Co-transport of phenanthrene and pentachlorophenol by natural soil nanoparticles through saturated sand columns

Mobile colloids such as nanoparticles (NPs) are often considered to affect the fate and transport of various contaminants by serving as carriers. Many studies have focused on the effect of engineered NPs on contaminant transport. To date, very little information is available on the co-transport of natural soil NPs with typical organic contaminants. This study investigated the co-transport of phenanthrene (PHE) and pentachlorophenol (PCP) by three soil NPs through saturated sand columns. Soil NPs with high organic matter and particle concentration were the most effective in transporting PHE through columns. In addition, soil NPs significantly increased the transport of low-level PHE (0.2 mg L^-1) but there was no obvious increase at 1.0 mg L^-1 PHE. This is attributed to a higher ratio of NP-associated PHE to total PHE at a low-level than at a high-level during transport. In contrast to PHE, the chemical speciation of PCP determined its mobility, which was highly dependent on solution pH. At pH 6.5, anionic PCP became dominant and soluble in the effluent. This could account for the negligible effect of soil NPs on PCP mobility. At pH 4.0, however, neutral molecular PCP dominated and, as expected, decreased mobility of PCP occurred. Soil NPs considerably enhanced the transport of neutral PCP in NP-associated forms compared to controls, due to the high hydrophobicity and sorption affinity of PCP to NPs. The mobility of soil NPs was little affected by PHE and PCP under tested conditions. This study indicated that highly mobile soil NPs may be effective carriers for organic contaminants and give a new direction to polluted site remediation by using a natural material, e.g., soil.