LIMITATIONS IN APPLYING SINGLE-ION SORPTION CONSTANTS FOR FERRIHYDRITE IN NATURAL SOLUTIONS: POTENTIAL EFFECT OF MULTIPLE COMPONENTS AND TEMPERATURE

Mobility of metals in ground and surface water is affected by the presence of iron precipitates due to sorption, surface precipitation, and co-precipitation. Proper prediction of the composition of iron precipitates is essential to fate and transport studies and successful remediation of a variety of metal-contaminated sites such as battery acid or metal plating spills, landfills, as well as natural acid rock drainage and acid mine drainage.

Currently, modeling the composition of iron precipitates often use sorption constants calculated from solutions containing one sorbing ion at temperatures between 20-30 C (Dzombak and Morel, 1990). These single-ion constants are not able to fully describe the behavior of metals in the presence of iron precipitates in natural systems (Tessier et al., 1996; Tonkin et al., 2002; Smith et al., 1998; Webster et al., 1998). Our models of data from a stream in Alaska that receives acid rock drainage also show limitations in applying the single-ion constants to natural systems. Analyzed aqueous chemistry from the stream was entered into the geochemical program PHREEQC, which incorporates the Dzombak and Morel (1990) sorption constants. The model-predicted composition of the solid phase that formed from the water was compared to the actual composition of the solid phase collected from the stream. The differences between analyzed and model-predicted values for the S, Zn and Mn composition were 17%, 38% and 70%, respectively. Modest modification of the sorption constants was required to match analyzed and model-predicted compositions. The constants for S, Zn and Mn required changes of 0.1, 0.5 and 0.95 log units, respectively, in order to match model-predicted and analytical values. These changes are within the margin of error reported by Dzombak and Morel (1990) due to their choice of surface properties for hydrous ferric oxide. This demonstrates that predictions with single-ion sorption constants are not always accurate for natural materials, perhaps due to potential competitive and/or synergistic effects in solutions containing multiple sorbing species or the lower temperatures of natural systems. Future work for this study will include sorption experiments conducted with multi-component solutions at temperatures from 5-25 C.