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Characterization and leaching toxicities of mercury in flue gas desulfurization gypsum from coal-fired power plants in China
Fuel 177 (2016) 157–163
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Characterization and leaching toxicities of mercury in flue gas desulfurization gypsum from coal-fired power plants in China Ying Hao, Simiao Wu, Yun Pan, Qiu Li, Jizhi Zhou, Yuebu Xu, Guanren Qian ⇑ School of Environmental and Chemical Engineering, Shanghai University, No. 99 Shangda Road, Shanghai 200444, PR China
h i g h l i g h t s Hg pollutant characters in FGD gypsums around China were evaluated. Total content and chemical species of Hg displayed regional characters. Hg in FGD gypsums mainly distributed in strong complex F4 phase. TCLP and SPLP tests indicated low mobility of Hg in FGD gypsum.
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Article history: Received 13 October 2015 Received in revised form 21 February 2016 Accepted 29 February 2016 Available online 5 March 2016 Keywords: FGD gypsum Mercury Chemical speciation Leaching toxicity
a b s t r a c t Flue gas desulphurization (FGD) gypsum samples were collected from 70 power plants in 20 provinces in China. The total Hg concentration, chemical speciation and leaching toxicity of Hg in the samples were determined. The total Hg concentrations ranged from not detectable (ND) to 4330 lg/kg with an average of 891 lg/kg and a median of 629 lg/kg. The Hg concentrations in the FGD samples had obvious regional characteristics. Provinces in the central part of China had higher average Hg concentrations in the FGD gypsum than other provinces. Selective sequential extraction (SSE) used for chemical speciation analysis showed that water soluble and human stomach acid soluble (F1 + F2) percentages ranged from ND to 25.2% of the total Hg, and strong complex Hg (F4) was the dominant chemical species, which accounted for more than 60% of the total Hg in most samples. Moreover, the leaching Hg concentrations from Toxicity Characteristic Leaching Procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP) tests were all below the regulation level of leaching toxicity. The average percentage of leaching Hg from TCLP and SPLP was approximately 3% of the total Hg, indicating limited Hg mobility in FGD gypsum. The Hg potential leaching from FGD gypsum was approximately 0.65 tons per year. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In China, coal-fired power plants produced large amounts of combustion residuals, including fly ash and flue gas desulfurization (FGD) gypsum [1]. Due to the increasingly strict air control policy, the amount of combustion by-products has increased rapidly. Specifically, FGD gypsum output reached 52 million tons per year in 2012, and the total reutilization rate was only approximately 56% [2]. A large amount of FGD gypsum ended up in landfills or outdoor storage. It is well known that Hg can be co-removed from flue gas during air pollution control and retains its combustion residuals [3,4], causing potential Hg release. Mercury in fly ash and its effects on the environment during management and application have been widely investigated [5–8]. Comparatively, there ⇑ Corresponding author. Tel.: +86 21 66137640; fax: +86 21 66137761. E-mail address: [email protected] (G. Qian). http://dx.doi.org/10.1016/j.fuel.2016.02.091 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
were fewer reports on Hg in FGD gypsum. However, its safe treatment and disposal have raised concerns [9,10]. Al-abed et al. [11] reported that Hg concentrations in FGD gypsum in Pennsylvania, USA, ranged from 1000 to 2300 lg/kg and had a higher leaching at acidic pH levels. Wang et al. [12] found Hg concentrations in plant stems increased when FGD gypsum is present in soil. It has been reported that Hg in flue gas released from coal during combustion exists primarily as Hg0, oxidized Hg (Hg2+) and particulate Hg (Hgp). Hgp and partial Hg2+ can be removed by particle control devices and enter fly ash, whereas FGD gypsum can retain up to 95% of Hg2+ [13]. Therefore, Hg in fly ash and FGD gypsum varies with the ratio of different Hg speciation in flue gas. Hg levels in flue gas depend on the Hg concentration and the chemical composition of coal as well as the method of air pollution control processes [14,15]. Thus, it is expected that there are different total Hg levels in FGD gypsum from different regions. However, the Hg concentrations in FGD gypsum and Hg concentration distributions
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throughout different regions of China haven’t received enough attention. There were only limited data regarding the Hg concentration of FGD gypsum from several power plants; the total Hg concentration was reported between 150 and 2470 lg/g [16–18]. Furthermore, there are a number of power plants, and a few samples may not reflect the actual levels of Hg in FGD gypsum in China. As previously mentioned, most Hg2+ is removed from flue gas during the FGD process. However, when Hg2+ enters FGD gypsum, it interacts with FGD gypsum through different mechanisms, forming a different chemical speciation. Sun et al. [17] used a modified sequential selective extraction method to determine the chemical speciation of Hg in FGD gypsum collected from four coal-fired power plants in Zhejiang Province, China. The method differentiated Hg into four groups. The results showed that Hg in FGD gypsum from the different power plants had different distribution characteristics. Al-abed et al. [11] reported that 99% of Hg in FGD gypsum stayed in a strong complex and residue phase. These results suggested that the Hg chemical speciation in FGD gypsum varied from place to place, thus causing different environmental behaviors during disposal and reutilization. Furthermore, chemical speciation of heavy metals in soil is closely correlated with its bioavailability [19,20]. Thus, determination of chemical speciation aside from the total concentration of Hg will be helpful to understand the biogeochemical cycle of Hg and predict its environmental fate in FGD gypsum from different regions. In this study, we collected FGD gypsum samples from 70 power plants in 20 provinces, and determined their Hg characteristics. The total Hg and the chemical speciation of Hg were analyzed with a sequential selective extraction (SSE) method. A toxicity characteristic leaching procedure (TCLP) and a synthetic precipitation leaching procedure (SPLP) were performed to evaluate the leaching toxicity of Hg. 2. Materials and methods 2.1. Sample collection FGD gypsum samples were collected from 70 coal-fired power plants in 20 provinces, China. The sample sites are shown in Fig. 1a. The collected samples were preserved in sealed plastic bags and sent to the laboratory as soon as possible. Upon arrival at the lab, the samples were stored at 4 °C in the refrigerator. Because of the possible fluctuations of Hg concentrations in FGD gypsum, samples from four power plants in Shanghai and Anhui Provinces were collected each day during monitoring periods of four to eight weeks. 