Letter Cite This: Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/estlcu
Comparison of Reactive Gaseous Mercury Collection by Different Sampling Methods in a Laboratory Test and Field Monitoring Xiaoge Bu,† Hefeng Zhang,† Guangkuo Lv,† Huiming Lin,‡ Long Chen,§ Xiufeng Yin,∥ Guofeng Shen,‡ Wen Yuan,‡ Wei Zhang,⊥ Xuejun Wang,‡ and Yindong Tong*,† †
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China College of Urban and Environmental Sciences, Peking University, Beijing 100871, China § School of Geographic Sciences, East China Normal University, Shanghai 200241, China ∥ State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China ⊥ School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China
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‡
S Supporting Information *
ABSTRACT: Accurately measuring reactive gaseous mercury (RGM) concentrations in the atmosphere is important to improve our understanding of the global mercury (Hg) cycle. In this study, we compared the RGM collection efficiencies of four sampling methods, including a 14 cm long KCl-coated denuder, a KCl-coated glass fiber filter, a KCl-coated quartz sand tube, and a cation exchange membrane. Both laboratory studies and field RGM monitoring were performed in environments with low humidity [relative humidity (RH) of ∼20%], medium humidity (RH of 50−70%), and high humidity (RH of ∼100%). Laboratory results showed that in environments with 0.05; n = 60). Large variations (≤20 during field monitoring) in RGM concentrations measured by different sampling methods indicate the need for a unified and standardized method for future RGM monitoring.
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INTRODUCTION Mercury (Hg) is regarded as a priority-control pollutant by the United Nations Environment Programme because of its neurotoxicity to human beings and negative impacts on the ecosystem.1−3 Hg in the atmosphere is generally released by anthropogenic sources (e.g., coal combustion, waste incineration, and metal smelting) and natural sources (e.g., volcano eruption, earthquake, and forest fire).4−7 A majority of humaninduced Hg emissions exist in the gaseous form,8,9 and different Hg species in the atmosphere [e.g., gaseous elemental Hg (GEM), reactive gaseous Hg (RGM), and particle-bound Hg (PBM)] have different chemical properties and long-range transport behaviors.10−12 GEM accounts for >90% of all atmospheric Hg species13−15 and is transported long distances through atmospheric circulation, affecting regions far from the emission sources.16 RGM and PBM in the atmosphere have a much shorter half-life, ranging from a few hours to several days.14,17 RGM and PBM have been extensively studied because of their rapid deposition in the local environment.18,19 © XXXX American Chemical Society
Under anaerobic conditions, deposited Hg is transformed into methylmercury,20 which further bioaccumulates in the aquatic food chain.21,22 To understand the global Hg cycle, the accurate measurement of different atmospheric Hg species is vital.8 In recent years, concerns have been raised about the accuracy of RGM measurements23−26 and some studies have pointed out that existing RGM sampling methods have seriously underestimated ambient RGM concentrations.24,27−29 Collection of RGM from the atmosphere is performed using mist chambers,30,31 KCl-coated glass fiber filters,32−34 KCl-coated annular denuders,35,36 and cation exchange membranes.24,25,27 Among these methods, the KCl-coated annular denuder is the most widely applied because it is a key component used in the Received: Revised: Accepted: Published: A
August 24, 2018 September 13, 2018 September 14, 2018 September 14, 2018 DOI: 10.1021/acs.estlett.8b00439 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters
Figure 1. Diagram of laboratory studies and different RGM sampling methods. Legend: KCl-S, KCl-coated quartz sand tube; KCl-D, KCl-coated denuder; KCl-F, KCl-coated glass fiber filter; CEM, cation exchange membrane.
efficiencies in the atmosphere at different RH levels (∼20, 50− 70, and ∼100%) and their performance in field monitoring were investigated. These results help to understand the gap in RGM monitoring data by different sampling methods and contribute to the development of a unified and high-efficiency sampling method for future monitoring.
