Mutual effects of dialkyl phthalate esters and humic acid sorption on carbon nanotubes in aqueous environments

Various reaction sequences of mutual sorptions of dimethyl phthalate (DMP) or diethyl phthalate (DEP) and humic acid (HA) on single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) were studied. The results indicate that the sorption of DMP and DEP on CNT decreases owing to its competition effect in the presence of HA. The competition is stronger at lower HA concentration. At higher HA concentration, weaker competition has occurred because there are steric hindrance and pore blockage of HA on CNT. The reaction sequences of DMP (or DEP) and HA sorption could affect their sorption mechanisms. The initial sorbed HA can modify the surface properties of CNT with a concomitant effect on the partial complexation of DMP (or DEP) with the sorbed HA. The Fourier transform infrared (FTIR) spectra indicate that SWCNT and MWCNT contain mainly -COO^- and -COOH moieties, respectively. In addition, the IR spectroscopic and thermogravimetric analyses illustrate that CNTs provide similar sorption capacity regardless of the interaction sequence of DMP (or DEP) and HA. Finally, the fluorescence quenching results indicate that DEP exhibits stronger binding to HA than that of DMP.