2.2. Total Hg content and Fe, Al analysis The moisture of the samples was measured by drying the samples at 105 °C overnight. The moisture was calculated as the weight difference of the sample before and after drying. The moisture ranged from 1.3% to 51%. The FGD gypsum samples were oven-dried at 40 °C before analysis. The total Hg concentrations were determined by mixing FGD gypsum with 5 mL distilled water and 5 mL aqua regia, then oscillating the samples on an end-over-end shaker for 24 h at room temperature [21]. The mixture was finally centrifuged at 3000 rpm for 15 min. The supernatant was filtered through a 0.45 lm membrane, before the Hg concentration was determined by cold vapor atomic absorption spectrometry (CVAAS) (HUAGUANG F732-VJ, Shanghai) with Hg2+ being reduced to Hg0 by the addition of SnCl2. For Fe and Al concentrations in FGD gypsum, the samples were placed in a 60 mL PTFE tube and digested with a mixture of 10 ml HNO3, 5 ml HClO4 and 10 ml HF on a graphite digestion block at
135 °C. The addition of the mixed acid was repeated several times until the solution was clear. The residual solution was filtered with a 0.45 lm cellulose acetate membrane and diluted to 50 mL in volumetric flasks with deionized water. Quantification of Fe and Al concentrations in the solution were determined by an inductively coupled plasma atomic emission spectrometry (ICP-AES) (LEEMAN Prodigy, USA). 2.3. Hg chemical speciation by SSE method A five-step sequential selective extraction method developed by Bloom et al. [21] was performed to determine the Hg speciation in FGD gypsum. This method differentiated Hg compounds into different behavioral classes instead of species-specific information. The five extraction solutions adopted included deionized water, 0.1 M CH3COOH + 0.01 M HCl, 1 M KOH, 12 M HNO3 and aqua regia. The extracted Hg was defined as water soluble Hg (F1), human stomach acid soluble Hg (F2), organo-chelated Hg (F3), strong complex Hg (F4) and residue Hg (F5). 2.4. Leaching tests The US EPA’s TCLP [22] and SPLP [23] were used to estimate the Hg leaching potential from FGD gypsum. TCLP simulates the leachability of solids in an acidic environment of municipal landfills. The TCLP extraction fluid used in this work included 17.25 mL of glacial acetic acid (CH3COOH) that was added to a glass volumetric flask, and then diluted with deionized water to 1000 mL (pH of 4.93 ± 0.05). One gram of gypsum was mixed with 20.0 mL of solution, maintaining the same solution-to-solids ratio of 20:1, according to the standard procedure. Samples were extracted for 18 ± 2 h in a 50-mL centrifuge tube at room temperature by end-over-end tumbling at 30 rpm. After extraction, the samples were centrifuged for 20 min at 3000 rpm, and the supernatant was filtered through a 0.45 lm filter. The filtrates were then oxidized and preserved by adding 2% (v/v%) of 0.2 M BrCl. The Hg concentration in filtrates was determined by CVAAS. For the SPLP, sulfuric acid and nitric acid (mass ratio 2:1; pH = 3.20 ± 0.05) were used as extraction fluids to conduct the solid waste-extraction procedure for leaching toxicity and estimate the effect of acid rain on FGD gypsum. Two grams of sample and 20 mL of extraction fluid were combined into a glass volumetric flask. The extraction and determination procedures were similar to those of the TCLP. 2.5. Quality control The Hg concentration was expressed as dry weight basis. To ensure accurate determination of the Hg concentration, the standard addition method was applied to the total Hg determination and the standard recovery rate was calculated. The recovery rates ranged from 80% to 106%. A blank sample was analyzed in each batch test to eliminate the effect of impurities in the reagent. All analyses were performed in triplicates and the results were expressed as the mean ± standard deviation. 3. Results and discussion 3.1. Spatial distributions of total Hg in FGD gypsum Fig. 1 displays the total Hg concentrations in the FGD gypsum samples from the different provinces and the frequency distribution of Hg in all of the FGD gypsum samples. The Hg concentration in each FGD gypsum sample is displayed in SI Table 1. The Hg concentrations ranged from not detectable to 4330 ± 620 lg/kg, and showed obvious regional differences. FGD gypsum from Shanxi,
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20
15
10
5
0
0
1000
2000
3000
4000
Hg concentration (µg/kg)
(a)
(b)
Fig. 1. Sampling sites and average total Hg concentration in FGD gypsum by province (a) and frequency distribution of Hg concentration in FGD gypsum of all the samples (b).
Henan, Anhui and Sichuan Provinces had relatively high average Hg concentrations > 800 lg/kg. The two samples with the highest Hg concentrations were from DT3 Shanxi Province (3976 lg/kg) and SH Anhui Province (4330 lg/kg). Samples from Shanghai, Shandong, Ningxia, Zhejiang, Guizhou and Yunnan Provinces had Hg concentrations at average levels between 500 and 800 lg/kg. Other provinces, such as three northeast provinces, Xinjiang, Inner Mongolia and Gansu, had average Hg concentrations < 500 lg/kg. In general, Fig. 1a shows that provinces in the northern part of China had lower average Hg concentrations, and provinces in the middle and southern parts of China had relative higher average Hg in FGD gypsum. Fig. 1b shows that 67% samples contained Hg at concentrations between 200 and 1000 lg/kg, and 30% of the samples were above 1000 lg/kg. The average total Hg concentration of all FGD gypsum samples was 891 lg/kg and the median was 629 lg/kg. As mentioned earlier, the Hg concentration in FGD gypsum depends on Hg in the burning coal and air pollution control system. Although Hg in FGD gypsum displayed regional characteristics, some provinces had higher average Hg concentrations in FGD gypsum. It is hard to build the correlation between Hg concentrations in FGD gypsum and Hg in the coal and air pollution control system based on information on Hg in coal in each province [24] and the air pollution control system adopted in each power plant (mainly NOx removal devices). Therefore, other factors, such as halogen in flue gas and the operational conditions of the oven and air pollution control system, may also play important roles in Hg retention in FGD gypsum. The relationship between Hg concentrations in FGD gypsum and these factors needs further investigation. For comparison, the concentrations of Hg in the USA ranged from 53 to 846 lg/kg (n = 11) [10,12,25–27], with an average of
352 lg/kg. Samples from Spanish power plants ranged from 150 to 310 lg/kg (n = 3) [28,29]. Average total Hg in FGD gypsum from China seemed to be higher than those reported from other countries. It was well established that the average Hg concentration in coal in China was reported between 160 and 220 lg/kg [30,31]. Therefore, it seems that FGD gypsum is enriched in Hg that is released from coal during the combustion process.
