Sorption and continuous filtration of heavy metals and radionuclides using novel nano-Farringtonite: Mechanisms delineation using EXAFS

Abstract

Simultaneous removal of a wide range of toxic heavy metal cations and potential radionuclides from water bodies and their continuous filtration with a single low-cost and eco-friendly material represents several scientific merits. Herein, for the first time, we report the simple and straightforward wet-chemical synthesis of novel nano-Farringtonite (FAR) composed of magnesium and phosphate ions. Further, the potential of known alternate nano-hydroxyapatite (HAP) and novel engineered nano-FAR including their non-stoichiometric variations was evaluated for the removal of mimicking radionuclide (Sr2+) and heavy metals (Cd2+, and Zn2+) from water bodies. Non-stoichiometric FAR (ns-FAR) have shown multifold higher contaminant removal capacities than HAP, i.e., Sr2+≈ 85 mg/g vs 49.5 mg/g, Cd2+≈ 560 mg/g vs 98.5 mg/g, and Zn2+ = 489 mg/g vs 62 mg/g in batch mode. NsFAR showed complete removal of Cd2+ and Zn2+ with <20% and 0% recovery, respectively in three consecutive sorption-recovery cycles, probing towards permanent incorporation of these ions. Spectroscopic analysis and extended x-ray absorption fine structure (EXAFS) spectroscopy fitting confirmed ion exchange and crystal incorporation as probable removal mechanisms. The high ionic potential of Mg2+ allows easy ion exchange with +2 charged metal toxins of varying ionic radius at both Mg1 and Mg2 sites of FAR. nsFAR showed instantaneous separation of these cations in continuous column mode with >2,00,000 L/kg of water filtration capacity (at 1 mg/L), justifying the adsorbent's candidature in water purification applications.