Characterization and Complexing Capacity of Humic Acid Extracted from Yakouren Soil with Heavy Metals by Conductimetry and Quenching of Fluorescence

摘要

New humic acid isolated from forest soil of Yakouren (YHA), Algeria, and purified commercial humic acid (PCHA) were analyzed and characterized. Elemental analyses, electronic microscopy, UV-Vis absorbance E-465/E-665 ratio and molecular spectroscopy investigation using CP-MAS C-13-NMR of YHA indicated the presence of a high content of aliphatic and carboxylic acid groups. The higher O/C, H/C and N/C ratios of YHA can be assigned to the low aromatization degree. A study of interaction between Ni(II), Zn(II), Cd(II) and humic acids (HAs) at different metal-ligand ratios has been made by conductimetric and fluorescence techniques. The results obtained by the conductimetry method are interpreted using an excess function (Delta k), which related the conductivity of the mixture and of the separated components. A positive value of this function is obtained. It indicates the complexation of HAs with metallic ions. The association degree of the heavy metals (M) with both HAs was in the following order: Ni > Zn > Cd and the binding capacity of PCHA is smaller than that of YHA. A fluorescence titration method and a single site model were used for determining metal ion complexing capacities (CCM) and stability constants (log K-M) of YHA and PCHA complexes. Titration of HAs with metal ions at pH 7 and ionic strength 0.1 mol/L resulted in a marked decrease of fluorescence intensities of untreated HAs. The quenching constants (K-SV)between HAs and M are obtained in terms of Stern-Volmer analysis. K-SV show that the YHA binds the higher amount metals than PCHA. CCM of YHA calculated by MATLAB program were higher than those of PCHA and other natural HAs found in the literature at pH 7. Based on these results, YHA may play the role of a biocaptor of polluting metals.