Article Cite This: Energy Fuels 2018, 32, 8340−8347
pubs.acs.org/EF
Increasing Recovery Ratios with an Improved European Community Bureau of Reference Method for Mercury Analysis in Flue Gas Desulfurization Gypsum Shuang Bian,† Jiawen Wu,† Yongsheng Zhang,†,* Tao Wang,† Pauline Norris,‡ and Wei-Ping Pan†,‡ †
Energy Fuels 2018.32:8340-8347. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 08/17/18. For personal use only.
Key Laboratory of Condition Monitoring and Control for Power Plant Equipment, Ministry of Education, North China Electric Power University, Beijing, 102206, China ‡ Institute for Combustion Science and Environmental Technology, Western Kentucky University, Bowling Green, Kentucky 42101, United States ABSTRACT: Coal-fired power plants produce flue gas desulfurization (FGD) gypsum as a byproduct of the sulfur removal process. This gypsum is contaminated with trace elements. Knowing the occurrence and release characteristics of trace elements from the FGD gypsum is very important if the gypsum is to be safely used in different applications. The sequential extraction procedure proposed by the European Community Bureau of Reference (BCR) can be used to provide mercury speciation information for FGD gypsum samples. Many researchers have used the BCR sequential extraction method to analyze the occurrence of mercury in FGD gypsum. Unfortunately, the recovery ratio is low, around 50−60%. The recovery is low because the BCR method does not include the portion of elemental mercury (Hg0) lost during each leaching step. In this work, modifications to the BCR method were made to more accurately determine the amount of mercury lost during the extraction. Using this “improved BCR method”, the Lumex RA-915+/PYRO-915 mercury analyzer measures the mercury content in the residues at each extraction step. The difference between the sample and residue concentrations is equal to the amount of Hg released. This indirect measurement method was used to overcome the shortcomings of the traditional BCR sequential extraction process. The results show that the “improved BCR method” has a recovery ratio of about 99%, which can more accurately evaluate the environmental stability of mercury in gypsum. Thus, an improved five-step sequential extraction procedure for analysis of mercury was successfully applied to FGD gypsum samples in this study.
1. INTRODUCTION Flue gas desulfurization gypsum and fly ash are two major byproducts of coal-fired power plants. The main mineral composition of the FGD gypsum is CaSO4·2H2O and can be used as a good substitute for natural gypsum in the production of wallboard, cement, and concrete additives.1 In 2013, China produced about 72 million tons of gypsum, and the production is expected to reach up to 100 million tons/y in 2020.2 Utilizing the byproduct gypsum from power plants would be advantageous. Unfortunately, the FGD gypsum is contaminated with mercury because of the coal combustion process.3 Understanding the occurrence and release characteristics of the mercury in the FGD gypsum is necessary for safe utilization of the product. Mercury, in its elemental form (Hg0), is insoluble in water and very volatile. It is also a harmful environmental pollutant.4−8 Studying the speciation of mercury in FGD gypsum is essential to determine the risk when the wastes are recycled or disposed.9 The sequential extraction method proposed by the European Community Bureau of Reference (BCR) in 1993 has been widely used to analyze soil and sediments.10−12 The BCR method is based on the solubility of mercury compounds and is commonly used for Hg speciation studies.13,14 The BCR method divides the mercury in the FGD gypsum into five different forms. The forms are water-soluble Hg (F1), weak acid extractable Hg (F2), reducible state Hg (F3), oxidizable state Hg (F4), and residual state Hg (F5). The F1 and F2 forms are extracted by deionized water and © 2018 American Chemical Society
CH3COOH, respectively. Mercury in these fractions is easily leached into the environment. The F3 and F4 forms are extracted with a reducing agent and oxidizing agent, respectively. These forms also have the potential to leach into the environment. The residual state, F5, is considered the most stable phase and should have minimal impact on the environment. Collectively, the first four forms (F1, F2, F3, and F4) are defined as the available state. Mercury present in the available state is considered easily leached into the environment and could impact biological activity and interactions among organisms and their environment.15,16 An alternative to the BCR method is EPA method 3200. This method distinguishes mercury speciation in solids and sediments. EPA method 3200 provides varying degrees of mercury species information.17 The mercury in gypsum can be divided into extractable mercury and nonextractable mercury. The extractable mercury includes organic and inorganic mercury. The nonextractable mercury includes semimobile mercury and nonmobile mercury. The extractable mercury is more labile. The nonextractable mercury represents the least labile form of mercury. The BCR method evaluates the release of mercury into the environment under different leaching conditions, which separates mercury compounds into different leachable Received: May 25, 2018 Revised: July 19, 2018 Published: July 20, 2018 8340
DOI: 10.1021/acs.energyfuels.8b01827 Energy Fuels 2018, 32, 8340−8347
Article
Energy & Fuels
Figure 1. Escaping gas phase mercury evaluation device.
