Selecting metal oxide nanomaterials for arsenic removal in fixed bed columns: From nanopowders to aggregated nanoparticle media

This paper investigates the feasibility of arsenate removal by aggregated metal oxide nanoparticle media in packed bed columns. Batch experiments conducted with 16 commercial nanopowders in four water matrices were used to select a metal oxide nanoparticle that both amply removes arsenate and can be aggregated using an inert binder. TiO₂, Fe₂O₃, ZrO₂ and NiO nanopowders, which exhibited the highest arsenate removal in all water matrices, were characterized with fitted Freundlich adsorption isotherm (q = K C_e^{1 / n}) parameters. In 10 mM NaHCO₃ buffered nanopure water and at both pH ≈ 6.7 and 8.4, K ranged from 1.3 to 12.09 (mg As/g(media)) (L/mg As)^1/n, and 1/n ranged from 0.21 to 0.52. Under these conditions, the fitted Freundlich isotherm parameters for TiO₂ nanoparticles aggregated with inorganic and organic binders (K of 4.75-28.45 (mg As/g(media)) (L/mg As)^1/n and 1/n of 0.37-0.97) suggested favorable arsenate adsorption. To demonstrate that aggregated nanoparticle media would allow rapid mass transport of arsenate in a fixed bed adsorber setting, short bed adsorber (SBA) tests were conducted on TiO₂ nanoparticle aggregates at empty bed contact times (EBCT) of 0.1-0.5 min and Re × Sc = 1000 and 2000. These SBA tests suggested that the binder has a negligible role in adsorbing arsenic and that mass transport is controlled by rapid intraparticle diffusion rather than external film diffusion.