Neem (Azadirachta indica) leaves was employed in a batch adsorption technique for the detoxification of congo red (CR) and methyl orange (MO) dyes from a single component model wastewater. The adsorption efficiency of the leave was judged based on changes of parameters like agitation time, adsorbent dosage, adsorbent particle size, and adsorbate initial concentration and pH. Adsorbate concentration during the adsorption process was monitored using T60-U UV-Visible spectrometer from TG Instruments. While FTIR and SEM were employed to study the surface functional group and morphological changes of the adsorbent before and after the adsorption process. Percent adsorption increased with increase in agitation time (5 - 240 minutes), adsorbent dosage (0.1 - 0.5 g), initial concentration (100 - 300mg/L), and with decrease in particle size (≥75 μm to ≤300 μm) of adsorbent. The adsorption processes were found to be pH-dependent, increasing or decreasing in acidic (pH 2 - 6) or alkaline (pH 8 - 12) range over the studied pH (2 - 12) range. The correlation coefficient (R2 – values) ranging from 0.9359 - 0.9998 shows good agreement of the experimental data for all the tested isotherms. The monolayer maximum adsorption capacity for Langmuir’s qm (18.62 - 24.75mg/g) and Dubinin-Radushkevich, qD (20.72 - 26.06mg/g) are comparable. Both Langmuir’s separation factor (RL) and Freundlich constant (nF) suggests unfavourable adsorption of the dyes onto the adsorbent. The mean free energy of adsorption, ?? (79.06 - 316.23 J/mol) calculated from Dubinin-Radushkevich equation suggest a physical adsorption. Restricted range of values for heat of adsorption, bT (?946.9 to 737.4 J/mol), was obtained from Temkin equation. Reduction in band intensities and vibrational changes observed in FTIR spectra indicate possible involvement of carbonyl (-C=O), carboxyl (-COOH), hydroxyl alcoholic (-OH) and amino (-NH2) functional groups on the adsorbents’ surfaces during the adsorption and interaction with the sulfonic acid groups (-SO3H) on the adsorbate molecules. The difference in irregular and porous texture surface morphology of fresh and dye-loaded adsorbents characterized the adsorption of the dyes by neem leaves. The study shows that neem leaves are potential alternative low-cost adsorbents for the effective removal of Congo red (CR) and Methyl orange (MO) from wastewater.
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