MICROBIAL TOXICITY OF CD AND HG IN DIFFERENT SOILS RELATED TO TOTAL AND WATER-SOLUBLE CONTENTS

Microbial toxicity tests with Cd and Hg and up to 18 soil samples were combined with sorption and solubility measurements. The data ought to indicate to what extent toxic effects of the metals are influenced by sorption and the other factors that may alter their biocidal action, The microbial reduction of Fe(III) oxides to Fe2+ ions was used as a p arameter of microbial activity to calculate the total amounts of Cd an d Hg that cause 10, 50, and 90% inhibition [effective doses (ED)(10), ED50, ED90]. The corresponding solution concentrations [effective conc entrations: (EC)(10), EC50, EC90] were derived from Freundlich adsorpt ion isotherms, The very large variability in ED values in different so ils (ED10: Cd, 5-95 mg/kg; Hg, 0.125-125 mg/kg) can be related to the sorption and solubility behavior of the metals. Nevertheless, the EC v alues of both metals also indicate a considerable variation (EC10: Cd, 0.01-1.13 mg/liter; Hg, <0.001-0.041 mg/liter), The influence of solu ble soilborne substances on metal speciation is the main reason for th eir varying toxic potential in different soil solutions. The strong an d complex influence of soil properties on the toxicity of heavy metals in soils indicates that both chemical and biological methods of soil analysis are needed to assess the risks of soil contamination adequate ly. (C) 1997 Academic Press.