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Photolytic degradation of methylmercury enhanced by binding to natural organic ligands.

Zhang T, Hsu-Kim H - Nat Geosci (2010)

Bottom Line: In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive.Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition.Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

View Article: PubMed Central - PubMed

Affiliation: Duke University, Department of Civil & Environmental Engineering, 121 Hudson Hall, Durham, NC 27708 USA.

ABSTRACT
Monomethylmercury is a neurotoxin that poses significant risks to human health1 due to its bioaccumulation in food webs. Sunlight degradation to inorganic mercury is an important component of the mercury cycle that maintains methylmercury at low concentrations in natural waters. Rates of photodecomposition, however, can vary drastically between surface waters2-5 for reasons that are largely unknown. Here, we show that photodegradation occurs through singlet oxygen, a highly reactive form of dissolved oxygen generated by sunlight irradiation of dissolved natural organic matter. The kinetics of degradation, however, depended on water constituents that bind methylmercury cations. Relatively fast degradation rates (similar to observations in freshwater lakes) applied only to methylmercury species bound to organic sulfur-containing thiol ligands such as glutathione, mercaptoacetate, and humics. In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive. Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition. Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

No MeSH data available.


Related in: MedlinePlus

Concentration dependence of MeHg photodegradation ratesa) MeHg degradation by sunlight in water containing Suwannee River humic acid (SRHA) and phosphate (1 mM, pH = 7). The high MeHg treatment (1500 nM) was exposed in October 2008 (15.2°C average noon temperature). The low MeHg treatment (15 nM) was exposed in December 2008 (8.9°C). Error bars represent ±1 s.d. for replicate measurements (n=2–3) of the same sample. b) MeHg degradation by artificial UV-A (λ=365 nm) in water containing SRHA (2.8 mg-C/L) and phosphate (10 mM, pH 7.4). Data points represent the average MeHg concentration (±1 s.d.) for duplicate samples.
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Figure 1: Concentration dependence of MeHg photodegradation ratesa) MeHg degradation by sunlight in water containing Suwannee River humic acid (SRHA) and phosphate (1 mM, pH = 7). The high MeHg treatment (1500 nM) was exposed in October 2008 (15.2°C average noon temperature). The low MeHg treatment (15 nM) was exposed in December 2008 (8.9°C). Error bars represent ±1 s.d. for replicate measurements (n=2–3) of the same sample. b) MeHg degradation by artificial UV-A (λ=365 nm) in water containing SRHA (2.8 mg-C/L) and phosphate (10 mM, pH 7.4). Data points represent the average MeHg concentration (±1 s.d.) for duplicate samples.

Mentions: We performed photodegradation experiments in a simulated freshwater containing MeHg, Suwannee River humic acid, and a phosphate buffer (pH 7 to 7.4). In two separate experiments, sample containers (Teflon FEP) were exposed to natural sunlight over 4 days in October and December 2008 on a building roof top in Durham, North Carolina. During the exposure period, MeHg did not degrade in appreciable amounts in samples that contained only the phosphate buffer (Figure 1a). MeHg degradation was observed only when humic acid was present and when the MeHg concentration was low (15 nM) relative to humic acid. The differences observed between the 15 nM and 1500 nM MeHg treatments could be caused by complexation between MeHg and thiols associated with the humic. In the sample with 2 mg-C/L humic, the reduced-sulfur concentration from the humic acid was estimated at 150 nM (assuming 7.3×10−5 moles reduced-S per g C20) and greater than total MeHg (15 nM). In contrast, the sample with 1.1 mg-C/L humic and 1500 nM MeHg contained 80 nM reduced-S, less than MeHg concentration.


Photolytic degradation of methylmercury enhanced by binding to natural organic ligands.

Zhang T, Hsu-Kim H - Nat Geosci (2010)

Concentration dependence of MeHg photodegradation ratesa) MeHg degradation by sunlight in water containing Suwannee River humic acid (SRHA) and phosphate (1 mM, pH = 7). The high MeHg treatment (1500 nM) was exposed in October 2008 (15.2°C average noon temperature). The low MeHg treatment (15 nM) was exposed in December 2008 (8.9°C). Error bars represent ±1 s.d. for replicate measurements (n=2–3) of the same sample. b) MeHg degradation by artificial UV-A (λ=365 nm) in water containing SRHA (2.8 mg-C/L) and phosphate (10 mM, pH 7.4). Data points represent the average MeHg concentration (±1 s.d.) for duplicate samples.
© Copyright Policy
Related In: Results  -  Collection

License
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getmorefigures.php?uid=PMC2902198&req=5

Figure 1: Concentration dependence of MeHg photodegradation ratesa) MeHg degradation by sunlight in water containing Suwannee River humic acid (SRHA) and phosphate (1 mM, pH = 7). The high MeHg treatment (1500 nM) was exposed in October 2008 (15.2°C average noon temperature). The low MeHg treatment (15 nM) was exposed in December 2008 (8.9°C). Error bars represent ±1 s.d. for replicate measurements (n=2–3) of the same sample. b) MeHg degradation by artificial UV-A (λ=365 nm) in water containing SRHA (2.8 mg-C/L) and phosphate (10 mM, pH 7.4). Data points represent the average MeHg concentration (±1 s.d.) for duplicate samples.
Mentions: We performed photodegradation experiments in a simulated freshwater containing MeHg, Suwannee River humic acid, and a phosphate buffer (pH 7 to 7.4). In two separate experiments, sample containers (Teflon FEP) were exposed to natural sunlight over 4 days in October and December 2008 on a building roof top in Durham, North Carolina. During the exposure period, MeHg did not degrade in appreciable amounts in samples that contained only the phosphate buffer (Figure 1a). MeHg degradation was observed only when humic acid was present and when the MeHg concentration was low (15 nM) relative to humic acid. The differences observed between the 15 nM and 1500 nM MeHg treatments could be caused by complexation between MeHg and thiols associated with the humic. In the sample with 2 mg-C/L humic, the reduced-sulfur concentration from the humic acid was estimated at 150 nM (assuming 7.3×10−5 moles reduced-S per g C20) and greater than total MeHg (15 nM). In contrast, the sample with 1.1 mg-C/L humic and 1500 nM MeHg contained 80 nM reduced-S, less than MeHg concentration.

Bottom Line: In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive.Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition.Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

View Article: PubMed Central - PubMed

Affiliation: Duke University, Department of Civil & Environmental Engineering, 121 Hudson Hall, Durham, NC 27708 USA.

ABSTRACT
Monomethylmercury is a neurotoxin that poses significant risks to human health1 due to its bioaccumulation in food webs. Sunlight degradation to inorganic mercury is an important component of the mercury cycle that maintains methylmercury at low concentrations in natural waters. Rates of photodecomposition, however, can vary drastically between surface waters2-5 for reasons that are largely unknown. Here, we show that photodegradation occurs through singlet oxygen, a highly reactive form of dissolved oxygen generated by sunlight irradiation of dissolved natural organic matter. The kinetics of degradation, however, depended on water constituents that bind methylmercury cations. Relatively fast degradation rates (similar to observations in freshwater lakes) applied only to methylmercury species bound to organic sulfur-containing thiol ligands such as glutathione, mercaptoacetate, and humics. In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive. Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition. Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

No MeSH data available.


Related in: MedlinePlus