Development of a biogeochemical reaction model to simulate net methylmercury production in sediments

Stefanie HelmrichUniversity of California, Merced

Mercury Cycle in Reservoirs

Alpers C. N., Hunerlach M. P., May J. T. and Hothem R. L. (2005) Mercury Contamination from Historical Gold Mining in California. Publ. US Geol. Surv. 61, 7.

Conceptual Model for Hg Reaction Model

Model implemented in PHREEQC(Bessinger B. a., Vlassopoulos D., Serrano S. and O’Day P. a. (2012) Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury. Aquat. Geochemistry 18, 297–326.)

Methylation rate

Methylation rate formulation in experiments

𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 =𝑑𝑑 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+

𝑑𝑑𝑑𝑑= 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � 𝐶𝐶𝐻𝐻2+

𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = Methylation rate constant𝐶𝐶𝐻𝐻2+ = Concentration inorganic Hg𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+ = Concentration methylmercury

Methylation rate

Methylation rate formulation in experiments

𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 =𝑑𝑑 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+

𝑑𝑑𝑑𝑑= 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � 𝐶𝐶𝐻𝐻2+

Methylation rate formulation in model Coupled withIron reduction 𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑓𝑓𝐹𝐹𝑚𝑚𝐹𝐹𝐹𝐹𝐻𝐻 � 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � 𝐶𝐶𝐻𝐻2+

Sulfate reduction 𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑓𝑓𝑆𝑆𝐹𝐹𝑆 � 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � [𝐶𝐶𝐻𝐻2+]Methanogenesis 𝑅𝑅𝐻𝐻𝐻𝐻𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑓𝑓𝐶𝐶𝐻𝐻𝑆 � 𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 � [𝐶𝐶𝐻𝐻2+]

𝑓𝑓𝐹𝐹𝑚𝑚𝐹𝐹𝐹𝐹𝐻𝐻 ,𝑓𝑓𝑆𝑆𝐹𝐹𝑆, 𝑓𝑓𝐶𝐶𝐻𝐻𝑆 = Fraction of electrons consumed by reduction half-reaction𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚𝑚 = Methylation rate constant𝐶𝐶𝐻𝐻2+ = Concentration inorganic Hg𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+ = Concentration methylmercury

Thank you.

Use of isotope tracers to calulcate methylation rate constants

Methylmercury (MeHg) concentration over time

Methylation Rate Pseudo first-order reaction

𝑑𝑑 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+

𝑑𝑑𝑑𝑑= 𝑘𝑘𝑚𝑚 𝐶𝐶𝐻𝐻2+

Assumptions• No demethylation• No change in 𝐶𝐶𝐻𝐻2+

concentration

Hintelmann H., Keppel-Jones K. and Evans D. (2000) Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability. Environ. Toxicol. Chem. 19, 2204–2211.

Spike of isotopocially labeled 𝐶𝐶𝐻𝐻2+ and 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+

Comparison measured and simulated MeHg concentration

• Good fit during 24 hours• Assumptions not correct for longer incubationtime

Use of isotope tracers to calulcate methylation rate constants

Biogeochemical reaction model• PHREEQC (Version 3), U.S. Geolological Survey• Model capabilities:

• Heterogenneous and homogeneous equilibrium speciation• Kinetic-based mineral dissolution and precipitation• Kinetic-based biodegradation organic matter via redox reactions• Methylation and Demethylation of mercury

Evaluating Nutrient Fate and Redox Controls in Groundwater in Riparian Areas, Michelle M. Lorah (USGS MD-DE-DC Water Science Center, Baltimore, Maryland), Isabelle M. Cozzarelli and J. K. Böhlke (USGS National Research Program, Virginia, 03/30/2017

Kinetic rates for redox reactions according to: Canavan R. W. et. al. (2006) Organic matter mineralization in sediment of a coastal freshwater lake and response to salinization. Geochim. Cosmochim. Acta 70, 2836–2855.Van Cappellen P. and Wang Y. (1996) Cycling of iron and manganese in surface and sediments: a general theory for the coupled transport and reaction of carbon, oxygen, nitrogen, sulfur, iron, and manganese. Am. J. Sci. 296, 197 – 243.

