Mesoporous Silica-Gelatin Aerogels for the Selective Adsorption of Aqueous Hg(II)

Herman, Petra and Fábián, István and Kalmár, József (2020) Mesoporous Silica-Gelatin Aerogels for the Selective Adsorption of Aqueous Hg(II). ACS APPLIED NANO MATERIALS, 3 (1). pp. 195-206. ISSN 2574-0970

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Abstract

Supercritically dried, mesoporous silica–gelatin hybrid aerogels of 4–24 wt % gelatin content show high selectivity for the adsorption of aqueous Hg(II) in the simultaneous presence of Cu(II), Cd(II), Co(II), Pb(II), Ni(II), Ag(I), and Zn(II), as demonstrated by batch adsorption experiments with multiple competing ions. The aerogels are characterized by SEM and N2 porosimetry, and their aqueous particle size distributions and zeta potentials are reported. The adsorption properties of the hybrid aerogels are studied as a function of their composition, initial aqueous Hg(II) concentration, contact time, and pH. The optimum pH for adsorption is 6.0, where the surface of the aerogel is already negatively charged, but Hg(II) does not completely hydrolyze. The Hg(II) uptake of the hybrid aerogels increases with increasing gelatin content and levels off at 24 wt % gelatin. The adsorption capacity of the 24 wt % gelatin hybrid is estimated to be S = 209 mg g–1 by fitting the isotherm with the Langmuir model (KL = 0.032 L mg–1). This translates to 91% Hg(II) removal at c0(Hg) = 1.0 mg L–1 and c0(aerogel) = 0.32 g L–1. Gelatin provides the active sites for Hg(II) binding; thus, higher gelatin content results in higher adsorption capacity. However, high gelatin content also induces the extensive swelling of backbone and the partial collapse of the open porous structure, which decreases the specific surface area. Time-resolved experiments show that the adsorption equilibrium is established within 15 min contact time with aqueous Hg(II). Washing the equilibrated aerogels with a 2.5 mM solution of EDTA complexing agent quantitatively liberates bound Hg(II). The regenerated aerogels demonstrate practically intact adsorption capacities in five cycles of reuse. Coordination chemistry based considerations reveal that Hg(II) is selectively complexed by the soft Lewis-base side chains of collagen.

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