Removal Of Volatile Methylsiloxane Based On Sulfuric Acid Absorption/Modification Of Biochar Adsorption

Volatile methyl siloxane（VMS）is a new type of environmental pollutants.In this paper,the removal of volatile methyl siloxane was studied based on sulfuric acid solution absorption/modified biochar adsorption.On the one hand,the experimental equipments and methods of VMSs removal by sulfuric acid solution were established and improved.Hexamethyldisiloxane（L2）was selected as the model pollutant of VMSs and the key factors that might influence the absorption of L2 in sulfuric acid solution system were explored;On the other hand,the effects of biomass species and ZnCl₂ modification on the adsorption and removal of L2 by biochar were investigated.A cheap and efficient ZnCl₂ modified wheat straw biochar adsorption material for removing volatile methylsiloxane was selected.The effects of pyrolysis temperature and concentration of ZnCl₂ solution on the properties of modified wheat straw biochar and the effect of adsorption on the removal of L2 were further studied.The main research contents are as follows:1.based on the absorption of sulfuric acid solution to remove L2 and its influencing factorsReferring to the research methods of physical and chemical absorption reported in the literature,starting with the simple acid absorption method to remove VMSs,the experimental equipment and analytical detection method for the purification of VMSs impurity gas in biogas were established and perfected.The key factors which may affect the absorption of L2in sulfuric acid solution system were investigated by using N₂ as the carrier gas and L2 as the model pollutant of VMSs.The results show that:（1）When the concentration of H₂SO₄ was less than 60%,the absorption of L2 was lower and the H₂SO₄ concentration increased to 70%or 80%,the amount of absorbed time increaseed almost linearly.By 180min,the total absorption of L2 could reach 8.1 and 15.6 g/L,respectively;（2）When the pore size of sand plate was reduced from 50μm to 4μm,the total amount of L2 absorbed by the 70%H₂SO₄solution for 180 min rose from 8.1 g/L to approximately 12.1 g/L;（3）The increase of the height of the liquid level made the residence time of L2 in the H₂SO₄ solution prolonged.the L2 inlet concentration increased,and the mass transfer driving force at the interface of gas-liquid contact rose,which leaded to the raise of L2 absorption;（4）The absorption of L2increased first and then decreased with temperature and gas flow rate,and the maximum absorption occurred at near room temperature and flow rate of 100 m L/min;（5）The effects of adding several kinds of manganese oxide and graphite oxide suspension on the absorption L2of sulfuric acid solution system were preliminarily investigated,and no effective factors were found to be beneficial to L2 removal.2.Effect of Biomass species and ZnCl₂ Modification on adsorption and removal of L2 by BiocharSix kinds of biomass,such as wheat straw,pine sawdust,Fraxinus mandshurica,cow dung,peanut shell and cow dung,were used as raw materials.The modified biochar-based adsorbents were prepared by ZnCl₂ solution pre-impregnation and co-pyrolysis.Meanwhile six kinds of raw biochar were prepared as control samples,labeled as Zn-WSB/WSB、Zn-PDB/PDB、Zn-FMB/FMB、Zn-MCB/MCB、Zn-PHB/PHB、Zn-CDB/CDB respectively.The performance of biochar adsorption and removal of L2 was evaluated by dynamic adsorption experiments,and the materials were characterized.The effects of biomass species and ZnCl₂ modification on the removal of L2 by biochar were explored.The results show:（1）The original biochar had almost no adsorption of L2,but the six kinds of ZnCl₂ modified biochar had a strong adsorption capacity.The order from small to large was Zn-CDB<Zn-PHB<Zn-MCB≈Zn-FMB<Zn-PDB<Zn-WSB;（2）The surface polarity of biochar was weakened and the hydrophobicity was enhanced by ZnCl2 modification.The degree of aromatization was higher than that of the corresponding original biochar;（3）Zn-PHB and Zn-FMB were composite biocarbon-based materials composed of amorphous graphite carbon and wurtzite ZnO microcrystals.Zn-WSB,Zn-PDB,Zn-MCB and Zn-CDB were composed of amorphous carbon and Amorphous ZnO composite biocarbon-based materials;（4）The N₂ adsorption-desorption isotherms of the original biochar belonged to the type III isotherm,and the Zn-WSB was a microporous-mesoporous hybrid composite biocarbon-based materials.The other five were microporous composite biocarbon-based materials;（5）The adsorption of L2 on ZnCl₂ modified biochar mainly occurred in microporous pores,and its adsorption mechanism was microporous filling.3.Effect of activation conditions on adsorption and removal of L2 by ZnCl₂ modified wheat straw biocharBased on the optimized ZnCl₂ modified wheat straw biochar（Zn-WSB）with the best L2adsorption capacity,the effects of pyrolysis temperature and ZnCl₂ solution concentration on the removal of L2 by modified straw biochar were studied in detail,and the impacts of structure,structure and properties were also discussed.The results show that:At low temperature and low concentration conditions,the B300 and B-0.1 were almost not adsorbed by L2.B400,B500 and B700 produced at high temperature had strong L2 adsorption effect,and the adsorption capacity was B500>B700>B400 from large to small.The biochar prepared by high concentration of ZnCl₂ had strong L2 adsorption effect,and the order of adsorption capacity was B-2>B-5>B-1;（2）B300 did not contain microporous,B400 contained rich pores,B500 and B700 had lots of microporous pores and mesoporous.Appropriate increase in temperature（500°C）could significantly improve the microporous structure of ZnCl₂ modified wheat straw biochar.B-0.1 and B-1 obtained at lower concentration belong to microporous materials,and B-2 and B-5 prepared at higher concentration had the characteristics of mixed micropore and mesoporous texture.（3）With the proper increase of pyrolysis temperature and ZnCl₂ concentration,the surface of modified biochar presents abundant spongy fine pores,which was beneficial to the formation of micropore structure and its adsorption effect on L2.The adsorption capacity of modified biochar（B500/B-2）for L2 was 880.11mg/g under optimized conditions.