Activated Neem Leaf: A Novel Adsorbent for the Removal of Phenol, 4-Nitrophenol, and 4-Chlorophenol from Aqueous Solutions

The adsorption of three phenolic compounds, phenol (P), 4-nitrophenol (4-NP), and 4-chlorophenol (4-CP), on activated neem leaves (ANL) was investigated from simulated wastewater in batch as well as fixed bed mode. The effect of various operating parameters on adsorption such as adsorbent dose, solution pH, contact time, and temperature effect was monitored, and optimal experimental conditions were determined. The Langmuir, Freundlich, Redlich-Peterson, Dubunin-Radushkevich, and Temkin models were applied to describe the adsorption isotherms in the three systems. The goodness of curve fitting in the various models was done in accordance with linear regression coefficients and various error functions. Thermodynamic parameters such as the Gibbs free energy change, enthalpy change, and entropy change of the adsorption processes were also evaluated for the three different phenolic systems. The thermodynamic study shows the exothermic nature of the adsorption. The pseudofirst-order, pseudosecond-order, and Elovich equations were used to describe the kinetics of the adsorption processes. The pseudosecond-order model described the data very well (R-2 > 0.99). A comparative study of the batch adsorption with fixed bed adsorption was also conducted. Column studies showed a higher efficiency of the adsorbent than in the batch mode. The order of the adsorption capacity of ANL from the batch study was found to be 4-NP > 4-CP > P. In all cases the experimental data showed a good fit with the Freundlich equation. The calculated adsorption capacity proved the feasibility of using ANL as a potential adsorbent for the treatment of water containing 4-NP, 4-CP, and P.