Surface functional groups and defects on carbon nanotubes affect adsorption-desorption hysteresis of metal cations and oxoanions in water

This study investigated the influence of the structure characteristics of carbon nanotubes (CNTs), such as surface oxygen-containing functional groups, specific surface area (SSA) and concentration of defects, on the adsorption-desorption hysteresis of a metal cation (Cu(II)) and two oxoanions (As(V) and Cr(VI)), from single, double and multi-walled CNTs (SWCNTs, DWCNTs and MWCNTs), and two oxidized MWCNTs with different oxygen concentrations (MWCNTs-O1, 2.51 wt% O and MWCNTs-O2, 3.5 wt% O). Oxygen-containing functional groups contributed to an increase in the adsorption capacity for Cu(II) from aqueous solutions, but a decrease in adsorption capacity for Cr(VI) and As(V). The order of adsorption capacities based on CNT SSA was MWCNTs-O2 > MWCNTs-O1 > MWCNTs > DWCNTs > SWCNTs, which was consistent with the order of CNT defect contents. Desorption hysteresis index (HI) values for Cu(II) increased as the number of functional groups increased. For Cr(VI) and As(V), however, HI values decreased as the number of functional groups increased. HI values decreased with an increase in metal ion surface coverage on CNTs. There may be a shift in the mechanisms of metal ion adsorption by CNTs, from more irreversible to more reversible processes, with an increase in adsorbed metal ions. An understanding of the desorption hysteresis of heavy metal ions is important and useful for the application and risk assessment of CNTs in the natural environment.

本研究探讨了碳纳米管（CNTs）的结构特征，如表面含氧官能团、比表面积（SSA）和缺陷浓度，对单壁、双壁和多壁碳纳米管（SWCNTs、DWCNTs和MWCNTs）以及两种不同氧浓度的氧化多壁碳纳米管（MWCNTs - O1，含氧量2.51 wt%；MWCNTs - O2，含氧量3.5 wt%）对一种金属阳离子（Cu(II)）和两种含氧阴离子（As(V)和Cr(VI)）吸附 - 解吸滞后现象的影响。含氧官能团有助于提高水溶液中Cu(II)的吸附能力，但会降低对Cr(VI)和As(V)的吸附能力。基于碳纳米管比表面积的吸附能力顺序为MWCNTs - O2 > MWCNTs - O1 > MWCNTs > DWCNTs > SWCNTs，这与碳纳米管缺陷含量的顺序一致。Cu(II)的解吸滞后指数（HI）值随着官能团数量的增加而增大。然而，对于Cr(VI)和As(V)，HI值随着官能团数量的增加而减小。HI值随着碳纳米管上金属离子表面覆盖率的增加而减小。随着吸附的金属离子增多，碳纳米管对金属离子的吸附机制可能从更不可逆转变为更可逆的过程。了解重金属离子的解吸滞后现象对于碳纳米管在自然环境中的应用和风险评估是重要且有用的。