The overlooked role of mineral pore geometry in the solid-solution partitioning of (poly)oxyanionic metals

Abstract

A high specific surface area (SSA) typically signifies a superior adsorption capacity. Nevertheless, minerals with high SSAs tend to possess tiny pores that may not be accessible to relatively large (poly)oxyanionic metals. Herein, we assessed the adsorption of (poly)oxyanionic metals on 2-line and 6-line ferrihydrite and goethite with distinct SSAs and pore geometries. SSA was estimated by BET isotherm using N2, while pore geometry was measured by N2 adsorption isotherms and positron annihilation lifetime spectroscopy. Tungstate and its polymers were chosen as representative (poly)oxyanions. Adsorption experiments were performed with constant mineral surface area, but different contact time, pH, and tungsten concentrations. Unexpectedly, the Langmuir adsorption capacities per unit surface area on 6-line ferrihydrite and goethite were 2–6 and 3–11 times as high as those on 2-line ferrihydrite, respectively. Adsorption on 2-line ferrihydrite was also severely kinetically limited. The varying rates and magnitudes of adsorption were attributed to distinct mineral pore widths: (poly)tungstates could hardly fit within the abundant tiny pores in 2-line ferrihydrite formed by voids between the primary particles/aggregates, but could enter the larger pores in 6-line ferrihydrite and goethite. Consequently, significant decreases of mineral microporosity (<2 nm) were observed following (poly)tungstate adsorption. Tungstate and polytungstate with different hydrated ion radii exhibited similar adsorption behavior, most likely due to the formation of adsorbed polytungstate directly on mineral surface. Our data demonstrate that mineral pore geometry controls the solid-solution partitioning of (poly)oxyanionic metals, which is crucial to comprehend their environmental fate and to mitigate their contamination.