Application Of Anaerobic Fermentation Straw Returning To Field In Mercury Polluted Farmland

Mercury is a highly toxic,persistent,global,and bioaccumulative heavy metal pollutant that can be converted into more toxic methyl mercury under specific conditions,thereby seriously threatening human health.China has a large amount of straw resources,producing up to 600 to800 million tons of various types of straw every year.Returning straw to the field is not only the main approach to the utilization of straw as a resource,but also an important content of China’s fertile soil project,harvest plan,and an important means to achieve the strategy of "cultivating the field with farmland".However,recent studies have found that direct straw return significantly enhances mercury methylation in paddy soil and promotes mercury enrichment in rice.It is urgent to change the utilization method of straw returning to the field and alleviate the increased mercury exposure risk caused by direct straw returning to the field in mercury polluted areas.The latest research by the research team found that under Waterlogged Conditions,compared to returning fresh straw to the field,aerobic fermentation of straw promoted the accumulation of mercury in plants,while returning straw to the field after anaerobic fermentation can reduce the mercury accumulation in crops caused by direct returning straw to the field,but the reduction effect is limited,and the underlying mechanism is still unclear.Therefore,it is particularly important to further strengthen anaerobic fermentation of straw to reduce soil mercury methylation potential and reduce crop mercury enrichment capacity.Based on the existing research foundation of the research team,sodium sulfide,sodium sulfide+sodium selenite,sodium sulfate,sodium sulfate+SRB active substances,goethite,mercaptoacetic acid,or ferrous sulfate+alum are added to the wet anaerobic fermentation process of straw for co fermentation to obtain modified straw.At the same time,through characterization techniques and pot experiments on different modified straw:(1)preliminarily explore the impact of different additive inputs on the structure and composition of straw fermentation products,and further determine whether there are functional groups produced;(2)The effects of straw anaerobic fermentation products with different additives on the bioavailability of mercury in soil were studied,and the mechanism of their effects on the bioaccumulation of mercury in soil was preliminarily explored.The main results of this paper are as follows:(1)The Fourier transform infrared spectroscopy characterization results showed that compared to the anaerobic fermentation products of straw(without addition),during the fermentation process,sodium sulfate,sodium sulfate+SRB active substances,or ferrous sulfate+alum were added for fermentation,and there were stretching vibration peaks of C-S or S-S and stretching vibration peaks of C=S or S=O on the fermentation biogas residue.The X-ray electron spectroscopy(XPS)characterization results showed that compared to the anaerobic fermentation products of straw(without addition),adding sodium sulfate,sodium sulfate+SRB active substances or ferrous sulfate+alum for fermentation,the organic sulfur groups of sulfide or mercaptan appeared on the fermentation biogas residue.(2)During the anaerobic fermentation of straw,adding sodium sulfide,sodium sulfide+sodium selenite,sodium sulfate,sodium sulfate+SRB active substances,goethite or ferrous sulfate+alum for co fermentation increased the humification index of the fermentation broth DOM and the relative content of protein and humic acid,while adding mercaptoacetic acid during the fermentation process significantly increased the E2/E3 value of the fermentation broth DOM.(3)Compared to returning straw directly to the field,anaerobic fermentation straw significantly reduced the acid mercury content in the simulated stomach of soil(61.6%),while the application of sodium sulfate,sodium sulfate+SRB active substances,or ferrous sulfate+alum co fermentation products further reduced the acid mercury content in the simulated stomach of soil,with a reduction rate of 70.41% to 80.41%.The reduction effect is better than the fermentation product(without additives)directly combined with sodium sulfate,sodium sulfate + SRB active substance or ferrous sulfate + alum.(4)Returning straw to the field after co anaerobic fermentation with additives significantly reduced the risk of soil methylmercury increase caused by direct straw return to the field.Compared to returning straw directly to the field,anaerobic fermentation products(without addition)significantly reduced the soil methylmercury concentration(72.52%),while co fermentation products added with sodium sulfide+sodium selenite,sodium sulfate+SRB active substances,goethite,ferrous sulfate+alum,or high concentrations of sodium sulfide and sodium sulfate further significantly reduced the soil methylmercury concentration,with a reduction rate of82.37% to 94.06%.The reduction effect increased with the increase in additive concentration.On the contrary,returning the co fermentation product of thioglycolic acid to the field significantly increased the concentration of methylmercury in soil(p<0.05).In the treatment group where anaerobic fermentation products(without addition)were directly mixed with various additives,except for the sodium sulfide+sodium selenite treatment group,other treatment groups increased the soil methylmercury concentration to varying degrees.(5)Returning straw to the field after co anaerobic fermentation with additives significantly reduced the risk of increased mercury in plants caused by direct returning straw to the field.Compared with the direct return of straw to the field,the anaerobic fermentation products of straw(without addition)significantly reduced the mercury content of water vine vegetables,with a reduction rate of 14.06%～18.04% for total mercury and 48.57%～53.72% for methyl mercury,respectively,while the co fermentation products of straw with sodium sulfide,sodium sulfide+sodium selenite,sodium sulfate,sodium sulfate+SRB active substances,goethite or ferrous sulfate+alum further significantly reduced the total mercury concentration in the stem(29.45%～66.19%),methyl mercury concentration(26.26%～81.97%),and methyl mercury concentration in the leaf(31.66%～89.41%)of water vine,among which the treatment group with high concentration of sodium sulfate and sodium sulfate+SRB active substances had the best effect,It can reach the level of soil without adding straw.In summary,returning straw to the field after anaerobic fermentation with the above additives can effectively reduce the risk of increased plant mercury caused by direct returning straw to the field,and is a straw returning method suitable for mercury contaminated paddy field planting areas.During the co anaerobic fermentation process of sulfate and straw,the properties of straw can be changed,generating organic sulfur groups of thioether or mercaptan on the fermentation biogas residue,and improving the humification degree of DOM in the fermentation product.After these products are added to the soil,they can complexe or chelate with more mercury,reducing the concentration of bioavailable mercury and methylmercury in the soil,thereby reducing the content of mercury and methylmercury in plants.