Effects of Acidification on Mobilization of Heavy Metals and Radionuclides from the Sediments of a Freshwater Lake

Abstract

Large (10 m) diameter enclosures were sealed to the sediments in 2–2.5 m of water in Lake 223. Two tubes were held at control pH (6.7–6.8), one was lowered to pH 5.7 and one to pH 5.1, using H₂SO₄. Aluminum, manganese, zinc, and iron were released from lake sediments at pH 5 and 6. Concentrations of zinc in the overlying water column exceeded 300 μg∙L⁻¹. Radioisotopes of several heavy metals added to the water of the enclosure showed the following: all metals were removed from the water at log-linear rates, with half-times of 5–25 d. Acidification caused several metals to become more soluble, including Fe-59, Co-60, Mn-54, and Zn-65. Solubility of V-48 and Hg-203 decreased with increasing acidity. Acidification also slowed the loss to sediments of Mn-54 and Zn-65. Losses of Ba-133, Se-75, Cs-134, and V-48 were more rapid under acid conditions. The fractions of any isotope retained by a 0.45-μm filter, activated charcoal and mixed-bed ion exchange resin remained constant throughout the experiment at any given pH.Key words: sediment–water interactions, heavy metals, radionuclides, lake acidification