Tekran monitoring system. This system has been widely applied in the existing global Hg monitoring network, such as the Atmospheric Mercury Network (AMNet) and the Global Mercury Observation System (GMOS).18,37,38 However, there have been debates about the RGM collection efficiency using the KCl-coated denuder method,24,25,27 which have raised the need for a unified and standardized RGM sampling method. Both laboratory studies and field monitoring results have indicated that annular denuders are not as effective or reliable as previously reported.24,25,28,29,39 Laboratory studies have suggested that KCl-coated denuders are efficient at capturing HgBr2 in a dry atmosphere29 but become unstable in field applications.29 RGM concentrations measured by KCl-coated denuders (20− 70 pg/m3) are lower than those measured by mist chambers (20−700 pg/m3).30 It is also suggested that RGM values with a cation exchange membrane were 2−12.6 times greater than the values measured by the Tekran system.24−26,40 The humidity and O3 concentration in the atmosphere are believed to be the main reasons for low RGM concentrations measured by annular denuders, although the chemical properties of different RGM species could also affect their capture by the denuder.25,26,39,41 The recovery of permeated HgBr2 by KClcoated denuders declined by 4−60% when the atmosphere had a relative humidity (RH) between 25 and 75%.25 The authors further demonstrated that KCl-coated denuders could become gradually passivated with increased sampling time and RH.25,27 In contrast to that of annular denuders, the RGM collection efficiency of a cation exchange membrane remains stable regardless of the environmental conditions.25,29 In field applications, the humidity in the atmosphere varies significantly with changes in meteorological conditions.37,42,43 To address inconsistencies of RGM monitoring data measured by different sampling methods, a systematic study that compares their performances in environments with different levels of relative humidity is necessary. In this study, we compared the RGM collection efficiencies of four sampling methods, including a 14 cm long KCl-coated denuder (KClD), a KCl-coated glass fiber filter (KCl-F), a KCl-coated quartz sand tube (KCl-S), and a cation exchange membrane (CEM), in both laboratory study and field monitoring. RGM collection
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MATERIALS AND METHODS RGM Sampling Methods. After reviewing RGM sampling methods that have been applied previously,24,26,35,40 we selected the following sampling methods to compare their RGM collection performances. KCl-Coated Annular Denuder. The KCl-D was a modified annular denuder that was applied in the Tekran 1130 RGM monitoring systems35 but was smaller for the sake of convenience. The manual denuder was composed of two separate quartz tubes, including an outside tube with a length of 140 mm and an inside tube with a length of 70 mm. The outer surface of the inside tube and the inner surface of the outside tube were etched to be coated with KCl solutions (Figure 1). Before RGM sampling, the quartz denuder was soaked in a 10% HNO3 solution for 48 h and then rinsed with deionized water three times. Then, the denuder was coated with a 2.4 M KCl solution and dried at 500 °C in the Hg-free atmosphere. KCl-Coated Quartz Sand Tube. KCl-S’s design was configured similar to the gold trap used to collect GEM.44 The whole tube (with a length of 120 mm) was made of quartz to reduce Hg residues. Approximately one-third of the sampling tube was filled with etched quartz sands (Figure 1), which were sieved by 10 and 18 mesh sieves in advance. A prebaked glass frit impactor plate and quartz wool were used to hold sands in the tube. The etched quartz sands were soaked in 2.4 M KCl solutions for 30 min and then dried at 500 °C in the Hg-free atmosphere. KCl-Coated Glass Fiber Filter. The KCl-F has been used to collect RGM species in the atmosphere in laboratory studies and field monitoring.33,34,45 The KCl-coated fiber filter was prepared using 47 mm glass fiber filters (with a pore size of 0.45 μm, produced by Jiangsu Lvmeng Science Instrument Co., Ltd.) and a 0.1 M KCl solution. The glass fiber filter was B
DOI: 10.1021/acs.estlett.8b00439 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters
Figure 2. Performance of different RGM sampling methods in different humidity environments. (A) Low- and medium-humidity environments, with RHs of 20 and 50−70%, respectively. (B) High-humidity environment, with an RH of 100%. The RGM collection efficiency is normalized by RGM amounts collected by the CEM. Group 1−3 represent 500, 1000, and 1500 ng/L aerosolized Hg(NO3)2 solutions, respectively.
of aqueous Hg(NO3)2 concentrations were applied, 500, 1000, and 1500 ng/L. In the low- and medium-humidity tests, the Hg(NO3)2 concentration for RGM generation was 2000 ng/L. The RGM generation rate was maintained at ∼0.8 L/min, and the argon flow rate, which was used to adjust the RHs in the sampling flows, was maintained at ∼4 L/min. The sampling rate was maintained at ∼1.5 L/min for all the sampling, and the sampling time of each group was 30 min. RGM Field Monitoring. The sampling site was on the Tianjin University campus (longitude 117.32°, latitude 39.00°), located in a suburban area of Tianjin (Figure S1). RGM sampling was conducted from October 6, 2017, to December 26, 2017. The atmosphere was collected in parallel with the KCl-S, KCl-D, KCl-F, and CEM at ∼1 L/min for 3 h. Before the atmosphere was passed through the RGM sampling tube, prebaked glass fiber filters (with a diameter of 47 mm and a pore size of 0.45 μm, held by a Teflon holder and sieve disk) were used to remove ambient particles. To avoid water condensation, the KCl-D and KCl-S were heated using the coils and remained at ∼50 °C during the entire sampling. Information about the meteorological parameters (e.g., temperature and RH) and air quality (e.g., O3, CO, NO2, SO2, PM2.5, and PM10) during the sampling was collected from a nearby national air quality monitoring station (Figure S1, http://www.cnemc.cn/). All atmospheric pollutant monitoring was performed by Environmental Monitoring of China according to China’s national standard method. Detailed information about the measurement method, the detection limits of pollutants, and the monitored values during the sampling is provided in the Supporting Information and Table S1.
