(117h) A Systematic Study of Hexavalent Chromium Adsorption and Removal from Aqueous Environments Using Chemically Functionalized Amorphous and Mesoporous Silica Nanoparticles

Silica-based materials have been an essential class of adsorbents widely employed due to their large surface areas and excellent chemical, thermal, and mechanical stabilities. Amorphous silica nanoparticles (ASNs) are one of the common nanomaterials being used in industrial, manufacturing, and biomedical applications due to their favorable physic-chemical properties, facile synthesis, and relatively easy surface modification.

Herein, we present the results of work aimed at preparing two types of bare and two types of 15 – 60 nm diameter NH₂-functionalized silica nanoparticles of similar sizes, that is, ASNs with an amorphous solid core and mesoporous silica nanoparticles (MSNs) with a nanoporous core. We demonstrate the adsorption of hexavalent chromium (Cr(VI)) ions on amino-functionalized ASNs (NH₂–ASNs) and MSNs (NH₂–MSNs) and their removal from aqueous environments. We show that the specific surface area (SSA) of NH₂–MSNs is four times as larger as that of NH₂–ASNs and that more than 70% of the total SSA of NH₂–MSNs is due to the presence of nanopores. Analyses of Cr(VI) adsorption kinetics on NH₂–ASNs and NH₂–MSNs exhibited relatively rapid adsorption behavior following pseudo-second order kinetics as determined by nonlinear fitting. NH₂–ASNs and NH₂–MSNs exhibited significantly higher Cr(VI) adsorption capacities of 34.0 and 42.2 mg·g^-1 and removal efficiencies of 61.9 and 76.8% than those of unfunctionalized ASNs and MSNs, respectively. Their kinetics, temperature-dependent isotherms, and thermodynamics of Cr(VI) adsorption were systematically analyzed.

Our findings are significant because they suggest a structural and chemical rationale for enhanced Cr(VI) adsorption, removal, and conversion to relatively non-toxic Cr(III). Understanding the detailed mechanism of Cr(VI) to Cr(III) conversion represents a meaningful step toward the goal of removing toxic Cr(VI) from wastewater environments.

[1] Jang et al. Sci. Rep. 10, 5558 (2020).