The mechanisms of mobility and retention of titanium dioxide nanoparticles (nTiO_2) in well-defined porous media composed of clean quartz sand in the presence of fulvic acid (FA) and humic acid (HA) were studied under acidic conditions.nTiO_2 were immobile in the porous media in the absence of FA and HA at pH 4.0.FA and HA could be adsorbed onto the surface of nTiO_2,change the electrokinetic properties of nTiO_2,and facilitate the transport of nTiO_2.The elution of nTiO_2 increased from 0.01 and 0.88 to 0.91 and 0.94 with the increase of FA and HA from 1mg/L to 10mg/L respectively.Compared to FA,more HA was adsorbed onto nTiO_2,and thus the facilitated effect of HA on transport of nTiO_2 was stronger.Ions inhibited the mobility of nTiO_2,and the effect of CaCl_2 was greater than that of NaCl in same concentration.The mobility of nTiO_2 was better in the presence of HA than FA.In addition,7%~56% nTiO_2 was deposited in the secondary energy minimum well in the presence of HA,higher than 4%~17% in the presence of FA,which could be easily released when the environmental conditions changed.High energy barriers between nTiO_2 and quartz promoted the mobility of nTiO_2,while a combination of the secondary minimum energy,straining,diffusion and gravitational deposition were involved in the retention of nTiO_2
