In this article, ZnO nanogranules were synthesised rapidly by the thorough solid state mixing of nitrate precursor with citric acid followed by aging and calcination at two different temperatures of 400 degrees C and 600 degrees C. Here, citric acid played a superior role in the development of porous ZnO nanostructures. The ZnO nanogranules obtained in this way were characterised by different techniques. X-ray diffraction patterns indicated a hexagonal wurtzite structure of ZnO with high crystallinity and nanocrystalline size. Scanning and transmission electron microscopy images indicated the uniformly well separated hexagonal morphology of the ZnO nanoparticles, with granular shape. BET surface area-porosity analysis confirmed the presence of a mesoporous network in both samples, with high surface area and a monomodal pore size distribution. Room temperature Photoluminescence spectra of the samples exhibited characteristic blue and green emission bands in accordance with the calcination temperature. The photocatalytic performance of the ZnO nanogranules was studied using methylene blue degradation and found that the ZnO prepared at 400 degrees C exhibited higher photocatalytic activity due to its high surface area and more oxygen vacancies. High concentrations of several hazardous dye pollutants were effectively degraded, when the solution was passed over 50 mg of ZnO nanogranules under UV-light illumination. (C) 2013 Elsevier B.V. All rights reserved.