two is the total amount of mercury released during that step. This amount of mercury includes the mercury in the leachate (liquid portion) and the mercury lost into the air (gas portion). The original BCR method does not measure the mercury present in the gas portion. A high recovery rate can be achieved through this improved BCR method. The BCR method and the improved BCR method both were used in this study to analyze the form of mercury in five gypsum samples.
categories of behavior rather than specific species. This method focuses on the toxicity and bioavailability of mercury. Many researchers use the BCR method to determine the form of mercury in gypsum. Ideally, the summation of the five different fractions should be relatively close to the total amount of mercury present overall. However, low recovery data from the BCR method indicates that some mercury is lost in the extraction process. Zhang et al.18 confirmed that the recovery ratio of mercury in gypsum is about 60% using the BCR method. Sun et al.19 found that the recovery ratio is less than 50% when studying the occurrence of mercury in four kinds of desulfurization gypsum by continuous extraction method. Alabed et al.20 found that the recovery ratio was about 50% when studying the occurrence and migration characteristics of trace elemental mercury in FGD gypsum. The relatively low recovery ratio means that the technique cannot accurately be used to determine the release characteristic of mercury in each form. Many scholars have determined that the Hg0 will be released when the FGD gypsum is exposed to air or water.21−24 Therefore, it is possible that the exposure of gypsum to the air, the addition of aqueous solution, and heating the solution in a water bath all lead to the loss of mercury and cause the low recovery ratio. In this study, gas phase evolution experiments were set up first to prove that elemental mercury was escaping during the extraction process. Then, a more accurate determination of the form of mercury in FGD gypsum could be made. First, the occurrence and the amount of gas phase mercury that escapes during the extraction process were assessed. Then, an “improved BCR method” was designed that would take the amount of mercury lost at each step into consideration. In this method, the sample and solid residues produced after each extraction step were analyzed and the difference between these
2. EXPERIMENTAL METHODS 2.1. Sample Collection and Processing. The gypsum samples for the sequential selective extraction were collected from a 300 MW coal-fired power plant. This plant burns bituminous coal and is equipped with selective catalyst reduction (SCR), electrostatic precipitation (ESP), and wet flue gas desulfurization systems (WFGD). The samples were dried for 8 h in a drying oven at a temperature of 45 °C. This temperature can remove excess moisture from the samples without releasing crystalline water. To ensure sample homogeneity, the samples were also milled for 5 min and passed through a 100-mesh sieve. These samples were collected and recorded as 1, 2, 3, 4, and 5. 2.2. Description of the Gas Phase Mercury Release Evaluation Device. The gas phase mercury release evaluation device was used to investigate the Hg0 release characteristics from the FGD gypsum. The amount of Hg0 released from the FGD gypsum was analyzed to further explain the reasons for the low recovery ratio observed with the BCR method. The mercury that escaped during the experiment consists of two parts: (1) Hg0 released by the addition of an aqueous solution; (2) mercury that escaped while heating the water bath. The first experiment was used to assess the amount of mercury released during the addition of the aqueous solution. For the purposes of this work, the mercury in the FGD gypsum was divided into three categories, gas phase escaping Hg0 (when gypsum is exposed to air and water), Hg2+ (which can be reduced by SnCl2), and residual mercury (mercury in gypsum particles). 8341
DOI: 10.1021/acs.energyfuels.8b01827 Energy Fuels 2018, 32, 8340−8347
Article
Energy & Fuels The gas phase Hg0 escaping system, the Hg2+ reduction system, the Hg0 detection system and the exhaust gas treatment system are shown in Figure 1a. The gaseous mercury escaping system and the Hg2+ reduction system consists of two bubbling reactors 1 and 2. The Hg0 detection system consists of a Lumex RA-915+ (Lumex, Russia) mercury analyzer and a gas pretreatment bottle. The gas pretreatment bottle contains 20% NaOH solution to filter the acid gas. The detection limit of the built-in optical path cell of Lumex RA-915+ mercury analyzer is 2 ng/m3. The exhaust treatment system has three scrubbers. The first two are filled with 20 mL 4% w/v KMnO4 and 10% v/v H2SO4 solutions and the last one contains 20 g of activated carbon. The first two absorption bottles are used to oxidize and absorb the Hg0 in the carrier gas. Then, the carrier gas is discharged after passing through the activated carbon. Experimental Procedure. (1) The calibration curve was made using 1, 2, and 3 mL of 10 ppb mercury standard solution, reduced with SnCl2. (2) A gypsum sample of 0.5 g and 3 mL deionized water was added to bubbling reactor 1. The air gas flow rate was 1 L/min. The gas passed through the NaOH solution into the Lumex RA-915+ mercury analyzer to determine the amount of Hg0. (3) A 3 mL portion of of SnCl2 solution was added to the abovementioned solution. SnCl 2 + Hg 2 + → Hg 0 + SnCl4
(1)
These experiments were done in triplicate. A Lumex RA-915+ mercury analyzer was used to measure Hg0. A RA-915+/PYRO-915 analyzer was used to measure mercury in the solid phase. A Lumex RA-915+ mercury analyzer was used to measure the amount of Hg2+ reduced by SnCl2. The gypsum slurry was filtered and dried in a vacuum dryer at room temperature and, then, analyzed using a Lumex RA-915+/PYRO-915 mercury analyzer. The concentration of residual mercury was measured in this step. Formula 2 is used to analyze the relationship between different forms of mercury in the gas phase mercury evolution experiment.
