Modeling metal removal onto natural particles formed during mixing of acid rock drainage with ambient surface water.

Studies have examined partitioning of trace metals onto natural particles to better understand the fate and transport of trace metals in the environment, but few studies have compared model predictions with field results. We evaluate the application of an empirical modeling approach, using surface complexation parameters available in the literature, to complex natural systems. In this work, the equilibrium speciation computer program PHREEQC was used along with the diffuse double-layer surface complexation model to simulate metal removal onto natural oxide particles formed during the mixing of acid rock drainage with ambient surface water. End-member solutions sampled in the Coeur d'Alene (CdA) Mining District in September 1999 from the Bunker Hill Mine and the South Fork Coeur d'Alene (SFCdA) River were filtered and mixed in several ratios. Solution chemistry was determined for end-members and mixed solutions, and X-ray diffraction (XRD) was used to determine the mineralogy of precipitate phases. Predicted amounts of Fe precipitates were in good agreement with measured values for particulate Fe. Surface area and reactive site characteristics were used along with surface complexation constants for ferrihydrite (Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; John Wiley & Sons: New York, 1990) to predict ion sorption as a function of mixing fraction. Comparisons of model predictions with field results indicate that Pb and Cu sorption are predicted well by the model, while As, Mo, and Sb sorption are less well-predicted. Additional comparisons with particulate metal and Fe data collected from the CdA Mining District in 1996 and 1997 suggest that sorption on particulate Fe, including amorphous iron oxides and schwertmannite, may be described using universal model parameters.