${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$ Adsorption Mechanism by Coconut Husk Powder with and without Amine Modification

The adsorbent derived from the processing of green coconut husk (powder) was chemically treated with a primary amine to investigate its adsorption potential for metal ions, ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$, present in an aqueous solution. The treatment of the adsorbent was carried out using monoethanolamine impregnation with a mass ratio of $1:1$. The adsorbent was characterized before and after treatment as well as after the monocomponent adsorption ions ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$ by Fourier transform infrared (FTIR) spectroscopy, thermogravimetry/differential thermal analyzer (TGA/DTA), specific surface area, volume, and pore size. The kinetic models’ pseudo-first-order, pseudo-second-order, and intraparticle diffusion were tested. The adsorption rate did not change after treatment; however, a greater adsorption capacity was observed for the treated adsorbent, i.e., the amine compounds in coconut husk powder favored its adsorption capacity. The interaction mechanism between the functional groups of the adsorbent and ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$ ions was investigated by adjusting the experimental data from the adsorption equilibrium studied to other theoretical models. Through the thermodynamic study, it was observed that the monocomponent adsorption of ${\mathrm{Cu}}^{2+}$ by the coconut husk powder treated with amine is endothermic and has a chemical mechanism, while the untreated material is exothermic and has a physical mechanism. ${\mathrm{Cd}}^{2+}$ adsorption, in turn, has a physical mechanism for both coconut shell powder samples (CP and CPA). The chemical treatment by impregnation of monoethanolamine improves adsorption capacity of coconut husk powder through the interaction between the functional groups present in the adsorbent and the metal ions ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cd}}^{2+}$.