Xi =
ci × m × 100% ∑i = 1,2,3 ci × m
Figure 2. Traditional BCR sequential extraction procedure flowchart.
Fraction 3. Reducible. Residue 2 from Fraction 2 was extracted with 0.1 mol/L NH2OH−HCl at room temperature and rotated in an end over end manner for 16 h and, then, centrifuged at 1500 rpm for 20 min. A 1 mol/L portion of HNO3 was used to adjust the pH of the leaching agent to 2. Fraction 4.Oxidizible. Residue 3 from Fraction 3 was extracted with 30% H2O2. A small amount of H2O2 solution was carefully added to the residue, then, heated to 90 °C for 1 h, and boiled for 5 min. The H2O2 evaporated and the sample was cooled to room temperature. Then, residue 4 was extracted with 1 mol/L CH3COONH4 at room temperature, rotated in an end over end manner for 16 h, and, then, centrifuged at 1500 rpm for 20 min. Fraction 5.Residual. Residue 5 from Fraction 4 was dried in an oven at 45 °C. Residue 6 is acid-digested with 5 mL HNO3 and 3 mL HF. All reagents (hydroxylamine hydrochloride, stannous chloride, and ammonium acetate) used in this study were of high quality. HCl (36− 38%), HNO3 (65−68%), HF (40%), H2O2 (33%), H2SO4 (95−98%), and CH3COOH (98%) were of high quality. They were purchased from Beijing Chemical Inc. The filtrate was analyzed by cold-atomic absorption spectroscopy (Hydra II, Leman Company, USA), which can directly provide the concentration of each form of Hg. The filtrate was filtered through a 0.45 μm membrane filter prior to analysis. Extractions were done in triplicate to obtain an average value. The shaded areas indicate places where the loss of Hg0 might be likely. As shown in Figure 2, the release of Hg0 may occur during the addition of H2O2 to 90 °C and the addition of the aqueous solution. Hg0 is not soluble in water and cannot be detected by the liquid mercury analyzer Hydra II. Thus, the mercury content of the sample and leaching solution is unbalanced, resulting a low recovery ratio. 2.4. Overview of the Improved BCR Method. The traditional BCR method does not have any way to measure the amount of gaseous Hg0 that escapes during the extraction due to air and water exposure. Thus, the improved BCR method was proposed to include this portion of missing mercury. A flow diagram for the improved BCR method is shown in Figure 3. A summary of the extraction method is as follows:
(2)
where Xi is the ratio of speciation; c1 (ng/mL), c2 (ng/mL), and c3 (ng/mL) represent the concentration of escaping gas phase Hg0, Hg2+ reduced by SnCl2, the mercury in filter residue; m (g) represents the mass of the gypsum sample used for the experiment. The second experiment was used to verify the amount of mercury that was released while heating the water bath during the leaching process. For the purposes of this work, escaped mercury can be captured using the device in Figure 1b. . In this system, sample was heated with water bath (90 °C), and the escaped mercury can be adsorbed by the activated carbon. Experimental Procedure. The residue from the third step in the BCR method (sample) was placed in a clean quartz glass chamber, which was fixed in a water bath at 90 °C. Ambient air with a flow rate of l L/min was used as the carrier gas to transport escaped mercury, which was subsequently captured by activated carbon. Heat tracing pipe, set at 140 °C, was used to prevent the Hg0 from condensing. These experiments were done in triplicate. The mercury that was released was measured by Lumex RA-915+. 2.3. Overview of the Traditional BCR Method. The traditional five-step BCR sequential extraction method simulates the release of mercury into the environment under different leaching conditions, as shown in Figure 2. This extraction method distinguishes mercury compounds into different leachable categories of behavior rather than specific species. A summary of the extraction method is as follows: Fraction 1. Water-Soluble. A gypsum sample of 1 g was extracted for 16 h with deionized water at room temperature and rotated in an end over end manner and, then, centrifuged at 1500 rpm for 20 min. Fraction 2. Acid-Soluble. Residue 1 from Fraction 1 was extracted with 0.11 mol/L CH3COOH at room temperature and rotated in an end over end manner for 16 h and, then, centrifuged at 1500 rpm for 20 min. 8342
