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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

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

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

Mercury Speciation

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

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

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

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)

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

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

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

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),

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

Sulfate media

Sulfate Column

Sulfate Column Effluent

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

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

MeHg vs DOC

Sulfate reducing column

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.

Questions? Thank you for listening!