Heterogeneous Adsorption Behaviors of Humic Acid Light-Absorbing Components on Multiple-Walled Carbon Nanotubes by Two-Dimensional Correlation Absorption Spectroscopy
YANG Cheng-hu1, LIU Yang-zhi2, SUN Xiu-mei1, LIU Qin1, LI Tie-jun1, GUO Yuan-ming1*
1. Marine and Fishery Research Institute of Zhejiang, Key Laboratory of Sustainable Utilization of Technology Research for Fishery Resource of Zhejiang Province, Zhoushan 316021, China
2. Department of Environmental Science, Zhejiang University, Hangzhou 310058, China
Abstract：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 the precursor of the Disinfection Byproducts (DBPs). Therefore, how to effectively remove DOM from water has become one of the most popular issues in the research of environmental pollution control and management technology. In this study, adsorption behaviors of the commercial humic acid (HA) light-absorbing components on pristine multiple-walled carbon nanotube (MWCNT), hydroxylated MWCNT (MWCNT-OH) and carboxylated MWCNT (MWCNT-COOH) were investigated using two-dimensional correlation spectroscopy (2D-COS) based on the ultraviolet-visible absorption spectra. The kinetics, isotherms and thermodynamics of adsorption of the HA light-absorbing components on the three MWCNTs were the focus of the present study. 2D-COS could enhance the resolution of the HA one-dimensional absorption spectra. Two types of absorption bands for the HA light-absorbing components 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 components on MWCNTs. The adsorption kinetic rate of the HA light-absorbing component at 275 nm was higher than that of the HA light-absorbing component at 400 nm, suggesting that the HA light-absorbing component at 275 nm was preferentially adsorbed to MWCNTs. The adsorption isotherms of MWNCTs and the HA light-absorbing components were nonlinear under 25 ℃ and 35 ℃. In comparison, the adsorption isotherms between the HA light-absorbing components and MWCNTs were fitted better to the Freundlich model than the Langmuir model by higher correlation coefficient R2. Furthermore, the saturation adsorption capacity (qmax) and the single point adsorption coefficient Kd of the HA light-absorbing component 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-absorbing components was heterogeneous adsorption. In addition, the adsorption affinity between MWCNTs and the HA light-absorbing component was stronger when the given equilibrium concentration (ce=0.5 cm-1 and ce=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 adsorption affinity sequence was in order of MWCNT8>MWCNT8-OH>MWCNT8-COOH when at the same equilibrium concentration and the specific HA light-absorbing component, demonstrating that various functional groups could contribute to different adsorption characteristics between MWCNTs and the HA light-absorbing component. Furthermore, an insignificant correlation between the surface area of MWCNTs and the Kd values under the same conditions suggested that the surface area 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 MWCNTs showed an insignificant correlation with the Kd values, which could be due 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 showed that the HA light-absorbing components adsorption on MWCNTs occurred spontaneously due to the negative values of Gibbs free energy change (ΔG0). The positive values of enthalpy change (ΔH0) indicated that the adsorption of the HA light-absorbing components on MWCNTs is an endothermic reactions, while the positive values of entropy change (ΔS0) reflected an increase in the randomness at the solid-solution interface during the adsorption process. The ΔG0 values of MWCNT8-OH and MWCNT8-COOH for the same HA light-absorbing component and temperature were higher than those of MWCNT8, further suggesting 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 various HA light-absorbing components with different adsorption behaviors, and two-dimensional correlation absorption spectroscopy could be successfully applied to investigate the heterogeneous adsorption behaviors of the HA light-absorbing components on MWCNTs. The conclusion of the present study could provide a deep understanding of the characteristics and mechanisms of the interactions between MWCNTs and DOM. These results have implications for eliminating DOM in the aquatic environment, and the findings obtained in this research are helpful to better predict the transport and fate of both DOM and MWCNTs in the natural environment.
杨承虎，刘洋之，孙秀梅，刘 琴，李铁军，郭远明. 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[J]. 光谱学与光谱分析, 2018, 38(11): 3334-3340.
YANG Cheng-hu, LIU Yang-zhi, SUN Xiu-mei, LIU Qin, LI Tie-jun, GUO Yuan-ming. Heterogeneous Adsorption Behaviors of Humic Acid Light-Absorbing Components on Multiple-Walled Carbon Nanotubes by Two-Dimensional Correlation Absorption Spectroscopy. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2018, 38(11): 3334-3340.
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