Fluorescence of the hydrophobic acid fraction (HPOA) of Suwannee River natural organic matter and Tb3+ excitation spectra were measured in tandem using the instantaneous and time-resolved emission modes. The intensity of HPOA fluorescence decreased in the presence of Tb3+, while the intensity of the emission from Tb3+ cations bound by HPOA increased by up to several orders of magnitude due to energy transfer (ET) from HPOA to Tb3+ ions. To determine intrinsic ET and fluorescence quenching (FQ) coefficients, NICA–Donnan modeling was carried. It showed that phenolic groups in HPOA dominated both the ET and FQ processes and that the binding of Tb3+ by HPOA could be described using the non-ideality parameter nTb, median binding constant log K~Tb for the phenolic sites and intrinsic ET and FQ coefficients (denoted as ηTbΦ and αTbΦ), and were 0.48, 8.5, 1385 and 0.12, respectively. The high value of the energy transfer coefficient of Tb3+ ions bound by the phenolic groups in HPOA is indicative of both the match between the electronic levels of the donor and acceptor, and the short distance between them. The deviation of the data of Nica–Donnan modeling of the ET and FQ dependence of versus [Tb]total for a 1.0 M ionic strength highlights the need to quantify the distribution of donor–acceptor distances in HPOA molecules in more detail.
