二维相关吸收光谱法研究多壁碳纳米管与腐植酸吸光组分间非均相吸附行为

Heterogeneous Adsorption Behaviors of Humic Acid Light-Absorbing Components on Multiple-Walled Carbon Nanotubes by Two-Dimensional Correlation Absorption Spectroscopy

溶解性有机质(DOM)是水环境中一类复杂的溶解性有机混合物,不仅可影响污染物归趋及生物有效性,且DOM自身属于消毒副产物(DBPs)前驱体。为此,如何高效去除水中DOM已成为当前环境污染控制与治理技术研究的热点问题之一。以商品化腐植酸(HA)为DOM典型代表物质,利用紫外可见吸收光谱结合二维相关光谱法(2D-COS)从动力学、吸附等温线及热力学等角度研究了原始多壁碳纳米管(MWCNT)、羟基化MWCNT及羧基化MWCNT与HA吸光组分间吸附行为。2D-COS提高了HA一维吸收光谱分辨率,经二维相关吸收光谱图分析显示3种MWCNTs对HA吸光组分的吸附变化顺序均为275nm→400nm,表明MWCNTs与HA吸光组分间为非均相吸附行为。动力学结果显示MWCNTs与275nm处HA吸光组分间吸附速率高于MWCNTs与400nm处HA吸光组分间吸附速率,表明275nm处HA吸光组分可优先吸附至三种MWCNTs上。在25和35℃条件下,MWNCTs与HA吸光组分间吸附等温线表现为非线性,且Freundlich模型拟合决定系数R2大于Langmuir模型拟合决定系数,表明Freundlich吸附等温模型比Langmuir吸附等温模型更有利于描述MWCNTs与HA吸光组分间吸附等温线。275nm处HA吸光组分的饱和吸附容量(qmax)及相同给定平衡浓度下单点吸附系数Kd高于400nm处HA吸光组分,进一步表明MWCNTs与HA吸光组分间为非均相吸附。此外,当给定平衡浓度(ce=0.5cm-1和ce=1.5cm-1)越低,MWCNTs与HA吸光组分间结合能力越大,即HA吸光组分浓度较低时更易与MWCNTs上高能吸附位点相结合。相同平衡浓度下MWCNTs与特定HA吸光组分间吸附能力顺序表现为MWCNT8> MWCNT8-OH>MWCNT8-COOH,表明功能化基团可影响MWCNTs与HA吸光组分间吸附特性。另外,相同条件下单点吸附系数Kd与MWCNTs比表面积间无显著相关性(p>0.05),表明比表面积不是造成MWCNTs与特定HA吸光组分间结合能力差异的主要因素;Kd与MWCNTs中孔孔隙度间呈显著正相关(p<0.05)而与微孔孔隙度间无显著相关性(p>0.05),这主要是由于HA吸光组分可进入MWCNTs中孔而难以通过MWCNTs微孔。最后热力学分析结果显示Gibbs自由能变(ΔG0)<0、焓变(ΔH0)>0和熵变(ΔS0)>0,表明MWCNTs吸附HA吸光组分的过程为吸热反应,可自发进行,升温可促进MWCNTs对HA吸光组分的吸附,且吸附过程中固液界面的无序性增加。相同温度及吸光组分下,MWCNT8的ΔG0值小于MWCNT8-OH及MWCNT8-COOH,进一步表明MWCNT8与特定HA吸光组分结合能力强于MWCNT8-OH及MWCNT8-COOH。证明了2D-COS可识别HA中具有不同吸附行为的吸光组分,二维相关吸收光谱技术可作为研究MWCNTs与DOM间非均相吸附行为的有效工具。同时,有助于更好地理解MWCNTs与DOM间相互作用特性及机理,不仅可为水环境中DOM去除研究提供基础数据及新的研究思路,且有利于更好地预测自然环境中MWCNTs及DOM迁移传输及环境归趋。

