碳酸钾添加比例对玉米秸秆生物炭表面特性的影响

在600℃的热解温度下通过添加不同比例的碳酸钾制备活性生物炭。对比分析了碳酸钾活化生物炭(KBC)和普通生物炭(BC)的特性,包括生物炭红外光谱特征、表面官能团、比表面积和孔径分布及吸附动力学过程等。结果表明,经碳酸钾活化的生物炭比表面积大为提高,最高达到566 m2 g-1(KBC-2-600),而普通生物炭(BC600)的比表面积仅为86.8 m2 g-1,KBC600系列的介孔容积和微孔容积均显著高于BC600,介孔容积平均扩大了16倍,微孔容积平均扩大了4倍,同时提高了微孔率。经碳酸钾活化的生物炭表面官能团的数量和饱和度发生改变,在1 256 cm-1~3 414 cm-1处,官能团的总体含量均低于普通生物炭,酯类羰基消失,形成氢键的能力减弱,非饱和醚类增加,芳香性和非极性提高,此表面特征更有利于吸附非极性芳香类污染物。对萘吸附的动力学实验表明,BC600和KBC-4-600均适合以二级动力学模型拟合,经碳酸钾活化后,生物炭的吸附性能提高,颗粒内扩散模型拟合结果表明,经碳酸钾活化后,生物炭内部孔隙复杂程度和数量均得到提高。碳酸钾活化的生物炭表面性质优良,作为高效吸附剂用于修复非极性芳香类污染有很大潜力。

Effect of Potassium Carbonate Addition Rate on Surface Characteristics of Corn Stalk Derived Biochar

[Objective]In recent years,biochar as an environment-friendly material has been arousing more and more attention the world over. The use of biochar as an adsorbent has been proved to be an effective way in managing polluted soil and water. Ordinary biochar is quite low in specific surface area and underdeveloped in internal pore structure,which greatly affects adsorption efficiency of the biochar. Therefore,usually biochar needs activating for higher adsorption capacity. In this study,effect of the amendment of potassium carbonate on surface properties of biochar was studied.[Method]Activated biochar was prepared under 600℃ from corn stalk amended with potassium carbonate at a varying ratio(stalk/potassium carbonate in mass=1:1,2:1 and 4:1). Potassium carbonate was prepared into solutions according to the impregnation ratio,0.60 mol L-1,1.2 mol L-1 and 2.4 mol L-1 in concentration,separately, and added into corn stalk at a rate of 3 ml per gram of corn stalk in preparation of activated biochar. The biochars prepared in such a way were labeled as KBC-1-600,KBC-2-600 and KBC-4-600 separately;the biochar prepared without the amendment of potassium carbonate was labeled as BC600,non-activated biochar. Basic properties of the biochars,BC(corn stalk biochar)and KBC(potassium carbonate activated biochar)were characterized,with infrared spectrum,surface functional groups,specific surface area,pore size distribution and adsorption kinetics. Naphthalene,as a typical small-sized molecule persistent pollutant (POPs),was selected as adsorbate to evaluate adsorption capacity of KBC and BC. Naphthalene adsorption capacities of KBC and BC and affinities of KBC and BC to naphthalene were analyzed,and prospect of the application of activated carbon was discussed. The pseudo first-order kinetic model,second-order kinetics model and intraparticle diffusion model were used to analyze dynamic process of the adsorption.[Result] With increasing potassium carbonate amendment rate,aromaticity of the biochars increased. When the impregnation ratio was increased from 4:1 to 2:1,hydrophilicity and polarity of the biochars improved, but when the impregnation ratio was further increased from 2:1 to 1:1,hydrophilicity and polarity of the biochars decreased,instead. Potassium carbonate amendment increased specific surface area of the biochards,with KBC-2-600 in particular reaching up to 566 m2 g-1,whereas specific surface area of the ordinary biochar(BC600)was only 86.8 m2 g-1;The KBC600 series of biochars were significantly higher or on average 16 and 4 times higher respectively than BC600 in mesopore volume and micropore volume,and also higher in mesoporosity. The amendment altered the number of functional groups and their saturation on the surface of biochar. KBC was lower than BC in total of functional groups at 1256 cm-1~3414 cm-1. With the increasing amendment rate of potassium carbonate,the ester C=O disappeared,reducing hydrogen bond forming capacity,and unsaturated C-O-C increased,enhancing aromaticity and non-polarity. Alteration of the cellulosic cleavage process caused by potassium carbonate is considered to be the main cause triggering changes in functional groups on the surface of the biochar. Kinetic analysis shows that the adsorption of naphthalene onto BC600 and KBC-4-600 could be described by the pseudo-second-order kinetic model. The adsorption process involved simultaneously external surface adsorption and intraparticle diffusion;After activation by potassium carbonate,adsorption capacity of the biochar improved significantly. The fitting of intraparticle diffusion model demonstrates that KBC gets more complicated in internal pore structure. [Conclusion]The amendment of potassium carbonate contributes significantly to improvement of surface active sites and pore complexity of the biochar,thus enhancing its adsorption capacity of nonpolar aromatic pollutants. The potassium carbonate activated biochar has well-developed pore structure,and hence a great potential to be used as highly-efficient adsorbent in remedying polluted soils in future.