MINERALOGICAL FORM CONTROL OF FERRIC/FERROUS PRECIPITSTS
FOR EFFECTIVE TREATMENT OF ACID MINE DRAINAGE
Chiharu Tokoro, Yugo Mitani, Tatsuya Kato, Abdullah Al Mamun
Department of Science and Engineering, Waseda University
ABSTRACT
Acid mine drainage (AMD), which is a major source for mining pollution, often contains ferric and/or
ferrous ions with several toxic elements. The ferric and ferrous ions in AMD precipitate as several kinds of
mineralogical forms after neutralization and the ferric/ferrous precipitates become main component of
sludge generated after AMD treatment. They are also good absorbent for several toxic elements. To avoid
generation of voluminous sludge and re-dissolution of toxic elements from the sludge, it is very important
to generate well-drained and efficient adsorbent from ferric/ferrous ions by mineralogical form control of
ferric/ferrous precipitates.
Green rust, which is ferric and ferrous hydroxide, is a layer double hydroxide and capable to sorb several
toxic elements. The green rust transforms to goethite or lepidochrocite by rapid oxidation, whereas to
magnetite in ambient temperature by slow oxidation. The magnetite is also good absorbent especially for
toxic cations and brings compact sludge due to its magnetic property. We investigated magnetite
generation process in ambient temperature in detail. Magnetite generation process could be divided into
three steps; (i) oxidation process of Fe(II), (ii) Fe(II) surface complexation to Fe(III) hydroxide, and (iii)
crystallization of magnetite. More magnetite could form when oxidation rate was slower than
crystallization rate and optimum conditions for it was at pH 9.5 in mild agitation.
Ferrihydrite is most popular precipitates and capable to sorb many kinds of toxic elements, although it is
badly drained and solid/liquid separation of it is difficult. We investigated removal mechanism of several
toxic elements in co-precipitation process with ferrihydrite and confirmed that removal mechanism in coprecipitation process partly involved surface precipitation when the initial ratio of toxic elements to ferric
ion was high, although main mechanism of it was surface complexation. When surface precipitation
formed, high efficiency for toxic elements removal to ferrihydrite could be achieved.
KEYWORDS
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