Effects of multi-walled carbon nanotubes on pyrene adsorption and desorption in soils: The role of soil constituents.

Once entering soil, carbon nanotubes (CNTs) can influence the fate of hydrophobic organic compounds (HOCs) in soil due to its strong adsorption capacity. This process may be influenced by the interactions between CNTs and soil constituents. The mechanisms therein were investigated in the present study by examining pyrene adsorption/desorption on one CNTs, two soils (black soil and paddy soil), and mixtures thereof. CNTs' amendment enhanced soil site heterogeneity and adsorption capacity of pyrene while it was less than that predicted by the sum of the individual adsorption on soils and CNTs, which was more obvious at low aqueous concentrations. This could be due to the interactions between soil constituents (dissolved organic matter (DOM) and clays) and CNTs. Modification of CNTs by DOM attenuated pyrene adsorption by 14.9%-66.1%, which was ascribed to occupying of surface adsorption sites, pore blockage of CNTs' aggregates, enhancement of surface polarity, and enhancement of pyrene solubility in aqueous phase. The coexistence of clay (kaolinite) also showed inhibition on pyrene adsorption onto CNTs with a reduction of 19.2%-40.2%. This could be ascribed to that the attachment of clay particles on CNTs' aggregates could cover the surface adsorption sites and enhance the surface polarity of CNTs. The effect of CNTs amendment on pyrene desorption hysteresis differed among soils. The hysteresis index of the black soil doubled after CNTs' amendment while that of paddy soil remained unchanged. The results of this study provide insights into the possible effects of CNTs on the fate of HOCs in real soil environment.