Plastic debris, in particular, microplastics and nanoplastics, is becoming an emerging class of pollutants of global concern. Aging can significantly affect the physicochemical properties of plastics, and therefore, may influence the fate, transport, and effects of these materials. Here, we show that aging by UV or O3 exposure drastically enhanced the mobility and contaminant-mobilizing ability of spherical polystyrene nanoplastics (PSNPs, 487.3 ± 18.3 nm in diameter) in saturated loamy sand. Extended Derjaguin–Landau–Verwey–Overbeek calculations and pH-dependent transport experiments demonstrated that the greater mobility of the aged PSNPs was mainly the result of surface oxidation of the nanoplastics, which increased not only the surface charge negativity, but more importantly, hydrophilicity of the materials. The increased mobility of the aged PSNPs significantly contributed to their elevated contaminant-mobilizing abilities. Moreover, aging of PSNPs enhanced the binding of both nonpolar and polar contaminants, further increasing the contaminant-mobilizing ability of PSNPs. Interestingly, aging enhanced binding of nonpolar versus polar compounds via distinctly different mechanisms: increased binding of nonpolar contaminants (tested using pyrene) was mainly the result of the modification of the polymeric structure of PSNPs that exacerbated slow desorption kinetics; for polar compounds (4-nonylphenol), aging induced changes in surface properties also resulted in irreversible adsorption of contaminants through polar interactions, such as hydrogen bonding. The findings further underline the significant effects of aging on environmental fate and implications of nanoplastics.

The authors conducted column experiments and a modeling study to understand the effects of several environmental factors on the aggregation and transport of graphene oxide nanoparticles (GONPs) in saturated quartz sand. The GONPs were negatively charged and stable under the test conditions (0-50 mM NaCl; pH 4.8-9.0), and the Derjaguin-Landau-Verwey-Overbeek (DLVO) calculation indicated that deposition of GONPs was under unfavorable attachment conditions. The GONPs exhibited high mobility even at an ionic strength of 25 mM NaCl. The transport of GONPs was insensitive to the changes of pH (from 5.1 to 9.0), but the presence of 10 mg/L Suwannee River humic acid (SRHA) considerably enhanced transport at high ionic strength (35 mM NaCl), likely via enhanced steric repulsion and significantly inhibited stacking of GO flakes. Varying flow velocity also enhanced transport at high ionic strength. In general, GONPs exhibit greater mobility compared with other carbon nanoparticles because the aggregation and transport of GONPs are more resilient to changes in solution chemistry and hydrodynamic forces that favor aggregation and deposition of nanoparticles. A 2-site transport model incorporating both the blocking-affected attachment process and straining effects can effectively model the transport of GONPs. The high mobility of GONPs should be given full consideration in assessing their environmental risks.

With the increasing production and use of graphene oxide, the environmental implications of this new carbonaceous nanomaterial have received much attention. In this study, we found that the presence of low concentrations of graphene oxide nanoparticles (GONPs) significantly enhanced the transport of 1-naphthol in a saturated soil, but affected the transport of phenanthrene to a much smaller extent. The much stronger transport-enhancement effect on 1-naphthol was due to the significant desorption hysteresis (both thermodynamically irreversible adsorption and slow desorption kinetics) of GONP-adsorbed 1-naphthol, likely stemmed from the specific polar interactions (e.g., H-bonding) between 1-naphthol and GONPs. Increasing ionic strength or the presence of Cu(II) ion (a complexing cation) generally increased the transport-enhancement capability of GONPs, mainly by increasing the aggregation of GONPs and thus, sequestering adsorbed contaminant molecules. Interestingly, modifying GONPs with Suwannee River humic acid or sodium dodecyl sulfate had little or essentially no effect on the transport-enhancement capability of GONPs, in contrast with the previously reported profound effects of humic acids and surfactants on the transport-enhancement capability of C60 nanoparticles. Overall, the findings indicate that GONPs in the aquatic environment may serve as an effective carrier for certain organic compounds that can interact with GONPs through strong polar interactions.

We describe how the reduction of graphene oxide (GO) via environmentally relevant pathways affects its transport behavior in porous media. A pair of sulfide-reduced GOs (RGOs), prepared by reducing 10 mg/L GO with 0.1 mM Na2S for 3 and 5 days, respectively, exhibited lower mobility than did parent GO in saturated quartz sand. Interestingly, decreased mobility cannot simply be attributed to the increased hydrophobicity and aggregation upon GO reduction because the retention mechanisms of RGOs were highly cation-dependent. In the presence of Na(+) (a representative monovalent cation), the main retention mechanism was deposition in the secondary energy minimum. However, in the presence of Ca(2+) (a model divalent cation), cation bridging between RGO and sand grains became the most predominant retention mechanism; this was because sulfide reduction markedly increased the amount of hydroxyl groups (a strong metal-complexing moiety) on GO. When Na(+) was the background cation, increasing pH (which increased the accumulation of large hydrated Na(+) ions on grain surface) and the presence of Suwannee River humic acid (SRHA) significantly enhanced the transport of RGO, mainly due to steric hindrance. However, pH and SRHA had little effect when Ca(2+) was the background cation because neither affected the extent of cation bridging that controlled particle retention. These findings highlight the significance of abiotic transformations on the fate and transport of GO in aqueous systems.

We examined the transport properties of graphene oxide nanoparticles (GONPs) in saturated sandy soil, under different solution chemistry conditions and flow velocities. GONPs exhibited high mobility in soil, even at 50 mM NaCl. While at relatively high ionic strength GONPs were less mobile in soil than in quartz sand, the differences were not significant. At a concentration of 0.5 mM, Ca(2+) significantly inhibited the transport of GONPs in soil, but only slightly inhibited the transport in quartz sand. This was because by complexing with the surface O-functionalities of both GONPs and soil components, Ca(2+) could enhance the aggregation of GONPs and bridge GONPs and soil grains. Increasing pH from 4 to 9 only slightly enhanced the transport of GONPs in soil, probably because the mobility of GONPs was already high at low pH. The presence of 10 mg L(-1) Suwannee River humic acid significantly enhanced the transport of GONPs in quartz sand at 35 mM, but only had a small effect for the transport in soil. This was possibly linked to the much smaller grain sizes and much more heterogeneous nature of the soil. Flow velocity had marked effects on the transport in soil, but essentially no effects on the transport in quartz sand. A two-site transport model incorporating both the blocking-affected attachment process and straining effects can effectively model the transport of GONPs. The high mobility of GONPs may have important implications for their environmental fate and effects.