Effects Of Raw Materials And Temperature On Physical And Chemical Properties And Cadmiumadsorption Capacity Of Biochar

Soil pollution of heavy metals becomes more and more serious, it can easily be absorbed and accumulated by crops, which is harmful to human health. The management of heavy metal contaminated soil is paid more and more attention.Biochar has developed internal pore structure and large specific surface area. Meanwhile, it also has rich oxygen functional groups and it shows great potential in the improvement of heavy metal pollution removal. In this paper,a series of biochars obtained from rice straw and rice husk were prepared at varies temperatures under an oxygen limited condition.In order to explore the influence of materials and temperature on the physical and chemical properties of biochars,modern analysis methods such as element analysis,scanning electron microscopy(SEM), BET specific surface area analyzer were used.Meanwhile,we investigated the effect of materials,temperature and acid soluble mineral on the adsorption capacity of cadmium. The results and conclusions are as follows.1) Both temperature and materials had significant effects on the physical and chemical properties of biochar. At the same temperature, the yield and ash content and carbon content of rice husk biochar were higher than that of rice straw biochar, while the pH value and volatile content of rice husk biochar were lower. With the increase of temperature,the changes of physical and chemical properties of the two kinds of biochars were similar.With pyrolysis temperature increasing,the production rate and the volatile matter content decreased, ash content,pH value and carbon content increased.Results of element analyzer showed that Nitrogen,Hydrogen,Oxygen element contents,H/C,O/C and(O+N)/C all decreased, the aromatic carbon increased while the hydrophilicity and the polarity decreased.2) The result of FTIR showed that there was no significant differences on the species of functional groups between rice straw and rice husk biochar.Both of them had rich functional groups such as hydroxyl, carboxyl and so on.The effect of temperature on the surface functional groups was significant.With pyrolysis temperature increasing,methyl s-CH3 tand methylenes-CH2 tgradually disappeared while more aromatization was shown by new aromatic compound.The results of Bohem titration showed that both materials and temperature may have some influence on the quantity of biochars’ functional groups.Compared to rice husk biochars,the total quantities of surface functional groups and the contents of the basic functional groups were higher in rice straw biochar.with the pyrolysis temperature increasing,the two kinds of biochars showed similar changes:the total quantities of surface functional groups and the contents of acidic surface functional groups decreased while the contents of the basic functional groups increased.3) The results of SEM showed that after pyrolysis,the structure of the two raw materials all changed a lot,and their structure were similar. They all had rich porous structure and compared to rice husk biochar, rice straw biochar had more pores,its pore structure developed more complete and more regular.Specific surface area test showed that the main pore structure of rice straw biochars and rice husk biochars was mesoporous.As pyrolysis temperature rising,themean pore size decreased.The raw material and pyrolysis temperature have significant influence on the surface characteristics of the biochar.At the same temperature, the surface characteristics of rice straw and rice husk biochar were different.At the temperature of300 L~500 L,the surface area of rice husk biochar was significantly larger than that of rice straw,while at 600 Lor 700 L it showed the opposite result.The best surface characteristics of rice straw and rice husk biochar were at 600 L and 700 L respectively.The two kinds of biochars showed a similar change on the surface characteristics with the increase of the pyrolysis temperature:the average pore size decreased while the sum of pores increased and the structure of pore structure was better. Total pore volume and the value of the BET specific surface also increased.4) Specific surface area test showed that the surface area and total pore volume of rice straw and rice husk biomass increased after acidification treatment.Compared with the rice husk biochar’ slowly increase, the influence of acid treatment on the rice straw biochar seemed more obvious, it greatly increased the rice straw biochar’ surface area.Before acidification treatment, The maximum surface area of rice straw biochar were at 600 L,while after acidification treatment, it was at 700 L.Therefore, acidification treatment can be used to increase the surface area of biochar, and optimize the surface properties of the biochar.5) Raw materials,temperature and acid soluble minerals all had a certain effect on the adsorption capacity of the biochar, and the effect of acid soluble minerals was more significant.The cadmium adsorption capacity of rice straw biochar was stronger than that of rice husk biochar, and both of their cadmium adsorption capacity enhanced with the increase of the temperature.The adsorption rate of cadmium was decreased significantly after acidification treatment, which indicated that the acid soluble minerals in the biochar had an important role in the process of cadmium adsorption.Therefore, we can increase the temperature and the mineral content of the biochar to increase the adsorption capacity of the biochar.