EFFECT OF COMPLEXATION ON COPPER MOBILITY IN LANDFILL LEACHATE POLLUTED GROUNDWATER: A EXPERIMENTALLY VALIDATED CONCEPTUAL MODEL FOR MODELING HEAVY METALS TRANSPORT

The mobility of trace metals in aquatic environment is often characterized by distribution coefficient, K_d, define as the ratio of metal concentration in the solid phase to that in solution at equilibrium. However, large variations of K_d with a given pH have been reported, thus making it difficult to select an appropriate K_d to use in trace metal transport modeling. A conceptual model, K_d  = K_{d, ion} * f_ion, was established for the purpose of predicting K_d values in a variety of aquatic environments, assuming that solid surface only sorbs free metal ion (i.e., organic and inorganic complexes can not sorb) in solution. Thus, for a sorbent material (i.e., sandy aquifer material or soils) the variation of observed trace metal K_d at a given pH value can be explained mainly as a result of the metal complexation in the solution.

Laboratory batch studies (two sandy aquifer materials, landfill leachate polluted groundwater with DOC (163 mg/l) and without DOC of salt matrix), ion-exchange speciation techniques and WHAM model (Tipping, 1994) speciation were conducted to investigate copper mobility in DOC contaminated groundwater in terms of K_d variation with pH to test the conceptual model. Also, K_d values and speciation from 82 copper contaminated soil samples reported in the literature were used to re-evaluate the conceptual model. The investigations confirmed the model was capable of predicting K_d values of copper, at a mean deviation of about 0.5 orders of magnitude at pH 5-8 in our study, and a fair prediction for a variety of contaminated soils at pH 5-8. Two empirical equations for K_d,ion values of copper correlated with pH at pH 5-8 were generated in this study for sandy aquifer materials and contaminated soils.

This study suggests that the combination of the conceptual model with empirical K_d,ion values might be an useful approach in a reactive transport model coupled with speciation model or geochemical model for modeling long-term metal contamination transport at field scale.