Spectral Characteristics Of Dissolved Organic Matter In Agricultural Organic Materials And Its Effects On Photoreduction Of Mercury In Water

Mercury(Hg),a highly toxic global pollutant,exists in various environmental media in various inorganic and organic forms.Due to the great differences in the environmental behavior and toxicity of various forms of mercury,its morphological transformation will inevitably affect the biogeochemical cycle and toxicity of mercury.Among them,the reduction of mercury in water is an important part of the mercury biogeochemical cycle.On the one hand,this process can limit the yield of methylmercury in water and reduce the enrichment of methylmercury;on the other hand,it can promote the release of zero-valent mercury in water and the global cycle of zero-valent mercury.The reduction of mercury in water mainly occurs under light induction,and various environmental parameters,such as light intensity,concentration and composition of dissolved organic matter(DOM),can affect this process.The application of organic materials(such as livestock and poultry manure and crop straw)is a common agricultural measure,and a large amount of DOM is generated during the decomposition process,which enters water bodies or aquatic-like ecosystems(such as flooded paddy fields)through surface runoff or flooding release,becoming an important source of DOM in water bodies or flooded paddy fields.DOM,which can lead to mercury reduction through photo-generated hydrated electrons,superoxide anions,organic free radicals or electron transfer in complexes,is one of the main factors controlling the reduction of inorganic mercury.However,the photoreduction of mercury in water mediated by DOM in agricultural organic materials is still unclear.Therefore,in this study,two typical agricultural organic materials of livestock and poultry manure(pig manure,cow manure and chicken manure)and crop straw(rice,corn,rapeseed,sesame,sorghum,peanut,soybean straw and corn cob)were used as the research objects.On the basis of understanding the spectral characteristics(ultraviolet-visible absorption and three-dimensional fluorescence spectrum)of DOM in agricultural organic materials,systematically carried out the study on the effect of representative DOM in agricultural organic materials(pig manure,cow manure and chicken manure;rice,corn and rapeseed straw)on mercury reduction in water bodies.Further exploring the role of superoxide anions in this process,the results can provide a basis for a clear understanding of the biogeochemical cycle of mercury in water bodies and aquatic-like ecosystems(such as paddy fields)by the application of agricultural organic materials.The main conclusions are as follows:(1)The degree of humification and the content of hydrophobic components of DOM in chicken manure and cow manure were higher,and the degree of humification of DOM in pig manure(with 30%straw added for composting)and crop straw was lower.Aromatic and conjugated double bond structures may exist in DOM in pig manure,corn stover,sorghum stover,rice stover and corncob stover.There is a significant linear correlation between the fluorophore and chromophore of DOM in the studied organic materials,and the degree of humification,hydrophobic component content,aromaticity and molecular weight are related to the colored dissolved organic matter(CDOM)and fluorescent dissolved organic matter(FDOM),and the aromatic structure in the DOM is related to the hydrophobic component content and the degree of humification,and the aromatic structure may mainly exist in the hydrophobic component.(2)For different kinds of agricultural organic materials,the ability of pig manure DOM to promote mercury photoreduction in water was much higher than that of chicken manure and cow manure at low DOM concentration(5 mg·L-1).However,the ability of chicken manure and cow manure DOM to promote mercury photoreduction in water were much higher than that of pig manure at higher concentrations of DOM(20,40 mg·L-1),which may be related to the humification degree and hydrophobic components of DOM of the three kinds of livestock and poultry feces.Some mercury dark reduction reaction occurred in chicken manure and cow manure system.The variation pattern of crop straws was consistent:the mercury photoreduction capacity and reduction rate constant k value of rice and corn straws DOM system were higher than that of rape straws,but the dark reduction capacity of the three straws was weak.The photoreduction of mercury in water mediated by different DOM in agricultural organic materials is a complex process which cannot be explained by a single spectral characteristic.(3)For DOM in agricultural organic materials with different concentrations,the change of DOM concentration of crop straws has little effect on the photoreduction of mercury in water,while the effect of DOM concentration in livestock and poultry manure on the photoreduction of mercury in water has an inhibitory or promotion effect.With the increase of DOM concentration,chicken and cow manure DOM showed the effect of promoting and then inhibiting the mediated photoreduction of mercury in water;the inhibition of photoreduction of water mercury in pig manure DOM system increased with the increase of DOM concentration.It is worth noting that the cow manure DOM system exhibits a certain mercury reduction reaction under dark conditions,and with the increase of DOM concentration,the dark reduction of mercury in water is enhanced.(4)Superoxide anions play an important role in promoting the mercury photoretrial reduction reaction of water bodies mediated by the DOM in livestock and poultry manure,especially pig manure,but the promotion effect of mercury photochemical reactions in water mediated by crop straw DOM is not obvious.In summary,the photoreduction of mercury in water mediated by DOM from livestock and poultry manure is more complex,with both promoting and inhibiting effects,while the photoreduction of mercury in water mediated by DOM from crop straws is more consistent.This may be due to the more complex DOM composition and structure of the decomposed organic material.Among them,chicken manure and cow manure had higher DOM molecular weight,higher degree of aromatization and humification and higher proportion of hydrophobic components.The results show that the application of agricultural organic materials has a certain influence on the photoreduction reaction of mercury in water,which in turn affects the biogeochemical cycle of mercury in water.