3.3. Zeta Potential Analysis
The adsorption of reagents can lead to changes in the charge of the specularite surface. Therefore, zeta potential testing was used to evaluate the charge changes on the surface of a mineral.
Figure 7 shows the relationship between the zeta potential and pH of specularite, aegirine, and chlorite when Pb
2+ was used as an activator.
Figure 7a shows that the isoelectric point (IEP) of specularite occurs at pH of 4.4. After adding NaOL, the surface potential of specularite decreases over the entire pH range, and the IEP shifts to 3.7. This is mainly because NaOL is a polar molecule that adsorbs and replaces water molecules on the mineral surface and arranges them directionally, thereby forming an additional adsorption dipole layer that changes the interphase potential difference between the remaining charges on the mineral surface. Consequently, a negative shift occurs in the mineral surface potential. After treatment with Pb
2+, the potential of the specularite shifts significantly and positively over the entire pH range, with an IEP of 8.9, indicating that Pb
2+ was adsorbed onto the surface of the specularite. After simultaneous treatment with NaOL and Pb
2+, the zeta potential of specularite decreased significantly over the entire pH range. Additionally, its IEP shifted from 8.9 to 5.1, indicating that a large amount of NaOL was adsorbed onto the surface of the specularite after lead ion activation. Comparing the zeta potential changes of natural specularite and specularite treated with Pb
2+ before and after NaOL treatment, the zeta potential difference of specularite treated with lead ions is greater than that of natural specularite, particularly in the pH range of 6–8. This indicates that more oleic acid species were adsorbed on the surface of specularite after lead ion treatment, which may have been due to the increase in the number of reaction sites on the surface of specularite after treatment with Pb
2+. Consequently, the adsorption of oleic acid substances on the surface of the modified minerals was promoted and the hydrophobicity of the specularite particles was improved.
As shown in
Figure 7b,c, the zeta potential changes of the aegirine and chlorite treated with sodium oleate or Pb
2+ are relatively small over the entire pH range compared with those of specularite. This indicates that the addition of Pb
2+ has a small impact on the adsorption quantity of sodium oleate on the surface of aegirine and chlorite.
3.4. DFT Results
Using the above research methods, the mechanism of action of lead ions and their effects on the adsorption of the NaOL collector on the mineral surfaces were analyzed. However, the underlying mechanisms remain unclear. Therefore, DFT was used to clarify the mechanisms of the interactions between the agents and mineral surfaces from an atomic point of view. The results of the solution chemistry calculations indicate that PbOH+ may be the main component of the selectively activated specularite. Therefore, this section discusses the adsorption configuration of PbOH+ on the mineral surface, further revealing the selective activation mechanism of metal ion activators on specularite.
Figure 8 and
Table 1 show the adsorption configuration and bonding characteristics of PbOH
+ on the surface of specularite.
According to the calculation results, PbOH+ can interact with the O atom and the Fe atom on the surface of specularite to form three chemical bonds, Fe1-O1, Pb1-O2, and Pb1-O3, with bond lengths of 1.922, 2.647, and 2.661 Å, and bond populations of 0.29, 0.05, and 0.04, respectively. The adsorption energy of the interaction between PbOH+ and specularite is −3.58 eV, indicating that the reaction occurred spontaneously.
Figure 9 and
Table 2 present the adsorption configuration and bonding characteristics of PbOH
+ on the aegirine surface.
According to the calculation results, PbOH+ can interact with the O and Fe atoms on the surface of aegirine to form two chemical bonds, Pb1-O2 and Fe1-O1, with bond lengths of 2.402 Å and 1.965 Å, and bond populations of 0.02 and 0.20, respectively. The adsorption energy of the interaction between PbOH+ and aegirine is −1.84 eV, which is lower than that of PbOH+ and specularite (−3.58 eV).
Figure 10 and
Table 3 present the adsorption configuration and bonding characteristics of PbOH
+ on the chlorite surface.
According to the calculation results, PbOH+ can interact with the O atom on the surface of chlorite, forming a Pb1-O1 bond with a bond length of 2.269 Å and bond population of 0.08. The adsorption energy of the interaction between PbOH+ and chlorite is −2.29 eV. These results indicate that the interaction strength between PbOH+ and the surface of chlorite is also lower than that of specularite, which may be the main reason why lead ions can selectively activate specularite and have a smaller effect on the flotation of aegirine and chlorite.
To further determine the electron transfer between atoms in the system during the surface bonding process of lead ions with specularite, aegirine, and chlorite, the Mulliken population and charge changes of the relevant atoms in each system were analyzed before and after the action of Pb ions.
