Simulation On The Remediation Of Lead-Contaminated Soil By Biochar Under The Influence Of Phosphate Solubilizing Microorganisms

Biochar can release phosphorus into soil to improve soil fertility and passivate soil lead,thus reducing the bioavailability of lead.The phosphorus content of biochar depends on the type of biomass material and the pyrolysis temperature.Most of the phosphorus in biochar is low in bioavailability,such as Al/Fe phosphate and insoluble phosphate-containing minerals.Phosphate solubilizing microorganisms（PSM）naturally present in soil can promote the release of phosphorus in biochar,thus enhancing the passivation effect of biochar on lead.Exploring the transformation of different phosphorus in the presence of PSM will help to understand the remediation mechanism of biochar on lead pollution.However,PSM may also transform the form of phosphorus in biochar,decompose the precipitate already formed on the surface of biochar,and reduce the long-term efficacy of biochar passivating lead.Therefore,the influence of PSM on biochar remediation of lead pollution is still unclear.Most of the studies mainly focused on the release of phosphorus in biochar under the physical and chemical conditions of natural soil,but there were few studies on the promotion of phosphorus release and form transformation by PSM,and the effect of PSM on biochar adsorption of lead in soil was still unclear.In this paper,biochar was prepared from sludge and rice straw at the pyrolysis temperature of 400/700℃（sludge and rice straw biochar were respectively denoted as SBC and RBC）.The obtained biochar was characterized and adsorption experiments were carried out to explore its adsorption performance for Pb²⁺.And a highly efficient phosphate-solubilizing fungus was screened from the soil and identified as Aspergillus Niger.Aspergillus Niger was cultured in biochar medium to explore the influence on the form transformation of phosphorus in biochar,and the adsorption effect of biochar on lead in the presence of Aspergillus Niger.The main conclusions acquired from the study were as follows:1)The surface functional groups of the two biochar from biomass sources prepared in this experiment decreased with the increase of pyrolysis temperature,among which sludge biochar was more stable than straw biochar,and the loss of surface functional groups was less.Sludge biochar contained pyrophosphate and other minerals,while straw biochar contained less mineral structure.Pyrolysis temperature and biomass raw materials had great influence on the formation of minerals in biochar.2)According to the adsorption kinetics results of biochar,the adsorption of Pb²⁺by straw biochar and sludge biochar reached equilibrium at 1.5 h and 3 h,respectively.For straw biochar,the pseudo-second-order kinetic equation can better fit the adsorption process of Pb²⁺,indicating that the adsorption process of straw biochar to Pb²⁺is mainly controlled by chemical adsorption.For sludge biochar,the pseudo-first-order kinetic equation can better fit the adsorption process of Pb²⁺.The adsorption efficiency of sludge biochar and straw biochar on Pb²⁺was affected by the pyrolysis temperature.The adsorption capacity and reaction rate of sludge biochar increased with the increase of pyrolysis temperature.According to the isothermal adsorption experiment results of biochar for Pb²⁺,Freundlich model can better describe the adsorption of Pb²⁺by straw biochar and sludge biochar.RBC700,RBC400,SBC700,and SBC400 were in the order of adsorption effect from high to low.In other words,straw biochar had a better adsorption performance for Pb²⁺than sludge biochar,and the biochar prepared from the same biomass raw material had a better adsorption performance at high pyrolysis temperature.Langmuir model can also describe the adsorption of Pb²⁺by biochar.The Langmuir equation was used to fit the maximum adsorption capacity Q_m of Pb²⁺by four kinds of biochar.The results showed that at25℃,the maximum adsorption capacity Q_m of RBC400,RBC700,SBC400 and SBC700 for Pb²⁺were as follows:88.7,102,32.4,56.1 mg·g^-1.3)In this study,a strain of Aspergillus Niger with phosphate solubilization ability was screened out,and the strain was cultured in the medium containing biochar to explore the influence of phosphorus solubilizing microorganisms on the release and form transformation of phosphorus in biochar.The results showed that sludge biochar released more phosphorus than straw biochar,and higher biomass pyrolysis temperature was beneficial to the release of phosphorus in biochar.In the absence of Aspergillus Niger,the phosphorus released by biochar was mainly pyrophosphate.Aspergillus Niger greatly promoted the release of phosphorus in biochar,and the form of phosphorus was mainly orthophosphate,indicating that part of pyrophosphate was converted to orthophosphate under the action of Aspergillus Niger.4)After a week of mixed culture of Aspergillus Niger in NBRIP medium with different calcium phosphate content and biochar,lead adsorption experiments were carried out with the obtained the mixture of Aspergillus Niger and biochar to simulate the adsorption of lead by biochar in the presence of phosphorus-solving microorganisms in the soil environment.The results showed that,the best removal effect of the mixture on lead was 274 mg·g^-1,and the removal rate of lead in the system reached 68.5%.In the presence of a small amount of Aspergillus Niger,the adsorption capacity of straw biochar on lead was affected,which was only 38.6-58.2%of that without Aspergillus Niger.However,the adsorption of Pb²⁺by sludge biochar was less affected by a small amount of Aspergillus Niger.When Aspergillus Niger was growing normally,the adsorption of lead by straw biochar was further limited.Although the sludge biochar mixture had a small increase in the removal of lead,it may be that the phosphate in sludge biochar is used as a phosphorus source by Aspergillus Niger,rather than that the adsorption effect of biochar itself is improved.And lead phosphate,lead oxalate and lead carbonate precipitate in the adsorption of lead by Aspergillus niger-biochar mixture.