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The Importance of a Nanoscience Approach in the Understanding of Major Aqueous Contamination Scenarios: A Case Study from a Recent Coal Ash Spill Yi Yang, Benjamin P. Colman, Emily S Bernhardt, and Michael F. Hochella Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505662q • Publication Date (Web): 17 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Environmental Science & Technology
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The Importance of a Nanoscience Approach in the Understanding of Major
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Aqueous Contamination Scenarios: A Case Study from a Recent Coal Ash
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Spill
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Yi Yang a,c,d*, Benjamin P. Colman b,d, Emily S. Bernhardt b,d, Michael F. Hochella, Jr.a*
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a
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24061, USA
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b
Biology Department, Duke University, Durham, NC, 27708, USA
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c
Department of Geosciences, East China Normal University, 3663 North Zhongshan Road,
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Shanghai, 200062, China
The Center for NanoBioEarth, Department of Geosciences, Virginia Tech, Blacksburg, VA
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d
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Durham, NC, 27517, USA
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Corresponding authors:
[email protected] (Y. Yang);
[email protected] (M. F. Hochella,
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Jr.)
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TOC
Center for the Environmental Implications of NanoTechnology (CEINT), Duke University,
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Abstract
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A coal ash spill that occurred from an ash impoundment pond into the Dan River, North Carolina,
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provided a unique opportunity to study the significance and role of naturally-occurring and
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incidental nanomaterials associated with contaminant distribution from a large-scale, acute
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aquatic contamination event. Besides traditional measurements of bulk watercolumn and
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sediment metal concentrations, the nanoparticle (NP) analyses are based on cross-flow
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ultrafiltration (CFUF) and advanced transmission electron microscopy (TEM) techniques. A
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drain pipe fed by coal ash impoundment seepage showed a high level of arsenic, with
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concentrations many times over the EPA limit. The majority of the arsenic was found sorbed to
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large aggregates dominated by incidental iron oxyhydroxide (ferrihydrite) NPs, while the
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remainder of the arsenic was truly dissolved. These ferrihydrites were probably formed in situ
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where Fe(II) was leached through subsurface flowpaths into an aerobic environment, and further
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act as a significant contributor to the elevated As concentrations in downstream sediments after
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the spill. In addition, we discovered and describe a photocatalytic nano-TiO2 phase (anatase)
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present in the coal ash impacted river water that was also carrying/transporting transition metals
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(Cu, Fe), which may also have environmental consequences.
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Introduction
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It is becoming increasing apparent that naturally occurring and anthropogenic incidental
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nanomaterials play an important and often completely overlooked role in regulating the behavior
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of contaminants in complex aquatic systems. This statement is applicable to both manmade
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aquatic systems, such as nanomaterials in water treatment plants and distribution systems,1-3 and 2 ACS Paragon Plus Environment
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natural aquatic systems, such as rivers and ground water.4-7 What has been generally missing is
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an example of a major, acute aqueous contaminant spill where the central role of naturally
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occurring and incidental nanomaterials has been clearly delineated. Our opportunity to study this
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underrepresented type of impact in a major aqueous contamination scenario came in February of
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2014 during a coal ash spill into the Dan River near the town of Eden, North Carolina. Although
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this spill was considerably smaller than previously studied coal ash spills in the North
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Carolina/Tennessee areas, 8, 9 it was still a major event. The resulting flow of ash and water was
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estimated to contain 27 million gallons of water and 30,000 to 39,000 tons of coal ash flowing
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into the Dan River from February 2nd to the 8th, 2014, when the leak was finally and successfully
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capped.10
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Coal combustion residues (CCRs) can cause elevated concentrations of toxic metals in surface
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water (streams, rivers, and lakes), and potentially in ground water.11,12 Previous studies of coal
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ash spills have documented long-term chemical impacts that may result in long-term
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environmental and health impacts on these aquatic systems.9,12 There has been limited research
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on these long-term impacts, however a recent study concluded that the amount of toxic metal
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released from coal ash and its further mobility in an aquatic system are considered the key
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factors that control the environmental risk.8
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While regulations in the United States for many contaminants commonly found in ash spills
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are based on total concentrations, studies on water quality and pollutants following coal ash
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spills often differentiate between solid particulate matter (SPM) and dissolved materials by
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separating them using membrane filters (e.g. having a pore size of 0.45 µm). In fact, the
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“dissolved” phase present in the filtrate of natural waters is a complex mixture including a range
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of sizes of colloids, nanoparticles (NPs), and the truly dissolved phase.13,14 In the last several
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years, growing attention has been given to NPs due to their inherent reactivity with contaminants
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and their implications for contaminant mobility.15,16 These NPs can be transported for long
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distances and serve as carriers for contaminants that may significantly alter their fate and
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transport relative to standard geochemical assumptions, calculations, and models.
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In order to systematically assess the role of NPs associated with toxic metals in a major
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aqueous contamination scenario, water samples from the Dan River and two inflows to the Dan
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River from the coal ash impoundment areas were sequentially separated using cross-flow
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ultrafiltration (CFUF) into > 0.45 µm, 0.45 um fractions were acid digested
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according to Standard Method 3030F.18 This method is satisfactory for most metals but is
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inefficient for titanium oxides, which are insoluble even under strong acidic condition. Sediment
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samples were acid digested in duplicates following EPA Method 3050B. Major and trace
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element concentrations were analyzed using a Thermo Electron X-Series inductively coupled
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plasma mass spectrometer (ICP-MS) per Standard Method 3125-B.18
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Electron Microscopy Analysis. Sediment and the retentate of the > 0.45 um fraction of
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inflow samples were investigated using an Environmental Scanning Electron Microscope (ESEM,
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FEI Quanta 600 FEG) equipped with an Energy-Dispersive X-ray Spectrometer (EDS,
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QUANTAX 400, Bruker). Selected colloidal solution samples were placed onto a 300 mesh
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copper TEM grad with lacey carbon support film (Electron Microscopy Sciences, PA).
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Morphology, aggregation, chemistry and crystal structure of the NPs in the colloidal solution
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(1kD-0.45µm fraction) were investigated using a JEM 2100 TEM/STEM (JEOL Corporation)
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operating at 200 kV and equipped with an energy dispersive X-ray (EDX) spectrometer. Electron
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diffraction patterns of the crystalline and semi-crystalline phases were recorded in selected area
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electron diffraction (SAED) mode.
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XRD. The SPM suspension from Inflow A was pipetted onto a zero background sample
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holder and analyzed by a Rigaku MiniFlex x-ray diffractometer (Ni-filtered Cu Kα radiation,
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scanned between 10° and 80° 2θ at a scan rate of 0.02° 2θ s-1). All raw data files were converted
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to an Excel file from which the relative intensity was plotted versus 2θ.
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Results and Discussion
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The highest concentrations of contaminants in water samples associated with CCR were
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measured not at the site of the spill, but from the two inflows which enter the Dan River both
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upstream (Inflow A) and downstream (Inflow B) of the ash containment basins. Concentrations
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in < 0.45 µm water of Na, S, Al, Cr, Mn, Co, As, Sr, and Mo were all significantly higher in both
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inflows than in any of the sampling locations within the Dan River (Table S2, t-test, p < 0.05).
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For both inflows we found that the majority of the trace metals were in the
