Bioleaching Of Middle-low Grade Rock Phosphates By Acidithiobacillus And Its Leaching Mechanism

Rock phosphates (RPs) in Hubei province are abundant, most of which aremiddle-low grade and refractory. In order to realize the sustainable development ofphosphorous industry in our province, it is of great significance for us to make aresearch on the bioleaching of RPs. Two autotrophic phosphate solubilizingmicroorgansims CK and TST3were isolated from an acid mine drainage of Yichangphosphate mines (Hubei, China) and the soil sample on acid mine drainage of DayeTongshankou copper mines (Hubei, China), respectively. According to their cellmorphology, physiological characteristics and16S rDNA gene sequence analyses, theisolates CK and TST3are identified as Acidithiobacillus ferrooxidans and A.caldus,respectively.The analyses of the residues leached by the isolates showed that gypsum crystalswere precipitated in these bio-leached residues. In9K leaching systems, the formationof jarosite precipitates was also observed on the surface of residues leached byA.ferrooxidans CK, which can strongly inhibit the erosion of RPs as a passivation layer.The crude ores mainly consist of fluorapatite, carbonate-fluorapatite, dolomite andquartz, which are associated with pyrite. Therefore, the process of the solubilization ofRPs and the formation of gypsum crystals through bacterial activity of A.ferrooxidansCK or A.caldus TST3was elucidated on the basis of these data.The optimum conditions for these isolates to solubilize low-grade RPs werestudied. Under the optimal condition for the bioleaching of RPs which the temperatureof30C, initial pH of1.5, rotation speed of135r/min, the inoculation of2%, particlesize of RPs of <74μm, the amount of RPs of10.0g/L, the concentration of pyrite of15.0g/L and the amount of FeSO4·7H2O of44.7g/L, the fraction of phosphorousleached by A.ferrooxidans CK was70.15%after22d of leaching. In addition, thefraction of phosphorous leached of95.24%was achieved by A.caldus TST3after11dof leaching under the optimal condition for the bioleaching which the temperature of33C, initial pH of1.5, rotation speed of120r/min, the inoculation amount of30%,particle size of RPs of150～300μm, the amount of sulfur powders of10.0g/L and the concentration of RPs of10.0g/L.The effects of amino acids on the bioleaching of low-grade RPs by two isolateswere investigated in this study. With the addition of0.5g/L of L-glycine,0.5g/L ofL-lysine,0.5g/L of L-leucine or1.0g/L of L-glutamic acid to leaching solution, thefraction of phosphorous leached after12d of leaching by A.ferrooxidans CK wasincreased by20.24%,6.44%,4.00%or13.17%compared to the normal processwithout these biochemical additives, respectively. However, these amino acids addedcouldn‘t significantly accelerate the bioleaching by A.caldus TST3. The results ofreal-time quantitative PCR showed mRNA expression levels of those ferrous ionmetabolism-related genes such as ctaR, cup and rus were increased when a certainamount of L-glycine (0.5g/L) was added into leaching solution, which played a keyrole in the enhanced solubilization of RPs by A.ferrooxidans CK.The solubilization of RPs by these bacteria was compared under the conditions ofdifferent leaching method. These maximum fractions of leached phosphorus in the9K,9K+S or Waksman medium by A.ferrooxidans CK with leachate leaching methodconstitute increases of36.66%,30.33%and131.04%over those achieved with batchleaching method, respectively. However, the fraction of phosphorus leached after7dof leaching in Waksman medium by A.caldus TST3with batch leaching method wasincreased by27.18%compared to that with leachate leaching method. The correlationbetween the solubilization of RPs and bacterial growth was discussed. ForA.ferrooxidans CK, the supplement of minerals into leaching solution with inocula inthe process of batch leaching could exert negative effect on the bacterial growth.However, the addition of RPs could facilitate the growth of A.caldus TST3as the resultof the highly upregulated mRNA expression of these genes coding sulfur oxidationproteins in the Sox, Sor, S4I pathways and terminal oxidases in the eletron transportchain.