The process of mono and binary removal of crystal violet and safranin dyes by Spirulina sp. (blue-green algae) modified with cationic surfactant is evaluated. The surfactant used was hexadecyltrimethylammonium bromide (HDTMA). Spirulina sp. was cultivated in Zarrouk's medium. The adsorptive properties of the modified Spirulina sp. (HDTMA-algae) were tested as a function of pH (2-10), contact time (5-180 min), temperature (25-45 degrees C, and initial dye concentrations (25-300 mg/L) and characterized with FTIR, SEM, EDX, XRD and BET analyses. The specific surface area of HDTMA-algae was 0.1990 m(2)/g. The data were fitted to non-linear Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models and non linear pseudo-first-order and pseudo-second-order kinetic models. The adsorption was 75% and 88% at pH 2 for Crystal Violet and Safranin dye, respectively. The maximum adsorption capacities were 101.87 mg/g and 54.05 mg/g and the k(F) values were 0.96 L/g and 3.56 L/g for Crystal Violet and Safranin, respectively. The kinetics of Crystal Violet and Safranin dyes onto HDTMA-algae were best described by the pseudo-second-order kinetic model. The negative values of free energy and enthalpy change indicated the feasibility, spontaneity, and exothermic nature of the adsorption process. Also, binary adsorption of Crystal Violet and Safranin dyes onto HDTMA-algae from binary dye mixtures is compared. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.