Study On The Removal Of Heavy Metals From Soil By The Water-Soluble Matter Of Brown Algae

In recent years,heavy metal pollution in farmland soil has become more and more serious,and it has become one of the environmental problems that need to be solved urgently.Among the physical,chemical,and biological remediation methods,the chemical extraction technique is simple,fast and efficient,and is a feasible remediation method.The key to the chemical extraction method is the choice of green and efficient chemical extractants.In this study,farmland soils with different contents of copper,cadmium and lead compound pollution were selected as the research object.The large seaweed Sargassum fusiforme and Sargassum thunbergii officinalis,which are rich in reserves along the coast of southern Zhejiang,were selected as plant raw materials to explore the impacts of their water soluble on the extraction of copper,cadmium and lead in soil under different concentration and pH extraction conditions;further use the kinetic equation to fit the extraction process to explore the kinetic characteristics under the optimal extraction conditions.By filtering and grading water soluble,distinguish the types of key substances.Use modern spectroscopy methods such as SEM-EDS,XRD,XPS,etc.to characterize the soil before and after extraction,and explore the changes in its physical and chemical properties;analyze the changes in the form of soil heavy metals before and after extraction,and demonstrate the potential threats to the soil environment according to soil heavy metal stability and mobility changes.The soil fertility changes before and after extraction were compared,and the superior performance of the large brown algae water soluble matter was explained.This study started from the extraction efficiency of Sargassum fusiforme and Sargassum thunbergii water soluble matter on soil heavy metals,and conducted a more comprehensive study on the extraction mechanism and effects on soil properties before and after the action,in order to provide a theoretical basis and reference for the study of heavy metal contaminated soil remediation.The main findings are as follows:1.Extraction effect of the water soluble matter of Sargassum fusiforme and Sargassum thunbergii on copper,cadmium and lead contaminated soilWhen the water soluble matter of Sargassum fusiforme and Sargassum thunbergii at a concentration of 80 g/L,a pH of 3,an extraction time of 240 min,a temperature of 55℃,and an extraction frequency of 3 times,they can effectively extract the farmland soils with different pollution levels.For copper,cadmium and lead,the extraction difficulty is cadmium>copper>lead from easy to difficult.In particular,the extraction efficiency of cadmium is 78.12%,66.84%,83.97%,83.71%,respectively.Under these conditions,the cadmium in the lightly polluted soil after the extraction of Sargassum fusiforme,the copper content in the heavily polluted soil after the extraction of Sargassum fusiforme,and the extraction of Sargassum thunbergii are all lower than the "Soil Environmental Quality Agricultural Land Soil Pollution"According to the risk screening value in the "Risk Control Standards(Trial)"(GB 15618-2018),the treated farmland soil can carry out normal agricultural production activities.2.Research on the water soluble matter of Sargassum fusiforme and Sargassum thunbergii extracts on copper,cadmium and lead contaminated soilThe Elovich equation,quasi-first-order kinetic equation,and quasi-second-order kinetic equation models were selected to fit the extraction process of soil heavy metals with Sargassum fusiforme and Sargassum thunbergii water soluble matter.The quasi-first-order kinetic equation and the quasi-second-order kinetic equation can better fit the soil heavy metal extraction process than the Elovich equation,indicating that this process is a complex chemical process mainly based on ion exchange and chelation reactions.The water soluble matter is filtered and graded by the filter membrane,and the extraction efficiency of heavy metals decreases with the decrease of the diameter of the filter membrane(p<0.05).The research results show that the content of water-soluble substances such as amino acids,polysaccharides and proteins decreases with the decrease of the diameter of the filter membrane.It is small and gradually decreases,and the main substances that affect the heavy metals in the soil are sugars and aliphatic alkanes.The soil before and after extraction was subjected to SEM-EDS,XRD and XPS characterization analysis.The research results showed that water soluble matter act on the dissolution and transformation of minerals in the soil where heavy metals are present,and the heavy metals are mainly absorbed and complexed from the soil colloid Surface removal.3.The effect of heavy metal extraction process on soil propertiesThrough the analysis of soil heavy metal forms before and after extraction,the extracts of Sargassum fusiforme and Sargassum thunbergii water soluble matter are mainly used to remove the exchangeable state of the heavy metals copper,cadmium and lead,the combined state of carbonate,and the oxidation state of iron and manganese.The stability and mobility of heavy metals in the soil are further analyzed.After the extraction of Sargassum fusiforme and Sargassum thunbergii,the stability and mobility of heavy metals such as copper,cadmium and lead are enhanced,and the mobility of heavy metals is reduced,which greatly reduces the potential threat of heavy metals in the soil.Compared with conventional chemical extractants,the soil after the action of Sargassum fusiforme and Sargassum thunbergii water soluble matter has improved fertility indexes such as total nitrogen,total phosphorus,total potassium,alkali hydrolyzed nitrogen,available phosphorus,and available potassium to varying degrees.In particular,the improvement of indicators such as alkaline hydrolysis nitrogen,available phosphorus,and quick-acting potassium was particularly significant(p<0.05),it further shows that the extraction performance of the large brown algae water soluble matter is better than that of the conventional chemical extractant.This study expanded the selection range of soil heavy metal extractants,and initially explores the mechanism of Sargassum fusiforme and Sargassum thunbergii water soluble matter acting on soil heavy metals.From the analysis of soil heavy metal morphology and fertility before and after extraction,it further demonstrated the multiple properties of water soluble matter superior to conventional chemical extractants.Sargassum fusiforme and Sargassum thunbergii water soluble matter have certain application prospects for removing complex heavy metal contaminated farmland soil.