# Application of Modern Research Methods for the Physicochemical Characterization of Ion Exchangers
## 4 Conclusions  
In this paper, the new trends for ion exchangers characteristics and application were presented. Special attention was paid to the ion exchangers with multifunctionality for heavy metal ions removal Dowex M 4195, Amberlite IRA 743 and Purolite Arsen X\({}^{\rm rp}\). The use of optical profilometry allowed for the physicochemical characterization of ion exchanger beads. The tests showed the two-layer structure of the Dowex M4195 ion exchanger and the homogeneous internal structure of Amberlite IRA 743 and Purolite Arsen X\({}^{\rm rp}\). These studies allowed us to determine the roughness parameters of the ion exchangers cut with the ultramicroton. Information on the roughness of the ion exchanger in future studies will be related to the depth of penetration of the sorbed ions into the ion exchanger beads. Moreover, the microscopic tests carried out using optical microscopy allowed the observation of the two-layer structure of Dowex M 4195 ion exchanger which is very important taking into account its application. XPS spectroscopy turned out to be an excellent tool for studying changes in the area of an ion exchanger molecule after the sorption processes. This enables the quantitative determination of the sorbed metal content on the ion exchanger surface and other analytical techniques should be used to quantify the amount of sorbed metals in the total volume of beads. However, XPS spectroscopy is very effective for the observation of sorption mechanisms. The XPS studies showed that the process of Cu(II) ions sorption on Dowex M 4195 occurs mainly with the share of nitrogen atoms present in the pyridine rings originating from the bis-(2-pyridylmethyl) amino) functional groups, which was confirmed by the theoretical calculations. Therefore, the optical profilometry and the X-ray photoelectron spectroscopy can prove beneficial for this purpose. The proposed methods were also helpful for the Amberlite IRA 743 and Purolite Arsen X\({}^{\rm rp}\) characterization.  
Conceptualization, D.K., Y.-G.C., S.-H.C., G.-Y.L. and W.S.-C.; methodology, D.K. and W.S.-C.; validation, D.K., W.S.-C. and Y.-G.C.; formal analysis, D.K., W.S.-C. and Y.-G.C.; investigation, W.S.-C., Y.-G.C. and D.K.; writing--original draft preparation, D.K., W.S.-C. and Y.-G.C.; writing--review and editing, D.K., visualization, D.K.; W.S.-C. and Y.-G.C.; supervision, D.K. All authors have read and agreed to the published version of the manuscript.  
Figure 10: Cu(II) ions complexation by Dowex M 4195 proposed on the basis of theoretical calculations with the bis-(2-pyridylmethyl)amino groups attached in the _para_ position.  
**Funding:** This research was founded by the National Key Research and Development Program (No. 2017 YFC1702200), the Key Research and Development Program of Zhejiang Province (No. 2017C03052 and No. 2015C02032), Ten-Thousand Talents Program of Zhejiang Province (No. ZJWR0102035).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Acknowledgments:** The authors are grateful to [PERSON], Maria Curie Sklodowska University, Poland for the theoretical calculations.  
**Conflicts of Interest:** The authors declare no conflict of interest.