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Combined Effects of Fe/Al Oxyhydroxide Coating and pH on Polystyrene Nanoplastic Transport in Saturated Sand Media
Summary
Researchers found that soil pH and iron or aluminum mineral coatings on sand grains strongly control how far nanoplastics travel through the ground, with acidic, mineral-rich conditions trapping particles more effectively — findings that could help predict nanoplastic contamination of groundwater.
Nanoplastics (NPs) in the subsurface environment have received great attention recently. In this study, column experiments were conducted to investigate the individual effect of Fe and Al oxyhydroxide, and also the combined effects of the two oxyhydroxides and pH on the transport of polystyrene nanoplastics (PSNPs) in water-saturated Fe/Al oxyhydroxide-coating sand. The results showed that compared with Fe oxyhydroxide, Al oxyhydroxide exhibited a more profound impact to retain PSNPs because of its higher pHPZC (point of zero charge; Fe: 7.2, Al: 9.9). More interestingly, for pH values (Fe: 4.5–7.0, Al: 4.5–9.7) lower than Fe/Al oxyhydroxide pHPZC, Fe/Al oxyhydroxide coating dramatically enhanced the retention of negatively charged PSNPs, attributing to the less negatively charged surface and thus reduced electrostatic repulsion. In contrast, for pH values (Fe: 9.7, Al: 11.0) higher than Fe/Al oxyhydroxide pHPZC, Fe/Al oxyhydroxide coating had negligible influence on PSNP retention, because the electrostatic repulsion dominated the transport process. These results clearly demonstrate that pH strongly mediates the transport of PSNPs in Fe/Al oxyhydroxide-coated sand, and emphasize the dominant role of electrostatic interaction in PSNP transport. Findings of this study provide new insight into understanding and predicting the fate and transport of NPs in natural medias with complex physicochemical properties.