An approach for evaluating nanomaterials for use as packed bed adsorber media: A case study of arsenate removal by titanate nanofibers

The primary goal of this paper is to propose a series of logical testing steps to determine whether a new adsorbent media is suitable for application in packed bed configurations for treating drinking water pollutants. Although the focus of the study is placed on titanate nanofibers, as a never before tested media for arsenate removal, the set of testing processes that encompasses nanomaterial characterization, equilibrium and kinetics tests, and modeling, can be used on any material to quickly determine whether these materials are suitable for water treatment applications in a packed bed configurations. Bundle-like titanate nanofibers were produced by an alkaline synthesis method with Degussa P25 TiO₂. The synthesized nanofibers have a rectangular ribbon-like shape and exhibited large surface area (126 m² g^-1) and high adsorbent porosity (ε_P ≈ 0.51). Equilibrium batch experiments conducted in 10 mM NaHCO₃ buffered ultrapure water at three pH values (6.6, 7.6 and 8.3) with 125 μg L^-1 As(V) were fit with the Freundlich isotherm equation (q = K × C_E^{1 / n}). The Freundlich adsorption intensity parameter (1/n) ranged from 0.51 to 0.66, while the capacity parameters (K) ranged from 5 to 26 μg g^-1. The pore diffusion coefficient and tortuosity were estimated to be D_P ≈ 1.04 × 10^-6 cm² s^-1, and τ ≈ 4.4. For a packed bed adsorbent operated at a realistic loading rate of 11.6 m³ m^-2 h^-1 with particles obtained by sieving the media through US mesh 80 × 120, the external mass transport coefficient was estimated to be k_f ≈ 8.84 × 10^-3 cm s^-1. In this study, surface diffusion was ignored because the adsorbent has high porosity. Pore surface diffusion model (PSDM) was used to predict the arsenate breakthrough curve, and a short bed adsorbent (SBA) test was conducted under the same conditions to verify validity of the estimated values. There was no titanium release in the treated effluent during the SBA test. The pore Biot number (Bi_P > 100) implied that pore intraparticle resistance controls the overall mass transport. The PSDM was used to predict arsenate breakthrough in a simulated full-scale system. The overall combined use of modeling, material characterization, equilibrium, and kinetics tests was easier, cheaper and faster than a long duration pilot tests. While the conclusion regarding the titanate nanofibers is that they are less suitable for arsenate removal from water than commercially available media, there may be other applications where this novel nanomaterial may be suitable because of unique surface chemistry and porosity.