Facilitated transport of titanium dioxide nanoparticles via hydrochars in the presence of ammonium in saturated sands: Effects of pH, ionic strength, and ionic composition

Abstract

The widespread use of nanoparticles (NPs) has led to their inevitable introduction into environmental systems. How the existence of hydrochars in crop soils will affect the mobility of nanoparticle titanium dioxide (nTiO2), especially in the presence of ammonium (NH4+), remains unknown. Research is needed to study the effects of hydrochars on the transport and retention of nTiO2 and to uncover the mechanisms of these effects on nTiO2 transport. Column experiments with nTiO2 and hydrochars were performed in various electrolyte (NaCl, NH4Cl, and CaCl2) solutions under a controlled pH (6.0 and 8.0). Additionally, the size distributions and scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the NPs were observed. The experimental results suggested that the mobility of the hydrochars was much better than that of nTiO2. Thus, the mobility of nTiO2 was improved upon their attachment to the hydrochars. The facilitated transport of nTiO2 in the presence of hydrochars was stronger at pH8.0 than at pH6.0, and facilitated transport was nearly independent of the electrolyte cation at pH8.0. However, at pH6.0, the facilitated transport in various electrolytes had the following order: NaCl>NH4Cl>CaCl2. The conversion from a completely reversible to a partially irreversible deposition of nTiO2 in sand was induced by the partially irreversible retention of hydrochars, and this phenomenon was more pronounced in the presence of NH4+ than in the presence of Na+. In particular, the irreversible deposition of nTiO2-hydrochars was enhanced as the cation concentration increased. The increased irreversible retention of nTiO2 was related to the greater k2 value (irreversible attachment coefficients) on site 2 for hydrochars based on two-site kinetic retention modeling. Thus, there is a potential risk of contaminating crops, soil, and underground water when nTiO2 exists in a hydrochar-amended environment, especially when associated with NH4-N fertilizer.