Article pubs.acs.org/IECR
Development of a Sustainable Enrichment Strategy for Quantification of Mercury Ions in Complex Samples at the Sub-Parts per Billion Level Matthew P. Tracey and Kazunori Koide* Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States S Supporting Information *
ABSTRACT: To limit environmental exposure of mercury species, government bodies restrict emissions of various environmental mercury sources to sub-parts per billion (ppb) levels. Current methods for detection of mercury are timeconsuming and expensive and suffer from many drawbacks. Optical methods are in principle less intensive but have not yet been implemented for real-world applications because of a lack of sufficient sensitivity and robustness. We previously reported a fluorometric method for quantifying mercury ions based on the oxymercuration of a vinyl ether with a detection limit of 1 ppb, not meeting the requirement by government bodies. To fill the gap between our previous method and the governments’ restrictions, we have developed a method to enrich complex samples with mercury ions through the use of a recyclable thiolbased resin and the novel chemistry of mercury release. The combination of our previous fluorometric method and the new enrichment chemistry allowed the detection of 0.1 ppb mercury in a complex synthetic sample.
■
INTRODUCTION Mercury species cause several health problems, such as liver and kidney failure.1 It is also potentially linked to neurodiseases2−6 and diabetes.7 Mercury was marked as the cause of the infamous Minamata disease in 1958, in which more than 2000 people were affected by various disorders, ranging from developmental disabilities to organ failure.8 Environmental mercury is a notable problem because marine life can convert inorganic mercury salts to more toxic methylmercury. 9,10 Sources of inorganic environmental mercury(II) include natural events, such as volcanic eruptions,11 and human activities, such as dumping of wastewater from coal-fired power plants12,13 and dental offices.14,15 The wastewater from coal-fired power plants alone accounts for up to 50% of environmental mercury globally.16 As such, the U.S. Environmental Protection Agency (EPA) has placed several restrictions on mercury emissions, an example being a 0.1 ppb (microgram per kilogram) mercury limit17 on the discharge of wastewater from coal-fired power plants. The Minamata Convention on Mercury, a United Nations resolution implemented in October 2013, expanded this limit to 147 countries.18 Analysis of environmental samples to quantify heavy metals is performed off-site, using inductively coupled plasma mass spectrometry (ICP-MS)19 and cold vapor absorption spectroscopy (CVAS).20 Samples, such as those from power plants, are often “scrubbed” using metal-scavenging treatments or diluted to appropriate levels of heavy metals. Dental offices are able to use an amalgam separator to remove mercury-rich amalgam from general wastewater discharge. The analysis of these samples, to ensure appropriate levels of heavy metals, is not routine; instrumentation requires trained individuals to operate, and repair of broken instruments may take days, and sometimes up to several months, severely slowing the analysis of samples. The cost of an inductively coupled spectrometer is more than © 2014 American Chemical Society
$180000, with annual maintenance estimated at $20000. Supplies required for operation, namely liquid argon, require replenishing every 2−3 weeks, and the cost has been estimated at $250−300 per refill. A more cost-effective, on-site method would alleviate many of these problems and expedite the detection of mercury from various sources such as power plant wastewater. The list of Hach methods has proven that optical methods allow rapid and cost-effective on-site analysis of complex samples. More than 90 colorimetric or fluorometric methods have been reported for mercury detection;21,22 however, none of these have been used on site. This is not surprising because in real-world samples, the concentrations of other metal ions and anions far exceed those of mercury species. For example, in a wastewater sample from a coal-fired plant, concentrations of magnesium, calcium, and chloride ions can be as high as 20, 1.1, and 1.6 g/L, respectively, while mercury concentrations can be as low as 0.1 μg/L. Even the most selective chemosensor would not detect such a low concentration of mercury ions in the presence of other components. We hypothesized that a more realistic approach might be to enrich samples with mercury ions before applying a fluorometric method. Here, we report our efforts toward implementing such a technique in the field.
