The carbon modified BiOI/BiOIO3 (CSs-BiOI/BiOIO3) composites with heterostructures have been prepared via a facile hydrothermal method, using carbon spheres, bismuth nitrate and potassium iodate as precursors. Moreover, a new synthesis mechanism has been proposed. The photocatalytic activities of as-prepared catalysts are evaluated by oxidation of gaseous elemental mercury under visible light irradiation. It is found that the CSs-BiOI/BiOIO3 exhibits highly enhanced visible photocatalytic activity, compared with that of pure BiOIO3. The results show that the Hg degrees removal efficiency of CSs-BiOI/BiOIO3 reach up to 92.9% and 83.0% under LED light irradiation and simulated sunlight irradiation respectively. The improved photocatalytic activity can be ascribed to the increased the absorption of visible light, enhanced charge separation and transfer and the surface properties. This work points to CSs-BiOI/BiOIO3 nanocomposites as new and efficient visible light photocatalysts for environmental remediation applications. (C) 2016 Elsevier B.V. All rights reserved.