Characterization and application of alkali-soluble polysaccharide of Carica papaya seeds for removal of indigo carmine and Congo red dyes from single and binary solutions

Adsorption of Congo red (CR) and indigo carmine (IC) dyes by alkali-soluble polysaccharide (ASP) extracted from Carica papaya seeds was studied in single as well as in binary mixtures. The adsorption study was performed by varying operating variables such as pH, initial dye concentration, adsorbent dosage, contact time and temperature. Fourier-transform infrared, X-ray diffraction, elemental analysis and scanning electron microscopy images were used for characterization of ASP and dye-loaded ASP. Kinetic results fitted well the pseudo-second order and Elovich models. The Langmuir-Freundlich isotherm shows the best fit to the experimental data of both L-shape in single component whereas, the partial competitive Langmuir isotherm gives a best adjustment of binary experimental data. The maximum adsorption capacities of CR and IC in single/binary mixtures at 30 degrees C were 319.08/635.99 mg/g and 168.51/102.65 mg/g respectively. The thermodynamic study showed that the adsorption process is thermodynamically favorable, spontaneous, with exothermic process for IC and endothermic one for CR. The adsorption mechanism behaviors were dominated by the electrostatic interactions between adsorbent surface functional groups and dyes molecules. The desorption study showed that ethylene-diaminetetraacetate ion (EDTA) was the best eluant for both dyes recovery. A good removal efficiency obtained during the five executed cycles suggests a good reusability of ASP.