Adsorption Mechanism Of Pb²⁺ By Cotton Stalk Biochar

As an environmentally friendly material due to its rich functional groups,well-developed pores and easy availability of raw materials,biochar has potential advantages in removing heavy metal from wastewater.Agricultural waste biomass has the characteristics of large amount of resources,short regeneration cycle and carbon neutrality,which can be used as an ideal raw material for producing high-quality biochar adsorbents.At present,biochar has low selectivity for heavy metal,and there is a lack of research on the application in industrial wastewater and the safe treatment of waste biochar adsorbent materials containing heavy metal.How to improve the sorption performance of biochar based materials from the aspects of biochar quality control,heavy metal adsorption mechanism and harmless treatment of waste adsorbents is a bottleneck that limits the promotion and application of biochar.The conclusion of qualitative and quantitative analysis of heavy metal adsorption mechanism of cotton stalk biochar was used to guide the quality control and modification of cotton stalk biochar.The industrial wastewater was treated with modified adsorbents.Harmless treatment methods for waste adsorbents have also been studied.The problems of poor adsorption performance of heavy metals and the safe disposal of waste biochar materials have been solved.The functional regulation and application of biochar can be realized.The main research contents and conclusions are as follows:（1）The sorption characteristics of Pb²⁺by cotton stalk biochar produced by slow pyrolysis at different temperatures were systematically studied.The relationship between physicochemical properties of cotton stalk biochar and temperature,and the relationship between Pb²⁺sorption capacity and physicochemical indexes were analyzed.The results showed that the yield,ash content,p H value and C content of cotton stalk biochar increased,while the acid functional group content,O and H content decreased when the temperature increased.The Pb²⁺removal rate was more than 98%when the temperature was 450-650℃.The lead sorption capacity of cotton stalk biochar was positively correlated with ash content,p H value,C content and average pore size,and negatively correlated with yield,O content,O/C and（N+O）/C ratio.The adsorption of Pb²⁺by cotton stalk biochar was an endothermic process dominated by chemical sorption,and the theoretical maximum sorption capacity of lead fitted by Langmuir model was 146.78 mg/g.The regeneration test shows that cotton stalk biochar has good recycling performance.It lays a foundation for the preparation of cotton stalk biochar and the study of heavy metal adsorption mechanism.（2）The role of organic and inorganic components in the process of adsorbing Pb²⁺was clarified,and the increment of complexation adsorption capacity was calculated.The influence of pickling on the calculation of adsorption capacity was excluded,and the contribution of complexation,πbond,ion exchange,precipitation and physical adsorption in the process of Pb²⁺adsorption by cotton stalk biochar was clarified.The results show that pickling can increase the sorption capacity of complexation.Inorganic substances play a leading role in the adsorption process through ion exchange and precipitation and its proportion of Pb²⁺adsorption capacity was≥72.49%.Organics are rich in oxygen-containing functional groups,aromatic bonds,pore structures,which can adsorb Pb²⁺through complexation,πbonds and physical interaction.With the increase of pyrolysis temperature,precipitation andπ-bond interaction were enhanced,while ion exchange and complexation interaction were weakened.Pickling has no effect on physical and ion exchange adsorption,but has an effect on complexation,πbonding and precipitation adsorption.It provides a theoretical basis for quality control and modification of cotton stalk biochar.（3）Under the guidance of the mechanism research conclusion,the direction of improving the efficiency of adsorbent was to increase the content of inorganic components,improve the pore structure and increase the oxygen-containing functional groups.The spherical composite aerogel adsorbent with excellent structure and performance was prepared by impregnation co-pyrolysis and sol-gel method using cotton stalk biochar and alginate.The results showed that the optimal preparation process parameters of aerogel were as follows:the addition amount of modified cotton stalk biochar was 6 g/L,the mass concentrations of sodium alginate and Ca Cl₂ were 3%and 1%,respectively.The composite aerogel spheres have the advantages of stable structure,wide p H application range,well developed pores,rich exchangeable ions and oxygen-containing functional groups,and its adsorption process of Pb²⁺was a chemical endothermic reaction of monolayer.The maximum adsorption capacity of lead was 664.6 mg/g.The adsorption mechanism study showed that the complexation,ion exchange and physical interaction of the composite aerogel were enhanced,and the two materials had synergistic promotion effect on the Pb²⁺adsorption.This is the extension and application of the research conclusion of heavy metal adsorption mechanism by cotton stalk biochar,and provides a new kind of efficient adsorbent for the treatment of industrial wastewater containing heavy metal.（4）A new adsorbent was developed for treating industrial wastewater.The performance of composite aerogels and the competitive adsorption relationship between heavy metals were investigated.The effectiveness of reduction and harmless treatment of waste adsorbent by calcination was verified.It has been shown that the concentrations of Pb²⁺,Cu²⁺and Zn²⁺in the treated wastewater all reach the emission limits stipulated in the national standards when the applied amount of composite aerogel reaches 35 g/L.The adsorption kinetics showed that there was competitive relationship among the three heavy metals,and the order of adsorption rate was:Pb>Zn>Cu.After calcination（700 and900℃）,the volume of the adsorbent was significantly reduced,the toxicity and bioavailability of heavy metals were weakened due to the formation of stable oxides and sulfides.In addition,the calcined material is rich in magnesite,cerussite and zigueline,which are important raw materials for smelting zinc,lead and cooper,making the adsorbed heavy metals have the potential of recycling and utilization.It provides support and experience for the application and harmless treatment of biochar based adsorption materials.This study provides theoretical basis and support for solving problems of poor performance,difficult recovery and lack of harmless treatment technology of biochar-based materials for heavy metal adsorption from wastewater.The research results are the basis for clarifying the improvement direction of biochar based adsorption materials for specific pollutants.The research content combines laboratory research with production practice,that is,the research,development,application and harmless treatment of biochar based adsorbent materials forms a closed loop management.It has improved the resource utilization level of agricultural waste biomass and reduced agricultural non-point source pollution.