3.2. Fluctuant profile of Hg concentration in FGD gypsum To assess the possible fluctuations of Hg in FGD gypsum, samples from four power plants in Shanghai and Anhui province were collected each day during four to eight weeks and the total Hg concentration was analyzed. The results are displayed in Fig. 2 and provide a general description of Hg concentration fluctuations in FGD gypsum that reflect the effect of daily operations of the coal power plants. The results indicated that the total Hg concentration in all samples from the four power plants during the monitoring period had large variations. The Hg content in SZ Anhui FGD gypsum varied between 1290 and 3600 lg/kg, with an average of 2669 lg/kg and a median of 2709 lg/kg. Moreover, the samples from XH Shanghai, SHZ Shanxi and DT2 Shanxi power plants also had large fluctuations. The largest influence on Hg in FGD gypsum was the fluctuation of the daily coal constituents there were burned in the power plants. Power plants might burn coal from different coalmines and there are differences in the Hg concentration of the parent coal. There may even be differences in the Hg concentration in coal from the same coal mine that varies largely due to the different mining sites [32]. The combustion temperature, unburned carbon in flue gas and the operational conditions of air
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Fig. 2. Variation of Hg contents in FGD gypsum. The bottom and top of the box represent the 25th and 75th percentiles, respectively. The band between the bottom and top of the box represents the 50th percentile. The ends of the whiskers represent the 10th and 90th percentiles.
pollution control devices might be other factors influencing the Hg concentration in FGD gypsum [33]. 3.3. Hg chemical speciation in FGD gypsum from SSE process Bloom’s five-step sequential selective extraction (SSE) procedure was performed to glean information of the Hg chemical speciation in FGD gypsum. The five groups demonstrated the mobility and leachability of Hg in solids under different conditions. Fig. 3 displays the Hg distributions in the five extraction fractions of the FGD gypsum samples from different parts of China. The results show that although strong complex Hg was the dominant phase in most of the samples, the chemical speciation of Hg in FGD gypsum from different provinces was different and also varied within the samples from same province. F1 and F2 were extracted by water and weak acid, respectively. Both conditions were commonly encountered in the natural environment. Therefore, F1 + F2 were often considered to be easily leachable and potentially bioavailable to the environment. For all of the samples, F1 + F2 accounted for percentages ranging from ND to 25.2% of the total Hg, with an average of 7.4%. The percentages of these two parts extracted from the total Hg were displayed in SI Table 2. In general, the samples from Shanghai and Gansu Provinces had relatively higher F1 + F2 percentages with averages of 11% and 13.5%, respectively. Samples from different power plants in the same province also had different F1 + F2 percentages. For example, the F1 + F2 percentage in most of the samples from Shanxi Province was lower than 10%. However, the F1 + F2 of the DT2 sample was 21%. The F1 + F2 percentage clearly varied for samples from Shandong Province, too, with the highest sample at 21% and the lowest sample less than 1%. F1 and F2 normally included HgCl2, Hg(NO3)2, HgSO4, and some Hg that weakly absorbed to materials [21]. The variation of Hg in F1 + F2 was likely due to the flue gas composition and the pollution control process method. An F1 + F2 fraction average of 7.4% of the total Hg suggested low Hg mobility in FGD gypsum in China. The F3 percentage ranged from ND to 82%, with most samples containing less than 10% (n = 54). It is worth noting that DT Shandong, XH Shanghai and BG Xinjiang FGD gypsum had 36%, 58% and 82% of Hg in F3, respectively. F3 was identified by Bloom et al. [21] as organo-chelated Hg. However, during the desulfurization process, it was unlikely to form organo-chelated Hg. Furthermore, it was found that Hg2Cl2 was also extracted during this step [21]. One study discovered that Hg2+ was partially reduced to Hg0
instead of being removed by the FGD process [17]. Instead of being reduced to Hg0, Hg+ may have partially formed and it may have been likely that Hg remained primarily Hg2Cl2 in the F3 species. Because Hg2Cl2 easily formed under reduction atmosphere, the operational conditions of the FGD procedure may be the key factor affecting the Hg level in F3 species. Bloom et al. [21] observed that Hg in the F3 species was strongly correlated with the methylation potential. Therefore, for FGD gypsum samples with high F3 percentages, avoiding disposal under the conditions which are enriched with microorganisms is recommended. The percentage of F4 varied from 10% to 98.5% of the total Hg concentration. Fig. 3 shows that 84% (n = 59) of the samples had more than 60% of the F4 species. Therefore, most Hg in FGD gypsum in China stayed in the strong complex phase. Bloom et al. [21] defined this phase as elemental Hg because Hg0 was removed at this step. Other chemical species may also be included in this fraction, such as Hg2Cl2, Hg0-metal amalgam and Hg2+ complexes. Kairies et al. [26] suggested that the Hg species of FGD-derived gypsum appeared to correlate with the Fe concentrations. Hg2+ could form a Hg–Al-precipitate and a Hg–Fe-precipitate at the presence of Ca, SO2 4 and Fe or Al [34]. FGD gypsum had relatively high Fe and Al concentrations [35]. We also measured the abundance of Al and Fe elements in the FGD gypsum samples, which ranged from 1000 to 10,000 mg/kg and 100 to 3000 mg/kg, respectively (see SI Table 1). Pearson correlation coefficient analysis revealed the concentrations of Hg and Fe were significantly correlated and the concentrations of Hg and Al were significantly correlated below concentrations of 5000 mg/kg (P < 0.05). This correlation suggested that Hg in FGD gypsum might exist primarily as Hg-complexity with Fe and Al and show a strong affinity to the materials. The F5 species, the most stable phase in FGD gypsum during the extraction procedure, accounted for percentages ranging from ND to 53% of the total Hg. Fig. 3 shows that samples from Shanxi and Shanghai had relatively higher percentages of the F5 species than the samples from other parts of China. For most of the samples, the F5 species accounted for only small parts of the total Hg. HgS was identified as the main form of Hg in the F5 species [21].
