The relationship between speciation and release ability of mercury in flue gas desulfurization (FGD) gypsum

Fuel 125 (2014) 66–72

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The relationship between speciation and release ability of mercury in flue gas desulfurization (FGD) gypsum Mingyang Sun, Jiaai Hou, Guanghuan Cheng, Shams Ali Baig, Lisha Tan, Xinhua Xu ⇑ Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, People’s Republic of China

g r a p h i c a l a b s t r a c t

complex phase in FGD gypsum.  Chlorine content in coal might influence water soluble Hg in FGD gypsum.  Hg release ability from FGD gypsum was related to its speciation.  Water soluble Hg was easily to release, while residual Hg was relatively stable.  Hg emission from FGD gypsum during its disposal in Zhejiang province was estimated.

slow release Water soluble Diluted acid extractable Strong complex resistant release Residual rapid release

rapid release

resistant release rapid release

a r t i c l e

i n f o

Article history: Received 29 November 2013 Received in revised form 21 January 2014 Accepted 6 February 2014 Available online 19 February 2014 Keywords: Mercury Speciation Sequential extraction FGD gypsum Release ability

Frap : F(water souble+acid souble)

Fr : F residual

y=x

y=1.193x+17.56; 2 R =0.963

40

y=0.992x+13.69; 2 R =0.818

35 30

90

25

y=x

85

y=0.985x+12.83; 2 R =0.808

20 15

80 75

y=1.305x- 46.97; 2 R =0.820

70 65 60 55

10

50 78

80

82

84

86

88

90

the ratio of extractable mercury to total extract, %

5 5

10 15 20 25 30 35 40 45 the ratio of mercury speciation to total extract, %

50

Multipurpose utilization of flue gas desulfurization (FGD) gypsums releases mercury into environment and poses threats to public health. Determining Hg speciation is essential not only for predicting its toxicity and mobility but also for designing effective remediation strategies. Sequential chemical extraction (SCE) method was used to analyze Hg speciation in this study. The total Hg concentration in four samples ranged from 0.61 to 1.63 lg/g. XRD and EDX analysis revealed that the main chemical composition of FGD gypsum was calcium sulfate (CaSO4). SCE result indicated that Hg was mainly distributed in the strong complex phase, ranging from 60% to 80%. Water soluble mercury in Sample SX accounted for 30% of the total extract, which might be attributed to the relatively high chlorine content in coal. Moreover, the mobility of Hg from FGD gypsum was also investigated in this study, which exhibited biphasic kinetics. The rapid release of Hg was related to the ratio of water soluble Hg at some extent (R2 = 0.818), which signified of more attentions for its stabilization. This study also suggested theoretical framework for the environmental risk associated with FGD gypsum during its usage and disposal. Ó 2014 Elsevier Ltd. All rights reserved.

Due to the high toxicity, volatility and bio-accumulation, mercury contamination in local, regional and global environments

⇑ Corresponding author. Tel./fax: +86 571 88982031.

http://dx.doi.org/10.1016/j.fuel.2014.02.012 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

Frap : Fwater souble 45

a b s t r a c t

1. Introduction

E-mail address: [email protected] (X. Xu).

the ratio of mercury in release fraction to total mercury, %

 Hg was mainly distributed in strong

Frap+Fslow

h i g h l i g h t s

has received growing attention of legislative bodies, policy makers and researchers in recent years [1,2]. Human health hazards from Hg compounds are associated with their persistence in the environment and potential of bio-accumulation in the food chain [3]. Coal-fired power plants are responsible for most of the anthropogenic mercury emissions into the environment. The world’s coal reserves are the most abundant fossil energy source, which are

