Adsorption and co-precipitation of heavy metals on hydrous ferric oxides (HFO) from acidic waters
Major species and trace elements were analyzed in acidic waters (pH = 3.8 — 5.8) and in coexisting hydrous ferric oxides (HFO). Samples were collected from two sites, McAdoo, PA (acid mine drainage, AMD) and the Wharton State Forest (WSF), NJ (naturally acidic region). At McAdoo, Fe, Ni, Cd, and Mn concentrations exceeded USEPA and U.S. Public Health Service drinking water standards. The geochemical computer model PHRHEQC and laboratory-derived complexation constants (K) were used to predict the amount of heavy metals that should be adsorbed on HFO. The predicted values are generally 1 to 2 orders of magnitude greater than measured values. This difference may be due to organic complexation of heavy metals and other factors in the natural environment. In situ surface complexation constants have been calculated by this study. These in situ sorption constants can give better prediction of metal sorption on HFOs and mobility of trace elements in natural waters. Differences in sorption and calculated sorption constants between the two areas may reflect natural variations in organic complexing and species availability. Trace element concentrations in New Jersey bog iron ore (50 years old) were used to back-calculate metal contents in the historic waters from which it formed. Dissolved trace metal concentrations depend on pH and sorption constants, and they were equal or less than present values. Unrealistic remediation goals would result from using sorption constants that do not compensate for organic complexation and other factors in the natural environment.
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