Impact of iodine treatment on the thermal combusition of activated carbon

摘要

It is well known that iodine is strongly adsorbed on activated carbon. The adsorption of iodine on carbon from aqueous solution is the basis of the well-known "iodine number" method for characterizing the surface area of activated carbon [1]. Activated carbon has been utilized for the removal of radio-iodine from nuclear facilities [2]. Iodine-treated activated carbon has also been applied to the removal of mercury from air [3]. One of the limitations associated with the application of iodine-treated activated carbon is the potential for loss of iodine at elevated temperatures. In order to better understand the safe range of application of iodine-treated carbons, we undertook thermogravimetric analysis (TGA) on variably iodine-treated carbons. Since most of the applications of activated carbon involving iodine are performed in air, TGA analysis was also conducted in air. Temperatures in excess of 175oC were required to desorb iodine. The maximum iodine desorption temperature decreased linearly with increasing loading. Increasing the loading results in population of sites of lower iodine affinity and the adsorption maximum decreases accordingly. Regardless of the loading, most iodine was lost at temperatures significantly below the combustion temperature of activated carbon. Surprisingly, the combustion of activated carbon occurred at significantly lower temperatures on iodine-treated carbon than on un-treated carbon. Even more surprising, there appeared to be a linear correlation of carbon combustion temperature and iodine loading. If the adsorption of iodine is primarily a physical process, it is surprising that iodine treatment of carbon leads to a change in the thermal stability of activated carbon. Kinetic investigation showed no change in activation energy for combustion with and without iodine loading, suggesting the absence of a catalytic process. X-ray photoelectron spectroscopy (XPS) has been applied to understand changes taking place on the carbon surface following adsorption and desorption of iodine. In order to probe the interaction of iodine with activated carbon, iodine-treated carbon was analysed by electron paramagnetic resonance spectroscopy (EPR). Certain activated carbons possess unpaired electrons that can be observed by EPR spectroscopy [4]. It is well known that the adsorption of oxygen on activated carbon leads to the broadening of the EPR signal [4], [5]. This effect is known to be reversible. The adsorption of iodine was also shown to broaden the EPR signal of activated carbon. We will report the line-broadening of the EPR signal as a function of iodine loading and compare that line broadening with the reduction in the thermal decomposition temperature, as observed by TGA.