Surface Complexation and Packed Bed Mass Transport Models Enable Adsorbent Design for Arsenate and Vanadate Removal

Abstract

Co-occurrence of metal oxo-anions (e.g., arsenate) in drinking water poses human health risks. To understand and predict competition and breakthrough for individual or mixtures of oxo-anions in continuous-flow packed bed adsorption systems, we linked equilibrium surface complexation models (SCMs) with a pore surface diffusion model (PSDM). After parametrization, using data for two commercial adsorbents, the SCM and PSDM predicted well the adsorption isotherm data and column breakthrough curves, respectively, for single-solute (arsenate) and bisolute water chemistries (arsenate, vanadate) as well as chromatographic displacement of previously adsorbed arsenate by vanadate. Surface and pore diffusivities for both commercial adsorbents were 3.0 to 3.5 x10^–12 cm²/s and 1.1 to 0.8 x10^–6 cm²/s, respectively. After validation, SCM + PSDM was used in silico to evaluate adsorbent media characteristics, variable water chemistries, and reactor configurations. When contrasting hypothetical crystalline versus amorphous metal (hydr)oxide adsorbents, increasing surface site density resulted in higher Freundlich isotherm capacity (K_F) but did not impact 1/n. Increasing surface binding affinities beneficially impacted both the K_F and 1/n isotherms and would improve the performance of point-of-use (POU) adsorbent system applications. In silico simulation results suggest prioritizing enhancing adsorbent capacity (q) through improved surface reactivity in the design of new POU adsorbent materials rather than focusing on reducing mass transport limitations through intraparticle pore design. For municipal-scale adsorption systems, the PSDM simulation of the mass transfer zone shape was evaluated for hypothetical adsorbent pore designs (i.e., intraparticle porosity (ε_p) and tortuosity) and demonstrated that ε_p control was a key strategy to improve performance.