Simultaneous removal of Pb2+ and methylene blue from aqueous solution by a new carboxylic acid functionalized walnut shell: Optimization by multivariate method

In the present work, we explore the modification of walnut shell using a solvent-free reaction, in which walnut is mixed with malonic acid cyclic isopropylidene ester, generating carboxylic acid-functionalized walnut shell. Using different analysis techniques, the presence of carboxylic acid groups on the adsorbent surface is confirmed. The efficiency of the modified adsorbent for the simultaneous removal of Pb2+ and methylene blue (MB) is studied. Optimization of the experimental parameters involved is carried out using the Box-Behnken design. Under the optimum experimental conditions, high removal percentages of 97.14% and 92.61% were acquired for Pb2+ and MB, respectively. Investigations made on the FT-IR spectra show that electrostatic forces play a major role in the adsorption of Pb2+, while pi-pi interactions and hydrogen bonding play more important roles in the adsorption of MB on the proposed adsorbent.