Role of surface functionalities of nanoplastics on their transport in seawater-saturated sea sand

The transport and retention of nanoplastics (NP, 200 nm nanopolystyrene) functionalized with surface carboxyl (NPC), sulfonic (NPS), low-density amino (negatively charged, NPA(-)), and high-density amino (positively charged, NPA(+)) groups in seawater-saturated sand with/without humic acid were examined to explore the role of NP surface functionalities. The mass percentages of NP recovered from the effluent (M-eff) with a salinity of 35 practical salinity units (PSU) were ranked as follows: NPC (19.69%) > NPS (16.37%)> NPA(+) (13.33%)> NPA(-) (9.78%). The homoaggregation of NPS and NPA(-) was observed in seawater. The transport of NPA(-) exhibited a ripening phenomenon (i.e., a decrease in the transport rate with time) due to the high attraction of NP with previously deposited NP, whereas monodispersed NPA(+) presented a low M-eff value because of the electrostatic attraction between NPA(+) and negatively charged sand. Retention experiments showed that the majority of NPC, NPS and NPA(+) accumulated in a monolayer on the sand surface, whereas NPA(-) accumulated in multiple layers. Suwannee River humic acid (SRHA) could remarkably improve the transportability of NPC, NPS, and NPA(-) by increasing steric repulsion. The strong attraction between NPA(+) and the deposited NPA(+) in the presence of SRHA triggered the weak ripening phenomenon. As seawater salinity decreased from 35 PSU to 3.5 PSU, the increase in electrostatic repulsion of NP-NP and NP-sand enhanced the transport of NPC, NPS, and NPA(-), and the ripening of NPA(-) breakthrough curves disappeared. In deionized water, NPC, NPS, and NPA(-) achieved complete column breakthrough because the electrostatic repulsion between NP and sand intensified. However, the M-eff values of NPA(+) in 3.5 PSU seawater and deionized water presented limited increments of 15.49% and 23.67%, respectively. These results indicated that the fate of NP in sandy marine environments were strongly affected by NP surface functionalities, seawater salinity, and coexisting SRHA. (C) 2019 Elsevier Ltd. All rights reserved.