Biosorption of Malachite Green dye by the brown alga Dictyota cervicornis: Kinetics and isotherm study

Malachite Green (MG) dye was removed from aqueous solutions by the brown alga Dictyota cervicornis. The effects of initial solution pH (3, 4, 5, 6, 7, 9 and 11), contact time, initial dye concentration (5, 25, 50, 75, 100 and 125 mg l−1) and biomass concentration (0.2, 0.6, 1.0 and 1.4 g l−1) on MG adsorption were studied. The highest adsorption capacity was observed at pH 9. Biosorption kinetics were described by the pseudo‐second order model. The Langmuir model predicted the maximum adsorption capacity occurring at a relatively high value of 230 mg g−1. The chemical modification of functional groups, potentiometric titration, Fourier Transform–infrared (FTIR) analysis of the biomass before and after biosorption, and ICP analysis of the solution after MG adsorption, were also performed. The quantities of strong, weak and very weak acidic sites obtained by potentiometric titration were 1.23, 0.88 and 0.29 meq g−1 of dry biomass, respectively, with a total acidity equal to 2.4 meq g−1. FTIR spectra of the pristine biomass showed the presence of carboxylic, sulphonic, hydroxyl and amine functional groups on the biomass structure. There were no obvious differences in the spectra of the biomass before and after MG adsorption. Chemical modification of the functional groups did not affect biosorption. Therefore, MG was removed via a physical mechanism. K(I), Ca(II), Na(I) and Mg(II) ions were released into the solution upon MG adsorption.