Modification of corncob with citric acid to enhance its capacity for adsorbing cadmium(II) from water solution

► Corncob was modified with citric acid to enhance its Cd(II) adsorption capacity. ► Adsorption capacity of modified corncob was 26.5 times greater than that of natural corncob. ► Carboxylic sites played a key role in the adsorption of Cd(II) on modified corncob. ► Adsorption capacity increased considerably by raising the solution pH. ► Cd(II) was mainly adsorbed on chemically modified corncob by ion exchange. The effect of the concentration of the citric acid (CA) on the Cd(II) adsorption capacity of corncob was investigated in this work. The concentrations of acidic sites of the natural corncob (NC) and chemically modified corncobs (OXCs) were determined by acid–base titration, and the acidic sites were mainly carboxylic and hydroxylic sites. The functional groups on the surface of the NC and OXCs were identified by infrared spectroscopy. The surface charge of the NC and OXCs was also evaluated by a titration method and the pH of points of zero charge (pH PZC) of NC and OXCs ranged from 2.4 to 5.0. The adsorption capacity of OXCs presented a maximum when the concentration of CA was 1.0 mol/L, this corncob was designated as OX1.0. The adsorption capacity of OXCs was linearly proportional to the concentrations of carboxylic sites. Hence, the carboxylic sites played a very important role on the adsorption capacity of OXCs. The adsorption capacity of OX1.0 was augmented by raising the solution pH from 3 to 7. This trend was attributed to the electrostatic interactions between the OX1.0 and the Cd 2+ cations in the water solution. The adsorption capacity of corncob can be considerably enhanced by chemical modification with CA.