M. Sun et al. / Fuel 125 (2014) 66–72

widely used for power generation and domestic heating [4]. In China, coal constitutes about 70% of the total primary energy consumption [5]. Although Hg concentration in coal is relative low (average concentration is about 0.1 ppm), significant amounts of Hg are released during the combustion process, and then redistributed among the flue gas and coal utilization byproducts, which are generated from the cleaning of flue gases [6]. Examination of Hg speciation in flue gases revealed that Hg exits in three forms: mainly elemental Hg (Hg0), oxidized Hg (Hg2+) and particulate Hg (Hgp) [7]. Among these forms: Hgp can be captured by the typical air pollution control devices (APCD), while water soluble Hg2+ can be efficiently removed by flue gas desulfurization (FGD) system, a proven and effective method used for the removal of sulfur dioxide (SO2). However, Hg0 is hard to remove due to its volatile, inert and virtually insoluble nature. Therefore, the oxidation of Hg0 in flue gas has received much attention. For example, Galbreath et al. [8] found that the injected c-Fe2O3 into the flue gas could increase the extent of mercury oxidation. These studies further enhanced the Hg capture efficiency of FGD system. Meanwhile, the amount of Hg in FGD gypsums may also increase in future if these units are optimized for co-capture. Hence, the associated environmental risk of FGD gypsum increased during its disposal and the subsequent utilization, such as soil amendments and road subbase [9]. For this reason, it is important to make sure the chemistry of the Hg–gypsum interaction, to be able to predict the environmental fate of the Hg bearing FGD gypsums. Most of the studies reported Hg in FGD gypsums concern the total Hg concentration. However, few attempts have been made to evaluate the speciation that it may exist [10–12]. Use of total concentration as a criterion to assess the potential risk of Hg-laden FGD gypsum presumes that all forms of mercury play an equal role on the environment, such an assumption is clearly untenable and not justified. Different chemical forms and oxidation states of Hg in FGD gypsum define its toxicity, mobility and biological availability. Thus, its determination is important not only to determine the risk when the FGD gypsums are finally disposed of but also to understand the behavior of Hg during the combustion process and therefore to develop the appropriate Hg removal technology. Methods for determining Hg speciation have increased in both number and sophistication over time. The most widely used analysis methods are sequential chemical extractions (SCE) [10,13–14], temperature programmed decomposition [12], capillary electrophoresis [15] and in situ X-ray adsorption spectroscopic analysis [10,11]. Among these, X-ray absorption spectroscopy and X-ray fluorescence spectroscopy are frequently used to provide direct, in situ information on metal bonding and coordination in solid samples, but one limitation of this method is the measurement of Hg LIII edge XAFS data using currently available synchrotron radiation sources and the X-ray detectors when total Hg concentration in nature samples was below 100 lg/g. SCE has been most widely used to determine Hg speciation due to the ease, efficiency and reproducibility of the procedure. The features detection levels of SCE is low enough (0.5  103 lg/g) to measure Hg in ambient sediments and soils. This paper focuses on the speciation and mobility of Hg in FGD gypsum collected from different representative coal-fired power plants in Zhejiang province. Moreover, this study was also concerned with the evaluation of potential risk of Hg emission during disposal of FGD gypsum. Considering the Hg concentration in FGD gypsum was relatively low [9], SCE method was selected to investigate the Hg speciation in FGD gypsum. In addition, Hg emission as a function of time during disposal process was simulated. Useful knowledge can be achieved to evaluate appropriate disposal and reuse scenarios of the FGD gypsum.

67

2. Experiments and methods 2.1. Sampling and characterization Multiple samples of FGD gypsums, labeled as Sample SX, HZ-1, HZ, and HZ-2, respectively, were obtained from four different coalfired power plants located in Zhejiang province, China. Bituminous coal produced from different provinces was used in these plants, and the mean coal-Hg concentration was about 0.23 lg/g. Other information of the selected power plants was listed in Table 1. The moisture content of the collected gypsum samples was about 20–30% and all the samples were air-dried before analysis. Surface area of the samples was performed with a 100CX surface area analyzer (Coulter Omnisorp, USA) using BET method [16]. The dry samples were examined with X-ray diffraction (XRD, D/Max-2550 pc, Rigaku Inc., Japan) using CuKa radiation at a scanning rate of 8° min1 in the 2h range from 5° to 80°. The structure and morphology of each sample were examined using a scanning electron microscopy (SEM, SU-70, Hitachi, Japan) at ambient temperature and 3.0 kV. Additionally, Energy dispersive X-ray spectroscopy (EDX) coupled with SEM was used to confirm the chemical composition of the sample matrixes and the association of Hg with specific matrix phases. Besides, the surface organic functional groups of FGD gypsums were recorded between 4000 and 400 cm1 wavenumber using a Fourier transform infrared spectrometer (IRPrestige-21, Japan). All samples were mixed with spectroscopic grade KBr in the ratio of 1:150 to produce sufficient absorbance. All the spectra were recorded at a resolution of 4.0 cm1 using a minimum of 400 scans.

