Adsorption behavior of Direct Red 12B and Rhodamine B from water onto surfactant-modified coconut coir pith

The surface of coir pith, an agricultural solid waste, was modified using a cationic surfactant hexadecyltrimethylammonium bromide. Adsorption of two dyes namely Direct Red 12B (acidic dye) and Rhodamine B (basic dye) on surfactant-modified coir pith has been investigated in a series of batch adsorption experiments. Effects of process variables such as contact time, concentration of the dyes, adsorbent dose, temperature and pH have been studied to understand the kinetic and thermodynamic parameters of the process. Kinetics of adsorption obeyed second order rate equation. Equilibrium adsorption data were fitted into the Langmuir, Freundlich and Dubinin and Radushkevich isotherms and the system followed all the three-adsorption isotherms. It was found that modified coir pith yields adsorption capacity of 76.3 mg g−1 and 14.9 mg g−1 for Direct Red 12B and Rhodamine B, respectively. Chemisorption might be responsible for the uptake of dyes. The adsorbent was also tested for the removal of dyes from synthetic effluent.