Limitations to the reductive dissolution of Al-substituted goethites by Clostridium butyricum

The effect of increasing Al substitution of synthetic goethites on chemical reductive dissolution is well known. But little is known about the bacterial reductive dissolution of Al-substituted goethites, although Fe(III) oxides are Al substituted in most natural sites. Batch cultures of the fermentative Fe(III) reducer Clostridium butyricum were used to reduce goethites with 1.4 (AL1), 5.2 (AL5), 8.1 (AL8), and 32 (AL32) mol% Al for Fe substitution. As known for chemical reductive dissolution experiments, maximum bacterial dissolution rates (normalized to initial specific surfaces) were negatively linear correlated with the degree of Al-substitution. Although there was no deficiency of carbon substrate, the iron oxides were not dissolved completely and the final degree of dissolution decreased with increasing Al-substitution. Because the medium lacked an effective complexing agent, Fe and Al dissolved incongruently: only 11–55% of the Al released during reduction of Fe(III) was found in solution at the end of incubation. It was calculated that the remaining Al was mainly associated (80–93%) with the goethites but only to a minor extent with the bacteria. Therefore, one possible reason for the total inhibition of dissolution of goethites is that with increasing Al-substitution a minor degree of dissolution is sufficient to cover the goethite surface with undissolved, non-reducible Al. In fact, it was calculated, that 1.1–1.4 μmol Al m−2 of initial surface were associated with the goethites. Other possible reasons for inhibition of dissolution include direct inhibition of the microbial activity. Due to the slower reduction of the higher Al-substituted goethites the pH decreased to lower values and even less glucose was fermented. Toxic effects on bacteria could result from (a) the acidity itself or its influence on (b) Al solubility and shifting speciation of Al towards toxic Al3+ and (c) on the speciation of butyric acid. As indicated in the literature, all these factors are within the bounds to inhibit activity of C. butyricum. On the basis of our data it is not possible to decide whether one factor was of major importance or all acted together.