2.2. Reagents and analytical methods All reagents used in this study were of analytical or higher grade. Deionized (DI) water was supplied by Zhejiang University throughout this study. The gases Ar (P99.95%) stored under pressure in the steel cylinders were provided by the Jin-Gong Gas Co., Ltd. Guaranteed grade HCl (36–38%) and HNO3 (65–68%) was bought from Sinopharm Chemical Reagent Co., Ltd. The mercury stock solution of 1.0 g/L was prepared by dissolving mercury chloride (HgCl2, Pharmaceutical Group Shanghai Chemical Reagent Company, China) in 100 mL stationary liquid (HNO3–K2Cr2O7 solution) and then stored in the refrigerator. The Hg content obtained from the experimental solutions was analyzed using cold-atomic fluorescence spectroscopy (AFS-230E, Beijing Kechuanghaiguang Instrument Co., Ltd.). KBH4 (guaranteed reagent, Aladdin Reagent Co., Ltd., Shanghai, China) was used as a reducing reagent. Other heavy metals concentration, such as As, Se, Fe, Al, Mn, Ni and Cr were determined by inductively coupled plasma atomic emission spectrometry (ICPE-9000, Shimdzu, Japan).

2.3. Experimental approach 2.3.1. Total heavy metal content Total Hg content was determined by digesting the gypsum samples in aqua regia (3:1 HCl:HNO3) according to the national standard methods of China (GB/T 22105.1-2008). Two-step microwave digestion method was used to determine other heavy metals concentration in FGD gypsums. In this method, HNO3/HF (9 + 1 mL) was added to 0.2 g gypsum in specific bottle, then closed the cap and shaken the bottle, and heated at 210 °C. After cooling down, 10 mL of 4% H3B3O3 was added. Close the cap, shaken and heated in 180 °C. After the two-step microwave digestion, there should not be any precipitation in the solution and the solution appears crystal clear.

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M. Sun et al. / Fuel 125 (2014) 66–72

Table 1 General information of selected coal-fired power plants.

Location Installed capacity (MW) Type of combustion unit FGD type Absorber additives Coal type Coal source Chlorine content in coal (mg/kg)

SX

HZ-1

HZ

HZ-2

Shaoxing 135 Pulverized-fuel WFGD Limestone Bituminous Shanxi 360

Hangzhou 130 Pulverized-fuel WFGD Limestone Bituminous Yunnan 75

Huzhou 135 Pulverized-fuel WFGD Limestone Bituminous Shaanxi, inner Mongolia 200

Hangzhou 135 Pulverized-fuel WFGD Limestone Bituminous Anhui 150

2.3.2. Sequential chemical extractions A slightly modified sequential chemical extraction (SCE) procedure was employed to identify associations of Hg in the FGD gypsums [14]. Since the organic Hg could be translated into Hg0 or Hgp during combustion, the extracted step was slightly improved to mainly extract the strong complex Hg0. At the end of each step, the extracted solution centrifuged, filtered through 0.45 lm membrane filters and then analyzed for Hg concentration. Prior to each step, the solid samples were freeze-dried to remove moisture and also minimize solid losses. The details of four extraction steps are described below, all extractions were treated in triplicates and the mean value was used. Fraction 1: Water soluble. The FGD gypsum samples were extracted for 2 h with DI water at room temperature with agitation. Fraction 2: Diluted acid extractable. The residue from Fraction 1 was leached with 0.2 M HCl and 1% CuSO4 at room temperature and continuously agitated for 2 h. Fraction 3: Strong complex(Hg0). The residue from Fraction 2 was extracted at room temperature for 8 h with 30% H2O2 and agitated with concentrated HCl. Fraction 4: Residual. The residue from Fraction 3 was digested with aqua regia (3:1 HCl:HNO3) according to the procedure described earlier for total Hg analysis.

