Adsorption of Sulfate by Kaolinite and Amorphous Iron Oxide in the Presence of Organic Ligands
- William P. Inskeep *
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.
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