DOI: 10.1021/acs.energyfuels.8b01827 Energy Fuels 2018, 32, 8340−8347
Article
Energy & Fuels
Figure 3. Improved BCR method sequential extraction flowchart. Fraction 1: F1. A gypsum sample of 1 g was extracted for 16 h with deionized water at room temperature and rotated in an end over end manner and then centrifuged at 1500 rpm for 20 min. The residue 1 was dried in an oven at 45 °C and the mercury concentration in the slag was measured with Lumex RA-915+/PYRO-915. Fraction 2: F1+F2. A gypsum sample of 1 g was extracted with deionized water at room temperature and rotated in an end over end manner for 16 h. Then, the residue was extracted with 0.11 mol/L CH3COOH at room temperature and rotated in an end over end manner for 16 h and then centrifuged at 1500 rpm for 20 min. Residue 2 was dried in an oven at 45 °C, and the mercury concentration in the slag was measured with Lumex RA-915+/PYRO915. Fraction 3: F1+F2+F3. The sample procedure was followed as described in Fraction 2 to produce residue 2. The residue was leached with 0.1 mol/L hydroxylamine hydrochloride at room temperature, rotated in an end over end manner for 16 h, and, then, centrifuged at 1500 rpm for 20 min. The residue 3 was dried in an oven at 45 °C, and the mercury concentration in the slag was measured with Lumex RA-915+/PYRO-915. Fraction 4: F1+F2+F3+A. (A: After adding the H2O2 solution, the mercury escaped during the water bath heating process.) The sample procedure was followed as described in Fraction 3 to produce residue 3. A small amount of H2O2 solution was carefully added to residue 3 and then heated to 90 °C for 1 h. The H2O2 evaporated, and the sample was cooled to room temperature. Residue 4 was dried in an oven at 45 °C, and the mercury concentration in the slag was measured with Lumex RA-915+/PYRO-915.
Fraction 5: F1+F2+F3+F4. The sample procedure was followed as described in Fraction 4 to produce residue 4. The residue was extracted for 16 h with 1 mol/L CH3COONH4 and centrifuged for 20 min. Residue 5 was dried in an oven at 45 °C, and the mercury concentration in the slag was measured with Lumex RA-915+/PYRO915. Fraction 6: F5. Residue 5 was analyzed by Lumex RA-915+/ PYRO-915 mercury analyzer to obtain the residual state F5. Extractions were done in triplicate to obtain an average value. The release of mercury at each step of the leaching process can be expressed by eq 3. 5
∑ Fi + i =A4 = M − Mi i=1
(3)
Where M represents mercury content in the gypsum samples; Mi represents the residual mercury content of step i; and A represents the mercury released after adding H2O2 solution during the water bath heating process. The mercury difference between sample and residue in each step is the total amount of released mercury during the leaching process. For example, the mercury difference between sample and residue 1 in the first step is F1. The mercury difference between sample and residue 2 in the second step is F1+F2. Then, the true value of F2 is obtained by the difference between F1+F2 and F1 from the previous test. The improved BCR method can accurately overcome the shortcomings in the original BCR method. 2.5. Sample Quality Control for Preparation and Analysis. 8343
DOI: 10.1021/acs.energyfuels.8b01827 Energy Fuels 2018, 32, 8340−8347
Article
Energy & Fuels (1) Method blank: A method blank is prepared using the same reagents and quantities used with samples and analyzed in each batch test to eliminate the effect of impurities in the reagent. (2) Duplicate: The experiment was done in triplicate to get an average value to minimize the experimental error. (3) Equipment accuracy: The accuracies of the Hydra II and Lumex RA-915+/PYRO-915 mercury analyzers were verified by analyzing a national reference soil material with a certified mercury concentration of 460 ppb. Figure 4 shows the
Xi = 1,2,3,4 =
X5 =
∑i = 1,2,3,4 ci × vi ∑i = 1,2,3,4 ci × vi + m5 × c5′ × v5′/m5′
× 100% (6)
m5′ × c5 × v5/m5 × 100% ∑i = 1,2,3,4 ci × vi + m5 × c5′ × v5′/m5′
(7)
where Xi=1,2,3,4 represents the ratio of available state mercury and X5 represents the ratio of residual state mercury.