spiked with 1 mL of a 0.1 M KCl solution and baked in the oven at 500 °C for 3 h to remove the Hg residues before sampling. Cation Exchange Membrane. Mustang S membranes (MSTG-25S6, produced by Pall Corp., with a pore size of 0.8 μm and a diameter of 25 mm) were used to collect RGM. The Mustang S membrane is a strong cation exchanger that effectively binds positively charged proteins and viral particles, and it also exhibited a good performance in RGM collection.24,25,29 For instance, CEM had a high and stable RGM collection efficiency regardless of humidity and O3 concentrations in the atmosphere.24,25,29 In this study, CEM was used as a standard method to be compared with other sampling methods. Laboratory Study and Field Monitoring. Laboratory Study. As in our previous studies,34,44 air flows containing the RGM compounds were generated using a collision style nebulizer (TSI 9302). We used Hg(NO 3 ) 2 solutions (analytical reagent, Guizhou Tongren Chemical Reagent Factory) to generate RGM compounds because Hg(NO3)2 is a RGM species that is poorly collected by existing methods.26 To avoid potential chemical reactions between RGM and other major components, the nebulizer was operated using compressed ultra-high-purity argon (>99.99%, Tianjin Huanyu Gas Co., Ltd.). To generate flows with different RHs, an additional dry argon flow was used to mix with flows out of the nebulizer (Figure 1). We generated three sampling flows with RHs of ∼20% (low humidity), 50−70% (medium humidity), and ∼100% (high humidity). The sampling flow rate was regulated by a combination of the argon supply, mass flow meter, and air pump (Figure 1). RH was measured by a hygronom, produced by Arco Electronics Ltd. In the high-humidity test, three levels C
DOI: 10.1021/acs.estlett.8b00439 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters
Figure 3. RGM concentrations measured by different methods and the corresponding major air pollutants during field monitoring.
Hg Measurement and Quality Control and Quality Assurance. RGM Measurement. RGM amounts collected by the KCl-S, KCl-D, and KCl-F were determined by cold vapor atomic fluorescence spectrometry (Takran 2500 CVAFS) based on our previous studies.34,44 In general, the KCl-F, KCl-D, and KCl-S containing the collected RGM were first heated at ∼500 °C for 5 min, and then, an argon flow was passed through the pyrolyzer to convert all the Hg species into Hg(0).35 The reduced Hg(0) was then captured by a goldcoated bead trap (Brooks Rand Laboratories). Then, the goldcoated bead trap containing Hg(0) was heated to 500 °C to desorb the amalgamated Hg(0) and introduced into the CVAFS instrument. The simplified procedure for Hg analysis is described in Figure S2. The RGM collected by the CEM was measured according to EPA method 1631E.46 The cation exchange membranes were first digested in the 50 mL digestion solution (a mixture of 48 mL of Milli-Q water, 1 mL of Optima HCl, and 1 mL of a BrCl solution) for 12 h.24,25 After digestion, 0.1 mL of NH2OH·HCl was added to destroy the free halogens and 0.5 mL of a SnCl2 solution was added to convert all the Hg species into Hg(0). The converted Hg(0) in the solution was purged with argon, captured by a gold-coated bead trap, and then analyzed with the Tekran 2500 instrument. Quality Control and Quality Assurance. The method detection limit (MDL) of Hg measurement with the Tekran 2500 instrument was calculated as 3 times the standard deviation of the measured Hg in the prebaked gold-coated bead trap, which was 3.0 pg in this study (n = 11). The Hg blank value of the gold-coated bead trap was 5.9 ± 1.0 pg (n = 11). Three types of standard reference materials, including TORT-2, SRM-2976, and a certain amount of Hg(0) vapor, were measured to verify the accuracy and stability of Hg measurements taken with the Tekran 2500 instrument. The
average recovery of the three standard reference materials was 97 ± 6% (Table S2). To check if there was any leak from system, a certain amount of Hg(0) vapor (∼100 pg) was directly injected into the chamber through the inlet, and the recovery was 95 ± 9% in this test (n = 11). Before and after each group of tests, the ultra-high-purity argon flow, previously filtered by the gold-coated bead trap, was passed through the entire system for 30 min to remove potential Hg residue in the tubing. During the field monitoring, we randomly replicated the sampling four to six times to validate the stability of the sampling methods (Figure S3). Generally, the RGM collection efficiency of the same sampling method was stable during field monitoring. The standard deviation of measured RGM by the parallel samples was usually
