Effect of dissolved organic carbon on the mobility of cadmium, nickel and zinc in leachate polluted groundwater

The ability of dissolved organic carbon (DOC) from landfill leachate polluted groundwater to form complexes with the heavy metals cadmium (Cd), nickel (Ni) and zinc (Zn) was investigated. The DOC samples originated from the leachate pollution plume at Vejen Landfill, Denmark and were studied in the original matrix with a minimum of manipulation.

The experiments were performed as batch sorption experiments and the metal distribution between the aquifer material and the solution (Kd) was determined in the leachate polluted groundwater samples and in reference solutions of synthetic inorganic leachate. The difference in distribution coefficients was a direct indication of complex formation between DOC and heavy metals.

The results showed, that DOC from landfill leachate polluted groundwater has the ability to form complexes with Cd, Ni and Zn, and the distribution coefficients were a factor of 2–6 lower in the presence of DOC. Based on the distribution coefficients, the relative migration velocities of the heavy metals were estimated.

The migration velocity of the metals was increased by the presence of DOC but did not exceed 1.2% of the water migration velocity, indicating that the effect of DOC on the mobility of Cd, Ni and Zn may have only minor environmental importance. Conditional complex formation constants (log Kc) were estimated from the Kd-values.

The constants for 1:1 complexes increased slightly in the following order Zn < Cd < Ni. However, increasing the metal concentration into the mg l−1 level decreased the constants by about one order of magnitude, showing the importance of determining the constants at environmentally relevant metal concentrations.