Experimental Study of Trace Metal Chemistry in Soft-Water Lakes at Different pH Levels

Abstract

The biogeochemistry of Hg, Zn, Co, Fe, Mn, Cr, V, Th, Ba, Cs, As, and Se in two soft-water lakes of the Canadian Shield was investigated by means of carrier-free γ-emitting isotopes introduced into limnocorrals in which the pH of the water was varied from 6.8 to 5.1. The residence times of the radionuclides in the water were determined, and the partitioning of the nuclides among different metal-binding agents in the water and sediments was studied with the aid of membrane filtration, dialysis, solvent extractions, and fractionation on Sephadex columns.Metal behaviour varied systematically with metal properties. Metals of high crystal field stabilization energy, high electronegativity, or small ionic radius were most readily scavenged by> 0.45 μm suspended particles and dispersed colloids in the water, disappeared most rapidly from the water column, and were preferentially accumulated by sedimentary binding agents, including organic substances. Which property of a metal had the dominant effect on metal behavior depended on environmental factors, such as the ambient pH and the nature of the binding agents. Thus, Hg was removed fairly rapidly from the water at pH 6.7–6.8 owing to its high electronegativity but was removed more slowly than any other metal at pH 5.1 owing to its large ionic radius.Most metals were strongly chemisorbed or complexed by mud and organic ooze, and only Ba could be redissolved by cation exchange. Therefore, 0.1 N NaOH and 0.1 N or 1 N HCl were more efficient extractants than H2O, 1 N CaCl2, or benzene/methanol, and the metals were largely nonextractable by any of these solvents. The major NaOH-extractable metal-binding agents were the higher molecular weight fractions of a group of organic (probably humic) substances characterized by a UV absorption band at [Formula: see text]. The affinity of metals for the highest molecular weight "humic" (NaOH-extractable) substances increased with electronegativity, being greatest for Hg. Hg and Se were nonextractable by HCl but were the elements most readily extracted by NaOH, implying exceptionally strong covalent bonding by humic matter. The more electropositive metals were more readily extracted by HCl than by NaOH, suggesting weaker, less highly covalent bonding or preferential sorption by acid-soluble hydrated oxides.Acidification of lake water to pH 5.1 interfered with accumulation of Hg and other metals by organic ooze, probably owing in part to interference with the deposition or formation of the complexing agents with the 265 nm absorption band. Acidification also lowered the concentration of NaOH-extractable colloidal phosphate in the ooze but had no effect on NaOH-extractable orthophosphate content.Key words: metals, lakes, chemistry, pH, humic matter, organic matter, phosphorus, radioactive tracers, acidification