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Green macroalga Caulerpa lentillifera was found to have reasonable adsorption capacity for basic dyes,Astrazon(R) Blue FGRL(AB),Astrazon(R) Red GTLN (AR),and Astrazon(R) Golden Yellow GL-E(AY).The initial dye concentration was in the range of 100-1,800 mg/L.The dried algal sorhent was ground and sieved into 3 sizes:S(0.1-0.84 mm),M(0.84-2.0 mm),and L sizes(1arger than 2.0 mm).For all conditions examined in this work(at 25℃ in batch systems).the adsorption reached equilibrium within the first hour.The kinetic data corresponded well with the pseudo second order kinetic model where the rate constant,k2,decreased as the sorhent size increased for all dves.The adsorption isotherms followed both Langmuir and Freundlich models.Among three sorbent sizes.S size gave the highest adsorption capacity followed by M and L sizes.A reduction of sorhent size increased the specific surface area for mass transfer,and also increased the total pore volume.thus providing more active sites for adsorption.The adsorption of AB was adversely influenced by the protonation of algal surface at low pH.On the other hand,the adsorption of AR and AY could he due to weak electrostatic interaction.which was not significantly affected by pH.Increasing saliIlitV ofthe system caused a decrease in adsorption capacity possibly due to the competition between Na+ and the dye cations for the binding sites on algal surface.Moreover, all increase in salinity generated a compressed electrical double layer on the algal surface which exerted repulsive force,retarding the adsorption of positive charged molecules such as the basic dyes.