Effects Of Molecular Weight Fractionated Humic Acid On The Transport And Retention Of Quantum Dots In Porous Media

Over the past decade,engineered nanoparticles（ENPs）have been increasingly applied to a variety of commercial products due to their unique physical-chemical properties.During the production,application and disposal processes,ENPs could be inevitably released into surrounding environment.Once entering environment,ENPs could undergo complicated physical-chemical transformations and pose a threat to nature environment and human health.Thus,it is important and urgent to understand the fate and transport of ENPs in environment and evaluate the potential impact of ENPs on environment and human health.Although humic acid（HA）plays an important role in the fate and transport of nanoparticles in environment,HA consists of various mixtures of organic compound with different sizes of molecular,it is still not well understood how the specific fractions of HA influence the transport and fate of nanoparticles in environment.Quantum dots（QDs）,as a kind of ENPs,have been well-known for their distinguished tunable optical properties and widely used in solar cells,biomedical imaging,luminescent probes,chemical analysis,display and lighting technologies.Quantum dots（QDs）with carboxyl and amino modification（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 of QDs were also investigated the effect of cation type and the coating of nanoparticles on the transport and retention behaviour of nanoparticles.The results show that the M_f-HA had distinct effects on the transport and retention behavior of QDs,which were highly dependent on the MW of M_f-HA.In addition,the surface coating of 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 QDs more significantly than the low MW M_f-HA（10-30 kDa,3-10 kDa,and<3kDa）.High adsorption and strong steric repulsion of high MW M_f-HA could explain this MW-dependent effect.However,the 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 high MW M_f-HA（>100 kDa and 30-100 kDa）could enhance the mobility of QDs more significantly than the low MW M_f-HA（10-30 kDa,3-10 kDa,and<3 kDa）.However,the transport behavior of QD-NH₂ was not altered by pristine-and M_f-HA.Most of QD-NH₂ desposited in the sand column.