Marine Macroalga-Mediated Green Synthesis of Silver Nanoparticles Using Ulva reticulata for Sunlight-Assisted Photocatalytic Degradation of Synthetic Dyes

Marine macroalgae provide renewable biological resources for the eco-friendly synthesis of functional nanomaterials for environmental remediation. This study synthesized silver nanoparticles (AgNPs) using an aqueous extract of the green macroalga Ulva reticulata and evaluated their sunlight-driven photocatalytic activity against synthetic dyes. Five seaweed species were screened for AgNP synthesis, and U. reticulata was selected based on visible color change and UV–visible spectral response. The effects of light conditions, incubation time, AgNO3-to-extract ratio, temperature, pH, and AgNO3 concentration were optimized. Maximum AgNP synthesis was achieved under light conditions using a 7:3 ratio of AgNO3 solution to algal extract, 1 mM AgNO3, pH 8, and incubation at 60 °C for 24 h. UV–visible spectroscopy showed a characteristic surface plasmon resonance peak at approximately 440 nm. FTIR analysis indicated that algal functional groups contributed to silver ion reduction and nanoparticle stabilization. SEM analysis showed predominantly spherical AgNPs with an average size of 3.1 nm, while EDS analysis confirmed the presence of elemental silver. The biosynthesized AgNPs degraded methylene blue, methyl orange, aniline blue, and rose bengal under sunlight, achieving 60–90% decolorization after 24 h. These findings support U. reticulata as a promising marine biological platform for sustainable AgNP synthesis and dye-contaminated wastewater treatment.