Study On Microbial Solubilization Of Middle-low Grade Rock Phosphate

There are abundant of rock phosphates(RPs) in China,but most of them are middle-low grade and often abandoned today.Heterotrophic and autotrophic microorganisms were used to solubilize middle-low grade RP, respectively,in this study.It is expected that the results of this study will provide a theoretical basis to solve the utilization problems of middle-low grade RPs in China.The isolation and screening of heterotrophic phosphate solubilizing microorganisms and phosphate solubilization by these microorganisms were studied in this study.A bacterial strain YC and three fungal strains of HB1,HB2 and HB3 were isolated from phosphate mines in Hubei province(PR China).They are identified as Stenotrophomonas maltophilia(S.maltophilia),Penicillium expansum(P.expansum),Mucor ramosissimus(M ramosissimus) and Candida krissii(C.krissii), respectively.It is the first time to report these strains as phosphate solubilizing microorganisms in this study.The mechanisms of phosphate solubilization by the isolates were discussed.Results show that organic acids including gluconic acid,citric acid and oxalic acid,which were released by the isolates,played key roles on the solubilization of tricalcium phosphate and RP.Moreover,there have a positive correlation between concentration of soluble phosphorus and population of the isolates,and a negative correlation between concentration of soluble phosphorus and pH in the culture medium.The optimum conditions for the isolates to solubilize middle-low grade RP were studied.The optimum conditions to release soluble phosphorus from RP by S.maltophilia YC are:culture temperature, 29-32℃;initial pH,6.5-7.0;RP concentration,2.0 g/L;RP particle size, 0.04 mm;shaking speed,140 r/min;and inoculation of S.maltophilia YC, 25%.The maximal contents of soluble phosphorus released by three fungal isolates(P.expansum HB1,M.ramosissimus HB2 and C.krissii HB3) are attained when cultured at 32℃,shaked at 160 r/min,and added with RP concentration and particle size at 2.5 g/L and 0.04 mm, respectively.In addition,it will facilitate the solubilization of RP by the isolates to minimize the existence of toxic ions such as silver ion,or add some chelating agents such as EDTA,etc.Wheat seedling growth promoting experiments by three fungal isolates under pot culture conditions were investigated in this study. Results show that all the fungal isolates promoted the length and dry weight of the shoot and root,and phosphorus and nitrogen uptake of wheat seedlings in field soil containing RP,thus demonstrating the capability of the isolates to convert insoluble forms of phosphorus into plant available forms from RP,and therefore hold great potential for development as biofertilizers to enhance soil fertility and promote plant growth.Autotrophic microorganisms,Acidithiobacillus ferrooxidans(At.f) and Acidithiobacillus thiooxidans(At.t) were used to bioleach soluble phosphorus from middle-low grade RP containing pyrite and sulfur, respectively,in this study.Sulfuric acid was produced by the bio-oxidation of pyrite and sulfur by At.f and At.t,respectively.The production of sulfuric acid not only benefits the growth of At.f and At.t, but also solubilizes RP.Under the same pH condition(pH is 2.5),the bioleaching of soluble phosphorus from RP by At.f and At.t are more effective than sulfuric acid leaching.The optimum conditions for At.f and At.t to bioleach soluble phosphorus from RP were studied.The optimal technological parameters of bioleaching soluble phosphorus from RP by At.f are:culture temperature,30℃;initial pH,2.0;shaking speed,140 r/min;RP concentration,5 g/L;inoculation of At.f,25%;pyrite addition,25 g/L; HPO₄^2- concentration,1.0 g/L;NH₄⁺ concentration,1.5 g/L;Mg²⁺ concentration,0.5 g/L.The optimal technological parameters of bioleaching soluble phosphorus from RP by At.t are:inoculation of At.t, 25%;RP concentration,5 g/L;initial pH,2.0;sulfur addition,25 g/L.In order to promote the bioleaching of soluble phosphorus,RP and pyrite should be ground as fine as possible;the accumulation of Fe³⁺ in the culture medium should be minimized,and used adapted bacteria as inoculants.In addition,the addition of surfactants such as Tween 80 will promote the bioleaching of soluble phosphorus from RP.The bioleaching of soluble phosphorus from RP by mutagenic and immobilized At.f were studied.Results show that the treatments of At.f by mutagenesis and immobilization could enhance the oxidative activity of At.f,and increase the leaching rate of soluble phosphorus.The optimal technological parameters of bioleaching soluble phosphorus from RP by mutagenic At.f are:irradiating with 15 W ultraviolet ray lamp for 5 min at a distance of 15 cm;radiating with 800 W microwaves for 40 sec; mixing with 1.0%diethyl sulfate for 15 min.The immobilized At.f by sodium alginate has the biggest leaching rate of soluble phosphorus among the immobilized At.f by sodium alginate,polyvinyl alcohol and carrageenan.The oxidative activity and leaching rate of soluble phosphorus of immobilized At.f by sodium alginate are of 3.5 and 1.28 times than that of bioleaching by free At.f,respectively.