2.3.3. Mercury release test Based on the acidic precipitation in Hangzhou [36], leaching tests were conducted by adding 1.0 g sample and 20 mL DI water at the initial pH value of 4.5 into a 25 mL beaker consecutively; meanwhile, a plastic film was used to prevent solution evaporation at 25 °C at predetermined time intervals. Control tests showed that Hg adsorption on the plastic film was negligible. During the leaching process, the pH value was not altered and the reaction was lasted for 24 h. After the reaction, Hg concentration left in gypsum was determined.

2.5. Quality control In this study, quality control procedure was adopted to ensure reliability of the results. Hg analysis was carried out with standard materials of 1 lg/L Hg concentration to check the accuracy of the Hg analyzer. Experimental result indicated that the error limit is around 5–10%. 3. Results and discussion 3.1. Characterization of FGD gypsum 3.1.1. Chemical composition of FGD gypsum Surface area, pH and sulfite content for FGD gypsum samples are presented in Table 2. No significant differences were found in surface area and pH among all the investigated samples. The mineralogy characterization of FGD gypsums was studied. As shown in Fig. 1, the XRD analysis on bulk FGD gypsum showed that calcium sulfate (CaSO4) was the predominant crystalline phase, which was in agreement with the reported studies [11,17]. In Ref. [17], the

Table 2 Chemical and physical property of FGD gypsum samples.

Specific surface area (m2/g) Sulfite content (mg/g) pH

SX

HZ-1

HZ

HZ-2

6.58 0.68 7.86

3.84 0.22 7.93

6.68 0 8.08

6.33 0.35 7.79

Note: sulfite content of 0 means can detect by our method.

Sample SX Sample HZ

G G

Sample HZ-1 Sample HZ-2 G: Gypsum C: Calcite Q: Quartz

G G G

2.4. Data analysis

St =S0 ¼ F r þ F rap expðkrap tÞ þ F slow expðkslow tÞ

Q

C

The following first-order, modified two Domains Model (MM) was used to fit the Hg releasing data using Origin 8.0.

G

G Q

G

C

C

G G

ð1Þ

where St and S0 represent Hg concentration in leachate (lg/g) at time t and the initial Hg content in gypsum, respectively. Frap, Fslow and Fr represent the fractions of rapid, slow and resistant release domains, respectively. The sum of Frap, Fslow, and Fr should almost equal one. krap and kslow represent the kinetic constants (103/ min) of the rapid and slow fractions, respectively.

0

10

20

30

40

50

60

70

Angle Fig. 1. XRD patterns of different FGD gypsum samples.

80

M. Sun et al. / Fuel 125 (2014) 66–72

contents of CaO and SO3 in FGD gypsum were 25.83% and 38.71%, respectively, which correspondingly indicated the contents of CaSO4 were 79.3% and 83.23%, respectively. However, other less intense peaks appeared to be those for silica, ferric oxide and some other minor minerals, such as calcium sulfite and calcite. Flue gas desulfurization system employs lime or limestone as reagent to reduce SO2 in coal-fired power plants. During this process, the lime coverts to calcium sulfite first, then CaSO3 could be rapidly oxidized to CaSO4 and little remained in the oxygenated environment [18]. Additionally, no Hg-bearing phases are detectable by XRD, as they represent a very minor proportion of the crystalline phases in the samples. SEM images along with EDX spectra of individual particles were collected to determine the particle morphology and chemical information. As mentioned above, the main chemical composition of Sample SX was similar to others, Sample SX was selected as a representative example, the SEM images shown in Fig. 2 revealed that the FGD gypsum was regular monoclinic crystalline, which confirmed the results that the main content of FGD gypsum was CaSO4. EDX analysis of FGD gypsum shown in Table 3 found that the main element of the FGD gypsum was Ca (13.89 wt.%), S (12.04 wt.%) and O (65.34 wt.%). Almost all the other elements (Si, Al, and Fe) were below 1% and they may exist in the gypsum as silica, alumina, iron oxide, which was proved by the XRD patterns. 3.1.2. The relationship between mercury and other heavy metals Coal contains numerous trace heavy metals [6], such as Hg, As, Se, Al, Fe, Pb, Cr, Mn and Ni. Flue gas desulfurization system facilitated the capture of these heavy metals and increased their concentrations in FGD gypsum. The heavy metal concentration in FGD gypsum was listed in Table 4, which revealed that their concentrations varied with the FGD gypsum samples. We took Sample SX as an example, the contents of Fe (1791.75 lg/g) and Mg

