Experimental Study On The Promotion Of Obsidian Weathering By Bacillus Safensis HR-2 And Bacillus Cereus LV-1

Microorganisms promoting rock/mineral weathering is one of the most important geochemical processes occurring on the earth’s surface.At the same time,microorganisms use these elements that excreted for their own growth and reproduction,and provide nutrients to plants.Secondary minerals produced during weathering are important component of soil.Therefore,studying the formation and evolution of secondary minerals is beneficial to deeply understand the process of soil development.At the same time,the weathering process of silicate minerals usually consume atmospheric CO₂and form carbonate minerals.Therefore,the study on the mechanism of rocks/minerals weathering promoted by microorganisms can provide a theoretical basis to reveal the global carbon cycle.Previous research on rock weathering with microorganisms mostly focused on the dissolution rate and sequence of elements and the role of microorganisms in process of rock weathering,etc.,Lacking of attention to secondary minerals formed by weathering.The obsidian,as a glassy material,is beneficial to the study of secondary minerals produced during weathering.At present,there are few studies on obsidian weathering,only a few studies have been done under high temperature and pressure conditions,and there are no reports on obsidian weathering under the action of microorganisms.In this paper,a long-term weathering simulation experiment was carried out using Bacillus safensis HR-2 and Bacillus cereus LV-1 as microbial materials and obsidian as rock material.The contents of six kinds of Si,Al,Na,K,Fe,Ca ions,the content of extracellular polysaccharides and the activity of carbonic anhydrase and iron carrier were determined.According to Stokes’law,the particle size<1μm was extracted from the solid products.The composition,surface morphology and elemental composition of mineral that the particle size<1μm were analyzed and observed by X-ray diffractometer,scanning electron microscope and energy dispersive spectrometer separately.In the end of the paper,we discuss the leaching mechanism of elements and the formation mechanism of secondary minerals in obsidian under strains HR-2 and LV-1.These experimental results may provide a scientific basis for the obsidian weathering process with microorganisms.The main conclusions are as follows:（1）The concentrations of Si,Al,K,Fe and Na ion in the process of obsidian weathering with Bacillus safensis HR-2 and Bacillus cereus LV-1 was significantly higher than that of the sterile control groups.This suggests that two Bacillus strains can all promote the weathering of obsidian.（2）There was no significant difference on the dissolution amount of Si,Al,K,Fe and Na ion during the experiments of strains HR-2 and LV-1.These results indicate that the two Bacillus species promote the weathering of obsidian by the same mechanism.（3）The maximum dissolution rates of the elements（Na,K,Fe）between the Si-O tetrahedron skeletons/Al-O octahedron.are higher than the elements（Si,Al）in the Si-O tetrahedron/Al-O octahedron.The position of elements in the Si-O tetrahedron/Al-O octahedron of obsidian determines its dissolution rate.（4）There was a significant positive correlation between the dissolution of Si and the content of extracellular polysaccharide in the medium（r=0.75）.The dissolution of Al was positively correlated with the content of extracellular polysaccharide（r=0.53）,and negatively correlated with p H（r=-0.80）.The dissolution of K was positively correlated with p H value（r=0.57）and positively correlated with siderophore（r=0.61）.The dissolution of Na was negatively correlated with p H value（r=-0.65）,and positively correlated with the content of extracellular polysaccharide（r=0.63）.There was a significant negative correlation between the dissolution of Ca and siderophore（r=-0.67）.Therefore,it is suggested that the two strains promote the dissolution of Si and Al in obsidian by secreting extracellular polysaccharides and the dissolution of Al,K and Na in obsidian by H⁺substitution.（5）Under the effect of strain HR-2,the dissolved elements Si,Al,Na,K and Ca in obsidian were rearranged to form montmorillonite.Elements such as Al,K,Ca,Fe and anions in the medium(SO₄^2-,PO₄^-)combine to form plagioalosite,potassium gypsum,platyosite and phosphoblastite.Oxalic acid secreted by microorganisms combined with NH₄⁺in the medium to form ammonium oxalate.Under the effect of strain LV-1,the obsidian was weathered to form quartz,ammonium oxalate and hydromagnesite.Co₃^2-combines with Mg²⁺to form hydromagnesite,while Si and O combine to form quartz.