Effects of molecular weight fractionated humic acid on the transport and retention of quantum dots in porous media

Although humic acid (HA) plays an important role in the fate and transport of nanoparticles in subsurface environments, the roles of specific fractions of HA in the transport of nanoparticles are still not well understood. In this study, quantum dots (QDs) with carboxyl and amino modifications (QD-COOH and QD-NH2) were used as typical nanoparticles to understand the effects of molecular weight (MW) fractionated humic acid (M-f-HA) on the transport and retention behaviour of nanoparticles. Transport experiments on QDs were performed both in the absence and presence of pristine- and M-f-HA in both NaCl and CaCl2 solutions at pH 7.0. The M-f-HA had distinct effects on the transport and retention behavior of QDs, which were highly dependent on the MW of the M-f-HA. In addition, the surface coating of the QDs and cation types also showed significant effects on the transport behavior of QDs. In NaCl solution, the transport of QD-NH2 was dramatically enhanced in the presence of pristine- and M-f-HA, and the high MW M-f-HA (>100 kDa and 30-100 kDa) could enhance the mobility of the QDs more significantly than the low MW M-f-HA (10-30 kDa, 3-10 kDa, and <3 kDa). QD-COOH was readily mobile in the sand column and the recovery of injected QD-COOH was significantly higher in the presence of pristine- and M-f-HA. In CaCl2 solution, the transport of QD-COOH was enhanced in the presence of pristine- and M-f-HA, and the enhancement significantly correlated with the MW of the M-f-HA. However, the transport behavior of QD-NH2 was not altered by the pristine- and M-f-HA. Overall, these findings can improve our understanding of the effects of M-f-HA on the transport and retention of nanoparticles in subsurface environments, and suggest that the HA, surface coating and cation types are likely key factors which govern the stability and mobility of nanoparticles in the natural environment.