Significant Influences Of Light On The Microbial Oxidation Of Pyrite By Acidithiobacillus Ferrooxidans

The microbial oxidation of Pyrite,an abundant sulfide and semiconducting mineral in the Earth's crust,plays a considerable role in global geochemical cycling of iron and sulfur,and is regarded as a promising engineering material.It is evident that light can affect iron cycling in many natural environments and accelerate oxidative dissolution of Pyrite through photochemical reactions.Nevertheless,the effect of light was neglected when proposing the mechanism mode of the microbial oxidation of Pyrite.To disclose this intricate mechanism on illumination,batch experiments in the presence of typical iron-oxidizing bacteria A.ferrooxidans and Pyrite were conducted under light and dark,with various measurements,e.g.solution chemistry,SEM observation,XANES,XPS and so forth.Our results showed that in chemical and microbial oxidation,iron was first oxidized to Fe3+ while sulfur is stepwise oxidized to S2O32-,then through several intermediates,e.g.M S0,SO32-,and finally to SO42-.However,light did accelerate the iron redox cycle through the light-induced oxidation of Fe2+ and photo-reduction of Fe3+ species,consequently influenced the behavior of A.ferrooxidans and the microbial oxidation kinetic and depth of Pyrite.First,light promoted the growth of A.ferrooxidans with the regenerated Fe(II)but hindered the initial attachment and formation of biofilm,resulting in the slower bio-oxidation of Pyrite surface in the short experimental time.However,the biotic oxidation of pyrite surface in the light outweighed that in the dark,for the biofilm in the light was continuously developing whereas dispersed in the dark.The light-induced electron transfer from Pyrite to solution and the cells,as well as the potential of enhanced oxidation by radicals and light-induced hole-electron pair finally contributed together to the enhancement in Pyrite microbial corrosion and oxidation depth.Considering those,a conceptual mode for microbial oxidation of Pyrite on illumination has been proposed based on the results integrated with early relevant studies:the photoreduction of Fe(III)in solution and EPS besides the weaker development of biofilm hinder the microbial oxidation of pyrite in the short term.However,light promotes the biotic oxidation of pyrite in the long term,through:(i)the better growth of A.ferrooxidans for the photoreduction of Fe(III);(ii)ligh-induced radicals;(iii)photocatalysis of semiconducting minerals,as well as the well-developed biofilm at last.Therefore,light will promote the biotic oxidation of pyrite for the long term.This study also provides a new insight for understanding the influence of light on the interaction between bacteria and semiconducting minerals.