Carica Papaya Seed as a Biosorbent for Removal of Cr (VI) and Ni (II) Ions from Aqueous Solution: Thermodynamics and Kinetic Analysis of Experimental Data

Abstract

Abstract The use of Carica papaya seeds as an adsorbent for decontamination of toxic metal ions like Ni (II) and Cr (VI) in solution was investigated. The maximum biosorption capacities of Cr (VI) and Ni (II) were observed at pH 2 and pH 6 respectively. The experimental isotherm data were analyzed using the Langmuir, Freundlich, Dubinin–Radushkevich and Temkin equations. In order to determine the best fit isotherm, five error analysis methods were used, namely the sum-of-squared errors (SSE), the composite functional error (HYBRID), the Marquardt’s percent standard deviation (MPSD), the average relative error (ARE) and the sum of absolute error (EABS). The error values demonstrated that the Langmuir isotherm model provided the best fit to the experimental data for Cr (VI) and the Freundlich model for Ni (II). Adsorption kinetics data were tested using pseudo-first-order and pseudo-second-order models. Kinetic studies showed that the adsorption followed a pseudo-second-order model. Thermodynamic parameters such as changes of standard free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were determined. The maximum metal uptake of 5.85 mgg–1 for Cr (VI) and 5.58 mgg–1 for Ni (II) from Langmuir model demonstrates the ability of Carica papaya seed to absorb heavy metals. Fourier transform infrared spectroscopy analysis was used to evaluate the binding characteristics of Carica papaya seed with metals.