Selenium enrichment in floating particles after water float-sink procedure: Insights into risk control of...

This study revealed selenium (Se, 190-388 μg/g) enrichment in floating particles (FP) formed during a water-based float-sink procedure applied to alkaline coal fly ash and discussed possible mechanisms. Comprehensive analyses (X-ray fluorescence spectroscopy, inductively coupled plasma mass spectrometry, Brunauer-Emmett-Teller surface area analysis, X-ray pHotoelectron spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy) revealed that Se in raw fly ash was mainly present as elemental Se⁰, with minor selenite (SeO₃^2-). Under alkaline conditions, Se⁰ was partially oxidized to SeO₃^2- and interacted with Ca²⁺, SO₄^2-, and Na⁺. Floating particles, characterized by unburned carbon, fine bubbles, and water films, provided hydrophobic interfaces that facilitated the capture of possible insoluble (e.g., CaSeO₃·CaSO₄) and soluble (e.g., Na₂SeO₃) Se species through co-precipitation and surface adsorption. Se enrichment in floating products signals both environmental risks and opportunities. Assessing Se leaching only under acidic conditions is inadequate, and the leaching kinetics of Se in alkaline coal fly ash and floating products require further study. Moreover, strategies that promote greater Se concentration in floating products could potentially enable more effective control and recovery.