During fungal rock phosphate (RP) solubilization, a significant quantity of fluoride (F) is released together with phosphorus
(P), strongly inhibiting the process. In the present study, the effect of two F adsorbents [activated alumina (Al2O3) and biochar]
on RP solubilization by Aspergillus niger was examined. Al2O3 adsorbed part of the F released but also adsorbed soluble P,
which makes it inappropriate for microbial RP solubilization systems. In contrast, biochar adsorbed only F while enhancing
phosphate solubilization 3-fold, leading to the accumulation of up to 160 mg of P per liter. By comparing the values of F measured
in solution at the end of incubation and those from a predictive model, it was estimated that up to 19 mg of F per liter can
be removed from solution by biochar when added at 3 g liter1 to the culture medium. Thus, biochar acted as an F sink during
RP solubilization and led to an F concentration in solution that was less inhibitory to the process. In the presence of biochar, A.
niger produced larger amounts of citric, gluconic, and oxalic acids, whether RP was present or not. Our results show that biochar
enhances RP solubilization through two interrelated processes: partial removal of the released F and increased organic acid
production. Given the importance of organic acids for P solubilization and that most of the RPs contain high concentrations of
F, the proposed solubilization system offers an important technological improvement for the microbial production of soluble
P fertilizers from RP.
During fungal rock phosphate (RP) solubilization, a significant quantity of fluoride (F) is released together with phosphorus
(P), strongly inhibiting the process. In the present study, the effect of two F adsorbents [activated alumina (Al2O3) and biochar]
on RP solubilization by Aspergillus niger was examined. Al2O3 adsorbed part of the F released but also adsorbed soluble P,
which makes it inappropriate for microbial RP solubilization systems. In contrast, biochar adsorbed only F while enhancing
phosphate solubilization 3-fold, leading to the accumulation of up to 160 mg of P per liter. By comparing the values of F measured
in solution at the end of incubation and those from a predictive model, it was estimated that up to 19 mg of F per liter can
be removed from solution by biochar when added at 3 g liter1 to the culture medium. Thus, biochar acted as an F sink during
RP solubilization and led to an F concentration in solution that was less inhibitory to the process. In the presence of biochar, A.
niger produced larger amounts of citric, gluconic, and oxalic acids, whether RP was present or not. Our results show that biochar
enhances RP solubilization through two interrelated processes: partial removal of the released F and increased organic acid
production. Given the importance of organic acids for P solubilization and that most of the RPs contain high concentrations of
F, the proposed solubilization system offers an important technological improvement for the microbial production of soluble
P fertilizers from RP.
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