3. RESULTS AND DISCUSSION 3.1. Analysis of Escaping Gas Phase Hg0. Hg0 escaping experiments were performed on samples 1, 2, 3, 4, and 5 using the gas phase mercury escaping evaluation device shown in Figure 1. Experimental results are shown in Table 1. In three Table 1. Forms of Mercury in Gypsum and Proportional Relationship form and proportion of mercury (%) sample
escaping Hg0
Hg2+
residual mercury
1 2 3 4 5
28.80 32.97 29.00 29.80 30.10
18.15 17.13 16.97 17.11 17.43
53.05 49.89 54.03 53.09 52.47
different runs, between 28.80% and 32.97% of the Hg0 escaped from the gypsum upon exposure to air and water. A water molecule is polar and could compete with Hg0 for binding sites on the surfaces of solid particles. Hg0 adsorbed on solid particles would be desorbed and become free Hg0 in the pores when water was added, so as to increase the Hg0 pool available for release.22 The percentage of Hg2+ (that can be reduced by SnCl2)varied from 16.97% to 18.15%. The percentage of residual mercury varied from 49.89% to 54.03%. Figure 5 displays the percentage of Hg0 that escaped while heating the water bath. Around 19%−35% of the mercury
Figure 4. Standard reference material analysis results determined using Hydra II and Lumex RA 915+ analyzers. measurement results. Replicates 1−3 were measured by Hydra II, and 4−6 were measured by Lumex RA-915+/PYRO-915. Both instruments have an accuracy greater than 90%, which is within the allowable standard deviation range for the instrument. The measured Hg concentration was 430 ± 10 ppb for the Hydra II and 470 ± 5 ppb for the Lumex RA915+/PYRO-915. (4) Corresponding uncertainties: The corresponding uncertainties for the improved BCR method sequential extraction method are defined as 5
σ=
∑ [σ(ci)]2 (4)
i=1
Where σ is corresponding uncertainty; σ(ci) represents the respective errors of the residue concentrations resulting from each of the improved BCR method steps. 2.6. Mass Balance. Recovery Ratio Calculations. The mercury recovery ratio for the BCR sequential extraction method is defined as X=
∑i = 1,2,3,4 ci × vi + m5 × c5′ × v5′/m5′ m0 × c0′ × v0′/m0′
× 100%
(5)
where X (%) is the recovery ratio; m0 (g) and m′0 (g) are the masses of the FGD samples for leaching and digestion, respectively; ci (ng/mL) and c′0 (ng/mL) represent the concentrations of mercury in the leaching solution for each step and digestion solution, respectively; vi (mL) and v′0 (mL) represent the volumes of leaching solution for each step and digestion solution, respectively; m5 (g) represents the quantity of the residue after drying; m′5 represents the quantity of residual state used for digestion; c5′ (ng/mL) represents the concentration of mercury in the residual solution; v′5 (mL) represents the volume of residual solution. Formulas 6 and 7 are used to calculate the ratio of mercury present in the available and residual fractions, respectively.
Figure 5. Percentage of Hg0 escaped during the heating of water bath.
escaped while heating the water bath, which is consistent with the percentage of escaped mercury from the improved BCR method, in the range of 22−39%. Sui et al.25 determined that the initial pyrolysis temperatures of Hg2Cl2 and HgCl2 were 30 and 50 °C, respectively. The temperature of the water bath is 90 °C in the BCR method, which would also increase the Hg0 8344
DOI: 10.1021/acs.energyfuels.8b01827 Energy Fuels 2018, 32, 8340−8347
Article
Energy & Fuels Table 2. Speciation and Proportion of Mercury in the BCR Methoda form and proportion of mercury % sample
F1
F2
F3
F4
F5
sum (ng/g)
total mercury (ng/g)
recovery ratio (%)
1 2 3 4 5
11.40 12.37 10.12 16.54 17.93
0.27 0.50 0.61 0.43 0.09
0.28 0.53 0.61 0.73 0.57
47.81 17.84 23.46 19.89 21.37
7.03 11.14 10.90 12.34 8.29
440.29 154.28 138.06 344.79 68.03
659.22 364.03 302.03 690.54 141.00
66.79 42.38 45.71 49.93 48.25
a
Sum: F1+F2+F3+F4+F5. Total mercury: the Hg content in gypsum samples was measured using the Hydra II.
Table 3. Speciation and Proportion of Mercury in Improved BCR Methoda form and proportion of mercury % sample
F1
F2
F3
A
F4
F5
sum (ng/g)
total mercury(ng/g)
recovery ratio (%)
1 2 3 4 5
14.15 14.83 11.90 17.22 19.20
1.72 2.72 6.35 5.09 1.53
23.07 10.16 24.33 25.35 24.68
25.78 39.06 29.36 23.05 21.92
58.71 68.53 53.98 41.20 47.63
1.65 1.95 1.49 9.02 6.72
607.33 365.96 292.19 701.50 142.98
611.67 372.67 298.00 716.67 143.33
99.29 98.20 98.05 97.88 99.76
a
A: after adding H2O2 solution, the mercury released during the water bath heating process. Sum: F1+F2+F3+F4+F5. Total mercury: the Hg content in gypsum samples was measured using the Lumex RA915+.
