Azadirachta indica leaf powder as an effective biosorbent for dyes: a case study with aqueous Congo Red solutions

In the present work, the leaves of Azadirachta indica (locally known as the Neem tree) in the form of a powder were investigated as a biosorbent of dyes taking aqueous Congo Red solution as a model system. The sorbent was made from mature Neem leaves and was investigated in a batch reactor under variable system parameters such as concentration of the aqueous dye solution, agitation time, adsorbent amount, pH, and temperature. An amount of 0.6 g of the Neem leaf powder (NLP) per litre could remove 52.0–99.0% of the dye from an aqueous solution of concentration 2.87×10−2 mmol l−1 with the agitation time increasing from 60 to 300 min. The interactions were tested with respect to both pseudo first-order and second-order reaction kinetics; the latter was found to be more suitable. Considerable intra-particle diffusion was found to occur simultaneously. The sorption process was in conformity with Langmuir and Freundlich isotherms yielding values of the adsorption coefficients in the following ranges: Freundlich n: 0.12–0.19, Kf: 0.1039–0.2648 L g−1; Langmuir qm: 41.24–128.26 g kg−1, b: 443.3–1898.0 l mmol−1, which supported favourable adsorption. The Langmuir monolayer capacity (qm) was high and the values of the coefficient b indicated the equilibrium, dye+NLP=dye⋯NLP being shifted overwhelmingly towards adsorption. Thermodynamically, the sorption process was exothermic with an average heat of adsorption of −12.75 kJ mol−1. The spontaneity of the sorption process was also confirmed by the favourable values of Gibbs energy (mean values: −1.09 to −1.81 kJ mol−1) and entropy of adsorption (range: −18.97 to −56.32 J mol−1 K−1). The results point to the effectiveness of the Neem leaf powder as a biosorbent for removing dyes like Congo Red from water.