木质素-聚乙烯共热解生物炭对Cd(Ⅱ)的吸附性能

Cd(Ⅱ)Adsorption Capability of the Biochar Derived from Co-pyrolysis of Lignin and Polyethylene

为了寻求木质素的资源化利用途径和开发低廉、高效的重金属吸附材料,该研究将木质素和聚乙烯混合物在600℃下热解制备生物炭(LG/PE-600C),以单独木质素在相同条件下所制备的生物炭作对比(LG-600C),利用扫描电镜(SEM)、N_(2)吸附/脱附、傅里叶红外光谱(FT-IR)和元素分析等对比分析LG-600C和LG/PE-600C在表观形貌、孔隙特性、表面官能团、物质组成等的差异,并进一步考察两种生物炭对Cd(Ⅱ)的吸附性能。相对于LG-600,LG/PE-600C表面较粗糙,孔隙结构较发达。LG/PE-600C的比表面积达213.87 m^(2)·g^(-1),约为LG-600的2.5倍。聚乙烯的介入,导致LG/PE-600C的O元素含量和表面含氧官能团较LG-600C有所降低。吸附结果显示,LG/PE-600C对Cd(Ⅱ)的吸附性能明显优于LG-600。在一定范围内,Cd(Ⅱ)初始浓度、环境温度、吸附时间、溶液pH的增加均能够促进LG/PE-600C对Cd(Ⅱ)的吸附。Langmuir模型和准二级动力学模型能够很好地拟合Cd(Ⅱ)的吸附过程,表明LG/PE-600C对Cd(Ⅱ)的吸附为化学作用主导的单分子层吸附。基于Langmuir模型所得到的理论最大吸附量达到40.82 mg·g^(-1)。热力学分析证实,LG/PE-600C对Cd(Ⅱ)的吸附为自发的吸热反应。机理分析表明,LG/PE-600C具有优于LG-600C的吸附性能,其主要原因在于聚乙烯介入能够强化木质素的分解,促进LG/PE-600C孔隙的发育,使得LG/PE-600C表面暴露大量Cd(Ⅱ)的吸附位点,如-OH、C=C、-C-O-C等。这些吸附位点能够通过配位、沉淀、离子交换等作用对Cd(Ⅱ)进行吸附。由此证实,添加聚乙烯与木质素进行共热解制备生物炭用于Cd(Ⅱ)的吸附可作为木质素重要的资源化利用方式。

In order to explore a way to utilize lignin and develop low-cost and highly-efficient adsorbents for the removal of heavy metals,this study prepared biochar by lignin and pyrolysis of lignin(LG-600C)and polyethylene mixture(LG/PE-600C),respectively,under the same condition at 600℃.The physicochemical properties involving surface morphology,textural characteristics,surface functional groups,and composition of LG-600C and LG/PE-600C were determined by scanning electron microscope(SEM),N_(2) adsorption/desorption,fourier transform infrared spectroscopy(FT-IR),and ultimate analysis.The biochar adsorption capability of LG-600C ad LG/PE-600C for Cd(Ⅱ)were investigated as well.Compared to LG-600C,LG/PE-600C presented a rougher surface and a more developed pore structure with specific surface area reaching 213.87 m^(2)·g^(-1),which was about 2.5 times that of LG-600C.The introduction of polyethylene led to the decrease of O content and O-containing surface functional groups of LG/PE-600C.The adsorption test results confirmed that the adsorption capability of LG/PE-600C for Cd(Ⅱ)was apparently superior to that of LG-600C.The adsorption capability of LG/PE-600C for Cd(Ⅱ)can be promoted with the increase of Cd(Ⅱ)initial concentration,ambient temperature,adsorption time,and solution pH within a certain range.Langmuir isotherm model and pseudo-second-order kinetic model can be employed to well simulate the adsorption of LG/PE-600C for Cd(Ⅱ),indicating that the adsorption process belongs to a monolayer adsorption and is dominated by chemical actions.The theoretical maximum adsorption capacity of LG/PE-600C for Cd(Ⅱ)calculated by Langmuir model was 40.82 mg·g^(-1).Thermodynamic analysis showed that the adsorption of LG/PE-600C for Cd(Ⅱ)was a spontaneous and endothermic reaction.According to mechanism analysis,the introduction of polyethylene could intensify the degradation of lignin and promote the growth of pore structure of LG/PE-600C.As a result,abundant adsorption sites,such as–OH,C=C,and C-O-C,for Cd(Ⅱ)were exposed,leading to a higher adsorption capability of LG/PR-600C via complexation,precipitation,and ion exchange.Results confirmed that the conversion of lignin into biochar with the addition of polyethylene could be considered as an alternative way to use lignin.