Waste Litchi Peels for Cr(VI) Removal from Synthetic Wastewater in Batch and Continuous Systems: Sorbent Characterization, Regeneration and Reuse Study

Cr(VI) removal was investigated under batch and continuous mode using dry Litchi chinensis as the low-cost sorbent. Batch biosorption experiments displayed a maximum Cr(VI) removal of 99% with litchi peels, suggesting that the litchi peels can be utilized as a prime sorbent for removal of Cr(VI). Fourier transform infrared spectroscopy (FTIR) profiles confirmed the occurrence of NH, CH, CH3, CH2, and CO functional groups in the litchi peels for the biosorption of Cr(VI). Cr(VI) sorption thermodynamics by litchi peels indicated that the reaction is endothermic and spontaneous in nature. The effect of various co-ions on Cr(VI) biosorption in the batch experiments revealed a significant effect due to Fe2+(p=0.021) and PO43- (p=0.043) ions on biosorption of Cr(VI). By fitting the experimental data on Cr(VI) biosorption to different sorption isotherm models, a very high coefficient of determination (R2) and very high biosorption capacity values were obtained for the Freundlich model. Cr(VI) sorption kinetic followed the pseudo-first-order kinetics with a R2 value of 0.914. The breakthrough curves from the continuous flow-through column study for Cr(VI) removal demonstrated that litchi peels in the fixed-bed column are suitable for decreasing the concentration of Cr(VI) from 10mg/L to a value below the recommended detection limit of 0.02mg/L. Furthermore, the sorbent regeneration followed by Cr(VI) biosorption demonstrated that litchi peels could be reused for at least two consecutive cycles without any significant change in the Cr(VI) biosorption capacity.