Adsorption Controls on Releases

Silva and Nitsche (1996) note that distribution coefficient (K i) values for adsorption of the actinides are roughly equal for actinide cations of similar charge and structure. Based on earlier work, they report average actinide (An) cation KjS for 12 different minerals and 4 rock types of 500, 50, 5, and 1 respectively, for An , An , AnO +, and AnOt ions. The sequence of adsorption edges with increasing pH forTh(IV), Am(III), Np(V), and Pu(V) adsorption by y-Al203 in Fig. 13.34 (Bidoglio et al. 1989) is consistent with this order, with pH values of 2.5,5.8, and 7.3, respectively, at 50% ad- 

The adsorption edge plots in Fig. 13.34 show that adsorption of actinide cations increases with pH and has a strong pH dependence. This reflects that the actinide cations must compete with protons for adsorption sites. Cations whose adsorption edges occur at lowest pH values must, therefore, form the strongest bonds with sorbent surface sites. The adsorption edges of weaker-bonding cations then occur at successively higher pH values. 

Because units of weight adsorbed and dissolved cancel out in the expression, we can use any weight units we choose. At pH = 6, assuming 6% adsorbed is 6 on 0.2 g/L sorbent, then 94% dissolved in 1 L is equivalent to 94 fig/L or 0.094 ug/ml. then equals 

The pH-dependence of Kj values is also apparent from Fig. 13.13, which describes U(VI) adsorption by HFO. Based on this figure, ranges from about 10 to 10 between pH 4 and 9. Finally, values for Th(lV) adsorption by quartz shown in Fig. 10.24 are seen to increase from 10 to 10 between pH 3 and 6. 

The pH-dependence of radionuclide cation adsorption partly reflects competition with H ions for adsorption sites. Competition for adsorption sites by major electrolyte ions also limits radionuclide adsorption in saline waters (cf. Mahoney and Langmuir 1991). Also, competition for sites with Ca can significantly reduce Ra adsorption by quartz and kaolinite (Riese 1982), as well as NpOJ adsorption by smectite clays (Kozai et al. 1995). 

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