Surface Complexation\nand Packed Bed Mass Transport\nModels Enable Adsorbent Design for Arsenate and Vanadate Removal

Co-occurrence\nof metal oxo-anions (e.g., arsenate) in\ndrinking\nwater poses human health risks. To understand and predict competition\nand breakthrough for individual or mixtures of oxo-anions in continuous-flow\npacked bed adsorption systems, we linked equilibrium surface complexation\nmodels (SCMs) with a pore surface diffusion model (PSDM). After parametrization,\nusing data for two commercial adsorbents, the SCM and PSDM predicted\nwell the adsorption isotherm data and column breakthrough curves,\nrespectively, for single-solute (arsenate) and bisolute water chemistries\n(arsenate, vanadate) as well as chromatographic displacement of previously\nadsorbed arsenate by vanadate. Surface and pore diffusivities for\nboth commercial adsorbents were 3.0 to 3.5 x10<sup>–12</sup> cm<sup>2</sup>/s and 1.1 to 0.8 x10<sup>–6</sup> cm<sup>2</sup>/s, respectively. After validation, SCM + PSDM was used in silico\nto evaluate adsorbent media characteristics, variable water chemistries,\nand reactor configurations. When contrasting hypothetical crystalline\nversus amorphous metal (hydr)oxide adsorbents, increasing surface\nsite density resulted in higher Freundlich isotherm capacity (<i>K</i><sub>F</sub>) but did not impact 1/<i>n</i>.\nIncreasing surface binding affinities beneficially impacted both the <i>K</i><sub>F</sub> and 1/<i>n</i> isotherms and would\nimprove the performance of point-of-use (POU) adsorbent system applications.\nIn silico simulation results suggest prioritizing enhancing adsorbent\ncapacity (<i>q</i>) through improved surface reactivity\nin the design of new POU adsorbent materials rather than focusing\non reducing mass transport limitations through intraparticle pore\ndesign. For municipal-scale adsorption systems, the PSDM simulation\nof the mass transfer zone shape was evaluated for hypothetical adsorbent\npore designs (i.e., intraparticle porosity (ε<sub>p</sub>) and\ntortuosity) and demonstrated that ε<sub>p</sub> control was\na key strategy to improve performance.