Biochar-Mediated Se(â…£) Reduction By Shewanella And Se(0) Nanoparticles Oxidation

Selenium is one of the essential trace elements of humans and animals and is of great significance to human health.In soil environment,transformation among different forms of selenium such as Se（IV）and Se（0）through redox reactions affects the migration,fate and bioavailability of selenium.Due to its simple preparation process and low application cost,biochar has great potential for application in the fields of contaminated soil restoration and soil fertility improvement.Recent studies have shown that biochar matrix can directly participate in electron transfer.The rich surface functional groups of biochar also have good redox properties that can mediate microbial reduction of hematite and a variety of organic and heavy metal contaminants.However,the impacts of biohar on the redox transformation of selenium have rarely been demonstrated.Here,the promotion effects of biochar on selenium transformation under redox fluctuation were studied.It was found that biochar can accelerate the electron transfer between Shewanella oneidensis MR-1 and Se（IV）and promote the bioreduction of Se（IV）.When the pyrolysis temperature,concentration or average particle size of biochar was 700℃,2.0 g/L or 0.1-0.6μm respectively,the effects of Se（IV）biological reduction were The most significant stimulation of Se（IV）bioreduction was achieved with biochar prepared at 700℃,average particle size of 0.1-0.6μm and at a dosage of 2.0 g/L.With the increase of biomass,the reduction rate of Se（IV）increased,but the effects of biochar on reduction decreased gradually.Soluble fraction of biochar showed no significant effects on bioreduction of Se（IV）.The mediating activity was mainly associated with insoluble fraction of biochar.Biochar addition also significantly improved the repeated operation of Se（IV）bioreduction.It was shown that biochar can drive the oxidation of Se（0）nanoparticle under aerobic conditions.At p H=7.0,the highest rate of Se（0）nanoparticle oxidation was reached in the presence of BC700 and 66%of Se（0）nanoparticle（10μM）was oxidized in 24 h.PFRs in biochar is vital for Se（0）nanoparticle oxidation.p H value of the oxidation system has an important influence on biochar-driven oxidation of Se（0）nanoparticle.With the increase of p H from 3.0 to 9.0,the oxidation rate and extent of Se（0）increase significantly.Based on analyses of accumulation of H₂O₂and·OH under different p H conditions and results of reactive oxygen species quenching experiments,it was shown that biochar could activate molecular oxygen to produce reactive oxygen species such as H₂O₂and·OH,and therefore promote the oxidation of Se（0）nanoparticle.Under anaerobic conditions,almost no Se（0）nanoparticle was oxidized at p H=3.0 and p H=7.0,while biochar-driven oxidation of 82%of10μM Se（0）nanoparticle occurred at p H=9.0,indicating that biochar could directly oxidize Se（0）even in the absence of O₂under alkaline conditions.It was speculated that the redox-active groups on the surface of biochar was involved in this process.