  The effects of pH, ionic strength and temperature on sorption of Eu(III) on
attapulgite were investigated in the presence and absence of fulvic acid (FA)
and humic acid (HA). The results indicated that the sorption of Eu(III) on
attapulgite was strongly dependent on pH and ionic strength, and independent of
temperature. In the presence of FA/HA, Eu(III) sorption was enhanced at pH < 4,
decreased at pH range of 4 - 6, and then increased again at pH > 7. The X-ray
photoelectron spectroscopy (XPS) analysis suggested that the sorption of
Eu(III) might be expressed as ?X3Eu0 ?SwOHEu3+ and ?SOEu-OOC-/HA in the ternary
Eu/HA/attapulgite system. The extended X-ray absorption fine structure (EXAFS)
analysis of Eu-HA complexes indicated that the distances of d(Eu-O) decreased
from 2.451 to 2.360 {\AA} with increasing pH from 1.76 to 9.50, whereas the
coordination number (N) decreased from ~9.94 to ~8.56. Different complexation
species were also found for the different addition sequences of HA and Eu(III)
to attapulgite suspension. The results are important to understand the
influence of humic substances on Eu(III) behavior in the natural environment.
