Bacterial and Chemical Reductive Dissolution of Mn-, Co-, Cr-, and Al-Substituted Goethites

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Abstract
Goethite samples in which 5% of the iron atoms were replaced with different noniron metal (NIM) atoms (Mn, Co, Al, Cr) were synthesized and characterized by X-ray diffraction. Their dissolution (a) by bacterial (enzymatic) reduction and (b) by chemical dissolution through reduction and complexing with a citrate-bicarbonate-dithionite reagent was investigated and compared with the response of similarly treated nonsubstituted goethite. Chemical dissolution differed in rate of iron solubilization depending on NIM substituents. Initial rates of iron dissolution were faster with goethite-lacking metal substituents than with Al-goethite. Bacterial fermentation and reduction processes were not significantly modified by the presence of Mn, Co, Al, or Cr, but the dissolution rates of iron decreased with substitution. During bacterial iron reduction, Al-goethite was more resistant than Mn-goethite, Co-goethite, Cr-goethite, and pure goethite. Both chemical and bacterial reduction of iron resulted in simultaneous solubilization of the NIMs. In the chemical treatment, Mn, Al, and Co were dissolved congruently with respect to Fe, whereas Cr was not. However, in the bacterial treatment, only Mn and Co solubilization were congruent with respect to iron. The fate of the substitution metals is of fundamental interest to an understanding of microbial weathering and in assessing the potential, after solubilization, for their uptake by plants and pollution of ground water.

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