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ABSTRACT
In this study, batch removal of hexavalent chromium from aqueous solutions by powdered Colocasia esculenta leaves was investigated. Batch experiments were conducted to study the effects of adsorption of Cr(VI) at different pH values, initial concentrations, agitation speeds, temperatures, and contact times. The biosorbent was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectrometer analysis. The biosorptive capacity of the adsorbent was dependent on the pH of the chromium solution in which maximum removal was observed at pH 2. The adsorption equilibrium data were evaluated for various adsorption isotherm models, kinetic models, and thermodynamics. The equilibrium data fitted well with Freundlich and Halsey models. The adsorption capacity calculated was 47.62 mg/g at pH 2. The adsorption kinetic data were best described by pseudo-second-order kinetic model. Thus, Colocasia esculenta leaves can be considered as one of the efficient and cheap biosorbents for hexavalent chromium removal from aqueous solutions.
Acknowledgments
All the authors heartfully thank the Editor and anonymous reviewers for their valuable comments and suggestions which improved the quality of the paper.
List of symbols
| qe | = | Amount of Cr(VI) ions adsorbed per unit mass of biosorbent (mg/g) |
| q | = | Amount of Cr(VI) adsorbed at any time t(mg/g) |
| R | = | Atmospheric gas constant (8.314 J/mol K) |
| RL | = | Separation factor |
| V | = | Volume of the solution (L) |
| T | = | Temperature (K) |
| Co | = | Initial Cr(VI) concentration (mg/L) |
| Ce | = | Cr(VI) concentration in solution at equilibrium (mg/L) |
| m | = | Amount of biosorbent (g) |
| n | = | Adsorption intensity |
| ΔG° | = | Gibbs free energy (kJ/mol) |
| ΔH° | = | Enthalpy (kJ/mol) |
| ΔS° | = | Entropy (J/mol K) |
| kc | = | Distribution coefficient |
| R2 | = | Coefficient of determination |
| ϵ | = | Polanyi potential |
| k1 | = | Pseudo-first-order constant (min) |
| k2 | = | Pseudo-second-order constant (g/mg/min) |
| kid | = | Intra-particle diffusion rate constant (mg/g/min1/2) |
| C | = | Intercept of intra-particle diffusion model |
| AT | = | Temkin isotherm equilibrium binding constant (L/mg) |
| bT | = | Temkin isotherm constant (J/mol) |
| bL | = | Langmuir isotherm constant (L/mg) |
| E | = | Mean adsorption energy (kJ/mol) |
| Kf | = | Freundlich isotherm constant (mg/g)/(mg/L)1/n |
| K | = | Coefficient related to the mean free energy of adsorption (mol2/kJ2) |
| KE | = | Elovich equilibrium constant (L/mg) |
| Kj | = | Jovanovic isotherm constant (L/g) |
| Q0 | = | Monolayer coverage capacity(mg/g) |
| Qm | = | Maximum adsorption capacity (mg/g) in D-R model |
| qm | = | Elovich maximum adsorption capacity (mg/g) |
| qmj | = | Maximum adsorption capacity in Jovanovic model (mg/g) |
| KH | = | Halsey isotherm model constant |
| nH | = | Halsey isotherm model exponent |
| KFH | = | Flory–Huggins equilibrium constant |
| nFH | = | Flory–Huggins model exponent |
| Q | = | Degree of surface coverage |