69

(291.50 lg/g) were highest. Besides, the trace heavy metals, As (3.08 lg/g), Pb (7.84 lg/g), Hg (1.63 lg/g) concentration were also comparatively high. Among these heavy metals, Hg poses threats to public health due to its toxicity. As shown in Table 4, the total Hg concentration in the investigated samples follows the order: Sample SX > Sample HZ-2 > Sample HZ-1 > Sample HZ. Many factors could be presumed affecting Hg content in FGD gypsum, such as the coal types, the technology used to capture Hg in coal-fired power plants and the type of particulate control. Liu et al. [33] found that bituminous coal results in higher Hg concentration in FGD gypsum due to the higher chlorine content, which is in good agreement with our results. Moreover, the total Hg content in FGD gypsums might also related to sulfite [19], Se [20] and Fe [21] contents based on previous studies, which need to be extensively researched in future studies. 3.2. Mercury speciation in FGD gypsum Fig. 3 presents the results of Hg speciation in FGD gypsum samples. It can be seen that the proportion of four Hg extract varied with FGD gypsum samples. The first two fractions were corresponding to water soluble and diluted acid extractable Hg theoretically, such as HgCl2, HgSO4, Hg(NO3)2 and HgO. They are the most likely species to be formed during coal combustion and in FGD system [22]. The data showed that Hg concentration was relatively low in the first two fraction steps (about 10%), which was also confirmed by previous study that the first two fraction steps can be ambiguous at low Hg concentrations [13]. However, in Sample SX, the Fraction 1 which is mainly presenting HgCl2 accounted for about 30% of the total Hg extract. As shown in Table 1, coal chlorine content in Sample SX was highest. It was reported that more than 90% of chlorine in coals was liberated as HCl gas during coal combustion at the temperature ranging from 300 °C to 600 °C [23]. In this temperature range, elemental Hg can undergo homogeneous

Fig. 2. SEM image and EDX spectra, taking Sample SX as example.

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M. Sun et al. / Fuel 125 (2014) 66–72

Table 3 The element compositions of different FGD gypsum samples by EDX.

C O F Al Si S Cl Fe Ca

SX

HZ-1

HZ

HZ-2

Weight (%)

Atom (%)

Weight (%)

Atom (%)

Weight (%)

Atom (%)

Weight (%)

Atom (%)

3.8 65.04 3.04 0.86 1.07 12.04 0.2 0.06 13.89

5.92 76.12 2.99 0.59 0.72 7.03 0.1 0.02 6.49

2.96 60.47 2.33 1.78 2.54 13.83 – 0.37 15.56

4.8 73.6 2.39 1.29 1.76 8.4 – 0.13 7.56

4.67 64.44 3.2 – 0.26 13.77 – – 13.66

7.24 75.09 3.14 – 0.17 8 – – 6.36

3.75 62.37 2.77 0.31 0.7 14.48 0.21 – 15.41

5.97 74.47 2.79 0.22 0.48 8.63 0.11 – 7.34

Table 4 The total heavy metal concentration in FGD gypsum samples.

Sample SX

Sample HZ-1

Sample HZ

Sample HZ-2

SX

HZ-1

HZ

HZ-2

Mg Cr Mn Fe Ni Cu Zn As Se Cd Pb Hg

291.50 7.37 19.98 2872.50 3.65 2.44 23.23 3.08 2.68 0.34 7.84 1.63

289.50 9.39 31.65 1791.75 5.86 5.22 33.68 2.93 1.71 0.26 13.13 0.78

355.25 4.98 11.06 758.00 2.02 1.46 16.77 2.87 0.95 0.14 2.82 0.61

233.70 15.61 18.46 2476.50 4.70 4.62 26.10 4.73 2.91 0.29 13.31 1.26

Extraction percentage, %

140

Element (lg/g)

80 60

120

40

100

20 80

0 -20

60

-40

40

-60 20

-80 -100

0

or heterogeneous oxidation via reaction with chlorine radicals. The possible mechanism was yield by Sliger et al. [24] (Eq. (2)).

