Paramagnetic mesoporous carbon spheres with diameters of 1-3 mu m were synthesized through the hydrothermal carbonization of carboxymethylcellulose with nickel acetate, followed by high-temperature carbonization in a N-2 atmosphere. Monodisperse Ni particles of average size of 2-5 nm were doped into the carbon matrix, and covered the entrances of pores. Ni particles existed as metallic nickel and nickel oxide with ordered lattice structures. The effect of Ni content on the specific surface area, mesopore percentage, and magnetic and adsorption properties were investigated. The highest vitamin B-12 adsorption capacity of 103 mg/g was achieved for the sample prepared using 0.04g of nickel acetate. The Freundlich and Langmuir isotherm models were used to determine the equilibrium uptakes of vitamin B-12. Vitamin B-12 was physically adsorbed as a monolayer on the carbon spheres. The carbon spheres were easily separated on account of their magnetism. (C) 2015 Elsevier B.V. All rights reserved.