A surface adsorption-diffusion-reaction model is proposed and tested by Monte Carlo simulations. The solvent shell effect is considered as a geometric constraint and the Langmuir adsorption mechanism is also incorporated. An anomalous power law of t(1/2) is obtained for the amount of material removed from the surface vs time. The result agrees well with etching experiments of SAM on a silica surface. We conclude that the unusual power law results from an interplay of vacancy growth and fusion at the surface.