pool for release. Both forms of mercury cannot be measured in the BCR extraction solutions, causing a mercury input and output imbalance and lower mercury recovery values. 3.2. Experimental Results for BCR and Improved BCR Methods. Tables 2 and 3 show the experimental data obtained from the BCR and improved BCR methods, respectively. Tables 2 and 3 display the Hg speciation and proportion in the FGD gypsum samples. The results show small differences between the two methods for the F1 (water-soluble mercury) and F2 (acid-extractable mercury) fractions. The difference may be due to the physically absorbed mercury released during the extraction process but not captured in the liquid portion.26 Most of the mercury in the F1 and F2 fractions exists as HgCl2, HgSO4, Hg(NO3)2.27 A small portion of elemental mercury could be escaping during the acid extraction of HgCl2. The mercury measured in the F3 fraction increased from about 0.5% to about 20% using the improved BCR method. Most of the mercury in the F3 fraction exists as Hg2Cl2.27 Hg2Cl2 is an unstable compound that is easily reduced back to elemental mercury. The elemental mercury then escapes during processing to cause the lower concentration in the leachate in step 3. Between 54% and 69% of the mercury present in the FGD gypsum samples exists in the oxidizable mercury form (F4). The F4 fraction contains complex mercury compounds, including Hg0, Hg1+, and Hg−Fe compounds.28−30 The formation of complex mercury compounds is due to the forced oxidation conditions (conditions such as pH of solutions, temperatures, Ca/S ratios, and air ratios) in the FGD system.31 Additional heating during the F4 step causes significant elemental mercury losses; the value is between 25% and 40% (Table 3). Most of the mercury loss in the traditional BCR method is believed to occur during this step. HgS is the major mercury species in gypsum. Previous studies have shown that the oxidative dissolution of HgS can happen and form Hg(OH)2(aq) via eq 8.32,33 Hg(OH)2 could then be reduced to Hg0 via eq 9 (10) which would also increase the Hg0 pool for release.34
HgS(s) + 2O2 (aq) + 2H 2O ↔ Hg(OH)2 (aq) + SO24 − + 2H+
(8)
hv
Hg(OH)2 → [Hg(OH)]* ↔ Hg(OH)aq + OH· Hg(OH)aq + H+ + e− → Hg 0 + H 2O
(9) (10)
The amount of mercury measured in the available fractions (F1, F2, F3, and F4) increased in the improved BCR method. This is due to the inclusion of the elemental mercury that escaped during processing. In the traditional BCR method, the F5 fraction is about 10%. However, the F5 fraction is less than 10% in the improved BCR method. The lower F5 percentage in the improved BCR method is due to the greater accuracy in determining the F1−F4 fractions using that method. In addition, measurements made using the direct combustion method have fewer experimental errors than the corresponding measurements made using the extraction method. Figure 6 shows the Hg distributions in the five extraction forms of the FGD gypsum samples. Most of the mercury in the
Figure 6. Speciation of trace elements in FGD gypsum. 8345
DOI: 10.1021/acs.energyfuels.8b01827 Energy Fuels 2018, 32, 8340−8347
Article
Energy & Fuels
In summary, repeated leaching and heating will change the stability of the FGD gypsum. Knowing the release potential of Hg is important for long-term utilization, storage, and disposal of FGD gypsum.
FGD gypsum is present in the F1, F3, and F4 fractions. Most of the mercury exists in an oxidizable form due to favorable conditions in the FGD system. All the FGD gypsum samples in this study had a leachable mercury fraction (F1+F2) at about 18%. Mercury in this state will be observed in landfills and soil amendments using FGD gypsum. Around 78% of the mercury exists as potentially leachable mercury (F3+F4) and will be reemitted during the heating process while making cement and drywall. This research indicates that the mercury present in FGD gypsum has an enhanced potential migration ability. The most stable phase F5 accounts for about 2%, which is in good agreement with the results from previous studies.35 The difference in coal types, combustion conditions, and desulfurization equipment is the main reason for the large range of mercury in FGD gypsum. The main form of mercury in the residual fraction (F5) is HgS. Recovery ratios from the two extraction methods were given in Tables 2 and 3. The BCR method recovery ratio is comparatively low, varying from 42.38% to 66.79%. However, the recovery ratio of the improved BCR method is about 99%. The corresponding uncertainties of the improved BCR method are given in Table 4. The corresponding uncertainties were less than 2.7. In summary, the release potential of each form of mercury is better obtained using the improved BCR method.
■
Corresponding Author
*E-mail:
[email protected]. ORCID
Yongsheng Zhang: 0000-0002-1104-5605 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (No. 2016YFB0600205), the National Natural Science Foundation of China (51706069), and the Fundamental Research Funds for the Central Universities (2017JQ002).
