Conditions for biological precipitation of iron by Gallionella ferruginea in a slightly polluted ground water

A sand filter has been built as a pilot plant with the purpose of biologica l precipitation of Fe from ground water polluted with mainly chlorinated al iphatics, The ground water is pumped directly from a well in a polluted gro und water aquifer in Esbjerg, Denmark. The pollution includes trichlorethyl ene and tetrachlorethylene together with smaller amounts of pesticides. Fur thermore the best conditions for Fe precipitating bacteria were not expecte d to be present because of a relatively high O-2 content, up to 6.7 mg/l, a low Fe content, 0.2 mg/l and a pH of similar to5 in the ground water. Adde d FeSO4 increased the Fe content of the ground water to about 4 mg/l. These rather extreme conditions for precipitating Fe were observed over a period of 3 months. The goal of the research was to observe the mechanism of Fe p recipitation in a sand filter in the above mentioned conditions comparative to normal conditions for biotic as well as abiotic Fe mineralization in sa nd filters of fresh water treatment plants. The Fe precipitating bacterium Gallionella ferrugenia was found to dominate the biotic Fe oxidation/precip itation process despite the extreme conditions. A huge amount of exopolymer from Gallionella was present. The precipitated Fe oxide was determined to be ferrihydrate. The rate of the Fe oxidation/precipitation was found to be about 1000 times faster than formerly found for abiotic physicochemical ox idation/precipitation processes. The hydrophobic pesticides and some of the ir degradation products were not adsorbed in the filter. An added hydrophil ic pesticide was adsorbed up to 40%. Trichlorethylene was not adsorbed in t he filler. The reason for the poor adsorption of the hydrophobic compounds and trichlorethylene is due to the pronounced hydrophilic property of the e xopolymers of Gallionella and the precipitated ferrihydrite. (C) 2001 Elsev ier Science Ltd. All rights reserved.