Metallic iron (Fe0)-based materials for aqueous phosphate removal: A critical review

The discharge of excessive phosphate from wastewater sources into the aquatic environment has been identified as a major environmental threat responsible for eutrophication. It has become essential to develop efficient but affordable techniques to remove excess phosphate from wastewater before discharging into freshwater bodies. The use of metallic iron (Fe-0) as a reactive agent for aqueous phosphate removal has received a wide attention. Fe-0 in-situ generates positively charged iron corrosion products (FeCPs) at pH > 4.5, with high binding affinity for anionic phosphate. This study critically reviews the literature that focuses on the utilization of Fe-0 -based materials for aqueous phosphate removal. The fundamental science of aqueous iron corrosion and historical background of the application of Fe-0 for phosphate removal are elucidated. The main mechanisms for phosphate removal are identified and extensively discussed based on the chemistry of the Fe-0/H2O system. This critical evaluation confirms that the removal process is highly influenced by several operational factors including contact time, Fe-0 type, influent geochemistry, initial phosphate concentration, mixing conditions, and pH value. The difficulty in comparing independent results owing to diverse experimental conditions is highlighted. Moreover, contemporary research in progress including Fe-0/oxidant systems, nano-Fe-0 application, Fe-0 material selection, desorption studies, and proper design of Fe-0-based systems for improved phosphate removal have been discussed. Finally, potential strategies to close the loop in Fe-0-based phosphate remediation systems are discussed. This review presents a science-based guide to optimize the efficient design of Fe-0-based systems for phosphate removal.