Mercury Adsorption on Phosphoric Acid- and Nitric Acid-Modified Activated Carbon

In this work, the influence of phosphoric groups on the surface of activated carbon on the adsorption of elemental mercury (Hg0) is systematically investigated for the first time. The influence of functional oxygen groups–controversially discussed in the literature–on the single-component adsorption of Hg0 as well as the co-adsorption of water and Hg0 are thoroughly analyzed, leading to a new mechanistic proposal. Single-component adsorption of Hg0 and co-adsorption of mercury and water on modified activated carbons are investigated by breakthrough curves as well as by coupled adsorption and temperature-programmed desorption (TPD) experiments. The modification of a basic activated carbon with phosphoric acid evolves phosphoric acid esters on the surface, which significantly increase the chemisorptive capacity for Hg0. TPD experiments reveal that only a single chemisorptive mechanism is involved. Based on these findings, a schematic mechanism for the chemisorptive attachment of Hg0 is proposed. In contrast to this, physisorptive interactions between Hg0 and surface groups dominate at the basic activated carbon and at the activated carbon modified with nitric acid. Oxygen-containing functional groups formed by nitric acid do not significantly enhance the one-component adsorption of elemental mercury. The experiments on the co-adsorption of mercury and water prove that the surface complexes of oxygen functional groups and Hg0 are strongly stabilized by water from the gas phase. A schematic chemisorptive mechanism is also proposed here. The chemical reaction of Hg0 with phosphoric acid esters, on the other hand, is not influenced by water from the gas phase in the concentration range investigated.