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 2 soft-water lakes of the Canadian Shield was investigated by carrier-free .gamma.-emitting isotopes introduced into limnocorrals in which the pH of the water was varied from 6.8-5.1. 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 behavior 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 .mu.m 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. Most metals were strongly chemisorbed or complexed by mud and organic ooze, and only Ba could be redissolved by cation exchange. NaOH (0.1 N) 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 .simeq. 265 nm. 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 Pi content.