3.4. Leaching toxicity The Hg concentrations in the extracted solutions from the TCLP and SPLP tests were all below the US EPA’s regulation level of the maximum concentration of Hg to be characterized as toxic (0.2 mg/L), which classified FGD gypsum as non-hazardous waste. Fig. 4 displays the TCLP and SPLP results as a variation of the total Hg. For the TCLP, total Hg concentrations below 800 lg/kg had Hg in the leaching solutions lower than 3.0 lg/L. As the total concentration of Hg increased, some samples had higher leaching concentrations. The highest leaching concentration was 19 lg/L in the sample from SZ Anhui; this sample also had the highest total Hg concentration. Fig. 4 also shows that although the highest TCLP concentration was observed in the sample with the highest total Hg concentration, other FGD gypsums did not have a similar trend. For example, samples from DT3 Shanxi, HN Anhui, JN Shandong had very low leaching concentrations and higher total Hg concentrations, whereas samples from SZ Anhui, DT1, DT2 Shanxi had relatively lower total Hg concentration yet still had a little higher leaching concentrations. This result was likely due to the different binding modes of Hg in FGD gypsum. In general, the leaching level for all of the FGD gypsum samples was low. Pasini and Walker [25] also found that the FGD gypsum from the Midwestern United States contained 355 lg/kg of the total Hg, but only leached less than 0.1% of Hg during the TCLP.
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F1
F2
F3
F4
F5
100
Percentage (%)
80
60
40
DT JN1 JN2 JN LY QD RZ TA ZP JS1 JS2 JS3 JY SZH TZ ZJ1 ZJ2
BS SDK JS MH WGQ1 WGQ2 WGQ3 WGQ4 WJ XH
0
TY1 TY2 DT1 DT2 DT3 YQ PY YC SHZ LF YCH JZH
20
Shanxi
Shandong and Jiangzhe
Shanghai
100
Percentage (%)
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Center part
North Part
QJ
BG
SC2
NX
SC1
GZ1
GZ2
JQ
GS3
GS1
GS2
HS
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SY2
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SY1
CF
AS
BT
NM
BJS
BJ
HB
HN4
HN3
HN2
AQ
HN1
SZ
0
HN
20
West Part
Sampling sites Fig. 3. Chemical speciation of Hg in FGD gypsum from sequential selective extraction procedure.
Compared with the TCLP, the SPLP results had a more scattered distribution. However, similar to the TCLP, there was no clear correlation between leaching activity and total Hg concentration. The samples with high total Hg concentration certainly did not have higher leaching Hg. Generally, there was no significant difference between the leaching results from the TCLP and the SPLP. The slight variation may result from the different nature of the two extraction solutions. The comparison of the total Hg leaching percentages from the TCLP, the SPLP and the SSE procedure (F1 + F2) was displayed in SI Table 2. Comparatively, extractable F1 + F2 percentages were more than that from the TCLP and the SPLP. Because F1 + F2 was well accepted as the easily mobile phase during natural conditions, Hg in FGD gypsum may leach more than what is predicted by the TCLP and SPLP methods.
3.5. Estimation of Hg release from FGD gypsum through the leaching process Most of FGD gypsum byproducts have been randomly piled up on the ground before reutilization, either for storage or land reclamation. Although the percentage of the potential Hg release is
small, Hg in FGD gypsum originally scrubbed from flue gas still possibly reentered the environment through leaching. We estimated the potential Hg release using the following equation:
m¼
CRM 109
50%
where m is the amount of released Hg (tons); C is the average Hg concentration (lg/kg); R is the average release rate of Hg from the TCLP and the SPLP (%); and M is the total production of FGD gypsum per year in China (tons); 50% is the general reutilization rate of FGD gypsum in China [1]. The average concentration of Hg from the collected samples was 891 lg/kg. The average percentage of leaching Hg from the TCLP and the SPLP, which was 2.75%, was used to estimate the potential release of Hg from FGD gypsum. The total production of FGD gypsum per year in China was approximately 52 million tons. Therefore, the amount of Hg released from FGD gypsum was estimated to be 0.65 tons per year. The estimation of the potential release of Hg from FGD gypsum was based on the results from the TCLP and the SPLP, which simulated the landfilling condition and the effects of acid rain, respectively. Under natural conditions, there are other variables, such as redox conditions, ambient temperature, microbial conditions
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ples from different power plants. Leaching toxicity tests indicated low mobility of Hg in FGD gypsum. Hg potential leaching from FGD gypsum in China was estimated to be 0.65 tons per year. Acknowledgments The research was funded by National Natural Science Foundation of China (Grant No. 41402311) and we would also like to give our grateful acknowledgement to those who help us collect FGD gypsum around the country. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.02.091. References
Fig. 4. Leaching characteristics as the variation of total Hg.
and the light conditions, that may also affect the amount of Hg released from FGD gypsum. However, the equation permits an estimation of the potential release of Hg from FGD gypsum. If that amount of Hg does release, the potential environmental risks should be considered. The output of FGD gypsum from power plants varies widely for the different provinces. Provinces located in the central part of China, such as Anhui, Shanxi and Henan, have more coal-fired power plants, thus producing higher amounts of FGD gypsum than provinces in the eastern part of China, where other types of electricity generation methods account for part of the total electricity production [1]. However, the reutilization rates of industrial byproducts in the provinces in the central part were lower compared to those located in the eastern part of China [1]. Furthermore, as previously discussed, the FGD gypsum in the central part had higher Hg concentrations. Thus, it is expected that the Hg leaching risk of FGD gypsum in the central part of China will be larger than the Hg leaching risk in the eastern part of China. Yang et al. [36] has expressed concerns that ignoring the emissions of Hg from coal fly ash and FGD may undermine the efforts of Hg control. Therefore, it is urgent to adopt a standardized management procedure to minimize the potential risks of FGD gypsum.
