Mercury Speciation in FGD: Assessing Transport and

Bioavailability Risk

Kirk Scheckel1, Souhail Al-Abed1, Thabet Tolaymat1, Gautham Jegadeesan2, Aaron Williams1 & Bruce Ravel3

1 US EPA2 Pegasus Technical Services

3 MR CAT

Atomic MolecularMicroscopicMacroscopic Field

• Field Plots• Equilibrium Studies• Kinetic Studies• Extractions

• Enhanced Visual Analysis: 1. SEM 2. TEM 3. AFM

• Visual/ Intuitive Insight• Field Plots

• XRD• TGA• FTIR• DRS

• XRF• XPS• XAS Requires synchrotron radiation.

Adaptation of Bertsch and Hunter, 1996.

The Research Continuum

Samples

• FGD samples were provided from locations with historically high levels of Hg (up to ~ 2 ppm)

• Simple density separation method to concentrate the Hg

• Employed XAS and Mössbauer spectroscopies

Location Source Average CompositionHeavy Fraction µg/kg %

1 A 1922.97 0.92

1 B 1058.56 0.91

1 C 619.06 0.44

Location Source Average CompositionLight Fraction µg/kg %

1 A 207937.65 99.08

1 B 115357.02 99.09

1 C 140716.49 99.56

Advanced Photon Source(Argonne National Laboratory, Argonne, IL)

Principal Synchrotron Techniques Used in Environmental Science

• X-ray Fluorescence (XRF): chemical composition (quantification, mapping)

• X-ray Absorption Fine Structure (XAFS) Spectroscopy: chemical speciation (oxidation state, coordination, nearest neighbors)

• Surface Scattering and Diffraction: surface structure, sorption processes

• Microtomography: 3D imaging of internal microstructure (porosity, fluid flow, composition)

Surface Reactions

Arsenic on Bangladesh

Biotite

AsAs3+3+ Arsenic in Cattail Root Plaque

495oCCl

2.09Å

Cu1+

Copper Speciation in

Fluid Inclusions

X-ray Absorption Spectroscopy: Measure energy-dependence of the x-ray absorption coefficient(E) [either log(I0 /I) or (If / I0 )] of a core-level of a selected element

Element Specific: Elements with Z>20 can be examined.

EXAFS = Extended X-ray Absorption Fine-Structure

XANES = X-ray Absorption Near-Edge Spectroscopy

Valence Probe: XANES gives chemical state and formal valence of selected element.

Natural Samples: samples can be in solution, liquids, amorphous solids, soils, aggregates, plant roots, surfaces, etc.

Low Concentration: concentrations down to 10 ppm for XANES, 100 ppm for EXAFS.

Small Spot Size: XANES and EXAFS measurements can be made on samples down to ~5 microns in size.

Local Structure Probe: EXAFS gives atomic species, distance, and number of near-neighbor atoms around a selected element..

X-ray Absorption Spectroscopy

X-ray Absorption Near Edge Spectroscopy

• Chemical state is critical in determining toxicity and mobility

Cr(VI) is highly carcinogenic and highly mobile in ground water.

Cr(III) is not carcinogenic or very toxic, and is not mobile in ground water.

Hg X-ray Absorption SpectroscopyD

eri

vati

ve

Inflection point difference (IPD)

Referencecompound

First derivative XANESpeak separation (eV)

HgO (red) 13.2

HgSO4 11.4

Hg(NO3)2 (aq) 10.7

Hg2SO4 9.6

Hg2Br2 8.8

Hg2Cl2 8.1

HgCl2 8.0

HgBr2 7.7

HgS (red) 7.4

HgI2 6.5

Hg X-ray Absorption Spectroscopy

IPD = 6.3 - 6.5 eVSpeciation: Hg(I)

SampleDetector Transducer 57 Fe*

sample mount

Mössbauer Spectroscopy

What can we learn?

Mössbauer Spectra

Isomer Shift

Quadrupole Splitting

Magnetic Splitting

Oxidation State Coordination #

Oxidation State Site Symmetry

UnperturbedMagnetic

Properties Particles size

298 K

4 K

Nuclear Transitio

ns

C. L. Kairies, K. T. Schroeder, C. R. Cardone. Mercury in gypsum produced from flue gas desulfurization. Fuel 85 (2006) 2530–2536

Fe Influence in FGD

0.6

0.4

0.2

0.0

ab

sorp

tion

(%

)

100-10velocity (mm s

-1)

data fit Fe(III) silicate/clay Ferrihydrite

Fe Chemistry in FGD

Top Layer85% Ferrihydrite15% Fe(III)-Clay

The 503 Rule

The 503 Rule

Hazard Identification: Can this pollutant harm human health and/or the environment?

Exposure Assessment: Who is exposed, how do they become exposed, and how much exposure occurs?

Dose-Response Evaluation: If a person, animal or plant are exposed to this pollutant, what happens?

Risk Characterization: What is the likelihood of an adverse affect in the population exposed to a pollutant under the conditions studied?

The 503 Rule

Consider the Amount of Hg

4’ X 8’ piece of drywall weighs 54 lbs (24.55 kg)Estimate 330 sheets of drywall for walls & ceilingsTotal drywall weight = 8106 kgIf FGD contains 2 ppm Hgmax = 0.016 kg or 0.036 lbs of HgThe “what-if”Katrina Effect: 100,000 homes

640 Ac/mile2

40 Ac

Land Application:40 Ac X 2 T/Ac = 80 tons of FGDCould have as much as 0.15 kg or 0.32 lbs of Hg

Consider the Amount of Hg

Land Application of FGD with 2 ppm Hg:Application rate @ 2 tons/Ac yields 0.0036 kg Hg/Ac

One acre furrow slice (20 cm) weighs 1,052,183 kg

One application results in 0.00035 mg Hg/kg soil

347 applications would approach the non-residential clean-up standard of 0.12 mg Hg/kg soil. This does not account for Hg loss.

Hourly Hgo emissions from a sludge amended soil plot over    24-hrs.  Hgo emissions were more strongly correlated with 

  solar radiation than soil temperature.  Peak background soil    Hgo emissions at the same site were < 25 ng m-2 hr-1.

Carpi, A., Lindberg, S.E. (1997) "Sunlight-Mediated Emission of Elemental Mercury from Soil Amended with Municipal Sewage Sludge," Environmental Science & Technology 31(7):2085-2091.

Hg Loss from Land Application

Conclusions

• Hg speciation can be characterized as Hg(I) in a high Fe matrix; perhaps a direct association with Fe oxides or a Hg-C-Fe oxide ligand bridge

• Fe chemistry in FGD consists of ferrihydrite and clay-based Fe likely from the CaCO3 source

• Can the addition of Fe enhance the FGD process?

• Hg can be easily concentrated via water separation – Erosion?

• The objectives of Rule 503 are not geared towards land application of FGD material

• Loss from microbial, solar radiation, and dust must be understood

Discussion/QuestionsDiscussion/Questions