Arsenate removal by nanostructured ZrO₂ spheres

A new zirconium oxide-based media for arsenate removal from water was fabricated and evaluated in batch and continuous flow experiments. Highly porous (ε_P ≈ 0.9) nanostructured zirconium oxide spheres were fabricated by the impregnation of macroporous ion-exchange media (CalRes 2103, Calgon) with zirconium salt; the media was then ashed at T > 750 ± 50°C to remove the organic polymer resin and obtain ZrO₂ spheres. The spheres generally ranged from 200 to 800 μm in diameter, and consisted of ZrO₂ nanoastructures generally ranging between 20 and 100 nm. They also exhibited monoclinic and tetragonal crystalline structures, and had an isoelectric point of 5.6. Equilibrium batch experiments were conducted in 10 mM NaHCO₃ buffered nanopure water atthree pH values (6.4, 7.3, and 8.3) with 120 μg/L As(V). Data were fit with the Freundlich isotherm equation (q_e = K x C_E^1/n), resulting in an intensity parameter (1/n) of ∼ 0.33 and capacity parameters (K) ranging from 115 to 400 (μg As(V) g^-1 dry media)(L μg^-1)^1/n. The pore diffusion coefficient and toruosity were estimated to be 6.4 x 10 ^-6 cm² s^-1 and 1.3, respectively. For a packed bed adsorbent operating at a loading rate of 11.5 m³ nr^-2 hr^-1 in a realistic continuous flow experiment the external mass transport coefficient was estimated to be k_f ≈ 6.3 x 10 ^-3 cm s^-1. The pore diffusion coefficient and the external mass transport coefficient were used with the pore surface diffusion model (PSDM) to predict the arsenate breakthrough curve. A short bed adsorbent (SBA) test was conducted under the same conditions to validate the model. In this study, surface diffusion was ignored because the particles have a very high porosity. The validated model was used to predict arsenate breakthrough in a simulated full-scale system. The overall combined use of modeling, material characterization, equilibria, and kinetics tests determined the suitability of the media for arsenate treatment cheaper, easier, faster, and with less media than a long duration pilot test would have. Although the fabricated zirconium oxide spheres exhibited adsorption capacity comparable to some commercially available media such as iron based (hydr)oxides, the high cost of fabrication may render the media not feasible for wide use in commercial applications. However, the very high porosity of this media provides for improved pore diffusion and faster overall mass transport, which may be critical for applications where mass transport is the limiting factor.