Abstract
The choice of an appropriate organic substrate as electron donor for sulphate-reducing bacteria (SRB) is a critical step on the performance of the in situ biological treatment of acid mine drainage (AMD). The relative ability of sheep manure to sustain the activity of SRB was demonstrated in a previous study where dissolved metals were deliberately omitted. The present work evaluates a sheep manure/limestone mixture in the treatment of a synthetic acid mine drainage to ascertain the effect of dissolved heavy metals on the bacterial activity. The effluent solution was systematically analyzed throughout a column experiment, at the end of which precipitates in the column were withdrawn for chemical and mineralogical analysis. Sorption onto sheep manure was examined by Fourier-transform infrared (FTIR) by comparing the general features of its spectra before and after the experiment. The results showed that although pH was neutralised and heavy metals were significantly depleted (with the exception of Zn), sulphate reduction did not evolved. This finding is in contrast with the results obtained in the previous study where heavy metals were excluded, presumably due to their toxic/inhibitory effect on the bacterial activity. Although a significant metal removal occurred (through pH-dependent mechanisms rather than bacterial activity), the results have emphasized that dissolved metals might impact the SRB metabolism and may pose significant secondary difficulties from a toxicity viewpoint. This constraint implies that organic matter degradability tests should be performed with acidic waters containing both sulphate and metals for the subsequent design of a SRB-based treatment.
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