Adsorption mechanism and behavior of methylene blue and Congo red dyes on modified bentonites

Abstract

The present study aims the investigation of modifying bentonites in the elimination of both cationic and anionic dyes; Methylene blue and Congo red were chosen as models for each kind of dyes. Comprehensive characterization through FTIR, XRD, BET, SEM, and pH_PZC demonstrated the suitability of bentonites in the removal of dyes owing to theirs remarkable morphology, and texture. Several parameters that effect adsorption process of dyes were investigated, to establish the behavior and the removal mechanism such as: effect of modification of bentonite (purification, activation, and organophilization), effect of pH (2–12), and effect of temperature (10–40 °C). For both dyes, MB and CR adsorption kinetics are best described by the pseudo-second-order model, and Langmuir model for the isothermal study. pH parameter affects both dyes. MB Adsorption process is favorable in basic medium. CR adsorption process is favorable in acidic mediums for PBent, ABent and favorable in basic medium for OBent. MB/PBent, CR/ABent, and CR/OBent adsorption processes were found to be spontaneous and exothermic in nature owing to the negative values of the thermodynamic parameters. For MB/ABent, MB/OBent, and CR/PBent adsorption processes were found to be spontaneous and endothermic because the values of the thermodynamic parameters were found to be positive. The bentonites have achieved maximum MB adsorption capacities of 345.8, 152.6, and 347.2 mg/g for PBent, ABent, and OBent, respectively and CR adsorption capacities of 315.2, 4.96, and 205.85 mg/g for PBent, ABent, and OBent, respectively. From the obtained results, it can be said that all prepared bentonites are a very promising adsorbents of both cationic and anionic dyes and can be used successfully in the industrial of textile to remediate their wastewater.