4. Conclusions We have investigated Hg contamination in FGD gypsum from 70 power plants in 20 provinces in China. Both the total Hg concentration and the chemical speciation had significant regional characteristics. The total Hg concentrations in FGD gypsum ranged from non-detectable to 4330 lg/kg, with an average of 891 lg/kg and a median of 629 lg/kg. FGD gypsum from the central part of China had relatively high average Hg concentrations. Chemical speciation analysis demonstrated that most Hg remained in the strong complex phase in FGD gypsum and Hg in each phase varied with sam-
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Characterization and leaching toxicities of mercury in flue gas desulfurization gypsum from coal-fired power plants in China Ying Hao, Simiao Wu, Yun Pan, Qiu Li, Jizhi Zhou, Yuebu Xu, Guanren Qian ⇑ School of Environmental and Chemical Engineering, Shanghai University, No. 99 Shangda Road, Shanghai 200444, PR China
h i g h l i g h t s Hg pollutant characters in FGD gypsums around China were evaluated. Total content and chemical species of Hg displayed regional characters. Hg in FGD gypsums mainly distributed in strong complex F4 phase. TCLP and SPLP tests indicated low mobility of Hg in FGD gypsum.
a r t i c l e
i n f o
Article history: Received 13 October 2015 Received in revised form 21 February 2016 Accepted 29 February 2016 Available online 5 March 2016 Keywords: FGD gypsum Mercury Chemical speciation Leaching toxicity
a b s t r a c t Flue gas desulphurization (FGD) gypsum samples were collected from 70 power plants in 20 provinces in China. The total Hg concentration, chemical speciation and leaching toxicity of Hg in the samples were determined. The total Hg concentrations ranged from not detectable (ND) to 4330 lg/kg with an average of 891 lg/kg and a median of 629 lg/kg. The Hg concentrations in the FGD samples had obvious regional characteristics. Provinces in the central part of China had higher average Hg concentrations in the FGD gypsum than other provinces. Selective sequential extraction (SSE) used for chemical speciation analysis showed that water soluble and human stomach acid soluble (F1 + F2) percentages ranged from ND to 25.2% of the total Hg, and strong complex Hg (F4) was the dominant chemical species, which accounted for more than 60% of the total Hg in most samples. Moreover, the leaching Hg concentrations from Toxicity Characteristic Leaching Procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP) tests were all below the regulation level of leaching toxicity. The average percentage of leaching Hg from TCLP and SPLP was approximately 3% of the total Hg, indicating limited Hg mobility in FGD gypsum. The Hg potential leaching from FGD gypsum was approximately 0.65 tons per year. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In China, coal-fired power plants produced large amounts of combustion residuals, including fly ash and flue gas desulfurization (FGD) gypsum [1]. Due to the increasingly strict air control policy, the amount of combustion by-products has increased rapidly. Specifically, FGD gypsum output reached 52 million tons per year in 2012, and the total reutilization rate was only approximately 56% [2]. A large amount of FGD gypsum ended up in landfills or outdoor storage. It is well known that Hg can be co-removed from flue gas during air pollution control and retains its combustion residuals [3,4], causing potential Hg release. Mercury in fly ash and its effects on the environment during management and application have been widely investigated [5–8]. Comparatively, there ⇑ Corresponding author. Tel.: +86 21 66137640; fax: +86 21 66137761. E-mail address: [email protected] (G. Qian). http://dx.doi.org/10.1016/j.fuel.2016.02.091 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
were fewer reports on Hg in FGD gypsum. However, its safe treatment and disposal have raised concerns [9,10]. Al-abed et al. [11] reported that Hg concentrations in FGD gypsum in Pennsylvania, USA, ranged from 1000 to 2300 lg/kg and had a higher leaching at acidic pH levels. Wang et al. [12] found Hg concentrations in plant stems increased when FGD gypsum is present in soil. It has been reported that Hg in flue gas released from coal during combustion exists primarily as Hg0, oxidized Hg (Hg2+) and particulate Hg (Hgp). Hgp and partial Hg2+ can be removed by particle control devices and enter fly ash, whereas FGD gypsum can retain up to 95% of Hg2+ [13]. Therefore, Hg in fly ash and FGD gypsum varies with the ratio of different Hg speciation in flue gas. Hg levels in flue gas depend on the Hg concentration and the chemical composition of coal as well as the method of air pollution control processes [14,15]. Thus, it is expected that there are different total Hg levels in FGD gypsum from different regions. However, the Hg concentrations in FGD gypsum and Hg concentration distributions
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throughout different regions of China haven’t received enough attention. There were only limited data regarding the Hg concentration of FGD gypsum from several power plants; the total Hg concentration was reported between 150 and 2470 lg/g [16–18]. Furthermore, there are a number of power plants, and a few samples may not reflect the actual levels of Hg in FGD gypsum in China. As previously mentioned, most Hg2+ is removed from flue gas during the FGD process. However, when Hg2+ enters FGD gypsum, it interacts with FGD gypsum through different mechanisms, forming a different chemical speciation. Sun et al. [17] used a modified sequential selective extraction method to determine the chemical speciation of Hg in FGD gypsum collected from four coal-fired power plants in Zhejiang Province, China. The method differentiated Hg into four groups. The results showed that Hg in FGD gypsum from the different power plants had different distribution characteristics. Al-abed et al. [11] reported that 99% of Hg in FGD gypsum stayed in a strong complex and residue phase. These results suggested that the Hg chemical speciation in FGD gypsum varied from place to place, thus causing different environmental behaviors during disposal and reutilization. Furthermore, chemical speciation of heavy metals in soil is closely correlated with its bioavailability [19,20]. Thus, determination of chemical speciation aside from the total concentration of Hg will be helpful to understand the biogeochemical cycle of Hg and predict its environmental fate in FGD gypsum from different regions. In this study, we collected FGD gypsum samples from 70 power plants in 20 provinces, and determined their Hg characteristics. The total Hg and the chemical speciation of Hg were analyzed with a sequential selective extraction (SSE) method. A toxicity characteristic leaching procedure (TCLP) and a synthetic precipitation leaching procedure (SPLP) were performed to evaluate the leaching toxicity of Hg. 2. Materials and methods 2.1. Sample collection FGD gypsum samples were collected from 70 coal-fired power plants in 20 provinces, China. The sample sites are shown in Fig. 1a. The collected samples were preserved in sealed plastic bags and sent to the laboratory as soon as possible. Upon arrival at the lab, the samples were stored at 4 °C in the refrigerator. Because of the possible fluctuations of Hg concentrations in FGD gypsum, samples from four power plants in Shanghai and Anhui Provinces were collected each day during monitoring periods of four to eight weeks. 