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ABSTRACT
Chromium(VI) was removed from aqueous solution using sulfuric- and phosphoric-acid-activated Strychnine tree fruit shells (SSTFS and PSTFS) as biosorbents. Effects of various parameters such as adsorbent dose (0.02–0.1 g/L), temperature (303–333 K), agitation speed, solution pH (2–9), contact time, and initial Cr(VI) concentration (50–250 mg/L) were studied for a batch adsorption system. The optimum pH range for Cr(VI) adsorption was determined as 2. Equilibrium adsorption data were analyzed with isotherm models and the Langmuir and Freundlich models got best fitted values for SSTFS (R2 value – 0.994) and PSTFS (R2 value – 0.996), respectively. The maximum adsorption capacities of SSTFS and PSTFS were 100 and 142.85 mg/g, respectively. The biosorption process was well explained by pseudo-second-order kinetic model with higher R2 value (SSTFS – 0.996, PSTFS – 0.990) for both biosorbents. Characterization of biosorbents was done using Fourier transform infrared spectroscopy, scanning electron microscopy, elemental analysis, energy-dispersive X-ray analysis, and thermogravimetric analysis. Thermodynamic studies revealed the spontaneous, endothermic, and randomness in nature of the Cr(VI) adsorption process. Different concentrations of NaOH solutions were used to perform the desorption studies. The results demonstrated that both SSTFS and PSTFS can be used as an effective and low-cost biosorbent for removal of Cr(VI) from aqueous solutions.
Nomenclature
| Q0 | = | Monolayer coverage capacity (mg/g) |
| bL | = | Langmuir isotherm constant (L/mg) |
| RL | = | Separation factor |
| C0 | = | Initial Cr(VI) concentration (mg/L) |
| Ce | = | Cr(VI) concentration in solution at equilibrium (mg/L) |
| 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) |
| m | = | Amount of biosorbent (g) |
| Kf | = | Freundlich isotherm constant (mg/g)/(mg/L)1/n |
| n | = | Adsorption intensity |
| K | = | Coefficient related to the mean free energy of adsorption (mol2/kJ2) |
| E | = | Mean adsorption energy (kJ/mol) |
| Qm | = | Maximum adsorption capacity (mg/g) in the D–R model |
| qmj | = | Maximum adsorption capacity in Jovanovic model (mg/g) |
| Kj | = | Jovanovic isotherm constant (L/g) |
| AT | = | Temkin isotherm equilibrium binding constant (L/mg) |
| bT | = | Temkin isotherm constant (J/mol) |
| R | = | Atmospheric gas constant (8.314 J/mol K) |
| V | = | Volume of the solution (L) |
| T | = | Temperature (K) |
| ΔG° | = | Gibbs free energy (kJ/mol) |
| ΔH° | = | Enthalpy (kJ/mol) |
| ΔS° | = | Entropy (J/mol K) |
| kc | = | Distribution coefficient |
| R2 | = | Coefficient of determination |
| 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 |
Funding
Many thanks to Kerala State Council for Science, Technology and Environment, India (Grant No. ETP/02/2014/KSCSTE) for their financial support and to the Research Council for Engineering and Technology Programmes, to pursue this study.