A model for design and scale-up of an external light irradiation slurry photoreactor was developed on the basis of a model for the average light intensity in the photoreactor. In order to assess the design parameters in the proposed model, experiments for UV light or sunlight-photoassisted Fenton discoloration of azo-dye Orange II in water with iron ion eluted from the natural mineral tourmaline powder containing 4.49 wt% Fe2O3 were conducted using an external light irradiation cylindrical column photoreactor. The efficiency of discoloration increased as initial Orange II concentration decreased and the tourmaline concentration and UV light intensity increased. It was found that the optimal hydrogen peroxide concentration and pH are 500 mg L-1 and 3, respectively. The proposed model, in which the change of light intensity with degradation time due to the discoloration of Orange II and the light scatter due to solid photocatalysts are taken into account, could simulate the experimental results both for UV light and sunlight irradiation conditions reasonably well. The model is applicable commonly both for UV light and sunlight irradiation. The results will be very useful for designing slurry photoreactors in the treatment of textile effluents. (c) 2006 Elsevier Ltd. All rights reserved.