HUMIC ACID-METAL IONS AFFINITY THROUGH MODIFIED CARBON PASTE ELECTRODES. STUDY BY ANODIC STRIPPING VOLTAMMETRY

Carbon paste electrodes (CPE) are built by a mixture of graphite powder with either mineral or vegetal oil. Its main characteristic is its easily renewable surface but it also has characteristics like large working windows in both cathodic and anodic potentials, low cost, and facility of manipulation. Humic substances (HS) are polyelectrolytic organic structures resulting from natural substance degradation. Due to their anionic functional groups, HS have affinity and make coordination complexes with metal ions. In this paper, a carbon paste electrode was modified with humic acids (CPE-HA) and used as a working electrode. The objective was to evaluate the complexation behavior of the HA/solution interface using an electrochemical tool. The anodic stripping voltammograms of CPE-HA, obtained at pH 4.5 and 5 mV s(-1), show a quasi-reversible process with a well defined oxidation step at about 0.01 V (SCE) and two reduction steps at -0.12 and -0.17 V. Cyclic voltammetry shows that the intensity of the second cathodic step increases compared to the first one, after each cycle. The first reduction step is more pronounced in concentrated solutions. All potentials are repetitive at pH values between 3.0 and 4.5, but from 5.0 up to 6.0, the second reduction step shifts in the cathodic direction. The two-step reduction behavior is not observed with a non-modified CPE. According to the results, the two reductions steps are representative, respectively, of linked and free (or weakly linked) Cu2+ with the HA present on the electrode surface. CPE-HA tool presents a good way to evaluate metal-HA interactions in heterogeneous systems.