The Cu, Cd and Zn sorption capacity of formaldehyde-crosslinked seaweed biomass (Ascophyllum nodosum : FCAN) was studied using equilibrium methodology. The shape of sorption isotherms and biosorbent mass losses indicated that in the first uptake cycle the biosorbent leached cellular polysaccharides that formed precipitate with metal solutions influencing the sorption study and application. However, since the sorption levels of washed and non-washed biosorbents were comparable, the leached cell wall polysaccharide(s) (alginate) could not be the main component responsible for metal sorption in this case.