Removal of Methyl Orange Dye from Aqueous Solution Using Bioadsorbent Modified with Polyaniline Nanofibers;Isotherm, Kinetic and Thermodynamic Studies

Hypothesis: 

Methyl orange is a dye used in various industries such as printing and dyeing. Since methyl orange is not affected by conventional biological treatment and is not easily decomposed, the release of effluents containing this toxic compound into environment has adverse effects on human life and the aquatic ecosystem. Therefore, it is necessary to remove this dye from various wastewaters Among the common methods of water purification, the method of adsorption by bioadsorbents is considered as a simple, efficient and inexpensive method. In the present study, walnut shell modified with polyaniline nanofibers (PANI/WNS) has been used for removal of methyl orange dye. This modified adsorbent is a good candidate since it has favorable characteristics including suitable morphology and positively charged functional groups for removal of anionic dyes.

PANI/WNS was prepared through aniline polymerization reaction on the surface of walnut shell particles and characterized using FTIR, FESEM, EDX, BET and XRD techniques. The effect of various physical and chemical parameters such as initial pH, contact time, adsorbent dose and initial dye concentration on the removal rate of methyl orange was studied and optimized. Also, thermodynamic parameters such as ΔH, ΔS and ΔG were investigated.

The adsorbent is able to remove methyl orange dye in acidic environment. Optimization of the effective parameters on the adsorption process showed that the maximum methyl orange removal was obtained under conditions of pH 3, contact time of 140 min, adsorbent dosage of 0.35 g/L and dye initial concentration of 50 mg/L. Evaluation of laboratory data by different isotherm models showed that the highest correlation coefficient is related to Langmuir isotherm model and is equal to 0.9975, which indicates that the dye adsorption process is limited to a monolayer Also, the maximum adsorption capacity for methyl orange dye was obtained as 243.9 mg/g at 25°C, which is a significant amount compared to similar adsorbents reported in the literature and indicates the high ability of the studied modified bioadsorbent for removal of anionic dyes such as methyl orange. In addition, based on the results of the kinetic studies, the pseudo-second order model showed a better description of the laboratory data. Thermodynamic studies showed that the adsorption of the dye is a favorable, spontaneous and endothermic process.