The Effect of Fluorine on the Precipitation of Hydrous Iron Oxides from Groundwater Using Re-Infiltration

Abstract

The behaviour of Fe, Si, Cu, Zn and F was studied in a plant constructed for purification of groundwater from excess Fe, Mn and organic matter by aeration and subsequent re-infiltration at Taivassalo, south-western Finland. The aquifer of the groundwater intake comprises the Laitila rapakivi area and the water has a relatively high F content. The groundwater purification plant consist of two units. In the pre-treatment unit the water was discharged onto a macadam contact filter for aeration, after which it flowed through two clarification basins to the infiltration basin of the re-infiltration unit. Fe oxidized during the treatment and precipitated as hydrous oxides. Precipitates were filtered from water and dried at 20°C, both immediately after sampling and after two year's aging in natural water at 4°C. To clarify the mode of occurrence of the elements the samples were subjected to extraction by 1 M ammonium acetate (NH4Ac) at pH 7.3 for exchangeable and loosely bound elements, at pH 4.8 for loosely bound and sorbed elements, by 1 M hydroxylammonium chloride plus 2 M acetic acid (HX1 - HAc) at 18°C for reducible loosely bound elements and Fe oxyhydroxides, and by HXl-HAc at 80°C for total Fe oxyhydroxide bound F. The precipitates were extracted with NH4Ac (pH 4.8) and HX1-HAc during different time intervals. The F content in water decreased along the flow path but increased during the re-infiltration step. The removal of F from water would obviously be more efficient if the plant arrangements like retention time were more suitable for this purpose. F and Si interacted strongly with Fe in the precipitates, preventing their ordering, and F, Si and Fe correlated well with each other. The loosely bound forms of these elements decreased along flow path and increased during aging of the precipitates. F and Cu were strongly, and Zn less strongly sorbed and scavenged by the precipitates. Owing to the relatively low pH of the raw water (5.74) Ca2+and other cations were not sorbed as was observed at higher pH's (over 7) and Cu and Zn were sorbed instead.