## 2 Results and Discussion
### The Chemical Nature of Zirconium in the L-Zr-X Samples  
The chelate-forming ability of the iminodiacetate group resulting in the coordination of a nitrogen lonely electron pair to the zirconium d orbitals stabilizes the chelate structure. In this way, the protonation possibility of a tertiary nitrogen atom with HX acids in the chelate ring is lost. Therefore, the presence of foreign anions (chloride, nitrate and sulfate) due to the protonation of tertiary N atoms cannot be expected. The iminodiacetate group is a divalent-ion selective group; thus, one iminodiacetate group neutralizes only two charges of a multivalent cation, and one or more foreign ions neutralize the surplus charges, as was observed in the case of L-Fe-X complexes containing Fe(III) ions [6; 7; 8; 9]. In principle, the counter-ions in the salts used for Zr loading (chloride, nitrate or sulfate) may neutralize foreign anions. However, the elemental analysis data and IR spectroscopy results on L-Zr-X compounds show the absence of any foreign anion; thus, the tertiary amino group of the -CH\({}_{2}\)N(CH\({}_{2}\)COOH)\({}_{2}\) part must be self-protonated with one of its carboxylic acid group (-CH\({}_{2}\)NH(CH\({}_{2}\)COO)(CH\({}_{2}\)COOH) (Scheme 1b)). The almost identical N content in the three L-Zr-X samples or the lack of sulfur indicates the absence of nitrate or sulfate anions (Supplementary Materials, Table S1). The matching IR patterns for L-Zr-X samples (X=Cl, N and S, Supplementary Materials, Figure S2) confirm that the chemical nature of these samples is practically the same. Most of the sulfate and nitrate IR bands would coincide with the IR bands of the iminodiacetate group and STY-DVB skeleton. A comparison of the IR spectra of the HCl, HNO\({}_{3}\) and H\({}_{2}\)SO\({}_{4}\) salts of the iminodiacetic acid (Supplementary Materials, Figure S3) and L-Zr-Cl (Supplementary Materials, Figure S2) resins showed that the v\({}_{\rm as}\)(S-O) and v\({}_{\rm d}\)(N-O) modes of sulfate and nitrate ions could be identified in the IR spectra of [HN(CH\({}_{2}\)COOH)\({}_{2}\)].HX (X=NO\({}_{3}\) and \(\frac{1}{2}\)SO\({}_{4}\)) salts as a doublet at 1143/1042 cm\({}^{-1}\) and a singlet at 1057 cm\({}^{-1}\), respectively. These bands should appear in these spectral ranges to show the presence of sulfate and nitrate ions in the IR spectra of L-X and L-Zr-X resins if the tertiary N atom of the iminodiacetate fragment is protonated with the HX (X=N or S) acids. However, a comparison of the L-X and L-Zr-X compounds (Supplementary Materials, Figures S4-S6) showed that neither the L-X compounds nor the L-Zr-X compounds contain foreign anions (CI, NO\({}_{3}\)- or SO\({}_{4}\)\({}^{2}\)-). The presence of the Zr=O group or its hydrated form (Zr(OH)\({}_{2}\)) with oxide- or hydroxide-neutralizing anions cannot be confirmed because of the absence of a strong and wide band at 1020-870 cm\({}^{-1}\) characterizing the zirconyl ion [14; 15; 16; 17]. [PERSON] et al., assumed the presence of Zr species of the type L-Zr(OH)\({}_{2}\) in a Zr-saturated IDA resin, but we did not observe any strong and sharp bands of ionic OH [17; 18] above 3500 cm\({}^{-1}\) in the IR spectra of our L-Zr-X resins (Figure 1). Therefore, a similar chemical environment of the N atom is expected in all six L-X and L-Zr-X (X=CI, N and S) compounds. The IR spectra of L-X and L-Zr-X resins showed the typical C-H (aromatic and aliphatic) and C-C (aromatic ring) and other vibrational modes of the polystyrene-divinylbenzene copolymers [5; 19; 20], and the typical normal modes of iminodiacetate groups as well [21]. The evaluation of the spectra imposes a challenge due to the substantial overlaps of the polystyrene network and iminodiacetate group modes. The most critical peaks agree with the reference values [5; 14; 22; 23; 24; 25; 26; 27; 28] given for polystyrene-DVB samples. There is no peak at 1680 cm\({}^{-1}\) associated with the free vinyl groups of the crosslinker. Thus, there are no pendant vinyl groups in the samples.