Abstract
Adsorption phenomenon is affected by the nature and the presence of different functional groups on the adsorbates. Present study focuses on the influence of the groups (–CH3 and –NHCOCH3) linked to the model pollutants i.e. anthraquinone acid dyes such as acid blue 25 (AB 25), acid blue 40 (AB 40) and acid blue 129 (AB 129) in adsorption process. For this adsorptive study, a microporous activated carbon developed from demineralised kraft lignin (DKL) in an oxidizing atmosphere (surface area 1305 m2g−1; micropore volume 0.374 cm3g−1) was utilized as adsorbent and surprisingly a small but visible difference was observed in the experimental adsorption values of the chosen dyes. The dyes followed an adsorption order: AB 25 < AB 40 < AB 129 with experimental values 0.501, 0.561 and 0.601 mmolg−1 for AB 25, AB 40 and AB 129, respectively. Results indicated that the methyl groups attached to the AB 129 dye cause an increase in the hydrophobicity which further enhances its adsorption as compared to other acid dyes, whereas, hydrophillicity as well as tautomerism is responsible for the adsorptive removal of AB 40 onto the developed adsorbent. Since AB 25 doesn’t have any side group at phenyl ring, therefore, shows minimum removal efficiency. Furthermore, the effect of temperature was also investigated and the adsorption of dyes on activated carbon increase from 0.501 to 0.547, 0.561–0.610 and 0.601–0.665 mmolg−1, for AB 25, AB 40 and AB 129, respectively on increasing the temperature from 25° to 45°C. The small positive values of enthalpy change (ΔH°) obtained, reveals the physiosorptive nature of adsorption and confirms its endothermic nature. The adsorption isotherm data was examined by Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models and the isotherm data fitted well to the Langmuir model. The kinetic study showed that the adsorption equilibrium was attained in 240 min for each dye. Pseudo-first order, Pseudo-second order and Elovich kinetic models were applied and Pseudo-second order model followed the adsorption kinetic process. The results of this study will pave the way to utilize carbons from wastes for the noxious acid dyes removal. Moreover, the work carried out could open an ample space for utilising porous carbons for treatment of the wastewater/sea water containing dyes.
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•Nature and groups attached to adsorbate effects the adsorption process.•Hydrophobicity and tautomerism play role in adsorption of anthraquinone acid dyes.•Amount of adsorption is in the order: acid blue 25 < acid blue 40 < acid blue 129.•Adsorption process is physiosorptive and endothermic in nature.