Kinetic study and sorption mechanism of methylene blue and methyl violet onto mansonia (Mansonia altissima) wood sawdust

Batch sorption kinetic experiments were performed to study the adsorption of methylene blue and methyl violet dyes from aqueous solution using mansonia wood sawdust as adsorbent. Operating variables studied include solution pH and particle size of sawdust. Maximum dye removal was observed at a basic pH 10 for both dyes. Sorption kinetic data was fitted to both pseudo-first and pseudo-second-order kinetic models. The experimental data were found to deviate from the straight line of the pseudo-first-order plots after 30 min for the methylene blue and after 15 min for the methyl violet sorption. The pseudo-second-order kinetics was found to fit the experimental data and was able to consistently predict the amount of dye adsorbed over the sorption period and for the range of sawdust particle size used.The process mechanism was found to be complex, consisting of both mass transfer and intraparticle diffusion. Analysis of the sorption data using the Boyd's plot confirms mass transfer as the dominant mechanism for both methylene blue and methyl violet sorption. Mass transfer mechanism was more predominant in the methyl violet sorption and became more dominant as the particle size of the sawdust material increased. This was confirmed by the values of effective diffusion parameter Di, which were found to be higher for methylene blue, and reduced with increasing sawdust particle size. The molecular sizes of the basic dyes and the stability of the positive charge on the dyes played a significant role in their sorption onto the sawdust material.