Speciation of Eu3+ bound to humic substances by time-resolved laser fluorescence spectroscopy (TRLFS) and parallel factor analysis (PARAFAC)

Three factors corresponding to different Eu3+ species were obtained at 70 μM Eu3+ for all HSs investigated except for one humic acid. One of the factors corresponded to free Eu3+ ion interacting with HSs via diffusion. The remaining two factors were thought to be Eu3+ bound to HSs: one bound to acidic functional groups of HSs and the other to the sites of HSs influenced by the carbon backbone structures. It was also found that the latter factor exhibited strong energy transfer from the excited Eu3+ center to HSs. At lower Eu3+ concentration (10 μM), two factors having similar fluorescent characteristics to those of the second and third factors were obtained.