Interaction of oxygen-containing functional groups on reduced graphite oxide (rGO) surface and zinc oxide (ZnO) has been investigated; and critical functionalities of rGO for enhancing H2S adsorption were studied. The abundant oxygen functional groups attached on the surface of rGO promoted the dispersion of nano-sized ZnO; then a higher surface area on the adsorbents was achieved. In addition, they prevented the aggregation of ZnO particles during regeneration at 600 degrees C. Beside those physical property changes, the presence of rGO modified the chemical properties of ZnO, as confirmed by XPS analysis. Due to those oxygen functional groups, the amount of zinc ions (Zn2+) are placed at the oxygen vacant sites, not only in the Zn-O lattice on ZnO/rGO composite. It was determined that the amount of oxygen ions in the Zn-O lattice decreased; and loosely bonded oxygen ions near the Zn-O lattice and on the surface were generated. Therefore, it was found that the presence of rGO plays a critical role to provide appropriate conditions for H2S adsorption, which was confirmed through the H2S adsorption breakthrough and regeneration tests. The ZnO/rGO composite showed about fivefold that of pure ZnO. The capacity after 1st regeneration for ZnO/rGO decreased by about 57% but then was maintained constant over 8 recycles while that on ZnO decreased dramatically to reach nearly zero capacity after 5 cycles. (C) 2014 Elsevier B.V. All rights reserved.