Multistage optimization of the adsorption of methylene blue dye onto defatted Carica papaya seeds

Carica papaya seeds were defatted and used for the adsorption of methylene blue dye (MB dye). The pHPZC and specific surface area (SSA) of the defatted and undefatted Carica papaya seeds were found to be 6.25 and 143.27 m2 g−1 respectively. The adsorption capacities of the defatted Carica papaya seeds were 1250 and 769.23 mg g−1. Fourier transformed infrared (FT-IR) spectra analysis of defatted seeds suggests the presence of α,β-unsaturated ketone, β-keto (enolic) esters and lactones, quinones and carboxylic acids which were absent in the undefatted sample. The presence of carboxylic acid, phenolic and lactone functional groups were confirmed by surface chemistry studies. Adsorption of MB dye onto DPS adsorbent was found to be exothermic and spontaneous. FT-IR spectra of various particle sizes indicate the presence of functional groups on every particle size necessary for the adsorption of MB dye.Pseudo-second-order kinetic rate constant increased with increasing initial MB dye concentration. The adsorption reaction was observed to be very fast perhaps because of the presence of multiple functional groups with some of them having lone pair of electrons. Increasing pH was observed to have very little positive effect on the adsorption of MB dye on DPS adsorbent. Increasing particle size decreased the adsorption capacity of DPS adsorbent for MB dye. It is suggested that the mechanism for the adsorption of MB dye onto DPS adsorbent might not only be solely controlled by film diffusion but also by pore diffusion.Mathematical models for the optimization of adsorbent dose, number of adsorption stages and minimum contact time were developed. The optimum conditions for the adsorption of 99% of 100 mg L−1 MB dye from 10 m3 of aqueous solution of the dye were five adsorption stages, ≈21 kg for each stage with a total minimum contact time of ≈71 min.