2.2. Total Hg content and Fe, Al analysis The moisture of the samples was measured by drying the samples at 105 °C overnight. The moisture was calculated as the weight difference of the sample before and after drying. The moisture ranged from 1.3% to 51%. The FGD gypsum samples were oven-dried at 40 °C before analysis. The total Hg concentrations were determined by mixing FGD gypsum with 5 mL distilled water and 5 mL aqua regia, then oscillating the samples on an end-over-end shaker for 24 h at room temperature [21]. The mixture was finally centrifuged at 3000 rpm for 15 min. The supernatant was filtered through a 0.45 lm membrane, before the Hg concentration was determined by cold vapor atomic absorption spectrometry (CVAAS) (HUAGUANG F732-VJ, Shanghai) with Hg2+ being reduced to Hg0 by the addition of SnCl2. For Fe and Al concentrations in FGD gypsum, the samples were placed in a 60 mL PTFE tube and digested with a mixture of 10 ml HNO3, 5 ml HClO4 and 10 ml HF on a graphite digestion block at
135 °C. The addition of the mixed acid was repeated several times until the solution was clear. The residual solution was filtered with a 0.45 lm cellulose acetate membrane and diluted to 50 mL in volumetric flasks with deionized water. Quantification of Fe and Al concentrations in the solution were determined by an inductively coupled plasma atomic emission spectrometry (ICP-AES) (LEEMAN Prodigy, USA). 2.3. Hg chemical speciation by SSE method A five-step sequential selective extraction method developed by Bloom et al. [21] was performed to determine the Hg speciation in FGD gypsum. This method differentiated Hg compounds into different behavioral classes instead of species-specific information. The five extraction solutions adopted included deionized water, 0.1 M CH3COOH + 0.01 M HCl, 1 M KOH, 12 M HNO3 and aqua regia. The extracted Hg was defined as water soluble Hg (F1), human stomach acid soluble Hg (F2), organo-chelated Hg (F3), strong complex Hg (F4) and residue Hg (F5). 2.4. Leaching tests The US EPA’s TCLP [22] and SPLP [23] were used to estimate the Hg leaching potential from FGD gypsum. TCLP simulates the leachability of solids in an acidic environment of municipal landfills. The TCLP extraction fluid used in this work included 17.25 mL of glacial acetic acid (CH3COOH) that was added to a glass volumetric flask, and then diluted with deionized water to 1000 mL (pH of 4.93 ± 0.05). One gram of gypsum was mixed with 20.0 mL of solution, maintaining the same solution-to-solids ratio of 20:1, according to the standard procedure. Samples were extracted for 18 ± 2 h in a 50-mL centrifuge tube at room temperature by end-over-end tumbling at 30 rpm. After extraction, the samples were centrifuged for 20 min at 3000 rpm, and the supernatant was filtered through a 0.45 lm filter. The filtrates were then oxidized and preserved by adding 2% (v/v%) of 0.2 M BrCl. The Hg concentration in filtrates was determined by CVAAS. For the SPLP, sulfuric acid and nitric acid (mass ratio 2:1; pH = 3.20 ± 0.05) were used as extraction fluids to conduct the solid waste-extraction procedure for leaching toxicity and estimate the effect of acid rain on FGD gypsum. Two grams of sample and 20 mL of extraction fluid were combined into a glass volumetric flask. The extraction and determination procedures were similar to those of the TCLP. 2.5. Quality control The Hg concentration was expressed as dry weight basis. To ensure accurate determination of the Hg concentration, the standard addition method was applied to the total Hg determination and the standard recovery rate was calculated. The recovery rates ranged from 80% to 106%. A blank sample was analyzed in each batch test to eliminate the effect of impurities in the reagent. All analyses were performed in triplicates and the results were expressed as the mean ± standard deviation. 3. Results and discussion 3.1. Spatial distributions of total Hg in FGD gypsum Fig. 1 displays the total Hg concentrations in the FGD gypsum samples from the different provinces and the frequency distribution of Hg in all of the FGD gypsum samples. The Hg concentration in each FGD gypsum sample is displayed in SI Table 1. The Hg concentrations ranged from not detectable to 4330 ± 620 lg/kg, and showed obvious regional differences. FGD gypsum from Shanxi,
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20
15
10
5
0
0
1000
2000
3000
4000
Hg concentration (µg/kg)
(a)
(b)
Fig. 1. Sampling sites and average total Hg concentration in FGD gypsum by province (a) and frequency distribution of Hg concentration in FGD gypsum of all the samples (b).
Henan, Anhui and Sichuan Provinces had relatively high average Hg concentrations > 800 lg/kg. The two samples with the highest Hg concentrations were from DT3 Shanxi Province (3976 lg/kg) and SH Anhui Province (4330 lg/kg). Samples from Shanghai, Shandong, Ningxia, Zhejiang, Guizhou and Yunnan Provinces had Hg concentrations at average levels between 500 and 800 lg/kg. Other provinces, such as three northeast provinces, Xinjiang, Inner Mongolia and Gansu, had average Hg concentrations < 500 lg/kg. In general, Fig. 1a shows that provinces in the northern part of China had lower average Hg concentrations, and provinces in the middle and southern parts of China had relative higher average Hg in FGD gypsum. Fig. 1b shows that 67% samples contained Hg at concentrations between 200 and 1000 lg/kg, and 30% of the samples were above 1000 lg/kg. The average total Hg concentration of all FGD gypsum samples was 891 lg/kg and the median was 629 lg/kg. As mentioned earlier, the Hg concentration in FGD gypsum depends on Hg in the burning coal and air pollution control system. Although Hg in FGD gypsum displayed regional characteristics, some provinces had higher average Hg concentrations in FGD gypsum. It is hard to build the correlation between Hg concentrations in FGD gypsum and Hg in the coal and air pollution control system based on information on Hg in coal in each province [24] and the air pollution control system adopted in each power plant (mainly NOx removal devices). Therefore, other factors, such as halogen in flue gas and the operational conditions of the oven and air pollution control system, may also play important roles in Hg retention in FGD gypsum. The relationship between Hg concentrations in FGD gypsum and these factors needs further investigation. For comparison, the concentrations of Hg in the USA ranged from 53 to 846 lg/kg (n = 11) [10,12,25–27], with an average of
352 lg/kg. Samples from Spanish power plants ranged from 150 to 310 lg/kg (n = 3) [28,29]. Average total Hg in FGD gypsum from China seemed to be higher than those reported from other countries. It was well established that the average Hg concentration in coal in China was reported between 160 and 220 lg/kg [30,31]. Therefore, it seems that FGD gypsum is enriched in Hg that is released from coal during the combustion process.
