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Transport of functionalized nanoplastics in goethite-coated saturated porous media: Synergistic effect of polyacrylamide coupled with solution chemistry
Summary
A common soil conditioner used in farming, called polyacrylamide, can make tiny plastic particles move more easily through soil and potentially into groundwater — meaning the products we use to improve crops might actually help pollution spread further and faster than expected. This matters because these nanoplastics could end up contaminating water supplies, and the study found that factors like soil acidity and the type of plastic surface (positively vs. negatively charged) all change how far and fast this contamination can travel.
Polyacrylamide (PAM) is extensively applied to farmland as both a pesticide spray adjuvant and soil conditioner, resulting in co-exposure with widely detected micro- and nanoplastics (MNPs). However, the mechanism by which PAM influences transport behavior of MNPs in soil is still poorly understood. We systematically investigated the influence of PAM coupling physicochemical factors (pH and anion) on the migration behavior of functionalized nanoplastics (PSNPs-COOH and PSNPs-NH) within goethite-coated (FOS) saturated sand columns. Experimental results revealed that PSNPs-NH (45.05 ± 1.28% to 97.71 ± 0.13%) demonstrated superior mobility compared to PSNPs-COOH (16.10 ± 0.05% to 93.44 ± 2.01%) across all conditions. PAM enhances the transport of PSNPs in FOS through hydrogen bonding and surface modification. pH-dependent transport shows non-linear patterns, with maximum mobility at pH 7.0. Under anionic conditions, SO exhibits a stronger retention effect than NO due to enhanced charge shielding and hydrophobic interactions, which is consistent with the Hofmeister series. The molecular specificity of phosphate ions is demonstrated by their distinct forms and varying interaction modes of interaction with the medium and PSNPs. Quantum chemical calculations revealed binding energies of -17.84 kcal mol for PSNPs-COOH-PAM and -7.67 kcal mol for PSNPs-NH-PAM complexes, indicating stronger interactions between PSNPs-COOH and PAM. These findings provide crucial insights into the fate and transport of functionalized NPs in agricultural soils and groundwater systems, highlighting the complex interplay among surface functionalization, mineral coatings, and dissolved polymers in controlling the mobility of NPs.