ð2Þ

HgCl is subsequently oxidized by HCl, Cl2, or chlorine radicals. This mechanism was in good agreement with the observation that the extent of Hg oxidation (expressed as the fraction of Hg2+) increases with HCl concentration and coal-Cl content [25,26]. However, it should be noted that coal-Cl was not the only determining factor in the extent of Hg oxidation. For example, laboratory-scale tests verified that SCR catalysts oxidize Hg0 to Hg2+ [27]. Unfortunately, since no power plant investigated in this study was equipped with SCR to reduce NOx, the effect of SCR on Hg speciation was unknown, which need further study in future. Besides, mercuric oxide, nitrate, and sulfate may also be formed [28,29]. In real flue gas, the fraction of oxidized Hg ranges from nearly 0% to 100%, these fractions of Hg (HgCl2, HgSO4 and Hg(NO3)2) may be captured by the gypsum. Nevertheless, Hg in the FGD gypsum was the highest in Fraction 3 (60–80%), representing strongly complex Hg (Hg0). However, Hg(I) and Hg–Fe compound was also extracted in this Fraction. Al-Abed et al. [11] proved that Hg was strongly associated with Fe oxide materials in FGD residue by XRF. Since Hg in the flue gas is more likely to be in the forms of Hg (I), Hg (II), or Hgp [30]. Thus, their elevated release in Fraction 3 is expected. Besides Fraction 3, the residual Hg representing mercuric sulfide accounted for almost 20% of total Hg extracted. The Hg recovery percentage of all the samples shown in Fig. 3 was higher (60–80%) and it was in agreement with the previously reported study [11]. Quantitative analysis by FTIR spectrometry of representative FGD gypsum (Sample SX) was reported. According to previous study, fundamental sulfate vibrations occur for sulfate ion due to symmetric (t1, 981 cm1) and asymmetric (t3, 1104 cm1) stretching and symmetric (t2, 451 cm1) and asymmetric (t4, 613 cm1) bending vibrations [31]. As shown in Fig. 4, the band centered

Sample SX Residual

Sample HZ-1

Strong complex

Sample HZ-2

Sample HZ

Diluted acid extractable

Water soluble

Fig. 3. Mercury speciation in FGD gypsums, along with percent distribution among the extracted phase.

100 2914 2846

80

Transmittance (%)

HgðgÞ þ ClðgÞ ! HgClðgÞ

100

Recovery, %

Element

60

2210 2326

40

3240

20

1682

0

1620

3540 3400

4000

3500

1140 1109

3000

2500

2000

1500

449 669 602

1000

500

Wavenumber (cm-1) Fig. 4. FTIR spectra of FGD gypsum, taking Sample SX as example.

around 1140 cm1 which splits into two components at around 1140 cm1 and 1109 cm1 and the small peaks at 669 cm1, 602 cm1 and 459 cm1 were assigned to the stretching and bending modes of sulfate or silica, which may be attributed to the formation of HgSO4 or Hg–silica compounds. Sample showed a wide band located around 3400 cm1, which was typically ascribed to O–H stretching mode of hydroxyl groups or adsorbed water. The band at 1620 cm1 was assigned to O–H vibrations of water in the starting chemical form of the binder or calcium sulfate hemihy-

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M. Sun et al. / Fuel 125 (2014) 66–72

1.1

Frap : Fwater souble

0.8

the ratio of mercury in release fraction to total mercury, %

0.9

St /S0

45

Sample SX Sample HZ-1 Sample HZ Sample HZ-2

0.7 0.6 0.5 0.4 0.3 0.2 -200

0

200

400

600

800

1000

1200

1400

y=1.193x+17.56; 2 R =0.963

40

y=x y=0.992x+13.69; 2 R =0.818

35 30

90

25 20 15

80

1600

75

y=1.305x- 46.97; 2 R =0.820

70 65 60 55

10

50 78

80

82

84

86

88

90

the ratio of extractable mercury to total extract, %

5 5

T, min

y=x

85

y=0.985x+12.83; 2 R =0.808

Frap+F slow

1.0

Fr : Fresidual

Frap : F(water souble+acid souble)

