Chemical, Mineralogical and Microbiological Factors Affecting the Precipitation of Fe and Mn from Groundwater

Precipitation of hydrous Fe and Mn oxides was investigated in 12 plants, constructed for purification of groundwater from excess Fe and Mn. The re-infiltration method was applied in 7 plants and the slow sand filtration in 5 plants (Hatva et al., 1985; Hatva 1987, this symposium)

Water was analysed for aquatic chemistry and for the speciation of Fe, Mn, Zn and Cu. The distribution and mode of occurrence of the elements in the precipitates were studied by successive extraction and dissolution kinetics techniques. The mineralogy of the oxidic Fe and Mn precipitates was studied using XRD techniques.

The results suggest the presence of Fe(II), Mn and Zn in groundwater mainly in cationic form. Cu was in colloidal and organically bound forms.

The precipitation of hydrous Fe and Mn oxides from groundwater is affected by chemical and microbiological factors. The oxidation of Fe(II) took place mainly chemically with increasing O₂ content, pH and Eh of the water. Ferrihydrite, a poorly ordered oxidic Fe mineral was the dominant product. Environmentally controlled colloidal and surface chemical properties of the precipitating species enhanced the precipitation causing sorption and scavenging of Si and cations. Owing to the relatively high Si content (over 4 mg Si/l) of groundwater in Finland and the interaction of Si and Fe the crystallization of lepidocrocite that is formed in Si-free systems was inhibited and ferrihydrate was formed instead.

15-20% of total Fe and above 50% of total Mn were in sorbed form in the precipitates. Only 15-20% of Mn was in sorbed form in the precipitates that contained MnO₂ minerals, vernadite and birnessite (in two plants).

The oxidation of Mn(II) appeared to involve microbial catalysis because it was adversely influenced by high pH and copper sulphate.

The freshly precipitated MnO₄ strongly sorbed and scavenged heavy metals (e.g., Ag, Sn, Ni, Co, Zn and Cu). Cations (Ca²⁺, Mg²⁺, K⁺ and Na⁺) were sorbed by the hydrous Fe oxides at high pH (above 7) whereas Cu tended to be sorbed at low pH levels.

Gallionella was the dominant type of bacteria in the raw water and the bacterial density was low. In the filtration basins Leptothrix, Siderocapsa and Ochrobium were the typical microorganisms. The bacterial density was highest in the upper part of the sand in the infiltration basins and the bacteria were covered by Fe (and Mn) precipitate encrustations.