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This study explored the kinetics of metanil yellow (MY) dye biosorption onto the water hyacinth plant (Eichhornia crassipes) and its vegetative organs (leaves, roots, and stems). The water hyacinth's leaves exhibited the highest capacity for and initial volumetric rate of MY biosorption, followed by the entire plant, roots, and stems. Modeling the kinetics for MY biosorption onto the entire plant and vegetative organs showed that the best agreement of experimental data was achieved with the pseudo-second-order kinetic model, suggesting that the rate-determining step in the overall reaction of MY biosorption onto the biosorbents might be chemisorption. Fourier-transform infrared spectroscopy studies suggest that the amide I and II functional groups, which are present in the biosorbent proteins, participated in the biosorption of MY from aqueous solutions. A linear dependence of MY biosorption capacity at equilibrium on total protein content was observed, confirming that MY molecules bind to biosorbent proteins. Scanning electron and confocal laser scanning microscopy studies corroborated the presence of MY on the biosorbents' surface.

In the present work, the kinetics of amaranth dye biosorption onto the roots, stems, leaves and the whole plant of water hyacinth (Eichhornia crassipes) were studied for differential evaluation of the plant's biosorptive potential to remove the toxic dye from aqueous solutions. E. crassipes´ leaves showed the highest level of amaranth dye biosorption (43.1 mg/g), followed by the entire plant (31.18 mg/g), the roots (28.51 mg/g), and finally by the stems (23.97 mg/g). The same differential trend was observed for the initial volumetric rate of amaranth dye biosorption. The kinetics modeling of amaranth dye biosorption by the roots, stems, leaves and entire E. crassipes plant showed good agreement of experimental data with the pseudo-second-order model, which indicates that the rate-limiting step of the biosorption process is the most probably chemisorption. FTIR analysis results suggest that amaranth dye molecules interact with the amide I and amide II functional groups, which are present in the proteins of the vegetative organs and entire aquatic plant. Proximate chemical analysis revealed higher content of total protein in E. crassipes´ leaves than in other vegetative organs. A linear relationship was found between total protein content and amaranth biosorption capacity at equilibrium, which indicates that the proteins play a crucial role in amaranth dye biosorption from aqueous solution by E. crassipes. E. crassipes' leaves may be used as a low-cost, effective and environmentally friendly biosorbent to detoxify amaranth dye-polluted wastewaters.

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