Insights into enhanced adsorptive removal of Rhodamine B by different chemically modified garlic peels: Comparison, kinetics, isotherms, thermodynamics and mechanism

Abstract

Garlic peel, a typical agricultural by-product, was modified by three chemical modification methods, including oxidation with H2O2, acid oxidation with HNO3/C6H8O7, and alkaline saponification with NaOH, respectively. Unmodified garlic peel (unmodified-GP) and as-synthesized chemically modified garlic peels (H2O2-GP, HNO3-GP, CA-GP and NaOH-GP) were fully characterized and evaluated as biosorbents for the removal of Rhodamine B (RhB). Results showed that acid oxidation could lead to the increase of surface area and acidic oxygen groups, which were beneficial for the adsorption for toxic dyes. Notably, HNO3-GP turned out to be most efficient among the five studied garlic peels, the adsorption capacity of which was 3.5 times of unmodified-GP when initial RhB concentration was 15 mg L−1. Adsorption kinetics, isotherms and thermodynamics of HNO3-GP were investigated to gain insights into the adsorption behaviors. In addition, the adsorption of RhB was pH dependent, and 95.8 ± 1.1% of the adsorbed RhB could be eluted by 0.1 mol L−1 NaOH. Adsorption-desorption studies showed HNO3-GP was reusable. Moreover, on the basis of experimental data and analysis of FT-IR and XPS of HNO3-GP before and after adsorption, electrostatic interaction and hydrogen bonding played an important role during the adsorption process. This work systematically studied the effect of oxidation, acid oxidation and alkaline saponification as modification methods, and demonstrated acid oxidation with strong acid was an effective method to improve adsorption capacities of agricultural by-products for toxic dyes.