3.2. Fluctuant profile of Hg concentration in FGD gypsum To assess the possible fluctuations of Hg in FGD gypsum, samples from four power plants in Shanghai and Anhui province were collected each day during four to eight weeks and the total Hg concentration was analyzed. The results are displayed in Fig. 2 and provide a general description of Hg concentration fluctuations in FGD gypsum that reflect the effect of daily operations of the coal power plants. The results indicated that the total Hg concentration in all samples from the four power plants during the monitoring period had large variations. The Hg content in SZ Anhui FGD gypsum varied between 1290 and 3600 lg/kg, with an average of 2669 lg/kg and a median of 2709 lg/kg. Moreover, the samples from XH Shanghai, SHZ Shanxi and DT2 Shanxi power plants also had large fluctuations. The largest influence on Hg in FGD gypsum was the fluctuation of the daily coal constituents there were burned in the power plants. Power plants might burn coal from different coalmines and there are differences in the Hg concentration of the parent coal. There may even be differences in the Hg concentration in coal from the same coal mine that varies largely due to the different mining sites [32]. The combustion temperature, unburned carbon in flue gas and the operational conditions of air
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Fig. 2. Variation of Hg contents in FGD gypsum. The bottom and top of the box represent the 25th and 75th percentiles, respectively. The band between the bottom and top of the box represents the 50th percentile. The ends of the whiskers represent the 10th and 90th percentiles.
pollution control devices might be other factors influencing the Hg concentration in FGD gypsum [33]. 3.3. Hg chemical speciation in FGD gypsum from SSE process Bloom’s five-step sequential selective extraction (SSE) procedure was performed to glean information of the Hg chemical speciation in FGD gypsum. The five groups demonstrated the mobility and leachability of Hg in solids under different conditions. Fig. 3 displays the Hg distributions in the five extraction fractions of the FGD gypsum samples from different parts of China. The results show that although strong complex Hg was the dominant phase in most of the samples, the chemical speciation of Hg in FGD gypsum from different provinces was different and also varied within the samples from same province. F1 and F2 were extracted by water and weak acid, respectively. Both conditions were commonly encountered in the natural environment. Therefore, F1 + F2 were often considered to be easily leachable and potentially bioavailable to the environment. For all of the samples, F1 + F2 accounted for percentages ranging from ND to 25.2% of the total Hg, with an average of 7.4%. The percentages of these two parts extracted from the total Hg were displayed in SI Table 2. In general, the samples from Shanghai and Gansu Provinces had relatively higher F1 + F2 percentages with averages of 11% and 13.5%, respectively. Samples from different power plants in the same province also had different F1 + F2 percentages. For example, the F1 + F2 percentage in most of the samples from Shanxi Province was lower than 10%. However, the F1 + F2 of the DT2 sample was 21%. The F1 + F2 percentage clearly varied for samples from Shandong Province, too, with the highest sample at 21% and the lowest sample less than 1%. F1 and F2 normally included HgCl2, Hg(NO3)2, HgSO4, and some Hg that weakly absorbed to materials [21]. The variation of Hg in F1 + F2 was likely due to the flue gas composition and the pollution control process method. An F1 + F2 fraction average of 7.4% of the total Hg suggested low Hg mobility in FGD gypsum in China. The F3 percentage ranged from ND to 82%, with most samples containing less than 10% (n = 54). It is worth noting that DT Shandong, XH Shanghai and BG Xinjiang FGD gypsum had 36%, 58% and 82% of Hg in F3, respectively. F3 was identified by Bloom et al. [21] as organo-chelated Hg. However, during the desulfurization process, it was unlikely to form organo-chelated Hg. Furthermore, it was found that Hg2Cl2 was also extracted during this step [21]. One study discovered that Hg2+ was partially reduced to Hg0
instead of being removed by the FGD process [17]. Instead of being reduced to Hg0, Hg+ may have partially formed and it may have been likely that Hg remained primarily Hg2Cl2 in the F3 species. Because Hg2Cl2 easily formed under reduction atmosphere, the operational conditions of the FGD procedure may be the key factor affecting the Hg level in F3 species. Bloom et al. [21] observed that Hg in the F3 species was strongly correlated with the methylation potential. Therefore, for FGD gypsum samples with high F3 percentages, avoiding disposal under the conditions which are enriched with microorganisms is recommended. The percentage of F4 varied from 10% to 98.5% of the total Hg concentration. Fig. 3 shows that 84% (n = 59) of the samples had more than 60% of the F4 species. Therefore, most Hg in FGD gypsum in China stayed in the strong complex phase. Bloom et al. [21] defined this phase as elemental Hg because Hg0 was removed at this step. Other chemical species may also be included in this fraction, such as Hg2Cl2, Hg0-metal amalgam and Hg2+ complexes. Kairies et al. [26] suggested that the Hg species of FGD-derived gypsum appeared to correlate with the Fe concentrations. Hg2+ could form a Hg–Al-precipitate and a Hg–Fe-precipitate at the presence of Ca, SO2 4 and Fe or Al [34]. FGD gypsum had relatively high Fe and Al concentrations [35]. We also measured the abundance of Al and Fe elements in the FGD gypsum samples, which ranged from 1000 to 10,000 mg/kg and 100 to 3000 mg/kg, respectively (see SI Table 1). Pearson correlation coefficient analysis revealed the concentrations of Hg and Fe were significantly correlated and the concentrations of Hg and Al were significantly correlated below concentrations of 5000 mg/kg (P < 0.05). This correlation suggested that Hg in FGD gypsum might exist primarily as Hg-complexity with Fe and Al and show a strong affinity to the materials. The F5 species, the most stable phase in FGD gypsum during the extraction procedure, accounted for percentages ranging from ND to 53% of the total Hg. Fig. 3 shows that samples from Shanxi and Shanghai had relatively higher percentages of the F5 species than the samples from other parts of China. For most of the samples, the F5 species accounted for only small parts of the total Hg. HgS was identified as the main form of Hg in the F5 species [21].
