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

摘要

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 X 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, K_f: 0.1039-0.2648 L g~(-1); Langmuir q_m: 41.24-128.26 g kg~(-1), b : 443.3-1898.0 1 mmol~(-1), which supported favourable adsorption. The Langmuir monolayer capacity (q_m) 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.