Technetium Sorption in Surface Soils

Abstract

Technetium from a number of sources has the potential for entering soils in initially volatile or solubilised forms [1,2]. Several assessments [3] have sugg.ested that subsequent soil solubility and uptake by plants will be important factors in governing Tc radiation dose to man. However, prediction ofTc behaviour in soils and availability to plants is complicated hy its complex chemistry relative to soil processes. Technetium is known to exist in all valence states from Tc+7 to Tc- I and in chemical species representing a wide range in volatilities and solubilities [4-10]. The solubility of Tc in soil will be a function of Tc chemical speciation and distribution between the solution and solid phase. Soil factors which have been implicated in Tc sorption have been the degree of aeration, the nature and quantity of sorptive surfaces and microbial activity. Under aerobic conditions the most soluble chemical species in water is Ihe pertechnetate ion (Tc04-), which is highly stable over broad pH nmcentration ranges. When trace quantities of the pertechnetate ion (in 0·01 M CaCI2) were equilibrated with 22 soils for 24h under aerobic conditions, Kd values (pCi sorbed per g soil/pCi solute per ml equilibrating solution) ranged from 0·007 to 2·8 [11], indicating high solubility relative 10 hydrolysable elements such as Pu (Kd > 1000). In another soil equilibration study over longer time periods (2-5 weeks), Xof II surface soils sorbed 98% of pertechnetate from solution [12]. The longer equilibration times also provided an opportunity for microbial (cont in article)