10

15

20

25

30

35

40

45

50

the ratio of mercury speciation to total extract, % Fig. 5. The mercury release curve via leaching time at initial pH of 4.5. Fig. 6. The correlation between different Hg extract of its release ability.

drate, while the infrared band at 1680 cm1 was assigned to O–H vibrations of water in calcium sulfate dehydrate [32]. The bonds observed at 2846 cm1 and 2914 cm1 indicate the presence of carbonate species (such as CaCO3 and HgCO3) in the sample.

3.3. The release ability of mercury from FGD gypsum For Hg speciation in FGD gypsum defines their toxicity and mobility and it is essential to determine the risk when the wastes were disposed [11,33]. Therefore, in this study Hg release kinetics was also studied. First-order, modified two Domains Model was used to fit the release kinetics data. As shown in Fig. 5, the release of Hg from FGD gypsum used in the study exhibited biphasic kinetics. The experimental data was in good agreement with MM (Formula (1)), which indicated of high values for the determination of coefficient (i.e., R2 = 0.850–0.978 for each sample). The release curves (plotted as St/S0 versus time) were all characterized by a rapid decrease of the amount of Hg in the early stage of the reaction, after which they were leveled off. The parameters fitted by MM involving Fr, Frap, Fslow, krap and kslow are listed in Table 5. It is assumed that the sorption of Hg in FGD gypsum can not only occur by physical adsorption through weak binding force, but also occur by chemical adsorption. The condensation of Hg on FGD gypsums as the water soluble form HgCl2 may belong to physical adsorption and it is easy to release [33]. Other chemical adsorption Hg, such as strong complex phase and residual phase seems to be more stabilized [9]. As mentioned above, Hg could form a direct chemical bond with the goethite surface [21], which might be extracted in F3. The experimental results exhibited that Hg could not be completely released from FGD gypsum, and almost 30–45% of Hg residues still remained in gypsum after releasing for 24 h in this study. It means that fraction might have low bioavailable, labile and

environmental risks. Further in depth discussion is elaborated in Section 3.4. 3.4. The relationship between speciation and mobility of mercury A correlation analysis between different Hg extracts and their release potentials (exhibited by Frap, Fslow and Fr) was studied. As shown in Fig. 6, the regression analysis indicates a general, although not perfect, correlation between water soluble Hg and Frap with a correlation coefficient (R2) of 0.818. Relatively low R2 value might attributed to the low extract concentrations, even though the sequential chemical extractions for Hg has been widely reported to be successful [10,14]. Our result was similar to the reported study [33], which showed the Hg releasing percentage was related to the ratio of HgCl2 and Hg2Cl2 to the total Hg contents with the coefficient of 0.841. Moreover, the line of correlation between Frap and water soluble Hg was above the line of y = x, which means that the content of rapid release fraction was higher than water soluble fraction. In another word, a little portion of Hg in Fraction 2 and Fraction 3 could also rapidly release into environment. Especially, the diluted acid extractable Hg might release into the environment rapidly under acidic condition. The correlation coefficient was 0.808 between Frap and the sum of water soluble and diluted acid extractable Hg. Among all the extractions, Hg in Fraction 4 presenting HgS or crystal lattice Hg possesses the lowest mobility. It could be proved by the correlation analysis between residual Hg and Fr with the high coefficient value (R2 = 0.963), which means the environmental risk of this extract was relatively low. Therefore, researchers had already developed some sulfite materials to stabilize Hg sediment or FGD residues [34]. Moreover, the sum of the first three fractions was labile and was able to release into the environment. The coefficient was 0.820 between extractable Hg and Frap + Fslow, which means the ability of Hg release to the environment.

