Removal of methylene blue, methyl violet, rhodamine B, alizarin red, and bromocresol green dyes from aqueous solutions on activated cotton stalks

The adsorption behavior of methylene blue (MB), methyl violet (MV), rhodamine B (RB), alizarin red (AR), and bromocresol green (BG) dyes from aqueous solution on cotton stalks-activated carbon (CSAC) was investigated. The experimental procedure used activated carbon produced by chemical activation and physical activation using 50% ZnCl₂ at 850°C. For this purpose, batch tests were carried out as a function of solution pH value, contact time, dye concentration, agitation rate, temperature, and particle diameter. The experimental findings showed that the maximum adsorption capacities of adsorbent for BG and AR dyes were at pH 5. As comparison, the maximum adsorption capacity of adsorbent for MB dye was at an initial pH 12. In addition, the maximum adsorption capacities of adsorbent for RB and MV dyes were at pH 2 and 7, respectively. The maximum adsorption capacities of adsorbent for RB, MV, MB, AR, and BG were found to be 133.33, 135.14, 153.85, 196.08, and 222.22 mg/g, respectively. Kinetic studies of five dyes followed the pseudo-second-order model. Freundlich isotherm described the equilibrium data of BG, AR, and RB dyes on CSAC better than Langmuir isotherm, but Langmuir isotherm showed a better fit to the equilibrium data of MB and MV dyes. The R_L results show that the adsorption of five dyes onto CSAC is favorable.