Unravelling the efficiency removal of 2,4-dinitrophenol on coconut shell biomass-derived activated carbons theoretical and experimental investigation

The effectiveness of activated carbons in the adsorptive removal of 2,4-dinitrophenol (DNP) in aqueous solutions was evaluated both theoretically and experimentally after they were made from coconut shells. Coconut shell was subjected to pyrolysis (600 °C) and chemically activated with various solutions containing phosphoric acid (H3PO4); H3PO4+ and iron (II) salt solution; and H3PO4+ and iron (III) salt solution. Physicochemical analysis: iodine number, pH, pHPZC, bulk density, BET, and surface groups of samples obtained were determined. Some aqueous solution parameters, such as pH, contact time, and absorbent quantity, that have an impact on 2,4-DNP adsorption have been studied. The favorable pH for the adsorption process was found to be 2.0–3.0. The maximum adsorption of 8.0, 10.6, and 14.9 mg/g of 2,4-DNP onto activated carbon was observed at pH 3.0 for CAHP, CAHP@FeII, and CAHP@FeIII, respectively, 120 min contact time for all activated carbon samples. After fitting adsorption data to various kinetic models, it was clear that nonlinear Elovich kinetics was better suited to describe adsorption compared to the linear models with the correlation coefficient R2 greater than 0.96. The DFT results further demonstrated that the compounds under investigation were adsorbed by the process of chemical bonding, and the negative values of the interaction energy confirmed this conclusion. Additionally, the negative adsorption energy values imply that all chemicals have significantly adsorbed to the activated carbon surface by physisorption (electrostatic interactions), chemisorption (n-π bonds), and diffusion into the pores. The exothermic and spontaneous nature of 2,4-DNP adsorption was found, as demonstrated by the increase in adsorption energies and the proximity of the molecules being studied to the activated carbon surface.