Efficient removal of anionic Amaranth azo dye from the aqueous environment using papaya leaf stalks: Studies on batch adsorption, mechanism, and desorption

Papaya leaf stalks (PLS) were utilized in the current research to eliminate Amaranth (AM) dye from an aqueous medium. PLS was characterized through various techniques, including BET/BJH, pH(PZC), XRD, FTIR, FESEM, and EDX. The FTIR analysis unveiled that -OH (hydroxyl), -C=O (carbonyl), and -COOH (carboxylic) groups were involved in binding AM to the surface of PLS. Batch mode adsorption tests were conducted, and various operational variables like pH, the mass of PLS, AM dye concentration, contact duration, stirring speed, temperature, and regeneration of the PLS were investigated. The maximal AM removal efficiency of 91.1 +/- 2.601% was observed at pH 2.0. The pH(PZC) of the PLS was determined to be 5.5. The experiment results were analyzed using isotherm models (Freundlich, Temkin and Langmuir) and kinetic models (pseudo-second-order and pseudo-first-order). The pseudo-second-order kinetic and Langmuir isotherm models provided the best fit. The maximal sorption uptake of AM dye on PLS was achieved at 121.3 +/- 1.493 mg/g at 298 K. The thermodynamic variables indicated that the sorption process was spontaneous (Delta G(o) < 0), exothermic (Delta H-o > 0), and feasible (Delta S-o > 0) at different temperatures. The AM adsorption on PLS was mainly due to electrostatic interactions. Desorption tests were also carried out to investigate the feasibility of regenerating PLS, and 89.2 +/- 1.184% of the adsorbed AM was recovered using 0.2 M-NaOH. The reusability results indicated that 76.9 +/- 2.204% of the adsorption efficiency could still be maintained even after six cycles. These results demonstrate that PLS is an environmentally friendly and suitable material for removing AM from wastewater.