Tri-Carboxylic Acid Red Onion (Allium cepa) Skin Extract Resin for the Removal of Chromium (VI) Ion from Aqueous Solution

This study investigates the removal of chromium (VI) ions from aqueous solution using resin synthesized by modifying red onion skin extract with a tri-carboxylic acid (citric acid). The performance of citric acid red onion skin extract resin (CRER) in the removal process was examined based on the effect of temperature (302 to 343 K), agitation time (10 to 60 min) and initial ion concentration (2 to 100 mg/L). Data analysis from agitation time experiments showed that equilibrium adsorption was attained within 30 min. Maximum percentage adsorption was observed at 313 K, while the percentage ion adsorbed increased with increase in initial concentrations of Cr (VI) ions. The experimental data were subjected to Langmuir, Freundlich and Temkin isotherm models evaluation; and the deductions from the correlation coefficient (R2) of the plots showed that Freundlich isotherm model represented the best fitting for the batch experiment. Thermodynamic parameters were determined and the results indicated that the sorption process was spontaneous with ΔG values as -9.433, -8.811, -7.745, -5.400 and -3.395 kJ/mol for 302, 313, 323, 333 and 343 K respectively. Adsorption with CRER was exothermic with respect to its negative ΔH (-55.610 KJ/ mol) value, while decrease in disorderliness of the adsorption system was reflected by the negative ΔS (-150.816 J/K/ mol) value. Pseudo-second order kinetic process was resolved for the reaction rate investigations, with the values of activation energy and pre-exponential factor as -118.167 kJ/mol and 5.749 × 10-17 g/mol/min respectively. Dubinin- Radushkevich isotherm model was used to evaluate the sorption mechanism, and it was observed that physisorption is the dominant mechanism for the reaction with the E values ranging from 6.51 to 6.95 kJ/mol at the different experimental temperatures.