Dried prickly pear cactus (Opuntia ficus indica) cladodes as a low-cost and eco-friendly biosorbent for dyes removal from aqueous solutions

The biosorption of Methylene Blue (MB), Eriochrome Black T (EBT) and Alizarin S (AS) from aqueous solutions by dried prickly pear cactus cladodes as a low-cost, natural and eco-friendly biosorbent was investigated. The study was carried out under various parameters, such as average biosorbent particle size, pH, biosorbent dosage, contact time, initial dye concentration and temperature. The experimental results show that, the percentage of biosorption increases with an increase in the biosorbent dosage, and the decrease of particle size. The biosorption was pH dependent with a high biosorption of cationic dye (MB) in basic range and a high biosorption of anionic dyes (EBT and AS) in acidic range. The equilibrium uptake was increased with an increase in the initial dye concentration in solution. Biosorption kinetic data were properly fitted with the pseudo-second-order kinetic model. The experimental isotherm data were analyzed using Langmuir, Freundlich, Redlich-Peterson, Temkin and Toth isotherm equations. The best fit was obtained by the Redlich-Peterson model and the Langmuir model with a Langmuir maximum monolayer biosorption capacity of 189.83 mg/g for Methylene Blue, 200.22 mg/g for Eriochrome Black T and 118.35 mg/g for Alizarin S. The biosorption was exothermic in nature (Delta H degrees = -31.035 kJ/mol for MB, -10.25 kJ/mol for EBT and -11.69 kJ/mol for AS). The reaction was accompanied by a decrease in entropy (Delta S degrees = -94.76 J/K mol for MB, -38.44 J/K mol for EBT and -41.93 J/K mol for AS). The Gibbs energy (Delta G degrees) increased when the temperature was increased from 25 to 60 degrees C indicating a decrease in feasibility of biosorption at higher temperatures. (C) 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.