Free chlorine induced phototransformation of graphene oxide in water: Reaction kinetics and product characterization

Herein, the reaction of graphene oxide (GO) in the presence of free chlorine, exploring a variety of water chemistries, is described under both dark and ultraviolet (UV) irradiated conditions. For all conditions evaluated, GO reacts with free chlorine and the reaction rates are considerably enhanced under UV irradiation. Relative reaction kinetics are observed to be pseudo second order overall, while being first order to GO and pseudo first order to initial chlorine concentration. In addition to UV light irradiation, relative rate constants and reaction extents are affected by pH, dissolved oxygen, and the size of initial GO coupon itself. Advanced product characterizations using UV-vis spectroscopy, TEM, FTIR, TOC, Raman spectroscopy, mass spectrometry (MALDI), and XPS indicate GO is significantly transformed, being further oxidized - including significant mineralization to CO2 in some cases. Further, a fraction of the product(s) is identified to eventually stop reacting with free chlorine, suggesting a final, relatively recalcitrant portion of products. For all reactions, carbon mass conservation between the gas (as CO2) and liquid (as GO products) phases is established. Observed reactivity has direct implications for GO life cycle assessments and for material performance in water treatment technologies, such as membrane and disinfection processes, which utilize chlorine and/or UV irradiation, in addition to other GO based technologies.