Hydrogels are hydrophilic 3D networks that are able to ingest large amounts of water or biological fluids, and are potential candidates for biosensors, drug delivery vectors, energy harvester devices, and carriers or matrices for cells in tissue engineering. Natural polymers, e.g., cellulose, chitosan and starch, have excellent properties that afford fabrication of advanced hydrogel materials for biomedical applications: biodegradability, biocompatibility, non-toxicity, hydrophilicity, thermal and chemical stability, and the high capacity for swelling induced by facile synthetic modification, among other physicochemical properties. Hydrogels require variable time to reach an equilibrium swelling due to the variable diffusion rates of water sorption, capillary action, and other modalities. In this study, the nature, transport kinetics, and the role of water in the formation and structural stability of various types of hydrogels comprised of natural polymers are reviewed. Since water is an integral part of hydrogels that constitute a substantive portion of its composition, there is a need to obtain an improved understanding of the role of hydration in the structure, degree of swelling and the mechanical stability of such biomaterial hydrogels. The capacity of the polymer chains to swell in an aqueous solvent can be expressed by the rubber elasticity theory and other thermodynamic contributions; whereas the rate of water diffusion can be driven either by concentration gradient or chemical potential. An overview of fabrication strategies for various types of hydrogels is presented as well as their responsiveness to external stimuli, along with their potential utility in diverse and novel applications. This review aims to shed light on the role of hydration to the structure and function of hydrogels. In turn, this review will further contribute to the development of advanced materials, such as “injectable hydrogels” and super-adsorbents for applications in the field of environmental science and biomedicine.

Sulphuric acid (H₂SO₄) was utilized as an activator for the preparation of activated carbon from a biomass solid waste, coconut leaves. The sulphuric acid-treated activated carbon (SAC) was used as low-cost adsorbent for the removal of methylene blue (MB) cationic dye from aqueous solution. Adsorption of MB using SAC from aqueous solution under equilibrium and kinetic conditions in batch mode was evaluated by varying adsorbent dose (0.2–2.5 g/L), solution pH (3–11), initial dye concentration (30–400 mg/L), contact time (0–180 min) and temperature (303–323 K). The Langmuir isotherm model showed better fit to the equilibrium data than the Freundlich model. The adsorption capacity (q_m) of SAC increased with temperature where q_m varied as follows: 126.9 (303 K), 137.0 (313 K) and 149.3 mg/g (323 K). The kinetic uptake results were well described by the pseudo-second-order (PSO) kinetic for each temperatures. The thermodynamic adsorption parameters (∆G°, ∆H° and ∆S°) were driven by favourable entropic factors, in accordance with the low activation energy (29.70 kJ/mol) of the system. This study reveals that SAC is an effective and low-cost adsorbent for the removal of MB from aqueous solution.

Graphene oxide (GO) was cross-linked with chitosan to yield a composite (GO-LCTS) with variable morphology, enhanced surface area, and notably high methylene blue (MB) adsorption capacity. The materials were structurally characterized using thermogravimetric analysis and spectroscopic methods (X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and ¹³C solid-state NMR) to support that cross-linking occurs between the amine groups of chitosan and the −COOH groups of GO. Equilibrium swelling studies provide support for the enhanced structural stability of GO-cross-linked materials over the synthetic precursors. Scanning electron microscopy studies reveal the enhanced surface area and variable morphology of the cross-linked GO materials, along with equilibrium and kinetic uptake results with MB dye in aqueous media, revealing greater uptake of GO-LCTS composites over pristine GO. The monolayer uptake capacity (Q_m; mg g^–1) with MB reveals twofold variation for Q_m, where GO-LCTS (402.6 mg g^–1) > GO (286.9 mg g^–1). The kinetic uptake profiles of MB follow a pseudo-second-order trend, where the GO composite shows more rapid uptake over GO. This study reveals that the sorption properties of GO are markedly improved upon formation of a GO–chitosan composite. The facile cross-linking strategy of GO reveals that its physicochemical properties are tunable and versatile for a wider field of application for contaminant removal, especially over multiple adsorption–desorption cycles when compared against pristine GO in its highly dispersed nanoparticle form.