The biosorption efficiency and capacity of Methylene Blue (MB) dye from aqueous solution by untreated and pretreated (physical and chemical) brown (Nizamuddinia zanardinii), red (Gracilaria parvispora) and green (Ulva fasciata) algae was investigated. Among the seventeen chemicals tested for pretreatment, sodium thiosulfate, isooctane and 1-butanol were found to be the most effective chemicals for brown, red and green macroalgae, respectively. Different concentrations of these chemicals and sodium chloride were further examined in the range of 0-2 M to obtain the best concentration for pretreatment. Pretreatment with sodium chloride (0.1 M) was able to enhance the dye removal by macroalgae. Also, MB dye biosorption efficiency was studied as a function of salt water, initial dye concentration and contact time onto macroalgae treated with sodium chloride. Equilibrium data were best described by Langmuir isotherm model. The biosorption kinetics was well fitted by the pseudo second order kinetic model. Fourier transform infrared spectroscopy (FT-IR) analysis revealed the involvement of different functional groups, mainly hydroxyl, carboxylic aliphatic groups during the biosorption process. The morphology of algae surface was characterized by Scanning Electron Microscopy (SEM). Results from this study demonstrated the potential utility of macroalgae for MB dye removal from water. (C) 2016 Elsevier B.V. All rights reserved.