Redox reactions

Evaluating Nutrient Fate and Redox Controls in Groundwater in Riparian Areas, Michelle M. Lorah (USGS MD-DE-DC Water Science Center, Baltimore, Maryland), Isabelle M. Cozzarelli and J. K. Böhlke (USGS National Research Program, Virginia, 03/30/2017

Kinetic rates for redox reactions according to: Canavan R. W. et. al. (2006) Organic matter mineralization in sediment of a coastal freshwater lake and response to salinization. Geochim. Cosmochim. Acta 70, 2836–2855.Van Cappellen P. and Wang Y. (1996) Cycling of iron and manganese in surface and sediments: a general theory for the coupled transport and reaction of carbon, oxygen, nitrogen, sulfur, iron, and manganese. Am. J. Sci. 296, 197 – 243.

Use of isotope tracers to calulcate methylation rate constants

Methylmercury (MeHg) concentration over time

Hintelmann H., Keppel-Jones K. and Evans D. (2000) Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability. Environ. Toxicol. Chem. 19, 2204–2211.

Spike of isotopocially labeled 𝐶𝐶𝐻𝐻2+ and 𝐶𝐶𝐶𝐶3𝐶𝐶𝐻𝐻+

Use of isotope tracers to calulcate methylation rate constants

Number of studies using isotope tracers to calculate methylation rate constants

36

Calculated methylation rate constant

(𝑑𝑑−1)

0.0006 – 1.5

Use of isotope tracers to calulcate methylation rate constants

Number of studies using isotope tracers to calculate methylation rate constants

36

Calculated methylation rate constant

(𝑑𝑑−1)

0.0006 – 1.5

Why such a variation in magnitude?Which order of magnitude is plausible?

Use of isotope tracers to calulcate methylation rate constants

Fraction of MeHg produced from isotopically labeled HgJonsson S., Skyllberg U., Nilsson M. B., WestlundP., Shchukarev A., Lundberg E. and Bjo E. (2012) Mercury Methylation Rates for Geochemically relevant Hg(II) species in Sediments. Environ. Sci. Technol. 46, 11653–11659.

0

0.05

0.1

0.15

0.2

0 5 10 15 20 25

MeH

g/TH

g fr

actio

n

Time in days

MeHg/THg fraction, first batch MeHg/THg fraction, second batch

0

0.005

0.01

0.015

0.02

0.025

0.03

0 2 4 6 8

MM

Hg (n

mol

/g)

Time in days

MeHg produced from isotopically labeled HgMartin-Doimeadios R., Tessier E., Amouroux D., Guyoneaud R., Duran R., Caumette P. and Donard O. F. X. (2004) Mercury methylation / demethylation and volatilization pathways in estuarine sediment slurries using species-specific enriched. Mar. Chem. 90, 107–123.

Influence incubation time used to caluclate rate constant

Start: Methylation >> Demethylation

After approximately 24 to 48 hours:Methylation = Demethylation

Use of isotope tracers to calulcate methylation rate constants

Influence incubation time used to caluclate rate constant

2 - 5 18 - 24 48 960.0

2.0

4.0

6.0

8.0

10.0

12.0

Harm

onic

mea

n fo

r max

km

, 1*

10^-

2 (d

-1)

Incubationtime in hours

Hintelmann H., Keppel-Jones K. and Evans D. (2000) Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability. Environ. Toxicol. Chem. 19, 2204–2211.

Reasonable order of magnitude:1 � 10−2𝑑𝑑−1

Preleminary Results

Influence of organic matter on methylation rate

Influence of organic matter on methylation rateSensitivity analysisHigh ROM Reactive Organic matter = 500 𝑚𝑚𝑚𝑚Low ROM Reactive Organic matter = 5 𝑚𝑚𝑚𝑚