■
RESULTS AND DISCUSSION We previously reported the use of chemodosimeter 1 for detecting mercury ions in water with a sensitivity of 1 ppb.23 In this method, the nonfluorescent chemodosimeter was converted to the fluorescent phenoxide 2 as depicted in Scheme 1. Received: Revised: Accepted: Published: 14565
May 16, 2014 August 23, 2014 August 29, 2014 September 9, 2014 dx.doi.org/10.1021/ie502003f | Ind. Eng. Chem. Res. 2014, 53, 14565−14570
Industrial & Engineering Chemistry Research
Article
Scheme 1. Previously Published Fluorometric Method for Mercury Detection
can be efficiently captured by DeloxanMP at any of the pH values mentioned above. Next, we monitored the mercury concentrations of resintreated solutions at pH 2, 4, and 9 at times indicated in Figure 1 at 25 °C to determine the kinetics of capturing. Resin-untreated 19 ppb mercury(II) solutions showed an apparent gradual decrease as measured by the conversion of 1 to 2 as the incubation time increased, likely because of the conversion of soluble mercury(II) species to insoluble mercury(II) species during storage, such as mercury(II) oxide or binding of the mercury to the walls of the flask. In both pH 2 and 4 buffers, ∼70% of the mercury was bound to the resin after 2 h. It took an additional 14 h to fully capture the metal ions. Capturing in a pH 4 buffer showed a similar trend. In a pH 9 buffer, however, the adsorption was notably slower. Because of the decrease in mercury concentrations at all pHs when samples were treated with resin, we chose pH 4 buffer for future capturing experiments because wastewater samples are generally slightly acidic. To accelerate the capturing of metal, we heated the samples; at 70 °C in pH 4 buffer, most of the mercury(II) ions appeared to bind to the resin after only 2 h (Figure 2). We sought to further accelerate capturing and tested microwaving the solution. This required only 90 s for complete capturing in solutions with various mercury(II) concentrations, with heating for 1 min and cooling for 30 s at 0 °C (Figure 3). We concluded that the fastest capturing protocol was to treat a mercury-containing pH 4 solution with DeloxanMP resin and microwave the solution for 1 min, followed by cooling on ice for 30 s. When a sample is too large to be microwaved, capturing could be accomplished through incubation at 70 °C for 2 h. Thus, we have established a rapid capturing procedure for mercury(II) on DeloxanMP resin. To confirm that the lower-magnitude fluorescence signals from resin-treated samples were due to the reduced level of mercury ions and not other changes or interference, we spiked the filtrates from capturing experiments to 10 and 30 ppb mercury(II) and analyzed the filtrates with vinyl ether 1. Unexpectedly, the filtrates from both displayed a fluorescence similar to that of a mercury-free sample, indicating unknown substances leaching off the resin might be interfering with the oxymercuration of 1 (Figure 4). After this experiment, we revisited our previous results that indicated apparent capturing, considering them as potentially false positives. To alleviate this problem, we turned to treatment of the filtrates to mitigate the interference. We hypothesized that the interfering substance may be a thiol or a derivative, because the resin being used is thiol-based. To test this hypothesis, we treated the filtrates with oxidants to oxidize the unknown sulfur-based interfering substance and dissociate mercury ions from the sulfur atom. Employing the capturing conditions at 40 °C for 1 and 4 h, the filtrates were treated with either NCS or hydrogen peroxide, followed by spiking with known concen-
Although the accuracy was moderate, we were able to detect any type of mercury species after oxidative digestion of samples with N-chlorosuccinimide (NCS) to form inorganic mercury ions in situ. NCS proved to be easier to handle than and as effective as Br-Cl that the U.S. EPA uses. To enrich complex mixtures with mercury species, we chose to use a thiol-based resin, as thiols are routinely used in heavy metal removal.24,25 This strategy would allow for the increase in the mercury concentration, capturing mercury ions from large, complex samples and eluting them into a smaller volume while simultaneously removing interfering species, such as chloride ions,23 from the sample (Scheme 2). Scheme 2. Enriching with Mercury Ions Using Thiol Resin
The first milestone was to develop a rapid capturing methodology for mercury onto DeloxanMP thio-functionalized resin. DeloxanMP thio-functionalized resin consists of abrasionproof spheres, manufactured by Evonik, with a reported density of 0.55−0.65 kg/L (wet form), a pore volume of 2.5−3.5 mL/g, and a surface area of 300−450 m3/g.26 Initially, we investigated the potential pH dependence of mercury capture. Mercury(II) solutions (30 ppb) were treated with DeloxanMP resin at pH 9, 6, and 4 and 2% HNO3 at 25 °C. After 16 h, the solutions were filtered, and the filtrate was analyzed for mercury ions by using vinyl ether 1 (Table 1). The mercury concentrations of the filtrates of the mercury-containing solutions were near zero in all cases; in other words, the mercury ions were quantitatively captured on the resin. This result indicates that mercury ions Table 1. Capturing of Mercury(II) with DeloxanMP Resin in Various pH Solutions pH of the capturing solution
estimated [Hg] (ppb) after resin treatmenta
2 4 6 9
