# A Polymer Inclusion Membrane for Sensing Metal Complexation in Natural Waters
## 3 Results
### Effect of the Metal Complexation  
To test the feasibility of the developed PIM as a tool to evaluate metal complexation, two organic ligands have been used: EDTA, forming stable complexes with divalent metal ions, and humic acids, which are present in natural waters.  
For that, we previously checked using Visual MINTEQ software and an appropriate range of EDTA to obtain solutions with different free metal content when fixing the total metal concentration at 0.03 mM. Therefore, the evaluated EDTA range was 0-30 \(\upmu\)M. In the case of Zn, free metal was 98% when no EDTA was added, 66% at 10 \(\upmu\)M EDTA, 34% at 20 \(\upmu\)M EDTA, and 2% at 30 \(\upmu\)M EDTA. In the case of Cu, the free metal was 97%, 66%, 35%, and 5%, respectively.  
Figure 3: Metal accumulation at different times (individual studies). Feed phase: 100 mL of a 0.03 mM Zn or Cu in 0.01 M KNO\({}_{3}\), pH 6.0; PIM: M2; receiving phase: 5 mL 0.01 M HNO\({}_{3}\) (\(n=3\)).  
Figure 2: Effect of PIM composition on Zn accumulation at different times. Feed phase: 100 mL of a 0.03 mM Zn in 0.01 M KNO\({}_{3}\), pH 6.0; PIMs: M1–M3; receiving phase: 5 mL 0.01 M HNO\({}_{3}\) (\(n=3\)).  
Figure 5 shows the results of the accumulated metal using the PIM sensor vs. EDTA concentration in the feed solution, (a) for Zn and (b) for Cu. As can be observed, in both cases, the metal accumulated in the receiving phase diminished when the amount of EDTA increased, being almost zero at 30 \(\upmu\)M EDTA (where only 2% of Zn and 5% of Cu are present as a free species according to the MINTEQ calculations). These findings support the fact that the PIM is able to distinguish between strongly complexed metal species and free metal ions. Moreover, the variation of metal accumulated seemed to correlate well with the content of both free Zn or Cu initially present in the feed solution, estimated with the calibration curves shown in Figure 4.  
Figure 4: Effect of the initial metal concentration in the feed phase (individual studies) on metal accumulation. Feed phase: 100 mL of a 0.001 to 0.03 mM for Zn and 0.008 to 0.03 mM for Cu in 0.01 M KNO\({}_{3}\), pH 6.0; PIM: M2; receiving phase: 5 mL 0.01 M HNO\({}_{3}\) (\(n=3\)).  
Figure 5: Effect of EDTA on metal accumulation. Feed phase: 100 mL of 0.03 mM of Zn (**a**) or Cu (**b**) in 0.01 M KNO\({}_{3}\), pH 6.0 and EDTA: 0 to 30 \(\upmu\)M; PIM: M2; receiving phase: 5 mL 0.01 M HNO\({}_{3}\) (\(n=3\)).  
HA were also tested as a ligand for both Zn and Cu, using as a feed solution a 60 mg L\({}^{-1}\) HA in 0.01 M KNO\({}_{3}\) at pH 6 with a total metal concentration of 0.03 mM. It was found that, under these conditions, the metal accumulated in the receiving phase was 0.09 mM for Zn and 0.02 mM for Cu. The fact that, in the absence of HA, the metal accumulated was higher (0.12 mM for Zn and 0.19 mM for Cu) gives more evidence that the PIM sensor is sensitive at the presence of ligands that either diminish the free metal content or form complexes that cannot pass through the PIM.  
Consequently, these results show the feasibility of the developed PIM sensor to measure metal complexation in a controlled medium.