Fluorine adsorption on floating iron nanoparticles confined to gigaporous structure of large adsorbent water spheres

Abstract

Effective defluoridation of artesian well water can be performed with iron nanoparticles confined to expanded reticulated polymer network of polyvinyl alcohol (PVA) containing a high quantity of water. For this purpose, large (3433 ± 63 μm) adsorbent water spheres, meaning spheres containing Fe2O3 (15.8 wt%)-PVA (8.2 wt%)-H2O (76.0 wt%), were manufactured (sol–gel process) and characterized (TG/DTA, XRD, MÖSSBAUER, SEM/EDS, BET, FTIR, and XPS). In the resulting spongy water spheres, gigapores (140,000 nm) were filled with loosely agglomerated and floating naked nanoparticles (<20 nm) of octahedral [Fe2O2(H2O)8]2+ and/or [Fe2O3(H2O)6]0 complexes without prejudice to the chemical and mechanical (35.0 ± 0.4 kPa) stability of the nanostructured adsorbent. The built-in mesoporous and gigaporous structure provided good pore connectivity, which significantly reduced the adsorption rate limitation imposed by boundary-layer and intraparticle diffusion effects as well as it has enhanced the adsorptive capacity (from 77.9 to 204.9 mgF/g) of the referred adsorbent water spheres. XPS proved the chemical interaction mechanism in F–Fe–PVA–H2O system. Fluoride chemisorption occurs on PVA (marginally; (-CH2CHF-)n, (-CH2CF2-)n, and (-CF2CF2-)n) and Fe (predominantly; octahedral Fe2F5.7H2O complex) particles. Adsorbent water spheres can defluorinate natural waters in multiple adsorption cycles in fixed bed columns after fluorine desorption with NH4OH.