The pseudo-second-order (PSO) model, popularized by Ho and McKay's work on divalent metal sorption onto peat, is routinely misinterpreted as proof of a two-site chemisorption mechanism. This article provides a rigorous, multi-faceted refutation of that inference. It demonstrates that a PSO fit offers no mechanistic proof through three converging lines of evidence. First, the PSO rate law is mathematically non-unique: it emerges as a limiting case from Langmuir kinetics, intraparticle diffusion, and statistical rate theory. Most critically, the two-site Coleman mechanism itself yields a mixed-order rate equation in which pure PSO appears only under restrictive conditions; a formal reductio ad... More >