Speciation analysis of arsenic in coal and its combustion by-products in coal-fired power plants
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摘要: 燃煤电厂煤中砷（As）的形态在燃烧过程中不可避免地会发生转化。煤及其副产物中砷的形态与人体健康和环境安全密切相关，亟待鉴别。然而目前针对煤燃烧相关产物中砷形态的前处理手段和分析方法尚缺乏。本研究采用高效液相色谱-氢化物发生-原子荧光光谱法（HPLC-HG-AFS）成功测定了电厂煤、粉煤灰和石膏中砷的形态，优化了仪器参数、提取试剂和前处理方法（超声和微波辅助）。优化后，无机砷的分离时间缩短至7 min，As（Ⅲ）和As（Ⅴ）的检出限分别为1.8 ng/g和4.6 ng/g。砷形态的高效提取剂为1.0 mol/L磷酸和0.1 mol/L抗坏血酸的混合溶液。微波辅助（2000 W、80 ℃、40 min）和超声辅助（40 kHz、20 ℃、40 min）分别是煤/粉煤灰和石膏样品中砷形态的最佳提取方法。在微波和超声波辅助提取条件下，As（Ⅲ）/As（Ⅴ）的回收率分别为95.8%/104.5%和90.6%/89.7%。样品分析结果表明，煤中砷主要以As（Ⅴ）形式存在，As（Ⅲ）所占比例很小，而在粉煤灰和石膏中只观察到As（Ⅴ）。该研究揭示了As（Ⅲ）向As（Ⅴ）的转化是气态砷捕获的关键，可以为控制电厂砷排放提供科学依据。
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关键词:
- 砷 /
- 形态 /
- 煤 /
- 燃烧副产物 /
- HPLC-HG-AFS
Abstract: The speciations of Arsenic (As) in coal will inevitably convert during the combustion process. The As speciations in coal and its by-products are closely related to human health and environmental safety which is urgent to be identified. However, there is a lack of pretreatment procedure and analysis method on the As species in coal-related products in power plants. In this study, the As species in coal, fly ash (FA), and gypsum were successfully determined by high performance liquid chromatography coupled with hydride generation atomic fluorescence spectrometry (HPLC-HG-AFS). The instrument parameters, extract reagents, and pretreatment methods (i.e. ultrasound and microwave-assisted) were optimized. The whole separation time of inorganic As was shorten to 7 min after optimization, with the detection limit of 1.8 and 4.6 ng/g for As(Ⅲ) and As(Ⅴ), respectively. The efficient As extract reagent was the mixture of 1.0 mol/L H3PO4 and 0.1 mol/L ascorbic acid solution. Microwave-assisted (2000 W, 80 ℃, 40 min) and ultrasound-assisted (40 kHz, 20 ℃, 40 min) schemes were the optimal extraction methods for coal/FA and gypsum samples, respectively. Under the proposed microwave and ultrasound extraction procedure, the recovery of As(Ⅲ) and As(Ⅴ) could reach to 95.8%/104.5% and 90.6%/89.7%, respectively. The dominant occurrence of As species in coal was As(Ⅴ) with a small percentage of As(Ⅲ), while As(Ⅴ) was the only occurrence form observed in FA and gypsum. It is indicated that revealing the transformation of As(Ⅲ) to As(Ⅴ) is the key for gaseous As capture. The As species distribution investigation provides a scientific insight to the controlling of As emission from power plant.-
Key words:
- arsenic /
- speciation /
- coal /
- combustion by-products /
- HPLC-HG-AFS
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Figure 3 (a) As concentrations extracted by different reagents and (b) the effect of H3PO4 concentrations on the As extracting
Figure 6 Extraction efficiencies and species distributions of As in six gypsum samples
Table 1 HPLC-HG-AFS instrument conditions for As species determination
Instrument Parameter HPLC analytical column Hamilton PRP-X100 (250 mm×4.1 mm, 10 μm) guard column Hamilton PRP-X100 (25 mm×2.3 mm, 12-20 μm) mobile phase 30 mmol/L (NH4)2HPO4 (pH value 6.0) mobile phase flow rate 1.3 mL/min AFS carrier liquid 5% HCl carrier liquid flow rate 65 r/min reducing agent 0.5% KOH+ 2% KBH4 carrier gas flow rate 400 mL/min shielding gas flow 500 mL/min lamp current main cathode 60 mA auxiliary cathode 30 mA lamp voltage 270 mV 
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Table 2 As extraction efficiency of different methods (n=5)
Method Extraction efficiency coal fly ash gypsum Microwavea (74.8±9.1)% (92.3±7.5)% (36.2±5.9)% Ultrasoundb (63.7±8.6)% (53.1±7.4)% (57.6±6.0)% Shakingc (72.7±8.2)% (40.3±6.1)% (42.3±7.2)% a: 80 ℃, 2000 W for 40 min; b: 20 ℃, 40 kHz for 40 min; c: shaking at 20 ℃ for 18 h
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Table 3 Recovery of As speciation after the microwave and ultrasound extraction (n=3)
Microwave Ultrasound As(Ⅲ) As(Ⅴ) As(Ⅲ) As(Ⅴ) Average recovery 95.8% 104.5% 90.6% 89.7%
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Table 4 Linearities, LOD and LOQ of the proposed method
Speciation Linear ranges /(μg·L-1) Regressive equations Linearities LOD /(ng·g-1) LOQ /(ng·g-1) As(Ⅲ) 10-200 y=954.21x-432.69 0.9995 1.8 6.3 As(Ⅴ) 10-200 y=434.55x-2683.74 0.9979 4.6 14.7
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Table 5 The total As concentrations in coal, FA, and gypsum samples
Sample Concentration w/(μg·g-1) coal FA gypsum 1 9.17 22.47 3.92 2 5.23 26.77 4.57 3 9.02 41.35 4.28 4 6.27 16.21 4.15 5 7.46 14.51 4.39 6 8.75 42.18 3.92
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