THE CONTRIBUTION OF MICROBIAL BIOMASS TO THE ADSORPTION OF RADIOIODIDE IN SOILS

The contribution of soil microbial biomass to the sorption and migration of radioiodide in soil has been investigated in batch and column experiments. In two arable soils, a chernozem and a podzol, the numbers of microorganisms were either reduced by biocidal treatment or increased by addition of nutrient sources. Radioiodide (I-125-) adsorption by the pretreated soils was measured, relative to untreated soil samples, in aqueous suspensions containing 10(-8) mol litre-1 iodide by estimating the distribution coefficient (K(d)) after eight days of incubation. A reduction of biomass to about 10% of its original level drastically decreased adsorption. Elevated levels of microbial biomass (up to 126%) increased adsorption but the increase was not always correlated with biomass level. A closer correlation between soil biomass and adsorption was observed when the concentration of radioiodide in the suspension was increased by several orders of magnitude. Conditions which are known to influence the activity and survival of microorganisms also exerted an effect on radioiodide sorption. Anaerobiosis during the incubation period of eight days clearly lowered adsorption. Temperatures above 20-degrees-C and air-drying of soils, which also reduce survival of soil microorganisms, had the same effect. In accordance with the relationship described here between radioiodide adsorption and microbial biomass, migration in water saturated soil columns was influenced by the quantity of microorganisms present. However, high biomass contents obviously caused anaerobic conditions in the system, leading to increased leaching of radioiodide.