Photocatalytic Urea–In₂S₃ Hydrogel Beads for Wastewater Treatment Followed by Nitrogen Fixation to Plants

Abstract

Water pollution and nitrogen scarcity are interconnected global challenges that demand integrated solutions. In this study, we present photocatalytic urea–In2S3 hydrogel beads, fabricated using a sustainable pectin-based matrix loaded with indium sulfide (In2S3) nanoparticles and urea, that simultaneously remediate wastewater and function as self-fertilizing systems for sustainable plant growth. Structural and optical characterizations (XRD, SEM, FTIR, FESEM, PL, UV–Vis DRS) verified the crystalline nature of In2S3, its uniform dispersion in the pectin hydrogel, and strong visible-light absorption. The beads demonstrated outstanding photocatalytic activity, achieving 99% degradation of the pharmaceutical contaminant sulfamethoxazole (SMX) within 20 minutes under visible light, with radical quenching studies confirming the involvement of •O2⁻ and •OH species. After wastewater treatment, the beads were transferred into aquaponic setup, where water and sunlight exposure triggered photocatalytic nitrogen fixation, reducing atmospheric N₂ into bioavailable ammonium. Quantitative analysis confirmed the generation of 402 μM NH₄⁺ within 60 minutes, alongside controlled release of urea from the hydrogel, resulting in enhanced soil nitrogen availability. Also, 7 days of monitoring showed a steady buildup of NH₄⁺ in the soil system, increasing from an initial 402 μM on Day 1 to about 2100 μM by Day 7, which highlights the prolonged nitrogen-fixing activity and controlled nutrient release of the hydrogel beads. Overall, these multifunctional beads represent a circular water–energy–food platform, providing a scalable and sustainable pathway that unites wastewater remediation, nitrogen fixation under natural conditions, and improved soil fertility for environmentally friendly agriculture.