■
σ(c1)
σ(c2)
σ(c3)
σ(c4)
σ(c5)
σ
1 2 3 4 5
1.25 1.25 0.82 0.47 0.82
1.63 0.94 1.63 1.25 0.47
0.94 0.82 0.47 1.70 1.70
1.25 0.94 1.70 0.47 0.47
0.47 1.25 0.47 1.25 0.47
2.62 2.35 2.58 2.53 2.05
REFERENCES
(1) Zhao, F. Q.; Liu, H. J.; Hao, L. X.; Li, Q. Water resistant block from desulfurization gypsum. Constr. Build. Mater. 2012, 27 (1), 531− 533. (2) Wang, J.; Yang, P. Potential flue gas desulfurization gypsum utilization in agriculture: A comprehensive review. Renewable Sustainable Energy Rev. 2018, 82, 1969−1978. (3) Beatty, W. L.; Schroeder, K.; Beatty, C. L. K. Mineralogical Associations of Mercury in FGD Products. Energy Fuels 2012, 26 (6), 3399−3406. (4) Srivastava, R. K.; Hutson, N.; Martin, B.; Princiotta, F.; Staudt, J. Control of mercury emissions from coal-fired electric utility boilers. Environ. Sci. Technol. 2006, 40 (5), 1385. (5) Su, S.; Liu, L.; Wang, L.; Syedhassan, S. S. A.; Kong, F.; Hu, S.; Wang, Y.; Jiang, L.; Xu, K.; Zhang, A.; Xiang, J. Mass Flow Analysis of Mercury Transformation and Effect of Seawater Flue Gas Desulfurization on Mercury Removal in a Full-Scale Coal-Fired Power Plant. Energy Fuels 2017, 31 (10), 11109−11116. (6) Wang, F.; Shen, B.; Yang, J.; Singh, S. Review of mercury formation and capture from CO2 enriched oxy-fuel combustion flue gas. Energy Fuels 2017, 31 (2), 1053. (7) Wang, S.; Zhang, L.; Zhao, B.; Meng, Y.; Hao, J. Mitigation Potential of Mercury Emissions from Coal-Fired Power Plants in China. Energy Fuels 2012, 26 (8), 4635−4642. (8) Wang, T.; Liu, J.; Zhang, Y.; Zhang, H.; Chen, W. Y.; Norris, P.; Pan, W. P. Use of a non-thermal plasma technique to increase the number of chlorine active sites on biochar for improved mercury removal. Chem. Eng. J. 2018, 331, 536−544. (9) Liu, X. L.; Wang, S. X.; Zhang, L.; Wu, Y.; Duan, L.; Hao, J. M. Speciation of mercury in FGD gypsum and mercury emission during the wallboard production in China. Fuel 2013, 111 (9), 621−627. (10) Delgado, J.; Barba-Brioso, C.; Nieto, J. M.; Boski, T. Speciation and ecological risk of toxic elements in estuarine sediments affected by multiple anthropogenic contributions (Guadiana saltmarshes, SW Iberian Peninsula): I. Surficial sediments. Sci. Total Environ. 2011, 409 (19), 3666−3679. (11) Martínez-Fernandez, M.; et al. Matrix solid phase dispersionassisted BCR sequential extraction method for metal partitioning in surface estuarine sediments. Talanta 2011, 83 (3), 840. (12) Soliman, N. F.; El Zokm, G. Z.; Okbah, M. A. Risk assessment and chemical fractionation of selected elements in surface sediments from Lake Qarun, Egypt using modified BCR technique. Chemosphere 2018, 191, 262−271. (13) Ciceri, E.; Giussani, B.; Pozzi, A.; Dossi, C.; Recchia, S. Problems in the application of the three-step BCR sequential
Table 4. Corresponding Uncertainty of the Improved BCR Methoda sample
AUTHOR INFORMATION
a σ(ci): the respective errors of the residue concentrations resulting from each of the improved BCR method steps. σ: corresponding uncertainty of the improved BCR method.
4. CONCLUSIONS In this paper, the release characteristics of mercury in FGD gypsum were studied using the BCR method and the improved BCR method. (1) Gas phase mercury evolution evaluation experimental results show that Hg0 was released when the gypsum was exposed to air and water. About 25−40% of the mercury is lost after the addition of H2O2 during the water bath heating step. These factors help to explain the low recovery ratio observed with the BCR method. (2) The improved BCR method can get an almost 100% recovery rate using the indirect combustion method. (3) Most of the mercury present in FGD gypsum is in the oxidizable state, followed by the reducible state, the water-soluble state, the weak acid extractible state, and the residual state, in that order. The oxidizable state may be more dependent on the FGD control conditions. (4) The leachable mercury content is around 18%, the potential leachable mercury is around 78%, and the most stable phase is about 2−9%. Mercury in FGD gypsum has a strong migration ability and poor environmental stability. 8346
DOI: 10.1021/acs.energyfuels.8b01827 Energy Fuels 2018, 32, 8340−8347
Article
Energy & Fuels
cinnabar (α-HgS) and metacinnabar (β-HgS). Chem. Geol. 2007, 240 (3), 313−325. (34) Nriagu, J. O. Mechanistic steps in the photoreduction of mercury in natural waters. Sci. Total Environ. 1994, 154 (94), 1−8. (35) Hao, Y.; Wu, S. M.; Pan, Y.; Li, Q.; Zhou, J. Z.; Xu, Y. B.; Qian, G. R. Characterization and leaching toxicities of mercury in flue gas desulfurization gypsum from coal-fired power plants in China. Fuel 2016, 177, 157−163.
