Research On The Migration And Transformation Mechanism Of Organic Phosphorus Driven By Iron Minerals In Shallow Lakes

Organic phosphorus（OP）is an important component of phosphorus in lakes and one of the main forms of endogenous phosphorus load in sediments.More and more evidence shows that OP in sediments has become a major but underestimated problem of lake eutrophication and harmful algal blooms.Iron minerals play an important role in the transport and transformation of organic phosphorus.The adsorption of organic phosphorus by iron minerals directly affects the migration of organic phosphorus in water,and the semiconductor properties of iron minerals can affect the transformation process of surface adsorbed organic phosphorus.However,the existing research has insufficient understanding of these processes,resulting in the lack of an important link and basis for objectively describing the migration and transformation process of organic phosphorus.Based on this,this paper takes two different crystal forms of iron minerals as the research object,studies the photodegradation mechanism of organophosphorus compounds under the action of iron minerals,and compares the effects of two crystal forms of iron minerals on the photochemical transformation of organophosphorus in water from the perspective of free radicals.Then Fourier transform ion cyclotron resonance（FT-ICR MS）technique was used to study the adsorption fractionation characteristics of iron minerals on organophosphorus components at the molecular level.At the same time,the adsorption and fractionation characteristics of organic matter（OM）,an important carrier of organic phosphorus,on iron minerals were studied,and the molecular composition of organic matter was explored under light conditions.These results deepen the understanding of the effect of iron mineral composition on the migration and transformation of organic phosphorus in lake water,in order to supplement the relevant theoretical basis in improving the release of endogenous phosphorus in eutrophic lakes.The main results of this study are as follows:（1）The synthesized goethite and ferrihydrite are quite different in morphology and structure.The crystal goethite is short rod with smooth surface,and the BET specific surface area is 91.3 m²/g.The weakly crystalline ferrihydrite has an amorphous structure,and the surface is covered with ravines with a larger specific surface area（283.5 m²/g）.（2）The structure type of iron minerals affects the photodegradation process of organic phosphorus.In the light experiment involving two kinds of iron minerals,goethite promoted the photodegradation of adenosine triphosphate（ATP）,and the natural semiconductor photocatalyst goethite produced·OH and O₂^·-under light,resulting in the degradation of organic phosphorus and the release of more soluble inorganic phosphate.The concentration of phosphate in the goethite system increased by 0.12 mg/L.The weakly crystalline ferrihydrite is not an effective photocatalyst.No obvious organophosphorus degradation was observed in the ferrihydrite system,and the adsorption capacity of ferrihydrite was strong,which inhibited the release of phosphate.（3）The adsorption fractionation selectivity of the two iron minerals to the organic phosphorus molecules in the mixed system is similar,and the adsorption difference is mainly reflected in the adsorption capacity.Under weak acidic conditions,ferrihydrite and goethite had the best adsorption effect on organic phosphorus molecules.The results of FT-ICR MS showed that 91 DOP molecules were detected in the sediment extract samples.After adsorption by ferrihydrite,40 DOP molecules remained in the extract,and the adsorption rate was 51.91%.There were 67 DOP molecules left after goethite adsorption,and the adsorption rate was 26.37%.The adsorption selectivity of the two iron minerals is the same,mainly adsorbing unsaturated hydrocarbons,lignin-like,tannin-like compounds and carbohydrate-like organic phosphorus components,but the adsorption capacity of goethite is weaker than that of ferrihydrite.Lipid and aliphatic/protein are two kinds of organic phosphorus components that are not easily adsorbed by iron minerals.The double bond equivalent（DBE）is the main factor affecting the adsorption of organic phosphorus components by iron minerals.Ferrihydrite and goethite mainly preferentially adsorb DOP molecules with high unsaturated degree and high DBE value,and the DBE value is mainly affected by the number of hydroxyl and carboxyl functional groups on DOP molecules.In addition to adsorption,goethite also plays a role in catalytic hydrolysis,resulting in the degradation of DOP with high degree of unsaturation to produce DOP molecules with low degree of unsaturation,low O/C and high H/C.（4）The two iron minerals have similar adsorption fractionation selectivity for DOM molecular components,but play different roles in the photodegradation process.FT-ICR MS analysis showed that a total of 7148 DOM molecules were detected in the sediment extract,and 2776 and 2585 DOM molecules were adsorbed by ferrihydrite and goethite,respectively.The adsorption rates of DOM were 38.84%and 36.16%,respectively.The DOM of macromolecules（m/z>487.8）was mainly composed of lignin-like and aliphatic compounds.The FT-ICR MS results after illumination showed that the overall distribution of DOM components adsorbed by the two minerals after illumination was characterized by an increase in H/C and a decrease in O/C.The aromaticity in the two systems was significantly reduced,and the macromolecular DOM in lignin-like and aliphatic-like groups was significantly reduced,while the macromolecular DOM in lipids increased significantly.In the iron mineral-mixture system,the photodegradation of DOM is affected by both iron minerals and DOM components,including the adsorption and fixation ability of iron minerals,the ability to produce reactive oxygen species（ROS）,and the ROS production of DOM itself under light.About 58.26%of DOM in the goethite system was strongly degraded under light,resulting in the degradation of DOM originally adsorbed on goethite and detachment from the mineral surface.The degree of degradation of DOM molecules on ferrihydrite is low,and the new DOM molecules are not desorbed from the mineral surface,resulting in an increase in the number of originally adsorbed DOM molecules from 2776 to 3099.After illumination,the aromaticity index(AI_mod,wa)in the two systems decreased significantly,from 0.35 for ferrihydrite and 0.31 for goethite to0.024 and-0.0016,respectively.In addition,Kendrick mass defect（KMD）analysis showed that the DOM molecules with higher KMD values were preferentially photodegraded.