The Mulliken populations of the related elements before and after the interaction between PbOH
+ and the surface of the specularite are shown in
Figure 11 and
Table 4.
According to the results, there is a strong interaction between the Fe1 and O1 atoms, and between the Pb1 and O2 atoms, after the adsorption of PbOH+. This is accompanied by an evident shared electron behavior, indicating an interaction between the two forms of Fe-O and Pb-O bonds.
After the interaction between PbOH+ and the surface of specularite, the charge of the Fe1 atom on the surface of specularite increased from 0.84 e to 1.02 e, an increase of 0.18 e, and its electron loss mainly occurs in the Fe 4s orbital. The charge of the O1 atom in PbOH+ decreased from −0.95 e to −0.97 e, a decrease of 0.02 e, and its electrons are mainly located in the O 2p orbital. These results indicate that the charge in the Fe-O bond formed by the adsorption of PbOH+ on the surface of specularite is mainly transferred from the 4s orbital of the Fe1 atom to the 2p orbital of the O1 atom. Moreover, the charge of the O2 atoms on the surface of specularite decreased from −0.58 e to −0.60 e, a decrease of 0.02 e. The charge of O3 atom decreased from −0.59 e to −0.60 e, a decrease of 0.01 e, indicating that the electrons of the O atom on the surface of specularite are mainly located in the O 2p orbital. The charge of the Pb1 atom is 0.81 e, and compared to that before adsorption, the electron loss is mainly in the Pb 6p orbital. Based on the charge analysis between the above atoms, when PbOH+ is adsorbed on the surface of the specularite, the charge of the Pb-O bond formed by the interaction is mainly transferred from the 6p orbital of the Pb1 atom to the 2p orbitals of the O2 and O3 atoms.
The Mulliken populations of related elements before and after the interaction between PbOH
+ and the aegirine surface are shown in
Figure 12 and
Table 5.
After the adsorption of PbOH+, the electronic ability of Fe1 on the aegirine surface is significantly reduced. In addition, there is a clear shared electronic behavior between the Pb1 atom in PbOH+ and the O2 atom on the surface of aegirine, indicating that the two interacted to form Fe-O and Pb-O bonds.
After the interaction between PbOH+ and the surface of aegirine, the charge of Fe1 on the surface of aegirine increases from 0.82 e to 0.96 e, an increase of 0.14 e, and its electron loss mainly occurs in the Fe 3d orbital. Additionally, the charge of the O1 atom in PbOH+ decreases from −0.95 e to −0.98 e, a decrease of 0.03 e, and its electrons are mainly located in the O 2p orbital. These results indicate that the charge in the Fe-O bond formed by the adsorption of PbOH+ on the surface of aegirine is mainly transferred from the 3d orbital of the Fe1 atom to the 2p orbital of the O1 atom. The charge of the O2 atom on the surface of aegirine decreases from −0.91 e to −0.93 e, a decrease of 0.02 e, indicating that the electrons of the O atom on the surface of specularite are mainly in the O 2p orbital. The charge of the Pb1 atom is 1.09 e, and compared to that before adsorption, the electron loss is mainly in the Pb 6p orbital. Based on the charge analysis between the above atoms, when PbOH+ is adsorbed on the surface of aegirine, the charge in the Pb-O bond formed by this interaction is mainly transferred from the 6p orbital of the Pb1 atom to the 2p orbital of the O2 atom.
The Mulliken populations of the related elements before and after the interaction between PbOH
+ and the chlorite surface are shown in
Figure 13 and listed in
Table 6.
After the adsorption of PbOH+, the electron acquisition ability of the O1 atom on the surface of chlorite is significantly reduced, and there is a clear shared electron behavior between the Pb1 atom and O1 atom on the surface of chlorite, further confirming the formation of the Pb-O bond.
After the interaction between PbOH+ and the surface of chlorite, the charge of the O1 atom on the surface of chlorite decreases from −0.52 e to −0.67 e, a decrease of 0.15 e, indicating that the electrons of the O1 atom on the surface of chlorite are mainly in the O 2p orbital. The charge of the Pb1 atom in PbOH+ is 1.08 e, and compared to that before adsorption, the electron loss is mainly in the Pb 6p orbital. Based on the charge analysis between the above atoms, it can be concluded that when PbOH+ is adsorbed on the surface of the green mud, the charge in the Pb-O bond formed by the interaction is mainly transferred from the 6p orbital of the Pb1 atom to the 2p orbital of the O1 atom.