A facile ammonia-atmosphere prehydrolysis post-synthetic route that can uniformly and selectively deposit Fe(2)O(3) nanoparticles in the predefined mesopores (5.6 nm) of a bimodal (2.3, 5.6 nm) mesoporous carbon matrix is demonstrated (see figure). The mesoporous magnetic Fe(2)O(3)@C encapsulates show excellent performance for arsenic capture with remarkable adsorption capacity, fast uptake rate, easy magnetic separation, and good cyclic stability.