Blue Algae-derived Biochar On Passivation Remediation Effect For Cadmium Contaminated Paddy Soil

In recent years,the large-scale outbreaks of blue algae have occurred frequently.As a large volume and high organic biomass waste,it is imperative to realize the reduction and recycling treatment of blue algae,and to solve the problem of ecological environmental pollution.The harmless incineration of blue algae mud is the final disposal of blue algae at present.The preprocessing requires a large amount of flocculants to achieve deep dehydration of blue algae,which fails to make rational use of biomass resources.In addition,it is not optimistic about the Cadmium（Cd）contaminated paddy soil in our country.“Cd-polluted rice”incidents are not uncommon.Taking effective measures to repair Cd contaminated paddy soil is of great significance for ensuring the quality of agricultural products and maintaining food security.Therefore,in this study,considering the characteristics of blue algae that rich in nutrients and organic matter,the orthogonal experiment,with pyrolysis temperature and time as factors,was designed to find the optimal preparation conditions of blue algae biocars.Corn straw-derived biochar（CSBC）and rice husk-derived biochar（RHBC）,main biochars in the market,were used as the comparison materials.Some characterization methods were used to evaluate the structural properties of biochars.The adsorption performance and quantitative analysis of adsorption mechanisms of blue algae biochars on Cd（Ⅱ）from aqueous solution were studied through adsorption-desorption experiments and material characterization analysis before and after adsorption.The paddy pot experiment was used to explore the actural passivation and remeditation effects of blue algae biochar on Cd contaminated paddy soil(Cd content:2.22mg kg^-1)and evaluate its application potential.The main fingdings are as follows:（1）The results of the orthogonal experiment:pyrolysis temperature had a greater impact on the Cd（II）adsorption from aqueous solution by blue algae biochar than pyrolysis time.The optimal preparation consition was:pyrolysis temperature 600℃,pyrolysis time 2 h.Under the above condition,the blue algae biochar（BC600-2）had a larger specific surface area(15.75m² g^-1)and a more develpoed pore volume(0.046 cm³ g^-1).A honeycomb structure could be observed on the surface of BC600-2,which was rich in functional groups including C-O,-OH,aromatic C=C and C=O.Thus,BC600-2 had the adsorption potential of Cd（Ⅱ）in aqueous solution and soil.（2）The results of adsorption-desorption experiments:compared with CSBC and RHBC,BC600-2 had a larger saturated adsorption capacity for Cd（Ⅱ）(135.7 mg g^-1),a faster adsorption rate,a wider range of pH（5～8）and a stronger adsorption stability.In the quantitative analysis of the adsorption mechanism,both ion exchange and reaction with functional groups decreased with the increase of pyrolysis temperature of blue algae biochars,and the proportions were relatively small.Precipitation with minerals had a more important role during adsorption from 68.7%to 89.5%,and Cd₃（PO₄）₂ and Cd CO₃ were the dominant precipitates analyzed by XRD.Therefore,precipitation with minerals could be determined to play the most important role in the Cd（II）adsorption process on BC600-2.（3）The results of cultivation experiment:the passivation effect of heavy metal Cd in soil was more significant with the addition of 5%biochar.The paddy pot experiment with 5%as the addition amount showed that the BC600-2,CSBC and RHBC all significantly improved the basic properties of soil and built up soil fertility.At the same time,they all effectively reduced the transference of Cd in soil as well as the toxic effect of Cd on crops.Hence,the biochar treatments had dual effects on promoting the growth of rice and passivating Cd contaminated soil.Specifically,the application of BC600-2 significantly increased soil pH by0.40～0.93 units.It significantly increased the soil organic matter content and CEC by107.03%～143.88%and 7.74%～19.58%,respectively.The contents of soil Ca Cl₂-Cd were significantly reduced by 80.71%～91.39%.The soluble Cd was significantly changed to insoluble Cd and the content of Cd in rice husk as well as in grain decreased by 38.22%and34.02%,respectively.Compared with CK treatment,the BC600-2 treatment increased the rice yield by 41.78%.Overall,the remediation effect of BC600-2 was better than CSBC.（4）Considering the passivation effect and growth of rice by BC600-2 on Cd contaminated paddy soil,although 5%was added to make the content of rice grains Cd(0.253mg kg^-1)exceed the national food safety hygiene standard(0.200 mg kg^-1),it met the limit standard on the content of rice Cd(0.400 mg kg^-1)by JECFA of WHO.In general,the blue algae was used to produce biochar and the optimal preparation condition was found by exploring the pyrolysis temperature and time.It was effective to reduce the availability of Cd in soil and rice by applying BC600-2,which was pyrolyzed at600℃for 2 h.As for Cd soil remediation,it has the characteristics of remarkable effect,simlpe preparation and convenience for large-scale production.BC600-2 meets the requirments of passivation materials,but it is essential to further improve the material to meet the standard content of rice Cd in China.