Complexing capacity of different molecular weight fractions of sedimentaryhumic substances

In this paper we studied the complexing capacity of different molecular wei ght fractions of humic substances, subdivided into humic (HA) and fulvic ac ids (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 oxyge n and nitrogenous functional groups such as carboxylic, alcoholic and/or ph enolic and peptide groups. The higher degree of condensation of humic acids than fulvic acids is confirmed by their different molecular weight distrib ution; HA molecules are characterised by a narrow range of molecular weight s while fulvic acids are distributed over a wider range of molecular weight s. The complexation capacity for HA and FA increases with increasing pH val ue and at the same pH value the CC for HA and FA is equal even if structura l features and molecular weight distribution are different. Moreover, the c alorimetric measurements relative to fulvic acids at different pH values sh ow that by increasing the pH, the quantity of heat involved in the metal io n-fulvate interactions changes from an exothermic response to an endothermi c one. This shows that pH value influences the different reactions involved in the binding process, such as coordination reactions, electrostatic inte ractions, deprotonation due to exchange of copper with proton, interactions between hydrolysis products of copper and fulvic acids, in different ways.