Table 5 Mercury release parameters: the rapidly, slowly and resistant-releasing fractions (Frap, Fslow, Fr, respectively; %) and their corresponding rapid, slow release rate constants (krap, kslow, respectively; 103/min) are presented.

SX HZ-1 HZ HZ-2

Fr (%)

Frap (%)

krap (103/min)

Fslow

kslow (103/min)

R2

0.306 ± 0.0203 0.351 ± 0.021 0.382 ± 0.053 0.435 ± 0.033

0.456 ± 0.064 0.246 ± 0.094 0.231 ± 0.157 0.152 ± 0.071

1014.21 179.09 330.68 140.64

0.243 ± 0.049 0.374 ± 0.088 0.401 ± 0.141 0.398 ± 0.073

17.521 12.376 13.239 6.047

0.956 0.978 0.850 0.959

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3.5. Environmental implications of FGD gypsum during its disposal According to recent statistics, 48% of coal-fired power plants in China would be equipped with WFGD system by the end of 2008. It has been projected that WFGD applications would keep rising until at least 2020. In Zhejiang province, the FGD gypsum production was 3.69 million tons in 2010 [35]. As shown in Table 4, the average Hg concentration in FGD gypsum was 1.07 lg/g; it assumes that the Hg release potential for FGD gypsum in Zhejiang province was about 3.95 tons in 2010. The multipurpose utilization of FGD gypsums in China started only from last century. However, the multipurpose utilization was around 45%, mainly in the field of soil amendment, landfill materials, cement additives and wallboard production, about 55% were disposed as waste materials [35]. Among the issues that arise are the potential risks for groundwater and atmospheric emissions of Hg during post-dispersal mobilization from FGD gypsums. Liu et al. [33] assumed all the recycled FGD gypsum were used in wallboard production and cement additives and estimated Hg emissions during wallboard manufacturing process in 2008 was 4.7 tons in China (about 0.4 tons in Zhejiang province only). However, Hg releases during FGD gypsum disposal process were not considered in the reported paper. Considering that 55% of the FGD gypsum was disposed as waste materials and the average release percentage was 70%, Hg release from FGD gypsum during its disposal in 2008 was 1.52 tons in Zhejiang province. In conclusion, the researchers and policy makers should pay more attention to the stabilization of Hg in FGD gypsums to decrease the environmental risk during usage and disposal. 4. Conclusion In this paper, research about the FGD gypsum chemical composition, the relationship of total Hg concentration via sulfite, Se and Fe content, Hg speciation and its environmental risk during the disposal of FGD gypsum is presented. Result revealed that the main composition of FGD gypsum was calcium sulfate; Hg concentration in FGD gypsum was more likely to be related to S(IV), Fe and Se contents. More importantly, Hg was mainly distributed in the strong complex phase, representing Hg0 and Hg(I) as well as Hg– Fe compounds. Relatively higher coal-Cl concentration might increase the content of water soluble Hg. The rapid release of Hg from FGD gypsum might dominated by the ratio of water soluble Hg, which reiterated that we should pay more attention to the stabilization of the water soluble Hg in FGD gypsums to decrease the potential environmental risks during usage and disposal. Acknowledgments We would like to extend our thanks to the Science and Technology Project of Zhejiang Province, China (No. 2012C23061) and the National Natural Science Foundation of China for the financial support (No. 21277119). References [1] Pavlish JH, Hamre LL, Zhuang Y. Mercury control technologies for coal combustion and gasification systems. Fuel 2010;86:838–47. [2] Wang JX, Feng XB, Anderson CWN, Xing Y, Shang LH. Remediation of mercury contaminated sites – a review. J Hazard Mater 2012;221–222:1–18. [3] Fang GC, Yang IL, Liu CK. Estimation of atmospheric particulates and dry deposition particulatebound mercury Hg(p) in Sha-Lu, Taiwan. Aerosol Air Qual Res 2010;10:403–13. [4] Pavageau MP, Pécheyran C, Krupp EM, Morin A, Donard OFX. Volatile metal species in coal combustion flue gas. Environ Sci Technol 2002;36:1561–73. [5] You CF, Xu XC. Coal combustion and its pollution control in China. Energy 2010;35:4467–72.

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