With the increasing production and wide utilization of graphene oxide (GO), the nanomaterials are expected to be released into the environment, and end up in surface waters, and/or wastewater treatment plants. This study explored the changes in the physicochemical properties of GO resulting from photochlorination- simulating the reactions that occur during water and wastewater treatment. Photochlorination resulted in significant changes in the surface oxygen-functionalities of the nanomaterials, and fragmenting of the graphenic carbon sheets was observed. We found that photochlorination can enhance the decomposition of GO through the formation of reduced GO. The changes in surface oxygen-functionalities of GO were attributed to the oxidation by chlorine of the nanomaterials' quinone groups, and further oxidation by and/or radicals. The surface charge of GO, measured by its zeta potential, increased in magnitude with chlorination but decreased in magnitude with photochlorination, leading to the decrease in the colloidal stability of the photochlorinated nanomaterials. The antibacterial effect of the nanomaterials increased with both chlorination and photochlorination. This study clearly shows how the physicochemical properties, and environmental fate and effect of GO are modified by photochlorination.