Graphene oxide - polymer nanocomposites for efficient water hardness removal: a step towards healthier drinking water

A new, non-toxic, environmentally friendly nanocomposite, based on graphene oxide and biopolymers was developed as an effective adsorbent for water hardness removal. Graphene oxide was synthesized by a modified Hummers' method, whereas crosslinked carboxymethyl cellulose (CMC) and chitosan were used as biopolymers. Montmorillonite (MMT) was utilized as an additive to enhance the adsorbent's performance. The solubility and adsorption behaviours of the prepared materials were investigated in respect to calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). Among the prepared materials, a film containing graphene oxide (GO), crosslinked carboxymethyl cellulose (CMC), and montmorillonite (MMT), denoted as GO-CMC-MMT-3, exhibited the highest water softening capacity. The successful synthesis of the materials was confirmed through scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and ultraviolet-visible spectroscopy (UV-Vis). The optimal pH for the adsorption process was around 6-7. Kinetic studies showed that second-order kinetic model described the adsorption process. The thermodynamic analysis indicated that the process was endothermic in nature and showed a reduced degree of spontaneity at the given conditions. The Langmuir isotherm model was the best fit, and the adsorption capacities were 6.46 mg g^-1 for Mg²⁺ and 7.98 mg g^-1 for Ca²⁺, which indicated the formation of a monolayer of cations on homogeneous adsorption sites. Further, an investigation on the reusability of gravity filtration was carried out, demonstrating the practical utility of the GO-CMC-MMT-3 membrane in real-life water treatment.