High‐rate ferric sulfate generation by a Leptospirillum ferriphilum‐dominated biofilm and the role of jarosite in biomass retainment in a fluidized‐bed reactor

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Abstract
The aims of this work were to develop a high‐rate fluidized‐bed bioprocess for ferric sulfate production, to characterize biomass retainment, and to determine the phylogeny of the enrichment culture. After 7 months of continuous enrichment and air aeration at 37°C, the iron oxidation rate of 8.2 g Fe2+ L−1h−1 (4.5·10−12 g Fe2+ cell‐1 h−1) was obtained at a hydraulic retention time (HRT) of 0.6 h. However, oxygen supply became the rate‐limiting factor. With gas mixture (99.5% O2/0.5% CO2 (vol/vol)) aeration and HRT of 0.2 h, the iron oxidation rate was 26.4 g Fe2+ L−1h−1 (1.0·10−11 g Fe2+ cell−1 h−1). Leptospirillum sp. was predominant in the mesophilic fluidized‐bed reactor (FBR) enrichment culture as determined by fluorescent in situ hybridization, while Acidithiobacillus ferrooxidans was not detected. Denaturing gradient gel electrophoresis (DGGE) of the amplified partial 16S rDNA showed only three bands, indicating a simple microbial community. DGGE fragment excision and sequencing showed that the populations were related to L. ferriphilum (100% similarity in sequence) and possibly to the genus Ferroplasma (96% similarity to F. acidiphilum). Jarosite precipitates accumulated on the top of the activated carbon biomass carrier material, increasing the rate of iron oxidation. The activated carbon carrier material, jarosite precipitates, and reactor liquid contained 59% (or 3.71·109 cells g−1), 31% (or 3.12·1010 cells g−1) and 10% (or 1.24·108 cells mL−1) of the total FBR microbes, respectively, demonstrating that the jarosite precipitates played an important role in the FBR biomass retainment.

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