Complexing capacity of different molecular weight fractions of sedimentary humic substances

In this paper we studied the complexing capacity of different molecular weight fractions of humic substances, subdivided into humic (HA) and fulvic acids (FA), extracted from Arno river sediment. Humic acids are characterised by a high degree of aromaticity and a low nitrogen content whereas fulvic acids display aliphatic features and are characterised by a number of oxygen and nitrogenous functional groups such as carboxylic, alcoholic and/or phenolic and peptide groups. The higher degree of condensation of humic acids than fulvic acids is confirmed by their different molecular weight distribution; HA molecules are characterised by a narrow range of molecular weights while fulvic acids are distributed over a wider range of molecular weights. The complexation capacity for HA and FA increases with increasing pH value and at the same pH value the CC for HA and FA is equal even if structural features and molecular weight distribution are different. Moreover, the calorimetric measurements relative to fulvic acids at different pH values show that by increasing the pH, the quantity of heat involved in the metal ion-fulvate interactions changes from an exothermic response to an endothermic one. This shows that pH value influences the different reactions involved in the binding process, such as coordination reactions, electrostatic interactions, deprotonation due to exchange of copper with proton, interactions between hydrolysis products of copper and fulvic acids, in different ways.