The pure components (CaO, Al₂O₃∙SiO₂, CaCO₃ and etc.) of PWDS (Paper Waste Derived Sorbent) rearrange to gehlenite (Ca₂Al₂SiO₇) during the mercury adsorption process at 600 – 1100 °C. The rearranged gehlenite surface captures mercury on the active sites and then is deactivated. At high temperatures compared to mercury adsorption on activated carbon, this sorbent captures 83 – 90 % of elemental mercury according to bench-scale Hg control experiments. The mercury removal efficiency increases with increasing temperature. In this work, computational chemistry is used to investigate how mercury is adsorbed on the gehlenite surface and to theoretically predict the temperature affects in an effort to mechanistically understand the unusual adsorption behavior.

The adsorption of Hg on the gehlenite surface was investigated with density functional theory (DFT). The gehlenite structure was modeled with a periodic slab and was optimized with the local density approximation (LDA) with the Perdew – Wang (PWC) functional. The adsorption energies of Hg adsorbed on the gehlenite surface was calculated with the gradient corrected (GGA) method with the Becke-Lee-Yang-Parr (BLYP) functional using the lower level optimized structure. The fine basis set, core treatment and spin unrestricted methods were applied to all calculations to lead higher level and hopefully more accurate results. The equilibrium constants with respect to the temperature effects were also predicted, the results from this work show how Hg adsorbs on an atop site of oxygen with a shorter distance and stronger adsorption energy on gehlenite compared to other adsorbates.