Kinetic and equilibrium studies of methylene blue biosorption by Posidonia oceanica (L.) fibres

Abstract

Batch biosorption experiments were carried out for the removal of methylene blue, a basic dye, from aqueous solution using raw Posidonia oceanica (L.) fibres, a marine lignocellulosic biomass. A series of assays were undertaken to assess the effect of the system variables, i.e. contact time, solution pH, biosorbent dosage and initial dye concentration. The results had showed that biosorption capacity was optimal using 6–9 solution pH range and by increasing the biosorbent concentration up to 1 g/L. The biosorption kinetics were analyzed using irreversible-first-order, reversible-first-order and pseudo-second-order and the sorption data were very well described by the pseudo-second-order model for the entire adsorption time with squared correlation coefficients equal to unity for all experimented initial dye concentrations. Besides, equilibrium data were very well represented by both Langmuir and Redlich–Peterson isotherm models followed by Freundlich, which confirm the monolayer coverage of methylene blue molecules onto P. oceanica fibres.

Introduction

Many industries often use synthetic dyes to colour their products including textile dyeing and colouring paper and wool. Over 7 × 10⁵ tons of these dyes are produced annually worldwide. It is estimated that 10–15% of these chemical compounds are discharged into waste streams by the textile industry. Some dyestuffs, among them methylene blue, are not strongly hazardous, but an acute exposure could make them harmful to fish and other aquatic organisms [1]. Furthermore, their presence in aquatic systems, even at low concentrations, is highly visible, reduces light penetration and has a derogatory effect on photosynthesis [2]. Therefore, decolourisation of dye-containing effluent is becoming an obligation both environmentally and for water re-use.

Previously, several research works had been performed to search for efficient and low-cost materials to remove methylene blue and other basic dyes from aqueous solution, including rice husk [3], beech sawdust [4], agro-industry wastes [5] and activated carbon from date pits [6].

The main focus of this study was to evaluate the biosorption aptitude of a novel, low cost, and renewable biomass, Posidonia oceanica fibres for the removal of methylene blue as a model compound for basic dyes. The effects of pH, contact time, initial dye concentration and biomass dosage on the biosorption capacity were investigated. Moreover, kinetic and equilibrium models were used to fit experimental data.