Sorption of phenanthrene by nanosized alumina coated with sequentially extracted humic acids

Sorption of hydrophobic organic compounds (HOCs) to natural organic matter (NOM) is an important process that affects the transport, transformation, bioavailability, and fate of HOCs in the environment. Manufactured nanoparticles (NPs) such as nano-oxides will inevitably enter the environment in the processes of their production, transfer, and use and could be coated by the ubiquitous NOM. Thus, sorption of HOCs to NOM in the environment could be affected by the NP interactions with NOM. Furthermore, the toxicity of nano-oxides could be increased due to the adsorbed HOCs. Therefore, sorption of phenanthrene by nano-Al2O3 coated with humic acid (HA) was examined in this study to explore the possible effect of nanoparticles (NPs) on the environmental behavior of HOCs and the potential environmental and health risks of NPs. Four HAs were sequentially extracted with 0.1 mol/L NaOH from a peat soil. HAs, nano-Al2O3, and HA-coated nano-Al2O3 were characterized by techniques such as elemental analysis, solid-state C-13 NMR, N-2 surface area analysis, and zeta potential measurement. Adsorption isotherms of HAs by nano-Al2O3 and phenanthrene by HAs and HA-coated nano-Al2O3 were obtained using a batch equilibration technique at 25 +/- 1A degrees C. HA concentrations were measured by total organic carbon analysis. Phenanthrene concentrations were measured by liquid scintillation counting. The adsorption maxima of HAs by nano-Al2O3 was one order of magnitude higher than that by soil inorganic minerals. Phenanthrene isotherms of HA-coated nano-Al2O3 were more nonlinear than that of their respective bulk HAs. Concentration-dependent organic carbon-normalized sorption coefficients (K' (oc)) of phenanthrene by HA-coated nano-Al2O3 were lower than those for their respective bulk HAs, especially at relatively high concentrations. Isotherm nonlinearity of phenanthrene could be interpreted by a combination of partitioning accompanied by linear isotherm with adsorption accompanied by nonlinear isotherm. HA conformation changes during their adsorption on nano-Al2O3 could play an important role in phenanthrene sorption and were responsible for higher nonlinearity of phenanthrene isotherms and lower phenanthrene K' (oc) on the adsorbed HAs than their respective bulk HAs. Adsorption of HA on nano-Al2O3 would form a more condensed HA state with higher pi-polarity/polarizability and lower partitioning affinity than the respective bulk HA, leading to an increase of relative contribution of adsorption to the total sorption and more nonlinear phenanthrene isotherms in the adsorbed HA due to the increase in phenanthrene adsorption affinity and decrease in phenanthrene partitioning affinity. Adsorption of HA on nano-Al2O3 was much higher than that on soil oxide minerals and could form a more condensed HA state with higher pi-polarity/polarizability and lower partitioning affinity than the bulk HA, causing the significant difference in phenanthrene sorption between the adsorbed HA and the respective bulk HA. Therefore, once released in the environment, NPs such as nano-Al2O3 will strongly alter the environmental transport, fate, and bioavailability of HOCs and could be potentially more toxic due to the adsorbed toxic chemicals. Due to the high adsorption of HA on nano-Al2O3 and its significant effect on phenanthrene sorption, interactions of NOM with nano-oxides and their mechanistic relations with NOM conformation changes and HOC sorption merit further research. In addition, due to the higher sorption of phenanthrene on the HA-coated nano-Al2O3 than the pure counterpart, the effect of NOM and HOCs on the ecotoxicity of NPs should be addressed in the future.