The Kinetics of Aqueous Mercury(II) Reduction by Sulfite Over an Array of Environmental Conditions
2015
Feinberg, Aryeh I. | Kurien, Uday | Ariya, Parisa A.
The reaction of Hg²⁺with sulfite is a major identified reduction pathway in the atmosphere. UV absorption spectroscopy was used to study the kinetics of Hg²⁺reduction by sulfite (Na₂SO₃) in the presence of fly ash. Upon the addition of Cumberland and Shawnee fly ash samples, the reduction rates were 0.0071 ± 0.0008 and 0.0009 ± 0.0006 s⁻¹, respectively. This represents c.a. 40 and 90 % decreases in the homogeneous rate, 0.013 ± 0.007 s⁻¹. The reduction reaction was also observed when Cumberland was added without Na₂SO₃. Sulfur elemental analyzer and high-resolution field emission scanning microscopy with energy dispersive X-ray spectroscopy (HR-FE-SEM-EDS) characterization confirmed that Cumberland fly ash particles were rich in sulfur. Nanoparticle Tracking Analysis (NTA) determined the mean particle size in solution to be 246 ± 25 nm for Cumberland fly ash and 198 ± 14 nm for Shawnee. To obtain further insight on observed Hg²⁺homogeneous reduction rates by sulfite, the effects of several environmental variables were investigated. Hg(NO₃)₂and HgO were used as the sources of Hg²⁺. Extended pH (1–7) and temperature (1.0–45.0 °C) ranges were studied for the first time. The enthalpies of activation for the HgO reduction were 94 ± 3 kJ mol⁻¹at pH 1 and 92 ± 4 kJ mol⁻¹at pH 3, while the entropies were 33 ± 9 J mol⁻¹ K⁻¹at pH 1 and 30 ± 10 J mol⁻¹ K⁻¹at pH 3. It was determined that increasing ionic strength, especially with nitrate species, slows down the reaction at pH = 7. Significance of the results on the variability of mercury reduction by sulfite at various environmental conditions, and its implication in modelling are discussed.
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