , Mg 2+ can compete with Ca 2+ for Al 3+ to form hydrotalcite-type compound, and the higher the concentration of magnesium ions, the stronger the inhibitory effect of magnesium ion on the SO 4 2− removal.

Feasibility and Application Prospect of the Method.
e removal of sulfate ions and other ions in actual flue gas desulfurization wastewater by using the ettringite precipitation method was evaluated.e composition of wastewater and the results of purification various ions are shown in Tables 1 and 2.
e results indicate that the ettringite precipitation method had high removal efficiencies for SO 4 2− , F − , and heavy metal ions, with average removal efficiencies of more than 98%, 90%, and 99%, respectively.In addition, Cl − and NO 3 − could also be removed minimally by the method.
e results show that the removal of high-concentration sulfate ions from the sodium alkali FGD wastewater is feasible by using the ettringite precipitation method.
e SO 4 2− 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.erefore, 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.

Conclusions
In this paper, the ettringite precipitation method was used to remove the sulfate ions, and the influences of experimental parameters on SO 4 2− removal were investigated.Based on the results of the experiments, the following conclusions can be obtained: (1) e ettringite precipitation method can effectively synergistically remove SO 4 2− , F − , and heavy metal ions with removal efficiencies of more than 98%, 90%, and 99%, respectively.In addition, Cl − and NO 3 − also can be removed minimally by the method.
(3) e 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.

Table 1 :
Results of anions removal.