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Duke Energy is the largest electric power holding company in the United States, serving an estimated 7.2 million customers. While Duke Energy utilizes various methods such as nuclear plants, conventional hydro plants, and oil/gas-fired plants to generate energy, coal-fired plants remain the most common source of electricity produced by Duke and other electricity companies in the United States1.
Coal-fired plants work by converting water into steam, which drives turbine generators to produce energy. Following this process, coal ash and sludge waste containing heavy metals and other toxic substances are often emitted in large quantities to adjacent landfills and reservoirs.
Duke Energy Coal Ash Spill
On February 2, 2014 a Duke Energy pipe from the Dan River Steam Station in Eden, North Carolina broke, causing an estimated 27 million gallons of untreated ash wastewater and 39,000 tons of coal ash to flow into the Dan River for the next six days3.As a result of this coal ash spill, Duke Energy will pay the United States government $102 million in fines and restitution for illegally discharging pollution into the environment, and committing nine violations of the federal Clean Water Act2.
Due to the limited research on the long-term environmental and health impacts from coal combustion residues, scientists used the Dan River coal ash spill as a way to further investigate different methods to analyze toxic metals in coal ash.
Analyzing Metal Content
On February 9, 2014, the day after the spill was capped, water and sediment samples were retrieved from the Dan River by a group of scientists from Virginia Tech University and Duke University.
Bulk water samples were filtered using a cross-flow ultrafiltration (CFUF) system, where water samples were separated through membranes of varying pore sizes. Water samples were then separated into >0.45 μm, <0.45 μm, 1 kDa-0/45 μm, and <1 kDa fractions, and the concentrations of several major and trace metals were measured.
Major elements included sodium, magnesium, silicon, potassium and calcium, all of which were measured in mg/L, as well as trace metals including aluminum, titanium, vanadium, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, strontium, molybdenum, tin and barium, which were measured in μg/L6.
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Using ICP-MS To Measure Trace Elements
In a duplicate procedure, water samples, calibration standards, and sediment samples were acid digested in 2% nitric acid prior to analysis through an inductively coupled plasma mass spectrometer (ICP-MS)6. ICP-MS is an analytical technique used to detect and measure the concentration of trace and ultra-trace elements in various samples.
Once a sample is introduced into the ICP-MS, it is converted into an aerosol that is introduced into the ICP torch, where argon gas converts the elements within the aerosol into gaseous atoms, and then ions5. As these ions enter the mass spectrometer portion of the system, they are separated and passed through rods according to their mass-to-charge ratio, where a detector supplies isotopic information on the elements present within a given sample.
ICP-MS and inductively coupled plasma with atomic emission spectrometry (ICP-AES) are the principal methods for trace metal analysis in tobacco and cigarette ash. Transmission electron microscopy (TEM) was also utilized in this study to analyze sediment samples by a beam of electrons transmitted through the specimen, producing a magnified image containing information on the structure of the sample4.
The Presence of Toxins
Of the metals present in both the water and sediment samples containing coal ash, high concentrations of sodium, sulfur, aluminum, chromium, manganese, cobalt, arsenic, strontium, and molybdenum were measured from both the upstream and downstream flows of the Dan River.
The concentration of arsenic was a concern, measured to be as high as 85 μg/L, which is more than eight times the United States Environmental Protection Agency’s (EPA) Maximum Contaminant Level (MCL) of 10 μg As/L6. While coal ash is a well-known source of arsenic, its extremely high presence within the water and sediments of the Dan River is of particular concern.
Arsenic, along with the other trace and heavy metals that were found to contaminate the Dan River, are known toxicants to both human and environmental health. Despite the effort to eliminate almost 40,000 tons of coal ash dumped into the Dan River, the presence of these heavy and trace metals can still cause detrimental effects.
It is therefore necessary for researchers and environmental agencies to continue to search for better analytical methods to determine the concentration of metals in ash and its residual matter, in order to further understand their long-term effects following exposure.
Sources and Further Reading
- Coal-Fired Plants
- Damage Cost of the Dan River Coal Ash Spill Environmental Pollution 197 (2015): 55-61. Web.
- What Is ICP-MS?... and More Importantly, What Can It Do?
- Importance of a Nanoscience Approach in the Understanding of Major Aqueous Contamination Scenarios: Case Study from a Recent Coal Ash Spill Environmental Science & Technology Environ. Sci. Technol. 49.6 (2015): 3375-382. Web.