extraction to low amounts of sediments: An alternative validated route. Talanta 2008, 76 (3), 621−626. (14) Fernándezmartínez, R.; Rucandio, M. I. Study of extraction conditions for the quantitative determination of Hg bound to sulfide in soils from Almaden (Spain). Anal. Bioanal. Chem. 2003, 375 (8), 1089−96. (15) Heebink, L. V.; Hassett, D. J. Mercury release from FGD. Fuel 2005, 84 (11), 1372−1377. (16) Kerolli-Mustafa, M.; Fajković, H.; Rončević, S.; Ć urković, L. Assessment of metal risks from different depths of jarosite tailing waste of Trepça Zinc Industry, Kosovo based on BCR procedure. J. Geochem. Explor. 2015, 148, 161−168. (17) U.S. EPA. Mercury species fractionation and quantification by microwave assisted extraction, selective solvent extraction and/or solidphase extraction; 2005. (18) Zhang, Y. S.; Shang, P. F.; Wang, J. W.; Norris, P.; Romero, C. E.; Pan, W. P. Trace element (Hg, As, Cr, Cd, Pb) distribution and speciation in coal-fired power plants. Fuel 2017, 208, 647−654. (19) Sun, M. Y.; Hou, J. A.; Cheng, G. H.; Baig, S. A.; Tan, L. S.; Xu, X. H. The relationship between speciation and release ability of mercury in flue gas desulfurization (FGD) gypsum. Fuel 2014, 125 (3), 66−72. (20) Alabed, S. R.; Jegadeesan, G.; Scheckel, K. G.; Tolaymat, T. Speciation, Characterization, and Mobility of As, Se, and Hg in Flue Gas Desulphurization Residues. Environ. Sci. Technol. 2008, 42 (5), 1693. (21) Gustin, M.; Ladwig, K. Laboratory investigation of Hg release from flue gas desulfurization products. Environ. Sci. Technol. 2010, 44 (10), 4012. (22) Gustin, M. S.; Stamenkovic, J. Effect of Watering and Soil Moisture on Mercury Emissions from Soils. Biogeochemistry 2005, 76 (2), 215−232. (23) Sun, M. Y.; Cheng, G. H.; Lu, R. J.; Tang, T. M.; Baig, S. A.; Xu, X. H. Characterization of Hg0 re-emission and Hg2+ leaching potential from flue gas desulfurization (FGD) gypsum. Fuel Process. Technol. 2014, 118 (2), 28−33. (24) Zhu, Z. W.; Zhuo, Y. Q.; Fan, Y. M.; Wang, Z. P. Mechanism of mercury vapor release from flue gas desulfurization gypsum. Fuel Process. Technol. 2017, 156, 485−490. (25) Sui, Z. F.; Zhang, Y. S.; Li, W. H.; Orndorff, W.; Cao, Y.; Pan, W. P. Partitioning effect of mercury content and speciation in gypsum slurry as a function of time. J. Therm. Anal. Calorim. 2015, 119 (3), 1611−1618. (26) Kost, D. A.; Bigham, J. M.; Stehouwer, R. C.; Beeghly, J. H.; Fowler, R.; Traina, S. J.; Wolfe, W. E.; Dick, W. A. Chemical and physical properties of dry flue gas desulfurization products. J. Environ. Qual. 2005, 34 (2), 676−686. (27) Bloom, N. S.; Preus, E.; Katon, J.; Hiltner, M. Selective extractions to assess the biogeochemically relevant fractionation of inorganic mercury in sediments and soils. Anal. Chim. Acta 2003, 479 (2), 233−248. (28) Kairies, C. L.; Schroeder, K. T.; Cardone, C. R. Mercury in gypsum produced from flue gas desulfurization. Fuel 2006, 85 (17− 18), 2530−2536. (29) Kim, C. S.; Rytuba, J. J.; Brown, G. E., Jr. EXAFS study of mercury(II) sorption to Fe- and Al-(hydr)oxides II: I. Effects of pH. J. Colloid Interface Sci. 2004, 270 (1), 9−20. (30) Serrano, S.; Vlassopoulos, D.; Bessinger, B.; O’Day, P. A. Immobilization of Hg(II) by coprecipitation in sulfate-cement systems. Environ. Sci. Technol. 2012, 46 (12), 6767−6775. (31) Rallo, M.; Heidel, B.; Brechtel, K.; Maroto-Valer, M. M. Effect of SCR operation variables on mercury speciation. Chem. Eng. J. 2012, 198−199, 87−94. (32) Barnett, M. O.; Turner, R. R.; Singer, P. C. Oxidative dissolution of metacinnabar (β-HgS) by dissolved oxygen. Appl. Geochem. 2001, 16 (13), 1499−1512. (33) Holley, E. A.; James McQuillan, A.; Craw, D.; Kim, J. P.; Sander, S. G. Mercury mobilization by oxidative dissolution of 8347
DOI: 10.1021/acs.energyfuels.8b01827 Energy Fuels 2018, 32, 8340−8347
