Feasibility of hard acid-base affinity for the pronounced adsorption capacity of manganese(II) using amino-functionalized graphene oxide

The present study reveals the feasibility of using graphene oxide (GO) functionalized with 3-mercaptopropyl-trimethoxysilane (APTMS) for the removal of Mn(II) from aqueous solution. The APTMS bound on GO's surface was successfully confirmed by FTIR and EDX. For an optimal adsorption study, the effects of pH (pH 1-6), incubation time (5-80 min), temperature (30-60 degrees C) and initial concentration of Mn(II) (0.1-60 mg L-1) were investigated in association with Mn measurement by AAS. The optimum conditions for Mn(II) removal included a 20 mg L-1 initial concentration with 0.02 mg adsorbent at pH 5.5, and complete adsorption equilibrium was reached within 50 min. Their adsorption models are well described by both Langmuir and Freundlich isotherms. The maximum adsorption capacity of GO-NH2 for Mn(II) was 161.29 mg g(-1), about 10 and 4 times higher than those of GP (16.45 mg g(-1)) and GO (41.67 mg g(-1)) according to their relevant functional groups. For the GO-NH2 adsorbent, the adsorption process of Mn(II) follows a pseudo-second-order kinetics model, indicating that the overall rate of Mn(II) uptake is controlled by external mass transfer at the initial stage of the adsorption. Later, the driving forces that control the adsorption rate are attributable to an intra-particle diffusion. A thermodynamic adsorption process reveals mainly an exothermic spontaneous reaction. Therefore, the present study provides an excellent adsorbent for Mn(II) from aqueous solution. This adsorbent can also potentially be used for wastewater treatment.