Dissolved Organic Matter(DOM)is a heterogeneous mixture of dissolved material found ubiquitously in aquatic systems.DOM not only influences greatly the fate and the bioavailability of pollutants in natural waters,but also belongs to t he precursor of the Disinfection Byproducts(DBPs).Therefore,how to effectiv ely remove DOM from water has become one of the most popular issues in the resea rch of environmental pollution control and management technology.In this study,adsorption behaviors of the commercial humic acid(HA)light-absorbing compo nents on pristine multiple-walled carbon nanotube(MWCNT),hydroxylated MWCNT(MWCNT-OH)and carboxylated MWCNT(MWCNT-COOH)were investigated using two-d imensional correlation spectroscopy(2D-COS)based on the ultraviolet-visible absorption spectra.The kinetics,isotherms and thermodynamics of adsorption o f the HA light-absorbing components on the three MWCNTs were the focus of the p resent study.2D-COS could enhance the resolution of the HA one-dimensional a bsorption spectra.Two types of absorption bands for the HA light-absorbing co mponents were distinguished by two-dimensional correlation absorption spectra,which changed sequentially in the order of 275 nm→400 nm.Thus,the 2D-COS revealed that heterogeneous adsorption behaviors of the HA light-absorbing comp onents on MWCNTs.The adsorption kinetic rate of the HA light-absorbing compon ent at 275 nm was higher than that of the HA light-absorbing component at 400 n m,suggesting that the HA light-absorbing component at 275 nm was preferential ly adsorbed to MWCNTs.The adsorption isotherms of MWNCTs and the HA light-abs orbing components were nonlinear under 25℃and 35℃.In comparison,the ads orption isotherms between the HA light-absorbing components and MWCNTs were fit ted better to the Freundlich model than the Langmuir model by higher correlation coefficient R 2.Furthermore,the saturation adsorption capacity(qmax)and the single point adsorption coefficient Kd of the HA light-absorbing compon ent at 275 nm were higher than those of the HA light-absorbing component at 400 nm,further suggesting that the interaction of MWCNTs and the HA light-absorb ing components was heterogeneous adsorption.In addition,the adsorption affin ity between MWCNTs and the HA light-absorbing component was stronger when the g iven equilibrium concentration(c e=0.5 cm-1 and c e=1.5 cm-1)was lower,indicating that the HA light-absorbing component tended to occupy high energy adsorption sites on MWCNTs at lower HA concentration.The ad sorption affinity sequence was in order of MWCNT8>MWCNT8-OH>MWCNT8-COOH when a t the same equilibrium concentration and the specific HA light-absorbing compon ent,demonstrating that various functional groups could contribute to different adsorption characteristics between MWCNTs and the HA light-absorbing compon ent.Furthermore,an insignificant correlation between the surface area of MWC NTs and the Kd values under the same conditions suggested that the surface ar ea is not the major factor that makes the adsorption differences between MWCNTs and the specific HA light-absorbing component.Meso-pores of MWCNTs presented a significantly positive correlation with the Kd values,but micro-pores of M WCNTs showed an insignificant correlation with the Kd values,which could be du e to the fact that HA light-absorbing component molecules can enter meso-pores in MWCNTs,while it was difficult for the HA light-absorbing molecules to enter micro-pores in MWCNTs due to large molecular size.Finally,thermodynamic analysis s howed that the HA light-absorbing components adsorption on MWCNTs occurred spon taneously due to the negative values of Gibbs free energy change(ΔG 0).The positive values of enthalpy change(ΔH 0)indicated that the adsorption of the HA light-absorbing components on MWCNTs is an endothermic reactions,whi le the positive values of entropy change(ΔS 0)reflected an increase in the randomness at the solid-solution interface during the adsorption process.TheΔG 0 values of MWCNT8-OH and MWCNT8-COOH for the same HA light-absorbi ng component and temperature were higher than those of MWCNT8,further suggesti ng that the adsorption affinities of MWCNT8-OH and MWCNT8-COOH were lower than those of MWCNT8.This study proves that 2D-COS could differentiate the variou s HA light-absorbing components with different adsorption behaviors,and two-dime nsional correlation absorption spectroscopy could be successfully applied to inv estigate the heterogeneous adsorption behaviors of the HA light-absorbing compo nents on MWCNTs.The conclusion of the present study could provide a deep understand ing of the characteristics and mechanisms of the interactions between MWCNTs and DOM.These results have implications for eliminating DOM in the aquatic enviro nment,and the findings obtained in this research are helpful to better predic t the transport and fate of both DOM and MWCNTs in the natural environment.