Co-transport behavior of nano-ZnO particles in the presence of metal-nanoparticles through saturated porous media

Abstract

The objective of this study is to evaluate the co-transport behavior of nZnO (193.0 nm) along with other metal-based ENPs [i.e., nTiO2 (198.6 nm), nAg (167.9 nm), and nFexOy (178.5 nm)] through saturated porous media in the presence and absence of humic acid (HA) under varying pH (8, 6.5 and 5). To achieve this objective a series of column experiments are performed where the nZnO particles are injected in the porous media in the presence of other metal ENPs. After each column experiment, the column was sectioned, and the retained mass of nZnO and the porosity along the length of the column were measured. The size and surface charge of ENPs were also assessed. The study indicates that negligible amount of nZnO get transported in the absence of HA whether injected as a single entity or along with other ENPs. However, in the presence of HA and at pH 8, significant amounts (C/C0 = 0.57 to 0.68) of nZnO get transported. The surface charge of nZnO or mix-ENPs is positive or close to neutral. While in the presence of HA, surface charge of nZnO or mix-ENPs becomes highly negative (-40.6 ± 0.9 mV), which probably resulted in unfavorable condition for particle deposition in the presence of HA. Presence of nAg and nTiO2 does not influence the transport efficiency of nZnO. However, in the presence of nFexOy and under HA medium a decrease in the eluted concentration of nZnO is observed. With a decrease in pH (within a range of 8 to 5), co-transport efficiency of nZnO particles (α = 0.05-1.00, C/C0 = 0.68-0.0) has reduced drastically even when suspended in HA medium. This is probably controlled by surface charge of ENPs. The study also indicates that the hyper-exponential retention of ENPs has insignificant implication on the alteration of porosity of the media. Overall it could be stated that the presence of HA and the solution pH are mainly controlling the transport behavior of nZnO within the experimental condition adopted in this study.