油料作物秸秆生物炭对水体中铅离子的吸附特性与机制

Characteristics and mechanism of lead adsorption from aqueous solutions by oil crops straw-derived biochar

为探索利用废弃生物质资源制备生物炭去除水体中Pb^(2+)污染的可行性,以农业废弃物胡麻秸秆和油菜秸秆为原材料,采用限氧裂解法在700℃条件下制备油菜秸秆(rape straw)生物炭和胡麻秸秆(flax straw)生物炭,通过2种生物炭对Pb^(2+)的批量吸附试验,利用4种吸附动力学模型(拟一级动力学、拟二级动力学、Elovich模型和颗粒内扩散模型)和4种等温吸附模型(Langmuir、Freundlich、Temkin和D-R模型)研究了胡麻和油菜秸秆生物炭对Pb^(2+)的吸附行为。同时,通过(brunauer emmett teller,BET)比表面积和孔径分析、扫描电子显微镜(SEM)、X射线衍射(XRD)和傅里叶变换红外光谱(FTIR)等手段对生物炭的结构和性质进行了表征,初步探讨了2种生物炭对Pb^(2+)的吸附机制。结果表明,胡麻和油菜秸秆生物炭分别在4 h和10 h达到吸附平衡,理论最大吸附量分别达到220.07和307.59 mg/g;2种生物炭对Pb^(2+)的吸附符合拟二级动力学模型,吸附等温线符合Langmuir等温吸附模型,表明其吸附过程为单分子层吸附;2种生物炭对Pb^(2+)的吸附作用为物理-化学复合过程,吸附机制主要包括静电作用、离子/配体交换、阳离子–π作用。研究结果可为油料作物秸秆的资源化利用和生物炭对水中重金属污染防治提供理论依据。

In recent years,with the continuous development of economy,heavy metal pollution becomes increasingly serious.Biochar is often obtained from agriculture wastes through pyrolysis,in which the biomass is subjected to thermochemical conversion under an oxygen-limited condition.As a new type of cheap efficient adsorbent,because of the larger specific surface area and high surface energy,and the surface containing rich-oxygen functional groups,biochar has a good application prospect in wastewater treatment,causing great interest for scientist.Flax(Sesamum indicum)and rape(Brassica campestris L.)are the typical oil crops,which are kinds of good raw material for producing biochars.However,large amounts of flax straw(FS)and rape straw(RS)are discarded or incinerated directly and cause resource waste and environmental pollution seriously.In order to utilize the waste biomass resources,2 types of biochars derived from FS and RS were prepared by pyrolyzing at a temperature of 700℃under oxygen-limited condition,and the adsorptions of Pb2+in aqueous solutions were evaluated.The effects of contact time,initial Pb2+concentration and initial pH value in batch experiments were investigated by the 4 kinds of adsorption kinetics models(Pseudo first-order,Pseudo second-order,Elovich and Intra-particle diffusion model)and the 4 kinds of isothermal adsorption models(Langmuir,Freundlich,Temkin and D-R model).The structure and properties of biochars were characterized by using Brunauer-Emmett-Teller(BET)surface area and pore size analysis,scanning electron microscope(SEM),X-ray diffraction(XRD)and Fourier transform infrared spectrometer(FTIR),which were used to preliminarily analyze adsorption mechanism.The results showed that the BET surface areas of RS and FS were 84.44 and 172.61 m2/g,respectively;SEM showed that the surface of 2 kinds of biochars was smooth,and the pore structure was compact and regular;FTIR indicated that the active functional groups such as CO32-(675-750,835-886 cm–1),secondary alcohol(1 017 cm–1),C-C(1 437 cm–1),-COOH or C=O(1 578 cm–1),C=C(1 687 cm–1),-CH2–(2 800-3 000 cm–1),-OH(3 307 cm–1)were found to be rich on the surfaces of 2 kinds of biochars,and all of the properties were good for Pb2+adsorption.The XRD patterns suggested that carbonate and basic carbonate containing Pb2+were present in these biochars after adsorption.The adsorption equilibrium of RS and FS was reached in 4 and 10 h,respectively,and the adsorption capacity of RS and FS reached 307.59 and 220.07 mg/g,respectively.The adsorption kinetics were best fitted by the Pseudo second order model,while the isothermal adsorption was best described by Langmuir isotherm.This indicated that Pb2+ions were adsorbed onto 2 kinds of biochars via monolayer.The adsorption mechanism for Pb2+was a complex interaction of physical and chemical factors,mainly including electrostatic interaction,ion exchange/ligand exchange,cationic-πand precipitation.The results obtained show that 2 kinds of oil crop biochars are both excellent adsorbents to Pb2+in aqueous solution and will