Plant-Based Natural Coagulants in Sustainable Water and Wastewater Treatment: Mechanisms, Performance, and Emerging Applications

Abstract

The increasing pressure on global water resources, driven by population growth, industrial expansion, and pharmaceutical and chemical contamination, has intensified the demand for sustainable and environmentally benign water and wastewater treatment technologies. Conventional coagulation–flocculation processes, largely dependent on inorganic coagulants such as alum and ferric salts, are effective but raise concerns related to sludge toxicity, residual metal content, secondary pollution, and long-term ecological and health impacts. In response, plant-based natural coagulants have emerged as promising alternatives, attracting significant academic and practical attention over the past two decades. This article provides an extensive, theory-driven and evidence-based examination of natural coagulants derived from plant seeds, leaves, gums, and agricultural by-products, with a particular emphasis on their application in turbidity reduction, pharmaceutical removal, and industrial wastewater treatment. Drawing strictly on the provided literature, the study elaborates on coagulation mechanisms, material characteristics, process optimization, comparative performance with alum, and sustainability implications. The methodology adopted is a comprehensive qualitative synthesis of experimental, modeling, and review-based studies, enabling a deep interpretation of trends, consistencies, and contradictions within existing research. Results discussed indicate that natural coagulants, such as Moringa oleifera, okra gum, rosehip seeds, papaya seeds, and various local plant leaves, demonstrate high removal efficiencies under optimized conditions, often approaching or matching conventional coagulants while offering superior environmental compatibility. The discussion critically evaluates limitations related to dosage variability, organic load addition, storage stability, and scalability, while also exploring future research directions including composite coagulants, hybrid systems, and circular economy integration. The article concludes that plant-based natural coagulants represent a viable and strategically important pathway toward sustainable water and wastewater management, particularly in developing and resource-constrained regions.