Effects of solution chemistry on the removal reaction between calcium carbonate-based materials and Fe(II)

Elevated iron concentrations have been observed in the groundwater underlying and surrounding several Florida landfill sites. An in situ groundwater remediation method for iron (present as soluble ferrous iron) using a permeable reactive barrier composed of calcium carbonate-based materials (CCBMs), such as limestone, was examined as a potentially effective and low-cost treatment technique. The effects of various environmental factors (i.e., pH, co-existing cations, and natural organic matter (NOM)) on the removal reaction were investigated using laboratory batch studies. Solution pH had a minor effect on iron removal, with superior iron removal observed in the highest pH solution (pH of 9). Sodium and calcium tended to impede the iron removal process by increasing the ionic strength of the solution. Manganese competes with iron ions at the adsorption sites on CCBMs; therefore, the presence of manganese prohibits iron removal and reduces removal effectiveness. NOM was found to decrease Fe(II) uptake by CCBMs and reduce the removal effectiveness by complexing Fe(II), most likely through the carboxyl group, thereby maintaining Fe(II) mobility in the aqueous phase.

► Removal of Fe(II) by limestone is not significantly affected from pH 7 to 9. ► Na+ significantly affected removal of Fe(II) at levels of 100 mg/L and above. ► Ca2 + and Mn2 + showed effect on removal as low as 10 mg/L Ca2 + and 5 mg/L Mn2 +. ► NOM premixed with Fe(II) (10 mg/L DOC) resulted in final Fe(II) levels above GCTL. ► NOM retained 0.05 mg Fe(II)/mg C for 2/3 sources and 0.032 mg/mg for 1/3.