Sorption of Cu(II) ions on dried red alga Pterocladia capillacea and its activated carbon have been studied. The sorption equilibrium was determined as a function of contact time at several Cu(II) ion concentrations and the effect of adsorbent concentration was also investigated. The pseudo second-order kinetic model provided the best correlation for the experimental data in compared to the pseudo-first order kinetic. Ion exchange was occurred in the initial reaction period. The experimental results were fitted to the Langmuir, Freundlich and Redlich-Petrson isotherms to obtain the characteristic parameters of each model. Both the Langmuir and Redlich-Peterson equations were significantly better for dried red alga biomass. On the other hand, Freundlich and Redlich-Peterson equation were significantly better than Langmuir for the activated carbon prepared from dried red alga. Error functions have been used to determine the alternative single component parameters by non-linear regression analysis. The error function method provided the best parameters for the isotherm equation in this system and is demonstrated for error comparison purposes. Red alga biomass and its activated carbon may be evaluated as an environmentally friendly and extra economic treatment. (C) 2011 Elsevier B.V. All rights reserved.