A novel polyvinyl alcohol (PVA)/titanium oxide (TiO2) nanofiber adsorbent modified with mercapto groups was synthesized by electrospinning. The adsorbent was characterized by Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) and Brunauer-Emmett-Teller (BET) analysis. The influence of several variables such as TiO2 and mercapto contents, adsorbent dose, pH, contact time, initial concentration of U(VI) and Th(IV) ions and temperature were studied in batch experiments. The results showed that the sorption capacities of both metal ions for the modified PVA/TiO2 nanofibers were remarkably greater than those of the unmodified nanofibers. The kinetic data were described with pseudo-first-order, pseudo-second-order and double-exponential models. Three isotherm models, namely Freundlich, Langmuir and Dubinin-Radushkevich were used for analysis of equilibrium data. The maximum sorption capacities of U(VI) and Th(IV) by Langmuir isotherm are estimated to be 196.1 and 238.1 (mg/g) at 45 degrees C with pH of 4.5 and 5.0, respectively. Calculation of Delta G degrees, Delta H degrees and Delta S degrees showed that the nature of both metal ions sorption onto the nanofiber was endothermic and spontaneous and was favored at higher temperature. The sorption capacity did not change remarkably after five cycles of sorption-desorption. The selectivity order of uranium and thorium sorption onto the adsorbent was Th(IV) > U(VI). The inhibitory effect of competitive Th(IV) ion on the U(VI) sorption was greater than the inhibitory effect of competitive U(VI) ion on the Th(IV) sorption in the binary systems. Also the inhibitory effect was increased with increase of concentration of metal ions. (C) 2013 Elsevier B.V. All rights reserved.