3.4. Leaching toxicity The Hg concentrations in the extracted solutions from the TCLP and SPLP tests were all below the US EPA’s regulation level of the maximum concentration of Hg to be characterized as toxic (0.2 mg/L), which classified FGD gypsum as non-hazardous waste. Fig. 4 displays the TCLP and SPLP results as a variation of the total Hg. For the TCLP, total Hg concentrations below 800 lg/kg had Hg in the leaching solutions lower than 3.0 lg/L. As the total concentration of Hg increased, some samples had higher leaching concentrations. The highest leaching concentration was 19 lg/L in the sample from SZ Anhui; this sample also had the highest total Hg concentration. Fig. 4 also shows that although the highest TCLP concentration was observed in the sample with the highest total Hg concentration, other FGD gypsums did not have a similar trend. For example, samples from DT3 Shanxi, HN Anhui, JN Shandong had very low leaching concentrations and higher total Hg concentrations, whereas samples from SZ Anhui, DT1, DT2 Shanxi had relatively lower total Hg concentration yet still had a little higher leaching concentrations. This result was likely due to the different binding modes of Hg in FGD gypsum. In general, the leaching level for all of the FGD gypsum samples was low. Pasini and Walker [25] also found that the FGD gypsum from the Midwestern United States contained 355 lg/kg of the total Hg, but only leached less than 0.1% of Hg during the TCLP.
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F1
F2
F3
F4
F5
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DT JN1 JN2 JN LY QD RZ TA ZP JS1 JS2 JS3 JY SZH TZ ZJ1 ZJ2
BS SDK JS MH WGQ1 WGQ2 WGQ3 WGQ4 WJ XH
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TY1 TY2 DT1 DT2 DT3 YQ PY YC SHZ LF YCH JZH
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Sampling sites Fig. 3. Chemical speciation of Hg in FGD gypsum from sequential selective extraction procedure.
Compared with the TCLP, the SPLP results had a more scattered distribution. However, similar to the TCLP, there was no clear correlation between leaching activity and total Hg concentration. The samples with high total Hg concentration certainly did not have higher leaching Hg. Generally, there was no significant difference between the leaching results from the TCLP and the SPLP. The slight variation may result from the different nature of the two extraction solutions. The comparison of the total Hg leaching percentages from the TCLP, the SPLP and the SSE procedure (F1 + F2) was displayed in SI Table 2. Comparatively, extractable F1 + F2 percentages were more than that from the TCLP and the SPLP. Because F1 + F2 was well accepted as the easily mobile phase during natural conditions, Hg in FGD gypsum may leach more than what is predicted by the TCLP and SPLP methods.
3.5. Estimation of Hg release from FGD gypsum through the leaching process Most of FGD gypsum byproducts have been randomly piled up on the ground before reutilization, either for storage or land reclamation. Although the percentage of the potential Hg release is
small, Hg in FGD gypsum originally scrubbed from flue gas still possibly reentered the environment through leaching. We estimated the potential Hg release using the following equation:
m¼
CRM 109
50%
where m is the amount of released Hg (tons); C is the average Hg concentration (lg/kg); R is the average release rate of Hg from the TCLP and the SPLP (%); and M is the total production of FGD gypsum per year in China (tons); 50% is the general reutilization rate of FGD gypsum in China [1]. The average concentration of Hg from the collected samples was 891 lg/kg. The average percentage of leaching Hg from the TCLP and the SPLP, which was 2.75%, was used to estimate the potential release of Hg from FGD gypsum. The total production of FGD gypsum per year in China was approximately 52 million tons. Therefore, the amount of Hg released from FGD gypsum was estimated to be 0.65 tons per year. The estimation of the potential release of Hg from FGD gypsum was based on the results from the TCLP and the SPLP, which simulated the landfilling condition and the effects of acid rain, respectively. Under natural conditions, there are other variables, such as redox conditions, ambient temperature, microbial conditions
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ples from different power plants. Leaching toxicity tests indicated low mobility of Hg in FGD gypsum. Hg potential leaching from FGD gypsum in China was estimated to be 0.65 tons per year. Acknowledgments The research was funded by National Natural Science Foundation of China (Grant No. 41402311) and we would also like to give our grateful acknowledgement to those who help us collect FGD gypsum around the country. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2016.02.091. References
Fig. 4. Leaching characteristics as the variation of total Hg.
and the light conditions, that may also affect the amount of Hg released from FGD gypsum. However, the equation permits an estimation of the potential release of Hg from FGD gypsum. If that amount of Hg does release, the potential environmental risks should be considered. The output of FGD gypsum from power plants varies widely for the different provinces. Provinces located in the central part of China, such as Anhui, Shanxi and Henan, have more coal-fired power plants, thus producing higher amounts of FGD gypsum than provinces in the eastern part of China, where other types of electricity generation methods account for part of the total electricity production [1]. However, the reutilization rates of industrial byproducts in the provinces in the central part were lower compared to those located in the eastern part of China [1]. Furthermore, as previously discussed, the FGD gypsum in the central part had higher Hg concentrations. Thus, it is expected that the Hg leaching risk of FGD gypsum in the central part of China will be larger than the Hg leaching risk in the eastern part of China. Yang et al. [36] has expressed concerns that ignoring the emissions of Hg from coal fly ash and FGD may undermine the efforts of Hg control. Therefore, it is urgent to adopt a standardized management procedure to minimize the potential risks of FGD gypsum.
4. Conclusions We have investigated Hg contamination in FGD gypsum from 70 power plants in 20 provinces in China. Both the total Hg concentration and the chemical speciation had significant regional characteristics. The total Hg concentrations in FGD gypsum ranged from non-detectable to 4330 lg/kg, with an average of 891 lg/kg and a median of 629 lg/kg. FGD gypsum from the central part of China had relatively high average Hg concentrations. Chemical speciation analysis demonstrated that most Hg remained in the strong complex phase in FGD gypsum and Hg in each phase varied with sam-
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