A Hybrid {Silk@Zirconium MOF} Material as Highly Efficient AsIII-sponge

Abstract

Exposure of humans to Arsenic from groundwater drinking sources is an acute global public health problem, entailing the urgent need for highly efficient/low-cost Arsenite (As^III) up-taking materials. Herein we present an innovative hybrid-material, ZrMOF@SF_d operating like an "As^III-sponge" with unprecedented efficiency of 1800 mg As^III gr^-1. ZrMOF@SF_d consists of a neutral Zirconium Metal-Organic Framework [ZrMOF] covalently grafted on a natural silk-fiber (SF_d). ZrMOF itself exhibits As^III adsorption of 2200 mg gr^-1, which supersedes any -so far- known As^ΙΙΙ-sorbent. Using XPS, FTIR, BET-porosimetry data, together with theoretical Surface-Complexation-Modeling (SCM), we show that the high-As^ΙΙΙ-uptake is due to a sequence of two phenomena:[i] at low As^III-concentrations, surface-complexation of H₃AsO₃ results in As^III-coated voids of ZrMOF, [ii] at increased As^III-concentrations, the As^III-coated voids of ZrMOF are filled-up by H₃AsO₃ via a partitioning-like mechanism. In a more general context, the present research exemplifies a mind-changing concept, i.e. that a "partitioning-like" mechanism can be operating for adsorption of metalloids, such as H₃AsO_3, by metal oxide materials. So far, such a mechanism has been conceptualized only for the uptake of non-polar organics by natural organic matter or synthetic polymers.

Figure 1

FE-SEM images for ZrMOF (a1,a2), As^III-loaded ZrMOF (b1,b2), natural-silk SF_d (c),ZrMOF@SF_d (d), As^III-loaded ZrMOF@SF_d (e).

Figure 2

(a) PXRD data for ZrMOF and ZrMOF @ SF_d, (b) PXRD data for ZrMOF, and As^III@ ZrMOF. (c)TGA (solid line) and first derivative (DTG) (dashed line) plots for ZrMOF,(d) FT-IR spectra for pristine (−), and As^III-loaded ZrMOF material (), (e) XPS As3d analysis for ZrMOF after As^III loading.

Figure 3

(a) Nitrogen sorption isotherms at 77 K for ZrMOF and As^III@ZrMOF for different As^III loadings (b) BET surface area vs. As^III loading.

Figure 4

As^III adsorption kinetics for (a) ZrMOF and (b) ZrMOF@SF_d at pH 7. Symbols (, ) are experimental data. Lines are theoretical curves calculated using Eq. 2, with the parameters listed in Table S1. In (a) the added As^III concentration was 50 mg L⁻¹. In (b) the added As^III concentration was 15 mg L⁻¹.

Figure 5

As^III adsorption isotherms for ZrMOF (, , •), ZrMOF@SF_d (, , ■),pZrMOF(), and SF_d (▲) at pH 7.

Figure 6

Maximum As^III adsorption capacity (g g⁻¹) at waters’ near-neutral pH of some adsorbents reported in the literature compared with the present materials: 1) ZIF-8(cubic), 2) ZIF-8(leaf), 3) ZIF-8 (dodecahedral), 4) Fe₃O₄@ZIF-8,5) HCl-UiO-66(SH)₂, 6) CoFe₂O₄@MIL-100(Fe), 7) Fe₃O₄@MIL-101, 8) MIL-100(Fe), 9) M600, 10) M800, 11) M900, 12) ZrMOF(this work), 13) cationic pZrMOF (this work), 14) SF_d (this work), 15)neutral ZrMOF@SF_d (this work).