Synthesis of potential adsorbent for removal of malachite green dye using alginate hydrogel nanocomposites

Hydrogels are highly porous, hydrophilic, insoluble, 3D networks with a large capacity for water absorption. The goal of this research was to formulate sodium alginate/silica (SA/SiO₂) hydrogel and hydrogel nanocomposite (SA/SiO₂/ZnO-NPs) by impregnating the ZnO-NPs and cross-linking was furnished with siloxane network making use of the sol-gel method. The synthesized hydrogel/hydrogel nanocomposite was analyzed with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Zeta-sizer, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermo-gravimetric analyzer (TGA). Using the batch adsorption method, the hydrogel/hydrogel nanocomposite was examined for the ability to adsorb malachite green (MG) dye from aqueous media under different conditions like adsorbent dosage, contact time, pH and temperature. MG's maximum removal (97.31 %) was achieved by SA/SiO₂/ZnO-NPs adsorbent at pH = 8; the temperature was recorded as 333 K, utilizing 25 min at a dose level of 0.09 g. Langmuir and Temkin's models were utilized to assess the adsorption mechanism, and the maximum adsorption capacity (q_max) of 227.27 mg/g for SA/SiO₂ hydrogel and 322.58 mg/g for SA/SiO₂/ZnO-NPs hydrogel nanocomposite was obtained. At pH 8, the optimal adsorption was taken place in 25 min. The pseudo-second-order kinetic model deals with the adsorption process, and thermodynamic data reveals the endothermic and spontaneous nature of the adsorption process. The presence of -COOH groups in the synthesized hydrogel/hydrogel nanocomposite improved the cationic dye affinity towards hydrogel/hydrogel nanocomposite through H-bonding and electrostatic interactions. Thus, SA/SiO₂ hydrogel and SA/SiO₂/ZnO hydrogel nanocomposite could be efficient and promising adsorbents to deal with organic dye pollutants for a sustainable environment. Moreover, addressing the limitations such as SA and ZnO exhibit sensitivity to alterations in pH which could potentially influence the performance in practical scenarios where pH regulation is not maintained.