Phosphorus recovery from high concentrations of low-grade phosphate rocks using the biogenic acid produced by the acidophilic bacteria Acidithiobacillus thiooxidans

In this paper, we are presenting a two-step biological process to recover phosphorus solubilizing low-grade phosphate rocks. We start with a growing stage, where the biogenic acid produced by the acidophilic bacteria Acidithiobacillus thiooxidans endows the proper conditions for solubilization. It is followed by a recovery stage, where the phosphate rock is added to the culture once desired levels of acidity are reached. We varied the concentration of the phosphate rock and the bacterial growing time to elucidate optimal conditions and to get insight into the biological process. We found that 10% (w/v) of phosphate rock with 23 days of bacterial growth results in a recovery of 94%. FTIR spectrums and XRD-Rietveld quantification reveal that the high phosphorus solubilization is correlated with gypsum as the predominant phase, whereas, for less efficient conditions, a significant proportion of fluorapatite, predominant in the un-treated rock, is still observed. As a general trend, increasing the bacterial growing time results in higher amounts of phosphorus recovery. However, the biogenic acid is unable to efficiently solubilize rock concentrations higher than 20% due to an inhibition in the diffusion controlled reaction. We present a one-to-one comparison with a chemical acidulation using sulfuric acid. The biogenic acid shows better performance and achieves higher percentages of recovery at all conditions, thereby suggesting that additional metabolites in the A. thiooxidans culture enhance the bioleaching. Finally, we delineated an economical viability that indicates that the chemical recovery is four times more expensive than the biogenic method. From a technological perspective, our process offers fertile grounds for non-conventional and efficient phosphorus recovery in tropical soils or for low-grade phosphate rock reservoirs.