Fixed-bed column studies for the elimination of Cd2+ ions by native and protonated watermelon rind

Watermelon rind (WR) an agro-waste was protonated aiming to eliminate the alkali and alkaline metal ions and investigated towards adsorption of Cd2+ ions in a continuous process. The protonation step resulted in the elimination of K+ and Mg2+ ions from the watermelon rind surface, and the protonation was confirmed with FTIR and EDX techniques. Comparative investigations were carried out between native and protonated WR in a continuous process, and parameters such as rate of flow, feed concentration and bed heights were optimised. At higher bed depths and lower rates of flow and initial feed concentrations, the columns provided maximum adsorption of Cd2+ ions. Optimal conditions in this study were found to be 1 ml, 3 cm and 50 mg L−1, respectively, for flow rate, bed depths and initial concentrations. The adsorption capacity of native and protonated WR was experimentally calculated to be 97.9 and 107.3 mg g−1, respectively. The PWR exhibited higher adsorption capacity compared to WR, and this is due to the protonation step, which eliminated the alkali and alkaline metal ions bound onto the WR surface that blocked the active sites. The data obtained in the continuous columns were fitted to mathematical models such as Adams–Bohart, Thomas and Yoon–Nelson models, and the later models were able to explain the adsorption process well. Lastly, regeneration of the WR and PWR was investigated, and 0.1 M HCl exhibited higher desorption and regeneration efficiency compared to 0.1 M acetic acid. These results suggest that the protonation of watermelon rind results in enhanced adsorption of Cd ions, and both native and protonated are effective adsorbents.