In this study, we synthesized a hydrogel from the extract of gracillaria (GR), poly- (vinyl alcohol) (PVA), glutaraldehyde (GA), and carbon nanotube (CNT). To confirm the synthesis of hydrogel, several characterizations were done, including fourier transform infra red (FTIR) and scanning electron microscope (SEM). Analysis on the material performance under various parameters (such as swelling ratio and release behavior of potassium from hydrogel into aqueous media) was also carefully evaluated based on weight measurement and flame-photometry instrumentation. FTIR spectra showed that the interaction of hydrogel components (GR, PVA, GA, CNT and Kalium-related material) involved some functional groups such as C-O, C-N, C=O, C-H sp3 as well as O-H which predominantly occurred through hydrogen bonding. The swelling ratio of technical-grade PVA-based hydrogel was higher than that of pro-analytical-grade. The hydrogel showed a typical slow release behavior for potassium ion with the release rate is 8.89 ppm for first ten minutes and the release rate average is 0.0134 ppm/minutes for the normal phase.