1. Department Civil and Environmental Engineering
Mercury Interactions: The effect of redox conditions on mercury partitioning in subsurface systems
Stefan J. Grimberg
Department Civil and Environmental Engineering
Clarkson University
Potsdam, NY

2. Acknowledgments
People who actually did the work: Chase Gerbig (Honors Student)
Mat Fox (M.S. student)
Marilyn Mayer (technician)
Jon Schwenk (REU Student)

3. Mercury Background Mercury’s various forms Mercury Binding
Organic molecules → Complexation
Soil → Sorption
Effects of Mercury
Bioaccumulation
Define Complexation, Bioaccumulation analogy with DDT

4. Mercury Speciation

5. Mercury Competition in Soil
Hg-DOC
DOC + Hg+2
Hg+2
DOC

6. Controlling Factors of Bioaccumulation
Availability of Free Mercury (Hg0, Hg2+)
Complexed Mercury
DOC and Inorganics
Hg+2 + DOC Hg-DOC
Sorption of Mercury
What Controls availability?
Water Chemistry
pH, Redox potential
DOC Heterogeneity K1
Define pe here

7. Dialysis Method (Glaus, Hummel, Van Loon. Analytica Chimica Acta
Initial
Final
[Hg-DOC]
Hg Hg Hg
DOM
K1=
Hg Hg
HgDOM
DOM Hg
[Free DOC][Free Hg]
Binding Coefficient isn’t the same as equilibrium coefficient, although they are related

8. Soil Horizons & Redox Potential
Aerobic Zone
Eh ≈ 250 mV
Produced by bubbling DOC w/ Air
Nitrate Reducing Zone
Eh ≈ 150 mV
Produced with NH4+/NO3- Couple
Sulfate Reducing Zone
Eh ≈ -400mV
Produced with Titanium (III)

9. Sulfate Reducing Conditions – with Ti(III) ORP = -480 mV
Denitrifying Conditions with Ti(IV) ORP = 131 mV

10. Cinnabar Dissolution and Competitive Ligand Exchange - Sunday Lake DOM
Log KDOM = 38.2±0.2 (95% C.I.)

11. Complexation Summary
Complexation constants at low redox are significantly higher than at aerobic conditions.
Extremely high ‘apparent’ stability constant may be due to:
Mixed HS--Hg-DOM complexes
Nano-colloid sized HgS(s) particles
DOM diffusion through dialysis bag and subsequent surface reactions on cinnabar

12. Log Hg-DOC Complexation Constants, all aerobic conditions
Lamborg et al., 2003
21 – 23.9; pH = 7.5,
Benoit et al., 2001
10.6 – 11.8 for DOC, and
22.4 – 23.8 for fully ionized thiols in DOC,
Drexel, 2002
25.8 – 27.2 (strong binding sites) and
7.3 – 8.7 (weak binding sites),

13. Net significance of Hg-Complexation
Use watershed model developed for Sunday Lake
Effect of redox on Hg desorption
Assess transport of Hg as a function of varying complexation equilibrium

15. Sulfate media

16. Sulfate Column

17. Sulfate Column Effluent

19. High Redox Complexation Sorption log KDOC = 20 log KHg = 30 Hg-DOC
DOC + Hg+2
Hg+2
DOC
Hg remains sorbed
No Hg correlation with DOC

20. Low Redox Complexation Sorption log KDOC = 40 log KHg = 30 Hg-DOC
DOC + Hg+2
Hg+2
DOC
Hg Correlation with DOC

21. MeHg vs DOC

22. Sulfate reducing column

25. Conclusions
Hg complexation increases significantly at low redox potentials (up to 6 order of magnitude)
Small changes in Hg complexation equilibria result in significant changes in Hg mobility
MeHg concentration highest under sulfate reducing conditions and similar in aerobic and denitrifying columns.

26. Questions?
Thank you for listening!