**4. Microscopic measurement of metastable-equilibrium adsorption state**

It should be noted that, when the *C*p effect isotherm equations are used in the modeling of practical adsorption processes, they may be totally empirical and does not imply particular physical mechanism. The macroscopic adsorption behavior is fundamentally controlled by the microscopic reaction mechanism of adsorbed molecules on solid surfaces. Therefore, the direct Measurement on the microstructures at solid-water interfaces is crucial to verifying the MEA principle.

Macroscopic thermodynamic results19, 20 showed that Zn(II) adsorbed on manganite was largely irreversible (adsorption and desorption isotherms corresponding to the forward and backward reactions did not coincide, see Figure 9), but the adsorption of Zn (II) on δ-MnO2 was highly reversible (there was no apparent hysteresis between the adsorption and desorption isotherms, see Figure 10). This contrast adsorption behavior between the two forms of manganese oxides could be explained from the different microscopic structures

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Fig. 11. Corner-sharing linkage (a) and interlayer structures of Zn(II) adsorbed on δ-MnO2 (b). (a) RZn–O = 2.07 Å, RMn–O = 1.92 Å, RZn–Mn = 3.52 Å. (b) Squares were vacant sites, illustration diagram adapted from Wadsley,27 Post and Appleman,28 and Manceau et al..25

Fig. 12. Two types of linkage between adsorbed Zn(II) (octahedron and tetrahedron) and MnO6 octahedra on the γ-MnOOH surfaces. (a) Double-corner linkage mode; (b) edge-

Extended X-ray absorption fine structure (EXAFS) analysis showed that Zn(II) was adsorbed onto δ-MnO2 in a mode of corner-sharing linkage, which corresponded to only one Zn–Mn distance of 3.52 Å (Figure 11). However, there were two linkage modes for adsorbed Zn(II) on manganite surface as inner-sphere complexes, edge-sharing linkage and corner-sharing linkage, which corresponded to two Zn–Mn distances of 3.07 and 3.52 Å (Figure 12). The

linkage mode.

between δ-MnO2 and manganite, as well as the linkage modes of adsorbed Zn(II) on δ-MnO2 and manganite.19

Fig. 9. Adsorption (closed symbols) and desorption (open symbols) isotherms of Zn(II) on manganite. EXAFS samples were indicated by arrows.

Fig. 10. Adsorption (■) and desorption (□) isotherms of Zn(II) on δ-MnO2. EXAFS samples were symboled with blank triangles (Δ).

Manganite had a structure with rows of edge-sharing Mn(II)O6 octahedra linked to adjacent rows through corners. Due to the Jahn–Teller effect of Mn(II) ions and to the presence of both O and OH groups, the MnO6 octahedra were highly distorted: each Mn is bound to four equatorial oxygen and two axial oxygen atoms.21, 22 This distortion gave rise to a mild layered structure. Hydrolyzable Zn could be bonded on MnO6 octahedra of manganite surface via edge and corner-sharing coordination modes.21, 22 The basic structure of δ-MnO2 consisted of layers of edge-sharing MnO6 octahedra alternating with a layer of water molecules. One-sixth of Mn4+ positions were empty, which gave a layer charge that was compensated by two Zn atoms located above and below the vacancy.23, 24 Hydrolyzable Zn could be taken up in the interlayer to form tridentate corner-sharing complexes.25, 26 These differences in crystallographic structure resulted in different linkage modes for the adsorption of Zn on manganite and δ-MnO2.

between δ-MnO2 and manganite, as well as the linkage modes of adsorbed Zn(II) on δ-

Fig. 9. Adsorption (closed symbols) and desorption (open symbols) isotherms of Zn(II) on

Fig. 10. Adsorption (■) and desorption (□) isotherms of Zn(II) on δ-MnO2. EXAFS samples

Manganite had a structure with rows of edge-sharing Mn(II)O6 octahedra linked to adjacent rows through corners. Due to the Jahn–Teller effect of Mn(II) ions and to the presence of both O and OH groups, the MnO6 octahedra were highly distorted: each Mn is bound to four equatorial oxygen and two axial oxygen atoms.21, 22 This distortion gave rise to a mild layered structure. Hydrolyzable Zn could be bonded on MnO6 octahedra of manganite surface via edge and corner-sharing coordination modes.21, 22 The basic structure of δ-MnO2 consisted of layers of edge-sharing MnO6 octahedra alternating with a layer of water molecules. One-sixth of Mn4+ positions were empty, which gave a layer charge that was compensated by two Zn atoms located above and below the vacancy.23, 24 Hydrolyzable Zn could be taken up in the interlayer to form tridentate corner-sharing complexes.25, 26 These differences in crystallographic structure resulted in different linkage modes for the

manganite. EXAFS samples were indicated by arrows.

were symboled with blank triangles (Δ).

adsorption of Zn on manganite and δ-MnO2.

