Surface-bound humic acid increased Pb²⁺ sorption on carbon nanotubes

Solid humic acid (HA) particles were dissolved and subsequently coated on a type of multiwalled carbon nanotubes (MWCNTs). Pb(2+) sorption from water by the solid HA, the MWCNTs and the obtained HA-MWCNT complexes was compared. The underlying mechanism of the difference in the sorption was discussed with the data at different pHs, results of desorption in the presence and absence of Ca(2+) and the characterizations using inductively coupled plasma mass spectrometry, X-ray energy dispersion spectroscopy and X-ray absorption fine structure spectroscopy. The effect of MWCNT-contained impurities on the sorption was also examined. It was shown that the surface-bound HA introduced oxygen-containing functional groups and negative charges on the MWCNTs, thus greatly increasing Pb(2+) sorption on the MWCNTs. Pb(2+) could be electrostatically attracted into outer-sphere of the electric double layer of the HA-MWCNT complexes, a fraction of which would form coordination complexes with carboxyl groups in the inner- and/or outer-sphere.