Preparation And Lead (â…¡) Adsorption Characteristics Of Biochar

Conditioning soil with biochar can not only improve the qualification of soil environment, increase food production, capture greenhouse gases, but also show great potential on the issue of eliminating pollutants from soil. In this paper, twelve types of biochar, derived from rice straw, pig manure and fir tree bark generated at four different pyrolysis temperatures (300℃-600℃, referred as RC300-RC600, PC300-PC600 and WC300-WC600) were characterized by SEM, EDX, infrared spectroscopy(Fourier), Boehm titration and BET specific surface area analyzer and investigated for Lead(Ⅱ) removal from aqueous solution under different time, pH value, concentrations and temperature with reference to a commercial activated carbon(AC). Additionally, the influence of land application of biochar on the Lead (Ⅱ) adsorption characteristics of soil were also evaluated.Biochars generated at different pyrolysis temperatures displayed distinct properties. The net production rate, the content and abundance of surface oxygen functional groups such as carboxyl and phenolic hydroxyl of biochars prepared under different temperature varied in the following order: 300℃>400℃>500℃>600℃. While for the ash content, pH value, C/O value, N/O value, BET specific surface area, the trend was inversed. Raw materials showed larger amount of mass loss and more obvious physical and chemical variation between 300℃and 500℃than the temperature range 500℃to 600℃. Except of biochar WC300 and WC400, all the samples were alkaline. Biochars derived from different sources also displayed distinct properties. The net yield, pH value (RC300 not included), specific surface area (RC500, PC300, PC400 not included), total amount of surface functional groups (WC300 not included), surface functional groups abundance differed in the order of WC>RC>PC, RC>PC>WC, PC>RC>WC, PC>WC>RC, PC>RC>WC, respectively. RC and WC mainly contained micropore and macropore, but mesopore and macropore were dominant pores in PC. Compared with biochars, commercial activated carbon AC possessed less species of surface functional groups and the mount of surface functional groups was only 70%-96% of the biochar prepared at 300℃. However, AC had more micropore and mesopore and the BET surface area and total pore volume were 8-460 and 4-330 times of biochar, respectively.The adsorption characteristics of Lead(Ⅱ) on biochars and active carbon were also investigated. The results indicated that the sorption process of biochar could be described well by Lagergren Pseudo-second-order kinetics model (R2>0.9389). The Lead(Ⅱ) adsorption capacity of biochar increased significantly with increasing solution pH. The optimum solution pH for RC300, RC400 and PC500 was 3.5, for WC300 was 5.5, whereas for other biochars it was 6.5. The Lead(Ⅱ) adsorption by biochar gradually increased with the increasing concentration of Lead(Ⅱ). When the sample AC, RC300, RC400 (at 20℃, 25℃, 30℃), WC500, WC600 (at 25℃, 30℃), PC300, PC400 (at 30℃) could be well described by Langmuir isotherm equation model, the adsorption data of other biochars fitted the Freundlich isotherm equation model better. Contrasting the adsorption capacity qm and Kf of all the samples, we could conclude that the optimum preparation temperature for PC was 500℃, while for RC and WC it was 300℃. Generally, the Lead(Ⅱ) adsorption capacity of biochar varied in the following order: AC>RC>PC>WC.The adsorption characteristics of Lead(Ⅱ) on soil and Biochar-Conditioned soil were studied using batch adsorption experiments as well. The results showed that the sorption process of soil could be well characterized by Lagergren Pseudo-second-order kinetics model (R2>0.9389). After the application of 5%(W/W) WC500, the adsorption isotherm still fitted the Freundlich isotherm equation model, but the adsorption edge of soil moved to a higher pH value. Although the Kf value for biochar was only 41%-59% of soil, it increased by 2%-10% after the application, which indicated the promotion of the soil Lead(Ⅱ) adsorption capacity. The results of thermodynamic analysis showed that: under the given temperature 20, 25 and 30℃, whereas the Lead(Ⅱ) adsorption on biochar was a spontaneous endothermic reaction and the entropy increased in the adsorption process, the Lead(Ⅱ) adsorption on the soil and Biochar-Conditioned soil was a spontaneous exothermic reaction and the entropy reduced in the adsorption process.