Adsorption Behavior of Gold Ions on Nanofiber Webs Containing Protein Polyhedral Crystals

Abstract

Protein polyhedral crystals (PhCs), with well-defined three-dimensional structures and narrow channels/cavities, have the potential to be utilized as stable biobased adsorbents. However, their intrinsic adsorption abilities for metal ions, including precious metals, remain unclear. In this study, protein PhCs were immobilized in polymer nanofiber (PhC@NF) webs via electrospinning with poly-(ethylene-co-vinyl alcohol) (EVOH). The morphology and adsorption behavior of the PhC@NF web for precious metal ions such as gold (Au) were investigated by scanning electron microscopy and inductively coupled plasma mass spectrometry, respectively. At 25 °C, Au adsorbed only on PhCs and the PhC@NF web exhibited a maximum Au adsorption capacity of 51.7 mg g^-1 at pH 1 and a maximum Au adsorption equilibrium constant of 1.02 L mg^-1 at pH 3, the highest constant among reported Au ion adsorbents. By contrast, Au reduced by PhCs was deposited as particles on EVOH NF web at 65 °C. The study demonstrates that PhCs are promising materials for efficient recovery of Au from low-concentration Au ion solutions.

1

SEM image and size distribution of purified PhCs.

2

Surface SEM images and fiber diameter distribution of the prepared NF webs: (a) NF web without PhCs (EVOH NF) and (b) PhC@NF web. (c) High-magnification surface SEM image of PhC@NF web. (d) Bright-field STEM image of a typical immobilized PhC within the PhC@NF web. (e) Photograph of a typical PhC@NF web.

3

(a) Typical surface SEM image of the PhC@NF web after the Au adsorption experiment in 100 mg L^–1 Au ion solution at pH 3 and 25 °C. Corresponding EDS mapping of (b) Au-M and (c) Cl-K. (d) Corresponding EDS line analysis near a single PhC immobilized within the PhC@NF web.

4

Adsorption isotherms of Au on the PhC@NF web at 25 °C and (a) pH 1, (b) pH 2, and (c) pH 3. The blue circle indicates the Au adsorption capacity of the NF web without PhCs (EVOH NF). The solid lines correspond to theoretical fitting using the Langmuir model. The characteristic parameters are listed in Table .

5

Au 4f XPS spectra of Au-adsorbed PhCs with maximum adsorption capacity at (a) pH 1, (b) pH 2, and (c) pH 3.

6

Time-dependent Au adsorption on the PhC@NF web at 25 °C and pH 3 and theoretical fitting based on (a) pseudo first-order and pseudo second-order models and (b) the intraparticle diffusion model. The characteristic parameters are listed in Table . Three samples were measured for each condition, and the mean value (±standard deviation) is indicated.

7

(a) Amounts of Au-adsorbed and desorbed on the PhC@NF web. (b) Equilibrium adsorption capacity of the PhC@NF web in a mixed system containing Au, Mn, Zn, and Cu ions (50 mM). Three samples were measured for each metal, and the mean value (±standard deviation) is indicated.

8

Reaction energies of coordination interactions of Au species, [AuCl₄]⁻ (red circles) and [AuCl₂(OH)­(H₂O)] (green triangles), and energies of hydrogen bonding (orange stars) and reduction reactions (blue squares) of [AuCl₂(OH)­(H₂O)] calculated with various AAs and backbone groups.

9

Adsorption isotherms of Au on the PhC@NF web at pH 3 and (a) 25 °C, (b) 45 °C, and (c) 65 °C. The blue circle indicates the Au adsorption capacity of the NF web without PhCs (EVOH NF).

10

Typical SEM images of the Au-adsorbed PhC@NF web at pH 3 and (a) 25 °C, (b) 45 °C and (c) 65 °C. (d) EDS spot analysis of a single PhC in the web performed at the red spot in (a–c). The intensity was normalized to the peak intensity of carbon atoms.

11

Typical backscattered electron images of the Au-adsorbed PhC@NF web at pH 3 and 45 °C with (a) low Au adsorption capacity (30 mg g^–1) and (b) high Au absorption capacity (55 mg g^–1); and the Au-adsorbed PhC@NF web at pH 3 and 65 °C with (c) low Au adsorption capacity (25 mg g^–1) and (d) high Au absorption capacity (100 mg g^–1). (e) Typical EDS line analysis along a single NF [the data is for (b)].