荷梗生物炭理化性质及其对水中Cd的吸附机制

Physicochemical Properties of Biochar Derived from Lotus Petiole and Its Adsorption Mechanism of Cadmium in Aqueous Solution

为探究HBC(荷梗生物炭)的基本理化性质及其对水中Cd^(2+)的吸附机制,以荷梗为原料,在300～700℃热解温度下制得HBC,通过灰分分析、元素分析、SEM(扫描电镜)分析及FTIR(傅里叶红外光谱)分析初步探明HBC的基本理化性质并确定其最优热解温度,同时利用经典的吸附动力学模型和等温吸附模型对HBC吸附水中Cd^(2+)的内在机制进行分析.结果表明:HBC的最优热解温度为400℃,灰分物质显著影响了HBC的pH,进而对其吸附性能产生影响. SEM分析结果显示,HBC具有发达的多孔结构,其中400℃下制备的HBC(记为HBC-400)多孔结构最优.元素分析结果显示,HBC中φ(C)逐渐升高而H/C〔φ(H)/φ(C))〕、O/C〔φ(O)/φ(C)〕下降,表明随热解温度升高HBC逐渐失水且炭化程度逐渐增强. FTIR分析表明,HBC表面存在大量羟基、羧基和羰基等含氧官能团,随热解温度升高,HBC芳构化程度增强而表面官能团丰度降低.试验条件下,HBC对Cd^(2+)的平衡吸附量已达39. 239 mg/g,吸附平衡时间为600 min.通过对吸附动力学及等温吸附模型拟合结果分析可知,水中Cd^(2+)在HBC上的吸附是发生在多相异构表面的多分子层混合吸附.综合考虑HBC的理化性质及模型拟合结果可以推测Cd^(2+)在HBC上的吸附可分为3个过程:①Cd^(2+)在浓度梯度力作用下由溶液迅速扩散到HBC表面.②Cd^(2+)与HBC表面官能团发生络合、离子交换反应,与金属氧化物、碳酸盐等发生共沉淀反应.③Cd^(2+)扩散到HBC的多孔结构中,与苯环上普遍存在的π电子结构发生阳离子-π作用.研究显示,HBC具有碱性、发达的多孔结构、丰富的表面官能团和高稳定性等优良性质,HBC对水中Cd^(2+)的吸附是在其多孔介质表面进行的化学主导吸附过程,因此,可为生物炭类环境功能材料的研制提供选材依据.

In order to obtain more possible choices of biochar materials,the biochar prepared with lotus petiole was studied in current study.HBC(lotus petiole biochar)was obtained in the rang 300-700℃,the basic physicochemical properties of HBC were explored through ash analysis,elemental analysis,scanning electron microscope(SEM)analysis and Flourier-transform infrared spectrum(FTIR)analysis.Furthermore,the optimal pyrolysis temperature was determined.Adsorption kinetic and isothermal models were used to explore the adsorption mechanism of the HBC for Cd^2+in aqueous solution.The results showed that the optimum pyrolysis temperature was 400℃,and ash significantly affected the pH and the adsorption performance of HBC.SEM images showed that the HBC with porous structure was formed and the optimum HBC-400 was obtained at 400℃.The results of elemental analysis showed the content of C element in the HBC gradually increased while the element ratio of H/C and O/C decreased,which indicated that the HBC gradually lost water and the degree of carbonization gradually increased.FTIR analysis showed the presence of large amounts of phenolic and carbonyl functional groups on the surface of HBC.HBC aromatization degree increased and the content of the oxygen functional groups decreased with increasing pyrolysis temperature.The equilibrium adsorption capacity of HBC-400 was 39.239 mg/g and the adsorption equilibrium time was 600 min.The results of adsorption kinetic and isothermal models indicated that the adsorption of Cd^2+on the HBC-400 was a mixed adsorption of multi-molecular layer on the heterogeneous surface.The adsorption of Cd^2+on HBC could be divided into three processes by considering the physicochemical properties of HBC and the results of model:(1)Cd^2+rapidly diffused to the HBC surface from the solution affected by the force of concentration gradient.(2)Cd^2+was bonded to the HBC surface through ion exchange reaction and complexation with the functional groups,meanwhile,Cd^2+coprecipitated with the metal oxide and carbonate in the ash.(3)Cd^2+diffused into the porous structure of HBC and had a cationic-πeffect with theπelectron structure that prevalent in the benzene ring.The results showed that the HBC had alkaline properties,rich porous structure,abundant surface functional groups and high stability,HBC adsorption of Cd^2+occurred on the surface of the porous material,and the adsorption was a chemistry adsorption process.This research can provide a basis for the selection of raw materials in the development of biochar-like environmental and functional materials.