A Comparative Kinetic and Thermodynamic Adsorption Study of Methylene Blue and Its Analogue Dye on Filter Paper.

A comparative adsorption study was carried out for methylene blue (MB) and its 3,7-bis(N,N-(2-hydroxyethyl)amino)-phenothiazinium dye analog (MBI). Batch experiments employed aqueous solutions and commercial filter paper. Out of seven kinetic models tested by means of four quality statistical indicators, the pseudo-second-order, the double-exponential, and the bi-linear Weber-Morris equations were best fits. For both dyes, the process was described as a succession of two diffusion-controlled steps. The Freundlich isotherm was chosen from 11 models describing a variety of mechanism assumptions. Physisorption was considered responsible for the dye removal from liquid. Adsorption of MB is thermodynamically favored, whereas that of MBI is sterically hindered. Both processes are exothermic and exhibit reduced randomness at the S-L interface. The paper was found suitable for retaining MB but served rather filtration/purification purposes for MBI.