Modification of activated carbon using a NOx-containing gaseous by-product for enhanced Hg(II) removal from aqueous phase

In this study, activated carbon was treated using a gaseous by-product produced by the reaction of nitric acid and activated carbon. To determine the effectiveness of this treatment for enhancing the Hg(II) removal capacity of activated carbons, several batch experiments have been conducted for two different commercial activated carbons at different pH values, temperatures, and initial concentrations. In addition, Iodine number, BET surface area, total and micro-pore volume, as well as pHpzc amounts of treated and untreated samples were compared to determine the effects of this treatment on the porous structure and surface charge of activated carbons. After this modification, a decrease in porous characteristic and an increase in the surface acidity of activated carbons were observed. Hg(II) speciation for experimental conditions in this work showed that molecular species including HgCl2, HgClOH, and Hg(OH)2 are generally present in the solution. Additionally, thermodynamic calculations indicated that the Hg(II) removal using the modified activated carbons in this work is an endothermic process with increase in the entropy of the system. The positive entropy change was described by the chemisorption mechanisms during which some released species from the surface into the aqueous phase may increase the degree of freedom.