The existing species of uranium determines the design of novel sorbents towards uranium extraction from the natural waters. Herein, three composites based on waste commercially available polyacrylonitrile fiber (WPANF), namely WPANF/TiO·xHO, WPANF/CTAB-bentonite, and WPANF/NZVI, were first prepared and employed for the removal of U(VI) from the carbonate coexisted aqueous solutions. Among them, the WPANF/TiO·xHO exhibited the optimum sorption capacity of 40.6 mg·g (pH 8.0, C = 50 mg·L, and [CO] = 2 mmol·L), which is significantly greater than the WPANF/CTAB-bentonite (12.6 mg·g) and WPANF/NZVI (~10.3 mg·g). All sorption capacities decreased with the increases of initial pH, [NaCl], and [CO], due to the species transformation from UO(CO) and (UO)CO(OH) to UO(CO) that enhanced the electrostatic repulsion and the competitive sorption. The XPS analysis and DFT calculations indicated that in the composites, WPANF was a role in strengthening the mechanical properties of composites rather than the main sorption sites for uranyl carbonates. The sorption mechanisms were mainly involved in -OH group coordination, Br anions exchanges, and redox reactions. Desorption, reusability and U(VI) sorption test in the simulated seawater demonstrated that WPANF/TiO·xHO could be an alternative candidate for acquiring uranium resource. This work has screened the potential composites for U(VI) extraction from the natural waters, especially based on the practical U(VI) speciation, and provides a novel research approach for the removal of U(VI) towards U(VI)-CO systems.