Adsorption of Sulfate by Kaolinite and Amorphous Iron Oxide in the Presence of Organic Ligands
1989
Inskeep, William Parks
The ability of soils to adsorb SO₄ is an important factor in determining the effect of acidic deposition on the transport of H⁺ and cations in terrestrial ecosystems. However, the role of soluble organic acids and humic substances on the adsorption of SO₄ is poorly understood. Consequently, this study was conducted to determine the effects of soluble organic ligands on the adsorption of SO₄ by amorphous iron oxide (AIO) and kaolinite (KGa-2). Adsorption experiments were conducted by titrating 50 mM K₂SO₄ with suspensions of KGa-2 (146-583 m²L⁻¹) or AIO (324 m²L⁻¹) in 0.01 M KCl, at pH 4.3 and 25 °C. Sulfate adsorption was studied over a range in total SO₄ of 0.01 to 2.5 mM in the absence and presence of organic ligands including Seward humic and fulvic acid, tannic acid, citric acid, oxalic acid, and gallic acid. Sulfate adsorption capacities in the absence of organic ligands were 3.5 and 1.3 µmolc m⁻² at 324 m²L⁻¹ AIO and 583 m²L⁻¹ KGa-2, respectively. Humic and tannic acid present at 1.4 to 3.6 mM total soluble C reduced SO₄ adsorption by KGa-2 at total SO₄ levels below 0.12 mM. Humic, tannic, citric, gallic, and oxalic acid were all effective at inhibiting SO₄ adsorption by AIO. In several cases the percent of SO₄ adsorbed was reduced from near 100 to zero at total SO₄ levels below 0.12 mM. The amount of inhibition was related to the quantity of oxygen-containing functional groups added rather than the total soluble C. These results indicate that organic acids compete for SO₄ adsorption sites and that the presence of organic acids in soil solutions will influence SO₄ adsorption capacities. Contribution no. J-2225 from Montana Agric. Exp. Stn.
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