Printable PDF Version

July 6, 2005

Methylmercury Binding Sites in Humic Substances: X-ray Absorption Study

S. Yoon¹, L.M. Diener¹, P.R. Bloom², E.A. Nater², and W.F. Bleam^1
1Department of Soil Science, University of Wisconsin, Madison, WI; ²Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN

Methylmercury, a highly toxic and bioaccumulative form of mercury, is known to have a strong affinity for binding to organic matter in soil, sedimentary, and aquatic environments. The objective of our study was to determine the dominant ligands that bind to the methylmercury cation (CH₃Hg⁺) in humic acids (complex organic compound mixtures) by evaluating several CH₃Hg⁺-ligand complexation models. Mercury L_III-edge extended x-ray absorption fine structure (EXAFS) results show that CH₃Hg⁺ preferentially binds to thiol ligands (-SH), also known as sulfhydryl. After saturating reactive thiol ligands, the remaining CH₃Hg⁺ binds to carboxyl ligands rather than to amine or reduced-sulfur ligands other than thiol.

Soh-joung Yoon

Paul Bloom

William Bleam

In organisms, such as fish, the dominant form of accumulated mercury is the organometallic methylmercury cation CH₃Hg⁺, rather than the inorganic cation Hg²⁺. Scientists have reported that the biotic and abiotic methylation of inorganic mercury is affected by natural organic matter. Humic substances, for example, either stimulate mercury methylation, acting as methyl donors for Hg²⁺, or suppress it by forming complexes with Hg²⁺. Examining the nature of organic and inorganic mercury complexes with natural organic matter is important for understanding the biogeochemical cycle of mercury as well as the fate of mercury in the environment.

Both Hg²⁺ and CH₃Hg⁺ have strong affinities for organic matter in terrestrial and aquatic environments, with CH₃Hg⁺ having a lower affinity than Hg²⁺. The principle of hard and soft acids and bases (the HSAB principle) predicts the strong affinity of reduced-sulfur ligands for Hg²⁺ and CH₃Hg⁺. Previous mercury LIII-edge extended x-ray absorption fine structure (EXAFS) studies show that humic sulfur ligands bind both Hg²⁺ and CH₃Hg⁺. Scientists, however, have reported that only a small fraction of reduced sulfur in humic substances binds to CH₃Hg⁺, with oxygen or nitrogen binding to the remaining CH₃Hg⁺. They speculated that CH₃Hg⁺-binding sulfur ligands were, most likely, thiol and possibly sulfide, disulfide, and hydrogen disulfide groups. In this study, we determined the major CH₃Hg⁺-binding humic ligands using mercury L_III-edge EXAFS. Our spectra were obtained at NSLS beamlines X11A and X10C.

Figure 1. Experimental (solid lines) and least-squares fitted (dotted lines) Hg LIII-edge EXAFS of CH3Hg+-humic thiol complex (aquatic humic acid; CH3Hg+ to reduced sulfur ratio, 0.3, pH 5): (a) radial structure function and (b) EXAFS scattering curves (full scattering curve and Fourier-filtered scattering curves of peak A1 and peak B1).

We examined several CH₃Hg⁺-ligand models as potential CH₃Hg⁺-binding structures in humic acids: thiol (-SH), sulfide (-S-), disulfide (-SS-), hydrogen polysulfide (-SSH or -SSSH), carboxyl (-COOH), and amine (-NH2). We examined carboxyl and amine ligands, although they are hard Lewis ligands, because CH₃Hg⁺-amine complexes exhibit relatively high complexation constants, and carboxyl is the most abundant ligand in humic acids.

Figure 2. Experimental (solid lines) and least-squares fitted (dotted lines) Hg LIII-edge EXAFS of CH3Hg+-humic carboxyl complex (soil humic acid; CH3Hg+ to reduced sulfur ratio, 1.3, pH 4): (a) radial structure function and (b) EXAFS scattering curves (full scattering curve and Fourier-filtered scattering curves of peak A2 and scattering region B2).

We equilibrated two different humic-acid solutions (soil and aquatic) at CH₃Hg⁺-to-reduced-sulfur ratios ranging from 0.3 to 1.5, quantifying the reduced-sulfur (e.g., thiol, sulfide, and hydrogen polysulfide) content using sulfur K-edge x-ray absorption near edge structure (XANES) spectroscopy at beamline X19A.

Our results show that thiol is the dominant CH₃Hg⁺ complexing ligand among the reduced sulfur ligands (Figure 1). We did not observe EXAFS evidence of CH₃Hg⁺ complexation to sulfide, disulfide, or hydrogen disulfide ligands in any of our samples. CH₃Hg⁺ complexation by carboxyl ligands (Figure 2) becomes significant after CH₃Hg⁺ saturates the available thiol ligands. Carboxyl ligands, rather than amine, eclipse thiol ligands as the CH₃Hg⁺-to-reduced-sulfur ratio approaches and then exceeds 1. We also found evidence for proximately coordinated heavy atoms in a sample where the CH₃Hg⁺-to-reduced-sulfur ratio slightly exceeded 1. The heavy-atom backscattering behavior agrees the best with that of mercury or, possibly, other atoms with similar atomic numbers.

BEAMLINE
X10C, X11A, X19A

FUNDING
U.S. Department of Agriculture
Hatch Program
U.S. Department of Energy

PUBLICATION
Yoon, S., Diener, L.M., Bloom, P.R., Nater, E.D., and Bleam, W.F. X-ray Absorption Studies of CH₃Hg⁺-Binding Sites in Humic Substances, Geochimica et Cosmochimica Acta, 69, 1111-1121 (2005).

FOR MORE INFORMATION
Soh-joung Yoon
Department of Crop and Soil Sciences
Pennsylvania State University
University Park, PA
Email: syoon@psu.edu