Diatom-Based Multifunctional Biogenic Nanostructures in Water Purification: From Mechanistic Understanding to Innovative Applications

The intensifying global water crisis, driven by industrial expansion, population pressures, and climate variability, necessitates transformative and sustainable purification technologies. Diatoms—unicellular microalgae encased in intricately nano- and micro-structured silica frustules—and their fossilized derivative, diatomaceous earth (DE), have emerged as multifunctional platforms for water treatment. Their hierarchical porosity, exceptional surface-to-volume ratio, and chemical tunability underpin the removal of a wide spectrum of contaminants, including heavy metals, synthetic dyes, pathogenic microorganisms, and emerging pollutants such as microplastics and per- and polyfluoroalkyl substances (PFAS). This review consolidates current advances in understanding the physicochemical and biological pathways of diatom-mediated remediation, encompassing adsorption, ion exchange, photocatalysis, and biofilm-driven processes. Recent progress in surface functionalization, nanocomposite integration, and photocatalytic coupling is critically evaluated, along with innovations that link diatom systems to constructed wetlands, phytoremediation frameworks, and circular bioeconomy models. Case studies across global regions, with emphasis on Indian contexts, highlight techno-economic feasibility and socio-environmental relevance. Emerging frontiers, including nanotechnology-enabled hybrids, machine learning-assisted predictive modeling, and life cycle sustainability assessment, are identified as accelerators of translational deployment. Persistent challenges, notably regeneration efficiency, safe disposal of contaminant-loaded DE, and competition with membrane technologies, are also examined. Collectively, this synthesis positions diatoms as both biological sentinels and material platforms, charting strategic pathways for next-generation, climate-resilient water purification systems. This review provides a critical assessment of the physicochemical processes governing contaminant removal, highlights recent innovations in diatom functionalization that enhance reactivity, and explores the incorporation of diatom-derived biosilica into nanocomposite materials and advanced filtration platforms. Additionally, it assesses the scalability, economic viability, and ecological implications of implementing diatom-based technologies in real-world water treatment applications. By pinpointing existing research gaps and technological constraints, the article proposes strategic pathways for interdisciplinary investigations aimed at fully leveraging the potential of diatoms in shaping next-generation water purification solutions.