Interaction between Ca2+ and dissolved organic carbon: implications for metal mobilization

To investigate the role of dissolved Ca on the sorption and complexation behaviour of dissolved organic carbon (DOC), and the effect on dissolved Cu concentration, batch sorption studies and titration experiments have been performed. Both adsorption and desorption studies showed that the DOC concentration depends on the presence of Ca. With increasing Ca content, the DOC concentration decreased as a result of sorption and precipitation processes. The formation of aqueous Ca-DOC complexes could be described well with a 1:1 Ca:DOC ratio. Log KCaDOC values for the formation of Ca-DOC complexes ranged from 3.7 to 4 in the low pH range (3.5 to 6, interaction with carboxyl groups) and from 5.8 to 6 in the pH range from 7 to 9 (interaction with phenolic groups). The results indicate that a large part of the available DOC is in a Ca form at near neutral pH values. In batch sorption experiments, the addition of Ca2+ significantly reduced the DOC as well as the Cu concentration in solution. This indicates that sorption of Ca-DOC complexes at near neutral pH is more important in regulating the Cu solution concentration compared to cation exchange processes.