MnO2 and manganite.19

Fig. 11. Corner-sharing linkage (a) and interlayer structures of Zn(II) adsorbed on δ-MnO2 (b). (a) RZn–O = 2.07 Å, RMn–O = 1.92 Å, RZn–Mn = 3.52 Å. (b) Squares were vacant sites, illustration diagram adapted from Wadsley,27 Post and Appleman,28 and Manceau et al..25

Fig. 12. Two types of linkage between adsorbed Zn(II) (octahedron and tetrahedron) and MnO6 octahedra on the γ-MnOOH surfaces. (a) Double-corner linkage mode; (b) edgelinkage mode.

Extended X-ray absorption fine structure (EXAFS) analysis showed that Zn(II) was adsorbed onto δ-MnO2 in a mode of corner-sharing linkage, which corresponded to only one Zn–Mn distance of 3.52 Å (Figure 11). However, there were two linkage modes for adsorbed Zn(II) on manganite surface as inner-sphere complexes, edge-sharing linkage and corner-sharing linkage, which corresponded to two Zn–Mn distances of 3.07 and 3.52 Å (Figure 12). The

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Fig. 13. Adsorption and desorption isotherms of Zn(II) on anatase at various temperatures. Symbols, experimental data; solid lines, model-fitted adsorption isotherms; dashed lines, model-fitted desorption isotherms. S5, S20, and S40 indicate where desorption was initiated and samples selected for subsequent EXAFS analysis. Data given as mean of duplicates and

corner-sharing linkage mode at the Zn-Ti distance of 3.69 Å may be a mixture of 4 coordinated bidentate binuclear (BB, 3.48 Å) and 6-coordinated monodentate mononuclear (MM, 4.01 Å) MEA states. DFT calculated energies showed that the MM complex was an energetically unstable MEA state compared with the BB (-8.58 kcal/mol) and BM (edgesharing bidentate mononuclear, -15.15 kcal/mol) adsorption modes,13 indicating that the MM linkage mode would be a minor MEA state, compared to the BB and BM MEA state. In the X-ray absorption near-edge structure analysis (XANES), the calculated XANES of BB and BM complexes reproduced all absorption characteristics (absorption edge, post-edge absorption oscillation and shape resonances) from the experimental XANES spectra (Figure 15).13 Therefore, the overall spectral and computational evidence indicated that the cornersharing BB and edge-sharing BM complexation mode coexisted in the adsorption of Zn(II)

As the temperature increased from 5 to 40 °C, the number of strong adsorption sites (edge linkage) remained relatively constant while the number of the weak adsorption sites (corner linkage) increased by 31%.29 These results indicate that the net gain in adsorption capacity and the decreased adsorption irreversibility at elevated temperatures were due to the increase in available weak adsorption sites or the decrease in the ratio of edge linkage to corner linkage. Both the macroscopic adsorption/desorption equilibrium data and the molecular level evidence indicated a strong temperature dependence for the metastable-

errors refer to the difference between the duplicated samples.

on anatase.

equilibrium adsorption of Zn(II) on anatase.

edge-sharing linkage was a stronger adsorption mode than that of the corner-sharing linkage, which would make it more difficult for the edge linkage to be desorbed from the solid surfaces than the corner linkage.20 So adsorption of Zn(II) onto manganite was more irreversible than that on δ-MnO2. This implied that the adsorption reversibility was influenced by the proportion of different bonding modes between adsorbate and adsorbent in nature.

Due to the contrast adsorption linkage mode, Zn(II) adsorbed on δ-MnO2 and manganite can be in very different metastable-equilibrium adsorption (MEA) states, which result in the different macroscopic adsorption–desorption behavior. For example, the extents of inconstancy of the equilibrium adsorption constant and the particle concentration effect are very different in the two systems. Adsorption of metals on δ-MnO2 and manganite may therefore be used as a pair of model systems for comparative studies of metastableequilibrium adsorption.
