Frustules or the rigid amorphous silica cell wall of unicellular, photosynthetic microalgae with unique porous architecture has been used to synthesize a composite by immobilizing ZrO2 on its surface and in the pores. This was effective in water defluoridation. The average diameter of the composite was 80 +/- 2 nm and surface area was 140 m(2)/g. The adsorption isotherms followed both Langmuir and Freundlich models, and the composite was regenerable. Adsorption kinetics followed second order model and the adsorption capacity was as high as 11.32 mg/g, while the Langmuir maximum adsorption capacity (q(m)) reached 15.53 mg/g. The research findings highlight the potential of diatoms as hosts for nanomaterials for use in water treatment. (C) 2015 Elsevier Inc. All rights reserved.