The paper considers mathematical modeling in relation to the removal of low concentrations of impurities by gas-solid reactions in a fixed bed of absorbent. Models describing the following process mechanisms are analyzed: (1) gas diffusion through the outer shell of the reacted absorbent; (2) chemical reaction at the interface between the reacted shell and the non-reacted core; (3) reacted-shell diffusion together with the interface chemical reaction; (4) reacted-shell diffusion and reaction together with diffusion in the bulk of the internal non-reacted core. For an isothermal plug-flow reactor with spherical particles of absorbent, solutions by quadrature or even the exact ones are derived for each of the models. The solutions are analyzed for the process of gas sweetening over zinc oxide absorbents: H2S(g) + ZnO(s) = H2O(g) + ZnS(s). It is shown that the models may be distinguished through analysis of experimental gas content versus time data at the outlet of the absorbent packed beds rather than on the basis of conversion versus time curves. The requirements for experimental distinction of the models are determined. It is shown that model (3), describing reacted-shell diffusion together with interface chemical reaction, is the most adequate for the design of zinc oxide beds. (c) 2007 Elsevier B.V. All rights reserved.