Utilization of Agricultural Biomass as a Low-Cost Adsorbent for the Removal of Phenolic Compounds from Aqueous Solution

Abstract

The adsorption of hydroquinone and phenol from aqueous solution onto coconut shell activated carbon was investigated with emphasis on the effects of contact time, solution pH, and thermodynamic behavior. Batch adsorption experiments were carried out to evaluate the efficiency of the adsorbent under varying operational conditions. The results revealed that adsorption of both contaminants increased with contact time, with a rapid uptake observed in the initial stages due to the availability of abundant vacant active sites. Equilibrium was attained within approximately 120–140 min, after which no significant increase in adsorption capacity was observed. The effect of solution pH indicated strong pH dependency of the adsorption process. Maximum adsorption occurred under acidic conditions, with a progressive decrease in adsorption capacity as pH increased toward alkaline conditions. This trend was attributed to changes in surface charge of the activated carbon and the ionization state of the adsorbates, leading to reduced electrostatic attraction and increased competition with hydroxyl ions at higher pH values. Thermodynamic analysis showed that the adsorption process was spontaneous, with negative Gibbs free energy values of −18.45 kJ mol⁻¹ for hydroquinone and −15.30 kJ mol⁻¹ for phenol. The negative enthalpy values (−42.80 and −36.50 kJ mol⁻¹, respectively) indicated an exothermic process, while positive entropy values (81.20 and 71.60 J mol⁻¹ K⁻¹) suggested increased randomness at the solid–liquid interface during adsorption. Comparative analysis demonstrated that hydroquinone exhibited consistently higher adsorption capacity than phenol across all studied conditions, likely due to its dual hydroxyl functional groups, which enhance hydrogen bonding and π–π interactions with the activated carbon surface. Overall, the study confirms that coconut shell activated carbon is an effective and environmentally friendly adsorbent for the removal of phenolic compounds from aqueous systems, with adsorption behavior strongly influenced by contact time, pH, and thermodynamic factors.

 https://doi.org/10.5281/zenodo.20590785