The effect of head group of surfactant on the adsorption of methyl red onto modified coffee residues

The effect of the head group structure of surfactant on the adsorption of methyl red (MR) onto modified coffee residues (MCRs) has been investigated. The coffee residue (CR) was modified by a cationic surfactant, namely Dodecyltrimethyl ammonium bromide (DTAB) and a zwitterionic surfactant, namely N, N-Dimethyldodecylamine N-oxide (DDAO). The CR and MCRs (DTAB–CR and DDAO–CR) were characterized by using SEM, EDX, BET and FTIR analysis. The effect of pH, contact time and initial MR concentration on the adsorption process was studied. Langmuir and Freundlich isotherm models were used to evaluate the experimental data. Adsorption equilibrium data were fitted to Langmuir isotherm, and the maximum adsorption capacities with DTAB–CR and DDAO–CR was 66.66 and 76.72 mg/g, respectively. On the other hand, the results of kinetic studies showed that the data were in maximum agreement with the pseudo-second-order kinetic model. Based on the experimental data and the proposed adsorption mechanism, it was concluded that zwitterionic surfactant is the most efficient for the adsorption of MR.