Effects of particle size and hydroxyaluminum oligocations on tungstate immobilization by weathered clay minerals

In laboratory tests, metal (hydro)oxides showed greater adsorption amounts for tungsten (W)—a emerging contaminant—than common aluminosilicates in aqueous systems. However, in actual soils, the often-overlooked yet ubiquitous clay minerals have recently been proposed as important components in the retardation of W(VI) transport. To address this discrepancy, montmorillonite (Mt), kaolinite (Kt), and illite (Ilt) of different sizes were prepared by gradient centrifugation, freeze–thaw, and/or intercalation protocols and subsequently modified via hydroxyaluminum (HyA) and hydroxyaluminosilicate (HAS) oligocations to simulate their natural states and interactions with W(VI) in soil. For all three clay minerals, the smaller particles possessed higher HyA or HAS contents. Additionally, Mt showed the larger loading amounts for two oligocations compared to Kt and Ilt. HyA or HAS loading significantly increased W(VI) uptake, negligibly influenced the adsorption kinetics, and improved the pH resistance. At low concentrations, the clay minerals loaded by HAS showed greater adsorption amounts of W(VI) than those loaded by HyA. The adsorption of W(VI) on unmodified Kt and HyA- or HAS-loaded clay minerals were increased by increasing ionic strength, suggesting the complexation mechanism. HyA-loaded small Kt and Mt induced the polymerization of monomeric tungstate on their surfaces, corresponding to an open-chain and a proximal polymerization models, respectively. Release of the polymerized W(VI) was related to the clay mineral type and W(VI) concentration. These findings offer novel insights into the fixation characteristics and mechanisms of W(VI) by soil clay minerals, thereby shedding light on the colocalization of W with Si/Al in natural soils.