The organic dye pollution has become increasingly serious in recent years and removal of organic dyes from the water samples has received intense attention. In present work the acid modified purple knight (Alternanthera dentata) leaves powder (MALP) is used as an adsorbent in order to remove sodium 4-[(4-dimethylaamino) phenyldiazenyl) benzene sulphonate (methyl orange dye) from an aqueous solution. The absence of stretching vibrations (1008cm(-1) , 1400cm(-1),1600cm(-1) and 3184cm(-1)) of characteristic functional groups of modified purple knight plant in FTIR-analysis, as well as a significant decrease in the lambda max value from 600 to 420 in the electronic study, indicate that the texture of the purple knight (Alternanthera dentata) leaf is changed. The influences of pH and contact time on adsorption is also investigated at the actual experimental conditions as pH range of 2-4.5, a methyl orange concentration of 0.1M, and contact time of 5-60 minutes at 26 degree celsius. Elovich and intraparticle diffusion models accurately described the kinetics experimental data for various solutions. The kinetic results indicate that adsorption process occurs in two or more steps. The adsorption models such as Langmuir, Freundlich, Temkin, Brunauer-Emmett-Teller(BET)and Dubinin -Radusshkevich (D-R) are used to explain adsorption process at different temperatures ranging from 26 to 60 degree celsius . In comparison to others adsorption models, equilibrium data at 26degree celsius provide clear information about the regression coefficient, adsorption capacity, (RL= 0 to 1) and Freundlich constant (1/n <1). These values indicate Langmuir, and Freundlich models fit very well, while other fit at >26 degree celsius. Values for the free energy change, enthalpy change, and entropy change indicate that the adsorption process is spontaneous and endothermic. The negative value of Delta G(degrees) at different temperatures indicate that at low temperatures, physical adsorption occurs, whereas at higher temperatures, its value become more negative and chemisorption occurs, revealing the orbital interaction between adsorbate and adsorbent molecules.
