Sorption of phenanthrene on single-walled carbon nanotubes modified by DOM: effects of DOM molecular weight and contact time

The impacts on the sorption of phenanthrene (PHE) onto single-walled carbon nanotubes (SCNT) by loading of dissolved organic matter (DOM) fractions with different molecular weights (MWs) were studied. Moreover, the influence of contact time on the sorption capacity of DOM-modified SCNT was evaluated for the first time. Preloading of DOM on SCNT for 2 days led to a reduction in PHE sorption and the nonlinearity of the sorption isotherm; however further increasing the contact time between the DOM and SCNT from 2 days to 20 days enhanced the sorption capacity, and the sorption nonlinearity increased also. This is due to the transferring of DOM molecules from the external surface to interstitial channels trapped in SCNT particles, restoring the sorption sites. The loading of DOM fraction with MWs larger than 14 000 Da (DOM>14 000) brought a stronger suppression of PHE sorption by SCNT compared to smaller ones (DOM<1000 and DOM1000-14 000), with the Freundlich nonlinear index n increasing from 0.236 to 1.093 and the Freundlich affinity coefficient log K-F decreasing from to 7.34 to 4.57. These are similar to the sorption characteristics of DOM>14 000, suggesting a complete coverage of the SCNT by DOM>14 000 molecules. Though the loading of DOM fractions with smaller MWs made SCNT more polar, the reduction in PHE sorption on DOM-preloaded SCNT was limited due to their lesser sorbed amount and smaller molecular size.