The interaction of hafnium(TV) and thorium(IV) with humic acid, HA, is stud
ied using EXAFS and compared to the interaction of these metal ions with ca
rboxylate groups of the cation exchanger Bio-Rex70. Fourier-filtered Hf and
Th L-3 edge transmission EXAFS for the first oxygen shell is analyzed. For
both Hf(IV)HA and Hf(IV)-Bio-Rex70, in wet paste and air-dried forms, 6-7
oxygen atoms at R=2.14 Angstrom are found. Th(IV)-HA also exhibits coordina
tion similar to Th(IV)-Bio-Rex70. These observations support conclusions fr
om previous studies on U(VT)-HA that metal cations are sorbed primarily ont
o HA carboxylate groups; other HA functional groups play a less significant
role.