Chitosan is an aminopolysaccharide that has been widely studied for metal ion recovery. In most cases it is used as a sorbent in solid form, but the polymer can also be used in a dissolved form in the so-called Polymer-Enhanced UltraFiltration (PEUF) process. The present work focuses on the use of dissolved chitosan for the removal of mercury from dilute solutions using an Amicon ultrafiltration unit. Recovery performance is compared to that obtained with poly(ethylenimine) (PEI), a synthetic amine-bearing polymer. The pH, metal concentration, and polymer concentration are the principal parameters to be taken into account in evaluating the recovery process. The impact of these parameters was tested with respect to metal and polymer retention and the filtration flow rate. In the case of chitosan, the comparison of molar metal/amine group ratios at saturation of the polymer in its solid state (adsorption process) and dissolved state (PEUF process) shows that dissolving the polymer improves the accessibility of sorption sites and enhances the sorption capacity. Although the addition of chloride strongly decreased mercury retention, it hardly influenced PEUF performances when using PEI; this indicates a different binding mechanism or, at least, different contributions on the part of electrostatic attraction and chelating mechanisms at different pHs for these different polymers: linear polymer (chitosan) and branched polymer (PEI).