Assessing the Effects of Thermodynamic Parameter Uncertainty in Mercury Aqueous Speciation Modeling
Abstract
Mercury (Hg) aqueous speciation is important for determining how Hg enters the biosphere, the spatial and temporal distribution of Hg species, Hg fate and transport, bioavailability, and environmental toxicity. Predictions of Hg aqueous speciation are based on equilibrium speciation models that rely on experimentally determined equilibrium constants (log K values) associated with individual chemical reactions. However, the effects of the uncertainty inherent in experimentally determined equilibrium constants are rarely taken into account in Hg speciation modeling. Due to the critical role speciation modeling plays in Hg biogeochemical cycling models, a systematic investigation of the propagation of speciation uncertainty needs to be conducted. In this study, Monte Carlo simulations of aqueous and mineral equilibrium speciation have been carried out for a Hg(II)-S(-II)-Cl system using PHREEQC, integrated with Latin hypercube sampling of Gaussian distributions of the logarithm of equilibrium stability constants. The output concentration distributions of major Hg-containing species generally span multiple orders of magnitude in concentration and vary significantly depending on the environmental conditions modeled (e.g., pH and total sulfide concentration). The nonlinear behavior of the equilibrium system can lead to asymmetric or bimodal distributions. Strong positive correlations are found between the (absolute) skewness and uncertainty when only aqueous phase equilibrium is considered. When equilibrium with solid phase β-HgS (metacinnabar) is introduced, overall uncertainty increases while the output distributions are constrained to be close to symmetric in the model system. Analytical derivations of probability distributions aid in understanding the computational results. Under different modeling scenarios, we have identified areas of significant uncertainty in major output species concentrations for a range of environmental conditions. The systematic uncertainty analysis presented in this study assists and should be integrated into mercury biogeochemical cycling analyses.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B51K2100G
- Keywords:
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- 0409 Bioavailability: chemical speciation and complexation;
- BIOGEOSCIENCESDE: 0432 Contaminant and organic biogeochemistry;
- BIOGEOSCIENCESDE: 0454 Isotopic composition and chemistry;
- BIOGEOSCIENCESDE: 0491 Food webs and trophodynamics;
- BIOGEOSCIENCES