The complexity of natural organic matter (NOM) motivates determination of how specific components in a NOM mixture interact with and affect nanoparticle (NP) behavior. The effects of two Suwannee River NOM fractions (separated by a 100,000 g/mol ultrafiltration membrane) on gold NP aggregation are compared. The weight-average molecular weight, Mw, for the unfractionated NOM was 23,300 g/mol, determined by size exclusion chromatography with multiangle light scattering. The NOM was comprised of ~1.8 wt % of >100,000 g/mol retentate (NOMr, Mw = 691,000 g/mol) and 98 wt % of filtrate (NOMf, Mw = 12,800 g/mol). Ten ppm of NOMr provided significantly better NP stability against aggregation than 10 ppm of NOMf in 100 mM NaCl due to steric effects. In the unfractionated NOM, the relative importance of the two components was concentration-dependent. For a low concentration of unfractionated NOM (10 ppm), both fractions contributed to the NOM effects; for a high concentration (560 ppm), NP stability was controlled by the small amount (10 ppm) of NOMr present, rather than the higher amount (550 ppm) of NOMf. Therefore, large humic aggregates in a heterogeneous NOM sample can have disproportionately strong effects, and characterization of Mw distributions (rather than average Mw) may be required to explain NOM effects on NP behavior.
Iron hydroxides are important scavengers
for dissolved chromium
(Cr) via coprecipitation processes; however, the influences of organic
matter (OM) on Cr sequestration in Fe/Cr-OM ternary systems and the
stability of the coprecipitates are not well understood. Here, Fe/Cr-OM
coprecipitation was conducted at pH 3, and Cr hydroxide was undersaturated.
Acetic acid (HAc), poly(acrylic acid) (PAA), and Suwannee River natural
organic matter (SRNOM) were selected as model OMs, which showed different
complexation capabilities with Fe/Cr ions and Fe/Cr hydroxide particles.
HAc had no significant effect on the coprecipitation, as the monodentate
carboxyl ligand in HAc did not favor complexation with dissolved Fe/Cr
ions or Fe/Cr hydroxide nanoparticles. Contrarily, PAA and SRNOM with
polydentate carboxyl ligand had strong complexation with Fe/Cr ions
and Fe/Cr hydroxide nanoparticles, leading to significant amounts
of PAA/SRNOM sequestered in the coprecipitates, which caused the structural
disorder and fast aggregation of the coprecipitates. In comparison
with that of PAA, preferential complexation of Cr ions with SRNOM
resulted in higher Cr/Fe ratios in the coprecipitates. This study
advances the fundamental understanding of Fe/Cr-OM coprecipitation
and mechanisms controlling the composition and stability of the coprecipitates,
which is essential for successful Cr remediation and removal in both
natural and engineered settings.
This review discusses polymeric nanocarriers for agrochemical delivery, from synthesis, characterization, and release, to benefits for agrochemical efficiency and sustainability.
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