Complexing capacity of electroplating rinsing baths—a twist to the resolution of two ligand families of similar strength

Background The presence of ligands affects metal behavior when removing metals from wastewaters. So far, most of the attention has been paid to strong ligands; however, experimental observations indicate that also weaker complexing agents may play a key role in the availability of metals in waters and in the success of the treatment to be applied. In particular, we have analyzed wastewaters of an electroplating facility during an 8-h workday. Total metal content (copper, zinc, cadmium, and lead) was determined; ligands were characterized by concentration (Lt) and strength (conditional stability constant, K'f). This work focuses on ligands of moderate strength which, as far as we know, have been overlooked in the literature. Methods A two-moderate-ligand system was used to mimic the wastewaters. N-(2-hydroxyethyl)iminodiacetic acid and nitrilotriacetic acid were used as model ligands. Electrochemical titration data (obtained by square wave anodic stripping voltammetry) were analyzed combining the Scatchard linearization method with a standard non-linear curve fitting function to focus on the resolution of two ligand families of similar strength. Correctness was tested both for the analytical procedure and data analysis. Results Most significant changes in metal concentration were related to zinc and lead that increased along the workday. Copper and cadmium contents were negligible. The model system and wastewater samples were successfully characterized by this methodology. Two ligand families of constants K'f 1 (4.07 ± 0.69 )× 10 6 and K'f 2 (5.56 ± 0.78) × 10 5 were discriminated in the micromolar range using zinc in the titration of the wastewater sample. Conclusions A combined strategy involving electrochemical techniques, the Scatchard linearization, and a non-linear curve fitting function was successfully applied to the model system, within experimental error. Our main goal was to characterize two moderate ligand families of similar strength in the wastewater samples by means of the same strategy, a task that so far has not been described. The combined strategy used in this particular case may be of interest for different environmental matrixes. Graphical abstract Evolution of pH, Zn(II) and ligand concentration during a workday in an industrial wastewater.