Preparation Of Manganese Modified Biochar And Its Remediation Effect On Cadmium And Arsenic Contaminated Soil

Cadmium(Cd)and arsenic(As)are the main contaminants inducing farmland soil pollution in China.The central and southern region is one of the important main grain production area in China,whereas,also a relatively developed mining area.Due to the improper mineral resource development and smelting methods,the surrounding farmland soil was contaminated by heavy metals,such as Cd and As.Cd and As input into the farmland will not only lead to a decrease in soil fertility,quality,and crop yield and quality but will also result in the accumulation of heavy metals in the organism through the food chain pathway.In recent years,biochar has been widely used in farmland soil pollution remediation due to its advantages such as low price,easy to be obtained,fertilizer retention,and pollution mitigation.In order to furtherly expand the scope of application of biochar in soil pollution remediation,the modification was essential for biochar to meet the demand for mitigation of a variety of pollution at one time.This paper took Cd-As contaminated farmland soil in the central and southern regions as the research object.The corn straw was firstly modified by manganese and then pyrolyzed at different temperatures(400?600?)to obtain the manganese modified biochar(MBC1).Corn Straw biochar was modified with manganese,followed by secondary pyrolysis to obtain another manganese modified biochar(MBC2).The adsorption characteristics of Cd and As were compared between the above two modified biochars,and the optimal remediation material was selected.-Study the passivation characteristics of heavy metals in the soil,and verify its remediation capacity in field trials.The main research results of this article are as follows:(1)Two kinds of manganese modified biochar with better adsorption relative to the original biochar effect were successfully prepared.The modified biochar shows more irregular folds and more particle aggregate surfaces.The main component of these aggregated particles is manganese oxide(MnOx)with high adsorption capacity,for instance,the crystal structure of MnOx loaded on MBC2 is a mixture of ?-MnO2 and MnO.At the same time,under the secondary pyrolysis of the MBC2 preparation process,MBC2 has a higher degree of aromatization and more potential active adsorption sites for Cd and As.(2)Both of the manganese modification methods can significantly improve the adsorption capacity of biochar to Cd(?)and As(?),among them,the manganese modified biochar prepared at a modification ratio of 5:1 at 500? is more effective.Under this preparation condition,the adsorption removal rate of Cd(II)by MBC2 was 11.01%higher than that of MBC1,and 27.2%higher as a comparison with BC,while the adsorption removal rate of As(III)was 30.12%higher than MBC1,and 54.66%higher compared with BC.The MBC2-500-5:1 Cd(II)adsorption and binding process are more consistent with the Freundlich model,which shows that the adsorption process tends to multi-molecular layer adsorption;the As(?)adsorption process is more suitable for the Langmuir model,indicating that adsorption mainly occurs in the monolayer,that is,the MnOx molecular layer on the surface of biochar.(3)MBC2-500-5:1 shows a good passivation capacity for Cd and As in soil.The soil samples contaminated by Cd and As of two concentrations were subjected to a 35-day constant temperature soil stabilization treatment.The soil samples were analyzed through BCR sequential extraction method and sequential extraction method of Wenzel.It is clear from the results that all the application rates of MBC2(1%,2%,and 3%)can reduce the content of heavy metals that are easily absorbed and utilized by plants.Thus,the fraction of relatively stable heavy metal forms is on the rise,controlling the migration,transformation and enrichment of Cd and As in the soil.(4)The field test results show that the remediation treatment makes the heavy metals in the soil more stable and reduces their transfer to plants,and the heavy metal content in rice grains reaches the safty standard.The pH value and organic matter content(SOM)content of soil increased after the remediation treatment,indicating the increased soil fertility.The total amount of heavy metals in the soil did not change a lot,while the content of available Cd and As significantly decreased.Under the remediation treatments,the growth state of rice plants was greatly improved.The peroxidase(POD)in the plant leaves shows little change,but the content of superoxide dismutase(SOD)and catalase(CAT)have increased,meaning the improved ability of plants to resist external factor stress.The content of heavy metals in rice grains was significantly reduced and·meets the requirements of the National Food Safety Standard-Limits of contaminants in food(GB 2762-2017)(0.2 mg·kg-1),greatly reducing the risk of edible toxicity.Therefore,it can be concluded that MBC2 may be a promising passivating agent in the actual remediation application of Cd and As contaminated soil.